Energy Matching

Key towards the design of sustainable photovoltaic powered products

Sioe Yao KAN

Energy Matching

Key towards the design of sustainable photovoltaic powered products

proefschrift

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft, op gezag van de Rector Magnificus prof. dr. ir. J.T. Fokkema, voorzitter van het College voor Promoties, in het openbaar te verdedigen op dinsdag, 19 december 2006 om 10.00 uur door

Sioe Yao KAN

elektrotechnisch ingenieur en Diplom-Physiker aan de Universität Stuttgart

Geboren te ‘s-Gravenhage Dit proefschrift is goedgekeurd door de promotoren:

Prof. dr. ir. J.C. Brezet Prof. dr. W.C. Sinke

Samenstelling van de promotiecommissie:

Rector Magnificus, voorzitter Prof. dr. ir. J.C. Brezet, Technische Universiteit Delft, promotor Prof. dr. W.C. Sinke, Universiteit Utrecht, promotor Prof. dr. T.B. Johansson, Universiteit Lund, Zweden Prof. dr. J. Schoonman, Technische Universiteit Delft Prof. dr. W.J. Ockels, Technische Universiteit Delft Prof. dr. dr. h.c. M. Grätzel, Ecole Polytechnique Fédéral de Lausanne, Zwitserland Dr. ir. S. Silvester, Technische Universiteit Delft

Energy Matching - Key towards the design of sustainable photovoltaic powered products Sioe Yao Kan Thesis Delft University of Technology, Delft, The Netherlands Design for Sustainability Program publication nr. 14 ISBN-10: 90-5155-030-8 ISBN-13: 978-90-5155-030-6

The research was funded by NWO/SenterNovem Stimuleringsprogramma Energieonder- zoek (Stimulation Program Energy Research)

Coverdesign and layout by Duygu Keskin Printed by PrintPartners Ipskamp, Rotterdam, The Netherlands

Distributed by DfS [email protected] Tel +31 15 278 2738 Fax + 31 15 278 2956

Copyright © by Sioe Yao Kan. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted by any means, electronic, mechanical, photocopying, recording or otherwise without any written permission from the author.

Table of Contents

Prologue xiii

Acknowledgment xv

1 Introduction and problem definition 1 1.1 Introduction 1 1.2 PV power supplies versus power supplies based on other energy conversion methods 2 1.2.1 General considerations 2 1.2.2 Comparison between the power systems 4 1.3 General overview energy demand and trends of mobile products 6 1.3.1 Digital electronics 6 1.3.2 Emerging technologies 6 1.3.3 Status today 7 1.3.4 Other design issues 8 1.4 PV powered mobile/wireless product designs today and in the coming five years 8 1.4.1 General considerations 8 1.4.2 Renewable energy matching in PV powered products 9 1.4.3 Design integration 10 1.5 Problem definition and research question 10 1.5.1 General 10 1.5.2 Research question and sub-questions 11 1.6 Matching 12 1.6.1 General considerations of matching 12 1.6.2 The energy chain and the energy matching model (EMM) 12 1.6.3 Power matching and energy matching 15 1.6.4 The Figure of Matching (FM) algorithm 16 1.7 Research objective, goals and scope of this dissertation 19 1.7.1 Research Objective and Goals 19 1.7.2 The Scope of this dissertation 19 1.8 Methodology 20 1.9 Outline of this dissertation 21

vii Energy Matching - Key towards the design of sustainable photovoltaic powered products

2 Optimizing photovoltaic (PV) energy conversion systems for mobile/wireless products in outdoor/indoor user contexts 23 2.1 Introduction and general remarks 23 2.2 Characteristics of the user context defined incident light 26 2.2.1 Light Energy, irradiance and illuminance 26 2.2.2 Overview of outdoor light energy sources and spectra 26 2.2.3 Overview of indoor light energy sources and spectra 29 2.2.4 Resume of available incident light energy 33 2.3 Potential photovoltaic power converters performance 34 2.3.1 General PV Review 34 2.3.2 PV output parameters and MPP 34 2.3.3 Overview of photovoltaic cell efficiencies 39 2.3.4 Spectral response of PV cells 44 2.3.5 Résumé of the potential photovoltaic power converter performances 46 2.4 Optimizing the irradiance matching interface (MI:1) 47 2.4.1 The spectral Figure of Matching (MI:1) 47 2.4.2 Minimise shadows on the PV cells by proper design 55 2.4.3 Increase the incident light 57 2.4.4 User context dependent PV power output and the spectral dependent efficiency 57 2.4.5 Résumé on the irradiance matching interface MI:1 57 2.5 Optimizing the PV power output matching interface (MI:2) 58 2.6 Irradiance and PV type dependent power output 59 2.6.1 General Remarks 59 2.6.2 The PV cell power output of one day 59 2.7 Other relevant design aspects 61 2.7.1 General Considerations 61 2.7.2 Curved PV surfaces 61 2.7.3 Colour and PV cells 63 2.7.4 Matching of the user emotional experience options with the PV application 63 2.8 Conclusions 65

3 Mobile/wireless electrical energy storage media 67 3.1 Introduction 67 3.2 Energy storage media characteristics and performance 70 3.2.1 General overview electrical energy storage media 70 3.2.2 Battery characteristics and performance 71 3.2.3 Selecting batteries that match the energy chain of PV powered products 77 3.2.4 Capacitors characteristics and performance 80 3.2.5 Comparing battery and capacitor characteristics 82

viii Table of Contents

3.3 Matching photovoltaic energy converters and energy storage media (MI:2) 84 3.3.1 General 84 3.3.2 Figure of Matching between PV cell and battery 85 3.3.3 Suboptimal energy matching in the PV - battery matching interface (MI:2) 88 3.3.4 Improving the matching between photovoltaic cells and batteries by using capacitors 89 3.4.5 Efficiency of energy transfer from capacitors to Li-Ion batteries 91 3.3.6 Some suggestions for improving the Figure of Matching between PV and battery 93 3.4 Matching energy storage media and energy use in the functional application (MI:3) 94 3.4.1 General 94 3.4.2 Figure of Matching between Battery (storage medium) and energy use in the functional application 94 3.4.3 A suggested solution for improving the Matching between battery and energy use in the functional application with the aid of a capacitor 100 3.5 Other advantages of battery - capacitor combinations 101 3.5.1 General 101 3.5.2 Fast recharge options today 102 3.5.3 Fast and large discharge options in battery - capacitor systems 102 3.6 Conclusions 104

4 Optimal matching in the energy chain 107 4.1 Introduction 107 4.2 Summary of energy matching examples as found in the preceding chapters 109 4.2.1 General considerations 109 4.2.2 Irradiance matching interface (MI:1) 109 4.2.3 Charge energy matching interface (MI:2) 112 4.2.4 The energy use matching interface (MI:3) 113 4.3 Matching parameters outside the energy chain 115 4.3.1 General remarks 115 4.3.2 The construction and embodiment matching 115 4.3.3 Environmental design issues and element matching 116 4.3.4 User context design issues and element matching 117 4.3.5 Standardisation 118 4.4 Overall matching and energy balance in PV powered products 118 4.4.1 General considerations 118 4.4.2 Relating and matching the elements and interfaces in the entire energy chain 120

ix Energy Matching - Key towards the design of sustainable photovoltaic powered products

4.4.3 The PowerQuest tool 122 4.4.4 Design Methodology of PV powered products 126 4.5 Conclusions 126

5 Test cases of mobile/wireless PV powered products 129 5.1 Introduction 129 5.2 Master Graduation project examples at the Delft University of Technology, faculty of Industrial Design Engineering 131 5.2.1 The ‘Backpack’ PV battery charger 131 5.2.2 Solar Rudy and his amazing pupil localizer 135 5.3 Benchmarked existing products 138 5.3.1 The benchmark process 138 5.3.2 The cellular phone powered by a PV battery 139 5.3.3 The universal PV charger ‘Source’ 141 5.4 Test case studies in the framework of the SYN-Energy program 143 5.4.1 General remarks 143 5.4.2 The Solar Mobile Companion 143 5.4.3 The wireless PV mouse 146 5.5 Résumé design steps 148 5.6 Conclusions 150

6 Conclusions and Recommendations 151 6.1 Conclusions 151 6.1.1 General considerations for the conclusions 151 6.1.2 Research question, Energy Matching Model and Figure of Matching algorithm 151 6.1.3 Interface related conclusions 153 6.1.4 Spin-offs to other interfaces outside the energy chain 153 6.2 Design approach and guidelines 153 6.2.1 General approach 154 6.2.2 Energy matching 155 6.2.3 Mechanical physical design aspects 156 6.3 Recommendations for further research 157 6.3.1 Recommendations for the PV and battery suppliers 157 6.3.2 Recommendations for product designers and manufacturers and general research 158 6.3.3 Recommendations for fundamental research 158

Summary 161

Sammenvatting 163

Epilogue 165

Reference List 166

x Table of Contents

List of Figures 179

List of Tables 184

Appendix A: Symbols, Quantities and Units used in this dissertation 185

Appendix B: Abbreviations and Acronyms used in this dissertation 186

Appendix C: Basic Units and fundamentals of light energy and photovoltaic conversion 188

Appendix D: Measured lamp spectra 195

Appendix E: PV performance measurement set-up 196

Appendix F: Battery fundamentals 198

Appendix G: Electronic circuit components and diagrams 199

Curriculum Vitae 201

xi Energy Matching - Key towards the design of sustainable photovoltaic powered products

xii Prologue

Usually one writes a dissertation at the early start of one’s career. The dissertation - they say - will boost your career. Apparently, my karma has been otherwise. This dissertation is written after and despite of an industrial career of over 24 years. So, what have been the drivers. What triggered this project?

In the 2000 I won the first price in an Energy Innovation Competition called ‘INVENTEX’. The main theme of this competition was the big challenge of the coming millennium that would make Energy Conversion and Storage more efficient.

For this competition I submitted two inventions, i.c.; • The ‘Electrical chain motor and power generator’ that won the first price; • The ‘Smart photovoltaic (PV) battery’.

In the search for an application of the winning concept, I met Han Brezet who was at that moment involved in the Mitka project, an electrical power assisted bike. Of course at that moment the smart PV battery concept was not mature. But for me, this invention was too dear for just gathering dust. In addition, it turned out that this invention could eventually be extended in a more general context and could also be applied in more configurations than the one originally anticipated. The main focus shifted from one integrated device, the smart PV battery, towards a more general sustainable product design concept. In particular, the original idea of obtaining efficiency improvement by combining energy conversion and energy storage was transferred to the whole energy chain including energy usage. Therefore, gradually the wish grew to do more with this invention. A breakthrough was made possible by an explorative study for NWO applied for by Wim Sinke of ECN together with Han Brezet and Sacha Silvester of the Delft University of Technology as well as Wim Turkenburg of the Utrecht University. And as a spin-off, a PhD project could be start June 1st 2003 marking also the start of the SYN-Energy program. The first move was the production of a paper on SYN-Energy, [Kan and Silvester, 2003]; the next move was writing a dissertation in which also SYN-Energy evolved into energy matching. The realization of this was however a different story.

So, although, this dissertation is written against all odds, I hope you will enjoy reading it.

xiii Energy Matching - Key towards the design of sustainable photovoltaic powered products

xiv Acknowledgments

Since this PhD thesis is the result of a fruitful collaboration between and with many people at Delft University of Technology, Utrecht University, University Twente and the Energy research Center of the Netherlands (ECN) I owe a word of thanks to so many people that it is impossible to mention them all by names.

First I want to thank NWO/SenterNovem for funding this PhD research and my promoters Han Brezet and Wim Sinke for their support. Han I am grateful for your guidance along a road you have foreseen right from the start. Wim I appreciated your detailed scientific comments and you taught me how to think in small steps. Both promoters have given me positive encouragement and inspiration by their creativity and true involvement.

Also my deep gratitude goes to Sacha Silvester who challenged me by saying ‘I expect that you can make a model of the PV chain, not trying is missing the opportunity’. By reading and commenting the various versions of my draft dissertation gradually the emphasis changed from ‘synergy’ to the more precise ‘matching’. The results are indeed an Energy Matching Model and the associated Figure of Matching algorithm.

I acknowledge also all the members of our DfS program in ever changing formation, for the warm welcome in the beginning given by Alexandra, Linda, Sacha and Han. This applies also for DfS today in particular the warm attention of Susan who is a master in organising things and giving the finishing touch to this dissertation.

I want also to thank Herman Broekhuizen and Martin Verwaal for helping me with the experiments on PV cells, batteries, capacitors and benchmark samples.

Thanks to the ECN members especially Jan Kroon who assisted me with the measurements of PV cells and offered valuable discussions.

I would like to thank the dissertation reading group e.g. Wilfried van Sark and Ruben Strijk to mention a few, for commenting on the draft versions of the dissertation. Also I am grateful for the various graduating Master students from TUDelft’s Industrial Design Engi- neering, Electrical Engineering and Architecture faculties who provided me with relevant information and necessary insights needed in this dissertation.

xv Energy Matching - Key towards the design of sustainable photovoltaic powered products

A special acknowledgment should be given here to the people who assisted me in a practical sense to make this dissertation an Industrial Design Engineering worthy publication. Kelsey Snook has corrected the English text, Duygu Keskin took comprehensively care of the layout, enhancing the figures and printing of the book, with Jan Carel Diehl’s continuous support.

A special place in this book is for my grandmother, who would always stimulated education in our family.

Finally, dear Ems, thank you for everything!

xvi Chapter 1: Introduction and problem definition

1.1 Introduction

The link between light, energy and power has been recognised by mankind since ancient times. Examples can be found in mythology all over the world ranging from the Mayans in the Americas to the Egyptians in Africa [Gallenkamp, 1959; Spence, 1917]. Even today light has a symbolic meaning. For instance the Delft University of Technology in its logo gives tribute to Prometheus who stole the light and fire from the Gods to energize mankind. Could the solution for the energy hungry society of today be found in this light energy? Would the photovoltaic cell that converts light into (electrical) power be the modern equivalent of the ancient ‘fire’ of the Gods? The global use of energy by mankind has increased dramatically in the last decade. This increase has had an impact on the environment, for example the pollution caused by the combustion of fossil fuel such as natural gas and oil. In addition the rapid diminishing amount of fossil fuels in turn has both political and economical implications. Thus, the increasing energy use triggers an urge for more sustainable solutions in order to support this energy demand. In particular, this calls for solutions that do not rely on depletable resources. Although the main global energy use is in the field of industrial applications it can be noted that there is also a rapid increase of industrial designed electronic products that need energy. Energy use and in particular energy supply systems play a crucial role in the design of electronic products. Therefore research is ongoing at the faculty of Industrial Design Engineer- ing (IDE) of the Delft University of Technology (DUT) on the optimisation and sustainable design of energy supply systems of electronic product. Of all these electronic products, a growing part consists of so-called ‘portable’, ‘wireless‘ or ‘mobile’ products. Throughout this dissertation the term ‘mobile products’ will be used. To make these products portable, they typically operate on batteries, mostly rechargeable ones. In general these rechargeable batteries are charged by the electricity grid. Today however there is a tendency for increased mobility or independency from the grid. Thus, a logical step for mobile products will be to combine the energy storage media in these products with a recharge unit that is powered by some kind of mobile energy converter. This will introduce a new mobility concept in which each product is provided with its own truly grid independent power supply system. In a combined energy converter - recharge solution there is a clear difference between batteries recharged by the fixed electricity grid and those recharged by any other potentially more mobile energy converters such as photovoltaics, human power or fuel cells. At first sight this may seem contradictory

1 Energy Matching - Key towards the design of sustainable photovoltaic powered products

and one could say the drive towards more mobility is not compatible with grid power. At the moment however users are accustomed to this way of recharging even by their own non-standardised battery recharge adapter or grid adapter connected to fixed grid outlets. Would mobile products imply that the energy converter should be connected and integrated directly into the products? Could there be a more general approach which also allows fixed external recharging points? Here for the sake of promoting renewable energy the option of recharging with ‘green grid power’ could also be considered. Green grid power is generated by renewable energy sources such as photovoltaics, wind and bio- mass. Electricity from batteries are characterised by Direct Current or commonly known as ‘DC’ power. Electricity from the grid or sometimes called mains are characterised by Alternating Current or known as ‘AC’ power. From fixed green grid power recharge points it is just a small step to that of battery recharging docking stations connected to a DC net powered by photovoltaic (PV) panels mounted for instance on the façade or sunshade of a house. The main drawback of a DC net is the limited distance of a few meters between the output sockets to the façade. This distance is dictated by the allowed voltage drop induced by the resistance of the cables and the current. An advantage of a DC net is that one can get rid of all those AC/DC converters/adapters currently in use for grid connection. This will eliminate energy losses introduced by AC/DC conversion and unnecessary waste of primary products such as iron and copper. An additional advantage will be the potential innovation cascade of designing with DC electricity such as introducing new lighting concepts, new smaller and more energy efficient power supplies and even new security and reliability concepts [Friedeman, 2002]. As indicated above, optimisation of the energy supply will require an optimisation of the entire energy chain in a product. The main elements of the energy chain are the energy converter, some energy storage media and the energy consumption by the application. The integration of technologies and devices into products is common practice in Design, and is firmly embedded in the research and educational program of the faculty of Industrial Design Engineering (IDE) at Delft University of Technology (DUT) [Buijs and Valkenburg, 2000]. In particular, the research on renewable energy sources such as photovoltaic cells and efficient energy use is substantial in the Design for Sustainability program (DfS) [Brezet, 1995; Brezet and Silvester, 2004; LCEP course, 2005-2006].This PhD research project is executed within the framework of the SYN-Energy program which explores the feasibility of the transition towards the use of photovoltaic cells in consumer and professional products [Alsema et. al. 2005]. This program is a part of the ‘Energy Research Stimulation Program’ of the Netherlands Organisation for Scientific Research (NWO).

1.2 PV power supplies versus power supplies based on other energy conversion methods

1.2.1 General considerations In general energy and power consumption or use depends on its application. Thus typically in literature on consumer and professional products, applications are divided according to their instantaneous power consumption in: • Low power applications range from 1 mW up to 100 W; • High power applications range larger than 100 W.

2 Chapter 1: Introduction and problem definition

Using this division, many products in the extreme of the low power application range such as watches, meters, and calculators are today already powered by photovoltaic cells. As a result of this vast amount of examples, the general public perception of PV powered products is limited to these low power applications [Weitjens, 2003; Keukens, 2005]. Taking the value of 100 W as the upper limit of this low power range, one can divide this range even further as presented in Figure 1-1.

Figure 1-1 Products and their power consumption ranges [Flipsen, 2005]

Products with high power applications (>100W) such as electrical drills/screwdrivers, portable defibrillators and portable ultrasound systems are today either directly powered by the grid or by NiCd rechargeable batteries which are recharged by the grid. From interviews and brainstorm sessions [Weitjens, 2003; Langeveld et. al., 2004; Keukens, 2005] it became apparent that the general public perception is that these products can not be PV powered. However, by using the nanotechnology batteries as will be presented in section 1.3.3 b), the power range in the coming years will be stretched further up to values in the range of 200 W. Looking at the power range 10 mW up to 200 W, as is needed in mobile products; one could envisage several power source options as replacement for the non-rechargeable batteries. At the faculty of Industrial Design Engineering of Delft University of Technology, besides photovoltaic cells, human power and fuel cells are also investigated as building blocks for potential energy supply options [LCEP course, 2005-2006]. Before compiling a comparison between the above options, one should start with the common points of departure of these power sources: • All the electrical energy used in a product is in general, a ‘conversion result’ of energy sources around us such as sun-light. Conversion is needed since the available energy generally cannot directly be used as such to power a product. • Quite often, the converted energy cannot be utilized at the very moment of conversion to power a product. A logical and a practical solution is just to store this energy and therefore to delay the moment of use until a more convenient point in time. • Battery re-chargers with some kind of intelligence i.e. smart power management with the aid of ‘special chips’ have already existed for over 10 years [Swager, 1995]. How- ever, since in these examples no mobile energy converter such as PV cells are used,

3 Energy Matching - Key towards the design of sustainable photovoltaic powered products

still today there is a need of an additional apparatus such as AC-DC transformer to be carried and taken along for connection to the grid socket. Note that AC-DC transformers are not standardised, so each product needs its own transformer.

1.2.2 Comparison between the power systems Whether based on photovoltaics, human power or fuel cells, each of the three compared power systems consist of (1) an energy source; (2) an energy carrier and conversion part and; (3) an energy storage part. Elaborating the comparison between the power systems, these elements can be described as follows: (1) The energy source • Photovoltaics (PV): The sun or artificial light source (lamp). • Human power (HP): The human body. • Fuel cells (FC): Hydrogen atoms, however since these atoms as such are not readily available to be put into fuel cells some effort (energy) must be put into the gathering, chemical processing, and packaging of hydrogen compounds. (2) The energy carrier and the energy converter • PV: The energy carrier is light which is converted by a photovoltaic cell directly into electrical power. • HP: The energy carrier is mechanical human muscle force and body heat which can be converted with the aid of a generator or converter into electrical power. Due to the small power yield harnessed with body heat, human power (HP) is generally only associated with mechanical muscle force. • FC: The energy carrier is hydrogen which is converted together with oxygen by the aid of a fuel cell into electrical power. Hydrogen atoms are extracted from the stored ‘fuel’ such as hydrogen gas, methanol or other hydrogen compounds. The oxygen is either extracted from the air or stored separately. (3) The energy storage media • PV conversion will generally be associated with electrical or electro-chemical energy storage media such as batteries. However in cases in which photovoltaics is used to aid electrolysis, there will also be hydrogen gas storage, and sometimes even separate oxygen gas storage. Combining the two gasses in a FC again yields electricity. • HP could be stored as potential energy in a spring or in compressed gas or as kinetic energy in a flywheel. After conversion by a generator, it can be stored as electrical energy in a battery. • The electrical energy generated by FC could either be used directly in an application or stored as electro-chemical energy in batteries. The ‘fuel’ and energy carrier for the FC is stored as a hydrogen compound in a container. In this case there is a distinct difference between storage of electrical energy and energy carrier storage. In comparing electrical energy densities of various technologies this difference should be noticed.

Choosing a certain conversion method for an application will often be a compromise between energy demand, convenience, how the application will be used, location of use and duration of use. Here the method, location and time of use define the so-called ‘user context’. If users become tired of applying human muscle force, a hydrogen cartridge with a fuel cell will give relief. If however, for security reasons and explosion risks, carrying around a compressed hydrogen cartridge is prohibited, readily available daylight captured

4 Chapter 1: Introduction and problem definition by photovoltaic cells could be the proper choice as a power supply. Depending on the application however, the conversion methods will be complementary to each other. If there is no light available, human power can take over from the photovoltaic cells. An example is a radio which is powered both by human power and photovoltaic cells. Sunlight is abundant and available for free throughout the world. So an energy converter that can convert sunlight directly into electricity, such as photovoltaic cells, will compare favourably to a human powered one. Therefore, PV cells are one of the most promising and potentially sound sustainable options to meet the above-mentioned mobile renewable energy need. In certain applications such as electric cars the potential large energy density of compressed hydrogen gas will surpass that of any rechargeable battery making the fuel cells the candidate to power the electric car of the future. Today however, the hazards of compressed hydrogen gas are not yet dealt with nor has any sufficient storage solution been found. This is particularly in regards to security and reliability. The question of how to produce the needed hydrogen gas commercially is still open.

Figure 1-2 PV generated hydrogen fuel-cell car [by courtesy of F. Wouterlood, 2006]

In combining several energy conversion methods, one should keep the total efficiency in mind. This is illustrated by the comic design example from the Engineering School of Rijswijk, as seen in Figure1-2. In this model, light is converted to electricity in the solar panel with an efficiency of about 10% [Green et. al., 2006]. This electricity is used to produce Hydrogen from plain water (H O) by hydrolyse. The efficiency of this process will be 2 maximum 50%. The hydrogen gas is then used in a fuel cell to generate electricity with an efficiency of maximum 50%. The resulting electricity is used to power an electromotor engine which rotates the tires with an efficiency of less than 70%. The total efficiency will be 0.1x0.5x0.5x0.7x100% = 1.75% maximum. This is far less than the efficiency of petrol in a combustion engine of about 25%* [Flipsen, 2005].

* In this comparison it is not taken into account that petrol is a natural product that has taken hundred million of years to become petrol.

5 Energy Matching - Key towards the design of sustainable photovoltaic powered products

1.3 General overview energy demand and trends in mobile products

Before going into detail about the parameters that have had an impact on the design of mobile PV powered products one should first consider mobile products in general, their energy demand and the trends found with such devices. Because trends in mobile products are closely related with the progress and innovations of electronics in general, these items will also be analysed in this section.

1.3.1 Digital electronics The digital revolution has become an accomplished fact, demonstrating the triumph of microelectronics. Despite all pessimistic prediction, Moore’s law [Moore, 1965], which states that the number of transistors on an integrated circuit (IC) doubles roughly each year is today still valid. No sign of levelling or saturation has occurred. This can be seen in Figure 1-3, which displays the number of transistors per computer (Intel) processor (IC) throughout the last 35 years.

Figure 1-3 Moore’s Law [Moore, 2003]

1.3.2 Emerging technologies Polymer electronics is an emerging technology which will become a design opportunity to be taken into account within the next 5 years. Of particular interest are the simplified ways of manufacturing these polymer electronic components, namely by using inkjet printing methods. In addition the band-gap of semiconducting polymers may be ‘tuned’ by selective chemistry [Sol, 2005]. This tuning could for example enable the making of photovoltaic cells sensitive in a prescribed spectral range. Novel communication methods such as Ultra Wide Band (UWB) transmission are emerging. This Ultra Wide Band of communication no longer needs a carrier as with conven-

6 Chapter 1: Introduction and problem definition tional communication means. As a result, power consumption can be reduced to about 10% of that which today is commonly needed by, for example, cellular phones [Dijkstra and Westerhuijs, 2005]. Ambient intelligent systems i.e. complete systems including (micro)-computers, embedded software and transducers to fulfil a dedicated function in wireless interfaces, will become commonplace. In this case wireless is a must, since these systems are too small for connectors. This also means the absence of any connector to the power supply. In addition, even if the connectors could be fitted, the large amount and the vast diversity of locations for these systems will make connection by wiring to the grid expensive and inconvenient or even impossible. Therefore it is often suggested that these systems have to be powered by photovoltaic cells for instance [Sol, 2005]. The use of nanotechnology in electrical storage media is emerging rapidly, as can be seen in examples of novel Super Capacitors [Cap-XX, 2005] and Li-Ion Batteries [Toshiba, 2005; NEC, 2005].

1.3.3 Status today A. General mobility The exploding market of mobile/wireless products such as cellular phones but also Per- sonal Digital Assistants (PDAs) and Notebooks in combination with Bluetooth systems and Wireless Fidelity (WiFi) sets, demonstrate a well-defined trend in both the consumer and professional market towards increasing mobility and wireless freedom. The main high- lights in the field of Telecommunications are Universal Mobile Telecommunications System (UMTS) and Voice over IP (VoIP) or Web-phone, which have become increasingly attrac- tive for private users as well. As a side effect, mobile and wireless products have almost become a necessity in modern daily life. The products are no longer designed with their one prime function in mind. Integration with other functions has to be taken into account. Therefore, the replacement of a single function approach by a fusion or integration of several functions into one overall design will become a trend. An example of this is the integration of Global Positioning System (GPS), Wireless Application Protocol (WAP) and camera functions into cellular phones. However the consequence of this trend is an increase in energy demands. On the other hand, there is also a growing demand for the simplest design. For example, the functionality of cellular phones should become more transparent and simple to manage, particularly in light of increasingly large elderly sectors. The mobile office and higher flexibility in choosing office locations have become a topic. The result is not a fixed working place at the office, but also the possibility to work at home or other locations which reduces traffic jams. This trend has a direct link with the trend of increased mobile and wireless communication.

B. Energy storage The power sources of mobile products have become an increasing matter of concern and annoyance. In particular it is apparent that: • Energy consumption becomes a limiting factor for the functions offered by mobile products powered by batteries; • As mobile products become increasingly smaller, so will the available space for batteries and connecters for electricity grid adapters.

7 Energy Matching - Key towards the design of sustainable photovoltaic powered products

For mobile consumer and professional applications there is a growing demand of (light weight) rechargeable batteries, as a power source. As a consequence, there is also a growing demand by the product functions and user context to recharge these batteries, anywhere at anytime, without limits posed by the presence or absence of electricity grid outlets. Lately this tendency towards grid-independent power has become even more intense due to a series of electrical blackouts and increasing public concern about the reliability of electricity networks as a result of privatisation (well known blackouts are e.g.:Cali- fornia, USA, 2001/2004; Haaksbergen, NL, 2005; Twente and Eindhoven, NL, 2006; main part of west Europe, 5 November 2006). An ironic detail is that during such an occasion of an electrical blackout, no mobile communication was possible due to failure of the grid powered repeater/amplifier send masts. Also the docking stations and receivers of digital phones (DECTs) were without power. Li-Ion Batteries that can be quickly re-charged and provide large discharge currents (large power output) have recently been introduced [Toshiba, 2005; NEC, 2005]. These small (55x43x4 mm) and high power (about 35W) batteries are based on nano-technology. Due to their high power capability, these new Li-Ion battery types will open opportunities for various applications. For example they can be used in the energy supply of power tools such as drills, which until now were considered to be limited to energy sources fed either directly by the electricity grid or by NiCd rechargeable batteries which in turn are recharged by the grid.

1.3.4 Other design issues With the ample availability of powerful and fast electronic (micro)-controller or processor chips, integration of intelligence into the energy consumption chain becomes a must [Scherpen et. al., 1998]. Matching product perception and user contexts becomes an issue. In other words, it would be ergonomically preferable to design a product in such a way that the first impression indicates the way the product is meant to be used. There are already examples of polymer electronic technology in use, as is demonstrated by polymer Optical Light Emitting Diode (OLED) displays for example the display of the Philips ‘Sensotec’ shaver [Philips, 2005]. At the moment however, the flexibility of the polymer electronics casing is still dictated by the use of glass substrates which are needed to keep out moisture and gas.

1.4 PV powered mobile/wireless product designs today and in the coming five years

1.4.1 General considerations Today there is already a vast amount of products on the market that are powered by some kind of photovoltaic (PV) cells. Most of these products are sold just as gadgets or toys. These products are not well designed or sustainable, since they are quite often poorly manufactured [Kan, 2002c]. Sometimes there is a special emphasis on the use of renewable energy (PV cells) in these products, and this feature is even used as a special selling point. In general, one can say that in designing sustainable PV powered mobile products a good balance has to be found between their renewability, functions and user context. This includes ergonomics, and overall design integration, and materialisation.

8 Chapter 1: Introduction and problem definition

1.4.2 Renewable energy matching in PV powered products In analysing the renewability of energy supplies of products, a Life Cycle Assessment (LCA) is typically done. In the case of PV powered products, as a first order evaluation, only the energy-related aspects will be taken into account. This means that the energy payback time (EPBT) will be estimated. The EPBT is generally defined as the time a PV cell or module has to operate in order to generate the same amount of energy (in equivalent terms) as was needed to manufacture it. The environmental impact of a product is then determined by comparing the EPBT with the time the PV cell will be used for that product. The EPBT depends on the PV cell under investigation, its efficiency and user context.

Figure 1-4 The energy payback time (EPBT) of the main silicon PV technologies calculated for South-Euro- pean (S-Eur.) and Middle-European (M-Eur.) locations [Alsema and Wild-Scholten, 2005b]

With the aid of LCAs the EPBT of the main silicon PV technologies: mono-crystalline (mono), multi-crystalline (multi) and ribbon silicon wafer (ribbon) were analysed [Alsema and Wild-Scholten, 2005b] (see Figure 1-4). In the example of mono-crystalline silicon modules, an EPBT was estimated of 1.5-2.5 years for South-European (S-Eur.) locations (irradiance 1700 kW/m2/year). For the Middle-Europe (M-Eur.) region (irradiance 1000 kW/m2/year), a higher EBPT in the range of 2,6-4,4 years was obtained. However at a low irradiance level, with an indoor user context (about 1 kW/m2/year or even less) a corresponding longer EPBT could be estimated. In all of these EBPT calculations the energy to laminate and frame the PV cells and the energy input for Balance of System (BOS) components such as cables and inverters, are taken into account. To be able to claim energy related renewability there must at least be a proper match between the energy and EPBT of the used PV cells and the actual ‘duration of use’ of the product concerned. The duration of use will depend on four factors: 1. The time a design is in vogue; 2. Introduction of new functionalities; 3. Innovative designs; 4. User attachment to the product.

Some types of mobile product designs have a short life cycle and as a result they are not likely to survive long enough to realise the PV EPBT. For example some of today’s designs seem to be outdated tomorrow, ready for disposal. Quite often in these product designs, the incorporated PV cells still need to function for years in order to meet their payback target after the product has for example gone out of fashion. Therefore, it is necessary to

9 Energy Matching - Key towards the design of sustainable photovoltaic powered products

find ways to recycle PV cells. A product with a new functionality will be more in demand than the old one. Therefore this old, but still functional item is discarded. One solution for these outdated products would be a second life. In this case the PV cells would meet their payback target. In cases of personal attachment and innovative design, the user tends to keep the products longer. Thus, the energy payback target of the PV cells is more likely to be met. Other component factors such as batteries, touch screens and interconnection fatigue will then set the time limit. There are however some optimistic signs that the antagonism and gap between EPBT, duration of use and fashion seasons could be bridged by future technological improvements and new of innovations in PV cells. An indication could be found in innovations like Dye Sensitised Cells (DSC) made with spray techniques that might have an EPBT of about 1 year in the Netherlands [Broeders and Netten, 2004; Nanu et. al., 2005]. These PV cells are not yet commercially available.

1.4.3 Design integration The most important criterion for a good design is to assure that the product can fulfil its expected functionality. The functionality is defined as something that a product can perform, independently of an objective. In a certain context there may be an objective (application) for this functionality. Functionality becomes a function in case it can be used in an application [Poelman, 2005]. One of the considerations in designing PV powered products will be the appearance of PV cells. For the long-term this appearance should not be the prominent factor of design. The PV cells simply have to perform well in the intended application but they should neither be allowed to interfere with the design freedom of shape nor with the user friendliness. An example in architecture is the use of PV tiles with the colour of normal tiles. It will not be obvious at first glance that the roof is something special. In other words, it will be a virtually invisible PV application. Of course if the purpose is just to show-off the use of PV on the roof, this example will not be applicable. For the user context of PV on the roof, this user preference has been investigated among the inhabitants of the district Nieuwland in Amersfoort in the Netherlands. In this district a 1 MW PV project was realized. It turns out that the majority (75%) prefer an invisible PV application [Vries and Silvester, 2002]. To avoid the predicate ‘just a gadget’ the added value of the additional use of PV cell must be apparent.

1.5 Problem definition and research question

1.5.1 General Today quite a lot of ‘photovoltaic (PV) powered’ products are already on the market (see Figure 1-5). In most of these products, the PV cells are combined with some kind of rechargeable energy storage medium such as a secondary battery. The function of the PV cells in these products is that of a mobile charger. However, in these PV powered products, often the choice of the used PV cell types is random and these PV cells are simply add-on units to give the product a ‘green’ image. Because of this ‘add-on’ approach, PV cells remain foreign bodies which are not well integrated into the total product design. In other words, there is a sub-optimal matching between the PV cell characteristics, energy storage and the product user context.

10 Chapter 1: Introduction and problem definition

Therefore, the keyword to obtain a sustainable and well-designed product will be matching.

Figure 1-5 The diversity of PV powered products

1.5.2 Research question and sub-questions In formulating the research question one keyword is matching. The matching can, however, be achieved in different gradations ranging from barely matching, to a proper match and even an optimized match. There are also several points of views with regard to the expression matching such as matching in appearance, ergonomics etc. To focus in this dissertation, the matching concerned will be energy matching. In addition, as previously stated, the emphasis of this dissertation will be on the electrotechnical integration aspects in an optimally combined PV - electrical storage system. The core of such a system will be the energy chain. In particular the focus will be on the optimal matching of the elements and interfaces in the energy chain. Taking into account the well defined trend towards increasing mobility and wireless freedom, the Industrial Design Engineering challenge will be mobility. This means the design of PV powered mobile /wireless products. As a result the research question of this dissertation is:

What systematic matching can be achieved between the elements and interfaces of the energy chain of photovoltaic powered mobile/wireless products?

Notes: • Energy chain concerns the electrical elements: photovoltaic energy converter, energy storage media and the energy using application of the product; • Current and envisaged future mobile/wireless PV powered products.

To provide a convenient outline of this dissertation, the research question above is divided into specific ‘sub-questions’ (Sub Q.). The first two are in line with the two main elements of the energy chain under investigation. This is namely the photovoltaic converter and the energy storage media:

11 Energy Matching - Key towards the design of sustainable photovoltaic powered products

• What photovoltaic conversion systems are optimally matched with applications in an outdoor/indoor user context? Sub Q. 1. • What matching can be achieved between the electrical energy storage media and both the photovoltaic energy conversion systems in an outdoor/indoor user context (1) and the functional application (2)? Sub Q. 2.

In addition, to complete the matching analyses: • What optimisation can be achieved between the elements of the energy chain of a PV powered mobile wireless/products and how does this affects other design engineering aspects? Sub Q. 3. • What insight can be gained by analyses of the energy matching in test cases of PV powered products? Sub Q. 4.

The answers to these sub-questions constitute the contents of the consecutive chapters of this dissertation.

1.6 Matching

1.6.1 General considerations of matching In finding answers to the research question and sub-questions, the main concern will be ‘matching’. Therefore, for the sake of clarity first this subject matter will be discussed. Throughout this dissertation the search for answers to the research question, namely the analysis of matching, will be guided by an Energy Matching Model (EMM). To probe how well the matching is achieved, a Figure of Matching (FM) algorithm will be used to analyse and quantify the matching.

1.6.2 The energy chain and the Energy Matching Model (EMM) As mentioned above, in this dissertation optimal matching is sought between the elements of the energy chain of PV powered products. For this purpose a novel ‘Energy Matching Model’ (EMM) and a related ‘Figure of Matching’ (FM) algorithm have been developed. In Figure 1-6 this Energy Matching Model of the energy chain is presented. The main elements of the energy chain are: a) user context defined incident light, b) PV power converter, c) electrical energy storage media, d) energy use in the functional product application and e) user context defined power/energy use pattern of the product.

Ad. a) User context defined incident light The incident light upon the PV cells depends on the context of when and where the PV powered product is used. The main characteristics of the incident light are its intensity and its spectrum. A product that is used only outdoors requires a different type of used PV cells than one which is used only indoors. In Chapter 2 the matching between various types of PV cells and incident light types for both outdoor and indoor user contexts will be analysed. In this dissertation the incident light will be treated as a given exogenous element so all the light transmission losses due to encapsulation and cover glasses of the PV cells and windows are already taken into account in the incident light [Schmidhuber, 2003; Randall, 2003].

12 Chapter 1: Introduction and problem definition

Ad. b) Photovoltaic power converter system The photovoltaic (PV) power conversion system is comprised of photovoltaic cells, electrical interconnection and interfaces, and smart power management circuitry. In conjunction with the energy chain of a PV powered product, the photovoltaic (PV) cells are regarded as light dependent electrical current sources. Whenever there is light available, a current is generated which can either be drained by an electrical energy storage medium or be used directly in an energy consuming product application. In Chapter 2 this PV power converter system and its interfaces will be analysed.

Ad. c) Electrical energy storage media In an electrical energy storage medium, electrical energy is stored either chemically e.g. in a battery or as potential energy in a capacitor. In Chapter 3 the electrical energy storage media will be analysed.

Figure 1-6 The energy chain of a PV powered product and the Energy Matching Model (Note that the terms like MPPT, DC, surge etc. will be explained in the following chapters)

13 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Ad. d) Energy use in the functional product application Energy use in functional product application refers to the actual energy consumption needed to fulfil the tasks of the product. For the design of a new product one can analyse datasheets of existing products. In the data sheets and user manuals of these existing products one can usually find some information about power consumption. Information is also sometimes provided in the data sheets, on whether there are any stand-by and sleep modes, including the power demand for those modes. By knowing the duration of use, the energy demand of the newly designed product can be estimated for particular applications. But this is only valid for a limited moment in time and this calculation does not give enough data for the complete design of a new product. Thus the information about energy use in the functional application has to be supplemented by the ‘pattern of use’ of the new product to be designed.

Ad. e) User context defined power/energy use pattern By knowing the ‘pattern of use’ over a certain period of time by one particular user or user group and combining this information with the power demand from the data sheets of existing products the total energy consumption of the new product over a certain period of time (e.g. for one day of that particular user) can be calculated.

The elements in the energy chain interface with each other through so-called matching interfaces (MI: 1-3). MI 1: An incident light PV system matching interface which concerns the impact of the outdoor/indoor user context defined incident light on the choice of PV technology to be used in the designed product. For example, as mentioned above, the user context will define the type (spectrum) and magnitude of incident light that will energize the PV powered product. MI 2: A PV output storage input matching interface which concerns the matching between the PV cells and its output circuitry, in particular to facilitate the proper matching of the PV output with respect to voltage and signal shape, as required for proper charging of the energy storage media. MI 3: An energy use matching interface which concerns circuitry for the optimal energy transfer from the energy storage media into the application.

To obtain a functional totality in addition to the above interfaces, the overall energy balance also has to be analysed. There are two overall energy balance tracks that close the feedback-loop through the overall application matching interface (OMI), namely: Track (1): The matching between the outdoor/indoor user contexts defined incident light and the needed energy by the application in a specific user context. Track (2): The matching between the outdoor/indoor user context defined incident light, the output of the PV cells, the energy storage capacity and the user context defined energy use. In other words an overall matching and energy balance in which the user context in combination with all the elements in the energy chain are taken into account.

14 Chapter 1: Introduction and problem definition

In the majority of today’s designs of PV powered products, the designer makes up the energy balance by comparing the total incident light energy available in a certain user context with the predicted energy use in the functional product application of the product to be designed (track 1) [RetScreen, 2005]. In this comparison the elements and interfaces inside the cadre box of Figure 1-6 are not taken into account. In fact this box is treated as a ‘black box’. From test cases it became apparent that for a proper design one needs to probe inside this black box [Weitjens, 2003]. Therefore this dissertation will focus on the contents of the cadre box and the energy balance will follow track 2. Energy balances will have a direct impact on several other interface matching parameters such as the embodiment/dimensional matching and environment matching. Embodiment matching means the matching between mechanical product design parameters such as weight, volume, structure, shape and the available PV mounting area on one side and the other elements of the energy chain on the other. As an example, the energy balance will dictate the necessary size of the PV cells to power the application and the available area to support the PV cells. Environment matching means the matching between the product and the environment in which it has to function. As an example there will be discrepancy between the energy balance at ambient temperature and one at say -24 °C. Resuming, a PV powered product could be used in a certain user context and energy use pattern, that defines the magnitude and nature of the incident light, the amount of energy use, mechanical dimensions, ergonomics and other environmental parameters such as temperature, humidity etc.

1.6.3 Power matching and energy matching In this dissertation matching is sought between the elements of the energy chain of PV powered products. The analysis will focus on ‘electrical energy matching’ or more popu- larly known in literature as ‘load matching’ [Jay, 1977]. Load matching is defined as: • The process of adjustment of the load circuit impedance to produce the desired energy transfer from the power source to the load; • The technique of either adjusting the load circuit or inserting a network between two parts of a system to produce the desired power or energy transfer.

In the two definitions mentioned above, the measure of how optimal the matching has been accomplished is the ‘transfer efficiency’ ( ). This is through an interface be- ηtransfer tween two parts of the system under investigation, and is generally defined as:

= P / P x 100% Eq.1-1 ηtransfer load power source

Note: P and P are respectively the power transferred into the load and the power emanat- load power source ing from the power source. Since the investigated system is an energy chain in which energy storage media are also involved, the emphasis should be focused on the energy transfer efficiency instead of power.

15 Energy Matching - Key towards the design of sustainable photovoltaic powered products

So the energy transfer efficiency ( ) can be defined as: ηE-transfer

= ∫ P dt / ∫ P dt x 100% Eq.1-2 ηE-transfer load power source active-time1 active-time2 Here the active time is the time the power source or the power load is in use. In most cases active-time 1 would coincide with active-time 2, for a general approach however this option in Eq. 1-2 is still open and will be closed for each specific case. In the energy chain of PV powered products, the transfer efficiency would be: • What percentage of incident irradiance power is converted into electrical power in the PV cells? And over a certain time sequence, e.g. one day, what percentage of the incident light energy is converted into electrical energy? • What percentage of the converted irradiance energy is emitted from the PV terminals as electrical energy towards the load or in most cases as an energy storage medium? • What percentage of the energy emanated at the PV terminals can be transferred, will reach and can be stored in the electrical energy storage medium? • What percentage of the energy available at the energy storage medium terminals will be transferred, reach and be used in the application?

To define the energy transfer efficiency of the entire energy chain one will not only need all the transfer efficiencies between the elements but also a defined user context, a certain pattern of use and well defined environmental conditions. This total energy transfer of the energy chain will not be generally valid but will change in time in accordance to the actual user scenario.

1.6.4 The Figure of Matching (FM) Algorithm The power and energy transfer efficiency between the elements of an energy chain as presented in section 1.6.3 can not always be measured directly or are typically not available as such for the designer of PV power products. So to analyse, quantify and predict to what extent the matching through the matching interfaces (MI:1 till 3) as presented in Figure 1-6 actually is achieved, a novel generic Figure of Matching (FM) algorithm is developed and introduced [Kan et.al, 2006c]. The term ‘generic’ is stressed since the format of this novel Figure of Matching algorithm is applicable at all the matching interfaces (MI:1 till 3). In this Figure of Matching algorithm, the main criterion for how well two elements in an energy chain are matched will be how efficient the power transfer from one element to the next is conducted. This Figure of Matching algorithm can be seen as a combination, adaptation and extension of two analytical methods namely the Stimulus - Response transfer concept and the correlation concept known from the general theory of information transmission. The rationale for this adaptation is that, in fact, the ‘information transmission or transfer’ in this theory concerns a kind of ‘power transfer’ [Krul, 1976]. Therefore information transfer formulas and nomenclature can be used. In other words, the energy chain of a PV powered product is treated as an ‘energy transmission line’. In general the Stimulus - Response transfer concept is presented in Figure 1-7.

Figure 1-7 The Stimulus (S) - Response (R) concept applied on an element p

16 Chapter 1: Introduction and problem definition

Each Stimulus S applied to an element p will provoke a Response R . From the relation p between the Stimulus and the Response one can learn about the characteristics of the element under investigation. In comparing the Response of various different Stimuli it is appropriate to first determine the Response of a standardized Stimulus. In such a standardized Stimulus - Response verification test, it is common practice that the Response R is measured as a result of a ‘Step Stimulus’ Therefore it is named a ‘Step-Response’ [Blok, 1973], see Figure 1-8.

Figure 1-8 The presentation of a Step-Response

In the theory of signal transmission the step is presented as:

def 0 → t = 0 S = Eq. 1-3 step 1 → t > 0

With t the time sequence coordinate of the ongoing process. In the energy chain the output of one element will be the input of the next element in other words the output of one element will be a stimulus for the next element in the chain as presented Figure 1-9.

Figure 1-9 Energy flow between two elements s and p in the energy chain

To analyse the matching between two elements in the energy chain, the correlation between the two is investigated. In particular, the output of the first one is correlated with the step response of the second. With this correlation, according to the theory of transmission lines, the power transfer efficiency between the two is established. Having determined the Step-Response of one element p, R , the impact of various stimuli of the influencing ele- Step-p ments on this element p can be compared with the aid of the Figure of Matching algorithm as presented in Figure 1-10. Now the optimal matched pair can be selected.

Figure 1-10 The Figure of Matching (FM) algorithm between two elements s and p in the energy chain [Kan et.al, 2006c]

17 Energy Matching - Key towards the design of sustainable photovoltaic powered products

With: R = Response coming from Element s = S = Stimulus on Element p s p R = Response on the standardised Stimulus i.e. the Step-Response of element p Step-p FM = Figure of Matching

The Figure of Matching between two elements s and p is defined as:

FM = { Ø / ∫ S (var) d var } x 100% Eq. 1-4 p

Here Ø is the correlation between the Stimulus S emanated from element s towards p the next element in the energy chain, i.e. element p, and the Step-Response R of this step-p element p.

Ø = ∫ S (var) x R (var) d (var) Eq. 1-5 p step-p var range Since the Figure of Matching is a quotient of the correlation Ø and the overall stimulus S p it can be regarded as a normalised correlation. The Stimulus S and the Step-Response p R are both a function of a given variable ‘var’. The higher this ‘Figure of Matching’ is the Step-p better the matching. This is just the opposite of the convention in the mathematical Figure of Merit in which the best one will have the lowest value [Weisstein, 2006]. A generalisation of the original time frame based concept of step responses is introduced here by replacing the time t with a general variable ‘var’ instead. This Step-Response concept fits into the generic Matching Algorithm described above. A generalised Step- Response could be written as:

def 0 → var < var S = 1 Eq. 1-6 step 1 → var > var 1 Here the variable var is the starting point of the step. 1

As mentioned above this generic Figure of Matching algorithm can be applied at all three matching interfaces (MI:1 trough 3) of the energy chain, as presented in Figure 1-6. The Figure of Matching can be made specific for one particular matching interface by selecting the proper power or energy type that has to be transferred between the two elements under investigation and as such should be correlated and matched at that particular interface. The related variable ‘var’ can than be selected to fit the Figure of Matching formula. The variable ‘var’ can for example be the wavelength λ, in which case the Figure of Match- ing is used in the matching between the spectrum of the incident light (S ) coming from p a light source (element s) and the spectral response (R ) of the PV cell (element p), as Step-p in the matching interface MI:1. The validation of the Energy Matching Model of the energy chain model and the Figure of Matching algorithm is done by calculating the Figure of Matching (FM) and comparing the results with measurements of the energy transfer efficiencies across the Matching In- terfaces (MI:1 till 3) and elements, as presented in Figure 1-6. This validation is a two way process. In one direction with the FM algorithm the measured transfer efficiency or found effect at a matching interface can be ‘explained’. In the other the FM provides ‘predicted design options’ that can be implemented and tested.

18 Chapter 1: Introduction and problem definition

In the Chapters 2 and 3 for each matching interface (MI:1 till 3), the appropriate transferred power type, variable ‘var’ and Step-Responses will be identified and measured to calculate the Figure of Matching which then can be compared with the experimentally found transfer efficiency through the analysed matching interfaces. The Figure of Matching algorithm is conceived in the direct way as a straight forward solution, giving quantitative results. This solution is chosen in favour of other less direct ways such as first developing theoretical models etc. This direct quantitative result is expected to yield good input for designers of PV powered products since it contributes towards a clear understanding of the design parameters involved.

1.7 Research Objective, Goals and Scope of this dissertation

1.7.1 Research Objective and Goals The research objective of this PhD project is to contribute to the understanding of how, why and under what circumstances the introduction of the Energy Matching Model (EMM) with the Figure of Matching (FM) algorithm could aid industrial designers to design more properly matched photovoltaic (PV) powered products. These well designed products will enable the diffusion of PV powered products from the niche market towards the mainstream and in addition could even provoke an innovation cascade in which new yet unconceivable products become feasible. The goal of this PhD project is to pave the way towards more sustainably designed PV powered products.

1.7.2 The Scope of this dissertation In this PhD project the energy chain is treated as one system. Thus the optimisation of the elements and interfaces will be limited to a system level. Technological aspects are therefore treated only in depth (on a component level) merely in those cases in which the technological details will clarify the measures to be taken for optimisation of matching. In the other cases the technological aspects are presented just for sake of completeness. The research activities in this PhD project will be focused on the integration aspects for optimal combination and the proper matching between the elements and interfaces in the energy chain of mobile photovoltaic (PV) powered products: i.e. photovoltaic (PV) energy converter, electrical energy storage media, and energy using application. As a result the emphasis in this dissertation is on the electro-technical integration and matching aspects as presented by the items inside the cadre box in Figure 1-6. Therefore the scope of this dissertation is circumscribed by this cadre box. Only those elements and interfaces that are inside this box will be analysed in detail. All the elements and interfaces outside this box; namely the user context defined parameters, the energy use inside the product application and the environmental parameters, are all treated in this dissertation as given exogenous parameters. These parameters will be mentioned in this dissertation for sake of completeness. However, the realization and detailed analysis of the nature of these given exogenous parameters are beyond the scope of this dissertation. The mechanical - physical context parameters such as embodiment and thermal matching are partly treated as being exogenous and are partly contained in the empirical - scope of this study, but the analyses are not systematically and complete. Also the detailed analysis of the energy conversion process inside the PV power converter will be regarded in this dissertation as exogenous.

19 Energy Matching - Key towards the design of sustainable photovoltaic powered products

These detailed analyses and modelling of elements and parameters will be treated in other research programs and dissertations within the SYN-Energy program [Reich, forthcoming, 2008]. Also further optimisation of the overall efficiency of energy consumption, with the use of energy/power management tools are beyond the scope of this dissertation. A vast amount of literature on these topics can be readily found in open literature [e.g. Pouwelse, 2003, Havinga, 2000]. In addition the emotional experience and user interface aspects will be beyond the scope of this dissertation. In the framework of the SYN-Energy program the research in these fields will be treated in another dissertation [Veefkind, forthcoming, 2007] at Delft University of Technology and research at Twente University. The same applies for the Life Cycle Assessment and Costing aspects and PV Technology modelling which will also be beyond the scope of this dissertation. In the framework of the SYN- Energy program the research in these fields will be conducted at Utrecht University.

1.8 Methodology In writing this dissertation and in search of answers to the research question presented in section 1.6, a research methodology is used that can be seen as a mixture of the systematic engineering research methods employed during my 24 years in the industry as well as the design methodology from the faculty of Industrial Design Engineering of the Delft University of Technology [Roozenberg and Eekels, 1998]. Both methodologies have an Orientation and Analysis phase. In these phases both the research question and the related problem definition are crystallised. Important in these phases are literature studies or more general the gathering of data and expert views. Literature studies are needed to establish the baseline what is known at the start of the project and to avoid a ‘re-invention of the wheel’. An extension of these literature studies are the expert views gathered from interviews taken by the author and those taken by Master Project students but also discussions by the author with experts at conferences. In this dissertation the literature will be quoted as [Ref., year]. The expert views will sometimes be quoted as [Private Communication] but mostly they will be hidden as the source is from a Master Graduation Thesis. If the expert view is an additional explanation of a presented paper, then only the original paper will be quoted. Experiments will be done to fill the missing data gaps. For this dissertation the Orientation phase was triggered by the invention of the Smart PV Battery [Kan, 2002a]. In the Analyses phase, this idea is explored further and generalized eventually results in the Research Question (section 1.5.2). Both methodologies have a Synthesis phase. In this phase the found data is synthesised to yield ‘product concepts’ in the design methodology. On the other side; the Engineer- ing Research Methodology in this Synthesis phase will combine the found data with the common practice and the author experiences. The result will be several ideas on how to answer the research question and sub-questions, as well as provide directive for experiments. In this phase new inventions might also be proposed (Chapters 2 and 3). In the Verification phase of the Engineering Research Methodology, the energy balance will be simulated and some experiments will be done (Chapters 2, 3 and 4). Due to the needed iterations both Chapter 2 and 3 will report about the findings in the Synthesis and the Verification phase. In the Evaluation phase the new found knowledge is used to evaluate some tests cases (Chapter 5).

20 Chapter 1: Introduction and problem definition

1.9 Outline of this dissertation The outline of this dissertation is structured by the quest to discover consecutive answers for the research question and sub-questions with the aid of the Energy Matching Model and Figure of Matching algorithm, as stated in section 1.5.2. In the following chapters, therefore the elements and matching interfaces of the energy chain, as presented in Figure 1-6 are to be analysed. The complete structure and outline of this dissertation is visualised in Figure 1-11.

Figure 1-11 The structure and outline of this dissertation

The first part of the research question (Sub Q. 1) will be addressed in Chapter 2. This Chapter 2 deals with photovoltaic performance and methods for optimising the matching with respect to incident light and power output from the PV cells in outdoor/indoor user context. In particular the interaction between the user context defined irradiance and the PV power converter in the irradiance matching interface (MI:1) will be analysed. This will include some examples of the spectral Figure of Matching calculations. The second part of the research question (Sub Q. 2) will be addressed in Chapter 3. This Chapter 3 deals with energy storage media in conjunction with photovoltaic cells and energy use. In particular the electrical interfaces between the PV power converter and the electrical energy storage Media (IM:2) and between the electrical storage media and the functional application (IM:3) will be analysed. This will include calculations of the Figure of Matching and some suggestions for improvement which are validated by a comparison of the measured transfer efficiencies. Chapters 2 and 3 will describe the parallel efforts to be undertaken to make the PV powered product feasible and well designed. The diagram in Figure 1-11 illustrates that the matching presented in Chapters 2 and 3 have to be done in parallel simultaneously in order to achieve the goal of the overall energy matching, as presented in Chapter 4. Chapter 4 addresses the optimisation between the elements of the energy chain and

21 Energy Matching - Key towards the design of sustainable photovoltaic powered products how this effects the other design engineering parameters as stated in the third part of the research question (Sub Q. 3).The found optimisation is presented here as a consequence of combining the results of Chapters 2 and 3 with the overall matching for energy use and energy balance in the design. Chapter 4 is followed by Chapter 5 which presents an overview of some benchmark experiments, test case studies and product examples compiled from master graduation theses and the SYN-Energy program. In Chapter 5 the fourth part of the research question (Sub Q. 4) is addressed. This chapter demonstrates and illustrates the feasibility of using the Energy Matching Model and the Figure of Matching algorithm in analysing test cases of PV powered products. To complete this dissertation it is finished with several conclusions and recommendations for further research (Chapter 6). For the reader’s convenience, basic information is provided in the Appendices. In Appendix A the used symbols and related units are listed; in Appendix B the used abbreviations and acronyms are tabulated. In Appendix C the basic units and fundamentals of light energy conversion is presented. In Appendix D the measured indoor lamp light spectra are tabulated. In Appendix E the setup of PV performance measurement at low irradiance levels is presented. In Appendix F the Battery Fundamentals are summarised. In Appendix G some relevant electronic schemes and component lists are presented.

22 Chapter 2: Optimizing photovoltaic energy conversion systems for mobile/wireless products in outdoor/indoor user contexts

2.1 Introduction and general remarks

In this chapter the most important element in the energy chain of photovoltaic (PV) powered products, namely the photovoltaic power converter and its interfaces will be analysed. In particular this analysis will include the user context defined incident light flux, the incident light matching interface (MI:1) and the PV power converter. In addition part of the electrical interface that concerns the power transfer from PV cells to the system terminals and the electrical output power from those terminals to the energy storage media will also be treated. In doing so, the first part of the research question will be answered:

What photovoltaic conversion systems are optimally matched with applications in an outdoor/indoor user context?

A photovoltaic conversion system is comprised of photovoltaic (PV) cells, electrical interconnection and interfaces and smart energy management circuitry. In conjunction with the energy chain of a PV powered product, PV cells are regarded as light dependent electrical current sources. Whenever there is light available, a current is generated which can either be drained by an electrical energy storage medium or directly be used in an energy consuming product application. In an ideal case this current would be linearly dependent on the irradiance level while the output voltage remains constant. In practice, however, neither the output voltage nor the directly related conversion efficiency conforms to this ideal behaviour.

With optimally matched primarily energy matching is meant. This includes: 1. Maximal transfer or conversion efficiency from light power to electrical power by choosing the right type of photovoltaic cell to match the available light type (spectra) and level in accordance to the user context. 2. Maximal extraction of the electrical power from the PV cells (or modules) to the output terminals. 3. Optimal match with respect to the user as far as the minimum required availability pattern is satisfied. If that required minimum is satisfied then one can speak of an optimal match. 4. Differentiating what is a real must for the user and what is of secondary importance. For an optimal match weight factors for certain energy use have to be taken into account.

23 Energy Matching - Key towards the design of sustainable photovoltaic powered products

5. Matching between energy parameters of the used type of PV cells and the charge parameters of the energy storage media. Thus an optimal match with respect to the available energy coming from the PV cells and the capacity of the battery being used (see Chapter 3). 6. Mobile products are by logical consequence of being carried around applied in an outdoor/indoor user context. This means that to achieve the optimal match, PV performances under low irradiance levels, as occur at indoor conditions, will also be important design parameters to be taken into account.

The part that concerns the optimal matching of electrical interface with the electrical energy storage media however will be treated in Chapter 3. Figure 2-1 presents the Energy Matching Model of the energy chain marking the elements and interfaces analysed in this chapter by including the section numbers.

Figure 2-1 The energy chain of a PV powered product - the marked (section numbers) elements and interfaces will be analysed in this chapter

24 Chapter 2: Optimizing photovoltaic energy conversion systems for mobile/wireless products in outdoor/indoor user contexts

From literature [e.g. Randall, 2003] it becomes apparent that until now around the world much renewable energy research effort is put into designing and optimisation of PV cells for outdoor operation only. However in matching photovoltaic power conversion system characteristics with the energy use in mobile products different aspects regarding photovoltaic cells used outdoors have to be taken into account. This is namely: • At this moment the datasheets of photovoltaic cell manufacturers present only PV performances measured under Standard Test Conditions (STC). STC means that the tests are conducted at an irradiance level of 1000 W/m2, which equals that of a clear summer day, with an air mass (AM) 1.5 spectrum and temperature of 25 °C. Since in the Netherlands for instance, there is more often an overcast sky, the irradiance level will in practice be less than 1000 W/m2 and the available light spectrum will not comply to the defined AM 1.5 spectrum. Also indoor light characteristics do not comply with the STC conditions. In addition the PV cells could have been warmed up to a temperature far over 25 °C or cooled down below 25 °C (e.g. mobile consumer products specification: 0 °C up to 40 °C). • The incident angle of irradiance on PV cells mounted on a product will not always be exactly perpendicular. • The size of the PV cells or modules will in most cases not be the common module size used on roof-tops. • The price will depend on application, the more urgent the additional function is needed the more money that can be invested in the PV cells. • Mechanical flexibility is usually not a matter of concern for rooftop application. For integration on products it will determine the feasible design space.

To find answers for the above-mentioned research sub-question the matching through the following matching interfaces will be analysed (see Figure 2-1): • Irradiance interface and its energy transfer efficiency (MI:1); • Electrical PV output and storage input Interface and its energy transfer efficiency (MI:2).

The emphasis is on the energy transfer, therefore mechanical interfaces and user interfaces are treated on the second place as ‘other relevant design parameters’ (section 2.7). In this dissertation they will be regarded as ‘given exogenous parameters’. Data of these exogenous parameters is obtained from literature, estimations of test cases and other joint measurements within the framework of the SYN-Energy program. The outline of this chapter is shown inside the cadre box, namely marking the sections numbers of the elements and matching interfaces (MI:1,2) of Figure 2-1. To investigate the matching mentioned in the research question above, first those items that are to be matched have to be analysed. Therefore the first two items to be reviewed are: What is the nature of the available light power to be converted (section 2.2) and what are the potential photovoltaic power converters and the performance that could be achieved by these PV converters (section 2.3). Next the optimal matching between the photovoltaic and the irradiance takes into account the efficiency over the entire irradiance range including indoor levels (the irradiance matching interface MI:1 in section 2.4), and the electrical matching at the output terminals (electrical matching interface in section 2.5). The irradiance and PV type dependent power output available for storage in the energy storage

25 Energy Matching - Key towards the design of sustainable photovoltaic powered products

media is analysed in section 2.6. Other relevant design items such as mechanical interfaces and user interfaces are presented in section 2.7. Finally in section 2.8 the conclusions are summarised and a roundup of the answers for research sub question 1 are presented. Parts of the analyses and information presented in this Chapter have been published previously in papers presented at the Euro Sun/ISES conferences [Kan, 2002b; Kan 2003; Kan et. al., 2004b], IPSS [Kan et. al. 2005b] and European PV conferences [Reich, Kan et. al., 2005].

2.2 Characteristics of the user context defined incident light

2.2.1 Light energy, irradiance and illuminance In a photovoltaic (PV) cell light power is converted into electrical power. In this dissertation it is assumed that the reader is familiar with this basic knowledge on light power. Information on this can be read in common textbooks on physics, [e.g.; Taylor, 2000; Ryer, 1997]. For the reader convenience a resume of the fundamentals is presented in Appendix C 1-3. Both sunlight and lamplight can be converted by PV cells into electricity that can be used to power mobile products. In literature the expression for this incident light flux is ‘irradiance’ for sunlight and ‘illuminance’ for artificial lamplight. Irradiance is a measure for the radiometric light flux incident on a surface per unit area; expressed in W/m2. Illuminance is expressed in lm/m2 or more commonly lux (lx).

2.2.2 Overview of outdoor light energy sources and spectra The sun’s irradiance just outside the earth’s atmosphere is 1390 W/m2 at the mean earth- sun distance, 1438 W/m2 at perihelion (January), 1345 at aphelion (July), and the solar constant is 1353 W/m2. Of this total amount of solar power available from outside the earth’s atmosphere, between 30 % and 45 % will be lost due to reflection and absorption in the atmosphere. Thus, in net at sea level on the earth only about 1000 W/m2 of solar power could readily be converted into electricity by a solar cell [Hazen, 1996, Green, 1986, Engstrom, 1974]. The spectrum of this solar irradiance, as received by a photovoltaic converter at sea level, is inside a fixed wavelength-window. This window starts at a wavelength of about 300 nm (ultra violet, UV) and ends at a wavelength of about 4 μm (infra red, IR) or effectively up to a wavelength of about 1.8 μm. It encloses the visible range from 380 nm up to 750 nm as presented in Figure 2-2 and Figure 2-3. The solar spectrum of AM 1.5 is usually defined from a wavelength of 295 nm up to a wavelength of 2545 nm as presented in Figure 2-2. For Standard Test Conditions (STC) normalisation purposes also the more infra-red (IR) regions are taken into account. The sensitivity of the PV cels under investigation are however negligible in these IR wavelength regions. The attenuation of the sunlight by the atmosphere depends on the optical path length to the observation point. This path length is shortest at the moment the sun is directly overhead (see Figure 2-4). During actual use of the PV cells the incident light is not always perpendicular.

26 Chapter 2: Optimizing photovoltaic energy conversion systems for mobile/wireless products in outdoor/indoor user contexts

Figure 2-2 Spectral solar irradiance with absorption bands [Engstrom, 1974]

Figure 2-3 Spectral distribution of sunlight, the AM1.5, AM0 and 6000 K blackbody radiation [Green, 1986]

27 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Figure 2-4 The location of the observer and the PV powered product with respect to the sun

The ratio between ‘any path length through the air on earth’ and the path length at the moment that the sun is directly overhead is called the optical air mass or air mass (AM) for short. As a result in case the sun is directly overhead, the air mass is unity (AM1). The sun light flux spectrum in this case is called: air mass one (AM1). In any other case, the sun will make an angle θ to this exact overhead position and therefore the air mass is given by:

Air Mass (θ) = 1 / cos θ Eq. 2-1

To allow a consequential comparison between performance measurements of different PV cells throughout the world, a global standard named ‘Air Mass 1.5’ or AM 1.5 is often quoted in literature. The total amount of irradiance depends on the location on the globe and the time of the year. In summer, the amount of potential irradiance will clearly be larger than in winter [Velds, 1992]. In figure 2-5 it can be seen that the irradiance, due to the weather conditions, is not constant. On a variable and cloudy day, as a result of cloud formations passing in front of the sun, the irradiance will be fluctuating erratically. On a clear day there are only a few dips below the expected bell shape irradiance curve. On a variable day the irradiance varied from 0,2 kW/m2 (the sun obscured by clouds), to 1,2 kW/m2 (the sun surrounded by bright clouds), enhancing the irradiance level due to reflections, even above the maximum level measured at a clear day. Due to reflections on the surroundings and the state of overcast, the nature of solar irradiance will always be a mixture of direct irradiance and diffuse irradiance. Even on a clear day, the total irradiance will still has a diffuse irradiance part of about 10-20% at horizontal surfaces. The heavier the overcast, the larger the diffuse part in the total irradiance. If the irradiance has dropped below 30% of the maximum value on a clear day, it will mostly be diffuse [Li et. al., 2000]. Diffuse light is isotropic (uniform in all directions) therefore the contribution on vertical surfaces will relatively increase, and as a result the difference between the total irradiance on north-oriented and south-oriented surfaces will be reduced [Borg and Wiggelinkhuizen, 2001]. There are some spectral differences between direct and diffuse sunlight. The latter will have a tendency to enhance the shorter (blue) wavelengths. Thus the spectrum of diffuse sunlight will not completely comply with a solar spectrum of AM1.5. In addition the spectrum of the sunlight early in the morning and near sunset will be richer in the red region and therefore also not comply with the solar spectrum of AM1.5.

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Date: Weather Insolation Max irradiance Max T Max T ambient PV module June 2003 type year, kWh/m2 G kW/m2 °C °C 15th clear 8,07 1,05 28 55 20th variable 6,64 1,26 23 45 28th cloudy 2,28 0,1 24 34

Figure 2-5 The irradiance versus time throughout one day, measured every 15 minutes in Kassel Germany on three different days. [Ransome and Funtan, 2005]

2.2.3 Overview of indoor light energy sources and spectra For indoor applications the incident light will be a mixture of sunlight and artificial light. For artificial light the eye’s spectral sensitivity plays an important role. Therefore the luminous power of an artificial light source has its reference at the peak efficiency of the human daylight vision curve, namely at 555 nm. The luminous power unit is the lumen. One watt (W) of radiant power is 683 lumens at 555 nm [Taylor, 2000; Ryer, 1997]. For the PV cell the illuminance or the total lumens/m2 = lux will be an important measure. For example the absolute minimum illuminance recommended for reading is about 500 lux which is equivalent to about 3-4 W/m2 of radiant power. In comparison the maximum sunlight outdoors is about 100 000 lux. Therefore for one m2, the electrical output energy at this minimum office reading level in the cases of 3 and 4 W/m2 above are respectively about 5 Wh/day and 8 Wh/day. Unfortunately the performance of the PV cells depends on the illumination level. Therefore these indoor PV performances will be actually less, depending on the type of PV cell and lamps used. An important design restriction.

Artificial light General considerations Artificial light (lamplight) can be divided into three general classes namely: • Incandescent lamplights: e.g. light from lamp bulbs; • Arc/Discharge lamplight: e.g. light from arc and fluorescent tube lamps; • Solid state lamplight: e.g. light from white LEDs or combined coloured LEDs.

29 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Incandescent lamplight Incandescent lamps produce light by heating a filament until it glows. Electric current is used to heat a coiled Tungsten filament to incandescence. To prevent evaporation of the filament, the glass envelope (the bulb) is filled with a mixture of nitrogen and a small amount of other inert gas such as argon or xenon. Special cases are ‘halogen’ lamps. Unlike normal incandescent lamps, halogen lamps use halogen gas fill such as jodine or bromine. Due to this special gas fill a process called ‘Halogen Cycle’ will occur inside the lamp. Halogen gas combines with the tungsten that evaporates from the lamp filament, causing it eventually to re-deposit on the filament instead on the bulb wall as it does in standard incandescent lamps.

Figure 2-6 The spectrum of incandescent lamps as function of filament temperature [General Electric, 2006]

Figure 2-7 Effect of voltage and light output on lamp life and light output [Taylor, 2000]

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Incandescent lamps are strongly affected by input voltage which determines the filament temperature as can be seen in Figure 2-6 and 2-7 with respect to: • Lumens per watt (light output); • Emission spectrum; • Life time.

Arc/Discharge lamplight Discharge lamplight is quite common in offices. With the introduction of ECO lamps, discharge lamplight nowadays is also more and more often used at home. Therefore this type of light source has to be taken into account in the design of future indoor PV powered products. In Figures 2-8 and 2-9 the results of recent spectra measurement done at Philips Lighting can be seen [Altena and van der Burgt, 2005]. The presented spectra data are of lamps commonly in use indoors, namely of fluorescent tubes (TLD) and a number of HID (high intensity discharge) lamps. The spectral data range from 250 to 780 nm. The spectra were measured from wavelengths in the ultra violet (UV) region up to far red (FR). The complete set of spectra is presented in Appendix D.

Figure 2-8 The spectra of some TL(D) lamps [Altena and Burgt, 2005]

The TLD lamp spectra show some similarity namely the spectrum range from 300 nm up to 750 nm. But there are also some differences depending on the material used. At this moment the TL(D) 9xx series is most commonly used in homes. The spectra of this TL(D) 9xx series have distinct peaks at 375, 415, 445 and 550 nm, which is mainly in the green- blue and UV region and less in the red region.

31 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Figure 2-9 The spectra of some HID-CDML lamps [Althena and Burgt, 2005]

The Spectra of HID-CDML lamps are at this moment not yet common in home applications but for the coming five years there is a growth potential [Althena and Burgt, 2005].

LED light There is a growing market for LEDs to replace light bulbs. LEDs come in all kinds of colours as can be seen in Figure 2-10.

Figure 2-10 Several LED spectra [Althena and Saalberg-Seppen, 2005]

For indoor light mainly the white colour will be used. This white colour can be obtained by combining Red, Green and Blue LEDs and in some cases Amber or using special white LEDs. The spectra of such white LEDs are presented in Figure 2-11.

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Figure 2-11 Colour spectra of two typical white LEDs, integral measurement. Presented are the white LXHL-BW02 and warm–white LXHL-BW03 LEDs [Althena and Saalberg-Seppen, 2005]

The white Led has a strong blue component while the warm-white LED has a strong red one. Recently there are also LED tubes combining about 140 LEDs of different colours which can dynamically be lit. The tubes can also be interconnected with each other to form long lines.

2.2.4 Resume of available incident light energy • Depending on the indoor location, time of the day and the year there will be a point or region in which the irradiance contribution from outdoor via the windows will be counterbalanced by the irradiance by the artificial light. In this region the spectrum will be a mixture of indoor and outdoor. • The outdoor spectrum from the sun is from 300 up to 2545 nm. • The indoor spectrum of TL(D) and HID lamps is from 250 up to 750 nm. • The indoor spectrum of incandescent lamps is from 350 up to 1200 nm. • The indoor spectrum of white LEDs is from 400 up to 800 nm.

An overview of the light spectra is presented in Figure 2-12.

Figure 2-12 An overview of light spectral distributions

33 Energy Matching - Key towards the design of sustainable photovoltaic powered products

2.3 Potential photovoltaic (PV) power converters performance

In this section the reader is assumed to be familiar with this basic knowledge on semiconductors and photovoltaic principles. Information on this can be read in common textbooks on semiconductors and photovoltaic technology [e.g. Sze, 1969 Green, 1986]. For the reader’s convenience a resume of the fundamentals is presented in Appendix C-4.

2.3.1 General PV review In designing products powered by PV cells the relevant performance design parameters are: • The PV performance PV output parameters Operating voltage and current i.e. Maximum Power Point (MPP) Operating temperature Efficiencies Spectral Response • The maturity of the technology Are they already commercially available? Are they still laboratory samples? What stability of PV performance will there be in time?

2.3.2 PV output parameters and Maximum Power Point (MPP) General Figure 2-13 presents the properties of a photovoltaic (PV) cell in the dark and when illuminated. The PV output is characterised by the parameters of short circuit current I and sc the open-circuit voltage V . In addition each PV cell/module at a certain irradiance level oc has an unique optimal operating point ( I , V ) in its I-V characteristic in which it delivers m m maximum power, ergo the Maximum Power Point (MPP).

Figure 2-13 Location of V , I and MPP (I , V ) on the I-V curve of an illuminated PV cell [Green, 1986] oc sc m m

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This MPP depends on the available irradiance. Another parameter that characterise the PV output is the fill factor FF that is defined as:

FF = V I / V I Eq. 2-2 mp mp oc sc

By using the parameters above the power conversion efficiency η of the PV cell under investigation is given by:

= V I / P = V I FF / P Eq. 2-3 η mp mp in oc sc in with P being the total power contained in the light flux incident on the PV cell. in The operating temperature of PV cells integrated on products that are carried around will not be constant. It has been found that the dominant variation with temperature is that of V . With silicon for example [Green, 1986]: oc dV /dT = -2,3 mV/°C Eq. 2-4 oc

As a result the power output and the efficiency decrease with increasing temperature. So the MPP depends on the cell temperature. To obtain an efficient power output from the photovoltaic converter it is important to deal with the fluctuations in incident light by anticipating fluctuations of the MPP with the aid of an MPP tracker (MPPT). Usually the locus, or path, of the MPP as function of irradiance is traced with the aid of an electronic circuit. The controller in this electronic circuit is programmed to follow a certain Maximum Power Point Tracking algorithm. To- day several Maximum Power Point Tracking (MPPT) algorithms or methods are in use, quite often they are heavily patented and the exact working principles are not known in open publication. Some methods have become really common [Hohm and Ropp, 2003; Masoum et. al., 2002]: 1. Perturb-and-Observe (P&O) methods; 2. Incremental Conductance (INC) methods; 3. Constant Voltage Ratio (CV) methods; 4. Look-up-Table (L-u-T) methods.

A detailed description on MPPT is presented in Appendix C-4.

Ad. 1 and 2: The P&O and INC MPPT method In the Perturb and Observe (P&O) MPPT method the operating voltage is perturbed (in- cremented or decreased) by a small increment and the resulting change in output power is measured. With the Incremental Conductance (INC) MPPT method the MPP in the P-V curve is obtained by differentiating the power with respect to the voltage and setting the result to zero. Both the P&O and the INC MPPT have the drawback that the oscillations around the Maximum Power Point (MPP) will be larger at low irradiance levels since the I-V curve will be flatter and the increments are smaller. Noise and other disturbances will take over and the PV cell will not perform optimally.

35 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Ad. 3: The Constant Voltage Ratio MPPT Method The basis for the Constant Voltage Ratio (CV) MPPT method is the assumption that on an I-V curve the ration between maximum power point voltage, V , and its open-circuit MPP voltage, V is approximately constant or: oc

V / V = Constant < 1 Eq. 2-5 MPP oc

With the performance measurements presented in section 2.3.3 [Kan et.al, 2005b] this assumption can be analysed. The following PV cells have been investigated: Crystalline silicon (c-Si), micro-crystalline silicon (mc-Si), amorphous silicon (a-Si), Dye Sensitised Cell (DSC), Copper Indium di-Selenide (CIS) and Gallium Arsenide (GaAs).

Figure 2-14 The measured ratio V /V for various PV technologies [Kan et.al, 2005b] MPP oc

The value of the ‘Constant’ taken at the irradiance level of Standard Test Conditions (STC), namely at 1000 W/m2 deviates at lower irradiance levels within a margin of +/-10% for GaAs, c-Si, a-Si, mc-Si and DSC. For the CIS samples this margin is +/-20%. The translation of the margins into errors in MPP is via the I-V curve at the various irradiations. Therefore this +/-10% and +/-20% will actually result in a larger MPP error, in particular at low irradiance levels.

Ad. 4: The look-up-table (LUT) MPPT method In the look-up-table MPPT method, the MPP at certain defined irradiance levels are stored in a Table. During PV operation with an external sensor the irradiance level is measured and the corresponding MPP is set in the MPP tracker. The drawback of the look-up-table MPPT method is a lack of relevant data available to the designer of PV powered products. Up to now, manufacturers provide only data measured under STC or at irradiance of 1000 W/m2. For designing mobile products with PV cells also the performance such as MPP should be known at lower irradiance levels.

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Proposed improvements As was shown in section 2.2.2 the varying irradiance will influence the maximum available current from the photovoltaic cell and the open-circuit voltage. The amount of this influence depends on the type of photovoltaic (PV) cells used. Therefore it is a logical step to take into account the data obtained during performance tests at various irradiance levels (section 2.3.3) in the search for an improved MPPT. In these PV performance measurements it was found that the MPP path as function of irradiance is ‘PV cell type dependent’, as can be seen in Figure 2-15.

Figure 2-15 The Maximum Power Point path as function of irradiance of two different photovoltaic cells normalised at a irradiance of 100 W/m2 [Kan et. al., 2005b]

To find an explanation for this PV cell type dependency of the MPP one has to take a closer look at the PV cell itself. PV cells generally suffer from parasitic series R and shunt S R resistances. The magnitude of these parasitic resistances will be different at each type SH of PV cell. The main parameter that determined the MPP is the fill factor FF as defined in Eq. 2-2.

FF = V I / V I Eq.2-2 mp mp oc sc

To determine the effect of these parasitic resistances on the fill factor their values are compared to a characteristic resistance of a PV cell defined as [Green, 1986]:

R = V /I Eq. 2-6 CH oc sc

As a result, the fill factor FF with the influence of the series resistance can be expressed as:

FF = FF (1- R /R ) = FF (1- R I / V ) = FF (V - R I ) / V Eq. 2-7 0 S CH 0 S sc oc 0 oc S sc oc

37 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Here FF is the ideal fill factor in absence of any parasitic resistance. Combining Eq. 2-2 and 0 2-6 yields the maximum power point V I : m m

V I = I FF (V – R I ) Eq. 2-8 m m sc 0 oc S sc

For an ideal case:

FF = FF and R = 0 resulting in: 0 S

V I = I FF V Eq. 2-9 m m sc 0 oc

Solving the equation for V by using the ideal diode law yields: oc

V I = I FF kT/q ln I /I +1 Eq. 2-10 m m sc 0 L 0

In this equation I is the saturation current in the ideal diode law, and I is the light gener- 0 L ated current. This light generated current will in the ideal case be equal to I . Through sc measurements [Kan et.al, 2005] it was shown that there is a linear relation between irradiance G and I as seen in Figure 2-16. All the measured PV cells converge to the same sc straight line.

Figure 2-16 The linear relation between irradiance and Isc normalized at an irradiance of 1000 W/m2 or 1 sun of various PV cells [Kan et. al. 2005b]

Therefore Eq. 2-10 can be written as:

V I = const.G FF kT/q ln G/I Eq. 2-11 m m 0 0

In other words the maximum power point V I depends logarithmically on the irradiance m m G. This can be seen in Figure 2-15 for the PV case. In a less ideal case at PV the parasitic 1 2 resistances in Eq. 2-8 are not zero and the MPP is reduced accordingly.

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Another measured discrepancy between the two PV types in Figure 2-16 is the difference in the photovoltaic open-circuit voltage (V ) as a function of irradiance. For PV the oc 1 open-circuit voltage remains nearly constant as a function of irradiance on a logarithmic scale in accordance to the ideal diode law. Therefore the MPP follows a straight line with irradiance on the logarithmic scale. For PV however the open-circuit voltage drops drasti- 2 cally, particularly at low light irradiance levels. From Figure 2-16 it can be concluded that knowing the type of photovoltaic cell helps predict the MPP path. This could result in a simplification of the MPPT algorithms. In a novel MPP Tracking method this data should be taken into account. Measuring performance with a solar or sun simulator and neutral density filters is precise but time consuming and therefore costly. A simpler method is to use the inverse square law of distance between light sources - PV as a means to reduce the irradiance in a controlled manner. This method can be used to measure the Maximum Power Points for the look-up-table. In addition, since the short circuit current I is linearly dependent on sc irradiance (see Figure 2-16), not the actual irradiance level but Isc could be measured and registered instead. I is measured and calibrated at 1000 W/m2 and then the distance is sc changed between a test lamp and the PV cell under investigation. By monitoring I , the sc Maximum Power Points (MPPs) can be mapped for several irradiance levels. The mapped MPPs can then be stored in a reprogrammable memory (e.g. Flash memory) as a ‘look up table’. Due to aging the MPPs might change in time, a measurement can then be repeated and the look-up-table is updated. The novelty of this proposed method is not only its simplicity but also that the trigger levels for tracking are extracted from the very same PV cell under investigation and not from an external sensor. In other words the levels of irradiance incident upon a PV cell are measured by monitoring its I values which directly provide the feedback controls for sc ‘tracking’ the Maximum Power Point (MPP) of that same PV cell.

2.3.3 Overview of photovoltaic cell efficiencies The power conversion efficiency of a photovoltaic (PV) cell is an important input parameter in the design of PV-powered products. With this basic parameter, the system design trade-off options can be calculated. These trade-offs are for example between the feasible options for PV cell/module size and layout in relation to the needed energy. These trade- offs are carried out to obtain an energy balance between the delivered PV energy and the energy needed by the application during a certain time period. But these trade-offs also include the choice between cost related options of the various PV cells, the storage unit and related product lifetime. Since quite often the available surface area for integrating PV cells on the product is limited, the used PV cell efficiency can in most cases become a criterion of principal system feasibility. An overview of conversion efficiencies is presented in Figure 2-17 and in Table 2-1. In exploring the feasibility of integrating PV cells onto products, the analysed efficiencies will mainly be restricted to those of single layer PV cells. This restriction is based on observations and tests by the author who found the tendency of multi-junction cells is to perform below their specifications at indoor conditions. The reason found for this discrepancy is the absence of a full (AM1.5) light spectrum indoors. As a result of the missing part (or colour) of the spectrum, the particular junction related to that part of the spectrum will

39 Energy Matching - Key towards the design of sustainable photovoltaic powered products

perform sub optimally and therefore will become a large resistor in series with the other junctions. This resistance will drastically reduce the overall performance of the multi-junction PV cell [Dillen, 2003].

Figure 2-17 Crystalline silicon (c-Si) technology in historic perspective extended to the year 2006 [Sinke, 2004]

Table 2-1 Energy transfer related PV parameters (single junction cells)

Type Record cell Typical cell Cell Cell Cell Spectral efficiency efficiency at efficiency efficiency efficiency range at STC (1) STC, 1000 at 100 at 10 at 1 W/m2 (2) W/m2 (2) W/m2 (2) W/m2 (2) (2)

Mono-c-Si 24,7 14-16 12 -15 6-9 1-5 350-1200 20,5 (3) 20 (3) 17,6 (3) 16 (3) nm

Multi-c-Si 20,3 13-15 11-14 2-5 0-3 350-1200 nm

a-Si 11,7 4-8,5 (6) 4-7 (6) 3-6 2-4 300-800 nm

CIGS/CIS 18,4 10-13 4-6 1-2 0 300-1200 nm

CdTe 16,5 10 (4) 5-7 (4) 3-4 (4) 2-3 (4) 350-850 nm (4)

III-V 25,1 19 15-18 14-17 7-12 300-1000 crystalline nm (4)

III-V thin 24,5 16 14-16 9-14 1-2 300-1000 film nm (4)

DSC 11,1 (7) 5-8 4-7 3-6 2-4 300-800 nm (5)

40 Chapter 2: Optimizing photovoltaic energy conversion systems for mobile/wireless products in outdoor/indoor user contexts

Notes to Table2-1: (1) Compiled from Solar Cell Efficiency Table 28 [Green et. al. 2006]. (2) From own measurements confirmed by literature [e.g. Randal, 2003]. (3) Cells designed for increased weak light performance, e.g. Fraunhofer ISE PERC cell [Glunz et.al., 2002]. (4) From Literature [Randal,2003]. (5) From Literature [Sommeling et. al., 2000; Hohl-Ebinger et. al., 2004]. (6) The lower value applies to a-Si PV cells on flexible substrates. (7) From Literature [ Chiba et. al., 2006].

In general, the efficiency of a PV cell to convert incident light into electricity depends on the used material (technology). The efficiency of silicon solar cells is among others dependent on the crystal quality of the material used. Another type of photovoltaic cell, the so-called dye sensitised cell (DSC) is based on charge separation at the interface between a dye molecule and a wide band gap semiconductor rather than at a P-N junction in a semiconductor [O’Regan and Grätzel, 1991]. The laboratory efficiency of these molecular PV cells is over 11% [Grätzel, 2005; Chiba et.al, 2006]. The main advantage of these cells is that the open circuit voltage Voc remains nearly the same even at low light level irradi- ances, while the current reduces linearly with the reduced light intensity. This type of PV cell is therefore of special interest for indoor applications [Grätzel, 2001]. In Table 2-1 the term ‘typical’ refers to the average efficiency values that are (can be) achieved for (more or less) commercial products, on significant surface areas (>> 1 cm2). The different technologies are approached in a similar way by taking the data mainly from the international confirmed Solar Cell Efficiency tables [Green et.al, 2006]. Since mobile products are used both outdoors and indoors, it is important to know the outdoor performance of PV cells on a bright sunny summer day (1000 W/m2) but also down to indoor irradiance levels (1 W/m2). At the start of the analysis phase of this PhD research project there was an apparent lack of data available on photovoltaic performance at low irradiance levels. In particular the power conversion efficiencies of the PV cells were only known at Standard Test Conditions (STC), i.e. at 25 °C, irradiance of 1000 W/m2 and with an AM 1.5 spectrum. Because of this, a series of efficiency measurements were carried out at ECN [Kan, 2003].

Figure 2-18 Measured conversion efficiency versus irradiance level of: amorphous silicon (a-Si), dye sensitised cell (DSC) and Copper Indium di-Selenide (CIS) PV cells [Kan, 2003]

41 Energy Matching - Key towards the design of sustainable photovoltaic powered products

As can be seen from Figure 2-18 a PV cell with a high efficiency at STC irradiance of 1000 W/m2, will not automatically maintain its higher efficiency at lower irradiance levels. The same applies for the output voltage which is also irradiance level dependent. The PV cells presented in Figure 2-18 are laboratory samples and only served as examples to demonstrate the differences in performance at low irradiance levels, rather than be representative for the particular PV technologies as such. In an internal EPFL report in which a comparison is made between DSC and a-Si at a low irradiance level of 500 lux (about 5 W/m2) it is shown that in contrast to Figure 2-18 the DSC perform better in absolute terms than the a-Si PV cell [Grätzel, 2006]. There are several possible explanations for this. First of all, in the study typical samples have been selected rather than best (maximum efficiency) samples. Secondly, cells may be optimized for operation under very different operating conditions: outdoor (high irradiance levels of typically >100 up to 1000 W/m2) or indoor (roughly < 100 to << 100 W/m2). In the first case the PV cell will be designed with the lowest serial resistance to allow large currents. In the latter case the currents are in correspondence to the lower irradiance much lower so the serial resistance could be much higher. As a result according to this internal EPFL report the conversion efficiency of the DSC may increase from 9.4% at 300 W/m2 to 10.3 % at 1.4 W/m2, while the open circuit voltage decreases from 721 mV to 635 mV. The applicability of DSC and a-Si is dependent on the absolute value of the efficiency at low illumination values as well as on the variation with illumination level over the whole range of 1000 W/m2 down to 1 W/m2. In any case, in Figure 2-18 it is shown that DSC and a-Si appear to perform better over the useful illumination range than CIS. The highest decrease in efficiency seems to be at the CIS cells. To avoid possible errors in the CIS cell measurements for the SYN-Energy project PV cell survey [Reich, Kan et.al, 2005], commercially available samples were used. These commercial cells were sealed to avoid environmental influences. The new results were about the same as that of earlier measurements. This poor performance at irradiation levels below 100 W/m2 was confirmed by similar measurement results elsewhere [Kaufmann et.al, 2005]. In their paper they even came to the conclusion that CIS PV cells could perform well only outdoors. Figure 2-18 shows that the type of PV cell used and the irradiance level at the location of use (user context) are important criteria in the selection of the proper PV cells. In a case where the PV cells are only used outdoors, the highest STC efficiency will typically be the decisive criterion. However, in cases the PV cells are used both outdoors and indoors, a more important criterion will be to what extent the efficiency and operating voltage will decrease at low irradiance levels. There is also a large discrepancy between ‘Laboratory Results’ and ‘commercially available’ PV cells. Today [Green et. al, 2006], commercially available PV cells would yield efficiencies of 17%, 15%, and 10% for respectively mono-crystalline- Silicon (c-Si), multi-crystalline Silicon (mc-Si), and thin-film silicon (tf-Si). In search for cheaper PV cells there is a trend towards an increasing use of thin-film Si. During the syntheses phase of this PhD project, and within the framework of the SYN- Energy program more PV performance measurements were done at ECN and Utrecht University. The scale was extended over a larger group of mainly commercially available PV test samples, namely some 120 cells provided by 18 manufacturers [Reich, Kan et.al, 2005]. Also the spectral responses (SR) of these PV cells were measured. The test setup is presented in Appendix E.

42 Chapter 2: Optimizing photovoltaic energy conversion systems for mobile/wireless products in outdoor/indoor user contexts

The characteristics found by the efficiency measurements are as follows: The efficiency decreases logarithmically at lower irradiance levels, as V decreases with the oc well-known logarithmic slope. To illustrate this, the efficiencies are plotted as a function of irradiance level on a logarithmic scale. In Figure 2-19 the resulting PV cell efficiencies of selected mono-crystalline silicon (c-Si) cells received from two different cell manufacturers are shown.

Figure 2-19 Power conversion efficiencies as a function of various irradiance levels measured with several c-Si PV cell samples of two different cell manufacturers [Reich, Kan et. al., 2005].

The decrease of PV cell efficiency towards lower irradiance levels is extremely dependent on cell technology. This has been previously published in other surveys on low light level PV cell performance [Kan, 2003, Randall et.al, 2003], and in theoretical modelling of mono-c-Si, a-Si:H and CIGS cells [Gemmer, 2003]. As can be seen in Figure 2-19, the cell efficiency of c-Si in the highest irradiance levels (200-1000 W/m2) does not vary much. A 4% decrease in samples by supplier one and a slightly larger decrease towards lower irradiance levels in the samples from supplier two can be seen. In this irradiation level region, the series resistance determines the efficiency. Towards lower irradiation levels fewer losses are associated with the serial resistance, explaining the slight increase in cell efficiency in samples from supplier one. In the region below irradiance levels of about 100 W/m2, almost all samples show a linear decrease on a natural logarithmic scale. In this low irradiance level region (most often encountered indoors) it is the parallel resistance and the second diode characteristic in the two-diode model that affects cell efficiency. Because these parameters vary significantly dependent on the individual cell, a wider spread in cell efficiency towards lower irradiance levels compared to the STC irradiance value can be seen. This is especially true for the measured multi-crystalline silicon (mc-Si) cell samples, as can be seen in Figure 2-20. Beside the differences between the different overall performances, which can be related to the different manufacturers that provide those test samples, at low irradiance level (2,9 W/m2) even a difference by a factor two was found between cells provided by one single supplier. These cells show almost the same performance under STC conditions and are likely to belong to the same power class (even the same manufacturing batch).

43 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Figure 2-20 Measured efficiencies of mc-Si PV cells versus irradiance of various samples of different cell manufacturers [Reich, Kan et.al., 2005]

2.3.4 Spectral response of PV cells In order to match and choose the appropriate PV cell for the expected light spectra as presented in section 2.2, a series of PV spectral response (SR) measurements were undertaken. The measure set-up is presented in Appendix E. A measurement of the spectral response was typically achieved by measuring the short circuit current of the PV cell while it was illuminated with a bias light (0.5 sun) and an additional illumination of chopped monochromatic light. The monochromatic light was generated by a carousel of band pass filters. The complete measurement procedure of SR measurements is described in literature [e.g. Janski et. al., 2004]. Figures 2-21 to 2-25 show a selection of spectral response measurement results for cells of different technologies.

Figure 2-21 Spectral responses (SRs) of a-Si and mc-Si samples [Reich, Kan et.al., 2005]

44 Chapter 2: Optimizing photovoltaic energy conversion systems for mobile/wireless products in outdoor/indoor user contexts

In Figure 2-21 two differences can be seen between the SRs of a-Si and mc-Si. Both the magnitude and the spectral ranges of the mc-Si cells are about 1,5 times larger than that of a-Si. The decrease found in Figure 2-21 of SRs at mc-Si samples is marginal and too small to influence efficiency.

Figure 2-22 Spectral responses (SRs) of two extremes a-Si samples [Reich, Kan et.al., 2005]

Different peaks in the SRs of a-Si cells in Figure 2-21 and 2-22 can be explained among other things by different doping concentrations used in processing the cells. The indoor a-Si was sold as being specially made for indoor application with an enhanced blue - ultra violet sensitivity.

Figure 2-23 Spectral responses (SRs) of c-Si and CIS samples [Reich, Kan, et. al., 2005]

The CIS samples show a broader spectral range in their SRs than the c-Si ones. In addition the CIS samples demonstrate a greater sensitivity in the blue spectral region.

45 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Figure 2-24 Spectral responses (SRs) of DSC samples [Sommeling et.al., 2000].

The spectral responses of DSC samples shown in Figure 2-24 presented a spectral range from 300 up to 775 nm and a peak at about 550nm. This was confirmed with other measurements in which various chop frequencies and bias illuminations were used [Hohl- Ebinger et. al., 2004]. An overview of the various spectral responses is presented in Figure 2-25.

Figure 2-25 Overview of the various spectral responses

2.3.5 Résumé of the potential photovoltaic power converter performances The Standard Test Conditions do not provide a good selection criterion for designing PV powered mobile wireless products because: • A PV cell with a high efficiency at Standard Test Conditions (STC) of 1000 W/m2, will at lower irradiance levels not automatically maintain its higher efficiency; • At low irradiance levels (about 2 W/m2) a factor two difference in efficiency was found between cells provided by one single supplier.

46 Chapter 2: Optimizing photovoltaic energy conversion systems for mobile/wireless products in outdoor/indoor user contexts

Therefore a new test data set is proposed to be included into datasheets, which also takes into account the PV performance at low irradiance levels existing in an indoor user context as well as the spectral responses. The spectral responses of a-Si and DSC encompass about the same spectral range from 300 to 775 nm. The spectral responses of c-Si, mc-Si and CIS encompass a broader spectral range including the more red and even infra red region: the spectral range is from 350 to 1200 nm.

2.4 Optimizing the irradiance matching interface (MI:1)

The power output of a photovoltaic system depends on the spectrum and level of the irradiance upon the photovoltaic cells, the type of photovoltaic cells used and the effective area of the photovoltaic cells. Therefore the type and size of photovoltaic cells must be chosen in accordance with the application and user context. The user contexts can be categorised as outdoor only, combined outdoor and indoor and indoor only. Because the application and user context influences the choice of photovoltaic type and the maximum photovoltaic conversion efficiency they also influence the product design [Kan et. al., 2004]. Once the photovoltaic type, application and user context are known the output power of the photovoltaic cell could be maximized by logical reasoning as follows: 1. Matching the spectral response (SR) of the PV cells with the spectrum of the available incident light; 2. Minimising shadows upon the PV cells by proper design; 3. Increasing the incident light on the PV cell; 4. Using the spectral dependent power conversion efficiency.

2.4.1 The spectral Figure of Matching (MI:1) To match the spectral response (SR) of the PV cells to that of the user context defined incident light spectrum, the following measures are proposed: • Before the matching is calculated some obvious pre-matching between the spectral response (SR) of the PV cells as presented in Figure 2-21 through 2-25 and the transmission window characteristics of the (anti reflection) coating and/or cover glass must be established to guarantee a proper optical power transfer. Antireflection coating can improve the transmission of light by 8,9-11,7 % [Kumar et. al., 2005]. By this pre-matching the matching interface MI-1 is optimised. • Matching of the incident light (IL) spectrum to the spectral response (SR) of the PV cell will be achieved by determining the Figure of Matching as was introduced in Chapter 1.

The generic Figure of Matching was defined as:

FM = { Ø / ∫ S (var) d var } x 100% Eq. 2-12 p

Here Ø is the correlation between the Stimulus S emanated from element s towards p the next element in the energy chain i.e. element p and the Step-Response R of this Step-p element p. This generic Figure of Matching algorithm can be made specific for the incident light - PV

47 Energy Matching - Key towards the design of sustainable photovoltaic powered products cell matching interface MI:1 by a proper selection of the type of power that is transferred from a light source to the PV cell. As a result of the selected power type the related variable ‘var’ can be determined. The MI:1 matching interface concerns the spectral Figure of Matching (FM ). Thus the type of power that has to be transferred is light power. Spectral Here the variable ‘var’ is the wavelength λ. The step response of the PV cell with respect to the wavelength is the spectral response (SR). The Stimulus S is the spectrum of the λ p incident light source (S ). IL The integral over the entire spectrum of the incident light will yield the total incident light power upon the PV cell in W/m2. The spectral response as measured has the dimension A/W. In order to match the dimension of the spectral response (SR) values as a function of the wavelength to that of the incident light i.e. power, the SR still has to be multiplied with a voltage. According to the definition of efficiency as was presented in Eq. 2-3:

= V I / P = V I FF / P Eq. 2-3 η mp mp in oc sc in and the method of measuring the SR namely with the short circuit current I , the cor- sc responding voltage to fit the dimension should be the open circuit voltage V x FF of the oc PV cell at that particular irradiance level. As a result a spectral response based on power can be defined as:

SRP ( ) = SR ( ) x V x FF Eq. 2-13 PV λ PV λ oc

The correlation Ø between the spectrum of the incident light and the power based spectral response of the PV cell under analysis can be written as Ø:

Ø = ∫ S ( ) x SRP ( ) d ( ) Eq. 2-14 IL λ PV λ λ spectrum range And the spectral Figure of Matching becomes:

FM = { Ø / ∫ S ( ) d } x 100% Eq. 2-15 Spectral IL λ λ spectrum range Therefore the spectral Figure of Matching is a quotient of two power quantities i.e. the correlated power of the spectral response of the PV cell and the total light power incident on that PV cell. This spectral Figure of Matching therefore has the ‘format’ of a normalised transfer efficiency as was presented in Eq. 1-1 in Chapter 1. In other words the spectral Figure of Matching is the efficiency of the PV cell at that particular light spectrum and that particular irradiance level. This spectral Figure of Matching is a measure how well a certain PV cell is matched with the irradiance spectrum of the available light source. The available light source is defined by the user context. Figures 2-22 through 2-26 present examples of various pairs of light source - PV cells. Examples are presented of the spectral matching between crystalline silicon (c-Si) and amorphous silicon (a-Si) and DSC PV cells with the spectra of outdoor AM1.5, indoor TLD and incandescent light. For calculating these spectral Figure of Match- ing values the spectra presented in Figures 2-6, and 2-8 to 2-12 were combined with the Spectral Response presented in Figures 2-21 to 2-25.

48 Chapter 2: Optimizing photovoltaic energy conversion systems for mobile/wireless products in outdoor/indoor user contexts

For a proper comparison between the outdoor AM1.5 spectrum and the indoor TLD and incandescent lamp spectra, the power contents of the AM1.5, TLD and the incandescent spectra have all been scaled to 1000 W/m2.

Figure 2-26 AM 1.5 spectrum at 1000 W/m2, the spectral response (SR) of a crystalline silicon (c-Si) PV cell and the power based spectral correlation product of S ( ) x SRP ( ) with S ( ) the AM 1.5 spectrum IL λ PV λ IL λ [Kan et.al., 2006c]

In Figure 2-26 the spectral matching between a c-Si PV cell and an AM 1.5 spectrum is presented. The spectral range of the AM 1.5 is broader than that of the SR of the c-Si PV cell. The product will follow the spectral range of the SR. The resulting spectral Figure of Matching yields 14,9%. This spectral Figure of Matching corresponds with the measured efficiency of the c-Si PV cells at Standard Test Conditions i.e. AM 1.5 spectrum and irradiance level of 1000 W/m2, namely 14 - 16 %.

Figure 2-27 TLD lamp spectrum at 1000 W/m2, the spectral response (SR) of a crystalline silicon (c-Si) PV cell and the power based spectral correlation product of S ( ) x SRP ( ) with S ( ) the TLD lamp IL λ PV λ IL λ spectrum [Kan et.al., 2006c]

49 Energy Matching - Key towards the design of sustainable photovoltaic powered products

In Figure 2-27 the spectral Figure of Matching is presented between the above c-Si and with a TLD spectrum. The spectral Figure of Matching in this case is 14,6%. Since the efficiency measurements presented in section 2.3.3 were all conducted under Standard Test Conditions with an AM 1.5 spectrum only, no direct comparison between this spectral Figure of Matching with TLD spectrum and a measured efficiency was possible. On the other hand, this example illustrates the potential of the Figure of Matching algorithm to provide design information.

Figure 2-28 Incandescent lamp spectrum at 1000 W/m2, the spectral response (SR) of a crystalline silicon (c-Si) PV cell and the power based spectral correlation product of S ( ) x SRP ( ) with S ( ) the IL λ PV λ IL λ incandescent lamp spectrum [Kan et.al.,2006c]

In Figure 2-28 the spectral Figure of Matching is presented between the above c-Si and an incandescent lamp spectrum. The spectral Figure of Matching in this case is 10,9%.

Figure 2-29 The TLD lamp spectrum at 1000 W/m2, the spectral response (SR) of an amorphous silicon (a-Si) PV cell and the power based spectral correlation product of S ( ) x SRP ( ) with S ( ) the TLD IL λ PV λ IL λ lamp spectrum [Kan et.al.,2006c]

50 Chapter 2: Optimizing photovoltaic energy conversion systems for mobile/wireless products in outdoor/indoor user contexts

In Figure 2-29 the spectral Figure of Matching is presented between an a-Si and a TLD lamp spectrum. The spectral Figure of Matching in this case is 17,6%. The spectral range of the TLD spectrum coincides with the SR of the a-Si PV cell resulting in a higher spectral Figure of Matching than that of the c-Si PV cell. The c-Si PV cell has a broader SR but the centre at 900 nm does not coincide with that of the TLD spectral range of about 560 nm.

Figure 2-30 The TLD lamp spectrum at 1000 W/m2, the spectral response (SR) of a dye sensitised (DSC) PV cell and the power based spectral correlation product of S ( ) x SRP ( ) with S ( ) the TLD lamp IL λ PV λ IL λ spectrum [Kan et.al.,2006c]

In Figure 2-30 the spectral Figure of Matching is presented between a dye sensitised (DSC) PV cell with a TLD lamp spectrum. The spectral Figure of Matching in this case is 19,3%. The spectral range of the TLD spectrum coincides with the SR of the DSC PV cell, resulting in a higher spectral Figure of Matching than that of the a-Si PV cell. The DSC PV cell has a broader SR and the centre at 550 nm coincides with that of the TLD spectral range of about 560 nm. For comparison the results of the spectral Figure of Matching calculation are tabulated in the Table 2-2.

51 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Table 2-2 Spectral Figure of Matching of various light sources - PV technology pairs, at an irradiance level of 1000 W/m2

Light Source FM with FM with FM with Spectral Spectral Spectral AM 1.5 TLD Incandescent PV cell Technology

c-Si 14,9 % 14,6 % 10,9 %

a-Si 8,3 % 17,6 % 2,7 %

mc-Si 13,4 % 13,6 % 9,3 %

CIS 15,8 % 16,8 % 10,8 %

DSC 7,5 % 19,3 % 2,3 %

As mentioned above the spectral Figure of Matching (FM ) is a normalised power Spectral transfer efficiency of a light source - PV cell combination that is in theory identical to the conversion efficiency of that PV cell for the irradiance of the light source. This conformity is demonstrated in Table 2-3.

Table 2-3 Comparison between spectral Figure of Matching and the measured conversion efficiencies at an AM 1.5 spectrum and an irradiance level of 1000 W/m2

Method FM with Measured conversion efficiency Spectral AM 1.5 at 1000 W/m2 at STC with AM 1.5 spectrum PV cell Technology and irradiance 1000 W/m2

c-Si 14,9 % 14-16 %

a-Si 8,3 % 6-8,5 %

mc-Si 13,4 % 13-15 %

CIS 15,8 % 10-14 %

DSC 7,5 % 5-8 %

The only non-conformity is presented by the CIS cells and this will need further research. The spectral Figure of Matching is obtained by using data from the incident light spectra and PV spectral response measurements. On the other hand the PV conversion efficiency is obtained by measuring the I-V characteristics of the PV cells. Therefore the spectral Figure of Matching and the conversion efficiency are obtained by two different and independent measurements. As a result the conversion efficiency measurements validate the spectral Figure of Matching calculations. The FM values presented in Table 2-2 are only valid for an irradiance level of 1000 Spectal W/m2. To obtain the spectral Figure of Matching (FM ) for various light source - PV spec+irrad technologies pairs and also various irradiance levels, one has to proceed in two steps.

52 Chapter 2: Optimizing photovoltaic energy conversion systems for mobile/wireless products in outdoor/indoor user contexts

These two steps are justified by the following assumptions: 1. That the spectral response (SR) is independent of the irradiance level; 2. The existing equivalence between spectral Figure of Matching and actual measured efficiencies.

These assumptions are reasonable as they reflect the definition of spectral response (SR) and the data presented in Table 2-3. In the first step the spectral Figure of Matching has to be calculated with an AM1.5 spectrum at various irradiance levels. For this calculation the integral over the entire AM1.5 spectrum is scaled to values of respectively 100 W/m2, 10 W/m2 and 1 W/m2. The spectral Figure of Matching obtained in this way assumes a straight logarithmic slope in the relation efficiency versus logarithmic scaled irradiance. From PV conversion efficiency measurements however it is apparent that this exists only in an ideal situation. So the correct spectral Figure of Matching at a certain irradiance level is obtained by taking the spectral Figure of Matching (FM ) values as listed in the first column of Table 2-2 and multiplying them Spectral with the quotients between actual efficiencies at that certain irradiance level ( ) ηAM1.5+irrad and the efficiency at irradiance level of 1000 W/m2 ( ), both measured with ηAM1.5 + 1000W/m2 an AM 1.5 spectrum. Therefore the spectral Figure of Matching with an AM1.5 spectrum at an irradiance level ‘irrad’ is:

FM ≈ FM x / [%] Eq. 2-14 AM1.5+irrad AM1.5 + 1000W/m2 ηAM1.5+irrad ηAM1.5 + 1000W/m2

Here is the PV conversion efficiency at a certain non STC irradiance level. These ηAM1.5+irrad non STC irradiance levels are e.g. 100 W/m2 or 10 W/m2 measured with an AM1.5 spectrum. The results are presented in the first column of Table 2-4.

In the second step the spectral Figure of Matching for other spectra than AM1.5 like that of TLD lamps and incandescent lamps are calculated. The irradiance level and spectrum dependent spectral Figure of Matching (FM ) is the product of an equation which takes spec+irrad the spectral Figure of Matching at a certain irradiance level ‘irrad’ with an AM1.5 spectrum and the quotient between the spectral Figure of Matching values at a particular incident light spectrum ‘spec’ and those of an AM 1.5 spectrum, both of which are at an irradiance level of 1000 W/m2.These values are taken from the Table 2-2. Therefore:

FM ≈ FM x FM / FM [%] Eq. 2-15 spec+irrad AM1.5+irrad Spec, 1000W/m2 AM1.5, 1000W/m2

A résumé of the calculation results is presented in Table 2-4.

53 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Table 2-4 Spectral Figure of Matching of various light sources - PV technology pairs, corrected for their irradiance level dependent conversion efficiencies

Light Source FM with FM with FM with Spectral Spectral Spectral AM 1.5 [%] TLD [%] Incandescent [%]

Irradiance [W/m2] Irradiance [W/m2] Irradiance[W/m2] 1000 100 10 1000 100 10 1000 100 10 PV cell Technology STC STC STC

c-Si 14,9 11,9 5 14,6 11 2,6 10,9 8,9 3,9

a-Si 8,3 5,7 4,6 17,6 12,6 10 2,7 2,3 1,9

mc-Si 13,4 10,5 3,4 13,6 10 2,6 9,3 7,3 2

CIS 15,8 7,3 1,5 16,8 7,4 1,4 10,8 4,8 0,9

DSC 7,5 5.3 4,2 19,3 13,6 10,9 2,3 1,9 1,5

The spectral Figure of Matching (FM ) of various light source - PV technology pairs and Spectral irradiance levels, as presented in Table 2-4, can be compared is various ways. a) Comparison per spectrum at irradiance of 1000 W/m2 reveals: • For the AM 1.5 spectrum the best spectral Figure of Matching is obtained for the CIS (15,8%) and the c-Si (14,9%) PV cells. • For the TLD spectrum the best spectral Figure of Matching is obtained for the DSC (19,3%) and the a-Si (17,6%) PV cells. • For the incandescent spectrum the best spectral Figure of Matching is obtained for the c-Si (10,9%) and the CIS (10,8%) PV cells. b) Overall comparison at different user contexts: • For outdoor in full sunlight (AM 1.5 spectrum) the best spectral Figure of Matching is obtained with CIS and crystalline silicon (c-Si) PV cells. • For indoor this depends on the means of illumination. • Near the window, on the windowsill during the day, the main spectrum will be AM1.5 and the irradiance will be about 100 W/m2. In this case the c-Si (11,9%) and the mc-Si yield the best spectral Figure of Matching. • In an office with mainly TLD light and an irradiance of 100 W/m2 the best spectral Figure of Matching will be with the DSC (13,6 % - 10,9%) and a-Si (12,6 % - 10 %). In this case it becomes apparent why crystalline silicon (c-Si) PV cells do not perform as well indoors with artificial TLD light compared to a scenario with outdoor sunlight. • In a home environment in which there is a mixture of TLD and incandescent lamp light the best spectral Figure of Matching will be with c-Si (on average 10% at 100 W/m2). For mobile portable products the use of high grade c-Si PV cells such as the PERC cell (see Table 2-1) is suggested [Schmidhuber, 2003] since at low irradiance levels as can be seen in Table 2-4 this technology is still performing well.

54 Chapter 2: Optimizing photovoltaic energy conversion systems for mobile/wireless products in outdoor/indoor user contexts

If the spectral Figure of Matching for a certain light source - PV cell is known it can be compared with other options of light source - PV combinations as is presented in Table 2-4. As a result the best matched light source - PV combinations can be chosen. In designing PV powered products however, the mechanical properties will also play a role. These will particularly be the case if the design needs curved surfaces. Table 2-4 focuses only on the power matching between light source and PV technology, as defined in Figure 2-1 in matching interface MI:1.

2.4.2 Minimise shadows on the PV cells by proper design In a photovoltaic cell array, the cells in shadow will act as resistor causing a voltage drop. In addition PV cells that are connected in series must carry the same current. If some of those PV cells are shaded, the current in this shaded PV cell will diminish, causing an increase of their effective series resistance. Eventually they will become reversely biased. This means those particular shaded cells will cause energy losses as dissipated heat and over a period of time even failure could occur at those PV cells. For a photovoltaic cell array, the resulting output will typically be dominated by the weakest link in the chain, therefore in this case it will be the cells in the shadow. The drawbacks of shadows can be overcome by designing with photovoltaic use in mind. First the photovoltaic cell location on the product must be such that the amount of shadow is minimised. Second the layout of the PV cells in the array, in terms of interconnection in series and in parallel, must be in such a way that shadows will have a minimum impact. In addition it is necessary to detect and trace photovoltaic (PV) cells inside shadow areas and apply a bypass of these particular PV cells in order to reduce series resistance. Bypass switches can be made by using Field Effect Transistors (FETs) instead of conventional blocking diodes [Kan, 1996]. The selection of cells to be bypassed is done by an integrated processor. Finally the photovoltaic cells can be connected in parallel instead of a series. In this case it is recommendable to have a provision to bypass the PV cells that are in the shadow in order to avoid current leakage induced voltage drops. To match the low parallel PV output voltage to that value required by the load or energy storage medium, DC/DC converters are used instead of a summation of output voltages in a serial setting [TSA, 2006]. In taking measures to avoid shadows in a PV integrated solution, an implementation example for proper matching of the PV cells with the product functionality can be seen in Figure 2-32. The way an idea is implemented determines whether the PV cells will be in shadow. For example the batteries of consumer products such as Cellular Phones, CD players and PDAs can be recharged directly by PV modules during the time they lie idle. In Figure 2-31 the PV module is placed directly on the battery. This PV battery replaces the normal battery at the backside of a cellular phone.

55 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Figure 2-31 PV battery on the back side of cellular phone [Kan et.al, 2004b]

The idea is innovative but the implementation and materialisation is poor. Although the PV battery combination has an equivalent functionality as the standard battery for that cellular phone, the utilisation of PV cells to recharge the battery is sub-optimally matched from the point of view of avoiding shadows and therefore light power conversion efficiency of the PV cells. For instance the PV battery is placed on the traditional location namely at the back of the cellular phone, which means that in normal use the buttons are facing upwards and the PV cells downwards and away from the light. In other words, when attached to the cellular phone the PV cells simply do not function well. Another disadvantage of the implementation at the back location is that the surface is easily scratched, reducing the transparency of the sensitive surface and therefore reducing the PV functionality.

Figure 2-32 PV batteries on the cover side of a PV powered PDA to avoid shadows [Luther, 2003]

In Figure 2-32 the PV module is placed on the top-cover facing the incident light.

56 Chapter 2: Optimizing photovoltaic energy conversion systems for mobile/wireless products in outdoor/indoor user contexts

2.4.3 Increase the incident light Several solutions to increase the incident light are already on the market, such as sun tracking mechanisms which track the sun position for optimal light incidence angle and complicated mirror constructions. However these solutions are not practical in wireless/mobile products because of their sizes. Therefore they are beyond the scope of this dissertation.

2.4.4 User context dependent PV power output and the spectral dependent efficiency In calculating the power output in a specific user context it is important to take both the particular spectra and irradiance levels applicable in that user context into account. The power conversion efficiencies presented above and quoted by PV manufactures are only measured at an AM1.5 light spectrum. Thus a spectral dependent efficiency is needed. For this purpose the spectral Figure of Matching established in section 2.4.1 can be used. The spectral dependent Figure of Matching for a certain Irradiance level is:

FM ≈ FM x FM / FM [%] Eq. 2-16 spec+irrad AM1.5+irrad Spec, 1000W/m2 AM1.5, 1000W/m2

Due to the interchange ability between the spectral Figure of Matching and the PV power conversion efficiency Eq. 2-6 can be rewritten as:

≈ FM x FM / FM [%] Eq. 2-17 ηspec+irrad AM1.5+irrad Spec, 1000W/m2 AM1.5, 1000W/m2

For a user context the spectrum and the level of the irradiance G can be determined. n The converted power of the PV cell with an area of APV m2 at a certain irradiance G can n then be calculated as:

P = G x x A [W] Eq. 2-18 PV n ηspec+irrad PV

The instantaneous irradiation G will vary during the day. As a result the conversion ef- n ficiency , as was discussed in section 2.3.2, will also vary correspondingly. Therefore ηirrad the converted power will depend on the user scenario at the moment the PV cells are exposed to light. With the Figure of Matching algorithm, the power transfer efficiency and the converted power as a function of the irradiance level and spectrum is found. Therefore the matching interface MI:1 can be quantified.

2.4.5 Résumé on the irradiance matching interface MI:1 By combining the Spectra presented in Figures 2-6, and 2-8 to 2-12 with the Spectral Responses presented in Figures 2-21 to 2-25, it can be concluded that a-Si and DSC PV cells match well with TLD light; whereas c-Si, mc-Si and CIS due to their broader spectrum range match well with outdoor sunlight and incandescent light. By proper design the effects of shadows can be minimised. Matching at the irradiance matching interface MI:1 depends both on the spectral matching and the conversion efficiency in the user context. Therefore at low irradiance levels less than 100 W/m2 the a-Si and DSC PV cells match well with a TLD light user context.

57 Energy Matching - Key towards the design of sustainable photovoltaic powered products

With the spectral Figure of Matching it is possible to determine the spectrally dependent conversion efficiency of any PV cell towards any incident light spectrum. Therefore the irradiance level and spectrum dependent converted power P can be determined. PV

2.5 Optimizing the PV power output matching interface (MI:2)

The power output of a photovoltaic system depends on how well the electrical characteristics are matched with the load. This load could be the application; it could also be the buffer energy storage medium which in turn is connected to the application. In general the following measures are taken [Green, 1986]: • Optimizing the combination of PV cells in the module to fit the load; • Maximum Power Point Tracking (MPPT), see section 2.3.2.

In order to match the output voltage to the required load voltage, often some PV cells are connected in series or series-parallel combinations to constitute a PV ‘module’. By connecting the PV cells in series the total voltage is increased, while parallel connection increases the total current. This output voltage matching is needed to accommodate all kinds of power supplies and storage media like batteries.

By connecting n PV cells in series: The total output voltage of the module becomes about n times the output voltage of one PV cell. In case of shadows, the PV cell under the shadow will act as series resistance. The total efficiency of a module will, due to internal losses, be some percentages lower than that of one PV cell. For instance in case of a c-Si module the efficiency will be 22,7% versus an efficiency of 24,7% for a c-Si cell [Green et. al., 2006]. Another reason for these discrepancies is the difference in cell area used in the efficiency measurements and the losses in relative effective area due to inter-cell distances in the module.

By connecting m PV cells in parallel: The total output current of the module becomes about m times the output current of one PV cell. When only in parallel connected PV cells are used, the output voltage will be too low (0.4 V - 0.6 V) and insufficient to recharge a battery or to drive an application, therefore such a configuration needs a step-up DC/DC converter [TSA, 2006]. A module with parallel connected PV cells will be less vulnerable for shadows. The interconnection has to handle larger currents than in the case of a series configuration.

In section 2.4.2 it was found that in order to minimize the effect of shadows it would be advantageous to connect the PV cells in parallel and to use Field Effect Transistors (FETs) instead of blocking diodes to bypass those cells in the shadow. For the future the use of smart decision controllers are proposed which can be programmed to find ways of bypassing the non-optimally performing PV cells in the combination. The result is a higher total performance of the whole module. To switch the PV cells or strings of PV cells, FET

58 Chapter 2: Optimizing photovoltaic energy conversion systems for mobile/wireless products in outdoor/indoor user contexts

are used as adaptable blocking diodes [Kan, 1996]. In a dynamic optimization, all cells are compared at predefined sample times . The under performing cells are shut-off or bypassed in order to obtain the optimal performance of the whole module. A cell could for instance perform below the given threshold because it is at that moment in the shadow. At the next sample time shadow could cover a different cell which will then perform below the threshold. This higher performance of a combination of PV cells by using intelligence was demonstrated in a practical application at ‘The World Solar Challenge Race’ in Australia which was won three times by the NUNA team of the Delft University of Technology. Here the cells facing the sun were connected while cells in shadow were bypassed. In another possible example PV energy of different PV cell types can be smartly combined in such a way to ensure an optimal matching between the PV cell type and the instantaneous spectral distribution of available light.

2.6 Irradiance and PV type dependent power output

2.6.1 General Remarks In section 2.3.2 it was shown that the efficiency of a PV cell is a function of the irradiance level. For instance in Figures 2-14 through 2-16 it was demonstrated that the efficiency decreases the lower the irradiance level. In calculating the power output from a PV cell this irradiance level dependence of the efficiency must be taken into account.

2.6.2 The PV cell power output of one day In Figure 2-5 the irradiance is presented over the span of a day for three different types of daylight: clear, cloudy and variable, during the month of June. The irradiance will vary from about 40 W/m2 at 5 o’clock up to 1040 W/m2 at 12 noon. The result is that the efficiency of the PV cell will vary also during the day resulting in an additional reduction of power output in the early morning and at the end of the day.

Figure 2-33 PV electrical power output of a c-Si cell during a clear day calculated by using the irradiance pattern of a clear day in Figure 2-5 and the irradiance level dependent efficiency of c-Si PV cells as is presented in Figure 2-19

59 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Figure 2-34 Comparison between the power output of a c-Si PV cell as a function of time on a clear day, shown in two cases- where the low irradiance level efficiencies of the PV cell are taken into account, and a case in which only the Standard Test Condition (STC) efficiency is taken for the calculation

The difference between STC and irradiance dependent efficiency can be more clearly seen in case of a cloudy day as presented in Figure 2-35. In this case if the low light level efficiencies are taken into account the calculated total energy output of the PV cell in one day will be 355 Wh/m2. While in the case where the efficiency values measured under Standard Test Conditions (STC) are taken for the calculation, the total energy will be 420 Wh/m2. Between the two efficiencies a difference of about 65 Wh/m2 is found.

Figure 2-35 Comparison between the power output of a c-Si PV cell as function of time on a cloudy day in a case the low irradiance level efficiencies of the PV cell are taken into account and in a case just the Standard Test Condition (STC) efficiency is taken for the calculation

In order to calculate the Figure of Matching between a certain PV cell and a certain Bat- tery, the power output of the PV cell during a fixed time period (e.g. one day) is correlated with the charging power response of the battery. This Figure of Matching will be presented in Chapter 3.

60 Chapter 2: Optimizing photovoltaic energy conversion systems for mobile/wireless products in outdoor/indoor user contexts

More details will be analysed in Chapter 4, in which the information from this chapter is combined with that of the energy storage media (Chapter 3) to obtain a proper energy balance.

2.7 Other relevant design aspects

2.7.1 General Considerations Although in this dissertation the emphasis is on the electrical matching in the energy chain of PV powered products, other parameters outside the energy chain will have an impact on the matching in the energy chain. So in designing products with integrated photovoltaic cells, more aspects than the electrical power and energy will be of relevance. Without the pretence of providing a complete design concept, we consider that it should also take into account: • The Structural / Mechanical properties: Typical area dimensions for PV deployment. Encapsulation of the PV cells and Storage Media. Shape flexibility of the PV cells and Storage Media. Wiring. • Thermal design • User interface aspects: The colour range of the available PV cells. The impact of the colour and shape flexibility options on the PV performance and the product appreciation of the user.

All of these aspects are not analysed in detail in this dissertation and are annotated here as a first order impulse for further research in another dissertation within the framework of the SYN-Energy program.

2.7.2 Curved PV surfaces In general the PV technologies with potential for a curved surface can be divided into: 1. Thin wafer mono and multi crystalline silicon PV cells; 2. Thin Film amorphous silicon PV cells deposited on flexible substrates and Dye Sensi- tised PV cells on or between flexible substrates; 3. Other possible technologies.

Ad. 1

Figure 2-36 The bending of a thin silicon wafer with PV cells [Glunz et. al. ISE 2003]

61 Energy Matching - Key towards the design of sustainable photovoltaic powered products

By making the wafer thinner, the bending radius can safely be reduced as can be seen in Figures 2-36 and 2-37. These laboratory thin mono crystalline Silicon PV cells have been produced with a thickness of 37 um and a power conversion efficiency of 20.2%. In comparison, the common industrial cells have a thickness of about 225-250 μm. One could place thin micro-crystalline and multi-crystalline PV wafers in the same category. For safety and stability the thin wafers could be pressed and sandwiched in between two curved glass plates, as can be seen in Figure 2-37.

Figure 2-37 PV panel curved by using multi-crystalline wafers pressed between curved glass plates [Beers, 2003]

Colour can be added if functional either by colouring the individual PV cells or by colouring the thick glass plates. The prototype panel in Figure 2-37 consists of two 5 mm thick pre-bent glass plates laminated with resin. The back panel is coloured blue.

Ad. 2 Thin film PV cells deposited on flexible substrates such as metal foils. Example are Flexible CIS (copper, indium and selenium) PV cells.

Figure 2-38 Example of a thin bendable CIS PV module [Powalla et.al, 2005]

Ad. 3 Other possible solutions for curved PV cells are provided: • By using the so-called ‘Semiconductor on insulator by enamel techniques’ [Poelman and Kan, 2000]. In this option, doped silicon granules are imbedded in Indium doped enamels. The Indium has two functions, as a doping reservoir, and a conducting interconnection. The result is the possibility of large surface PV cells which could also be curved. The test samples performances were poor, with an efficiency below 1%. There is still a large need for further research but the idea is noteworthy since PV cells based on a similar concept are now proposed and fabricated [Scheuten Solar, 2006].

62 Chapter 2: Optimizing photovoltaic energy conversion systems for mobile/wireless products in outdoor/indoor user contexts

• The ‘Concrete Solar Cell’ [Arthur et. al., 1997]. In this solution doped Silicon granules are embedded in concrete, resulting in Building Integrated PV walls. • DSC PV cells made by a spray technique [Nanu et. al., 2005].

2.7.3 Colour and PV cells PV cells usually have a blue, brownish or black colour. The reason for this is the standard choice of antireflection coating or the surface finish. Colour of PV cells and modules can be altered by two methods, namely: 1. By varying the thickness of the anti-reflection coating. So the colour does not need always to be blue. It can be silver-like, blue, bronze, gold, green, and magenta. It can even be a rainbow if the thickness varies gradually. 2. By using coloured and or screen-painted glass to cover or sandwich the PV cells. Even opaque glass can be used for this purpose.

We perceive that a surface has one certain colour if that particular surface mainly reflects that colour. A blank uncoated Silicon surface will reflect nearly all the incident light, therefore it gives us a silver-like appearance. On the other hand a surface with a black finish will hardly reflect any colour and therefore absorbs the maximum amount of incident light. This would be the ideal PV surface finish. The typical cost for adding colour to cells is 2 to 3 times the price of normal cells (per cell). Colour can also cause problems. In the case of a mismatch between colour coatings and the Spectral Response of the PV cell, the result could be a degradation of performance over normal cells of about 20%!! [BP Solar, 2006].

2.7.4 Matching of the user emotional experience options with the PV application In comparing the user emotional experience of PV powered products, it must be kept in mind that the PV performance in a certain user-context should not be hampered by the user’s emotional experience. One of these emotional experiences is colour. By selecting the PV cell coating the colour and appearance can be adjusted. PV cells can have various colours. It will be clear that coloured PV cells illuminated by light with a spectral distribution outside the spectral transmission window of the PV cell coating is in fact illuminated by the wrong light. In addition each PV cell type has its spectral sensitive wavelength region defined by its specific spectral response (see section 2.3.4) If not taken into account all of these factors, will surely impair the function of the PV cell. For example a blue coated PV cell will reflect mainly the blue component of the light spectrum. In a case in which this PV cell is illuminated by an artificial light environment with a strong blue emissive spectrum, the result would not yield a good matching. On the other hand, a red coated PV cell would absorb mainly the blue component and is therefore preferred, since this coating transmits the main spectral contribution. However the PV cell must also be blue sensitive to yield a good match. A study on reflection as the source of optical losses of the PV cell [Parretta et. al., 2003] reveals that: • The blue mono-crystalline-silicon cells show the lowest reflection for white light (4-5%); • Due to the strong anisotropy of the multi-crystalline silicon cells, they have shown significant variations (5-11%) in reflectance values over the same cell and among the different cells even with the same colour;

63 Energy Matching - Key towards the design of sustainable photovoltaic powered products

• Modern high grade PV cells optimised for minimum reflection have a black appearance.

It will also be evident that bending a PV cell might reduce the performance of that PV cell. Apart from the non uniform light incidence and the inevitability of shadows, the bending itself might have some negative effects. Preliminary experiments done on ribbons of flexible DSC PV cells show that by multiple (>500x) bending only a small (less than 10%) reduction in performance was detected [Boschloo and Hagfeld, 2003]. These experiments need to be extended further. The overall impact and loss of PV cell performance as a direct result of bending is beyond the scope of this dissertation, is still under investigation and will be reported elsewhere. Up to now, the PV cells used in products have been mainly limited to cells with flat surfaces. The main reason for this is found in the early PV cells made from thick silicon wafers which were rigid and fragile. Also only flat laminations were feasible. Nowadays however there are several new emerging technologies enabling curved PV surfaces resulting in new product design opportunities for integrating PV cells into or onto products.

Table 2-5 Other relevant PV design parameters

Type Maturity Colour/Surface Typical area Typical Flexibility (1) dimensions thickness (cm2) (μm)

Mono-c-Si highly blue, black-grey, 15x15 cm2 150-250 low (3) commercially smooth, variable available colour (1), (2)

Multi-c-Si highly blue, granular 15x15 cm2 150-250 low (3) commercially variable colour available (1), (2)

a-Si some dark-brown 10 x10 cm2 <1 (5) high (4) commercial reddish, smooth production

CIGS/CIS some grey/black long strips, ca. 1-3 (5) high commercial smooth 10 cm wide production

CdTe some brownish low commercial production

III-V crystalline mainly space black 8x8 cm2 140-200 low (3) application and smooth also for use in combination with concentrators

III-V thin film terrestrial black 8x8 cm2 5 (5) high application smooth

DSC lab. brown 1-10 (5) high smooth

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Notes to Table 2-5: (1) Depends on anti-reflection coating which is optimised for cell material. (2) Colour depends on AR coating, coloured cells are available e.g. from Sharp, including gold/brown, green, dark-blue, light-blue, efficiency will be slightly less for alternative colours. (3) Possible for thinned cells (<150 um), however this will reduce efficiency. (4) Possible for thinned cells, less efficiency impact than with Si. (5) Substrate support structure (glass, metal foil) can vary from several microns up to mm’s.

2.8 Conclusions

The Figure of Matching algorithm can be validated at the incident light matching interface MI:1 by comparing the calculated PV spectral response (SR) based spectral Figure of Matching with the measured PV power conversion efficiency. This validation is done by using the results obtained by different and independent measure methods. Optimisation of the power output from the photovoltaic cells is possible by a proper choice of photovoltaic cell type and by taking into account the spectrum and irradiance level of the incident light. By combining the incident light spectra presented in Figures 2-3, 2-6, and 2-8 to 2-12 with the PV spectral responses (SR) presented in Figures 2-21 to 2-25 in the determination of the spectral Figure of Matching, it can be concluded that a-Si and DSC PV cells match well with TLD lamp light (modern indoor lamplight), whereas c-Si, mc-Si and CIS PV cells match well with outdoor sunlight. The power conversion efficiency of PV cells depends on the spectrum of the incident light. For proper user context matching the spectral dependent efficiency has to be taken into account. MPP tracking can be simplified by using precognition of the PV type dependent Maximum Power Point (MPP) values the. Look-up tables measured on samples will provide information on how the MPP varies with irradiance level. Together with the short circuit current I sc of these samples, this can be the basis for a novel Maximum Power Tracking method. The Standard Test Conditions do not provide good selection criteria for designing PV powered mobile wireless products. This is because: A PV cell with a high efficiency at Standard Test Conditions (STC) of 1000 W/m2, will at lower irradiance levels not automatically maintain its higher efficiency. Although the efficiency at STC was the same, at low irradiance levels (about 2 W/m2) a factor two difference in efficiency was found in between cells provided by one single supplier. Therefore a new set of test data is proposed to be included into the data sheets which also take into account low irradiance levels existing in an indoor user context and the spectral responses. A parallel interconnection of PV cells with DC/DC converters for the appropriate load voltage will be favourable for mobile/wireless products since this configuration will be less sensitive for shadows. However an additional design condition should be implemented to prevent leakage currents through cells in shadow by introducing a selective bypass of those cells.

65 Energy Matching - Key towards the design of sustainable photovoltaic powered products

This chapter has addressed the first part of the research question, namely:

What photovoltaic conversion systems are optimally matched with applications in an outdoor/indoor user context?

The answers found are: a-Si and DSC PV cells match well only with an indoor office user context. c-Si, mc-Si and CIS PV cells match well with bright outdoor sunlight user contexts. High efficiency c-Si can match both outdoors and indoors since at low irradiance levels this technology still performs well. For determining which photovoltaic conversion system is optimally matched, the use of new test conditions that include also irradiance levels and spectra found in indoor user contexts should be introduced. For optimal matching of photovoltaic conversion systems with applications, the use of precognition of the PV type dependent Maximum Power Point (MPP) values would be advised. For optimal matching of the photovoltaic converters the use of the short circuit current I as an indicator for the irradiance level would be a good option. This option enables sc the merging of two elements, namely the irradiance level detector and light power converter. Therefore two functionalities can be achieved with one single element, eliminating duplicitous elements.

66 Chapter 3: Matching of electrical energy storage media in PV powered mobile/wireless products

3.1 Introduction

Before going into detail about potential electrical energy storage media, one could ask whether there is a need for any energy storage medium in PV powered products, and if so, why and in what user context and application? The answer to these questions can be structured as follows: a) Cases in which no electrical storage medium is needed. b) Cases in which an electrical energy storage medium is needed.

Ad. a) In general in those cases in which the electrical power coming from the PV cell can be used directly, no storage medium is needed. This is the simplest solution. The PV cell provides power whenever there is sufficient illumination. A good example is the electronic calculator as is shown in Figure 3-3.

Figure 3-1 A PV powered calculator

This product needs to function at the very moment that the numbers on the display are to be read. Both photovoltaic cells and the reading needs light, which coincide with the very moment of functioning. Therefore a PV cell without a battery is a good match.

Ad. b) Due to the frequently erratically fluctuating irradiation (see section 2.2.2) and unpredict- ability of the solar irradiation in time, the converted power supplied by the PV cells is

67 Energy Matching - Key towards the design of sustainable photovoltaic powered products intermittent. Therefore an electrical storage medium is needed to ensure the continuity of the supply and electricity on demand. Also the electrical power coming from the PV cells quite often cannot be utilized to power the products at the very moment of PV conversion but is needed by the application at a later time. Since delay and storage are synonyms in the power transmission analysis, a logical solution is just to store this electrical power and therefore to delay the moment of use to a more convenient moment.

Thinking about electrical energy storage one would immediately recall the battery. Histori- cally however the first electrical storage medium was a capacitor by the name of ‘Leyden Jar’ invented by Pieter van Musschenbroek in 1745 (see Figure 3-2). Since the sound made by discharging a group of parallel connected jars resembled that of a canon fired in a ‘battery’ formation, the name battery was given to a group of Leyden jars at that time. Today the name battery is still in use but now it means a group of electro-chemical cells.

Figure 3.2 An example of Mussenbroek’s Leyden Yar and a ‘discharge tool’ (courtesy Boerhaven Museum, Leiden, NL)

Since not only batteries but also capacitors will be analysed as means to store electrical energy, throughout this dissertation the plural form ‘energy storage media’ will be used. In this chapter, the electrical storage media are taken as the central part of the energy chain. As a result the analysis of the matching between this element and two others in the energy chain can be divided along the two sides of the storage media. On the side of the input - a matching is sought that enables storage of as much energy as possible from the photovoltaic (PV) cells into the storage media. On the side of the output - the matching is optimised to enable the use of the available stored energy as efficiently as possible. The first mentioned concerns the energy transfer through matching interface 2 (MI:2) in Figure 3-1, while the other concerns the energy transfer through matching interface 3 (MI:3). To stress the integral approach of the whole energy chain, both tracks of matching are treated equally in this chapter. In this way the second part of the research question (sub- Q. 2) as formulated in Chapter 1 will be answered:

What matching can be achieved between the electrical energy storage media and both the photovoltaic (PV) power conversion systems in an outdoor/indoor user context (1) and the functional product application (2)?

68 Chapter 3: Matching of electrical energy storage media in PV powered mobile/wireless products

The energy storage medium determines the amount of deliverable energy. Especially in mobile products, the energy storage medium will be an important design item due to the influence it has on the product weight, size, and on the overall portability of the product. Future improvements might increase the design options. The outline of this chapter is marked by the section numbers in Figure 3-1.

Figure 3-3 The Energy Matching Model of the energy chain of a PV powered product. The marked (section numbers) links and elements will be analysed in this chapter

To be able to investigate the ‘matching’ as mentioned above, the electrical energy storage media options and the characteristics of a variety of electrical energy storage media will be analysed in Section 3.2. To probe how well the matching between particular chain elements with the energy storage is, both the calculated Figure of Matching algorithm and measured energy transfer efficiency results will be used. The energy transfer efficiency from the PV system into energy storage, and the matching between the PV energy converter

69 Energy Matching - Key towards the design of sustainable photovoltaic powered products

and the energy storage media is analysed in Section 3.3. Next the energy transfer from electrical energy storage media towards the application and matching of the storage media and energy use in the energy chain is analysed in Section 3.4. In Section 3.5 the advantages of battery-capacitor combinations are analysed. Finally in Section 3.6 the conclusions are summarised and a roundup of answers for the research sub-question 2 are presented. Parts of the information presented in this chapter have been published previously in papers presented at the International Power Source Symposium (Brighton, April 2005) [Kan et.al., 2005a; Kan and Struik, 2005b].

3.2 Energy storage media characteristics and performance

3.2.1 General overview electrical energy storage media In an electrical energy storage medium, electrical energy is stored either chemically, e.g. in a battery, or as potential energy in a capacitor. An overview of the existing energy storage technologies, selected in particularly for their suitability for photovoltaic systems has been investigated for the European Community [INVESTIRE Network, 2003]. Although this report mainly concern large PV systems it provides a complete overview of energy storage systems. The following technologies were addressed and explained in this report viz: • Lead Acid (Pb-A) batteries; • Lithium-Ion batteries; • Super capacitors; • Ni/Cd batteries; • Ni/Zn batteries; • Electrolyser + Hydrogen storage + Fuel Cells; • Flywheels; • Vanadium redox flow batteries; • Pneumatic compressed air storages; • Zinc/Air batteries.

The general conclusions in this INVESTIRE Network report were: • Lead Acid batteries are the cheapest options for off-grid applications. Future improvements are not very likely as these batteries are proven technology. Proven technology is an advantage but also will offer resistance to innovation. • Lithium-Ion technology is characterised by high efficiency, high cyclability, and capability to withstand low state of charge periods. Future improvements in technology and investment price are promising compared to all the other energy storage methods. • Super capacitors are mainly of interest for levelling power production and power quality. Relatively low losses occur due to low internal resistance. • Nickel based batteries perform well in levelling power production. However the losses are relatively high. • The electrolyser + Hydrogen + fuel cell storage technology is, due to its complexity, only evaluated for use within urban power supply settings. Costs are high and losses are very important due to double conversion.

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• Flywheels are only adapted for power levelling and power quality. They are as a consequence of their working principle, which is based on a large moment of inertia, very heavy. • Vanadium redox flow batteries are adapted for power levelling and power quality. They are very expensive. • Pneumatic storage is well suited for levelling of power production. Due to its conversion method it is an expensive method and suffers large losses (double conversion). • Zinc-Air batteries have a short life span, however important improvements are expected.

As a result of these conclusions, particularly in respect to the parameters of size, weight and energy density, flywheels, pneumatic storages and electrolyser + hydrogen + fuel cells will not be likely candidates for use in combination with photovoltaic cells in mobile/wireless products and therefore will not be analysed in this dissertation. As a result these technologies are not incorporated in Table 3-1.

Note: In this dissertation it is assumed that the reader is familiar with the basic knowledge on electrical energy storage. Information on this topic can be read in common textbooks on batteries, [e.g.; Buchmann, 2001; Linden and Reddy, 2002] and Energy Storage Reports [e.g. INVESTIRE Network, 2003]. For the reader’s convenience a resume of the battery fundamentals is presented in Appendix F.

3.2.2 Battery characteristics and performances A. General characteristics Batteries are presently the most common media for storing electrical energy. Two types can be distinguished, namely: primary or non rechargeable and secondary or rechargeable ones. Commercially available rechargeable batteries are: Lead Acid, NiCd, NiMH, Li-Ion and Li-Ion polymer batteries. However the latter two are at this moment mainly available as product dedicated batteries for example in cameras, cellular phones and laptops. Com- paring primary and rechargeable secondary batteries: • Primary batteries suffer less from self discharge than rechargeable secondary batteries. This parameter is particularly important in the design of products that are used only occasionally, but have to work reliably at the very moment they should be used. • Primary batteries have relatively low initial costs; however, in the long run secondary ones are more economical for the user. • Secondary batteries generally pose less of a burden for the environment than primary batteries, but within this group there can be a large discrepancy.

In this dissertation the focus will be on secondary rechargeable batteries which can be recharged by PV cells. Used as an electrical energy storage medium in products, batteries have two important properties namely their voltage and storage capacity. In an ideal battery the voltage remains constant throughout the whole discharge sequence and then instantaneously drops to zero at the moment the battery is empty. Also in an ideal battery the capacity is independent of the discharge rate, outside temperature and the way it is recharged. In real life the battery performance is far from ideal. Therefore, special precautions have to be taken into account in designing with real batteries in mind.

71 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Note: The rate of charge or discharge is expressed in relation to battery capacity. Known as the C-rate, this rate of charge equals a charge or discharge current. It’s defined as: I = M x C N where: I = Charge or discharge current in amps M = A multiple or fraction of C C = Numerical current value of rated battery capacity in Ampere-hours (Ah) N = Time in hours at which C is declared A battery discharging at a C-rate of 1 delivers its nominal rated capacity in one hour. For example, if the rated capacity is 1000 mAh, a discharge rate of 1 C corresponds to a discharge current of 1000 mA. Simi- larly, a rate of C/10 corresponds to a discharge current of 100 mA. For example a battery with a capacity of 6.8 Ah has a 1 C value of 6.8 A. In this context the charge or discharge rates are expressed in x times C. So for example C/2 will mean 1/2 times the C value.

Charge and discharge characteristics of batteries depend on the type and technology involved. With Li-Ion batteries for example there is a strict charging procedure to be followed as can be seen in Figure 3-4. These charge procedures are recommended by most battery manufacturers for Li-Ion batteries. Also most of the dedicated Li-Ion-charge integrated circuits (ICs) are designed to charge the battery in this manner. The charging of a Li-Ion battery consists of three phases: a pre-charge; a fast-charge constant current (CC), and a constant voltage (CV) termination.

Figure 3-4 Charge characteristics of Li-Ion batteries at various charge rates (C, C/2 and C/5 with C= 6.8 A. Shown is the typical constant current (CC) followed by a constant voltage (CV) charge characteristic needed by a Li-Ion Battery [Saft, 2006]

In the pre-charge phase, the battery is charged at a low-rate (typically 1/10 of the fast charge rate) when the battery cell voltage is below 3.0 V. This provides a recovery of the passivating layer which might be dissolved after prolonged storage in deep discharge state. It also prevents overheating at 1C charge when partial copper decomposition appears on anode-shorted cells in over-discharge. When the battery cell voltage reaches 3.0 V, the charger enters into constant current (CC) phase. In this second phase, maximum current is applied until the cell voltage limit is reached.

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After this voltage limit is reached a constant voltage (CV) is applied, and the current decreases in this third phase. The charging should be terminated when the current has dropped below 3% of the original current. Li-Ion batteries become unstable if charged at higher voltages (overcharge). The results of overcharging are lithium metal plating on the anode, oxygen release and heating up of the battery. Therefore most Li-Ion battery packs contain a protection circuit that prevents overcharging. However the drawback of this protection circuit is a larger internal resistance. To safely charge the Li-Ion battery, it can only begin to charge when the ambient temperature is between 0 °C to 45 °C. Charging the battery at lower temperatures promotes formation of metallic Lithium, which increases the battery impedance and causes cell degradation. On the other hand, charging the battery at higher temperatures causes accelerated degradation because this promotes a Li-electrolyte reaction [Buchmann, 2001]. The discharge characteristics of a Li-Ion are shown in Figure 3-5 and Figure 3-6.

Figure 3-5 Discharge characteristics of Li-Ion batteries at various discharge rates (2C, C, C/2 and C/5). The voltage decreases slowly and presents a good indication for the state of charge. [Saft, 2006]

Figure 3-6 Discharge characteristics of Li-Ion batteries at various discharge temperatures. The output voltage in the low temperature range (< 20 °C) decreases [Saft, 2006]

73 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Temperature affects the internal resistance of batteries [Buchmann, 2001]. For instance, in a Li-Ion battery the internal resistance will be 50 mΩ at a temperature of 25 °C. If the temperature increases, the internal resistance decreases. At 40 °C the resistance has dropped to a value of 43 mΩ, while at 60 °C it is 40 mΩ. Despite the better performance at an elevated temperature, this situation should be avoided since the consequence is a capacity fading [Ramadass et.al, 2002]. The effect of capacity fading is that the capacity of the battery diminishes each following cycle time. Cold temperatures will impair the performance since the internal resistance increases drastically. At 0 °C the internal resistance of the above mentioned Li-Ion battery has increased to a value of 70 mΩ, while at -10 °C and -20 °C there is an increase to a value of respectively 80 mΩ and 100 mΩ. This higher internal resistance will result in an output voltage drop as can be seen in Figure 3-6. This lower voltage will in turn hamper the functionality of the product at cold locations as for example as with a digital photo camera at a winter sports location [Keukens, 2005].

B. General performance Battery performance parameters needed to design PV powered products can be found in datasheets, as for instance Nominal voltage, rated capacity, maximum recommended charge current (usually 1C) and maximum continuous discharge current, in X times C. Note that for Li-Ion this maximum discharge current is usually limited by the electronic protection circuits for example to maximum 5 A. Sometimes additional information can be found about pulse discharge current, discharge cut-off voltage and energy densities. Less information is generally found about power densities. Power density depends on the pulse duration and depth of discharge (DOD) as can be seen in Figure 3-7.

Figure 3-7 Power density of the Li-Ion battery as a function of pulse duration and depth of discharge (DOD) [CRIEPI, 2005]

To compare the characteristics and performance of batteries available today, they are presented in Table 3-1.

74 Chapter 3: Matching of electrical energy storage media in PV powered mobile/wireless products

Table 3-1 A comparison of battery characteristics and performances < 0,2C 2% (2) Reus. Alkaline 60C 0,5C 150 250 (2) NanoT. Li-Ion (15), (16) 80 80 (initial) (1) > 2C < 1C 100-315 (11) 3000-9000 280 (9) 350 (10) Polymer 170 (9) 135 (10) > 2C < 1C (12) 1350 (13) 500 (19) 5C < 0,5C 155-310 (13) 880-2900 (13) 3700 (16) 20% (6) 30% (6)20C 10% (7)1C 10% (7) 0,3% flat very flat flat slope slope slope 2 V 1,25 V (8) 1,25 V (8) 3,6 V 3,6 V 1,5 < 1008-16hhigh 100-2005% 1h (min)4-8% (2) 200-300 moderate 2-4h 150- 250 (3)5C low< 0,2C 200-300 (3) 2-4h very low 2-4h low 200-2000 1 min 80% 2-3h moderate 75-140 (11) 50-200 (11) 150-250 (11) 300 (11) 500 30-50 (1) 45-80 (1) 60-120 (1) 110-160 (1) 100-130 90 (2) 100 (2) 240 (2) 478 (17) ) (1) Ω Peak Best results • • Discharge profile (var. Litt) Cell voltage (nominal) (1) Load current (1) Power density (W/l) Internal resistance (m (Fast) charge time (1) Overcharge tolerance (1) Self-discharge/month (room temperature) (1) Specific power (W/kg) Gravimetric energy density (Wh/kg) Parameter SLA (Pb-A) NiCd NiMH Li-Ion Li-Ion Volumetric energy density (Wh/l)

75 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Table 3-1 A comparison of battery characteristics and performances (continued) 50 (5) (50%) Reus. Alkaline NanoT. Li-Ion (15), (16) 2000 (13) 0° to 45° (18) 0° to 65° Polymer flammable yes yes 500-1000 (5) 300-500 (18) (5) 30 to 60 days 60-90 days not required not required not required toxic 80-95 (13) 95 (13) 1 2 2,4 4 4 0,2 cube1970 round 1950 round 1990 round + flat round + flat 1991 1999 round 2005 1992 2-8 2-53 to 6 months (6) 2-5 85 (14)200-500 (4) 1500 (1), (4) 300-500 (4), 98 (14) -20° to 60° -40° to 60° -20° to 60° -20° to 45° Typical costs (SLA=1) (1) Safety Environmental Shape Flexible Commercially used since (1) Calendar life [years] (2) Maintenance requirement (1) Storage efficiency % Energy efficiency % Cycles life (20% fading) (1) Operating temperature (°C) (discharge only) (1) Parameter SLA (Pb-A) NiCd NiMH Li-Ion Li-Ion

76 Chapter 3: Matching of electrical energy storage media in PV powered mobile/wireless products

Notes to Table 3-1: Compared batteries are Sealed Lead Acid (SLA, PB-A), Nickel Cadmium (NiCd), Nickel Metal Hybrid (NiMH), Lithium Ion (Li-Ion) Lithium Ion Polymer (Li-Ion Polymer), Nano Technology Li-Ion (Nano T. Li-Ion and Reusable Alkaline (Reus. Alkaline). (1) Buchmann, 2001; (2) Linden and Reddy, 2002; (3) Protection circuit of Li-Ion and Li-Ion Polymer adds about 100 mΩ to the internal resistance; (4) Cycle life according to (1) is based on the battery receiving regular maintenance. Failing to apply periodic full discharge cycles may reduce the cycle life by a factor three; (5) Cycle life is based on the dept of discharge. Shallow discharges provide more cycles than deep discharges. In ref. (1) no information is presented as to what extent the depth of discharge (DOD) was performed for the given cycle time, therefore these figures present upper limits. Also there is no information about the temperature during recycling, suggesting that the cycling has been performed under isothermal laboratory conditions. In practice this will not be the case, resulting in a lower cycle time; (6) The self-discharge is highest immediately after charging, and then tapers off. The NiCd capacity decreases 10% in the first 24h, then the self discharge reduces to a value of 10% every 300 days. Self discharge increases with higher temperatures; (7) Internal protection circuits typically consume 3% of the stored energy per month; (8) 1,25 V is the open cell voltage. 1,2 V is the commonly used value (to be used in calculations); (9) FULLRIVER Li-Polymer Battery, 2005; (10) Power Stream Batteries, 2005; (11) http://www.thermoanalytics.com/support/publications/batterytypesdoc.html; (12) http://criepi.denken.or.jp/en/e_publication/home348/Data348-2-e.html; (13) Green A., 2005; (14 ) Mourzagh et. al., 2005; (15) Satoh, 2005; (16) Toshiba, 2005; (17) Kariatsumari, Sony, 2005; (18) Capacity fading occurs at temperature above 45° C [Ramadas et. al., 2002]; (19) ATP, 2005.

3.2.3. Selecting batteries that match the energy chain of PV powered products The selection of batteries for optimal matching into the energy chain of wireless mobile PV powered products requires tradeoffs between battery type, type of controlling electronics, load and the PV cells: 1. Mechanical matching: for example battery energy capacity (Ah) versus battery size and weight. 2. User interface matching: battery type versus charging and protection requirements. 3. Electrical and electronic complexity matching: for example voltage regulation topology versus load requirements, such as energy demands. 4. Fluctuating charge sequence. 5. Environmental burden versus performance.

77 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Ad. 1 Mechanical matching • Mechanical parameters such as low weight, and small volume. • Good compactness and large energy storage capacity density or high volumetric energy density (Wh/l). • High power density (W/l). • Flexibility to adapt to shapes. • Rugged for safety. This is usually a compromise with low weight and small volume.

Ad. 2 User interface matching • Convenience of use such as short charging time, good overcharge tolerance, long cycle life. • Good actual battery life per cycle. • To be able to cycle at different depth of discharge (DOD). • Low maintenance. • Low self discharge. • Broad operating temperature range. • Good reliability. • Maturity and experience in the field. • Low costs.

Ad. 3 Electrical and electronic complexity matching Average voltage per battery cell is technology dependent. The electronic complexity must be tailored to the application. Adding complex electronics to a simple application will only increase costs and will therefore not be implemented. The required load voltage in applications is about 3,5 V +/- 0,5 V. For example with NiMH batteries which have a nominal voltage of about 1,2 V, an additional DC/DC converter is needed. Li-Ion Batteries have operational voltages between 2,5 V and 4,2 V, so they need no DC/DC converter. Therefore for sake of simplicity in design they will match well into the energy chain of PV powered products.

Ad. 4 Fluctuating charge sequence Fluctuating irradiance will cause a fluctuating output from the PV cells (see Chapter 2, section 2.6). Due to this fluctuating output, the charge sequence will be interrupted and continued several times. To match a battery in the energy chain of PV powered products, the battery must be able to recharge and to function in a fluctuating charge condition. Bat- teries that suffer the so-called memory effect, i.e. a decrease in capacity in the case where it is recharged before completely discharged, will not match well in the energy chain. Mainly NiCd and to a lesser extent NiMH batteries suffer this memory effect. Due to this handicap of suffering the ‘memory effect’ NiCd batteries are therefore not well suited to be partially recharged by a fluctuating PV Cell output or each time there is some light. The replacement could be Li-Ion batteries that have no memory effect. Li-Ion batteries allow multiple recharging after partial or shallow discharge, and therefore can even demonstrate cycle lives of up to 1.000.000 cycles [Green, 2005]. Therefore Li-Ion batteries are a better match than NiCd and NiMH batteries for the energy chain of a PV powered product.

78 Chapter 3: Matching of electrical energy storage media in PV powered mobile/wireless products

Ad. 5 Environmental burden versus performance Complying with the (European) legislation. As a result of these criteria and due to their toxic components (Cadmium), NiCd batteries would not be a match, as of July 2006 [EC, 2003/2004, de Wild-Scholten et. al. 2005]. There seems to be a contradiction between European legislation and the use of batteries under extreme conditions, since NiCd batteries show a very good low temperature performance even at extremely low temperatures. At < - 25 °C only NiCd batteries can still be used. Therefore they should be used in cases where lead acid and Li-Ion batteries cannot be used [Hamlen, 2005].

In selecting specific energy storage media that will match with photovoltaic powered mobile/wireless applications, the following criteria will be important: • If weight does not matter, Sealed Lead Acid (SLA) batteries would be good candidates because this is proven technology. SLA batteries are today the most economical option for large power applications since these batteries are based on the most overcharge tolerant technology. • The disadvantage of Li ion batteries is the higher cost but this issue will be less dominant in the coming years due to mass production. • Ni-based batteries show limited performance mainly caused by their poor cycle life time and their large self-discharge. • The main problem of Li-Ion is the short life time at full DOD and the low power density. However concerning the power density, there are some encouraging innovations [Toshiba, 2005 and Satoh, 2005]. • A point of attention with Li-Ion batteries is the capacity fading at temperatures above 45 °C. • Li-Ion polymer is potentially a lower cost version of the Li-Ion battery. By virtue of polymers this type of battery enables a very slim geometry and simplified flexible packaging [NEC, 2005]. • Reusable Alkaline batteries are suitable for limited low power applications. Its limited cycle life (< 50) could be compensated by the low self discharge (0,3%).

In résumé: Li-Ion and Li-Ion Polymer batteries as well as future nano technology based Li-Ion batteries would be good candidates to match into the energy chain of mobile PV powered products since this technology has: • The lowest weight. For mobile application the weight advantage of Li-Ion plays an important role. • The smallest size. • The highest energy density. • The lowest self discharge. • Operating voltages that match most electronic circuitry without the need of additional DC/DC conversion.

Though as was presented above still a point of concern is the limited operating temperature range of Li-Ion batteries in comparison to NiMH and NiCd batteries.

79 Energy Matching - Key towards the design of sustainable photovoltaic powered products

3.2.4 Capacitors characteristics and performances Another less common medium for electrical energy storage is the capacitor. One of the first uses of capacitors in PV powered products is, for example, in electronic solar pocket calculators to retain or to ‘hold’ the charge of the ‘memory’ storage. Capacitors exist in various sizes, from a few pico Farads (pF), up to 1 Farad (F). In recent years special energy storing ‘Super’ and ‘Ultra’ capacitors have been developed which can store up to several hundred Farads. This development has opened the way towards new applications. The charge and discharge characteristics of capacitors follow an exponential curve as can be seen in Figures 3-8b, 3-9 and 3-10.

Figure 3-8 Charge characteristics of capacitors [ELNA, 2005]

80 Chapter 3: Matching of electrical energy storage media in PV powered mobile/wireless products

Figure 3-9 Discharge characteristics of capacitors [ELNA, 2005]

Figure 3-10 Power discharge characteristics of capacitors [Varakin et. al., 1999]

Figure 3-11 Self discharge characteristics of capacitors [Tallner and Lannetoft, 2005]

As can be seen in Figure 3-11, the higher the capacitor temperature, the higher the self discharge.

81 Energy Matching - Key towards the design of sustainable photovoltaic powered products

3.2.5 Comparing battery and capacitor characteristics The first difference between batteries and capacitors concerns both their charge and discharge characteristics. In the case of a battery, the charge and discharge characteristics are type dependent. In general the end voltage of the battery is reached shortly after charging has commenced and remains more or less constant during the whole charge period, as can be seen in Figure 3-4. Conversely, in a capacitor the voltage increases and decreases ex- ponentially during charge and discharge cycles, as is shown in Figures 3-8 and 3-9. Due to this exponential characteristic, capacitors feature rapid charge and discharge characteristics (seconds instead of hours). These rapid discharge capabilities enable capacitors to absorb and deliver current bursts or peak currents. The capacitors’ voltage depends linearly on the amount of charge put into it. Therefore with a constant current (I) there will be linear discharge and charge curve, as can be seen in Figure 3-12. The Battery’s voltage during the discharge and charge sequence will remain nearly constant.

Figure 3-12 Comparing the charge – discharge characteristics at the constant current (I) of both batteries and capacitors [Epcos, 2005]

Other differences between batteries and capacitors are their power and energy densities. Capacitors have larger power densities than batteries, while batteries have larger energy densities than capacitors, as can be seen in Figure 3-13.

Figure 3-13 A comparison between the specific or gravimetric energy densities and specific or gravimetric power densities of various batteries and capacitors [Epcos, 2005]

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Table 3-2 A selection from Table 3-1 for comparison of characteristics and performances of Li-Ion, Li-Ion Polymer, Nano Technology Li-Ion Battery and Super Capacitors

Parameter Li-Ion Li-Ion NanoT. Li-Ion Super Polymer (15), (16) Capacitor (17)

Gravimetric energy density 110-160 (1) 100-130 80 2,4 (Wh/kg) 170 (9) 10 (18) 135 (10) 120 (19)

Volumetric energy density 478 (20) 280 (9) 150 3,8 (Wh/l) 500 (21) 350 (10) 160 (19) Specific power (W/kg) 300 (11) 100-315 300-900 470-660 500 (12) (11) 1000 (18) 4300 (19)

Power density (W/l) 880-2900 3700 (16) 1250-1390 (13) 7000 (19)

Pulse load (A) 0,5-2 0,5-2 60 1-100 (18)

Internal resistance (mΩ) 150-250 (3) 200-300 200-400 (1) (3) (Fast) charge time (1) 2-4h 2-4h 1 min 80% seconds

Overcharge tolerance (1) very low low

Self-discharge/month 10% (7) 10% (7) (room temperature) (1)

Load current (1) • Peak > 2C > 2C 60C 100 • Best results < 1C < 1C

Discharge profile (1), (2) slope slope slope exponential

Operating temperature -20° to 60° 0° to 60° (°C) (discharge only) (1)

Storage efficiency % 95 (13)

Energy efficiency % 98 (14)

Cycles life (20% fading) (1) 500-1000 (5) 300-500 1000 500 000

Calendar life (years) (2) 10

Safety flammable

Shape round + flat round + flat flat flat

Flexibility yes yes yes no

Commercially used since 1991 1999 2006 2003 (1) (15), (16)

83 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Notes to Table 3-2: (1) Buchmann, 2001; (2) Linden and Reddy, 2002; (3) Protection circuit of Li-Ion and Li-Ion Polymer adds about 100 mΩ to the internal resistance; (4) Cycle life according to (1) is based on the battery receiving regular maintenance. Failing to apply periodic full discharge cycles may reduce the cycle life by a factor of three; (5) Cycle life is based on the dept of discharge. Shallow discharges provide more cycles than deep discharges. In ref. (1) no information is presented as to what extent the depth of discharge (DOD) was performed for the given cycle time, therefore these figures present upper limits. There is also no information about the temperature during recycling suggesting the cycling has been performed under isothermal laboratory conditions. In practice this will not be the case, which will result in a lower cycle time; (6) The self-discharge is highest immediately after charging, and then tapers off. The NiCd capacity decreases 10% in the first 24h, then the self discharge reduces to a value of 10% every 300 days. Self discharge increases with higher temperatures; (7) Internal protection circuits typically consume 3% of the stored energy per month; (8)1,25 V is the open cell voltage. 1,2 V is the commonly used value (to be used in calculations); (9) FULLRIVER Li-Polymer Battery, 2005; (10) Power Stream Batteries, 2005; (11) http://www.thermoanalytics.com/support/publications/batterytypesdoc.html; (12) http://criepi.denken.or.jp/en/e_publication/home348/Data348-2-e.html; (13) Green A., 2005; (14) Mourzagh et. al., 2005; (15) Satoh, 2005; (16) Toshiba, 2005; (17) Maxwell, Ultra-capacitors, 2005; (18) Cap-XX, Super-Cap, 2005; (19) ELNA, 2006; (20) Kariatsumari, Sony, 2005; (21) ATP, 2005.

3.3 Matching photovoltaic power converters and energy storage media (MI:2)

3.3.1 General According to the iteration steps of the Engineering Research Methodology (see Chapter 1); the ‘Matching’ process of the photovoltaic power converter and the energy storage medium or media is done in two steps. First the Figure of Matching is determined for various irradiance patterns, PV cells and battery types. However, for demonstration purposes of the Figure of Matching algorithm, in this dissertation matching is only analysed between a limited number of PV cells and the most suitably matched battery family, which are namely Li-Ion Batteries. Then in the second step the energy transfer efficiency across the Matching Interface 2 (MI: 2) in Figure 3-1 is optimized. In fact this is a derivative of the ‘load matching’ as described in Chapter 1.

84 Chapter 3: Matching of electrical energy storage media in PV powered mobile/wireless products

3.3.2 Figure of Matching between PV cell and battery In Chapter 1 the generic Figure of Matching algorithm between two elements m and j was introduced and defined as:

FM = { Ø / ∫ S (var) d var } x 100% Eq. 3-1 p

Here Ø is the correlation between the Stimulus S emanated from element s towards p the next element in the energy chain, i.e. element p and the Step-Response R of this Step-p element p.

Ø = ∫ S (var) x R (var) d (var) Eq. 3-2 p step-p var range This generic Figure of Matching is made applicable for the matching interface MI:2 between a certain PV cell and a certain battery by first determining what type of power can be correlated between the two elements. Of the whole set of parameters that can be matched between a PV cell and battery, a group of parameters such as temperature, weight, size, cost and LCA have been identified as being given exogenous parameters (see Chapter 1). The main point of concern in this dissertation is the energy transfer between the elements in the energy chain. Therefore the common parameter will be the energy or power transferred during a certain period of time. For calculating the Figure of Matching between a PV cell and a battery, the joint parameter will be ‘electrical power’ and the variable ‘var’ will be the time ‘t’. The step response of the battery will be the power intake response deduced from the charge characteristics presented in the data sheets, for example as presented in Figure 3-4. This power intake response is presented in Figure 3-14.

Figure 3-14 Power intake response (PIR) during charging versus Time of Li-Ion Batteries at various charge currents [Kan et.al, 2006d]

For the step power response the power step is started at 15 minutes at the Time (X) axis with a constant current. The three presented currents are respectively C/5 (=1,4 A), C/2 (= 3,4 A) and C (= 6,8 A). The areas under the curves represent the energy intake by the battery during charging. This energy intake is respectively 28 Wh, 26 Wh and 24 Wh. In accordance to the datasheet [Saft, 2006] the nominal energy content would be 26 Wh.

85 Energy Matching - Key towards the design of sustainable photovoltaic powered products

For determining the Figure of Matching, this power intake response will be correlated with the PV power output during a certain time, for example the period of one day. In chapter 2 these power output signals have been calculated and for the reader’s convenience they are presented in this chapter under Figure 3-15.

Figure 3-15 Power output of a c-Si PV cell during both a clear (a) and a cloudy (b) day

The low light level corrected power output of a c-Si PV cell for both a clear and cloudy day were respectively 1248 Wh/m2 and 355 Wh/m2 (see section 2.6.2 in chapter 2). For a mobile portable product however one or two dm2 of PV cell area will more likely be available rather than one m2. Therefore the energy intake of the battery has to be compared with the values of 12,5 Wh/dm2 and 3,5 Wh/dm2 PV energy output. By looking at these energy values a straight forward matching would yield that from irradiance on a clear day, a PV surface area of 2 dm2 would suffice for charging the battery. The power output of a 2 dm2 is presented in Figure 3-16.

86 Chapter 3: Matching of electrical energy storage media in PV powered mobile/wireless products

Figure 3-16 The power output of a 2 dm2 c-Si PV cell during a clear and a cloudy day [Kan et.al, 2006d]

To comply with the constant current restraint for charging a Li-Ion battery one should match the instantaneous PV power output with the power intake response. As can be seen in Figure 3-14, a lower initial power intake value will result in a longer charging time. For the initial power intake of 25 W, 12,5 W and 5W the charging time will be extended from 150 minutes to respectively 210 minutes and 375 minutes. This charging time extension shows two parts. The first part is inversely and linearly proportional to the initial power intake, while the second part is roughly of the same duration of about 100 minutes. As presented in Figure 3-4, the battery during the latter part has already reached its ultimate operating voltage of 4,2 V and is now charged with constant voltage. The Matching between the output of the c-Si PV cell and the intake response of the Li-Ion battery is presented in Figure 3-17.

Figure 3-17 The power output of a 2 dm2 c-Si PV cell during a clear and a cloudy day [Kan et.al, 2006d]

The Figure of Matching can be determined in accordance to Eq. 3-1 by calculating the ratio between the areas under the Power Correlation curve and the PV power output curve. The Figure of Matching is in this case 86%. This means that even during one entire clear day in June the battery will not be charged completely. Validation of the Figure of Matching calculations at this matching interface MI:2 were

87 Energy Matching - Key towards the design of sustainable photovoltaic powered products achieved by comparing the calculated Figure of Matching with independent charge - discharge measurements on Li-Ion batteries by using both grid and PV recharging [Langeveld et.al., 2005]. Battery charging tests conducted on several clear summer days showed that indeed, by using a standard charge electronic circuit, the battery was still not recharged completely by the PV cells after one entire day. The amount of charging ranged from 80% up to 90% but never completely charged. This finding contradicts the straight forward calculation by using the data sheet values (STC PV performance + battery capacity), which indicate that a 2 dm2 would be more than sufficient. Therefore with the Figure of Match- ing algorithm, the PV - Battery matching interface (MI:2) can be analysed and quantified. For design purposes the design options and design space for this matching interface MI:2 can be found.

3.3.3 Suboptimal energy matching in the PV - Battery matching Interface (MI:2) Examples of suboptimal energy matching were found during benchmark tests made by the author [Kan et. al., 2005a] on PV phone battery rechargers and other life tests on photovoltaic cell - Li-Ion battery recharge combinations. It becomes apparent with Li-Ion batteries (see also Chapter 5) that storing photovoltaic energy directly without some precaution is not efficient. To find a logical explanation for these discrepancies, we have to return to the basics of Li-Ion battery cycling. As a start, one should pay attention to the main requirements for proper recharging of these batteries. These requirements are recommended by battery manufacturers and are generally implemented in the battery charge electronic circuits, i.e. the constant current and constant voltage (CC-CV) sequence. During recharging the current is kept constant until a voltage of 4.1 V is reached. The voltage is then kept constant at 4.1 V, and the current will diminish as a result of increasing internal resistance. Measure- ments by the author on these photovoltaic (PV)-battery combinations show that the PV recharge current delivered is smaller than the recommended 1C value [Kan et. al., 2005a]. It therefore results in a longer recharging time as can be seen in Figure 3-4. In addition, the current coming directly from a photovoltaic cell is not constant due to the weather conditions. Often as a result of cloud formations passing in front of the sun, the irradiance and resulting PV output current will fluctuate, as can be seen in Figure 3-18. The erratically fluctuating input current coming from PV cells results in a recharging sequence that is often interrupted. The battery is charged for a while but before it is fully charged it is already partially used or discharged. Whether this is causing capacity fading is not clear from literature, since each interruption could have been counted as one new cycle. Note however that these cycles are shallow discharge cycles. For Li-ion batteries it is claimed that the dept of discharge (DOD) would not determine the maximum number of charge - discharge cycles or life-time [Takei et. al., 2001; Choi and Lim, 2002]. There are claims that small discharges and pulsed charging even enhance a battery’s life-span [Li et. al., 2001] but this effect depends on the pulse frequency. There is no synchronisation of the fluctuating current as result of the clouds and relaxation or resettle time of intercalat- ing Lithium ions. Therefore these pulses do not contribute to an enhancement of the battery life-span. Further, just connecting a PV cell directly to a battery without any circuitry which keeps the PV cell in its Maximum Power Point (see section 2.5.2) will result in an inefficient way of re-charging the battery. A special point of attention is of course the ap-

88 Chapter 3: Matching of electrical energy storage media in PV powered mobile/wireless products

Figure 3-18 Irradiance at the 3 different days versus time measured every 15 minutes at Kassel, Ger- many [Ransome and Funtan, 2005] plied voltage. If this voltage is not larger than the actual battery voltage at that moment, no charging will occur. The actual voltage of the battery increases during the charging process, as can be seen in Figure 3-4. While the voltage of the PV cell will fluctuate in accordance with the irradiance, it will sometimes even drop below the actual battery voltage.

3.3.4 Improving the matching between photovoltaic cells and batteries by using capacitors In the benchmark example above, one of the main drawbacks for proper charging is the erratic fluctuating irradiance of the sun. In using PV cells as mobile chargers the output voltage can, with the aid of DC/DC converters, be maintained at a certain voltage which is high enough to enable proper charging. However the irradiance related output current will reflect the erratic behaviour of the irradiance. In electronic engineering design it is common practice to smoother erratic fluctuating signals with a ‘low-pass filter’, or in practical applications, with a capacitor. Therefore the question will be what features and improvements can be introduced by placing capacitors at the input circuitry of the battery between the PV cells and the battery? The central questions here are: • Can capacitors improve the matching between PV cells and the battery? • Can the addition of capacitors to the battery input improve the energy transfer from PV cells into the battery?

To test this novel matching improvement concept, several charging tests were performed with capacitor-battery combinations [Kan et.al. 2005a]. In this experiment the PV cell was kept in its Maximum Power Point (MPP) by adjusting the current flow to the capacitor with the aid of a variable resistor. For the first test the variable resistor was comprised of a PNP transistor. By adjusting the control voltage on the Base of the transistor, the output

89 Energy Matching - Key towards the design of sustainable photovoltaic powered products current was pinched to fit the MPP. In the second experiment the variable resistor was comprised of a Metal Oxide Semiconductor Field Effect Transistor (MOSFET). By adjusting the frequency of the voltage pulses on the Gate of the MOSFET, the duty cycle of conduction was adjusted and therefore the amount of current was regulated to fit the MPP conditions.

Charging a capacitor by a photovoltaic cell means that the charging current will be fluctuating in accordance to the irradiance as can be seen in Figure 3-18. Therefore we first investigate how the charging efficiency depends on the charging current. This charging was executed in two ways, as can be seen in Figure 3-19a and b: • Directly without any other additional interface component; • With the aid of an inductance (L ) which acts as a charge buffer. 1

a. Direct charging (linear) [Kan et.al., 2005a]

b. Via inductance (switching) [Kan et.al., 2005a]

Figure 3-19 Charging of capacitor

In both schemes Figure 3-19a and b for simplicity and clarity, the load behind the capacitor, as for example a battery that is to be charged is not shown. So these schemes only present the first part of the interface between the PV cell and the battery. Also for simplicity the

90 Chapter 3: Matching of electrical energy storage media in PV powered mobile/wireless products controller circuitry to adjust the frequency of the variable duty cycle in accordance with the MPP of the PV cell has been omitted from Figure 3-19b. The results are presented in Figure 3-20.

Figure 3-20 Capacitor voltage versus charge efficiency [Kan et.al., 2005a]

The direct method yields an efficiency dependence on the extent to which the capacitor has already been charged. The inductance (L ) improved the transfer efficiency significantly 1 and made it less dependent on the state of charge. The main source of power loss is in the variable resistor, i.e. the PNP transistor, and the MOSFET to adjust the MPP of the PV cell. The transfer efficiency to the capacitor is limited by the losses introduced by the interconnection resistance and the internal resistance of the capacitor.

In resume: to circumvent the drawbacks encountered during the benchmark experiments of the solar-battery one can use capacitors in combination with PV rechargeable batteries for the following cases: • To buffer the PV current and therefore reduce the number of cycles induced by fluctuating solar irradiance at the input of the battery; • To buffer the PV current and to allow a well defined recharging of the battery from the capacitor. The use of capacitors at the battery input in a combined battery-capacitor system would therefore improve the battery performance by improving the recharging sequence.

3.3.5 Efficiency of power transfer from capacitors to Li-Ion batteries Having the energy stored in a capacitor as a buffer, the next step would be to transfer this energy to a battery. To test transfer efficiency, we used a DC/DC converter that kept the input voltage of the battery constant at about 4.1 V. This voltage is chosen because it is just below the maximum voltage allowed to avoid over-loading of Li-Ion batteries. Secondly, this value of 4.1 V during charging is ample above any other possible battery voltages. To keep the PV cell in its Maximum Power Point (MPP) the output current from the capacitor will vary, resulting in an amount of power ranging from 802 mW down to 1,76 mW. The DC/DC converter has two modes, namely a buck-boost mode in which the voltage

91 Energy Matching - Key towards the design of sustainable photovoltaic powered products is boosted to 4.1 V and a burst mode in which the voltage is maintained at 4.1 V. Figures 3-21 and 3-22 show that efficiency depends on the current to the load, which is in this case is the battery. In order to maintain the higher transfer efficiency when the DC/DC converter is below 80 mW, it should switch from buck-boost mode to burst mode (see Figure 3-23).

Figure 3-21 Transfer efficiency with the DC/DC switching regulator in buck-boost mode [Kan et. al., 2005a]

Figure 3-22 Transfer efficiency with the DC/DC switching regulator in burst mode [Kan et. al., 2005a]

By introducing a capacitor between the PV cell and the battery, the transfer efficiency can be improved from less than 50 % up to about 90 %. This is an example of matching improvement with the aid of a capacitor.

92 Chapter 3: Matching of electrical energy storage media in PV powered mobile/wireless products

Figure 3-23 Comparison of the power transfer efficiency by using buck-burst (bb) and burst mode [Kan et. al., 2005a]

3.3.6 A method to establish the right Matching between PV and Battery

Figure 3-17 and 3-14 reprint: The Matching of PV output, Battery input and Power Input Response of a Li-Ion battery

93 Energy Matching - Key towards the design of sustainable photovoltaic powered products

In Figure 3-17 the matching was shown between the PV power output and the power input response of a battery. The matching was not optimal due to lack of correlated coverage of certain areas namely at the time interval between 5 and 7 o’clock and between 9 and 14 o’clock. For full coverage during these periods several different power input responses, as presented in Figure 3-14, should be used instead of just one as was done in Figure 3-17. For an appropriate switching between these power input responses, an electronic control circuit will be needed that detects the irradiance level and switches on the appropriate power input response. The design of this control system is a topic of future research.

3.4 Matching energy storage media and energy use in the functional application (MI:3)

3.4.1 General ‘Matching’ the energy storage medium (or combined media) and the energy use is exactly the ‘Load Matching’ as described in Chapter 1. In accordance to the iteration steps of the Engineering Research Methodology (see Chapter 1), the ‘Matching’ process is done in two steps. First the Figure of Matching can be determined between a battery type and an energy use pattern. In this dissertation, for the purpose of demonstrating the Figure of Matching algorithm, the presented examples are confined to the energy use patterns found on digital mobile products. In addition, the batteries under analysis will only be of the Li-Ion battery family. In the second step the energy transfer efficiency across the matching interface 3 (MI: 3) in Figure 3-1 is optimized.

3.4.2 Figure of Matching between battery (storage medium) and energy use in the functional application In Chapter 1 the generic Figure of Matching between two elements s and p was introduced and defined as:

FM = { Ø / ∫ S (var) d var } x 100% Eq. 1-5 p

Here Ø is the correlation between the Stimulus S emanated from element s towards p the next element in the energy chain, i.e. element p and the Step-Response R of this Step-p element p.

Ø = ∫ S (var) x R (var) d (var) Eq. 1-6 p step-p var range This generic Figure of Matching is made applicable specifically for the matching interface MI:3 by first determining what type of power has to be transferred between the battery and the application. The type of power determines how the battery and application can be matched and correlated. As mentioned before in this dissertation, the main point of concern is the energy transfer between the elements in the energy chain. Therefore the common parameter will be electrical energy, or the power transferred during a certain period of time. Thus for calculating the Figure of Matching between a battery and the energy use in an application, the common parameter will be ‘electrical power’. Next came

94 Chapter 3: Matching of electrical energy storage media in PV powered mobile/wireless products the determination of the used variable ‘var’ in the formula. Since the power is integrated over a certain time span, the variable to be used will be the time ‘t’.

The total amount of energy use to fulfil a functional application of a product is usually estimated from the data sheets by assuming a certain use pattern. For instance in the data sheet of a mobile cell phone it is stated that during standby x mW is used, for waiting incoming calls y mW is used, and during checking mode z mW is used. After assuming a certain use pattern then the energy consumption is calculated. However it turns out that to save energy and to prolong the battery life, the power demand from a battery for application in digital mobile products such as a cellular phone or PDA will usually not be continuous but rather pulsed as can be seen in Figure 3-24. Therefore, by taking this pulsed power demand into account the actual energy consumption will be less than if it was calculated with a continuous power demand.

Figure 3-24 Typical chart of power delivered by a cellular phone battery during calls [Nicolaescu and Hoffman, 2001]

By virtue of reversibility Figure 3-24 can be seen as the ‘battery delivery response’ to the demand stimulus by the load or application namely a pulse train. Having established this relation one can define a general ‘demand stimuli’ pattern for batteries that have to deliver power to digital products as pulse trains. For clarity in experiments and Figure of Matching determination, this general stimuli pulse train will be replaced by just one pulse. By taking some precautions, as is explained later, this single pulse suffices for determining the discharge characteristics and responses of a battery. The pulse width can be adapted to fit the actual demand of the product under investigation. To calculate the Figure of Matching between a battery and a product the power demand step-response of the battery first has to be measured. These step- responses are the battery responses to a power pulse demand as described above. It is important for mobile/wireless products, which are carried around, to have the ability to function under various temperature conditions. Therefore it is needed to determine at what temperature the application still functions well. For this purpose two thresholds

95 Energy Matching - Key towards the design of sustainable photovoltaic powered products are depicted in Figure 3-25 and 3-26, namely the 5% and the 10% tolerance boundaries. These tolerance boundaries are measures of to what extent the battery output voltage and current are allowed to drop before this reduction causes a malfunction of the application. Power delivery responses of a Li-Ion battery are measured respectively at + 20 °C and -20 °C as presented in Figures 3-25 and 3-26. For clarity instead of the ‘output power’, the battery ‘output voltage’ and the ‘output current’ are presented separately. The pulse width is 100ms. Here two temperatures samples are used since for some batteries such as Li-Ion batteries, it is a point of annoyance to be deprived of energy at low temperatures. For instance cameras and cellular phones cease to work properly at cold climate conditions. Therefore not only room temperature but also pulse trains at low temperatures were measured. The temperature of -20 °C is in this context not too extreme since this is a practical value in northern Europe and the USA, as was demonstrated in January 2006 e.g. Munich (BRD) - 30 °C.

Figure 3-25 The Li-Ion battery output voltage in response to a power pulse demand at temperatures of respectively + 20 °C and at – 20 °C [Kan, et.al, 2006d]

By taking in Figure 3-25 the output voltage demanded by the application as 100%, two pulse windows of operation can be defined namely with 5% and 10% tolerance margins. For a certain temperature, the correlation between the measured battery output voltage and the application voltage demand can be determined by calculating the area enclosed above the tolerance margin line and the battery output voltage in time. The Figure of Matching at this matching interface MI:3 can be determined according to the generic formula by calculating the ratio between the correlated battery output voltage area and the area enclosed by the voltage demand curve and the tolerance margins lines of respectively 5% or 10%. With the 5 % tolerance window only the + 20 °C pulses yield some correlation. The Figure of Matching of these pulses is about 50%. The - 20 °C pulses yield no correlation with the 5% tolerance window, therefore the Figure of Matching is zero. Tests were conducted with current pulses of 1A up to 5A. In Figure 3-26, by taking the

96 Chapter 3: Matching of electrical energy storage media in PV powered mobile/wireless products

Figure 3-26 Li-Ion battery output current in response to a power pulse demand at temperatures of respectively + 20 °C and at – 20 °C [Kan et.al, 2006d] demanded 5 A as 100% the effective value of the current component of this Li-Ion battery will be 100% at + 20 °C and 62% at - 20 °C. Therefore the Figure of Matching for the 5% tolerance window is 100% for + 20 °C and zero for -20 °C. The Figure of Matching for the power output will be the product of the Figure of Matching of the output voltage and output current. As a result the Li-Ion battery with a 5% tolerance window has a Figure of Matching of 50% at room temperature and zero at - 20 °C. For the Li-Ion Polymer the Figure of Matching is 60% at room temperature and zero at - 20 °C. At room temperature the battery output voltage will show a dip, while the current of 5A can just manage to be safely delivered [Buchmann, 2001]. At - 20 °C however, both the output voltage and the output current show larger dips. These are potential causes of malfunction of the application. Since these large dips correspond with a Figure of Matching of zero, this matching algorithm can be used to predict how well the application will match the chosen battery at a certain temperature. The depth of the voltage dip of single pulses increases at lower temperatures, as can be seen in Figure 3-27. The minima of the voltage dips in accordance to the values presented in Figure 3-27 are

97 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Figure 3-27 Temperature dependency of a Li-Ion battery output voltage for a single pulse [Kan et.al, 2006d] plotted as a function of the temperature in Figure 3-28 for two types of the Li-Ion battery family namely a normal Li-Ion and a Li-Ion Polymer one. In practice, the power demand will not be a single pulse but rather a pulse train in which the overall output voltage decreases to a saturated value, as was presented in Figures 3-29 and 3-30. Therefore the voltage dip minima’s for a pulse train are corrected by an additional voltage drop of about 0,05 V down to 0,4 V for the temperature range of + 20 °C down to - 20 °C.

Figure 3-28 The voltage dip minima of a 100ms pulse train as a function of temperature for a Li-Ion battery and Li-Ion Polymer [Kan et.al, 2006d]

A comparison of the two batteries types in Figure 3-28 shows a better performance of the Li-Ion Polymer in the higher temperature region (> 5 °C) but a better performance of the normal Li-Ion battery at low temperatures. The main reason for this poor performance of the Li-Ion Polymer battery at low temperatures is its higher internal resistance. If low temperature is an important issue in the user context, then this performance difference will be a ‘design constraint’.

98 Chapter 3: Matching of electrical energy storage media in PV powered mobile/wireless products

The relation between a pulse train and a semi-continuous single pulse power demand can be seen in Figures 3-29 and 3-30. In Figures 3-29 and 3-30 it can be seen that the overall output voltage drop of the consecutive pulses of a pulse train will saturate in accordance to the semi continuous power demand curve of a long single pulse (a 100 s pulse).

Figure 3-29 Comparison between a semi-continuous power demand (a 100 s pulse) and a pulse train of a 100 ms pulse power demand measured with a Li-Ion battery at 20 °C [Kan et.al, 2006d]

Figure 3-30 Comparison between a semi-continuous power demand (a 100 s pulse) and a pulse train of a 100 ms pulse power demand measured with a Li-Ion battery at - 20 °C [Kan et.al, 2006d]

Validation of the Figure of Matching algorithm between batteries and applications at matching interface 3 (MI:3) is accomplished by comparing the results of the Figure of Matching calculations with actual performance of functional applications at various temperatures. For electronics products with a 5% tolerance in battery output voltage this threshold will be reached at a temperature of about + 5 °C for both the Li-Ion and the Li-Ion Polymer battery. The value of + 5 °C corresponds to the threshold found for example during a field test with mobile products conducted at a ski run. It was found that a digital camera and a cellular phone start to cease working properly at this very temperature [Keukens, 2005]. For products with a 10% tolerance in output voltage this threshold will be reached by the Li-Ion and the Li-Ion polymer batteries at temperatures of about 0 °C. In this context it is

99 Energy Matching - Key towards the design of sustainable photovoltaic powered products noteworthy to mention that the value of 0 °C is usually quoted as the lower limit of the operating temperature of consumer products powered by rechargeable Li-Ion batteries (see e.g. Canon, 2006).

3.4.3 A method for improving the matching between battery and energy use in the functional application with the aid of a capacitor In Section 3.3.3 it was demonstrated that the matching between the PV cell and the battery could be improved with the aid of a capacitor. Also a case of matching improvement between a battery - capacitor combination and an application at low temperatures has been reported. In this case a lead acid battery was put in parallel with a large capacitor. It was shown that the capacitor can take over the current generation from the battery in case it suffers of a current deficit at low temperature [Catherino et. al., 2005]. The question is could the aid of a capacitor also achieve a similar matching improvement for instance at the interface between Li-Ion or Li-Ion polymer batteries and an energy using application? To answer this question a series of experiments were conducted. In the Li-Ion batteries experiments the application energy demand is simulated with a pulse. The response of the energy storage unit is recorded at various temperatures ranging from + 25 °C down to - 25 °C and at various pulse widths. In Figure 3-31 and Figure 3-32 the voltage drop in the energy demand response at a temperature of - 20 °C for Li-Ion and Li-Ion Polymer batteries are presented for cases where only a battery is used and cases in which combinations of a battery and a capacitor are used. The discharge is 5C=5A. At lower temperatures the internal resistance of the battery - capacitor combination increase from 0,1 Ω up to about 0.3 Ω. The used capacitors have a comparable internal resistance as the Li-Ion battery. Therefore the effect was less dramatic than in the case of a lead acid battery and capacitor above.

Figure 3-31 Comparison between the output voltages of a Li-Ion Battery as a result of a power pulse train delivery at - 20 °C. Two cases: with only a battery and with a combination Battery + Capacitor [Kan et.al, 2006d]

100 Chapter 3: Matching of electrical energy storage media in PV powered mobile/wireless products

Figure 3-32 Comparison between the output voltages of a Li-Ion Battery Polymer as a result of a power pulse delivery at - 20 °C. Two cases: with only a battery and with a combination battery + capacitor [Kan, et.al, 2006d]

The Figure of Matching for power transfer of the Li-Ion and the Li-Ion Polymer battery alone at a temperature of -20 °C will be zero since there is no correlation possible with either the 5% or the 10% tolerance window. With the aid of a capacitor the Figure of Matching of the combination for the 10 % tolerance window is still zero. The overall depth of the dips of the pulse train is however reduced by about 0.5 V with both the Li-Ion and the Li-Ion polymer battery. The advantage of combining a battery with a capacitor is clearly demonstrated in these examples. Therefore it could just be the difference between a product that still functions well and one that fails.

3.5 Other advantages of battery - capacitor combinations

3.5.1 General Before analyzing the other advantages of battery - capacitor combinations, let us look at the particular features of batteries and capacitors which could make this combination favourable for other cases than low temperature applications as described above. In applications of Li-Ion batteries, large discharge currents will generate heat which could result in an efficiency reduction of the conversion of chemical to electrical energy. More- over this elevated temperature will cause an increase of the internal resistance and capacity fading [Zhang et al., 2000; Shim et al., 2002; Ramadas et.al.,2002; Ning et al, 2003]. Delivering large current surges is not quite the favourite use of Li-Ion batteries. On the other hand, capacitors can discharge in an exponential way and can therefore deliver large current pulses. This feature is particularly advantageous for product applications in which large currents need to be drawn, for example at the initialization stage of electro-motors to overcome stiction, and products with large inrush currents. By placing the capacitor in the output circuitry of the battery, e.g. in parallel, the battery is prevented from unnecessary deep-discharge cycles, since in this combination the current can be drawn from the

101 Energy Matching - Key towards the design of sustainable photovoltaic powered products capacitor [Kan et al. 2004a]. Since the battery does not need a deep-discharge cycle, such a battery - capacitor combination results in a better reliability and longer battery life, or provides a more sustainable product. Note however that this advantage is only possible at the expense of increased weight and volume of the storage system [Sikha and Popov, 2004].

3.5.2 Fast recharge options today From a practical usability and emergency point of view, it would be preferable and advantageous for mobile applications in general, and for products like cellular phones and digital cameras in particular, to have an ability to quickly recharge its batteries. For example in the case of a personal emergency, it would be unacceptable to be unreachable for hours because the battery takes so long to recharge. The drawback however of these quick recharge sequences is an elevated temperature which results in the loss of battery capacity and in a reduced battery life [Choi and Lim, 2002]. A possible solution could be the use of capacitors [EPYON, 2005]. Here the exponential charge curve of capacitors is exploited for quick collection of the available current (within 30 seconds). After storing the current in the capacitor, the actual recharge of the battery then can take place in a much longer time scale. The capacitor buffer can be used for reducing the battery charge current and as a result the temperature of the battery will not surpass the critical limit during recharging. This in turn will reduce the capacity fading due to cycling at an elevated temperature [Shim et. al., 2002; Ramadas et. al., 2002]. Therefore the use of capacitors in combination with batteries in this case could yield: • Means for quick collection of the available energy which then can be slowly transferred and stored into a battery. By slow charging the battery temperature will not rise exces- sively, thus avoiding a decline in capacity and lifetime. • Means for quick emergency charging.

The advantage of this capacitor option will diminish in view of recent developments in the field of Li-Ion batteries with the use nano-technology [Satoh, 2005; Toshiba, 2005]. These batteries can be recharged within 30 seconds. Note however what implications such a quick charge process would put on the battery, the cables and the electronic circuits in its neighbourhood. For instance a 1Ah battery would need a current of 1 A to charge in one hour. This same battery would need a charge current of 60 A to charge in one minute and 120 A to charge it in 30 seconds. Apart from the heat dissipations in the cables and electronics introduced by such large currents, the EMC impact will exceed the tolerated norm value of even the EMC Military Standards [Mil-STD 461/E, 1999].

3.5.3 Fast and large discharge options in battery - capacitor systems There is already much literature on battery - capacitor combinations used in high current pulse applications [Menachem and Yamin, 2004; Choi et. al., 2004]. In these combinations the advantage of high power density or the ability to deliver large discharge rates by the capacitor is combined with the high energy density of the battery. The capacitors in these examples are placed at the output side of the battery. Test results for Li-Ion and Li-Ion Polymer batteries in combination with capacitors are presented in Figures 3-33 and 3-34.

102 Chapter 3: Matching of electrical energy storage media in PV powered mobile/wireless products

Figure 3-33 The voltage dips as result of a 2 A current pulse delivery from a Li-Ion battery at 20 ºC in cases of battery only and battery combined with a capacitor [Kan et.al, 2006d]

Figure 3-34 The voltage dips as result of a 2 A current pulse delivery from a Li-Ion Polymer battery at 20 ºC in cases of battery only and battery combined with a capacitor [Kan et.al, 2006d]

As can be seen in both Figures 3-33 and 3-34, with the aid of a capacitor the voltage dip will be reduced above the 5% tolerance margin. This means that an application that did not function properly with only a battery will with the aid of a capacitor be able to function well. This was experimentally verified and demonstrated with the prototype of a PV powered toy in which the irradiance level can be made visible by the rotation speed of an electro motor. Without the additional capacitor the motor can not start properly [Kamp, 2003].

103 Energy Matching - Key towards the design of sustainable photovoltaic powered products

3.6 Conclusions In this chapter the matching between the electrical energy storage system and the other two elements of the energy chain have been analysed. This analysis is on one side between the energy storage and the PV energy converter and on the other between the energy storage and the energy use in functional application.

Figure 3-35 The Energy Storage System and its interfaces with the other elements in the energy chain [Kan et. al., 2005a]

As shown in Figure 3-35, the functional application can obtain the energy directly from the photovoltaic energy converter (MI:0). A capacitor can be used as a kind of low pass filter to reduce the influence of the fluctuating irradiance (MI:3a). The application can also be powered via the battery (MI:3b) or via a battery + capacitor MI:3c). In résumé in Figure 3-35 the main energy transfer line as presented in the Energy Matching Model is extended with all the options described in preceding sections of this Chapter 3. This Chapter has addressed research sub-question 2, namely:

What matching can be achieved between the electrical energy storage media and both the photovoltaic power conversion systems in an outdoor/indoor user context (1) and the functional application (2)?

The answers are: The Figure of Matching algorithm can be used both to analyse and to quantify the matching through the matching interfaces MI:2 and MI:3. The best match for the energy storage medium in an energy chain of a PV powered product is that with the Li-Ion battery and related Li-Ion Polymer and ‘Nano technology’ based Li-Ion batteries. A definite improved matching from an energy transfer efficiency point of view can be achieved by using capacitors both at the battery input and output. Improved charging of Li-Ion batteries can be achieved by the use of a capacitor buffer at the input of the battery. Improved reliability of batteries can be achieved by using capacitors at the output of the battery due to the lack of deep discharges in cases of large current demands. The use of capacitors on the battery output would improve low temperature, battery surge performance and prolong battery life. Important criteria are the Figure of Matching and the urgency of need. The use of capacitors at the output side to improve the surge capacity of the battery will be at the expense of larger weight and volume and should therefore be tuned to the application. If no large currents are needed by the application or no large inrush currents are to be expected

104 Chapter 3: Matching of electrical energy storage media in PV powered mobile/wireless products then the additional capacitor will in most cases not be an advantage. The use of super capacitors will significantly reduce the volume and weight. On the input side however the buffer function of the additional capacitor will be the main advantage. There will be a break-even point reached in which it will be no more advantageous to have the complicated construction of combining batteries with capacitors simply because the technological performance improvement of the batteries have counterbalanced the additional complication in electronic circuitry, volume, weight and cost of this combination.

105 Energy Matching - Key towards the design of sustainable photovoltaic powered products

106 Chapter 4: Optimal matching in the energy chain

4.1 Introduction

This chapter concerns the entire energy chain of PV powered products. Answers are to be found for the third part of the research question as formulated in Chapter 1, namely:

What optimisation can be achieved between the elements of the energy chain of a PV powered mobile wireless/products and how does this affects other design engineering aspects?

To find answers to the above mentioned research question, we first have to recall the generic Figure of Matching between the two elements s and p as was defined in Chapter 1 as:

FM = { Ø / ∫ S (var) d var } x 100% Eq. 4-1 p

Here Ø is the correlation between the Stimulus S emanated from element s towards p the next element in the energy chain, i.e. element p and the Step-Response R of this Step-p element p.

Ø = ∫ S (var) x R (var) d (var) Eq. 4-2 p step-p var range The stimulus S and the Step-Response R are both a function of a given variable ‘var’. p Step-p In determining the Figure of Matching (FM) between the two specific elements s and p, the first step is to find the power type that is transferred between the two elements that are to be matched and correlated as well as their common relation to the variable ‘var’. The system under investigation in this chapter will consist of the Energy Matching Model (EMM) of the energy chain of a PV powered product as presented in Fig. 4 -1.

107 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Figure 4-1 The Energy Matching Model (EMM) of the energy chain of a PV powered product

For a sustainably designed PV powered product not only should the matching in the entire energy chain be analysed but also matching with respect to mechanical, the environmental design issues such as the used materials and energy payback versus active product life, and user context design issues such as temperature. Therefore in this chapter, for sake of completeness all of these matching aspects will be treated. However, as mentioned earlier in Chapter 1, in this dissertation the user context defined parameters, the energy use in the application, are all treated as given exogenous parameters and will be dealt with in more detail in other projects within the framework of the SYN-Energy program. The mechanical and thermal parameters are partly treated as being exogenous and are partly contained in the empirical - scope of this study, but they are not analysed systematically and complete. The reason to treat these parameters is that they interact with the matching in the energy chain.

108 Chapter 4: Optimal matching in the energy chain

The outline of Chapter 4 is as follows: First a resume of the energy matching examples found in the energy chain as described in the preceding Chapters (2 and 3) is presented in section 4.2. This summary will concern mainly the matching interfaces MI:1, MI:2 and MI:3 in Figure 4-1. Next in section 4.3, although outside the scope of this dissertation, for sake of completeness in product design, the role of matching is extended beyond the direct energy chain related design parameters. This is followed in section 4.4 by an analysis of the overall matching between the parameters, elements and interfaces of the entire energy chain. This analysis will also include parameters, elements and interfaces that are not treated in detail in this dissertation. In Figure 4-1 these latter items are depicted outside the boxed cadre. Finally in section 4.5 the conclusions are summarised and a roundup of the answers for the third part of the research question are presented.

4.2 Summary of energy matching examples as found in the preceding chapters

4.2.1 General considerations In Chapters 2 and 3 the matching interfaces MI:1, MI:2 and MI:3 have been analysed. As a result of this analyses the specific Figure of Matching (FM) of these matching interfaces were analysed and quantified. This was between respectively: the user context defined incident light and the PV power converter (MI:1), the PV power converter and the electrical energy storage media (MI:2) and electrical energy storage media and the energy use in the functional application (MI:3). In this section 4.2 the results will be summarised.

4.2.2 Irradiance matching interface (MI:1) In this matching interface MI:1 the light power emanated from a light source e.g. the sun or lamps, is transferred into the PV power converter. The main characteristics of the light source are its spectrum and its intensity or level of irradiance. Therefore in order to investigate the matching between the light source spectrum and the PV power converter the spectral Figure of Matching is calculated. For calculating this specific Figure of Matching at the irradiance matching interface MI:1 the used variable ‘var’ is the wavelength λ. The step response of the PV cell with respect to the wavelength λ is the spectral response (SR). The stimulus S is the spectrum of the incident light (IL) source. For dimensional matching IL the power based spectral response is used, as defined in Chapter 2:

SRP ( ) = SR ( ) x V x FF Eq. 4-3 PV λ PV λ oc

Substituted into Eq. 4-1 and 4-2 yields for the correlation Ø:

Ø = ∫ S ( ) x SRP ( ) d ( ) Eq. 4-4 IL λ PV λ λ spectrum range and the spectral Figure of Matching becomes:

FM = { Ø / ∫ S ( ) d } x 100% Eq. 4-5 IL λ λ spectrum range

109 Energy Matching - Key towards the design of sustainable photovoltaic powered products

The resulting Figure of Matching can be calculated as shown in Table 4-1:

Table 4-1 Spectral Figure of Matching (FM ) - PV technology pairs, of various light sources, at an Spectral irradiance level of 1000 W/m2

Light source FM with FM with FM with Spectral Spectral Spectral AM 1.5 TLD Incandescent PV cell technology

c-Si 14,9 % 14,6 % 10,9 %

a-Si 8,3 % 17,6 % 2,7 %

mc-Si 13,4 % 13,6 % 9,3 %

CIS 15,8 % 16,8 % 10,8 %

DSC 7,5 % 19,3 % 2,3 %

As a result, the best matched light source spectrum – PV technology combinations at an irradiance level of 1000 W/m2 are known. Since the spectral Figure of Matching (FM ) is a power transfer efficiency of a light spectral source - PV cell combination in theory it is identical to the conversion efficiency of that PV cell for the irradiance of the light source. This conformity is demonstrated in Table 4-2.

Table 4-2 Comparison between spectral Figure of Matching (FM ) and the measured conversion Spectral efficiencies at an AM 1.5 spectrum and an irradiance level of 1000 W/m2

Method FM with Measured conversion efficiency Spectral AM 1.5 at 1000 W/m2 at STC with AM 1.5 spectrum PV cell technology and irradiance 1000 W/m2

c-Si 14,9 % 14-16 %

a-Si 8,3 % 6-8,5 %

mc-Si 13,4 % 13-15 %

CIS 15,8 % 10-14 %

DSC 7,5 % 5-8 %

In the next step the spectral Figure of Matching dependency of the various PV cells on the level of irradiance G are taken into account. Measurements have shown that the PV cell n conversion efficiency depends on the level of irradiance. The decrease of PV cell efficiency towards lower irradiance levels is extremely dependent on the cell technology. As a result, the spectral Figure of Matching at the various irradiance levels is obtained by:

FM ≈ FM x / [%] Eq. 4-6 AM1.5+irrad AM1.5 + 1000W/m2 ηAM1.5 + irrad ηSTC

110 Chapter 4: Optimal matching in the energy chain

In this equation, and respectively mean the conversion efficiency at the ηSTC ηAM1.5+irrad Standard Test Conditions (STC) irradiance level and at a certain non STC irradiance level. The non STC irradiance levels are for example 100 W/m2 or 10W/m2 with the AM1.5 spectrum. Data-sheets of efficiency measurements on PV cells are based on STC so in general only the AM 1.5 spectrum is used. The conversion efficiencies of the various PV cells depend not only on the level of irradiance but also on the spectrum of the incident light. The spectral Figure of Matching at other spectra, like that of TLD lamps and incandescent lamps, can be calculated by:

FM ≈ FM x FM / FM [%] Eq. 4-7 spec + irrad AM1.5+irrad spec+1000W/m2 AM1.5+1000W/m2

The results are presented in Table 4-3.

Table 4-3 Spectral Figure of Matching (FM ) of various light sources spectra PV technology pairs Spectral corrected for their irradiance level dependent conversion efficiencies

Light source FM with FM with FM with Spectral Spectral Spectral AM 1.5 [%] TLD [%] Incandescent [%]

Irradiance [W/m2] Irradiance [W/m2] Irradiance[W/m2] 1000 100 10 1000 100 10 1000 100 10 PV cell technology STC STC STC

c-Si 14,9 11,9 5 14,6 11 2,6 10,9 8,9 3,9

a-Si 8,3 5,7 4,6 17,6 12,6 10 2,7 2,3 1,9

mc-Si 13,4 10,5 3,4 13,6 10 2,6 9,3 7,3 2

CIS 15,8 7,3 1,5 16,8 7,4 1,4 10,8 4,8 0,9

DSC 7,5 5.3 4,2 19,3 13,6 10,9 2,3 1,9 1,5

With Table 4-3 the designer of PV powered products can make a trade-off based on what type of PV cell is to be used in combination with a certain user context, i.e. certain light spectra and irradiance levels. The conversion efficiency at a certain spectrum (spec) and a certain irradiance level (irrad) is:

≈ FM x FM / FM [%] Eq. 4-8 ηspec + irrad AM1.5+irrad spec+1000W/m2 AM1.5+1000W/m2

And the spectral dependent converted power, by the PV cell with an area of APV m2 at a certain Irradiance G , can then be calculated as: n

P = G x x A [W] Eq. 4-9 PV n ηspec+irrad PV

The equation can be used in reverse to calculate the area required in the case that the available power is known. By using the Figure of Matching algorithm, the efficiency and the

111 Energy Matching - Key towards the design of sustainable photovoltaic powered products converted power as a function of the irradiance level and spectrum is found and therefore the matching interface MI:1 can be analysed and quantified.

4.2.3 Charge energy matching interface (MI:2) A. Figure of Matching In this matching interface (MI:2) the common parameter that can be matched and correlated is the electrical power coming from the PV cells that is transferred into an electrical energy storage medium. Since the power is transferred during a certain time span the variable used, for calculating this specific Figure of Matching (FM) at matching interface MI:2, will be the time ‘t’. The step response of the battery is the power intake response deduced from the storage medium charge characteristics. The time ‘t’ is integrated over a certain time sequence, so an important indicator in determining the Figure of Matching is the joint active time sequences of the two elements that are to be correlated. For instance, the active time sequences of sunlight are the daylight hours of one day. During this time sequence the PV has an electrical power output. If at that very moment this power output can not be used for battery charging, there will be a mismatch and thus a waste of power. So in this particular case the product S(t) x R(t), used in calculating the correlation part of the Figure of Matching formula, will be zero and as a result the related Figure of Matching is also zero. The Figure of Matching indicates here what percentage of PV power is really transferred into the storage medium. Since in Chapter 3, section 3.2.3 it was found that Li-Ion batteries and their family, provide the best match with PV cells. The analysis of matching interface MI:2 concerns mainly this type of battery. In section 3.3.2 an example of how the Figure of Matching can help aid in the resolution of contradictions is presented. In this case a Li-Ion battery was recharged by a combination of a c-Si PV cell and some standard charge electronic circuits during one entire day. The battery was found not completely charged. Straight forward calculation based on data sheets (STC efficiency + battery capacity) alone indicate that the PV cell should deliver sufficient power for a complete charge. Calculation of the Figure of Match- ing however yields a value of only 86%. Thus with the Figure of Matching algorithm the contradictory example of a battery not be charged completely during a clear day in June can be explained.

B. Methods for improving the energy transfer efficiency in a PV battery benchmark example It was found in Chapter 2, section 2.2 that the irradiance can fluctuate erratically. The erratically fluctuating irradiance has as a consequence an erratically fluctuating output from the PV cells. So as a result the charging conditions of the battery are not met all the time, causing interruptions, i.e. the battery is not charged during the entire time sequence that power could be transferred from PV cells to the battery. During the period in which no charging occurs, both the Figure of Matching and the power transfer is zero. The power transfer integrated over the entire period in which the PV cell is producing power is thus sub-optimal for such a PV-Battery combination. An alternative to the battery is another energy storage medium i.e. the capacitor. A capacitor however has a low energy storage capacity. So in this case the bottleneck will be that after a while the power transfer will be terminated because the capacitor is already full. During the period of no power transfer the Figure of Matching will be zero. The

112 Chapter 4: Optimal matching in the energy chain power transfer integrated over the entire period in which the PV cell is producing power is thus, for such a PV-capacitor combination, sub-optimal. In the case that the battery and the capacitor are combined in parallel, the capacitor will not have to cope with a limited energy capacity, since it can direct its overflow into the battery. In addition the capacitor, acting as ‘low pass filter’, will level out the PV output fluctuations. This will allow the battery to charge in a more stable way during the entire period of PV power conversion. A battery alone will have a Figure of Matching of less than 40%, while a capacitor alone will have a Figure of Matching of 10%. The combination battery + capacitor, demonstrated in section 3.3 of Chapter 3, will yield a Figure of Matching of 90%. This is a good example of improved matching between PV energy converter and energy storage media through interface MI:2.

4.2.4 The energy use matching interface (MI:3) Determination of the Figure of Matching With the matching interface MI:3, power is transferred from the battery into the application. In Chapter 3, section 3.4.2 it was shown that in order to save energy and to prolong the battery life, the power demand by an application in digital mobile products such as a cellular phones or PDAs will usually not be continuous but rather pulsed. So for calculating the Figure of Matching in this interface pulsed powered transfer signals will be used and the parameter to be correlated will be ‘power’. Since the power is integrated over an active time sequence the variable used in Eq. 4-1 and 4-2, for this interface MI:3, will be the time ‘t’. The step response of the battery will be the power delivery response of a battery with respect to a pulsed demand. Due to the pulsed power demands both the output voltage and the output current of the battery might decrease from its nominal value. This decrease in output voltage and output current could have an impact on the performance of the application. As a result, in relating an application and a source of energy, for example a battery, one can distinguish two types of input circuit characteristics necessary to achieve proper functionality with a product. Namely: • A tight voltage range, therefore it will not start below a specified voltage. For example at certain white LEDs this minimum is 2 V. • A tight current range, therefore it will not start below a specified current. For example at certain electro-motors this minimum is 1 A.

These two types will also become important when defining the active range of voltage and current, as used in calculating the Figure of Matching in the Matching Interface MI:3. To take the minima voltage and current at which the application still functions properly into account, two thresholds margins are defined. These margins are defined with respect to the pulse response, namely a 5% and a 10% tolerance margin below nominal. It is common practice that these 5% and 10% tolerance margin values are chosen in product specifications. Therefore these thresholds act as boundaries for the active range of the application.

The energy/power use matching interface at low temperatures At low temperatures both the output voltage and the output current of batteries decrease as a result of the increase of internal resistance due to decreasing temperature. In order

113 Energy Matching - Key towards the design of sustainable photovoltaic powered products to be able to function properly an application requires, within some tolerance margins, a certain minimum voltage and current. Important for mobile products that have to function under various temperatures conditions, is a criterion that identifies at what temperature the application continues to functions properly. For this purpose two thresholds are presented above, namely the 5% and the 10% tolerance boundaries. These tolerance boundaries measure to what extent the battery output voltage is allowed to drop before it is causes the application to malfunction. Résumé as found in section 3.4.2 of Chapter 3 A comparison between the Figures of Matching at two temperatures yields the following results: • Within the 5% tolerance window only the pulses at 20 °C have some correlation between the output power and the active power demand range. The Figure of Matching of these pulses is about 50%. The pulses at - 20 °C has no correlation within the 5% tolerance window therefore the Figure of Matching is zero. • The Figure of Matching of the power (the product of voltage and current) is only a first order indication. To come to an understanding of how to design a product that will function well by using the Figure of Matching algorithm, a division between the contribution of the voltage and that of the current will be a necessity.

Plotting the voltage dip minima yields: For products active within a 5% tolerance range in output voltage this threshold will be reached at a temperature of about 5 °C for both the Li-Ion and the Li-Ion Polymer battery. The value of 5 °C corresponds to the threshold found during a field test with mobile products conducted at a ski run [Keukens, 2005]. For products active within a 10% tolerance range in output voltage this threshold will be reached by the Li-Ion and the Li-Ion polymer batteries at temperatures of about 0 °C.

Methods for improving the capability to supply large power pulses by using battery- capacitor combinations Due to its internal resistance a Li-Ion battery alone can not provide large enough currents for some applications as for example to overcome stiction to start an electro-motor. In such a case there will be no current delivered. The Figure of Matching during that period is zero. In such cases the result will be poor or zero power transfer efficiency over the entire active period. A capacitor can deliver large current pulses, but due to its low energy capacity can not sustain high currents for a longer time and will terminate operation after a while. So in the case that a capacitor alone is used, the energy transfer efficiency integrated over the entire active operation period of the application will also be low. A battery – capacitor combination combines the advantage of the two. The result is higher power transfer efficiencies, even higher than the sum of the battery and capacitor separately. In section 3.5.3 of Chapter 3 it was found that with the aid of capacitors, at 20 °C both Li-Ion and Li-Ion Polymer batteries can deliver large current pulses (2A) while the output voltage dip as result of these pulses remains within the 5% tolerance boundary. An additional advantage of this combined battery – capacitor storage medium is that the battery does not need to go into deep discharges and is not heated up. Both of these events can cause battery capacity fading (see Chapter 3; section 3.3).

114 Chapter 4: Optimal matching in the energy chain

Improvement for the energy/power use matching interface at low temperatures In Chapter 3 it was shown that improved matching between energy storage and energy use in the energy chain can be obtained through the simultaneous application of batteries and capacitors in combined energy storage media. The results can be summarised as follows: • The Figure of Matching for power transfer of the Li-Ion and the Li-Ion Polymer battery alone at a temperature of -20 °C will be zero since there is no correlation possible with neither the 5% nor the 10 % tolerance window. • With the aid of a capacitor the Figure of Matching of the combination for the 10 % tolerance window is still zero. The overall depth of the dips of the pulse train is however reduced by about 0.5 V for both the Li-Ion and the Li-Ion polymer battery.

4.3 Matching parameters outside the energy chain

4.3.1 General remarks As mentioned before in designing products with photovoltaic cells, more aspects than efficient use of the electrical power and energy will be of relevance. In a total design concept we should also consider taking structural, mechanical, user interface aspects for instance into account. Although these aspects are beyond the scope of this dissertation, they are listed here for the sake of completeness of the overall design and stimulus for further research. In this dissertation the emphasis is on energy matching in the energy chain. Therefore in this section only those parameters outside the energy chain that have impact on the performance of the energy chain will be analysed. As a result the list of parameters presented is a ‘first inventory’ that needs to be extended in future research.

4.3.2 The construction and embodiment matching Size, weight and shape in embodiment matching For portable, mobile wireless products the size, weight and shape will be essential design parameters. These parameters have impact on several elements in the energy chain of PV powered products. a) PV cells The active area of a PV cell is directly proportional to their power output. The weight, size and shape of PV systems also depend on the encapsulation of the cells and the substrate used. For instance PV cells with a glass substrate will be bulkier and weigh more than those on plastic foil. The PV shape can be flat or curved and will depend on the product. As it was presented in Chapter 2 section 2.7.3 the curvature will have an impact on the PV performance. A design guideline used for curved PV cells is to use them if and only if this feature introduces a new added value. The mechanical matching is certainly improved due the flexibility in design introduced by the option of bendable PV cells. b) Batteries The size and weight of a battery will be defined by the technology used. Due to their lead component lead acid batteries will be heavier than Li-Ion batteries. Moreover Li is one of lightest elements in the universe (atomic number 3). c) User Interface For portable mobile and wireless products size, weight and shape are important design

115 Energy Matching - Key towards the design of sustainable photovoltaic powered products criteria in view of the user – product interaction.

Battery storage capacity as design criterion and energy interface matching Although battery storage capacity typically decreases proportionally with thickness, there is a definite trend towards thin batteries for example in portable phone applications [Nokia, 2005]. To calculate the limit storage capacity of a battery, the term volumetric energy density is defined. This volumetric energy density depends on the type of Li-ion battery used. In particular, it depends on the electrolyte used. a) Liquid Electrolyte Li-ion Batteries Early commercially available prismatic Li-ion batteries with liquid electrolytes quote a capacity of 290 Wh/liter [Hara, 1999]. Today capacities of over 470 Wh/liter are quoted [Kariatsumari, 2005]. Commercially available (NiMH) AA batteries have a capacity range between 1.5 Ah and 2.5 Ah. With a voltage of 1.2V this makes a capacity between 1.8 Wh/battery and 3 Wh/battery or for a cylindrical volume 270 Wh/liter and 450 Wh/liter. Thus they are comparable with the low end performance of Li-Ion batteries. In comparison by calculating the cubic volume the result will be a capacity of just 204 Wh/liter and 273 Wh/liter. Compared to the low end Li-Ion the AA battery performs, at a minimum, 6% less efficiently and close to 40% less compared to the high end Li-Ion of today. b) Polymer Electrolyte Li-ion batteries For early Li-ion batteries with polymer electrolytes, the quoted capacity is 470 Wh/liter [Roos, 1999]. Today similar capacities of 470 Wh/liter are quoted but with the added feature of being flexible and bendable [NEC, 2006]. Even for the lowest capacity value of polymer electrolyte Li-ion batteries, the 25% advantage is obvious when compared with AA batteries. The mechanical matching is certainly improved by the flexibility in design introduced with the option of the bendable battery.

4.3.3 Environmental design issues and element matching In analysing the environmental design issues and the renewability of energy supplies of products, a Life-Cycle Assessment (LCA) is typically done. a) PV cells In the case of PV powered products, as a first order evaluation, only the energy-related aspects will be taken into account. This means that the energy payback time (EPBT) will be estimated. The EPBT is generally defined as the time a PV cell or module has to operate in order to generate the same amount of energy (in equivalent terms) as was needed to manufacture it. The environmental impact of a product is then determined by comparing the EPBT with the time the PV cell will be used for that product. The EPBT depends on the PV cell under investigation, its efficiency and user context. For mono-crystalline silicon modules, an EPBT was estimated of 1.5-2.5 years for South-European (S-Eur.) locations (irradiance 1700 kW/m2/year). For the Middle-Europe (M-Eur.) region (irradiance 1000kW/m2/year), a higher EBPT in the range of 2,6-4,4 years was obtained. However at a low irradiance level, with an indoor user context (about 1kW/m2/year or even less) a corresponding longer EPBT could be estimated [Alsema and Wild-Scholten, 2005b]. b) Batteries In matching the energy of the battery with the other elements in the energy chain of PV powered products the main emphasis has been on the discrepancies between life cycles of the batteries versus the active life of the product. For example in case the battery

116 Chapter 4: Optimal matching in the energy chain performance is reduced and have to be discarded because of the memory effect this will have a direct environmental effect. But also the matching with the other elements in the energy chain will be poor (see section 3.2.3). Another environmental design issue will be the used materials in the battery. As was presented 3.2.3 the battery has to comply to the European legislations which banned toxic materials such as Cadmium [EC legislation, 2003/2004, Wild-Scholten, 2005].

4.3.4 User context design issues and element matching Temperature Temperature has an impact on several elements in the energy chain of PV powered products. a) PV cells Elevated temperatures could occur in a case where the PV cell is integrated into a product and it is sunbathing for re-charging the product battery. The PV performance will be de- graded at elevated temperatures. For example at silicon PV cells the open circuit voltage Voc will decrease by about 4% per each °C increase in temperature. As a direct result the efficiency and related power output decrease by 0,5% per each °C with increasing temperature. [Green, 1986]. b) Batteries Li-Ion batteries will at temperatures above 45 °C have an increased self-discharge and a fading of capacity (see Chapter 3, section 3.5) but also at low temperature (less than 5 °C) due to an increase of their internal resistance the battery output performance will be reduced. In section 3.4.3 of Chapter 3 it was shown that the remedy for the latter is the use of capacitors. The battery - capacitor energy storage combination demonstrates a more favourable Figure of Matching than that of the battery alone at low temperature. c) Electronics Electronic components in the energy chain have specified temperature ranges in which they will perform. If the electronic components exceed their specified temperature range due to high battery temperatures or sunbathing the PV cell then performance degradation and even a loss of functionality will occur. As a design guideline thermal matching should be taken into account. This means for the design: • For the PV cells a proper balance has to be found between irradiance and avoiding overheating. • For the batteries temperatures above 45 °C should be avoided and capacitors should be used for applications at temperatures below 5 °C. • For the electronics operate the components within the specified temperature limits.

Moisture Moisture can have an impact on several elements in the energy chain. Apart from the latent cause of short circuits, there are also element specific points of attention related to moisture. The general design guideline to minimise the influence of moisture is encapsulation. a) PV cells For PV cells moisture can cause degradation in performance due to imperfections introduced in the surface. This problem will be the case in PV cells that are not sealed by a natural oxide sealant i.e. a crystalline silicon cell is sealed with SiO or glass. 2

117 Energy Matching - Key towards the design of sustainable photovoltaic powered products

b) Batteries For Li-Ion batteries the Li atoms like to combine with water resulting in a hot gas that can cause an explosion.

Pressure Pressure should be a design consideration in the case that there are cavities involved. Cavi- ties can occur due to imperfect gluing or encapsulation for instance.

4.3.5 Standardisation Another point of concern is standardisation. There is matching possible between one kind of energy use and another, if and only if there is a general standardisation. Take for example the chaos of various non-standardized batteries and battery chargers and adapters. Even from one producer there is a jumble of battery voltages: 6 V, 6,3 V, 7 V, 7,3 V etc.

Figure 4-2 Various connectors for battery chargers

In Figure 4-2 it can be seen that standardisation of the connectors would also be needed. The lack of standardisations result in a waste of natural recourses since each time a new product is introduced the old connectors and adapters are discarded and thus wasted. Although the transformers in newly developed adapters become smaller, there is still a need for iron cored transformers and copper wires in net adapters. Another advantage of standardisation will be the possibility of mass production which in turn results in a cost reduction. Therefore international standardisation is most recommendable.

4.4 Overall matching and energy balance in PV powered products

4.4.1 General considerations To obtain a thorough description of the ‘user context defined energy use’ an inventory of the following parameters [Mestre and Diehl, 2003; Keukens, 2005; Kan et.al, 2006c] have been compiled: 1. Who will be the user(s) of the product? 2. In which context will the product be used?

118 Chapter 4: Optimal matching in the energy chain

3. What are the characteristics and functions of the product in question? 4. What is the product energy requirement: max, min, average? 5. Are there standby and/or sleep modes involved? 6. What is the frequency of use of the product indicated in a number per hour, day or week? 7. At each time of active use, what is the duration indicated in seconds, and minutes per unit time? 8. Are these parameters constant? If yes, then the result for a particular user can be refined as: 9. How many hours of use per day or part of the day in summer (if applicable)? 10. How many hours of use per day or part of the day in winter (if applicable)? 11. How many hours of use per day or part of the day on average? 12. Is the product used only indoors, only outdoors or mixed? This has an impact on the environmental requirements such as its operational temperature, its durability and its ability to be weather-proof, but also on the spectrum of incident light.

By combining all the information one obtains a pattern of use over time by one particular user. For more generalised data, the energy need is linked to ‘user profiles’ and ‘target groups’. For each user profile and target group, and the energy use pattern per time interval, an average and maximum energy demand can be calculated [Weitjens, 2003; Keukens, 2005; Geelen, 2006]. In other words, for the energy need per time interval of a target group both an average and a worst case scenario can be found. This energy need can then be compared and matched in the energy balance with the available light energy during the same time interval. In Chapter 2 and 3 matching of the interfaces between the PV energy converter, the energy storage media and the energy use in the functional application were treated as separate entities. In designing PV powered products by optimal matching of the available energy and the energy required one has to take into account all the parameters, elements and interfaces of the entire energy chain as presented in Figure 4-1. The matching of these parameters, elements and interfaces has to be done simultaneously and analysed together and interrelated to each other on a system level. This higher level tuning is needed to avoid that optimal matching in one interface will reduce the matching in another interface and vice-versa. These elements and interfaces will also include those outside the charted box in Figure 4-1 and are therefore outside the scope of this dissertation. By taking all these parameters, elements and interfaces of the entire energy chain together the designer will be able to close the feedback loop and establish an energy balance in the energy chain. In analysing the entire energy chain by aiming at achieving an energy balance one can distinguish two approaches. 1. In a PV technology driven energy balance approach: one starts with the available amount and type of light in a given user context and calculates along the entire energy chain the ultimate amount of energy reaching the application. This approach is usually followed in the common rooftop PV systems energy balance calculation [RetScreen, 2006]. Knowing this amount of energy the proper application can be chosen. In the case that a certain application is already chosen, the energy need of this particular application can be tuned to match the energy supplied as mentioned above.

119 Energy Matching - Key towards the design of sustainable photovoltaic powered products

2. In an Application driven energy balance approach: one starts with the energy need for a particular application in a given user context and calculates in the reverse direction the amount of energy that has to come from the PV cells to make this application feasible. This approach is followed in designing stand-alone PV systems today. Knowing this amount of energy the proper PV cell can be chosen for a particular user context and product dimensions.

For both matching approaches, if used in mobile/wireless product design, the locations of the energy storage media and the PV cells can either be integrated in or on the product or as stand-alone unit. This is still an open design variable. Also by design the PV cell area can be extended, and thereby stretch the maximum energy supply. The choice of the energy storage media location will also be driven by concern for the user’s convenience and routine. Part of this analysis is based on the findings in PV related Master Graduation projects [Beers, 2002; Weitjens, 2003; Gennip, 2005; Keukens, 2005; Geelen, 2006].

4.4.2 Relating and matching the elements and interfaces in the entire Energy Chain Time and timing a) Looking at the entire energy chain the most striking matching improvement is achieved by the combination of a PV power converter and an energy storage medium. In many PV powered products this combination currently does not occur in the energy chain i.e. the light energy is converted into electrical energy, directly followed by utilization or transportation towards a central collecting point [Kan, 2002c]. In contrast, in this dissertation the next step is ‘Storage’, for example in a battery, whereby the use of the harnessed electrical energy from the PV system can be delayed until a convenient moment for utilization or transport. The advantage of the latter solution is time-delay matching between the photovoltaic energy converter, the energy storage media and the application which results in: • A better time matching in relation to the abundance of light; • A better energy transfer efficiency between the elements; • A more efficient use of energy due to rechargeable battery process; • A more even and stable supply of energy flow for use i.e. no spikes which results in an improvement of the product reliability and therefore its sustainability. b) In Chapter 3, section 3.3 and 3.4 and this Chapter 4, section 4.2 the matching interfaces MI:2 and MI:3 were treated as separate interfaces. In fact a rechargeable battery has only one terminal interface that is used both for charging but also for discharging energy. In products recharged by the main grid, there is a distinct moment for use and a moment for charging. So in that case electronically the two matching interfaces MI:2 and MI:3 are separated. In a PV powered product in which the PV cell will recharge the battery the two matching interfaces MI:2 and MI:3 are less distinct. The options are: • An electronic switch that separates the two interfaces. • An electronic multiplexer in which the two interfaces are connected alternately to the terminals of the battery. • A system in which the energy storage medium is divided into two separate storage

120 Chapter 4: Optimal matching in the energy chain

locations interconnected with each other as two communicating vessels. • No switch or multiplexer at all just directly interconnected. • The choice of the option will depend on the application. The criterion will be price performance consideration such as: • For a cheap simple product in which it will be too costly to have additional electronics the most simple’s method will suffice. In this case there is no distinction between MI:2 and MI:3. This is the case in most of the PV powered products now on the market. The PV cells still remain foreign bodies not matched with the product. • For a more costly product that has to be functional and reliable an electronic switch or multiplexer would be preferable. • For an expensive product that has to be functional all the time electronic multiplexers and even controllers are integrated.

Multiple uses of available data Information that is gathered in one matching interface can be used to optimise the matching in another interface. This double use of information is demonstrated by information about the irradiations level in time. For an optimal Maximum Power Point Tracking (MPPT) the irradiance level is monitored as a function of the time of the day (see section 2.5.2 in Chapter 2). The same information however can be used to improve the matching between PV output and power intake characteristics of batteries during charging (see section 3.3.6 in Chapter 3). This is an example of the multi-dimensional matching of two separate elements and interfaces.

Location of PV converter and energy storage with respect to the functional part of the product In most mobile products today the power source is a rechargeable battery. This battery is integrated within the functional part of the product in one and the same embodiment. The embodiment is more or less modelled to fulfil the user interface requirements. In PV powered products the power source is extended by incorporating a PV cell. In designing such PV powered product the main aim is therefore to avoid that the added PV cell remains just an ‘add-on foreign body’. Therefore optimal matching between the PV cell and the other elements and interfaces of the product will be the design goal. PV powered products can be classified as: • A PV powered version of an existing product, in which the PV cell is just an add-on; • A completely new product.

For both classes of products the question can be asked where to place the PV cells and where to place the energy storage media. To answer these questions the matching through the matching interfaces MI:1, MI:2 and MI:3 did not provide sufficient information. The key to answer these questions properly will be the use of user interface surveys in which questions have to be asked like: • How will the product be used? • Where will the product be used? • How inconvenient is the option of having an additional plug-in unit to charge the standard battery?

121 Energy Matching - Key towards the design of sustainable photovoltaic powered products

The result of these surveys could be that optimal matching would be a separate click-on unit that can be sunbathed on the windowsill while the product is used indoors. This was the result in the case of the PV powered Solar Mobile Companion (see Chapter 5, section 5.3). On the other hand the results of these surveys could be that the PV cells and storage must be integrated completely in the product. This was the result in the case of the PV powered wireless Mouse (see Chapter 5, section 5.3).

4.4.3 The PowerQuest tool General remarks PowerQuest was developed as a simulation tool. This PowerQuest software tool was designed in the framework of a master graduation thesis project at the Design for Sustain- ability program (DfS) at the Delft University of Technology [Gennip, 2005]. The point of departure was a direct request from the SYN-Energy program to search for ‘test-case’ products that can be powered by PV cells. The objective of the graduation project was to design a software tool that could find applications and match product options with certain PV cell technologies. After graduation the same PowerQuest tool was extended as a search tool in the opposite direction namely to find PV technologies that match given applications or the matching between products and PV technologies. The PowerQuest tool is now developed as an intuitive online tool that matches design parameters of electrical mobile products to PV energy-system parameters and vice versa. This tool provides both PV technology developers and the designer means to find trade-off options for feasible product – PV energy-systems combinations. Therefore the PowerQuest Software tool can be used to simulate and estimate matching and power transfer throughout the entire energy chain matching model of PV powered products as presented in Figure 4-1 [van Gennip, Kan et. al., 2006]. Since the tool is intended to be used by the designer in the early phases of a product design and development process, a general set of parameters are used as input values. These general values are presented to guide and aid the tool user in the selection process. To facilitate the use of the parameters, they are intuitively linked to the knowledge of the user, e.g. clicking on pictures of existing products that resemble the new product will establish links to the concept’s required parameter max/min values and triggers the user to use the related values as input. With this approach the chance of introducing false input parameters or false input parameter ranges is minimised. PowerQuest is developed to communicate interactively and ‘creatively’ with a tool user to stimulate a way of thinking during the design process. The PowerQuest tool can handle two approaches and as a result has two user interfaces namely: a) The PV driven energy balance approach for technology developers; b) The Application driven energy balance approach for designers.

Power Quest database for PV powered products The starting point of this PowerQuest tool is that the product is used in a certain environment or user-context during a certain amount of time and at a certain location on the earth i.e. in/outdoor, temperature, humidity, illumination, etc are known. The PowerQuest tool is a knowledge base that could contain all kind of information; different energy-system technologies, product parameters, product user profiles, user-context data and matching functions. Together with its content management system (CMS), it creates a knowledge

122 Chapter 4: Optimal matching in the energy chain management system (KMS). With the KMS software the information is correlated and matched. This knowledge base and its CMS are situated on a web server to provide easy access for anybody with an administrator account and an internet connection with a normal web browser. All data is stored on and retrieved from a MySQL database using a tailor-made secure HTML/PHP CMS link. To be able to use the PowerQuest tool to simulate the energy matching in the energy chain of PV powered products the data base has to be fed with for example the following information: • Spectra tables and graphs of various light sources; • PV characteristics of various PV cells technologies such as their Spectral Responses (SR) and their power conversion efficiencies at various irradiance levels; • Battery characteristics of various battery types such as their charge responses and their discharge characteristics.

Various load characteristics of applications such as their power demand in time graphs. The user-context data, which is available in the database of the tool, is divided into three parts: 1. The activity that a user carries out with the product (described in percentage of maximum power demand); 2. The usage (duration and moment) specification; 3. The location where product usage takes place.

In the input sheet the user context is defined by selecting a usage scheme, a global location, and an indoor/outdoor use scenario. This data can be entered manually using the input sheet or one can select a predefined scenario. The input parameters on the input sheet depend on the selected technology and can be changed using the CMS by a data administrator.

The Simulation The PowerQuest tool is used in during this PhD project to simulate the first order energy balance in the energy chain as presented throughout this dissertation in the Energy Match- ing Model presented in Figure 4-1 and for reader convenience in Figure 4-3.

The Simulations steps • Point of departure is the user context. Once the user context ‘n’ is known the incident light type, i.e. spectrum f ( ) and irradiance level G are known. The matching be- ISn λ n tween this user context defined incident light and the various PV cells in the data base will yield, in accordance with Eq. 4-3, the spectral Figure of Matching (FM ) of the Spectral optimal matched light source - PV cell combination. • In this first order simulation only a finite amount of standard user contexts were defined. For example: outdoor sunny summer day, outdoor cloudy winter day, TLD illuminated office room, TLD + incandescent lamps living room, inside a train on a sunny day etc. If necessary however this set can easily be extended for future use. • In the database the light conversion efficiency of that particular chosen PV ηAM1.5+irrad cell at the user context defined irradiance level G and the conversion efficiency n ηSTC under Standard Test Conditions (STC) are also known. So the irradiance level dependent spectral Figure of Matching can be calculated as:

123 Energy Matching - Key towards the design of sustainable photovoltaic powered products

FM ≈ FM x / [%] Eq. 4-6 AM1.5 + irrad AM1.5 + 1000W/m2 ηAM1.5 + irrad ηSTC

• With the aid of the spectral Figure of Matching the spectral conversion efficiency can be found as:

≈ FM x FM / FM Eq. 4-8 ηspec + irrad AM1.5+irrad spec, 1000W/m2 AM1.5, 1000W/m2

The PV area APV is known by design to fit the available area of the product. So the spectral dependent converted power by the PV cell with an area of APV m2 at a certain irradiance level G can then be calculated as: n

P = G x x A [W] Eq. 4-9a PV n ηspec + irrad PV

• At the user context defined irradiance level G all the transmission losses induced for n example by window glass, cover-glass or encapsulation of the PV cell have been taken into account. • At this moment in Power Quest the actual power P converted using this user conv context - PV cell combination is mostly still calculated with the STC efficiency or ηSTC occasionally instead of the spectral dependent efficiency , so: ηAM1.5 + irrad ηspec + irrad

P = G x x A [W] Eq. 4-9b conv n ηSTC PV

• Together with the user context the target user group is known. So the daily pattern and duration of exposure to light and the consecutive charging scenario of the energy storage media is known and the Figure of Matching between PV cell and Battery can be determined in according to the analysis in section 3.3 of chapter 3. For example in section 3.3 it was shown that the circuitry to maintain the PV cell at its Maximum Power Point and match the PV output to the input voltage of a Li-Ion battery has a transfer efficiency of 90%. • The battery is chosen by application driven criteria and available space in the case that it will be put inside the product. Thus the total energy transfer efficiency of the battery is known. ηbat total • Combining the user activity and product usage will result in a power-time demand graph. This power-time demand graph will result in an overview of energy required during a period of time. This energy figure is subsequently matched with the energy storage output characteristics resulting in a transfer efficiency determination. • The Figure of Matching of the power that can be transferred from the battery to the application can be calculated in accordance with the analysis in section 3.3.2 of Chapter 3. • Finally since the daily pattern of energy use of the target group is known matching of energy that is supplied and used can be compared in the overall application matching interface (OMI) as presented in Figure 4-1 and Figure 4-5.

So an energy matching and energy balance can be calculated and simulated with the Pow- erQuest tool for a given scenario. In the framework of the SYN-Energy program, a more detailed simulation scenario is under development [Reich, forthcoming, 2008].

124 Chapter 4: Optimal matching in the energy chain

Figure 4-3 The Energy Matching Model (EMM) of the entire energy chain of a PV powered product

In the Figure 4-3 there are two overall energy balance tracks to close the feedback-loop through the Overall Matching Interface (OMI) namely: Track (1): The matching between the outdoor/indoor user contexts defined incident light and the needed energy by the application in a specific user context. Track (2): The matching between the outdoor/indoor user context defined incident light, the output of the PV cells, the energy storage capacity and the user context defined energy use. In other words an overall matching and energy balance in which the user context in combination with all the elements in the energy chain are taken into account.

Up to now the literature only reflects the use of the first track [Randall, 2003]. With the PowerQuest simulation also the second track can be used.

125 Energy Matching - Key towards the design of sustainable photovoltaic powered products

The PowerQuest status today At this moment the PowerQuest tool main contribution to the development of the En- ergy Matching Model is limited to the first matching interface MI:1. It needs to be extended to include the other matching interfaces in a future research program. The input parameters are at this moment limited to the set data that was measured in the framework of the SYN-Energy program and need to be updated and extended by new data obtained from new participants and future research. The choice of the battery is not yet incorporated as an option in the PowerQuest tool.

4.4.4 Design Methodology of PV powered products The design process of a PV powered product will generally be an iterative process. Start- ing point is the user context and the energy match in accordance with the Energy Match- ing Model (EMM) presented in Figure 4-1 or 4-3, by using the Figure of Matching algorithm at the matching interfaces MI:1-3 and the energy balance through the overall matching interface OMI. The first iteration step will be needed to eliminate contradictory matching requirements. For instance one optimal user context defined irradiance - PV cell combination has been found with the aid of the Figure of Matching algorithm. In the materialization stage however it turns out that the chosen PV cell is only available with a rigid glass substrate encapsulation but the surface for the product needs to be curved. If this is the case then the second best match that is flexible may be the best choice. Consequently, the first iteration step will be necessary to obtain the optimal mechanical match. In the next iteration step the user context defined parameters such as temperature range are fine tuned. More iteration steps will be needed to obtain the optimal user interface match. Each time a new iteration step is undertaken and another parameter is added to the design an inspection is needed to determine contradictory matching requirements. To speed up the design steps some iteration steps can be done in parallel resulting in a multi-dimensional matching. Such multi-dimensional matching has been analyzed in Chapter 3 section 3.4 in which both the power transfer and the influence of temperature were matched.

4.5 Conclusions

The question to be answered in this chapter is:

What optimisation can be achieved between the elements of the energy chain of a PV powered mobile wireless/products and how does this affects other design engineering aspects?

The answers found: Matching improvement can be achieved in the energy chain by combining components that are not yet optimally matched. An example of suboptimal matching that can be improved in such a way is that of matching battery – capacitor combinations with an application. Mechanical matching improvements can be achieved by proper selection of materials and utilisation of novel technologies such as flexible flat batteries and flexible PV cells. Calculation of the energy densities with cubic volumes demonstrates the improvement

126 Chapter 4: Optimal matching in the energy chain

of embodiment matching gained by using prismatic Li-Ion and Li-Ion polymer batteries instead of conventional AA shaped batteries. For a renewable energy design, international standardisation is recommended. The energy transfer in the energy chain is optimised by selecting the proper combination of elements with the aid of the Figure of Matching algorithm. For a multi- dimensional matching model the Figure of Matching algorithm has to be adapted and extended to applications beyond power correlation as for instance mechanical shape correlation. The PowerQuest tool can be used to simulate and estimate matching and power transfer throughout the Energy Matching Model of the energy chain. Design guidelines: Thermal matching should be taken into account. Curved PV cells are to be used if and only if this feature introduces a new added value. Exploit the time delay matching by using PV - energy storage media combinations. Avoid moisture in the elements of the energy chain by proper encapsulation.

127 Energy Matching - Key towards the design of sustainable photovoltaic powered products

128 Chapter 5: Test cases of mobile/wireless PV powered products

5.1 Introduction

In this chapter the application of the Energy Matching Model (EMM) and the Figure of Matching (FM) algorithm, as described in the preceding Chapters 1 through 4, is used to analyse a compilation of several PV powered product examples. By these analyses, the fourth part of the research question (sub Q. 4) as formulated in Chapter 1 will be answered:

What insight can be gained by analyses of the energy matching in test cases of PV powered products?

These examples were selected because they all demonstrate test cases that use PV power converter – energy storage combinations as power sources. This is in accordance to the title of the SYN-Energy program in which the central point is the analysis of combined PV cell – energy storage power sources. The analyses of the test cases can be divided into 3 groups. The first group consists of two examples of products that were designed within the framework of master graduation projects. The master graduation projects examples were executed within the Design for Sustainability program (DfS) in the faculty of Industrial Design Engineering of the Delft University of Technology. These projects were carried out during the early development stage of the Energy Matching Model (EMM). So the analyses of the two examples in this group can each be classified at this moment as a kind of ‘post-analysis’. In the first example the user context definition was the central theme. In the second example the influence of the irradiance spectrum and energy balance were explored. During this latter project no generic Figure of Matching algorithm has yet been defined, only an expression for the spectral Figure of Matching was found. The second group consists of benchmark results of some existing PV powered products selected for their PV – battery approach. The product benchmarking activities were carried out at the faculty of Industrial Design Engineering of Delft University of Technology. Benchmarking in this context refers to the process of improving performance by learning from others [Boks and Diehl, 2005]. In particular the benchmarking activities were undertaken to understand why the product functioned as it did. So the analyse of the examples in this group can also be classified as a post analysis. The third group consists of products that have been developed as demonstration models for the SYN-Energy research program. The designs of these products are based on the Energy Matching Model and the generic Figure of Matching algorithm.

129 Energy Matching - Key towards the design of sustainable photovoltaic powered products

In analysing these PV powered product examples, it would be convenient to keep in mind the following considerations for each product: • What was the point of departure in designing this product? • What was the objective to be met by the design of this product? • Have the design steps (DS) based on the energy matching approach as defined in Chapter 4 been followed and how? The results of these analyses are presented in Table 5-2. o DS-1: Was the design ‘PV’ (energy source) or ‘application’ driven’? o DS-2: Was the starting point of the design based on choosing or determining (a) a user profile and (b) a pattern of energy use, resulting in (c) a specified user context? o DS-3: Does the design takes into account the user context that defines both the nature of the incident light (spectrum, light level) and the energy use in the application? o DS-4: The matching between the elements (a till d in Figure 5-1) in the energy chain can be analysed and quantified with the aid of the Figure of Matching (FM) algorithm at the matching interfaces MI:1 through MI:3 as presented in the Energy Matching Model (EMM) (For reader convenience see Figure 5-1). o DS-5: Was there any kind of matching found between elements in this product? o DS-6: Could there be an energy balance achieved in the energy chain? o DS-7: Were efforts made to analyse items outside the energy chain such as the user’s emotional impression and user interface or were there some other specific targets to aim for? • What were the lessons learned from this example? o Have design strategies and design steps been adopted in the framework of a general design methodology? o What are the successes and failures in this design? • How can the matching in the product be improved?

The analysis of the cases is guided by the Energy Matching Model as presented in Figure 5-1.

130 Chapter 5: Test cases of mobile/wireless PV powered products

Figure 5-1 The Energy Matching Model (EMM) of the energy chain of a PV powered product

5.2 Master graduation project examples at the Delft University of Technology faculty of Industrial Design Engineering

5.2.1. The ‘Backpack’ PV battery charger The project This project was initiated by the Design for Sustainability program (DfS) in close coopera- tion with the Energy research Centre of the Netherlands (ECN) [Weitjens, 2003]. The point of departure was the design of a novel PV powered product. The objective was to explore the feasibility of using PV power supplies in products. There- fore a PV powered product had to be designed in which the added value of the use of PV cells was demonstrated. In addition the design should be an implementation example and an extension of the original ‘Smart PV battery’ idea [Kan, 2002]. In a practical sense

131 Energy Matching - Key towards the design of sustainable photovoltaic powered products the advantage of using a combined electrical energy storage buffer - PV system was to be explored. The design was PV (energy source) driven since the project started with a search for applications of PV cells (DS1). The product i.e. the backpack PV battery charger was chosen as a result of a user enquiry. For choosing this application the users where classified in different user groups (DS2). From these user groups a certain target group was chosen and a typical product associated with that target group was found. So the user context was defined in the beginning of the project (DS3).

Figure 5-2 The backpack PV battery charger in use

The chosen product is a PV powered battery recharge unit which is implemented as an accessory for an existing designed backpack as presented in Figure 5-3 (Boblbee, 2006). This battery recharge unit was implemented as a click-on system as presented in Figure 5-3. This system is comprised of a cover plate (2), a back plate, PV panel (5) to generate electrical power, a cylindrical compartment (9) that could contain the product to be charged, a battery pack (6) as an electrical energy buffer storage unit and a printed circuit board (7) to facilitate the charge-recharge electronic control electronics. To monitor the charging process a LCD readout (8) was included.

Figure 5-3 The lay-out of the backpack PV powered battery recharge unit ‘Solar Tergo’

132 Chapter 5: Test cases of mobile/wireless PV powered products

Since the battery pack functions as an energy buffer, the PV panel therefore does not directly recharge the battery of a mobile/wireless product. The battery buffer is consciously chosen in order to not accommodate the various non-standardized-voltages, currently in use in mobile/wireless products. Instead, the voltage was fixed on 12 V Direct Current (DC) in accordance with the common voltage available at the cigarette lighter socket in cars. As a consequence, all products still will require their own DC/DC converters to adapt the 12 V of the car cigarette lighter to the product required voltage. Although it would be more convenient for the user not to carry a separate DC/DC converter for each product, these car cigarette lighter adapters are commonly used today. Therefore, for the time being, this solution will not pose a problem. A more ergonomic solution in the future would incorporate intelligence for automatic voltage selection by a DC/DC converter incorporated in the charge control electronics of the charger. The recharge unit is detachable from the backpack. The backpack is therefore as such not modified nor impaired in any way by this design, since the recharge unit can be clicked on and off as an autonomous unit on top of the backpack. Since the target user group is fixed, the pattern of energy use could be determined and the user context was established. Therefore the nature of the incident light and the power required from the PV panel and the necessary energy storage figures could be roughly determined. In this design a flexible curved PV panel was used to accommodate the shape of the backpack and contribute to its convenient use, as well as to enhance its appearance. After the master project was finished, the backpack recharge unit was actually built by the faculty of Industrial Design Engineering of the Delft University of Technology as a test prototype under the name ‘Solar Tergo’ (see Figure 5-4).

Figure 5-4 The prototype of ‘Solar Tergo’ mounted on the Boblbe-e backpack

The Matching Although at the time this project was done no Energy Matching Model nor any Figure of Matching algorithm was available, for the sake of uniformity in Chapter 5 and throughout this dissertation, the energy matching in this product is analysed by using the matching interfaces as presented in the Energy Matching Model of Figure 5-1 (DS4).

133 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Matching interface MI:1 incident light - PV cell Triple junction amorphous silicon cells from Uni-Solar, Model US-3 (frameless) were used as the chosen PV type. The specifications of this PV cell are [Uni-Solar, 2003-2006]: • Rated Power: 2.68 (W) • Operating Voltage: 8.10 (V) • Operating Current): 0.33 (A) • Open Circuit Voltage: 12.00 (V)

The main driver for the choice of the PV cell was that they had to be flexible, robust and fit the shape and appearance of the backpack. The choice of PV cell type was not based on the Figure of Matching algorithm but merely on a PV performance datasheets found on internet. These datasheets from the internet provide only basic data such as efficiency, obtained under Standard Test Conditions (STC), and no proper spectral information. A triple junction PV cell can be regarded as 3 PV cells connected in series. Each cell has its in- trinsic wave length band. If one or more of these wave length bands is poorly represented in the spectrum of the incident light, the related PV cell will have a spectral mismatch and therefore will not function optimally. As a result, in such a system of in series connected PV cells, this particular PV cell with a spectral mismatch will act as a large series resistor. Therefore the whole combination will not function. In other words the “Solar Tergo” suffers from a poorly performing PV system. The active area of the PV was defined by the available surface area of the Boblbee backpack. Some energy balance calculations for probing the needed PV area were done with STC PV data and cellular phone (GSM) power use data (DS6).

Matching interface MI:2 PV cell - energy storage media In the original design 12 NiMH AA-batteries should be recharged by the PV cell. This amount of batteries was chosen to achieve a battery pack that can deliver the 12 V, the same as a standard car cigarette lighter adapter. Later the number of batteries was brought down from 12 to 6 since the PV cell operating voltage was not the intended 12 V but merely 8 V at an irradiation of 1000 W/m2. So 6x1.2 V NiMH AA-batteries should be used. In the implementation of the Solar Tergo however 6x1.2 V NiCd AA-batteries were used. The reason for this choice was the large overcharge tolerance of NiCd in comparison to NiMH batteries. As a result no overcharge protection circuit was necessary.

Matching interface MI:3 energy storage media - application All the mobile products were recharged with the aid of 12 V car cigarette lighter adapters. To obtain the required 12 V from the electrical buffer storage of 8 V a DC/DC up-converter is used. The complete circuit diagram is presented in Appendix G. Mobile products that could be recharged include a cellular phone, PDA, ect. The recharge process could be carried out while the mobile products are carried around in the backpack both outdoors and indoors, all throughout the day.

Items outside the energy chain Care was taken of a proper colour matching between the coating on the PV cell and the cover plate (DS7). The products that are to be recharged should be easily acces-

134 Chapter 5: Test cases of mobile/wireless PV powered products sible therefore they were placed either in the special cylindrical compartment outside the backpack (Figure 5-3 item 9) or just inside the backpack. For the latter option a longer connecting cable was needed.

Lessons learned from this design A practical design can be made by choosing a user target group and implement the use pattern of that particular target group in the energy balance calculations. The flexibility and robustness of the PV cell are also selection criteria in addition to the energy performance.

Improving the design The PV performance can be improved by using single junction amorphous silicon instead of multi-junction amorphous silicon. The reason for this improvement is the spectral mismatch of multi junction PV cells. The multi-junction PV cell on paper could perform well under Standard Test Conditions with an AM 1.5 spectrum. The real spectrum encountered by the PV cells on the backpack does however cover only a part of this spectrum. In particular the junctions optimised for Ultra Violet (UV) and Infra Red (IR) spectral regions show a Spectral Figure of Matching much below the 10 % that could be achieved by an AM 1.5 spectrum. At indoor conditions with artificial TL lamp light the spectral Figure of Matching of the IR junction drop to zero and for incandescent lamp light the Spectral Figure of Matching of the UV junction drops to zero. In both cases these junctions became large series resistors. The overall effect on the multi-junction PV cell will therefore perform more poorly than that of a single junction PV cell. A Maximum Power Point Tracker and some additional capacitors as presented in Chapter 3 that could improve the charging capability of the Solar Tergo.

Evaluation Today it can be seen that several PV - Backpack combinations are on sale [Poliness, 2004; Hamersley, 2005 and O’Neill, 2005].

5.2.2 Solar Rudy and his amazing pupil localizer The project This pupil localizer system was initiated at the Design for Sustainability programme (DfS) with some initial support from Austrian scientists [Keukens, 2005]. The point of departure was both a PV demonstration project for the SYN-Energy Program and the design of a product to participate in the ‘Energy Systems of the Future’ (EdZ) program of the Austrian Ministry of Transport Innovations and Technology [BMVIT, 2002].The objective was to design a PV powered product that has a direct relation with Austria. The product choice was guided by the idea of combining a major source of Austrian income, namely winter sports, with one of Austria’s potential fields of research and technological development, i.e. photovoltaics. The design was certainly energy source driven, as the development of the photovoltaic power converter was to be used as input for Austrian technological PV development (DS1). The Pupil Locator is a product that helps the ski instructor in counting the pupils in his group. With the Pupil Locator an instructor can see where the pupils of his group are

135 Energy Matching - Key towards the design of sustainable photovoltaic powered products located, even in case they are out of his field of vision (see Figure 5-5). Lost pupils can be located and found. This will enhance the feeling of security of the kids, their parents, the instructor and ski schools (DS2).

Figure 5-5 The pupil localizer system - the UWB sender and pupil jacket with PV cells to power the localizer/receiver

The Matching Although at the time of this project both the Energy Matching Model and Figure of Match- ing algorithm were available only in a preliminary state, for the sake of uniformity in this chapter. and throughout this dissertation, the energy matching in this product is analysed by using the matching interfaces as presented in the Energy Matching Model of Figure 5-1 (DS4).

Matching interface MI:1 incident light - PV cell As the user context was defined, an estimate for the incident light level and type can be made. The user context defined incident light - PV matching has been made with a preliminary correlation calculation between the spectral distribution of the sun at ski level on the mountain and the scarce available spectral responses found in literature. Although the results found during this master project could be improved with more precise spectral responses measured after the project was finished [Reich, Kan et al, 2005], the chosen PV cells of CIS (Copper-Indium-Selenide), c-Si and mc-Si are still practical choices as can be seen in Figure 5-6.

136 Chapter 5: Test cases of mobile/wireless PV powered products

Figure 5-6 The Spectral Matching of CIS, c-Si and mc-Si PV cells with the AM 1.5 spectrum

The matching of the CIS PV cell and the AM1.5 sun spectrum is particularly significant in the blue and UV region. At high ski altitudes the contribution of this blue and UV component will be even more significant. The Spectral Figure of Matching of the CIS PV cell and the AM1.5 spectrum is 15,5%. This value is slightly higher than that of c-Si and mc-Si which have a Spectral Figure of Matching respectively of 15 % and 14 %. At high ski altitudes the Spectral Figure of Matching of the CIS PV cell becomes 16,5 % while that of c-Si and mc-Si remain the same as at an AM1.5 spectrum (DS4). In choosing the PV cell still another criterion besides the spectral matching has to be taken into account. The PV cells are fit on the pupil jackets so that the mechanical properties are also a point of concern. The advantage of CIS over c-Si and mc-Si PV cells is their flexibility. This property makes the CIS cell more rugged and suitable for integration on the ski jacket (DS7).

Matching interface MI:2 PV cell - energy storage media No Maximum Power Point Tracker (MPPT) was envisaged because the defined user context is at a sunny ski level. Therefore a fixed Maximum Power Point was set. Li-Ion batteries were chosen because of the appropriate voltage range and shallow charge/discharge capability of these batteries (see Chapter 3 section 3.2.3). To minimise the low temperature influence on the output voltage of the battery as was discussed in section 3.4.2 of Chapter 3, the batteries were put on the inside of the jacket and warmed up by the body.

Matching interface MI:3 energy storage media - application The trade-off for the pupil localizer application was between a combination of GSM + GPS and an Ultra Wide Band (UWB) send/receiver. The UWB device has a dual purpose application as both a communication device and a position detector or tracking device. Since the UWB consumes only about a tenth of the common GSM, the energy balance matching has resulted in the choice of an Ultra Wide Band (UWB) instead of a conventional GSM, not to mention the combination GSM+GPS (DS 5/6).

137 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Items outside the energy chain In designing the jacket for the pupil both the ergonomic aspects and emotional impression of the user has been taken into account. The ergonomical aspects were for instance treated by the choice of the location of the electronics and battery, in order to minimise injury in a case where the pupil might fall down (DS7). Flexibility and robustness of the PV cell are also selection criteria.

Lessons learned from this project In this project it was demonstrated for the first time that proper energy matching in the energy chain by using the Figure of Matching algorithm was feasible. Not only energy matching but also mechanical matching is important in choosing the appropriate PV cell. In the user context, parameters such as temperature have to be taken into account. Direct results of this lesson were the battery tests at low temperatures.

Improving the design Improvements in the design are sought in three directions: 1. Improvements on the existing design e.g. the choice and use of material for the PV panel fixation. 2. Improvements on the electronic system for example by introducing a Maximum Pow- er Point Tracker that enables the product to also be used at less sunny sites. 3. Improvements in design to allow a broader use of this product. This has consequences for the PV cell types used.

5.3 Benchmarked existing PV powered products

5.3.1 The benchmark process At this moment there are various PV powered products on the market. An important group of products consist of the so-called ‘solar recharge units’ in which the product battery is recharged by PV cells. Therefore a series of benchmark test were conducted in which the PV recharge units were benchmarked against recharging by the main electricity grid. Two examples are presented here. To benchmark the products, the following steps were undertaken: The product was used during over one year by several users to analyse the real use in various practical user contexts. After the user trial the product performance was analysed by measuring the electrical parameters. A rationale was found for the discrepancy between the specifications by the manufacture and those found in real use. Since no circuit diagram of the products was provided by the manufacturer, the products could only electronically be analysed by opening and measuring the system. To come to an understanding of how the product was supposed to work, the components were traced back to their original manufactures and their functions were determined (see Appendix G). The functions of some components with no sign or code markings at all were deduced by measurements. Since the benchmark concerns existing products the benchmark questions are: • How convenient are PV chargers in daily use?

138 Chapter 5: Test cases of mobile/wireless PV powered products

• What adjustments have to be made in habitual patterns of use in order to achieve or at least come close to the gain claimed by the producer? • What design aspects are relevant in such a product?

5.3.2 The cellular phone powered by a PV battery A user survey has been conducted at DfS/DUT on the overall product design and user context of cellular phones powered by a PV battery. Cellular phones powered by battery- PV cell combinations are to date readily available on the market (see Figure 5-7).

Figure 5-7 An example of a PV battery combination used to power a cellular phone

In these PV-batteries which were designed to replace the regular batteries in the cellular phones, the battery is directly coupled and recharged with photovoltaic cells. The preliminary observations of the test user group were: 1. Positive design features - An attempt to introduce a combined PV battery solution seems to be a good step towards the broader diffusion of PV powered products. 2. Negative design features - The PV battery is thicker than the standard one, so the phone fit less conveniently into one’s pocket. The PV battery was apparently not intended to be recharged by lamplight. 3. Points of attention - The recharging by light alone was not sufficient to operate the battery and cellular phone as such. Hence it is important that the PV battery can also be recharged by conventional means i.e. grid recharging.

The Matching Although it is unknown whether this product is designed by using either a matching model or algorithm, for the sake of uniformity in this chapter, this product’s energy matching is analysed by using the matching interfaces as presented in the Energy Matching Model of Figure 5-1 (DS4).

Matching Interface MI:1 incident light - PV cell The PV cells used were mc-Si. Looking at the Spectral Figure of Matching as presented in section 2.4.1 of Chapter 2, this would be a proper choice in case the charging was done only outdoors. Since the manufacturer claims the possibility of indoor charging, an a-Si PV cell might have been a better choice. The transparent PMA PV cover scratches quite easily, particularly in use as a PV battery attached on the bottom plate of the cellular phone.

139 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Matching Interface MI:2 PV cell - energy storage media The PV cells were connected via a blocking diode directly to the battery. The same type of Li-Ion battery, like the regular GSM battery, was used in this PV battery. Neither interface electronics such as DC/DC converters, nor any Maximum Power Point Trackers (MPPT) has been found.

Matching Interface MI:3 energy storage media - application Since the same battery type as commonly used in a conventional GSM configuration was connected to the PV cell, no additional electronics were needed to match this battery to the application.

Items outside the energy chain The PV cells used in this design are too fragile. The first sample was cracked after one month’s use and the second after about six months. No proper encapsulation has been applied. Although the push-buttons of this cellular phone can be locked, quite often an uninten- tional activation of some functions occurs in cases where the phone is placed in the upside down position, allowing the PV cells to be facing towards the light for recharging the battery. Since the phone was unintentionally consuming power during the recharge process the recharging was not efficient.

Lessons learned Direct recharging of Li-Ion batteries by photovoltaic (PV) cells is possible. To obtain a fully charged battery however, it usually took a longer time than grid recharging. For instance placing the PV-battery on the windowsill in the summer, a minimum of 12 hours is needed to charge the phone’s battery, compared to the 3 hours of typical charge time when plugged into the electricity grid. During its lifetime, the battery capacity during life-time decreases more rapidly in the solar-batteries than in the standard cellular batteries. After one year of use, a fully charged solar-battery could power a cellular phone for four days, while the standard battery recharged on the grid can power the phone up to six days. Apparently, the heating up while ‘sunbathing’ (while absorbing energy from the sun) in the windowsill was causing some capacity fading in the PV battery, as described in Chapter 3 section 3.5.1.

Ways to improve this PV-battery charger The PV cells can be made more robust by a better encapsulation. The power yield of PV cells can be improved by electronic control and a Maximum Power Point Tracker. In other words there is a need to convert this product into a real Smart PV Battery. Note that during the design of products using such Smart PV Batteries, the following items should be kept in mind: • For recharging, the PV Battery could be placed anywhere in the (sun) light. This means provisions should be taken to avoid short circuit or electrical leakage to the supporting surface or structure it is place upon. • For transportation of the PV Battery, the PV Cell must have a transparent scratch-protection cover or holder. This holder could simultaneously provide electrical insulation and light concentration.

140 Chapter 5: Test cases of mobile/wireless PV powered products

5.3.3 The universal PV charger ‘Source’ The product (see Figure 5-8) is designed with the aim of functioning as a pocket-sized solar charger for cell phones and AA batteries. But the question is whether it performs as well as presented in the advertisement [LP Electric, 2006].

a. b.

Figure 5-8 A PV battery charge system ‘source’

Preliminary observations from the test user group were: 1. Positive design features An attempt to introduce a PV battery recharge solution seems to be a good step towards the broader diffusion of PV powered products. 2. Negative design features The size of the Universal PV Charger is much bigger than that of a standard cellular phone which is to be charged. It is not convenient to carry such a charger around. The PV battery was apparently not intended to be recharged by lamplight. 3. Points of attention The recharging by light alone was barely sufficient to charge the buffer batteries and not enough to charge the cellular phone. Hence the need to recharge by conventional grid recharging still remains. The added value off this product is doubtful.

Matching Interface MI:1 incident light - PV cell The PV cells used were mc-Si ones. Looking at the Spectral Figure of Matching as presented in section 2.4.1 of Chapter 2, this would be a proper choice in case the charging was done outdoors only. Since the manufacturer claims also indoor or combined indoor/ outdoor irradiance charging a-Si might have been a better choice.

Matching Interface MI:2 PV cell - energy storage media No Maximum Power Point Tracker (MPPT) was found. The buffer batteries in this product were NiMH, which suffer of a large self-discharge. No DC/DC converter was found also no connection for grid recharge.

141 Energy Matching - Key towards the design of sustainable photovoltaic powered products

The under/upper voltage detector (see Appendix G) was only used to drive the LEDS indicating the status of the NiMH buffer batteries. The area of the PV cells was sufficient for charging the NiMH batteries in two days on the windowsill (DS6).

Matching Interface MI:3 energy storage media - application The NiMH batteries that were charged by the PV cells have to deliver energy to charge the batteries of cellular phones. The buffer NiMH battery pack and internal electronics are however not well matched with the batteries they are supposed to charge. The buffer energy storage consists of two NiMH rechargeable batteries. Fully charged, the two NiMH batteries in series deliver an output voltage of 2,4V. With the aid of an electronic circuit the output becomes 3.5 V. Li-Ion Batteries however will need a voltage range of between 2,5 V and 4,2 V in order to recharge. A mismatch occurs between the output voltage of the energy buffer of NiMH batteries and the battery to be charged since the voltage of the Li-Ion battery will increase during charging processes above 3,5 V. The charge voltage as quoted in the manual (6 V) can not be delivered by the energy buffer of NiMH batteries, therefore the charge process stops before the Li-Ion battery is even charged to 50%. In addition, the NiMH batteries demonstrate a large self-discharge (30%/month, 10% in the first 24 hours). In full sun the PV cell can deliver 7 V, which is sufficient to charge the Li-Ion batteries of the phone. For this purpose the buffer of NiMH batteries has to be bypassed. This can be done by changing the position of a switch. However in fluctuating weather conditions Li-Ion batteries are supposed to be charged by the NiMH buffer. The result of this confusing situation is that the NiMH buffer batteries alone are charged but the cellular phone battery is not properly charged.

Items outside the energy chain More effort should be put into making the outside dimensions smaller, at least smaller than a standard cellular phone.

Lessons learned The concept of having an energy buffer from which devices like cell phones and PDAs can be charged is potentially a good concept, provided that the system is designed with optimal matching in mind. For example the system has to be designed with an automatic voltage control that can cope with the differences between buffer and application storage media.The choice of energy buffer type is crucial for the success of the application.

Improving the Universal PV charger design To be able to charge Li-Ion batteries in cellular phones, the energy buffer and the electronics must be designed in such a way that it can deliver a stable voltage of 4,2 V all the time. A DC/DC converter that converts any voltage larger than 0,5V to a value of say 4,2 V, would allow even a partially discharged energy buffer with an output voltage of only 1 V to still deliver the required voltage. The DC/DC converter will stabilize the charging output at 4,2 V and would improve the energy transfer from the energy buffer to the Li- Ion battery that is to be charged. The adapter cable must be able to optimise the interface to the charging electronics of the target device to be charged. Replacing the NiMH by other types of batteries could not be done simply in this device since the whole circuit board was designed for the 2,4 V max of the NiMH.

142 Chapter 5: Test cases of mobile/wireless PV powered products

5.4 Test case studies in the framework of the SYN-Energy program

5.4.1 General Remarks As presented in Chapter 1 this dissertation is embedded within the SYN-Energy program. In this SYN-Energy research programme the use of PV and storage media combinations (PVSM) as power supplies in products is explored in several test cases. In this dissertation two test case studies that were conducted in the framework of the SYN-Energy project are designed and analysed using the energy matching model (EMM) and Figure of Match- ing algorithm. The point of departure was a brainstorm session among the team members of the SYN-Energy research group. As a result of this brainstorm session a long list of potential PV powered products was generated. To come to a proper choice this long list was reduced to a shorter list of 10 products. From this short list a wireless PV mouse was selected as a demonstration test case. The preliminary energy matching trade-offs and results of the wireless PV mouse are presented in this section. The short list has also served as a source of inspiration for master graduation projects. One of these projects will be presented in this section, namely the Solar Mobile Companion. The two cases serve as a demonstration of the Energy Matching Model (EMM) and Figure of Matching (FM) algorithm.

5.4.2 The Solar Mobile Companion The project This PV powered personal mobile companion was designed within the framework of a Master Graduation Project at the Design for Sustainability programme (DfS) [Geelen, 2006]. The point of departure was the design of a novel PV powered product that can be used as a test case and validating case for the SYN-Energy program. The objective was to design a PV powered product in which the added value of the use of PV cells was demonstrated. In addition, the design has to be an implemented example of the Energy Matching Model (EMM) and Figure of Matching (FM) algorithm as presented throughout this dissertation. The design was PV or energy source driven, since the project started from the SYN-En- ergy program short list which was the result of a search for applications with PV cell energy systems. From this SYN-Energy program short list a PDA was chosen (DS1).

Figure 5-9 The Solar Mobile Companion in use

143 Energy Matching - Key towards the design of sustainable photovoltaic powered products

The starting point of this project was a user enquiry to determine the user profile and energy use. This provided a User Context. For choosing the application the users were classified in user groups. From these user groups a certain target group was chosen and a typical product associated to that target group was found (DS2). As a result of the user enquiry a specific PDA was chosen from the various PDA configurations. This chosen product was a PDA – cellular phone combination i.e. the Solar Mobile Companion (see Figure 5-9). The name Solar Mobile Companion (SMC) is chosen since ‘Solar’ is a more generally known concept than ‘PV’.

The Matching During this project both the matching model and algorithm were available, first in a preliminary state but later as a complete set of tools. For the sake of uniformity in this chapter, this product’s energy matching is analysed by using the matching interfaces as presented in the Energy Matching Model of Figure 5-1 (DS4).

Matching interface MI:1 Incident light - PV cell Spectral Matching between the user context defined incident light and the PV cell was sought for five locations (DS3): • Indoor office (50% sunlight + 50% TLD light); • Indoor train compartment during travel (60% sunlight + 40% TLD light); • Home (30% sunlight + 35% TLD light + 35% Incandescent light); • Home in the windowsill (100% sunlight at reduced level); • Outdoors with AM 1.5 spectrum (100% sunlight at maximum level).

The results can be summarised in Table 5-1.

Table 5-1 Spectral Figure of Matching of various light source spectra - PV technology pairs corrected for their irradiance level conversion efficiency in the user context of the Solar Mobile Companion

Light Source FM outdoor with FM indoor office FM indoor train FM indoor home AM 1.5 [%] [%] [%] [%] Irradiance Irradiance Irradiance Irradiance PV cell [W/m2] [W/m2] [W/m2] [W/m2] Technology 1000 100 10 1000 100 10 1000 100 10 1000 100 10

c-Si 15 12 5 14,5 11,5 3,5 14,5 11,5 3,5 13,5 11 3,5

a-Si 8 5,5 4,6 13 9 7 12 8,5 6,5 9,5 7 5,5

mc-Si 14 11 3,4 13,5 10,5 3 13,5 10,5 3 12 9 2,5

CIS 15,5 7 1,5 15,5 7 1,5 15,5 7 1,5 14 7 1

DSC 7 5 4,2 13 9 7,5 12 8,5 6,5 9,5 7 5,5

144 Chapter 5: Test cases of mobile/wireless PV powered products

Looking at the spectral Figure of Matching down to an irradiance level of 100 W/m2, the best choice would be c-Si PV cells. At irradiance levels lower than 100 W/m2 however the best matching would be with a dye sensitised PV cell, but this cell is not yet commercially available. There is however an optimistic sign that a new enterprise named AST will start commercial production of dye sensitised PV cells by the end of 2006 [Meester, 2006]. The layout of the chosen PV cell configuration was a foldable panel (see Figure 5-10).

a. b.

Figure 5-10 The foldable PV panel in folded (a) and unfolded (b) state

In addition to a spectral matching, in the next iteration step, design effort was put to avoid unnecessary thickness of the glass substrate of c-Si PV cells in folded conditions. A mechanical matching was undertaken to reduce the thickness of the stack. Choosing roll-up and foldable options would direct the choice towards PV cells that are laminated on thin foils. Amorphous silicon cells are today available as flexible thin film sheets. The efficiency of this type of PV cell is however poor. This would initiate a third iteration step, using custom-made dye sensitised PV cells laminated on thin flexible foils. The foldable panel has to be used in various stages of folding depending on the available irradiation and user convenience. The result is that some measures have to be made to circumvent the large series resistors presented by the PV cells in the dark. For this purpose the PV cells are connected in parallel and a light sensor of the same PV cell will initiate a by-pass circuit. For charging convenience the PV panel can be detached from the SMC as can be seen in Figure 5-11.

Figure 5-11 The layout of the elements of the SMC: the main part, headphone and detached PV panel. On the ground plate are shown the click-on electrical contact points as interface between the detachable PV panel and the main part of the SMC

145 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Matching interface MI:2 PV cell - energy storage media The Solar Mobile Companion (SMC) has two batteries: a standard Li-Ion battery that is placed inside the SMC, and a Li-Ion battery as an energy buffer that is integrated within the PV panel. Li-Ion batteries were chosen because of the appropriate voltage range and the shallow charge/discharge capability of these batteries (see Chapter 3 section 3.2.3). Because of the parallel connection of the PV cells the module output voltage is insufficient to charge a Li-Ion battery. Therefore a DC/DC converter is needed. The Maximum Power Point Tracker (MPPT) is combined with the DC/DC converter (DS5).

Matching interface MI:3 energy storage media - functional application The user context was established by a user enquiry. Two user scenarios were analysed in detail resulting in the needed energy pattern. Both the battery’s capacity and PV area are tuned to the energy need in the energy balance. The PV area calculated by using the energy output formula as presented in Eq. 4-9 in Chapter 4. The minimum area was calculated for the month December as a worst case scenario (DS6).

Items outside the energy chain The PV panel with the buffer battery is made detachable to improve user convenience and to accommodate various charge locations. The hands free phoning option allows simultaneous data entry into the PDA as well as communication (DS7).

Lessons learned The Figure of Matching algorithm can be used for optimising the matching between the elements in the energy chain of PV powered products. However after the PV cell has been chosen using the incident light spectrum as a driver, more iteration steps will be needed. For example a proper mechanical matching step could be introduced. The problem of PV cells that become resistors in the shadow or dark will not be solved completely by just using parallel connected PV cells. Due to leakage currents, the output voltage of the panel will drop. There will be a need for a sensor that detects shadows or dark PV cells in order to bypass that particular PV cell.

5.4.3 The wireless PV mouse The project The point of departure was the design of a novel PV powered product that can be used as a testing case and validating case for the SYN-Energy program [Reich et.al, 2006; Veefkind et.al, 2006]. The design was PV or energy source driven since the project started from the SYN-Energy short list which was the result of a search for applications with PV cells (DS1). From potential user enquiries it became apparent that the PV cell should be integrated into the mouse and not just an add-on unit (DS2).

The Matching During this project both the Energy Matching Model and Figure of Matching algorithm were available, first in a preliminary state but later as a complete set of tools. For the sake of uniformity in this chapter, this product’s energy matching is analysed by using the matching interfaces as presented in the Energy Matching Model of Figure 5-1 (DS3).

146 Chapter 5: Test cases of mobile/wireless PV powered products

Matching interface MI:1 incident light - PV cell The Wireless PV Mouse (WPVM) will be used indoors only. For battery charging the WPVM will be put on the windowsill for ‘sunbathing’. From Spectral Figure of Matching calculations as presented in Table 2-3 in Chapter 2 it becomes apparent that the best matching was with c-Si PV cells. The drawback of c-Si is the limited flexibility of these PV cells. This contradicts with the general preference during the user enquiry session for a smooth and rounded mouse. To solve this contradiction the PV cell is put under a smooth rounded transparent cover. Placing the PV cell under a round cover however introduced the problem of irradiance reduction by reflection. To avoid malfunction by shadows two options were chosen: • One large PV cell; • Two PV cells in parallel or in series with a light detection circuit and a by-pass circuit to shut down the PV cell in the shadow.

Matching interface MI:2 PV cell - energy storage media In the first instance for simplicity no Maximum Power Point Tracker (MPPT) was envisaged. In a second iteration step however the novel MPPT as presented in section 2.5.2 of Chapter 2 could be an option. The basis of this MPTT is a fixed irradiance related to Maximum Power Points. The selection of the actual Maximum Power Point was achieved by monitoring at certain time intervals the short circuit current I as a measure of the SC irradiance. In the first stage Li-Ion batteries were chosen because of the appropriate voltage range of the electronics inside the mouse and the shallow charge/discharge capability of these batteries (see Chapter 3 section 3.2.3). However since Li-Ion batteries suffer from capacity fading at elevated temperatures above 45 °C (see section 3.5.1) and it was estimated that during ‘sunbathing’ the temperature will surpass that 45 °C limit, other less optimal batteries were taken such as NiMH. NiMH batteries suffer from memory effect, an unknown capacity fading at elevated temperatures. They also have a large self-discharge. On the other hand the DC/DC converter has to bridge a smaller voltage gap since the existing mouse electronics were designed to be powered by a 1,2 V battery. By using the existing DC/DC converter in the wireless mouse the voltage level of the electronics inside the mouse can be attained (DS4/DS5).

Matching interface MI:3 energy storage media - application The Wireless PV Mouse was designed by using the existing electronic circuits of a wireless Microsoft mouse. The capacity of the battery was calculated for an active mouse life of 3 days. The areas of the PV cells were calculated for a culminated charge time of 3 hours (DS6).

Items outside the energy chain These design aspects will be dealt in another dissertation [Veefkind, forthcoming, 2007].

Preliminary results A DC/DC converter that boosts the output voltage of a single PV cell from 0,4 V up to 1,2 V has been successfully tested. To match the processor the voltage was later altered to 2.4 V.

147 Energy Matching - Key towards the design of sustainable photovoltaic powered products

5.5 Résumé of design steps

Table 5-2 presents a comparison of the taken design steps (DS) in all the reported test cases in the preceding sections 5.2 through 5.4. The design steps were introduced in section 5.1. The numbers in the table correspond to those in the design step list in section 5.1. In Table 5-2 it can be seen how gradually more design steps are taken into account in correspondence with the progress of defining the Energy Matching Model and the Figure of Matching algorithm during this PhD project. DS-1: All the products are clearly PV driven. DS-2a: Although it can be assumed that the user profile was determined in the benchmarked products, there was no certainty to what extent this was done, and therefore the score is ‘Unknown’. DS-2b: It is unknown to what extent the energy pattern of use in the application is analysed in the benchmarked products. DS-2c: In all the products the application was defined, as a result the energy use was determined. DS-3 + 4a + 4b: Due to the found mismatch between incident light and PV cells in the benchmarked products, it is unknown whether there was any determination of the user context defined incident light, or any matching PV characteristics. DS-4c: In determining the matching between the PV cell and the energy storage, a distinction must be drawn between the energy storage of the application and the energy storage that serves as a buffer between the PV cells and the energy storage of the application. In the Universal PV charger the buffer was sufficiently charged but the application not. DS-5: Only in the products designed within the framework of the SYN-Energy program was full attention paid to obtain an overall matching of issues such as energy, ergonomics and life cycle assessments. DS-6: Although it can be assumed that the energy balance was determined in the benchmarked products, there was no certainty as to what extent this was done and therefore the score is ‘Unknown’. DS-7: In all of the products presented in Table 5-2, attention was paid to other items than just the energy chain. In the benchmarked products however it concerns only appearance therefore these two products scored as having ‘No’ additional matching.

148 Chapter 5: Test cases of mobile/wireless PV powered products

Table 5-2 Overview of the design steps in projects and benchmarked products

Projects Master projects Benchmarked SYN-Energy products demonstration cases (5.2) (5.3) (5.4)

Design steps Backpack PV battery charger: Solar Tergo Solar Rudy and the pupil localizer Cell phone PV battery Universal PV charger Solar Mobile Companion Wireless PV mouse

DS-1: PV driven? YYYYYY

DS-2a: User profile determined? YYUUYY

DS-2b: Energy pattern of use in YYUUYY application analysed?

DS-2c: User contexts defined YYYYYY energy use determined?

DS-3: User context defined NY UUY Y incident light determined?

DS-4a: Light-PV matching NY NNY Y

DS-4b: PV characteristics NY UUY Y

DS-4c: PV – energy storage YYYNYY matching

DS-4d: Electrical storage media Y Y NNY Y characteristics

DS-4e: Storage - energy use NY Y NY Y matching

DS-5: Overall matching NNNNY Y

DS-6: Energy balance YYUUYY

DS-7: Additional matching YYNNYY

Y = Yes; N = No; U = Unknown

149 Energy Matching - Key towards the design of sustainable photovoltaic powered products

5.6 Conclusions

The differences between Standard Test Conditions (STC) at PV specifications and practical conditions of PV cells/modules utilization will need special attention in designing PV powered products. The degradation in PV performance such as efficiency with diminishing illumination level is in practical cases usually larger than logarithmic (see section 2.3.3). In combining rechargeable batteries with PV rechargers, beware of the occurrence of the memory effect (notorious in this respect are NiCd batteries) and large self-discharge (NiMH). Special attention must be paid to the optical attenuation of the light by the cover cap of the PV cells.

This chapter has addressed fourth part of the research question namely:

What insight can be gained by analyses of the energy matching in test cases of PV powered products?

The answers found are concluded as below. In the drive towards sustainable designed PV powered products one has to learn from the failures in existing products. Therefore quantifying the mismatches is the first step in the search for ways to improve matching in the energy chain. This can be done with the aid of the Figure of Matching algorithm. PV recharging - energy storage combinations provide promising solutions to cope with the trend towards increasing mobility and mains independency in consumer and professional products. The Energy Matching Model (EMM) and the Figure of Matching (FM) algorithm can contribute towards a more sustainable designed PV powered product, provided a systematic combination selection between the elements has been conducted. One of the main obstacles for achieving proper matching is the lack of standardization of the power supplies. In practice batteries can not deliver their entire specified capacity in Ah. The temperature in the user context will be an important design parameter. From the case study of the PV wireless mouse it appears that the flexibility/bent ability of the PV cell will quite often be an important design parameter and could also be an important item for future research. In addition the possibility to deposit directly PV cells on curved surfaces should be an important issue for future research. The gained energy improvements by optimal matching can stimulate designers to endeavour the other design aspects, beside the energy chain, as well.

150 Chapter 6: Conclusions and recommendations

6.1 Conclusions

6.1.1 General considerations for the conclusions The conclusions made are valid for those items investigated: 1. Only mobile/wireless products have been investigated. 2. Only the energy chain of PV powered products has been analysed. No other power supplies such as fuel cell and human power have been investigated in detail. 3. In this energy chain of PV powered products only certain elements and interfaces are analysed in detail. These are the PV power converter, energy storage, the matching interfaces between incident light and PV, PV and energy storage, and energy storage and application. The other elements and interfaces presented in the energy matching model of the energy chain such as the user context defined parameters, how the energy is used inside the product application and the environmental parameters are all treated in this dissertation as given exogenous parameters. To be able to perform energy balance calculations for example, the data of these exogenous parameters are gathered from literature, estimations of test cases and other joint measurements within the SYN-Energy program. 4. Detailed analysis of the energy conversion process inside the PV power converter will be regarded in this dissertation as exogenous. These detailed analyses and modelling of elements and parameters will be treated in other research programs and dissertations within the SYN-Energy program [Reich, forthcoming, 2008].

6.1.2 Research question, Energy Matching Model and Figure of Matching algorithm In this dissertation the following research question was posed:

What systematic matching can be achieved between the elements and interfaces of the energy chain of photovoltaic powered mobile/wireless products?

To answer the research question a novel energy matching model (EMM) of the energy chain of PV powered products and a generic Figure of Matching (FM) algorithm have been developed and introduced. It was further demonstrated that this generic Figure of Match- ing can be made specific for a certain matching interface in the energy chain, by defining both the type of power to be transferred and the applicable variable at that particular interface. The basic assumption made for this Figure of Matching algorithm is that the

151 Energy Matching - Key towards the design of sustainable photovoltaic powered products stimulus - response power transfer concept, as an analytical tool, can be extended and generalized with the aid of the general theory of information transmission. The rationale behind this assumption is that information transmission or transfer of both by cable or wireless is in fact some kind of power transfer. The validity of the Figure of Matching algorithm is in accordance with the research question demonstrated for a limited number of matching interfaces. Those three interfaces are between the incident light and the PV cells, between the PV cells and the batteries and between the batteries and the application. So the results are representative for this part of the energy chain only. Since this energy matching model and this Figure of Matching algorithm enable the designer to conduct a proper analysis and quantification of the matching at an interface this model and algorithm can contribute towards a more mature design of sustainable PV powered products. As a résumé of the conclusions in the preceding chapters the following answers were found for the research question: • The energy transfer in the energy chain can be optimised by selecting the proper combination of chain elements with the aid of the Figure of Matching algorithm. For example optimisation of the power output from the photovoltaic cells is possible by proper choice of photovoltaic cell type, taking into account the spectrum and irradiance level of the incident light. As a result the best matched pair: user context defined incident light - PV cells can be found. For example a-Si and DSC PV cells match well with an indoor office (TLD lamplight only) user context, while c-Si, mc-Si and CIS PV cells match well with bright sunlit outdoor user contexts. As a special case high efficiency c-Si can match both outdoors and indoors since at low irradiance levels (less than 100 W/m2) this technology is still performing well with efficiencies comparable with a-Si and DSC PV cells at 1000W/m2. The conversion efficiency of PV cells depends on the spectrum of the incident light. For proper user context matching this spectral dependent efficiency has to be taken into account. Therefore it is proposed that data sheets of PV cells present spectral responses of the PV cells. Efficiency alone is not always the only important parameter. Mechanical properties of PV cells such as flexibility could play an important role in the case that the design requires a curved surface. In this case efficiency comes second, since less-brittle bendable PV cells with lower efficiencies are preferred above brittle high efficiency ones. • By using precognition of the PV type dependent Maximum Power Point (MPP) values the MPP tracking can be simplified. Look-up tables measured on PV samples will provide information on how the MPP varies with irradiance level. In addition the instantaneous short circuit current I values of these samples can be used as an irradi- sc ance level indicator. This MPP precognition and I irradiance level indicator could be sc the base for a novel Maximum Power Point Tracking method. Measures to reduce cost and complexity will improve matching by reducing duplicitous elements in the combination and by standardisation. • PV recharging - energy storage combinations can, due to their ‘delay’ and ‘time integration’ capability, provide promising solutions to cope with the trend towards increasing mobility and main electrical grid independency in consumer and professional products. Therefore the main gain of introducing the concept of PV powered products combined with an energy storage medium is the increased freedom in space and time for the customer.

152 Chapter 6: Conclusions and recommendations

• In mobile/wireless PV powered product Li-Ion batteries and their family will, due to the shallow charge/discharge capability and lack of memory effect, provide the best match in combination with PV cells that deliver power whenever the cells are illuminated.

6.1.3 Interface related conclusions The Standard Test Conditions (STC) for PV cells do not provide good selection criteria for designing PV powered mobile wireless products since: • A PV cell with a high efficiency at Standard Test Conditions (STC) of 1000 W/m2, will at lower irradiance levels not automatically maintain its higher efficiency. • By measurement it was found that although the efficiency at STC was the same, at low irradiance levels (1W/m2) a factor two difference in efficiency was found in between cells provided by one single supplier.

Therefore it is proposed that data sheets of PV cells should also provide information about efficiencies at low irradiance levels existing in an indoor user context. A parallel interconnection of PV cells in combination with DC/DC converters for the appropriate load voltage will be favourable to PV cells in series for mobile/wireless products since this configuration will be less sensitive to shadows. An additional design condition however should be implemented in this case to prevent current leakage through the cells in the shadow by introducing a selective bypass of those cells. Both during power input and output, the battery can benefit from matching improvement achieved by combining batteries and capacitors. In addition, this combined power supply in PV powered products results in a greater reliability and longer battery lifetime.

6.1.4 Spin-offs to other interfaces outside the energy chain For a multi-dimensional matching model the Figure of Matching algorithm has to be adapted to applications beyond power correlation as for instance mechanical shape correlation. Mechanical matching improvement can be achieved by proper selection of materials and the utilisation of novel technologies such as flexible flat batteries and flexible PV cells. Some novel concepts and options of integrating PV cells directly on curved structural elements of consumer products have been introduced. These application examples presented in test cases demonstrate clearly the design potential that could be obtained by integrating solar cells on some structural elements of the products. The energy improvements gained by optimal matching can stimulate designers to address the other design aspects beyond those of the energy chain. This integrated approach also provides some incentive for innovative industrial design.

6.2 Design approach and guidelines

The design approach and guidelines presented in this section are based on the findings of this PhD thesis only, with its specific focus and limitations. They are preliminary, since in the framework of the SYN-Energy program an overall design approach for PV powered products is under development. An overview is presented in Figure 6-1.

153 Energy Matching - Key towards the design of sustainable photovoltaic powered products

6.2.1 General approach 1. Specify the design assignment and user context The designer starts by specifying the user context in which the new design is to be used. This includes determination of the user profile, pattern of use and the added values aimed for by using a certain technology (e.g. photovoltaics). 2. Optimise cost and complexity Reducing of duplicitous elements in the combination and standardisation will improve matching and even produce synergy. Therefore this guideline will reduce cost and complexity. 3. Reduce product energy consumption From LCA calculations for mobile consumer products, it becomes apparent that energy consumption is one of the major factors for determination the product’s sustainability [Alsema, 2004; Alsema et. al., 2005]. Efficiency improvement of energy consumption will enhance the feasibility of PV application and directly support its sustainability. 4. Optimise PV versus product lifetime The discrepancies found between the economical end of life of products and the technical end of life of PV cells and energy storage components give rise to a need for practical solutions. A guideline would be to integrate the PV cells in such a way that a second life of these cells in another product is enabled. 5. Iterate the energy and mechanical sub-system matching For mobile and wireless products size, weight and shape are important design criteria in view of the user – product interaction. As a result after energy matching has been established by choosing the proper combination between the elements in the new product, the next step will be embodiment matching. This step can be divided into several matching activities such as proper positioning of the elements in the product and trade-offs made with respect to the shape, size and weight. Proper positioning can be illustrated by a design approach that suggests that the embodiment should neither impair the proper working of the PV cells nor the energy storage media. For example, PV cells should be facing towards the incident light. A trade-off has to be made between the energy matching and embodiment matching in which the most efficient PV cell can be exchanged for a less efficient PV cell but mechani- cally better matched one. To ease the trade-off, a guideline is to give weight factors to the high efficiency requirement and the mechanical properties. As criterion: what is a real must for the user and what is of secondary importance. In designing a new product the use of the guidelines mentioned above will require some iteration steps after the initial optimal matching of the energy chain. With each iteration step the design is improved further with respect to the user and energy aspects. The first iteration step will be needed to eliminate contradictory matching requirements. For instance the optimal user context defined irradiance - PV cell combination has been found with the aid of the Figure of Matching algorithm. In the materialization stage however it turns out that the chosen PV cell is only available with a rigid glass substrate encapsulation. However, the design of the new product requires that the surface of this product needs to be curved. The second best match than from an efficiency point of view, for example, could be the optimal choice instead of the former one. So in this case the first iteration step will be needed to obtain the optimal mechanical match. In the next iteration step the other design parameters are matched. For example the

154 Chapter 6: Conclusions and recommendations design is matched to the temperature range of the elements used. This matching can for instance be achieved by the placement of the battery with respect to avoiding hot spots and providing proper cooling. Potential user involvement should play an important role in directing the iteration steps. More iteration steps will be needed to obtain the optimal user interface match. Each time a new iteration step is undertaken and another parameter is added to the design an inspection is needed to search for contradictory matching requirements. To speed up the design steps some iteration steps can be done in parallel resulting in a multi-dimensional matching.

6.2.2 Energy Matching A. General guidelines (procedure) 1. Benchmark with similar PV powered products In finding ways to design a new product a powerful and logical design strategy would be to learn from the successes and failures in similar existing products by analysis of the interface and quantisation of the matches and mismatches between the elements in these products. 2. Apply the Energy Matching Model (EMM) and the Figure of Matching (FM) algorithm In the concept phase of the new product one of the main concerns will be the determination of concept feasibility. The design approach for this determination will be the identification, analysis and quantification of matching between the elements and interfaces of the concept. The analysis and quantification of this matching can be executed with the aid of the Figure of Matching algorithm. One possible improvement for a new design could be the optimal matching in the energy chain by selecting the proper combination of chain elements with the aid of the Figure of Matching algorithm. 3. Interface guidelines a. Beware of the Standard Test Conditions (STC) limitations: • PV cells with a high efficiency at STC will not automatically maintain its higher efficiency at lower irradiance levels. • Although the efficiency at STC was the same, at low irradiance levels (1W/m2) a factor two difference in efficiency was found in between cells provided by one single supplier. b. Apply the FM algorithm to the incident light - PV interface: For proper user context matching this spectral dependent efficiency has to be taken into account. c. Apply the FM algorithm to the PV- energy storage interface. d. Apply the FM algorithm to the energy storage - application interface. e. Determine the energy balance by using the PowerQuest tool.

B. Specific design guidelines (technical) 1. Incident Light - PV matching a. Indoor (TLD light): use a-Si and DSC PV cells. b. Outdoor (bright sunlit): use c-Si, mc-Si and CIS PV cells. c. Indoor/outdoor (flat surface): use high efficiency c-Si PV cells. d. Use intelligent PV panel constructions.

155 Energy Matching - Key towards the design of sustainable photovoltaic powered products

2. PV – energy storage matching a. Use mainly parallel connected PV cells + by-pass mechanisms. b. Use smart energy storage recharge systems. c. Apply Li-Ion batteries and their family. d. Apply smart capacitor – battery combinations.

3. Energy storage – application matching The addition of capacitors at the output side can improve the surge capacity and the low temperature performance of the battery. A simple design strategy would be to use ‘always’ capacitors. This measure will however be at the expense of larger weight and volume and should therefore be tuned to the application. A more appropriate design strategy will be that if no large currents are needed by the application or no large inrush currents or low temperature (< 0 °C) applications are to be expected then the additional capacitor will not be an advantage. Some possible ways to alleviate this could be obtained by the use of super capacitors that will reduce the volume and weight. There will be a break-even-point reached in which it will be no more advantageous to have the complicated construction of combining batteries with capacitors. In this case the technological performance improvements of the batteries will have counterbalanced the additional complication in electronic circuitry, volume, weight and cost of the above combination.

4. Overall guidelines a. Exploit the time delay matching by using PV – energy storage media combinations. b. Use flexible PV cells at curved surfaces. However curved PV cells are to be used if and only if this feature introduces a new added value.

6.2.3 Mechanical and physical design aspects 1. Optimise the embodiment: a. Select proper materials. b. Use flexible flat batteries. c. Use flexible PV cells if necessary. d. Make trade-off of the PV efficiency of flat versus curved surfaces 2. Thermal matching should be taken into account. The negative effects of elevated temperature on PV cells and energy storage media should be minimised by design, e.g. by cooling or by avoiding hot locations for battery positioning in the product. 3. Avoid moisture in the elements of the energy chain by proper encapsulation.

The whole approach and guidelines are presented in Figure 2-1.

156 Chapter 6: Conclusions and recommendations

Figure 6-1 Approach and guidelines for the design of PV powered products. (*) In the scope of this PhD study; (**) Partly in the scope of this PhD study but not systematically analysed; (***) Outside the scope of this PhD study, part of the ongoing and follow-up studies within the SYN- Energy program.

6.3 Recommendations for future research

6.3.1 Recommendations for the PV and battery suppliers The information provided in data sheets on PV cells and batteries is in most cases insufficient to perform Figure of Matching calculations in the energy chain of PV powered products. To stimulate a better assessment for the designer it is therefore recommended to extend the information found on these data sheets. For the PV data sheets more extensive information is needed than the efficiency at Stan- dard Test Conditions (STC) alone. For proper Figure of Matching calculations the spectral response (SR), the fill factor (FF), the open circuit voltage Voc and the short circuit current Isc would be needed. In addition, data sheets of PV cells should also provide information on efficiencies at low irradiance levels that exist in an indoor user context including the variance between the samples. For the battery the data sheets should provide the power input and output responses. It would be most recommendable to have a periodically updated list of battery performance parameters like the PV efficiency tables to be published in an international journal. Lack of standardization of the power supplies (e.g. voltages, connectors) is the main obsta- cle for achieving proper matching. For a proper energy design of sustainable PV powered products, international standardisation is recommended.

157 Energy Matching - Key towards the design of sustainable photovoltaic powered products

6.3.2 Recommendations for the product designers and manufacturers and general research User context parameters such as ambient temperature and the temperature and the moisture inside the product have been identified as having influence on the performance of PV cells and batteries. In the framework of design optimization it would be recommendable to investigate these environmental influences further. In this dissertation the use of the Figure of Matching algorithm is demonstrated at three interfaces in the energy chain of PV powered products. In accordance with the design approach intent on including embodiment, environmental and user context matching in a multi-dimensional matching it would be recommendable to investigate how the Figure of Matching algorithm can be used to analyse and quantify this type of matching. The design approaches identified need further evaluation and extension beyond the energy chain. In the framework of the SYN-Energy program this is done for PV powered products. It would be recommendable to extend these approaches also to other power systems. It was found that the spectral Figure of Matching for the combination of TLD lamplight and Dye Sensitised PV cells is optimal. It would be recommendable therefore to further explore this optimally matched pair in novel product applications. An example could be the use of this combination to power Ambient Intelligence micro-sensors, too small and too numerous in covering entire buildings, to be powered by the mains grid. During the design of the PV wireless mouse it became apparent that contradictory specifications such as sunbathing for charging the battery and battery capacity fading by elevated temperatures requires further exploration to be resolved. As sunbathing of products with integrated PV cell – Li-Ion battery technologies heat up they risk performance degradation of the battery such as capacity fading. A more practical solution would be to separate means of energy conversion and of energy storage. An example of this solution could be the air cooled PV façade with a DC net to one central energy storage and recharge point located in the cooler shade. During the design of the PV wireless mouse the importance of the optical interface in the cover of the PV cell become apparent. This applies in particular to the attenuation of the light by the cover. So it would be recommendable for future research to investigate this interface in more detail. Also recommendable is to include in this future research topics like: direct deposition of PV cells on the cover, flexible PV cells and light concentrators integrated in the cover like the ones under investigation by DARPA USA [Kirkpatrick, 2006]. Still more research is needed to find a practical solution for the end of life discrepancies between PV cells and energy storage components. The recommended design optimization research efforts can be dealt with in the ongoing SYN-Energy program but can also be part of the activities during a possible follow-up project of this SYN-Energy program.

6.3.3 Recommendations for fundamental research Additional recommendations are presented which are the result of problems and possible opportunities identified that were encountered during research for this dissertation.

158 Chapter 6: Conclusions and recommendations

A. To explore further applications of the PV battery hybrids The original idea The SYN-Energy program was initiated with the design of the PV battery i.e. a direct cou- pling between a battery and a photovoltaic (PV) cell. This can for instance be the combination or integration of an inorganic silicon PV cell and a Li ion Battery [Kan, 2000] or an integrated organic molecular PV Cell and a Li-ion battery [Schoonman, 1998]. In a completely integrated version, one side of the PV cell is used simultaneously as an electrode of the battery. In other words, there will be a direct electrical and electrochemical contact at one PV cell electrode, which can function at the same time as a battery electrode. Up to now in Li ion batteries, quite often the anode is made of graphite or carbon alloy compounds. By a fortunate coincidence, carbon is in the Periodic System in the same column as silicon. Silicon is well known as basic material for electronic chips and is used in the silicon PV cell. Therefore the basic idea behind a possible solution of this electrical and electrochemical interface matching challenge is based on the similarity between carbon and silicon. In other words, it is a logical step in predictive analysis to suggest silicon or alloys like silicon carbide to be used as anode material instead of carbon [Kan, 2002a]. The use of silicon or silicon alloy anodes would also open the way to directly integrate electronic intelligence into the battery. This intelligence is needed to assure safe use and make possible the integration of a smart battery charger. The result is a ‘Smart PV Battery’. In the original idea the PV battery application was a flat card. The integrated silicon PV cell - battery units are shaped to suit ergonomics in small flat Credit Card like cartridges [Kan, 2002]. This solar battery card can be put on the windowsill or be placed under lamplight to recharge the battery. The size is in such a format that it can easily be taken along as a spare battery.

New Development The potential of this PV battery concept in product applications is illustrated by the new developments worldwide in the field of hybrid battery solutions. The idea of using silicon electrodes has been adopted worldwide. Examples are the use of amorphous silicon anode for a Lithium Ion rechargeable battery [Lee et. al, 2004], Si/C nano-composite anodes [Kim, 2004] and CaSi2 anodes [Wolfenstine, 2003]. Another hybrid is represented by the development of the photo capacitor which combines a dye sensitized PV cell (DSC) with a super-capacitor in one device [Miyasaka and Murakami, 2004]. The photo-generated charges are stored at the electric double layer of the capacitor. In view of the use of capacitors as a buffer to improve the transfer efficiency from PV cells into batteries, this device could be a potential candidate. To enhance the energy density of super capacitors, new concepts have emerged in which one of the electrodes of the capacitor is transformed in a battery-type electrode. These combinations are usually referred to as an ‘asymmetrical’ device [Pell and Conway, 2004].

B. Combinations of PV and other energy conversion systems The concept of synergy seems to be not restricted to PV powered products but can be applied to similar power systems such as human powered products as well e.g. power generating revolving doors, piezo-energy generation by typing and walking and the steel chain as energy generator and electrical motor. At this moment various products are on

159 Energy Matching - Key towards the design of sustainable photovoltaic powered products the market in which the energy system consists of a hybrid solution, for example PV + human power. Most of these designs however are ad-hoc solutions and therefore a general systematic research approach on the design of such hybrid energy systems and the generation of design strategies would be advisable.

C. Intelligent PV panel constructions In the Solar Mobile Companion a novel smart PV panel was developed which is foldable and by intelligence bypasses those PV cells that are in the shadow or covered by the folding. To come to a general design strategy it would be recommendable to explore the feasibility of a systematic approach for ‘Intelligent PV panel constructions’.

D. PowerQuest PowerQuest was demonstrated to be of use in simulations during the design of PV powered products. So the next step would be to refine the parameters in this tool to include for instance, irradiance level dependent spectral Figure of Matching. Also the matching at other interfaces besides spectral matching should be included in a future PowerQuest tool.

E. Energy/Power management and thermal control tools In calculating the energy balance in this dissertation the energy use is estimated from that of similar products. In these estimations no further progress in efficient energy consumption and energy savings are taken into account. Through the use of either novel energy/ power management tools or thermal control tool or both, it is expected that the energy consumption will become more efficient in the coming years. Since more efficient use of energy would extend the range of feasible PV powered products, an investigation of the possible positive contributions of such energy/power management and thermal control tools in the framework of design optimization is recommended.

160 Summary

Energy Matching Key concept for the design of sustainable photovoltaic powered products

PhD Thesis by Sioe Yao Kan, December the 19th 2006

This dissertation will present how, why and under what circumstances a novel Energy Matching Model and associated Figure of Matching algorithm can support industrial designers in developing sustainable photovoltaic powered products. This PhD research project is executed within the framework of the SYN-Energy program which explores the feasibility of a transition towards the use of photovoltaic (PV) cells in consumer and professional products. This program is a part of the ‘Energy Research Stimulation Program’ of the Neth- erlands Organisation for Scientific Research (NWO).

The starting point of this dissertation is the observation that today a vast amount of ‘PV powered’ products are already on the market. However, in these PV powered products quite often the choice of PV cells seems random and PV cells function mostly as add-on units to give the product a ‘green’ energy image. Because of this ‘add-on’ approach, the PV cells remain foreign bodies which are not well-integrated into the total product design. As a result in today’s PV powered products often a sub-optimal matching between the PV cell characteristics, energy storage and the product user contexts. Therefore, the keyword to obtain a mature and sustainably designed PV powered product will be matching.

As a result the research question is formulated as:

What systematic matching can be achieved between the elements and interfaces of the energy chain of photovoltaic powered mobile/wireless products?

The emphasis of this dissertation will be on the optimal matching in the energy chain of PV powered products. For this purpose, an Energy Matching Model of the energy chain is developed. The scope of this dissertation is limited to the PV power converter and the electrical energy storage media. All the other elements and interfaces in the energy chain such as the user context defined parameters, energy use inside the product application and environmental parameters will be treated as given exogenous parameters. To quantify how well the matching at the matching interfaces MI:1 through 3 will be, a Figure of Matching algorithm is developed and presented. The higher this ‘Figure of Matching’ the better the matching.

161 Energy Matching - Key towards the design of sustainable photovoltaic powered products

In Chapter 2 the PV power converter is analysed. In particular the analysis will focus on matching the interfaces between this PV converter with both the incident light flux and the energy storage media. For this purpose the spectral Figure of Matching is calculated. The link between this spectral Figure of Matching and the more familiar PV conversion efficiency is determined by the introduction of the power based spectral response SRP . PV It is shown that the spectral Figure of Matching is identical to the conversion efficiency.

In Chapter 3, the storage media are the central part of the energy chain. Therefore, the analysis of the matching between this element and the other two can be divided along the two sides of the storage media. This is on the side of the input - a matching that enables to store as much as possible energy coming from the PV cells into the storage media. On the side of the output matching is achieved that enables the use of the available stored energy as efficient as possible. The Figure of Matching can be determined by calculating the ratio between the areas under the power correlation curve and the PV power output curve.

The Figure of Matching for the battery output can be calculated at a certain temperature. The correlation between the measured battery output voltage and the application voltage demand can be determined by calculating the area enclosed above the tolerance margin line and the battery output voltage in time. The Figure of Matching of this matching interface MI:3 can be determined by calculating the ratio between the correlated battery output voltage area and the area enclosed by the voltage demand curve and the tolerance margins lines of respectively 5% or 10%. With a 5% tolerance window only the + 20 °C pulses have some correlation. The Figure of Matching of these pulses at + 20 °C is about 50%. The - 20 °C pulses have no correlation with the 5% tolerance window. Therefore, the related Figure of Matching is zero. In addition, it has been shown that the output can be improved by the combining of capacitors and batteries.

Chapter 4 presents the energy balance between available energy, energy demand as well as a - specially developed- simulation tool for selecting both products and energy converters namely PowerQuest. With the aid of the spectral Figure of Matching, the spectrally dependent converted power of the PV cell with an area of A m2 at a certain irradiance PV G can be calculated Or in the opposite case in which the from application demanded n power is known the needed PV area can be calculated.

In Chapter 5 a number of benchmarks and demonstration cases were analysed with the aid of the Figure of Matching algorithm. As a result of this analysis the Energy Matching Model was developed and the generic Figure of Matching algorithm was fine tuned.

In Chapter 6 a roundup of the research is presented by summing up the main conclusions and recommendations for further research.

The main outcome of this dissertation is an Energy Matching Model and a related Figure of Matching algorithm that is used to analyse and quantify the matching. As a result of the analysis, several matching improvements and design approaches and guidelines were found to facilitate the role of the industrial designer in designing sustainable PV powered products.

162 Samenvatting

Energie Matching Sleutelconcept voor het ontwerpen van duurzame fotovoltaisch gevoede producten

Proefschrift van Sioe Yao Kan, December the 19th 2006

Dit promotieonderzoek is uitgevoerd in het kader van het Syn-Energy programma dat onderdeel is van het duurzame energie stimulerings programma van de Nederlandse Or- ganisatie voor Wetenschappelijk Onderzoek (NWO). In dit proefschrift worden de condities vermeld waaronder een nieuw ontwikkeld Energy Matching Model en daarmee gerelateerd Figure of Matching algorithme kunnen worden toegepast bij het ontwerpen van fotovoltaisch (PV) gevoede producten.

Beginpunt van dit onderzoek is de constatering dat er tegenwoordig reeds een groot aantal PV gevoede producten op de markt is. Echter, bij tal van deze producten is de keuze van de PV-cellen willekeurig en wordt PV-technologie vooral vaak toegevoegd om het product een groene uitstraling te geven. Door dit willekeurig toevoegen van PV cellen zijn deze technisch veelal niet optimaal afgestemd op de toepassing van deze producten en het gebruik door de consument. Het sleutelwoord is dus energie matching.

Dit proefschrift beperkt zich tot de energieketen in mobiele PV-gevoede producten. De volgende onderzoekvraag staat centraal:

Welke systematische matching is te bereiken tussen de elementen in de energieketen van mobiele draadloze PV gevoede producten?

De nadruk ligt dus op het optimaal afstemmen van elementen in de energieketen. Om dit doel te bereiken is een Energie Matching Model van de keten ontwikkeld. De analyse in dit proefschrift beperkt zich tot de elementen: PV-cellen en energie- opslagmedia. An- dere elementen, zoals de gebruikers context, worden als exogeen beschouwd en daarom slechts zijdelings genoemd.

Om de afstemming tussen de elementen te analyseren en te kwantificeren zijn interfaces gedefinieerd: MI:1 tot en met MI:3. Tevens is een Figure of Matching algoritme ontwikkeld. Het getal als uitkomst van de Figure of Matching geeft aan welke mate van matching is bereikt: hoe hoger hoe beter.

163 Energy Matching - Key towards the design of sustainable photovoltaic powered products

In hoofdstuk 2 wordt de energie-overdracht tussen inkomend licht en de PV-cel geanalyseerd. Hierbij is de spectrale Figure of Matching berekend. Het verband tussen de spectrale Figure of Matching en de meer bekende PV-conversie efficientie wordt verkregen door de introductie van een op vermogen gebaseerde spectrale respons SRP . PV Ook wordt de afstemming tussen PV-cel en de opslagmedia van elektrische energie worden in dit hoofstuk geoptimaliseerd.

In hoofdstuk 3 komen de elektrische opslagmedia aan de orde. Het betreft hier batterijen en condensatoren. De matching tussen deze energie-opslag-elementen en de andere twee elementen in de energieketen, namelijk de PV-cel en het gebruik van deze energie, is geanalyseerd. Doel van de analyse is om aan de inputzijde van het systeem een zodanige matching te bewerkstelligen dat zo veel mogelijk energie van de PV-cel in de opslagmedia terecht komt. Tevens wordt onderzocht hoe aan de outputzijde van het systeem een zodanige matching kan worden bereikt dat zo veel mogelijk van de opgeslagen energie voor het gebruik van het apparaat beschikbaar komt. De Figure of Matching kan worden bepaald door de ratio te berekenen tussen het oppervlak onder de uitstroomkromme van de PV-cel en dat van de instroomkromme van de batterij te correleren.

Omdat bij mobiele/draadloze apparaten de vermogensvraag meestal gepulst komt, worden de Figure of Matching waarden hier berekend met de puls respons van batterijen. Dit kan bijvoorbeeld bij verschillende temperaturen zoals +20 °C en - 20 °C. Door twee tolerantiegrenzen te benoemen, resp. van 5% en 10%, kan men een gebied definieren waarbinnen deze batterij nog functioneert. De batterijspanning vertoont alleen voor de 5% tolerantie bij +20 °C nog een correlatie. De Figure of Matching is hierbij 50%. Bij - 20 °C is er geen correlatie met de 5% tolerantie, zodat de Figure of Matching nul is.

In hoofdstuk 4 wordt de energiebalans berekend door de beschikbare energie te vergeli- jken met de vraag. Hierbij worden simulaties met - het speciaal voor dit doel ontwikkelde computerprogramma - PowerQuest uitgevoerd en kan met de spectrale Figure of Match- ing het benodigde PV-vermogen worden berekend. Of, in de omgekeerde richting, als de energievraag bekend is, kan de benodigde PV-oppervlakte worden berekend.

In hoofdstuk 5 wordt een aantal demonstratievoorbeelden geanalyseerd met behulp van het Figure of Matching algorithme en kan als resultaat het Energie Matching Model worden verfijnd.

Tot slot wordt in hoofdstuk 6 een opsomming gegeven van de conclusies van het onderzoek en aanbevelingen voor verdere studie. De belangrijkste uitkomsten van dit proefschrift zijn een Energie Matching Model en een gerelateerd Figure of Matching algorithme die kunnen worden gebruikt voor analyse en ontwerpverbeteringen van de interfaces tussen de elementen van een PV-product alsmede ontwerpregels voor duurzame - op PV gebaseerde - productontwikkeling.

164 Epilogue

By now you might have read about PV or solar powered products but you are still con- fused what it is all about? As a consolation here are some anecdotes to finish.

As an acquaintance happily noticed that the received relation gift: ‘a solar powered laundry drier’ was in fact a clothesline and some clothespins. A remark of a Portuguese PhD student the other day that Mediterranean people are ‘solar powered’ since in winter here in the Netherlands they have to fight sleep and quite often fall back in a kind of ‘stand-by mode’.

After completion of this dissertation my personal battery is due for PV powered recharge.

Thank you for reading the summary.

Sioe Yao Kan, November 2006

165 Reference List

Alsema E.A. and Turkenburg W.C., 1989; Winning van Energie met Zonnecellen; een overzicht, PEO report no. NSS-86-30.

Alsema E.A., 2004; Spreadsheet evaluation tool for energy balance and LCA of solar products, SYN-Energy Memo on LCA aspects of solar products.

Alsema E.A., Reich N.H., Sark W.G.J.H.M van, Kan S.Y., Silvester S., Veefkind M.,Elzen B. and Jelsma J, 2005a; Towards an optimized Design method for PV-powered consumer and professional applications – The SYN- Energy project, Proceedings of the 20th European Photovoltaic Solar Energy Conference and exhibition, Barcelona, Spain, (Eds. W. Palz, H. Ossenbrink, P. Helm), WIP-Renewable Energies, Munich, Germany, 2005, pp. 1981-1984.

Alsema E.A. and Wild-Scholten M.J. de, 2005b; The real environmental impact of Crystaline Silicon PV modules: an analyses based on up-to-date manufactures data, Proceedings of the 20th European Photovoltaic Solar Energy Conference and Exhibition, Barcelona, Spain, (Eds. W. Palz, H. Ossenbrink, P. Helm), WIP-Renewable Energies, Munich, Germany, 2005, pp. 1981-1984.

Altena F. and Burg P. van der, 2005; Light Spectra of common TLD and HID lamps, Philips Lighting B.V.

Altena F. and Saalberg-Seppen A., 2005; Light Spectra of LEDs, Philips Lighting BV.

Arthur J.R., Robert K. Graupner, Tyrus K. Monson, James A. van Vechten and Ernest G. Wolf, 1997; US Patent no. 5672214 and no 5415700, Oregon.

ATP, Advance Technology Program of the National Institute of Standards and Technology, USA, 2005; Rationale for the Li-Ion case, http://www.atp.nist.gov/eao/wp05-01/chapt2.htm, accessed 30-10-2006.

Bahgat, A.B.G, Helwa, N.H, Ahamd, G.E. and El Shenawy, E.T., 2003; Estimation of the maximum power and normal operating power of a photovoltaic module by neural networks, Renewable Energy, vol 29, pp443-457.

Barbe C.J., Arendse F. Comte P., Jirousek, M., Lenzmann F. Shklover V. and Grätzel M., 1997a; Nanocrystalline Titanium Oxide electrodes for Photovoltaic Applications; J. Am. Ceram. Soc., vol. 80, no 12, pp 3157-3171.

Barbe C.J., Bonhote P. and Grätzel M., 1997b; Solar Window: a new type of Solar Cell based on dye-sensitized nanocrystalline semiconductor films; Verre, vol. 3, no 2, pp. 3-8.

Beers S. van, 2002; Zonne-mobiliteit DCMR, Milieudienst Rijnmond, graduation Master Thesis, Design for Sustainability program, Delft University of Technology.

Benini L,.Castelli G,.Macii A, Macii E., and.Scarsi R, 2000; Battery-driven dynamic power management of portable systems, Proceedings of the 13th international symposium on System synthesis, pp. 25 - 30, New York, NY, USA: ACM Press.

Benini L., Castelli G., Macii A., Macii E., Poncino M., and Scarsi R., 2001; Extending lifetime of portable systems by battery scheduling, Proceedings of the conference on Design, automation and test in Europe, pp. 197 - 203, Piscataway, NJ, USA: IEEE Press.

Blok H., 1973; Integraaltransformaties in de elektrotechniek, Report no Inf. T. 1973-10, Electrical Engineering, Delft University of Technology.

166 Reference List

BMVIT, 2002; Offentliche Ausschreibung fur das Schirmmanagement der Programmlinie “Energiesysteme de Zukunft”, Bundesministerium fur Verkehr, Innovation and Technologie, Wien, Austria.

Boblbee, 2006; Boblbe.ebags and rucksacks, http://www.ebags.com/boblbee/brand_search/index.cfm?brandid=253, last checked and accessed 08-11-2006.

Boks C. and Diehl J.C., 2005; EcoBechmarking for all, Proceedings of ECODESIGN 2005, 12-15 december 2005, Tokyo, Japan.

Borg N.J.C.M. van der and Wiggelinkhuizen E.J., 2001; Building integration of photovoltaic power systems using amorphourus silicon modules: irradiance loss due to non conventional orientations, ECN Report no. ECN-C-01- 068.

Boschloo G. and Hagfeldt A., 2003, Flexible DSC Solar Cells, Angstrom Solar Centre, Private conversation ISES Solar Congress 2003, Goteborg, Sweden.

BP Solar, 2004; http://www.bpsolar.com, last checked and accessed 11-07-2006.

CET SOLAR Products; 2004 http://www.unlimited-power.co.uk/Solar_Powered_Products_UK.html, last checked and accessed 11-07-2006.

Brezet J.C.,1995; Van Prototype tot Standaard, de Diffusie van Energiebesparende Technologie; Dissertation, Erasmus University, Rotterdam.

Brezet J.C. and Silvester S., 2004; Responsible Industrial Design Engineering – RIDE, Proceedings TMCE 2004, Lausanne, Switzerland.

Broeders A. and Netten M., 2004; Hoe groen is ‘GroeneEnergie’. Een analyse van Dye Sensitized Cell,Technische Milieu Analyse report, TUDelft, IO.

Bruton T.M., 2002; General trends about photovoltaics based on crystalline silicon; Sol. Energy Mater. Sol. Cells, vol. 72, pp. 3-10.

Buchmann I., 2001; Batteries in a Portable World, Cadec Electronics Inc.

Buijs, J. and Valkenburg, R., 2000; Integrale Product Ontwikkeling; Lemma, Utrecht.

Burnside S., Winkel S., Brooks K., Shklover V., Grätzel M., Hinsh A, Kinderman R. Bradbury C., Hagfeld A., and Pettersson H., 2000; Deposition and characterization of screen-printed porous multi-layer thick film structures from semiconducting and conducting nannomaterials for use in photovoltaic devices; J. of Materials, vol. 11, no. 4, pp. 355-362.

Canon, 2006; Canon Cameras, http://consumer.usa.canon.com/ir/controller?act=CamProductCatIndexAct&fcateg oryid=101.

Catherino H.A., Burgel J.F., Shi P.L., Rusek A and Zou X., 2005; Hybrid Power Supplies: A Capacitor Assisted Battery, Proceeding 24th IPSS, Brighton, UK.

Cap-XX, 2005; Nanotechnology Super-cap characteristics, http://www.cap-xx.com/resources/reviews/strge_ cmprsn.htm.

CET SOLAR Products, 2004 http://www.unlimited-power.co.uk/Solar_Powered_Products_UK.html.

167 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Chang J.M. and Pedram M., 1997; Energy minimization using multiple supply voltages., IEEE Transactions on Very Large Scale Integration (VLSI) Systems, Vol. 5, No. 4, pp. 436 - 443,.

Chapin D.M., Fuller C.S. and Pearson G.L., 1954; A new p-n junction photocell for converting solar radiation into electrical power, Journal of Applied Physics, 25, pp. 676-677

Chappel S. and Zaban A., 2002; Nanoporous SnO electrodes for dye-sensitized solar cells: improved cell 2 performance by the synthesis of 18 nm SnO colloids; Sol. Energy Mater. Sol. Cells, vol. 71, pp. 141-152. 2

Chiasserini C. F.and Rao R.R., 2001; Energy efficient battery management., IEEE Journal on Selected Areas in Communications, Vol. 19, No. 7, pp. 1235 - 1245.

Chiba Y., Islam A., Watanabe Y., Komiya R., Koide N. and Han L., 2006; Dye-Sensitized Solar Cells with Conversion Efficiency of 11.1%, Japanese Journal of Applied Physics, Vol. 45, No. 25, pp. L638-L640.

Choi S.S. and. Lim H.S; 2002; Factors that affect cycle-life and possible degradation mechanism of a Li-ion cell based on LiCoO2, J. Power Sources, pp.111 130.

Choi S.H., Kim J. And Yoon Y.S.; 2004; Fabrication and characterisation of a LiCoO2 battery-supercapacitor combination for hig-puls power systems, J. Power Sources, pp. 138, 360.

Corning P.A., 1998; The Synergism Hypothesis: On the Concept of Synergy and It’s Role in the Evolution of Complex Systems, JOURNAL OF SOCIAL AND EVOLUTIONARY SYSTEMS 21(2), pp. 133-172.

CRIEPI, 2005; To Aim at the Practical Use of Lithium Batteries for Power Storage, http://criepi.denken.or.jp/en/ e_publication/home348/index.html; accessed 19-08-2006.

Dalton A.B. and Ellis C.S., 2003; Sensing User Intention and Context for Energy Management, Workshop on Hot Topics in Operating Systems (HOTOS).

Dijkstra F. and Westerhuijs P., 2005; Wide Band communication, Tomorrow’s Electronics Symposium at Electronics Automation 2005, Utrecht, The Netherlands.

Dillen, 2003, Solar Powered Buggy, graduation Master Thesis, Design for Sustainability, Delft University of Technology.

EC, 2003/2004; EC Directive 2002/95/EC of the European Parliament and of the Council, 27 January 2003 on the restriction of the use of certain hazardous substances in electrical and electronic equipment (2003) and EC Proposal for amending directive 2002/95/EC of the European Parliament and of the Council for the purpose of establishing the maximum concentration value for certain hazardous substances in electrical and electronic equipment, http://europa.eu.int/eur-lex/pri/en/oj/dat/2003/1_037/1_03720030213en00190023.pdf.

EIC, 2000; International Electrotechnical Commision, Directive no. 891; Procedures for Temperature and Irradiance Correction to Measured I-V Characteristics.

Eikelboom J.A., Feijen P.H.G., Weeber A.W. and Steeman R.A., 1995; Absolute rendementbepaling van zonnecellen bij ECN.

Elzen B., 2004; Method of product selection, SYN-Energy report UT.

ELNA Batteries, http://www.elna-america.com/products/products.htm, last accessed 8-10-2006.

Engstrom R.W., 1974; RCA Electro-Optics Handbook, Harrison, NJ, USA.

168 Reference List

Epcos, 2006; Ultracapacitors technology and application. http://www.epcos.com/web/generator/Web/Sections/ ProductCatalog/Capacitors/Ultracapacitors/PDF/PDF__UltraCapTechnology,property=Data__en.pdf;/UltraCap_ technology.pdf, last accessed 8-10-2006.

EPYON, 2005, Quick mobile charger, FLASH, patent pending, http://www.epyon.nl/.

Fihn M., 2001; LCD vs. CRT energy consumption, A Market Study for DELL, www.displaysearch.com.

Friedeman M., 2002; De DC laagspannings-woning; graduation master thesis, Faculty of Building Technology, Delft University of Technology.

Flipsen S.F.J., 2005; Power sources compared: The ultimate truth, Proceedings International Power Source Symposium (IPSS).

FULLRIVER Batteries, 2005, http://www.fentbattery.com/english/products.htm.

Gallenkamp C., 1959; Maya, the riddle and rediscovery of a lost civilisation, David Mc Kay Company, New York.

Gebeyehu D., Brabec C.J., Sariciftci N.S., Vangeneugden D. Kiebooms R., Vanderzande D., Kienberger F. and Schindler H., 2001;Hybrid solar cells based on dye-sensitized nanoporous TiO sub 2 electrodes and conjugated polymers as hole transport materials; Synthetic Metals, vol. 125, no. 3, pp. 279-287.

Geelen D., 2006; Design of a photovoltaic powered personal mobile companion, graduation Master Thesis, Design for Sustainability program, Delft University of Technology.

Gemmer C., 2003; Analytische und numerische Untersuchungen von Solarzellen unter wechselnden Beleuchtungsbedingungen”, Ph.D. thesis, Universität Stuttgart, Germany.

General Electric, 2006; Light spectra of incanscent lamps, www.generalelectric.com; http://www.gelighting.com/na/ business_lighting/education_resources/learn_about_light/distribution_curves.htm, Last accessed August 18th 2006.

Gennip P. van, 2005; Power Quest an on-line software tool, graduation Master Thesis, Design for Sustainability program, Delft University of Technology.

Gennip P. van, Kan S.Y. and Silvester S., 2006; Power Quest an on-line software tool to find novel power system applications, proceeding of TMCE 2006, Lubiana, Slovenia.

Glunz S.W., Dicker j., Hermie M., Isenberg J., Kamerewerd F.J., Knobloch J. Kray D., Leimenstoll A., Lutz F., Oszwald D., Preu R., Rein S., Schaeffer E., Schetter C., Schmidhuber H., Schmidt H., Steuder M., Vorgrimler C. and Willeke G., 2002; High-Efficient Silicon Solar Cells for Low –Illumination Applications, 29th IEEE PVSC, paper no. 1P4.11, New Orleans, May 20-24th, 2002.

Glunz, S. and G. Willeke, 2003; Scientists make progress towards “paper thin” 20 % efficient crystalline Silicon Cell, Fraunhover Institut Freiburg ISE, http://www.solarbuzz.com/News/NewsEUTE10.htm.

Goetsberger A. and Hebling C., 2000 ;Photovoltaic materials, past present, future; Sol. Energy Mater. Sol Cells, vol. 62, no. 1 pp. 1-19.

Grätzel M., 2001; Molecular photovoltaics that mimic photosynthesis, Pure Appl. Chem., vol. 73, no 3, pp. 459- 467.

Grätzel M., 2005; Solar Energy Conversion by Dye-Sensitized Photovoltaic Cells, Inorganic Chemistry, vol. 44, pp. 6841-6851.

169 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Grätzel M., 2006; Private communication.

Green A., 2005; A Technical Comparison between new Battery Technologies available for Stand-alone Photovoltaic Application, Proceedings 24th Power Source Symposium, Brighton, UK

Green M.A. 1986 (December); Solar Cells, University of the New South Wales Publ.

Green M.A. [2005]; Price/Efficiency correlations for 2004 Photovoltaic Modules; Progress in Photovoltaics: Res. Appl, 13, pp. 85-87.

Green M.A., Emery K., King D.L., Hishikawa Y. and Warta W., 2006; Solar Cell Efficiency Tables (Version 28), Progress in Photovoltaics: Research and Application, 14: pp. 455-461.

Grunow P., et. al. 2004; Weak light performance and annual energy yields of PV modules and systems as a result of the basic parameter set of industrial solar cells, Proc. 19th European Photovoltaic Solar Energy Conference and Exhibition Paris, (Eds. W. Palz, H. Ossenbrink, P. Helm), WIP-Renewable Energies, Munich, Germany, 2004, pp. 1981-1984 pp. 2190-3.

Hagfeldt A. and Grätzel M., 2000; Molecular Photovoltaics; Acc. Chem. Res. vol. 33. pp. 269-277.

Hamlen R.P.; Portable Military Power-Sources Past and Future, Proceeding 24th Power Source Symposium (IPSS), Brighton, UK

Hammersley B., 2005 (January 6); Power dressing, The Guardian; http://technology.guardian.co.uk/online/ story/0,,1383610,00.html.

Hara Y., 1999; Portable phone makers press demand for thin Lithium-ion cells; http://www.eet.com/story, 30 September

Havinga P.J.M., 2000; Mobile Multimedia Systems, Dissertation, University of Twente.

Havinga P.J.M and Smit G.J.M, 2001; Energy-efficient wireless networking for multimedia applications., Wireless Communications and Mobile Computing, Vol. 1, No. 2, pp. 165 - 184.

Hazen M.E.; 1996, Alternative Energy; Howard W. Sams & Company USA, p. 42 ff.

Hinsh A., Kroon J.M., Spath M. van Roosmalen J.A.M., Bakker N.J., Sommeling P. van der Burg N. and Kinderman R.; Kern R. Ferber J. Schill C. and Schubert M; Meyer A. and Meyer T; Uhlendorf I., Holzbock J. and Niepmann R., 2000; Long - term Stability of dye-sensitized solar cells for Large Area Power Applications (Lots-DSC); Proceedings 16th European Photovoltaic Solar Energy Conference and Exhibition, Glasgow, UK, (Eds. W. Palz, H. Ossenbrink, P. Helm), WIP-Renewable Energies, Munich, Germany, 2000.

Hinsch A., Kroon J.M., Kern R., Uhlendorf I. Holzbock J., Meyer A. and Ferber J., 2001; Long-term stability of Dye- Sensitised Solar Cells, Prog. Photovolt. Res. Appl. vol. 9, pp. 425-438.

Hohl-Ebinger J., Hinsch A., Sastrawan R., Warta W. and Wurfel U., 2004; Dependence of Spectral Response of Dye Solar Cells on Bias Illumination, Proceedings of the 19th European Photovoltaic Solar Energy Conference and Exhibition, 7-11 June, Paris, France.

Hohm D.P. and Ropp M.E, 2003; Comparative Study of Maximum Power Point Tracking Algorithm., Progress in photovoltaics: Research and Applications, Vol. 11, pp. 47-62.

170 Reference List

Hua C., Lin J. and Shen C., 1998; Implementation of a DSP-controlled photovoltaic system with peak power tracking, IEEE Trans. Ind. Electron., vol. 45. pp. 99-107.

Hussein K.H., Muta I., Hoshino T. and Osakada M., 1995; Maximum photovoltaic power tracking: an algorithm for rapidly changing atmospheric conditions, IEE proceedings of Generation, Transmission, Distribution; 142 (1), pp. 59-64.

INVESTIRE-NETWORK, 2003; “Investigation on Storage Technologies for Intermittent Renewable Energies: Evaluation and recommended R&D strategy; Final Report, Alsema E. and Patyk A. ed., European Community project no. ENK5-CT-200-20336.

ITRS, 2004; International technology roadmap for semiconductors 2003 edition; http://public.itrs.net/, 2003, update 2004.

Janski S. et. al., 2004; Spectral photocurrent measurement techniques for thin film modules, Proc. 19th European Photovoltaic Solar Energy Conference and Exhibition Paris, (Eds. W. Palz, H. Ossenbrink, P. Helm), WIP- Renewable Energies, Munich, Germany, pp.1595.

Jay F. (ed.), 1977; IEEE Standard Dictionary of Electrical and Electronical Terms, Wiley-Interscience, New York.

Kan S.Y., 1996; The use of FETs instead of Diodes at Solar Arrays; Fokker-SER-HEOM.

Kan S.Y., 2000; De PV Batterij, INVENTEC Energieprijsvraag Technisch Weekblad, June.

Kan S.Y., 2002a; The Smart PV Battery, Semiconductor Battery and method of manufacturing, Patent no. 1015956.

Kan S.Y., 2002b; Reduction of hidden power consumption in consumer products; An application example of the Smart Photovoltaic Battery, Proceedings of EuroSun 2002, Bologna, Italia; June, 23-26 June, ISES ITALIA, Roma.

Kan S.Y., 2002c, N.W.O./Novem final report project 014-28-213; 2002, http://www.hersenenenleren.nl/nwohome. nsf/pages/SPES_5VEFH7/$file/Synergie-eindrapport.pdf.

Kan S.Y., 2003; PV powered mobility and mobile/wireless product design, proceeding ISES 2003 Solar World Congress, Gotenborg: ISES Sweden.

Kan S.Y. and Silvester S., 2003, Synergy in a Smart Photo Voltaic (PV) Battery: SYN-Energy; The Journal of Sustainable Product Design vol 3.

Kan S.Y., Silvester S. and Brezet H., 2004a, Design application of combined photovoltaic and energy storage units as energy supplies in mobile/wireless product; Proceedings TMCE 2004, Lausanne, Switserland, pp. 309 ff.

Kan S.Y., Beers S. and J.C. Brezet, 2004b; Maturely designed and sustainable PV powered products. Proceeding EUROSUN 2004 conference, June, Freiburg, Germany.

Kan S.Y., Verwaal M. and Broekhuizen H., 2005a; Battery – Capacitor combinations in photovoltaic powered products, Proceeding of the 24th IPSS 18-21 April 2005, Brighton, UK.

Kan S.Y. and Struik R., 2005b; Towards a more efficient energy use in maturely designed photovoltaic powered products, Proceeding of the 24th IPSS 18-21 April 2005, Brighton, UK.

Kan S.Y., Verwaal M. and Broekhuizen H., 2006a; Battery – Capacitor combinations in photovoltaic powered products; Journal of Power Sources, vol. 162 (December).

171 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Kan S.Y. and Struik R., 2006b; Towards a more efficient energy use in maturely designed photovoltaic powered products, Journal of Power Sources, vol. 162 (December).

Kan S.Y., Silvester S. and Brezet J.C.; 2006c; Energy Matching Model and Figure of Matching Algorithm for the Energy Chain of Photovoltaic Powered Products, Journal of Cleaner Production (in preparation).

Kan S.Y., Verwaal M. and Broekhuizen H., 2006d; On the use of the of the Figure of Matching algorithm for analysing and quantifying battery – capacitor combinations in photovoltaic powered products; Journal of Power Sources, (in preparation).

Kamp L.G. van der, 2003; Een educatief speelobject met zonne-energie; ‘De Zon’, graduation Master Thesis, Design for Sustainability program, Delft University of Technology.

Kambe S., Murakoshi K., Kitamura T., Wada Y. Yanagida S. Kominami H. and Kera Y. 2000; Mesoporous electrodes having tight agglomeration of single-phase anatase TiO nanocrystallites: Application to dye-sensitized solar cells; 2 Sol. Energy Mater. Sol Cells, vol. 61, pp. 427-441.

Kariatsumari K., 2005; Sony Li-Ion Battery Boosts Capacity 30% (May 2005 Issue, Nikkei Electronics Asia), http:// neasia.nikkeibp.com/neasiaarchivedetail/001100.

Kaufman C.A., Klenk R., Neisser A. Korber P., Scheer R. and Schock W., 2005; Towards Flexible Cu(In,Ga)Se 2 Bassed, Thin Film Solar Cell Module, Proceedings of the 20th European Photovoltaic Solar Energy Conference and Exhibition, Barcelona, Spain, (Eds. W. Palz, H. Ossenbrink, P. Helm), WIP-Renewable Energies, Munich, Germany, 2005.

Keukens B.M., 2005: Solar Rudi and his amazing Pupil Locator: Photovoltaics in ski clothing, graduation Master Thesis, Design for Sustainability program, Delft University of Technology.

Keukens B.M., Kan S. Y. and Brezet J. C, 2006; PV in clothing; Design Considerations of Wearable PV energy supplies, Journal of Cleaner Production (in preparation).

Kim I.S. and Kumta P.N., 2004; High capacity Si/C nanocomposite anodes for Li-Ion batteries, Journal of Power Sources 136 (2004), pp. 145-149.

KNMI, 2006; De ‘normalen’ voor de Bilt 1971-2000, http://www.knmi.nl/voorl/weer/normalen.html.

Kravets R. and Krishnan P., 1998; Power management techniques for mobile communication., Proceedings of the 4th annual ACM/IEEE international conference on Mobile computing and networking, pp. 157 - 168, New York, NY, USA: ACM Press.

Kroon J.M., Veenstra S.C., Slooff L.H. and Verhees W.J.H., 2005; Polymer Based Photovoltaics: Novel Concepts, Materials and State-of-the-Art Efficiencies, Proceedings of the 20th European Photovoltaic Solar Energy Conference and Exhibition, Barcelona, Spain, (Eds. W. Palz, H. Ossenbrink, P. Helm), WIP-Renewable Energies, Munich, Germany, 2005.

Krul L., 1977: Informatie transmissiewegen, lecture notes Electrical Engineering, Delft University of Technology.

Kumar B., Pandian B., Sreekiran E. and Narayanan S., 2005; Benefit of dual layer Silicon Nitride Anti-Reflection coating, proceedings IEEE PVSC, Orlando, Florida, USA.

Kirkpatrick D., 2006; Photovoltaics take a load off soldiers, Compound Semiconductor issue October 2006, section Technology, pp. 27 – 30.

172 Reference List

Lahiri K., Raghunathan A., Dey S. and Panigrahi D., 2002; Battery driven system design: A new frontier in low power design, Proceedings ASP-DAC/Int. conf. VLSI Design.

Langeveld S, Huis in’t veld C., de Klein N., van Vorsselen J. and L. van den Bosch, 2004; PV energy source for an ultrasound system, Project Advanced Project (PAP) report Faculty Industrial Design Engineering, Delft University of Technology.

LCEP, 2005-2006; Life Cycle Engineering and Product designs, Master Engineering Course, Faculty of Industrial Design Engineering, Delft University of Technology.

Lee K.L., Jung J.Y., Lee S.W., Moon H.S. and Park J.W., 2004; Electrochemical characteristics of a-Si thin film anode for Li-Ion rechargeable batteries, Journal of Power Sources 129 (2004), pp. 270-274.

Lettieri P. and Srivastava M.B, 1999; Advances in wireless terminals, Personal Communications, IEEE [see also IEEE Wireless Communications], Vol. 6, No.1, pp. 6-19.

Li D.H.W and Lam J.C., 2000; Measurements of solar radiation an illuminance on vertical surfaces and day lighting implications, Renewable Energy vol. 20, pp. 389-404,

Li J., Murphy E., Winnick J. and Kohl P.A., 2001; The effects of pulse charging on the cycling characteristics of commercial lithium-ion batteries, J. Power Sources 102 (2001), pp. 302.

Linden D. and Reddy T.B. (Ed.), 2002; Handbook of batteries, Mc Graw-Hill, New York LP Electric, 2006; Solar Charger ‘Source’, http://www.lpelectric.ro/en/products/source/solar_charger_en.html.

Luther J., 2003; Photovoltaic electricity generation – Status and perspectives, Proceedings of the ISES Solar World Congress 2003, Gotenborg, Sweden.

Macht B., Turrion M., Barkschat A. Salvador P. Ellmer K and Tributsch H., 2002; Patterns of efficiency and degradation in dye sensitisation solar cells measured with imaging techniques; Sol. Energy Mater. Sol Cells, 73, pp. 163-173

Mao D., Ibrahim M.A. and Frank A.J., 1998; A plasticized polymer-electrolyte-based photo electrochemical solar cell; J. of the Electrochemical Soc., vol. 145, no 1, pp. 121-124.

Masoum M.A.S., Dehbonei H. and Fuch E.F., 2002 ;Theoretical and Experimental Analyses of Photovoltaic Systems with Voltage- and Current-Based Maximum Power-Point Tracking, IEEE Trans. On Energy Conv., vol. 17 no 4

Maxwell, 2005; Supercapacitors-ultracapacitors, http://www.maxwell.com/ultracapacitors/products/datasheets.html.

Meester B., 2006; Interview + business plan, Advance Surface Technology (AST) BV, Bleiswijk, the Netherlands. Meijer A., Huijbregts M. A. J., Schermer J. J. and L. Reinders, 2003; Life-cycle Assessment of Photovoltaic Modules: Comparison of mc-Si, InGaP and InGaP/mc-Si Solar odules, Prog. Photovolt. Res Appl. No 11, pp 489.

Mestre A., and Diehl J.C., 2003; Design Guidelines for Integration of Renewable Energy into Consumer Products, Design for Sustainable program, Faculty of Industrial Design Engineering, Delft University of Technology.

Menachem C. and H. Yamin, 2004; High-energy, high-power pulses Plus battery for long-term application, J. Power Sources 136 (2004), 268.

Meyer T.B., Meyer A.F., and Ginestoux D., 2001; Recent developments in dye-sensitized solar cell technology, Proceeding of the Conference on Organic photovoltaics, San Diego CA, USA.

173 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Mil-STD-461/E, 1999; Measurements of Electromagnetic Interference Characteristics, Department of Defence USA.

Miyasaka T. and Murakami T. N., 2004; The photocapacitor: an efficient capacitor for direct storage of solar energy, Appl. Phys. L. 85, 17 (25 October 2004), 3932.

Moore G.E., 1965; Cramming more components onto integrated circuits, Electronics, vol. 38 no 8.

Moore G.E., 2003; No exponential is forever… but we can delay ‘forever’, Proceedings International Solid State Circuit Conference (ISSCC).

Mourzagh D., Martinet S., Perrin M., Mattera F., Fourmentel D., Lausenaz Y, Sarre G., Marcel J.C. and Laflaquire P, 2005; Lithium batteries in stand alone photovoltaic applications, Proceedings 24th Power Sources symposium, Brighton, UK.

Nanu M., Schoonman J. and Goossens A., 2005; Low Cost Nanocomposite nS /TiO Solar Cells, Proceedings 2 2 of the 20th European Photovoltaic Solar Energy Conference and Exhibition, Barcelona, Spain, (Eds. W. Palz, H. Ossenbrink, P. Helm), WIP-Renewable Energies, Munich, Germany, 2005.

NEC, 2005; 30 sec super charge battery, Digital World, Tokyo, Thu Dec 8, 2005, http://www.digitalworldtokyo. com/archives/2005/12/30second_charge.html.

Nicolaescu I.V. and Hoffman W.F., 2001; Energy consumption of cellular telephones, IEEE International Symposium on Electronics and the Environment, pp. 134 - 138, Denver, CA, United States.

Ning G., Haran B. and B.N. Popov, 2003; Capacity fade study of lithium-ion batteries cycled at high discharge rates, J. Power Sources 117 (2003) pp.160.

Nokia, 2002; Ultra Slim Battery type BPS-1; http://www.Nokia.com/phones.

Novem, May 2000; Thin-Film Solar Cells, Technology Evaluation and Perspective; Bossert R.H., Tool C.J.J., van Roosmalen J. A. M., Wentink C. H. M. and de Vaan M. J. M; report no. DV 1.1.170, Berenschot and ECN.

NUNA 3, 2005; World Solar Challenge Solar Race, http://www.wsc.org.au.

OkSolar, 2004; LEDs, www.OkSolar.com.

Otani K., 2006; Solar Irradiation data bases on the Web, http://www.pvsystem.net/~k.otani/en/doc/irrdata/index. htm.

O’Neill, 2005; PV charger on Backpack, http://www.bitstorm.org/2005-10/O_Neill_Solar_Backpack.html, last checked and accessed 11-07-2006.

O’Regan B. and Grätzel M., 1991; A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO films, 2 Natur, vol. 353, pp. 737-739.

Parretta A., Yakubu H., Ferrazza F., Altermatt P.P., Green M.A. and Zhao J., 2002; Optical loss of photovoltaic modules uder diffuse light, Solar Energy Materials & Solar Cells 75, pp.497-505.

Pell W.G. and Conway B.E., 2004; Peculiarities and requirements of asymmetric capacitor devices based on combination of capacitor and battary type electrodes. Journal of Power Sources 136: pp. 334-345.

174 Reference List

Pellegrino M., Flaminio G., Leanza G., Privato C. And A. Scognamiglio, 2002; Aesthetical Appeal of BIPV or Electrical Performance, Conference: PV in Europe, from PV Technology to Energy Solutions, Rome Italia.

Pettersson H. and Gruszecki T., 2001; Long-term stability of low-power dye-sensitized solar cells prepared by industrial methods; Sol. Energy Mater. Sol. Cells, vol. 70, pp. 203-212. Phani G., Tulloch G. Vittorio D. and Skryabin I., 2001;Titania solar cells: new photovoltaic technology; Renewable Energy vol. 22, pp. 303-309.

Philips, 2005; Philishave Sensotec catalog, http://www.consumer.philips.com/consumer/catalog/.

Photon Technologies; 2004; http://members.aol.com/photontek/photon/photon2a.html.

Poelman W.A., 2000; Onbedorven jonge ontwerpers; Product, Vol. 8 no. 1.

Poelman W.A and S.Y. Kan, 2000: Semiconductor on Insulator Enamel techniques, Registered Invention, Zeist.

Poelman W.A., 2005; Technology Diffusion in Product Design, Dissertation, Delft University of Technology.

Poliness E., 2004 (5 April); Recharge your mobile wherever you are; ABC Science Online; http://www.abc.net. au/science/news/stories/s1079991.htm.

Pouwelse J.A. 2003; Power Management for Portable Devices, Dissertation Delft University of Technology.

Powalla M., Dimmler B. and Gross K.H., 2005; CIS Thin-Film Solar Modules – An Example of Remarkable Progress in PV, Proceedings of the 20th European Photovoltaic Solar Energy Conference and exhibition, Barcelona, Spain, (Eds. W. Palz, H. Ossenbrink, P. Helm), WIP-Renewable Energies, Munich, Germany.

Powerstream Batteries, 2006; http://www.powerstream.com/li-pol.htm.

Rabaey J.M. and Pedram M., 1996; Low power design methodologies, The Kluwer international series in engineering and computer science 336 - 367, Boston: Kluwer Academic.

Randall J.F., J. Jacot, 2003; Is AM1.5 applicable in practice? Modelling eight photovoltaic materials with respect to light intensity and two spectra, Renewable Energy, Vol. 28, Issue 12 (October).

Randall J.F., 2003; On the use of photovoltaic ambient energy sources for powering indoor electronic devices, Dissertation, Laboratoire de Production Microtechnique, École Polytechnique Fèderal de Lausanne, Switzerland.

Ransome S. and Funtan P., 2005; Why Hourly Average Measurements Data is insufficient to Model PV System Performance Accurately, Proceedings of the 20th European Photovoltaic Solar Energy Conference and Exhibition, Barcelona, Spain, (Eds. W. Palz, H. Ossenbrink, P. Helm), WIP-Renewable Energies, Munich, Germany.

Ramadass P, B. Haran, R. White and B.N. Popov; Capacity fade of Sony 18650 cells cycled at elevated temperatures, 2002, Part I. Cycling performance, J. Power Sources 112 606

Reich N.H., Kan S.Y., Sark W.G.J.H.M.V., Alsema E., Silvester S., and Heide A.S.H.van der, 2005; Weak light performance and spectral response of different solar cell types: a first step towards design of PV powered consumer and professional products, Proceedings of the 20th European Photovoltaic Solar Energy Conference and Exhibition, Barcelona, Spain, (Eds. W. Palz, H. Ossenbrink, P. Helm), WIP-Renewable Energies, Munich, Germany, pp. 2120-2123.

175 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Reich N.H., Veefkind M., Alsema E. and Kan S.Y., 2006; Industrial Design of a PV powered consumer application – case study of a solar powered wireless computer mouse, Proceedings of the 21st European Photovoltaic Solar Energy Conference and Exhibition, Dresden, Germany, (Eds. W. Palz, H. Ossenbrink, P. Helm), WIP-Renewable Energies, Munich, Germany. Reich N., 2008; Fortcoming Dissertation, Utrecht University.

RETScreen, 2005; Power calculation in PV systems, www.RETScreen.com. Roozenburg N.F.M. and Eekels J., 1998; Productontwerpen, structuur en methoden, Lemma BV, Utrecht.

Roos G. ,1999; Lithium-ion polymer batteries get recharged; EE Times; http://www.eet.com/story; 29 November.

Ryer A., 1997; Light Measurement Handbook, http://www.intl-light.com/handbook.

Saft, 2005, http://www.saft.fr//130-Catalogue/PDF/mp_176065.pdf; Accessed on 19-08-2006.

Satoh M., 2005; Organic Radical Battery and its Technology, Nec J. Of Adv. Tech, Summer 2005, pp. 262-263.

Scherpen J.M.A., Kerk B. Van der, Klaassens J.B., Lazeroms, Kan S.Y., 1998; Nonlinear Control for Magnetic Bearing in PV Deployment Test Rigs: Simulation and Experimental Results, Proceedings of the 37th IEEE Conference on Decision & Control, Tampa, Florida, USA, December.

Scheuten Solar, 2006, http://www.scheutensolar.com/?location=home&t=Home&s=.

Schmidhuber H., 2003; Verkapselung von Kristallinen Silizium-Solarzellen, Dissertation, FernUniversitat – Gesamthchschule, Hagen.

Schmidt-Mende L., Friend R. H., MacKenzie J.D., Fechtenkotter A., Muellen K and Moons E., 2001; Efficiency organic photovoltaics; Science, vol. 293, no 5532, pp. 1119-1122.

Schoonman J. 1998 (October); Materiaaltechnologie voor opslag van duurzame energie; Chemisch weekblad, nr. 10.

Shim J., Kostecki R,, .Richardson T. Song X., and Striebel K.A., 2002; Electrochemical analysis for cycle performance and capacity fading of a lithium-ion battery cycled at elevated temperature., Journal of Power Sources, Vol. 112, No. 1, pp. 222 – 230.

Sikha G. and B.N. Popov, 2004; Performance optimization of a battery-capacitor hybrid system, J. Power Sources, pp. 134-130.

Sinke W.C., 2004; The CrystalClear Integrated Project: Next generation crystalline silicon technology from lab to production, Proceedings of the 19th European Photovoltaic Solar Energy Conference and Exhibition, Kranjska Gora, Slovenia.

Smulders B., 2000; Roadmap Human Powered Products, graduation Master Thesis, Design for Sustainability, Faculty of Design Engineering, Delft University of Technology.

Solar Style, 2003; “Solar Cases plug into the sun”, http:// www.solar-style.com.

Solar Products, 2004; http://solar-world.com/home_garden.htm.

Solarion CIS PV cells, 2004; http://www.solarion.de/, accessed 18-08 2006.

176 Reference List

Sol E.J., 2005; Plastic electronics: Tomorrow’s Electronics; Tomorrow’s Electronics Symposium at Electronics Automation 2005, Utrecht, The Netherlands.

Sommeling P.M., Rieffe H.C., Roosmalen van J.A.M., Schonecker A., Kroon J.M., Wienke J.A. and Hinsch A., 2000, Solar Energy Materials & Solar Cells, 62, pp.399-410.

Sony, 2002; Stamina battery for Sony camcorders; http://www.netshoppe2000.com/sony/battery.htm. Sony, 2005, Sony Li-Ion battery boost 300%, NE Asia, may, 2005, http://neasia.nikkeibp.com/ neasiaarchivedetail/001100.

Spence L., 1917; Myths and Legends of ancient Egypt, GG Harrap & Company, London.

Swager A. W., 1995; Smart-battery technology; EDN, March.

Sze S.M., 1969; Physics of Semiconductor Devices, Wiley International Edition, New York.

Tai W.P., Inoue K., and Oh J.H., 2002; Ruthenium dye-sensitized SnO /TiO coupled solar cells; Sol. Energy Mater. 2 2 Sol. Cells, vol. 71, pp. 553-557.

Takei K, Kumai K., Kobayashi Y, Miyashiro H. Terada N., Iwahori T. and T. Tanaka, 2001; Cycle life estimation of lithium secondary battery by extrapolation method and accelerated aging tests, J. Power Sources 97-98 (2001), pp. 697.

Tallner C. and Lannetoft S., 2005; Batteries or supercapacitors as energy storages in HEVs?, Doc. no. LUTEDX/ (TEIE-5194)/1-71/(2005), Dept. of Industrial Electrical Engineering and Automation, Lund University, Sweden.

Taylor A.E.F., 2000; Illumination Fundamentals; Rensselaer Polytechnic Institute Publ.

ThermoAnalytics, 2005; Battery Types and Characteristics, http://www.thermoanalytics.com/support/publications/ batterytypesdoc.html.

Toshiba, 2005; Toshiba’s New Rechargeable Lithium-Ion Battery Recharges in Only One Minute, Press Release 29 March 2005, http://www.toshiba.co.jp/efort/scb/spec.htm.

TSA, 2005; True Solar Autonomy DC/DC converters for parallel connected PV cells, http://www. truesolarautonomy.com/.

Uni-Solar, 2006, http://www.prometheusenergy.com/pdfs/us_specs.pdf, last checked and accessed 11-07-2006.

Varakin I.N., Klementov A.D. and Litvinenko S.V., 1999; Power characteristics of “ESMA” Capacitors, Proceedings 9th International Seminar on Double Layer Capacitors and Similar Devices, Florida, USA, http://www.esma-cap. com/FAQ/doc3eng?lang=English.

Veefkind M., Kan S.Y., Silvester. S., Verwaal M., Elzen B., Alsema E.A. and Reich N.H., 2006 ; The design of a solar powered consumer product : a case study ; proceeding of the 2006 Going Green – Case INNOVATION conference, Vienna, Austria.

Veefkind M., 2007; The Design of PV powered products, Forthcoming Dissertation, Delft University of Technology.

Veerachary M. Senjyu T. and Uezato, K, (January) 2002a; Voltage-based Maximum Power Point Tracking control of PV systems, , IEEE Tran. AeroSpace and Elec. Sys., vol 38, no 1.

177 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Veerachary M. Senjyu T. and Uezato, K, (July) 2002b; Feedforward Maximum Power Point Tracking of PV Systems using Fuzzy Controller, IEEE Tran. AeroSpace and Elec. Sys., vol 38, no 3.

Velds C.A., 1992; Zonnestraling in Nederland, Thieme Baarn / KNMI.

Vries G. de and Silvester S. 2002; Rapport Sociale Monitoring 1 Megawatt PV-Project Nieuwland Amersfoort, REMU no 78171.

Weiser M., Welch B., Demers A., and Shenker S., 1994; Scheduling for reduced CPU energy, Proceedings of the First Symposium on Operating Systems Design and Implementation, pp. 13 - 23.

Weitjens B., 2003; Met de zon in de rug, graduation Master Thesis, Design for Sustainability program, Delft University of Technology.

Weisstein E.W., 2006; Merit Function, MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/ MeritFunction.html.

Wolfenstine J., 2003; CaSi as an anode for Lithium-Ion batteries, Journal of Power Sources 124 (2003), pp. 241- 2 245.

Wu, W., Pongratananukul, N., Qiu, W., Rustom, K., Kasparis, T. And Batarseh I.,2003; DSP-based Multiple Peak Power Tracking for Expandable Power System, IEEE.

Xia Yp, 2004; Supercapacitor boosts current from small battery, EDN, 9/2/2004, http://www.edn.com/article/ CA446994.html.

Yu G., Srdanov G., Wang H., Cao Y. and Heeger A.J., 2000; High performance polymer photovoltaic cells and photo detectors; Proceedings of the Conference on Organic photovoltaic, San Diego, CA, USA.

Zhang D., Haran B.S., Durairajan A., White R.E., Podrazhansky Y. and B.N. Popov; 2000; Studies on capacity fade of lithium-ion batteries, J. Power Sources, pp. 91, 122.

178 List of Figures

Nr. Title Page 1-1 Products and their power consumption ranges 3 1-2 PV generated hydrogen fuel-cell car 5 1-3 Moore’s Law 6 1-4 The energy pay back time (EPBT) for South- and Middle-European locations 9 1-5 The diversity of PV powered products 11 1-6 The energy chain of a PV powered product and the Energy Matching Model 13 1-7 The Stimulus (S) - Response (R) concept applied on an element p 16 1-8 The presentation of a Step-Response 17 1-9 Energy flow between two elements s and p in the energy chain 17 1-10 The Figure of Matching (FM) algorithm between two elements s and p in the energy chain 17 1-11 The structure and outline of this dissertation 21

2-1 The energy chain of a PV powered product and the Energy Matching Model 24 2-2 Spectral solar irradiance with absorption bands 27 2-3 Spectral distribution of sunlight: AM 1.5, AM 1.0 and 6000 K blackbody radiation 27 2-4 The location of the observer and the PV powered product with respect to the sun 28 2-5 The irradiance versus time during one day measured every 15 minutes in Kassel Germany at three different days 29 2-6 The spectrum of incandescent lamps as function of filament temperature 30 2-7 Effect of voltage and light output on lamp life and light output 30 2-8 The spectra of some TL(D) lamps 31 2-9 The spectra of some HID-CDML lamps 32 2-10 Several LED spectra 32 2-11 White colour spectrum of two typical white LEDs, integral measurement. Presented are the white LXHL-BW02 and warm-white LXHL-BW03 33 2-12 An overview of light spectral distributions 33 2-13 Location of V and I and MPP (I , V ) on the I-V curve of a oc sc m m PV cell 34 2-14 The measured ratio V /V for various PV technologies 36 MPP oc 2-15 The Maximum Power Point path of two different photovoltaic cells 37 2-16 The linear relation between irradiance and I normalized at sc 1 sun of various PV cells 38

179 Energy Matching - Key towards the design of sustainable photovoltaic powered products

2-17 Crystalline silicon (c-Si) technology in historic perspective ectended to the year 2006 40 2-18 Measured conversion efficiency versus irradiance level of: a-Si, DSC and CIS PV cells 41 2-19 Power conversion efficiencies as function of various irradiance levels measured at several c-Si PV samples of two different cell manufacturers 43 2-20 Measured efficiencies of mc-Si PV cells versus irradiance level of various samples of different cell manufacturers 44 2-21 Spectral responses of a-Si and mc-Si samples 44 2-22 Spectral responses of two extremes a-Si samples 45 2-23 Spectral responses of c-Si and CIS samples 45 2-24 Spectral response of DSC samples 46 2-25 Overview of the various spectral responses 46 2-26 The AM 1.5 spectrum at 1000 W/m2, the spectral response (SR) of crystalline silicon (c-Si) PV cell and the power based spectral correlation product S ( ) x SRP ( ) with S ( ) the AM 1.5 IL λ PV λ IL λ spectrum 49 2-27 TLD lamp spectrum at 1000W/m2, the spectral response (SR) of a crystalline silicon (c-Si) PV cell and the power based spectral correlation product of S ( ) x SRP ( ) with S ( ) the TLD lamp IL λ PV λ IL λ spectrum 49 2-28 Incandescent lamp spectrum at 1000 W/m2, the spectral response (SR) of a crystalline silicon (c-Si) PV cell and the power based spectral correlation product of S ( ) x SRP ( ) with S ( ) IL λ PV λ IL λ the incandescent lamp spectrum 50 2-29 The TLD lamp spectrum at 1000 W/m2, the spectral response (SR) of an amorphous silicon (a-Si) PV cell and the power based spectral correlation product of S ( ) x SRP ( ) with S ( ) IL λ PV λ IL λ the TLD lamp spectrum 50 2-30 The TLD lamp spectrum at 1000 W/m2, the spectral response (SR) of a dye sensitised (DSC) PV cell and the power based spectral correlation product of S ( ) x SRP ( ) with S ( ) IL λ PV λ IL λ the TLD lamp spectrum 51 2-31 PV batteries on the back side of cellular phones 56 2-32 PV batteries on the cover side of a PV powered PDA to avoid shadows 56 2-33 PV electrical power output of a c-Si cell during a clear day calculated by using the irradiance pattern of a clear day in Figure 2-5 and the irradiance level dependent efficiency of c-Si PV cells as is presented in Figure 2-19 59 2-34 Comparison between the power output of a c-Si PV cell on a clear day in case the low light level efficiencies of the PV cell is taken into account and in case just the Standard Test Condition (STC) efficiency is taken for the calculation 60

180 List of Figures

2-35 Comparison between the Power Output of a c-Si PV cell on a cloudy day in case the low light level efficiencies of the PV cell are taken into account and in case just the Standard Test Condition (STC) efficiency is taken for the calculation 60 2-36 The bending of a thin Silicon wafer with PV cells 61 2-37 Curved PV panel by using multi-crystalline wafers pressed between curved glass plates 62 2-38 Example of a bendable thin CIS PV module 62

3-1 PV powered calculator 67 3-2 An example of Mussenbroek’s ‘Leyden Yar’ and a ‘discharge tool’ 68 3-3 The Energy Matching Model of the energy chain of a PV powered product 69 3-4 Charge characteristics of Li-Ion batteries at various charge rates 72 3-5 Discharge characteristics of Li-Ion batteries at various discharge rates 73 3-6 Discharge characteristics of Li-Ion batteries at various discharge temperatures 73 3-7 Power density of the Li-Ion battery as a function of pulse duration and depth of discharge (DOD) 74 3-8 Charge characteristics of capacitors 80 3-9 Discharge characteristics of capacitors 81 3-10 Power discharge characteristics of capacitors 81 3-11 Self-discharge characteristics of capacitors 81 3-12 Comparing the charge - discharge characteristics at the constant current (I) of both batteries and capacitors 82 3-13 A comparison between the specific or gravimetric energy densities and specific or gravimetric power densities of various batteries and capacitors 82 3-14 Power intake response (PIR) during charging v.s. time of Li-Ion Batteries at various charge currents 85 3-15 Power output of a c-Si PV cell both during a clear (a) and a cloudy day (b) 86 3-16 The power output of a 2 dm2 c-Si PV cell during a clear and a cloudy day 87 3-17 Matching between the power output of a c-Si PV cell and the power intake response of a Li-Ion Battery 87 3-18 Irradiance at the 3 different days vs. time measured every 15 minutes at Kassel, Germany 89 3-19 Charging of capacitor, a. Direct (linear), b. Via inductance (switching) 90 3-20 Capacitor Voltage versus Charge Efficiency 91 3-21 Transfer efficiency with the DC/DC switching regulator in buck- boost mode 92 3-22 Transfer efficiency with the DC/DC switching regulator in burst mode 92

181 Energy Matching - Key towards the design of sustainable photovoltaic powered products

3-23 Comparison of the power transfer efficiency by using buck-burst (bb) and burst mode 93 3-24 Typical chart of power delivered by a cellular phone battery during call 95 3-25 The Li-Ion battery output voltage in response to a power pulse demand at temperature of + 20 °C and at -° 20 C 96 3-26 The Li-Ion battery output current in response to a power pulse demand at temperature of + 20 °C and at - 20 °C 97 3-27 Temperature dependency of a Li-Ion battery output voltage for a single pulse 98 3-28 The voltage dip minima of a 100 ms pulse train as a function of temperature for a Li-Ion battery and Li-Ion Polymer 98 3-29 Comparison between a semi-continuous power demand (a 100 s pulse) and a pulse train of 100 ms pulses power demand measured with a Li-Ion battery at 20 °C 99 3-30 Comparison between a semi-continuous power demand (a 100 s pulse) and a pulse train of 100 ms pulses power demand measured with a Li-Ion battery at - 20 °C 99 3-31 Comparison between the output voltages of a Li-Ion Battery as a result of a power pulse delivery at - 20 °C. Two cases: with only a battery and with a combination battery + capacitor 100 3-32 Comparison between the output voltages of a Li-Ion Battery Polymer as a result of a power pulse delivery at - 20 °C. Two cases: with only a battery and with a combination battery + capacitor 101 3-33 The voltage dips as result of a 2 A current pulse delivery from a Li-Ion battery at 20 °C in cases of battery only and battery combined with a capacitor 103 3-34 The voltage dips as result of a 2 A current pulse delivery from a Li-Ion Polymer battery at 20 °C in cases of battery only and battery combined with a capacitor 103 3-35 The Energy Storage System and its interfaces with the other elements in the energy chain 104

4-1 The Energy Matching Model (EMM) of the energy chain of a PV powered product 108 4-2 Various connectors for battery chargers 118 4-3 The Energy Matching Model (EMM) of the entire energy chain of a PV powered product 125

5-1 The Energy Matching Model of the energy chain of a PV powered product 131 5-2 The backpack PV battery charger in use 132 5-3 The lay-out of the recharge unit ‘Solar Tergo’ 132 5-4 Prototype ‘Solar Tergo’ mounted on Boblbee backpack 133

182 List of Figures

5-5 The pupil localizer system: The UWB sender and pupil jacket with PV cells to power the localizer/ receiver 136 5-6 The Spectral Matching of CIS, c-Si and mc-Si PV cells and the AM 1.5 normalized spectrum 137 5-7 An example of a PV battery combination used to power a cellular phone 139 5-8 A PV battery charge system ‘source’ 141 5-9 The Solar Mobile Companion in use 143 5-10 The foldable PV panel in folded (a) and unfolded (b) state 145 5-11 The layout of the elements of the SMC 145

6-1 Approach and guidelines for the design of PV powered products 157

183 Energy Matching - Key towards the design of sustainable photovoltaic powered products

List of Tables

Nr. Title Page 2-1 Energy transfer related PV parameters 40 2-2 Spectral Figure of Matching of various light sources - PV technology pairs, at an irradiance level of 1000 W/m2 52 2-3 Comparison between spectral Figure of Matching (FM ) Spectral and the measured conversion efficiencies at an AM 1.5 spectrum and an irradiance level of 1000 W/m2 52 2-4 Spectral Figure of Matching of various light sources - PV technology pairs, corrected for their irradiance level conversion efficiencies 54 2-5 Other relevant PV design parameters 64

3-1 A comparison of battery characteristics and performances 75 3-2 A selection from Table 3-1 for comparison of characteristics and performances of Li-Ion, Li-Ion Polymer, Nano Technology Li-Ion batteries and super capacitors 83

4-1 Spectral Figure of Matching (FM ) of various light sources - Spectral PV technology pairs, at an irradiance level of 1000 W/m2 108 4-2 Comparison between spectral Figure of Matching (FM ) Spectral and the measured conversion efficiencies at an AM1.5 spectrum and an irradiance level of 1000 W/m2 118 4-2 Figure of spectral Matching of various light sources - PV technology pairs corrected for their irradiance level dependent conversion efficiency 125

5-1 Spectral Figure of Matching of various light sources spectra - PV technology pairs corrected for their irradiance level conversion efficiency in the user context of the Solar Mobile Companion 144 5-2 Overview of the design steps in projects and benchmarked products 149

184 Appendix A: Symbols, Quantities and Units used in this dissertation

Symbol Quantity SI Units Other Units

A PV surface area m2 dm2 PV C Capacity of a capacitor F C Discharge rate A/Ah E Energy J Wh Energy density (volumetric energy density) Wh/m3 Wh/l F Force N

G Irradiance W/m2 I Maximum Power Point Current A M I Peak Power Current A pp I Short Circuit Current A Sc λ Wavelength nm Å M Mass kg P Power W P Peak Power W pp Power density (volumetric power density) W/m3 W/l Specific power W/kg Specific energy Wh/kg Q Charge C

Ø Correlation R Resistance Ω t Time s h T Temperature K ºC V Open Circuit Voltage V OC V Maximum Power Point Voltage V M Conversion efficiency of PV (cell) at STC % ηSTC Conversion efficiency at a non standard % ηIrrad irradiance G W Work J

185 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Appendix B: Abbreviations and Acronyms used in this dissertation

Abbreviation/Acronym Full expression

AC Alternating Current

AM (Optical) Air Mass, i.e. the ration between any sunlight path through the air on earth and the light path in case the sun is directly overhead.

a-Si Amorphous silicon PV cell

BOS Balance of System

c-Si Crystalline silicon PV cell

CIS Copper Indium di-Selenide PV cell

CdTe Cadmium Telluride

CVD Chemical Vapor Deposition

DSC Dye Sensitised (PV) cell

DC Direct Current

DOD Depth of Discharge

DUT Delft University of Technology

ECN Energie Centrum Nederland, Energy research Centre of the Netherlands

EPBT Energy Payback Time

FC Fuel Cell

FF Fill factor

FM Figure of Matching

FM Spectral Spectral Figure of Matching GaAs Gallium Arsenide

GPS Global Positioning System

GSM Global System for Mobile communication

HP Human Power

IR Infra Red

KNMI Koninklijk Nederlands Meteorologisch Instituut, i.e. Royal Dutch Meteorological Institute

LCA Life Cycle Assessment

186 Appendix B

Abbreviation/Acronym Full expression

MPP Maximum Power Point

MPPT Maximum Power Point Tracking

mc-Si Micro/multi crystalline silicon PV cell

NiCd Nickel Cadmium battery

NOVEM Nederlandse Onderneming Voor Energie en Milieu, i.e. the Netherlands Agency for Energy and the Environment

NWO Nederlandse organisatie voor Wetenschappelijk Onderzoek, i.e. The Netherlands organization for Scientific Research

PDA Personal Digital Assistant derneming Voor Energie en Milieu, i.e. el of 1000 W/m

STC Standard Test Conditions, representing a irradiance level of 1000 W/m2 at a spectrum of AM 1.5 and PV cell temperature of 25 °C

SLA Sealed Lead Acid battery

S Step Step Stimulus SR Spectral Response

TLD Tubular Lamp Discharge

PV Photovoltaic

R Step Response on a standardised (Step) Stimulus i.e. a Step Response

UMTS Universal Mobile Telecommunication System

UV Ultra Violet

UWB Ultra Wide Band

VOIP Voice over Internet Protocol

WAP Wireless Application Protocol

WiFi Wireless Fidelity

187 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Appendix C: Basic Units and fundamentals of light energy and photovoltaic conversion

C.1 Light Energy

Before analysing the energy and power from light and conversion of light energy into electricity, one refresh their knowledge of the basics. The watt (W) is quoted in specifications of PV cells as the (SI) fundamental unit of power. One watt is defined as the rate of energy of one joule (J) per second. However, since light is actually a diversity of colours with energy depending on their wavelength, the total photo energy will be a function of both light intensity or the number of light quanta (photon) and the wavelength. The energy of a photon Q with a wavelength λ is give by Planck’s equation as:

Q = hc/λ Eq. C-1

Where ‘h’ is Planck’s constant (6.623 x 10-34 Js) and ‘c’ is the speed of light (2.998 x 108 m/s).

Dealing with illumination sources such as the sun and several types of lamps one has to distinguish between Radiometric and Photometric or visible light units. Photometric units are for example: candela (cd) for luminous intensity, lumens (lm) for the luminous flux and lux (lx) for illuminance.

The candela (cd) is the base unit in light measurement and is defined as: One candela light source emits in all directions (isotropically) one lumen per steradian. A steradian is a unit of solid angle. There are 4 ∏ steradians to a sphere. A lumen has the dimension of power and therefore the visible light flux that can emanate from a light source to illuminate a surface, for example a photovoltaic cell. The lumen is the photometric equivalence of the radiometric watt. As a photometric unit, the unit lumen is related to the natural response of the standard human eye, which is color sensitive. So to determine the light intensities of (artificial) light sources, the eye spectral sensitivity plays an important role. The peak of human eye sensitivity is at a wavelength of 555 nm (green). Therefore:

1 watt at 555 nm = 683 lumens Eq. C-2

Note that the color of an incandescent bulb may vary with input voltage. The reason for this is the direct link between the voltage and the temperature of the filament and therefore with the ‘black-body’ radiation. Irradiance is a measure for the radiometric light flux incident on a surface per unit area, expressed in W/m2. For the PV cell, both irradiance and its photometric equivalent, namely illuminance, will be an important measure. Illuminance is expressed in lm/m2 or more common lux (lx) usually in relation with artificial light.

188 Appendix C

An example: An incandescent light bulb, connected to the mains at 220V, which consumes 60 W of electrical power, produces only 710 lm light. A crude calculation reveals that at a wavelength of 555 nm, only about 1 W (710/683 W) of the original 60 W is converted in useful visible green light. Integrating over the eye response curve, the total yield will be about 5 W. Therefore most of the electrical power put into the light bulb (55 W of the 60 W), is wasted as heat. This information is unfortunately not always indicated. If the same light bulb is used to illuminate a photovoltaic (PV) cell at a distance of 1 m to recharge a battery, the illuminance will be 710 lx or about 1W/m2 at green. Integrated over the whole light bulb spectrum this illumination power will be 2-7 W/m2, depending on the type of light bulb and the medium in between. An overview of the radiometric and photometric units is presented in section C.2.

C.2 Converting radiometric and photometric units

For future reference, the relation between radiometric and photometric units is tabulated.

Table C-1 Relation between radiometric and photometric units

Definition Radiometric Photometric

Name Unit (SI) Name Unit (SI)

Energy Radiant joule = W.s Luminous lumen.sec Energy Energy Energy per unit Radiant flux watt Luminous flux lumen time = Power flux

Power incident Irradiance W/m2 Illuminance lm/m2 = lux per unit area Power per unit Radiant W/sterradian Luminous candela solid angle Intensity Intensity

Power per unit Radiance W/m2 sterradian Luminance candela/m2 solid angle per unit projected

C.2 Converting radiometric and photometric units

Of the total amount of solar power available from outside the earth’s atmosphere, about 30% up to 45% will be lost due to reflection and absorption in the atmosphere. Therefore at sea level on earth a net of only about 1000 W/m2 of solar power can readily be converted into electricity by a solar cell [Hazen, 1996]. In literature, this global irradiance of 1000W/m2 is quite often named the irradiance of ‘one sun’.

189 Energy Matching - Key towards the design of sustainable photovoltaic powered products

The spectrum of this solar irradiance, as received by a photovoltaic converter at sea level, is inside a fixed ‘wavelength-window’. This window starts at a wavelength of about 300 nm and ends at a wavelength of about 4 μm, or effectively at about 1.5 μm. It encloses the visible range from 380 nm up to 750 nm as presented in figure C-1. The attenuation of the sunlight by the atmosphere depends on the optical path length to the observation point. This path length is shortest at the moment that the sun is directly overhead. As a result of this attenuation certain wavelengths or colours will be filtered out. So the attenuation of the sunlight by the atmosphere will not only have impact on the amount of sunlight (the intensity, the W/m2) but also on the spectral distribution.

Figure C-1 Spectral solar irradiance [Engstrom, 1974]

Air Mass (θ) = 1 / cosθ Eq. C-3

To allow a consequential comparison between performance measurements of different PV cells throughout the world, a global standard named ‘Air Mass 1.5’ or AM 1.5 is often quoted in literature.

190 Appendix C

C.4 Summary of the main characteristics of a photovoltaic system

The main characteristics of photovoltaic cells can be found in most textbooks on photovoltaic cells (e.g. Green, 1986). For the reader’s convenience in this appendix a summary of the main characteristics of photovoltaic cells are presented.

a) I-V curve A photovoltaic cell is characterised by its I-V curve in which the current (I) is plotted against the voltage (V). A typical I-V curve of a photovoltaic cell is presented in Figure C-2.

Figure C-2 A typical I-V curve of a photovoltaic cell [Green, 1986]

In Figure C-2/2-26 the typical Diode I-V curve is mirrored against the X axis to clarity. In this I-V curve can be seen: • The open circuit voltage V ; oc • The Short circuit current I ; sc • The maximum power point with it voltage V and current I ; M M • The difference between dark current and current under illumination IL.

b) A General info on Maximum Power Point Each photovoltaic cell/module has at a certain irradiance level a unique optimal operating point of its I-V characteristic in which it delivers maximum power (see Figure C-2), ergo the Maximum Power Point (MPP).

This MPP is dependent on the cell temperature and the available irradiance. To obtain an efficient power output from the photovoltaic conversion it is important to deal with the fluctuations in incident light by anticipating the fluctuations of the MPP with the aid of a MPP tracker (MPPT). Usually the locus, or path, of the MPP as a function of irradiance is traced with the aid of an electronic circuit. The controller in this electronic circuit is programmed to follow a certain Maximum Power Point Tracking algorithm.

191 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Today several Maximum Power Point Tracking (MPPT) algorithms or methods are in use, quite often they are heavily patented and the exact working principles are not known in open publication. Some methods have become common [Hohm and Ropp, 2003; Masoum et. al., 2002]: 1. Look-up-Table (L-u-T) methods; 2. Constant Voltage (CV) methods; 3. Perturb-and-Observe (P&O) methods; 4. Incremental conductance (INC) methods.

Beside the methods listed above it is also conceivable to have for example: 1. DSP controlled MPPT methods; 2. Computational methods; 3. Fuzzy Logic methods.

These methods are however beyond the scope of this dissertation and will not be treated here. A summary of today’s common MPPT methods:

Ad. 1 Look-up-table (L-U-T) methods [Masoum et.al., 2002] In the look-up-table MPPT method the MPP at certain defined irradiance levels are stored in a Table. During PV operation with an external sensor the irradiance level is measured and the corresponding MPP is set in the MPP tracker. These methods are convenient and simple however in general they suffer the drawback of insufficient data available. This lack of data is partially due to the cell suppliers who only supply data measured under Standard Test Conditions (STC) and reluctantly supply performance data for other irradiance and temperature modes. In addition the degradation in time (aging, dirt) is also an uncertainty.

Ad. 2 Constant Voltage (CV) methods [Hohm and Ropp, 2003; Veerachary et al, 2002] The basis for the Constant Voltage (CV) method is the assumption that on a I-V curve the ration between maximum power point voltage, V , and its open-circuit voltage, V MPP oc is approximately constant or:

V / V = Constant < 1 Eq. C-4 MPP oc

The difficulty is choosing the proper constant.

Ad. 3 Perturb-and-Observe (P&O) methods [Hohm and Ropp, 2003] In the Perturb and Observe (P&O) method the operating voltage of the PV cell is perturbed (increased or decreased) by a small increment and the resulting change in output power, ∆P is measured. If ∆P is positive, then it is assumed that due to the perturbation the initial operating point value on the I-V curve is moving closer to the MPP. So the operating voltage is perturbated further in the same direction. If ∆P is negative, then the operating point is moving away from the MPP. Here the voltage perturbation has to be reversed. Since this method can not determine when it has actually reached MPP, it will oscillate around MPP rather than stay at MPP.

192 Appendix C

Ad. 4 Incremental conductance (INC) methods [Hussein et.al., 1995] The basis of this method is that the maximum power point in the P-V curve is attained by differentiating the power with respect to the voltage and setting the result to zero. The output power of the photovoltaic cell P can be expressed as:

P = V x I Eq. C-5

Differentiating P with respect to the voltage V yields:

dP/dV = d(VxI)/dV = I + V dI/dV or (I/V) dP/dV = (I/V) + dI/dV

since:

V = I x R = I x 1/G

The conductance G can be written as:

G = I/V Eq. C-6

And the incremental conductance ∆G as:

∆G = dI/dV Eq. C-7

The change in current dI will correlate with the change in irradiance. There are three options as can be seen in Figure C-3. • dP/dV > 0 if G > ∆G left of MPP Eq. C-8a • dP/dV = 0 if G = ∆G exactly on MPP Eq. C-8b • dP/dV < 0 if G < ∆G right of MPP Eq. C-8c

In this method the operating point voltage is moved until the conductance is equal to the incremental conductance.

Figure C-3 The maximum power point as the first derivative in the P-V curve of a photovoltaic cell

193 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Having established the Maximum Power Point MPP (V , I ) the characteristics of the PV m m cells can be calculated.

For instance with this MPP the fill-factor can be calculated by:

FF = (V x I ) / V x I Eq. C-9 m m oc sc

The efficiency can be calculated by:

= ( V x I x FF) / P = (V x I ) / P Eq. C-10 η oc sc in m m in

P is the light power incident on the PV cell. in

194 Appendix D: Lamp Spectra measured at Philips Lighting BV [Altena, 2005]

For calculations of the spectral Figure of Matching a number of spectral have been measured at Philips. A selection is presented below.

Figure D-1 TLD lamp spectra of the series 9xx

Figure D-2 HID-CDM-T Lamp Spectra

195 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Appendix E: PV performance measurement set-up

Spectral response measurements have been performed at the Energy Research Centre of the Netherlands (ECN), IV-measurements were performed using a SPECTROLAB sun simulator at ECN and a WACOM sun simulator at Utrecht University.

Figure E-1 A schematic presentation of the IV curve measure setup with AM1.5 filter at ECN [Eikelboom et. al. 1995]

The light source of the sun simulator was a Spectro-Lab XT-10 Xenon bulb-lamp with a DC power supply. The appropriate spectrum of AM1.5 and irradiance of 1000 W/m2 was obtained with filters and lenses. With additional gray or neutral density filter combinations the irradiance level was varied from 1000 W/m2 down till 1 W/m2 (see Table E-1) So IV- characteristics were measured in an irradiance range between 1-1000 W/m2, Dark current curves of selected cell samples have been measured as the parallel (shunt) resistance R p and can be determined by the linear slope of the reverse dark current. The serial resistance R can be calculated from two IV curves at light intensities of 500 W/m2 and 1000 s W/m2, respectively, according to IEC 891 [IEC, 2000]. In addition, the basic cell parameter set (diode saturation current and quality factor, short circuit and maximum power point voltages and currents) was calculated automatically in the PC at each measured IV-curve and has been fed into a database for future research.

A measurement of the spectral response was achieved by measuring the short circuit current of the PV cell while it was illuminated with a bias light (0.5 sun) and an additional illumination of chopped monochromatic light. The monochromatic light was generated by a carousel of band pass filters.

196 Appendix E

Table E-1Light Transmission and Filter Combination

The Measurements: • I-V characteristics versus irradiance level; • Spectral response versus irradiance level; • Efficiency versus irradiance level; • Maximum Power Point (MPP) versus irradiance level.

197 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Appendix F: Battery Fundamentals

• Capacity: The battery capacity is a measure of how much energy in Ampere-hour (Ah) the battery can store. The amount of energy that can be extracted from a fully charged battery depends on temperature, the rate of discharge, battery age, and battery type. • Ampere-hour: The Ampere-hour (Ah) denotes the current at which a battery can discharge at a constant rate over a specified length of time. • Discharge rate: This parameter is usually denoted by the nomenclature of C/h. For example a 60 Ah battery discharge at a rate C/20 battery will produce if new and fully charged 3 Amps for 20 hours. • Voltage: By definition a battery consists of two or more cells wired together. A lead- acid type cell produces approximately 2.1 Volts. A three cell lead-acid battery thus produces 6.3 V (6.3 = 2.1x3) and a six cell lead-acid battery produces 12.6 V. For a battery with fill caps, the number of cells can be determined by counting the number of fill caps. The voltage rating is that of a fully charged battery; its voltage will decrease as the battery is discharged. • Depth of Discharge (DOD) and Cycle Depth: Fully discharging a battery often destroys the battery or, at a minimum, dramatically shortens its life. Regardless of whether or not the battery is deep-cycle or not, deep discharges shorten the life of a battery. A deep-cycle battery that can last 300 discharge-recharge cycles of 80% DOD (depth of discharge) may last 600 cycles at 50% DOD. • Energy density/Specific energy: These parameters are measures of how much energy can be extracted from a battery per unit of battery weight or volume. • Power density/Specific power: These parameters are measures of how much power can be extracted from a battery per unit of battery weight or volume. • Operating temperature: Batteries work best within a limited temperature range. Op- erated above that range will cause capacity fading, while below that range the internal resistance will increase causing a decrease of output voltage. • Self-discharge: A battery that is left alone will eventually discharge itself. This is particularly true of secondary (rechargeable) batteries as opposed to primary (non-rechargeable) batteries.

198 Appendix G: Lamp Spectra measured at Philips Lighting BV [Altena, 2005]

G.1 Solar Tergo

Figure G-1 Electronic schema Tergo

G.2 The Universal PV Charger ‘Source’

Component List Universal PV charger ICs • IC 1: i 7665 SCBA 1.00FGR CMOS Over and Under Voltage Detector • IC 2: HEF 4093BT AB484 22 un 03223 Quadruple 2 input NAND Schmitt-trigger CMOS 4000/4500 series (Philips) • IC 3: 9934 SWWD .W624 Dual N- and P channel enhancement mode FET • IC4: 9926A SAKD .L32A Dual N-Channel 2.5V MOSFET, These N-Channel 2.5V specified MOSFETs has been optimized for power management applications with a wide range of gate drive voltage (2.5V – 10V). Optimized for use in battery protection circuits

199 Energy Matching - Key towards the design of sustainable photovoltaic powered products

Other components • C1: SS12 3x Schottky Barrier Rectifier • C2: L5 Inductance • C3: A7 sv oo • C4: 511 2x • C5: E89 • C6: I24 2x • C7: 333 • C8: 684 • C9: 220 μF • C10: Resistors

200 Curriculum Vitae

Sioe Yao KAN was born in ’s-Gravenhage the Netherlands on April 2nd 1948. He has a master degree in experimental physics of the University of Stuttgart in Germany and a master degree in electronics of Delft University of Technology.

Before joining the faculty staff of Industrial Design Engineering of Delft University of Technology he was involved in R&D activities at Philips and Fokker. Those activities were ranging from research for improvements in the video chain at Philips, to designing attitude control systems for satellites and the Ariane 5 rocket prototype and prototype building and testing of the Fokker 70 aircraft.

June 2003 he started a PhD research project at the section Design for Sustainability of the Faculty of Industrial Design Engineering of Delft University of Technology.

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