Development of a New Biomass Stove Integrated with a Stirling Engine

Petra Näsström Julia Kuylenstierna

Bachelor of Science Thesis KTH School of Industrial Engineering and Management Energy Technology EGI-2016 SE-100 44 STOCKHOLM

Bachelor of Science Thesis EGI-2016

Development of new Biomass Stove with integrated electricity generation

Petra Näsström Julia Kuylenstierna

Approved Examiner Supervisor

Anders Malmquist Catharina Erlich Commissioner Contact person

UMSS Joseph Adhemar Araoz Ramos

Abstract 1.2 billion people around the world, mainly in the developing countries, live without access to electricity. To increase the living standard for this people and make socio economic development possible, supplying rural areas with electricity is crucial. In rural areas around the world 2.7 billion citizens use biomass through simple stoves or open fire for heating, lighting and cooking in the households. This leads to indoor pollution which both is unhealthy and dangerous to inhale and ultimately leads to the prematurely death of 4 million people every year. Bolivia is no exception since it is one of Latin Americas poorest country. 1.2 million Bolivians still have no electricity access and fuel wood accounts for 47 % of the energy used for cooking in the country. One solution to minimize the premature deaths caused by pollutants is to improve the cook stoves to become cleaner and more efficient. A product that can contribute to both solve the problems with indoor pollution and no electricity access is an improved cook stove integrated with electricity generation. This thesis will present the first design approach in the development of a new biomass stove with an integrated Stirling engine, primarily adapted for rural Bolivians. The Stirling engine is an external combustion engine that can convert thermal energy to electricity. Through literature survey and field studies information to formulate a requirement specification was collected. A Quality Function Deployment (QFD) was formulated to transfer the requirements into technical measurable qualities. This constituted the guidelines for the development which led to five different concepts. With support from a Decision Matrix, where the different concepts where compared to each other, it was shown that the best concept was MAYA, a stationary stove with capacity for two pots, one combustion chamber, an ash trap and an attached chimney. Combustion and heat transfer processes appearing in a stove are complicated and depending on a number of different parameters. To evaluate a good stove design testing is required which is why the design approach was to look at successful improved stoves and to follow design principles and guidelines. The improved MAYA is a 70 cm high metal stove with a scaffold base constructed with double metal walls with insulation in between. The two pots can together fit 10 liters and are sunken down in the stove body. To create a good draft in the stove the combustion chamber, flue gas path and chimney are designed with a constant cross sectional area. Three different placements for a 95 We Stirling engine prototype are suggested but the final placement has to be further investigated since it depends on the temperatures in the stove. To increase the accessibility of the produced electricity a battery is advisable connected to the engine. A battery with a capacity of 25 000 mAh is shown sufficient to store the daily produced electricity. MAYA is a design draft based on design principles of wood burning cook stoves and can be further used in physical modeling and testing to create a final design.

Sammanfattning 1.2 miljarder människor runt om i världen, huvudsakligen i utvecklingsländer, har ingen tillgång till elektricitet. För att öka levnadsstandarden för dessa människor och möjliggöra socio- ekonomisk utveckling är det avgörande att försörja dessa områden med elektricitet. I de rurala områdena runt om i världen använder 2.7 miljarder människor biomassa genom enkla spisar eller öppen eld för matlagning, belysning och värme. Detta leder till luftföroreningar inomhus vilka både är ohälsosamma och farliga att inhalera. I förlängningen leder det även till dödsfall i en för tidig ålder för 4 miljoner människor varje år. Bolivia är inget undantag då det är ett av Latinamerikas fattigaste länder. 1.2 miljoner bolivianer lever fortfarande utan tillgång till elektricitet och ved står för 47 % av energin använd för matlagning i landet. En lösning för att minimera de för tidiga dödsfallen är att utveckla renare och effektivare spisar. En produkt som både kan lösa problemen med begränsad elektricitet och föroreningar inomhus är en utvecklad spis med integrerad elgenerering. Detta projekt kommer presentera första designförslaget i utvecklingen av en ny biomassaspis med integrerad Stirlingmotor, primärt anpassad till Bolivias rurala områden. Stirlingmotorn är en extern förbränningsmotor som kan konvertera termisk energi till elektricitet. Genom litteratur- samt fältstudier insamlades information för att formulera en kravspecifikation. En Quality Function Deployment (QFD) utformades för att översätta kraven till tekniskt mätbara kvalitéer vilka gav riktlinjerna för fortsatt utveckling och ledde till att fem olika koncept genererades. Med hjälp av en Decision Matrix valdes det mest lovande konceptet av dem alla ut, detta var spisen MAYA: En stationär spis med plats för två kastruller, en förbränningskammare, ett askuttag samt en skorsten. De förbrännings- och värmeöverföringsprocesser som sker i en spis är komplicerade och beror av många olika parametrar. Utvecklingen av en bra spisdesign måste ske via prototyptestning varför tillvägagångssättet för fortsatt design var att inspireras av lyckade utvecklade spisar och designprinciper. Detta resulterade i en 70 cm hög metallspis konstruerad med dubbla metallväggar med isolering emellan. De båda kastrullerna rymmer tillsammans 10 liter och är nedsänkta i spiskonstruktionen. För att skapa ett bra drag genom spisen har förbränningskammaren, rökgasgången och skorstenen samma tvärsnittsarea.

Tre olika placeringar föreslogs för en 95 We Stirlingmotorprototyp, slutgiltig placering måste utforskas vidare då den beror på temperaturerna i spisen. För att öka tillgängligheten på den producerade elektriciteten bör ett batteri kopplas till Stirlingmotorn. Ett batteri med kapacitet på 25 000 mAh visade sig vara tillräckligt för att lagra den dagligt producerade elen. MAYA är ett designförslag baserat på designprinciper för vedspisar och kan användas i fysisk modellering för att utveckla en slutlig design.

Acknowledgement We wish to thank our supervisor Dr. Catharina Erlich. Her points, advice and review comments helped us form this thesis from day one to the finishing day. We also want to extend our gratitude to our supervisor in Bolivia, Dr. Joseph Adhemar Araoz Ramos for the help to get settled in Bolivia as well as his support throughout the entire project.

The main people making our field studies possible, Mariana Butron Oporto and Ricardo Billarroel Camacho from Deutsche Gesellschaft für Internationale Zusammenarbeit (GIZ), we want to thank you for the shared information that helped us understand rural cooking habits. Also Marcelo Gorrity from the Stove Testing Center (CPC) in La Paz who contributed with information about existing improved cook stoves.

We are grateful for the financial support from The Swedish International Development Cooperation Agency (SIDA) through the exchange program Linnaeus-Palme and The ÅForsk Foundation who both made this project possible.

Finally, we would like to extend our deepest gratitude to Dr. Lucio Alejo, Jerry Luis Solis Valdivia, Ricardo Mendoza Sirpa, Pablo L. Cossio Rodriguez and Gabriela Peña Balderrama for all the help we received with our everyday life in Cochabamba.

Julia Kuylenstierna, Petra Näsström Lima, 2016-06-08

Nomenclature

CHP Combined heat and power DFM Design for manufacturing FP Free piston GHG Greenhouse gas GDP Gross domestic product HDI Human development index LHV Lower heating value [MJ/kg] LPG Liquid petroleum gas OECD Organization for economic co-operation and development PAH Polycyclic aromatic hydrocarbons PV Photovoltaic QFD Quality function deployment SE Stirling engine TAG Thermoacoustic generator TEG Thermoelectric generator UNFCC United nations framework convention on climate change

Currencies used 1 USD (American dollars) = 6.89 BOB (Bolivianos) [1]

Table of Contents

Abstract ...... 1

Sammanfattning ...... 2

Acknowledgement ...... 3

Nomenclature ...... 4

Table of Contents ...... 5

1 Introduction ...... 1

1.1 Background ...... 1

1.2 Objective ...... 4

Specific objectives for the final product are: ...... 4

1.3 University Cooperation ...... 4

1.4 Limitations ...... 5

1.5 Methodology ...... 5

2 Literature Survey ...... 7

2.1 Biomass ...... 7

Biomass in Bolivia ...... 8

2.2 Energy and Electricity in Bolivia ...... 9

Electricity Need ...... 10

2.3 Economy in Bolivia ...... 10

2.4 Microgeneration systems ...... 11

2.5 The Stirling Engine ...... 12

Technology ...... 12

2.7 Cooking Habits ...... 14

2.8 Different Kinds of Stoves ...... 15

The Rocket Stove Principle ...... 16

Improved Cook Stoves ...... 17

2.9 State of the Art Cook Stoves Integrated With Electricity Source ...... 18

The µPower Cookstove ...... 18

The TEGSTOVE ...... 19

The BioLite CampStove ...... 19

Score Stove ...... 20

3 Biomass Stove Design ...... 21

3.1 Requirement Specification ...... 21

3.2 Quality Function Deployment ...... 23

3.3 Concept Generation ...... 23

3.4 Concept Evaluation ...... 26

3.5 Further Design and Design Approach for MAYA ...... 27

InStove ...... 27

Uganda 2-pot ...... 28

Rocket Stove ...... 29

3.6 Stove Components ...... 30

Combustion Chamber, Grate and Ash Trap ...... 30

Insulation ...... 31

Stove Body ...... 33

Flue Gas Passage and Chimney ...... 33

Outer Dimensions ...... 34

3.7 Stirling Engine Placement ...... 34

3.8 Pricing for the stove ...... 36

4 Results and Discussion ...... 39

4.1 Dimensions of MAYA ...... 39

4.2 Pricing ...... 44

4.3 Requirement specification ...... 45

5 Conclusion and Further work ...... 46

Conclusion ...... 46

5.1 Further Work ...... 46

References ...... 48

Appendix 1 ...... 1

Sustainable Development Goals ...... 1

Appendix 2 ...... 2

Stove Testing Center (CPC) ...... 2

Appendix 3 ...... 6

Cook stoves integrated with electricity producing source ...... 6

Appendix 4 ...... 9

Requirement Specification ...... 9

Time frame ...... 9

Functional Requirements ...... 9

Requirements ...... 9

Aspirations ...... 9

Restrictive Requirements ...... 9

Requirements ...... 9

Appendix 5 ...... 11

Decision Matrix ...... 11

Appendix 6 ...... 12

Pictures of improved cook stoves ...... 12

Appendix 7 ...... 13

Quality Function Deployment ...... 13

1 Introduction This section presents the background information about traditional cooking and how energy is connected to development. Furthermore, general and specific objective and methodology will be presented together with the limitations for this study.

1.1 Background Because of cooking with bad equipped stoves 4 million humans worldwide die prematurely every year of illnesses caused by polluted air [2]. Women additionally tend to suffer with these health effects more than men [3]. Today, about 2.7 billion citizens in the rural areas around the world use biomass as primary energy source through simple stoves or open fire for cooking [4]. It is stated that a cleaner housing condition will reduce asthma, rhino conjunctivitis and eczema symptoms in school aged children [5].

About 1.2 billion of the world’s population has no access to electricity and the majority of these lives in developing countries [7]. In a developing country the majority of the population lives on far less money with much fewer basic public services compared to the population in the industrialized countries [8]. The Human Development Index, a composite statistics of life expectancy, education and income per capita, is also in comparison low in these countries [9]. In estimations 550 million humans will live without access to electricity by 2040. During the same time the electricity demand will increase with 70 % because of the industrialized countries needs [10]. Figure 1 below shows the links between energy and poverty and how these factors are connected.

Figure 1. The links between energy and poverty [7]

Figure 1 only offers a rough description of the key components for sustainable development. Therefore Figure 2 below offer a deeper explanation of the key concepts.

Figure 2. The links between energy and the connections to environment, income, education and health [7]

Education is a main factor for changing people’s behavior, as shown in Figure 1 and Figure 2, it is also important in order for the society to develop. It is shown that education about sustainability at all levels is crucial if the citizens should embrace sustainable development in their everyday lives [13]. It is therefore essential to integrate the education possibility in rural areas to reach a sustainable development and reduce poverty already from the beginning [7]. Students in ages between 6 to 18 years old study 14 minutes longer per day if they have electricity. It is also shown that the children doing well in school are more likely to stay in school longer which further is another reason why energy access is crucial for a sustainable development [14].

The links between electricity access and advancement in socio-economic conditions can be seen in Figure 3. One can also notice that electrification will directly reduce the respiratory illnesses because the reduction of pollutants used for heating and lighting with dirty fuel. This will benefit the society due to reduction of health care costs, both private and public [14].

Figure 3. Electricity access and its impact on development [15]

Sustainable development through an environmental, social and economic perspective is crucial for humanity and the planet [11]. For sustainable development and poverty reduction access to affordable and modern electricity is vital [12]. It is therefore important, in order to develop rural areas and reduce poverty, to make electricity more accessible. Further, develop improved cooking devices to avoid the pollution that today’s traditional stove emits.

1.2 Objective The 2030 Agenda for Sustainable Development by the United Nations states 17 development goals for economic, social and environmental sustainable development, which are presented in Appendix 1 [11]. Eradicating poverty is seen as the greatest global challenge and an indispensable requirement for sustainable development. The importance of healing and securing the planet and shift the world to a resilient and sustainable path is pointed out. Particularly relevant in this context is development goal number three and seven and follows [11]:

“Ensure healthy lives and promote well-being for all at all ages”

“Ensure access to affordable, reliable, sustainable and modern energy for all“

A step towards these goals could be to develop alternative electricity sources and sustainable cooking equipment for rural populations with deficient grid connection. One solution is a combined heat and power plant for the final consumer [16].

The overall objective for this project is to develop a new biomass stove design of household scale that is sustainable from an economic, health and environmental perspective. This stove shall also be integrated with a Stirling engine for electricity production using the waste heat of the biomass fuel used for cooking.

Specific objectives for the final product are: 1. Design a biomass stove that satisfies the rural cooking habits 2. The combustion process in the developed stove shall be complete 3. The electricity generating source shall cover basic electricity need for an average Bolivian family 4. Formulate a requirement specification with requirements respecting the rural population 5. Investigate the economical limitation for the stove in order to make it accessible

1.3 University Cooperation This project has taken place in Cochabamba, Bolivia, since there is a research cooperation between the universities Royal Institute of Technology (KTH) and Universidad Mayor de San Simon (UMSS). The Research Cooperation Program works to educate Ph.D. students for UMSS to get a better foundation of knowledge. Students from both universities also get the possibility to write bachelor or master thesis at the other university. This project is a bachelor thesis following the guidelines of the ongoing Linnaeus Palme program [17].

1.4 Limitations This project will focus its study on Bolivian rural cooking, although the results will be valid in other rural areas with similar difficulties regarding electricity and cooking. The design of the stove will be custom for biomass fuel and the size will be for a family household and indoor use.

The wanted Stirling engine is not available for commercial use which is why the biomass stove will be designed to become a reality in 5-10 years from now.

Production analysis, design details and final material decision are suggested for complementary future work.

1.5 Methodology The four phases in a product design process is investigation, planning, implementation and inspection and this project followed these steps as well [18]. The project was organized into four phases as seen in Figure 4. As showed in the figure the difference between the described process and this project was that the planning was made before the investigation phase, number one in the figure. Further the implementation phase was divided in two, phase two and three, because of its workload.

Figure 4. The methodology used in the project The planning phase included time, logistic and project planning and was made before phase one and two started. All the phases will be described accordingly.

The study began through phase one where existing stove solutions and today’s situation for the rural population in Bolivia were studied. The literature study also consisted of deepening the knowledge concerning Stirling engines, biomass stoves, rural cooking habits and stove design principles. The investigation regarding the subject was also made through a field study. This consisted of meetings and interviews with local manufacturers in Cochabamba to see the further possibilities of manufacturing a stove. Parallel with this work a collaboration with the German Cooperation, Deutsche Gesellschaft für Internationale Zusammenbeit, (GIZ) took place where ideas and experiences were exchanged. To understand rural cooking habits and traditions an improved cook stove was inspected through a field study with GIZ. Additionally The Stove Testing Center (CPC) in La Paz was visited to receive information about stove testing and to see different models of improved cook stoves. As seen in Figure 4, phase one progressed throughout the entire project; this because planning difficulties with field studies and also the need to deepen the information throughout the entire project.

The second phase, and first part of the implementation phase of the project, was to gather the collected data and turn it into technical requirements through a requirement specification and a Qualify Function Deployment (QFD). The work at this stage contained divergent designing techniques through different concept generating processes. The aim was to create a variety of concepts.

The second part of the implementation, and third phase, was into convergent designing methods where the most successful concepts were individually evaluated. The final overall concept was selected at this stage of the project. The decision was made through a Decision Matrix.

The project got to its fourth phase and also the inspection phase regarding the product development methodology. This consisted of material analysis, dimensioning suggestions and a realization in Solid Edge [105]. Finally the further work was analyzed.

All of these phases were successive documented in this thesis and can be read accordingly.

2 Literature Survey This section will analyze biomass, energy and economic aspects to lay a foundation for future development. The chapter also contains a review of stoves used in rural areas as well as basic facts about the Stirling engine and combined heat and power systems. Lastly the chapter will display a state of the art regarding stoves integrated with an electricity producing source.

2.1 Biomass Biomass includes all organic materials that are made from the photosynthesis. It is a natural method of storing solar energy in form of chemical energy in a plant. In terms of energy biomass is defined as organic material that can be converted to fuel and is therefore regarded as a potential energy source [19]. Wood, agricultural residues and dung are examples of traditional biomass [20].

Today, biomass represents 10 % of global annual primary energy supply, mostly traditional biomass used for cooking and heating which also constitutes the largest proportion of renewable energy [21][22]. For rural energy supply in developing countries, biomass often accounts for more than 90 % and is generally used with very low efficiency of about 10 - 20 % [23][24]. From 2010 to 2030, the number of people using traditional biomass is estimated to increase with over 100 million [25].

If the combustion of biomass is complete the fuel is classified as a sustainable energy source regarding CO2-footprint since it returns the same amount of CO2-molecules to the atmosphere when burned as it holds during its lifetime. Other advantages are low cost of growing, low level of technology for growing and using and can be considered a renewable energy source [26].

In case of incomplete biomass combustion it releases pollutants that are harmful for both environment and health such as carbon monoxide (CO), nitrous oxide (N2O), methane (CH4) and polycyclic aromatic hydrocarbons (PAHs) [27]. Residential sources, mainly from cook stoves, represent more than 25 percent of the global inventory of black carbon emissions [28].

Irresponsible and negligence biomass use can cause deforestation, a problem that occurs when forests are replaced with faster growing crops or fuel wood is used without re-plantation. Deforestation is in conflict with the United Nations Framework Convention on Climate Changes (UNFCCC) as well as United Nations (UN) 2030 agenda [29]. It is contributing to extermination of plants and animals, increased flooding and climate changes [30][11].

Depending on firewood species, zone of harvesting and dried or not the moisture content value in the wood differs. As seen in Table 1 below, the lower heating value (LHV) increases when the moisture content decreases. The lower heating value is defined as the heat amount of a fuel portion evolved when a unit weight of the fuel is completely burnt and water vapor leaves with the combustion products without being condensed [32].

Table 1. Lower heating value for firewood with different moisture rate [33]

Firewood, different Moisture Rate LHV properties [-] [ %] [MJ/kg] Firewood (wet, fresh out) 40 10.4 Firewood (air dried, humid 20 14.6 zone) Firewood (air dried, dry zone) 15 15.6 Firewood (oven dry) 0 20.0

After ignition, the moisture in the fuel requires energy in the evaporating process. First when the fuel is dried the temperature rises and the combustion can be fully developed. The residual ash outcome is correlated with the initial fuel moisture rate. Ash content for firewood with moisture rate between 10 - 60 % varies between 0.25 - 1.70 % [33]. A low moisture rate and a low rate of ash content are most preferable. There is a correlation between efficient biomass fuel and a high level of heating value which is why low moisture rate is important for the efficiency [33].

Biomass can hence be sustainable if used correctly but at the same time harmful for humans and environment if used incorrectly. Modern biomass based cooking options such as improved biomass-, biogas- and producer gas-/red stoves can potentially play an important role in mitigating greenhouse gas (GHG) emission [27][31].

Biomass in Bolivia Due to Bolivia’s variety in topography the accessibility and availability of fuel sources differ within the country. In Figure 5 the annual biomass production in different areas of Bolivia is displayed. In the tropical zone of the country primary firewood as fuel is chosen, because of the forest´s easy access. Dung is a common fuel source and liquid petroleum gas (LPG) is frequently used for long time cooking or during rainy season [6]. As seen in Figure 5 the biomass production is equal to zero in the mountainous south-western parts, despite this firewood is the

8 most common fuel source over all in rural Bolivia [35]. Year 2009 wood accounted for 47 % of fuel used for cooking in the country and it is said that fuelwood gives the food more of a character and is therefore preferred. Fuelwood is in general easy accessible in Bolivia and the most frequent used fuel source of today [6].

Figure 5. Map of the annual biomass productivity in Bolivia in the year of 2012 [34]

Even for the rural population in the fertile areas the collection of firewood can be both time consuming and dangerous. Therefore a fuel efficient and health promoting stove should make everyday life easier. To achieve the good consequences with this new biomass stove it has to match cooking habits and stove utilization of today. This leads to a number of criteria for the stove and the use of biomass concerning combustion, reforestation and smoke emission.

2.2 Energy and Electricity in Bolivia Bolivia does not have many big manufacturing industries and therefore, the country is not a big electricity consumer. In the country with only 493 kWh per capita yearly compared to the 8795 kWh per capita and year consumed by OECD Countries [41].

In the rural areas of Bolivia 66 % of the inhabitants have access to electricity compared to 99 % in the cities. In 2012 1.2 million Bolivians had no electricity access and traditional biomass was an important fuel for heating and cooking. For those who have access to electricity, natural gas- fired plants and hydropower are the dominant sources of Bolivia’s electricity supply [36].

The governmental plan for Bolivia is to access electricity for all citizens by the year 2025 [37]. Electrifying the rural areas of Bolivia has been proven to be more expensive and time consuming than originally considered, which is why the plan to electrify the entire country by 2025 can be seen as unreasonable [38].

The energy demand in different topographical zones has been investigated and the result showed that the average daily energy demand was 32 kWh for rural households, of which 30.5 kWh was used for cooking [6].

Electricity Need Daily electricity use in rural households connected to the grid varies between 1.7 - 4.2 kWh in existing studies [6][42]. In addition the wanted devices in rural households has been investigated. Even though the number of households in the study were few one can see a pattern; illumination, a refrigerator and the possibility to charge a cellphone were highly desirable. When looking at the economical assets in the same study it was seen that a fridge is a big investment for the rural population [6].

With today’s fuel-efficient devices both illumination and cellphone charging can be achieved with low electrical consumption. A simple cellphone has a battery capacity around 800 mAh and 3.7 V, which is equal to 2.96 Wh [44]. A LED light bulb with a power of 7 - 8.5 W is illumination wise equivalent to a 40 - 60 W incandescent bulb. The LED light of today also has a long lifetime of 30 000 - 50 000 hours [40].

2.3 Economy in Bolivia Bolivia is one of the poorest Latin American countries. It has no big manufacturing industries and depends in many sectors on foreign import. The economy of the country is traditionally based on subsistence farming, the production of hydrocarbons and mining [41]. Bolivia owns one of the biggest fields of natural gas in the world, it also contributes to 8 % of Bolivia’s Gross Domestic Product (GDP) and the export of the fuel accounted for 54 % of the country’s total export in 2014 [36]. Except natural gas Bolivia also has great mineral and oil reserves [36][39]. Due to increased mineral and natural gas exports the economic growth in Bolivia was in average 4.9% per year between 2004 -2014 [43].

Despite great natural resources about 39 % of the population 2013 lived, because of inequalities and a weak institutional framework, under the national poverty threshold [41][45].

Poverty can be described as:

“Poverty is associated with the undermining of a range of key human attributes, including health. The poor are exposed to greater personal and environmental health risks, are less well nourished, have less information and have a higher risk of illness and disability” [48]

2.4 Microgeneration systems Microgeneration systems are small scale decentralized energy generation of heat and/or electricity. These can be categorizes in the three following groups [71];

1. Microheat based on heat pumps (air, water and ground source), biomass or solar thermal 2. Microelectricity based on solar photovoltaic (PV), microwind turbines and microhydro 3. Micro combined heat and power (micro-CHP) or cogeneration systems based on internal combustion engines, Stirling engines or fuel cells

Where the third category micro-CHP based on Stirling engines is applicable for the biomass stove.

The combined heat and power (CHP) arrangement combines two forms of energy, thermal together with electrical or mechanical. The CHP is therefore able to use fuel in a way that is more efficient compared to conventional separate electric and thermal energy technologies [71]. The CHP arrangement appears in different scales. The micro - CHP is defined as a unit with a maximum capacity below 50 kWel which suits the individual households and thus the target group [79].

Small Stirling engines with a power output less than 1 kW barely exist in prototype or research stage and the same situation applies for Micro-CHP systems of a household scale. Some small Stirling engines or CHP-SE for military applications have been reported [80][81].

Bigger CHP-SE systems for commercial use seem to be more common. Cleanergy offers solutions powered on either biogas or solar power. Those have an electrical peak efficiency of 25-30 % and a power output of 9 - 11 kW, the life expectancy for the systems are 25 years [83].

Qnergy has though invented a CHP solution fueled on natural gas or propane. The free piston Stirling engine generates a power output between 3 - 6.5 kW [84]. However, the electrical efficiency for biomass driven CHP-SE systems is low, around 10 - 15 % [85][63]. The thermodynamic total efficiency is reported to vary between 59 - 73 % [65].

2.5 The Stirling Engine In the new biomass stove, fuelwood will be used for cooking and a Stirling engine will take advantage of waste heat to produce electricity which will increase the efficiency.

The first Stirling engine (SE) was patented in 1816 by Robert Stirling, who also gave the engine its name. [64]. Even if the engine has been relatively efficient and promising has the development has been slow. Literature from the last decades report a number of Stirling Engine applications, mostly for heat and power generation, which might be seen as the revival of the technology [67][68][69] [83][84]. Today the engine is hardly commercial but some can be found like the biofuel compatible Stirling engines of 1 respective 3 kWe that can be bought for 16 500 and 19 300 USD [136].

The engine is quiet running and fuel flexible, it can theoretically achieve high electrical efficiency. These are properties that make the Stirling engine suitable for combined heat and power arrangements. It does not require special infrastructure and suits therefore rural environment [71].

Technology The Stirling engine is an external combustion engine. It is based on the Stirling cycle, a closed thermodynamic cycle where the working gas is compressed in a cold cylinder volume or expanded in a hot cylinder volume [67]. The engine is working due to a temperature difference between a hot and a cold end of the engine where thermal energy is transformed to mechanical energy. The hot side is pictured in red and the cold in blue in Figure 6. The heat to the engine´s hot side is transferred through a heat exchanger, unlike the internal combustion engine where heat is supplied to the cycle by combustion of fuel inside the cycle [67].

Figure 6. A drawing of the Stirling engine with the hot side in red and the cold side in blue [72] The Stirling engine appears in three different types of cylinder couplings α, β and γ shown in Figure 7. The alpha engine has two pistons in separate cylinders; those are connected in series with heater, regenerator and cooler. As shown in Figure 7 the Beta engine has a displacer and the power piston in the same cylinder. The gamma configuration has, as the beta, a displacer and a power piston but in two different cylinders. The engine can also be categorized based on piston coupling that can either be rigid-, gas-, or liquid coupling [76]. One of the gas coupling engines is the free piston engine. It has gas dynamically, instead of mechanical, linkage for the piston and the displacer. Advantages with this type are reduction of maintenance and possibilities to develop more compact engines [65]. These qualities make it suitable for the combined heat and power arrangement [77]. A well-made free piston SE can achieve 50 % and above of the ideal Carnot efficiency [78].

Figure 7. The different cylinder couplings appearance in Stirling engines [66]

The free piston Stirling engine has a lifetime of about 10 years and the maintenance interval for small engines (capacity > 20 kW power) are 5000 - 8000 hours. This is low compared to internal combustion engines [65].

A battery will increase the accessibility of the electricity produced in the Stove. This means that the electricity can be used when needed and not only when the stove is running. The average daily time for cooking in rural households was estimated to be 130 min [6]. With a power output from the engine of 95 W the daily energy production in the households would be 206 Wh. A 12V and 25 000 mAh battery, only discharged to 70 %, would be sufficiently to store all daily produced electricity [86][87]. Further, Bolivia has great natural resources in lithium which is a component in the lithium battery. This may contribute to future industry and economic development.

2.7 Cooking Habits For a successful stove implementation cooking habits in a cultural and social perspective are crucial to consider. The technology is obliged to be customized for the user. A number of projects have failed tried to implement new stove-concepts in rural areas not considering these aspects [53][54]. Technology adoption is influenced by perceived difficulty, adoptive experiences, supplier’s commitment, perceived benefits, compatibility and enhanced value [55]. For a successful implementation of the new biomass stove it is therefore important to educate in maintenance and provide support during the lifetime of the stove, declare the advantages with the stove and make a new stove that allows the user to have the same cooking habits as today.

The economic situation is foundational for the possibility to cook and determine what families in general cook. This makes the cooking habits throughout rural Bolivia diverse. One can though see some similarities if the cooking habits are studied. One of them is that it is most common to eat three cooked meals a day and the biggest out of all of these three meals is the lunch. This lunch often consists of two dishes, first a soup and then a dish consisting of meat with rice, potatoes or quinoa. Total average daily cooking time was determined to be 130 minutes [6].

A long-lived tradition in the rural areas of Bolivia is to drink a maize beverage called Chicha, a socially important drink [56]. To make this traditional drink one has to boil the water together with spices and then add the maize flour and let the drink boil minimum 2 hours [57]. In Bolivia it is also mostly common to cook all of these meals in a standing position [58].

2.8 Different Kinds of Stoves The term stove means a device that burns fuel or uses electricity in order to heat a specific place [59]. As already stated, most of rural cooking is through simple biomass stoves and Bolivia is no exception. The rural citizens often use different kinds of fuels depending on the accessibility in the area. Therefore the stoves also vary in design [6].

The most common stoves in Bolivia are the open fire stove and the U-type stove which Figure 8 and Figure 9 shows. It is possible to see design changes for the stoves throughout the country because of the altitude changes [6].

Figure 8. The open fire stove [6]

For example on higher altitude, it is more common to have well-constructed stoves with a small risk of heat waste; this difference can be seen when comparing Figure 8 and Figure 9. But throughout Bolivia it is not common to see a chimney attached to the stove. It is also common to have the stoves located in the house, especially in the areas in high altitude where the temperature is lower and the need for heating is bigger [6]. This makes the problem with indoor pollution even more important to solve.

Figure 9. The open fire U-type stove that is one of the most common in Bolivia [6]

The Rocket Stove Principle There have been many projects where the firewood cooking stoves have been improved and new concepts developed. The rocket stove principle is one of the most successful new designs of a improved cooking stove [60]. As shown in Figure 10 the stove is integrated with a large combustion chamber. This chamber behaves a bit like a chimney which is good regarding the pollution problem previously discussed since the smoke removed from inside the house. The internal walls are isolated which reflects all the heat back into the chamber instead of warming up the stove body. The heat makes the chemical combustion complete which is why many of the improved fuel wood cook stoves follow the rocket stove principle [49].

Figure 10. The rocket stove principle [49] This principle is successful because of the stack effect. The hot air is lighter, less dense, than the cold air [46]. The hot air wants to rise and out which creates a diminished air pressure where cold air gets sucked in to the combustion chamber. These thermal forces create a draft throughout the construction, which formulates higher temperatures, and further a complete combustion [47].

Improved Cook Stoves To discover and investigate different kinds of improved cook stoves a field study was made to Stove Testing Center (CPC). These stoves design principles and thermal efficiency can be seen fully in Appendix 2. The discovery of the outer design was that there were two categories of stove designs on the market. One was a portable stove design and one was a stationary stove design in a standing height. There were also discoveries about the stove characteristics such as stoves with or without chimney, with or without fan, sunken down pots or not.

In addition to the different design categories analyzed during the visit to CPC an interesting implementation for an improved cook stove with traditional design named Malena was discovered. Knowledge about this stove and the way it is implemented was gathered during a field study with GIZ. The implementation regarding its sustainable way to introduce the Malena stove to the rural areas in Bolivia [63]. The users prepare the clay mix beforehand and then

17 locally trained stove builders construct the stove. After the stove is constructed there is also follow up monitoring of the installation [61]. The construction of the stove does not need any other tools than available in the house [62]. A principle sketch of the Malena stove can be seen in Figure 11.

Figure 11. Principle sketch of the Malena cook stove [62] 2.9 State of the Art Cook Stoves Integrated With Electricity Source There have been several projects where cooks stoves also have been working as an electricity source. Four improved cook- or camping stoves that follows this principle are presented below.

The µPower Cookstove An engineering team from Trinity College Dublin has developed a low cost, locally made biomass cook stove [92]. The µPower Cookstove can be seen in Figure 12 together with all its components. This stove can also produce electricity through locally made thermoelectric generator (TEG) devices [93]. TEG is a technology that converts heat into electrical energy without moving parts as in an engine [94].

Figure 12. The µPower Cookstove with all its components [93] The TEGSTOVE This is a portable gas stove which transforms the energy from the heat to electrical energy through TEG technology. As seen in Figure 13 the TEGSTOVE is integrated with a battery so that the electrical energy can be stored for later use. The TEG device consists of two ceramic plates connected with two different materials. The temperature difference between these two plates it used to generate electrical energy [95].

Figure 13. The TEGSTOVEs complete design [95] The BioLite CampStove The BioLite CampStove is a camp stove that also generates electricity. The camp stove is made of metal and can be seen in Figure 14. This stove uses fuelwood and has smokeless flames [96].

The smokeless flames are possible because of an integrated fan that leads to complete combustion [97]. The BioLite CampStove uses TEG technology to convert heat power into electrical power, this electricity can be used at the same time as the fire is burning [98].

Figure 14. The BioLite CampStove [96] Score Stove The Score stove is a biomass stove integrated with thermoacoustic generator (TAG) to create electrical power. As seen in Figure 15 the stove is accompanied with a chimney to avoid indoor pollution [99]. The TAG itself has only one moving part and uses the temperature difference to induce high amplitude sound waves [100]. Compression and expansion is driven by a gas which is motioned by these sound waves [101]. The display of the Score stove can be shown in Figure 15 together with description of the different stove parts.

Figure 15. The Score stove displayed together with all is stove parts [102] To create a deeper understating of material and usage for the stoves described above, pictures can be seen in Appendix 3. 20

3 Biomass Stove Design This section will describe the procedure to get to the final stove design through concept generation and evaluation based on the requirement specification. It also includes material suggestions for the different stove components and suggestions for placement of the Stirling engine. Lastly a price investment for the stove will be made.

3.1 Requirement Specification A requirement specification is a list of requirements which are independent of solutions [103]. All the requirements, wishes and needs formulated in the literature survey through field studies, observations and literature research, were summarized and formulated into the requirement specification for the further design of the stove. The complete requirement specification can be found in Appendix 4.

The aim and prerequisites for a successful stove were formulated into a design that avoids indoor pollution, makes complete combustion and provides easy removal of ashes. The literature survey showed that fuel wood is the most common fuel source for cooking in rural Bolivia why the stove shall be adapted for fuelwood burning. Further the size of the stove should be customized for a family of five persons since it is the average in Bolivia [62]. This also applies for the Stirling engine that should have a power output of minimum 100 W. This will cover the electricity that lightning and cell phone charging requires, stated in the literature survey as wanted devices likable for the rural population to get.

A central objective, and therefore a crucial requirement, was to develop a stove that satisfies the rural culture and cooking habits. Due this objective requirements of two pots, stove height for an average Bolivian woman and stove adopted for fuel wood were formulated.

The airflow inside the stove should be ideal in the sense of not cooling the fire with unnecessary air neither too little air for complete combustion. Further the importance of the flames not reaching the pot or plate to avoid a burned bottom was pointed out. In Table 2 all the functional requirements are displayed.

Table 2. Functional requirements from the requirement specification

Functional Requirements The size of the stove shall be customized for a family of five persons [62]

The combustion in the stove must be complete [5]

The stove shall have a solution for easy removal of ashes

The air flow inside the stove shall be ideal [60]

The fire shall not be in direct contact with the pot [60]

The stove shall have minimum two pots

The stove shall be designed for a standing position for an average woman in Bolivia [2]

An electricity generating engine shall be integrated in the stove

The engine shall be integrated with a battery

The stove shall satisfy the rural culture and cooking habits [54]

The stove shall be adapted for burning fuel wood [35]

To solve the problem with indoor air pollution the emission levels of the stove are essential.

Maximum emission levels for PM10, PM2.5 and CO are set with respect to WHO and European Commission´s guidelines and can be seen Table 3 together with the other restrictive requirements. Many of these requirements are impossible to evaluate without a physical model of the stove and will therefore not be evaluated in this project.

Table 3. Restrictive requirements from the requirement specification

Restrictive Requirement The lifetime of the entire stove concept shall be minimum of 10 years

The stove shall boil 5 liter of water in maximum 30 minutes [58]

The minimum thermal efficiency shall be 25 % [58]

Stirling engine shall maintenance interval of minimum 5000 hours [77]

Dimensions of the stove shall not be bigger than 3x1 m [62]

3 Not emit more than 0.035 µg/m per minute of PM10 to the indoor air [149]

The Stirling Engine shall have the capacity of minimum 100 W

The stove will need maximum 20 000 kJ to boil 5 l of water [58]

Emission rates from fuel combustion should not exceed 0.80 mg/min of PM2.5 (fine

particles) [148]

Emission rates from fuel combustion should not exceed 0.59 g/min of CO [148]

The outer temperature of the stove body shall not exceed 52 °C [150]

3.2 Quality Function Deployment A Quality Function Deployment (QFD) was made to systematically transfer the customer requirements and wishes into measurable technical qualities. In this way, the most important stove qualities could be identified for the coming design decisions [104]. The customer requirements and wishes used in the QFD were collected from the requirement specification. These wishes and requirements together with measurable product qualities formulated the foundation of the QFD. The customer requirements were weighted according to the customer experience of what was seen as most important. Further these requirements were weighted towards the product qualities given a number 1, 3 or 9 depending on how strong they were correlated. This weighting was, for respective product quality, summarized into a technical weight. The quality with the highest technical weight was the most important one to focus on in the design aspect.

This analysis showed that the most important product quality was thermal and overall efficiency in the stove. Another highly weighted quality was the stove´s height and that the stoves surface area could not be too warm. It is possible to find the full QFD in Appendix 7 together with all customer requirements and product qualities.

3.3 Concept Generation A first concept generation followed the feasibility research. At this stage the aim was to create a great variety of solutions and see the possibilities in them. Focus on the stoves fulfilling the requirement specification was not prioritized during the generation when the aim was the diversity. The concept generation was made after a field study to CPC discussed earlier. The characteristics were studied from the successfully tested stove design and furthermore a design concept from each of the discovered successful stove characteristics was formulated. From these discoveries two families of stove solutions were shaped, one portable and one stationary. The

23 most interesting solutions were three portable and two stationary stoves all sketched in Solid Edge and can be seen with description [105]. The different concept generated were named afterwards so it would be easier to separate them in further work.

An assumption was made that the integrated engine does not affect the basic stove design why a decision was made to first develop the fuel wood stove and then investigate the optimal placement for the engine.

The portable stoves were formulated in three different appearances and can all be seen in Figure 16. The first concept shown in Figure 16 a) named SOLAN is a portable stove with an integrated oven close to the combustion chamber. There are two hobs for two pots and the darker material seen in the figure is made of a conductive material. The rest of the stove consisted of a heat resistant metal combined with an insulating material.

Next portable stove is named IDA and can be seen in Figure 16 b). IDA also has a chimney but only place for one bigger pot. The pot has a skirt for potential reduction in fuel use and emissions [89]. The body of this stove was metal based with an insulating material around the pot and combustion chamber.

The third and last portable stove named ULLA can be seen in Figure 16 c) and is like the others, mainly made of metal and insulation. This stove does not have a chimney but instead a fan. The Stirling engine driven fan boosts the air flow and enables complete combustion. In general, improved stoves with a fan have a fuel efficiency of 60 % compared to improved cook stoves without a fan which has a fuel efficiency of 25 - 30 % [58].

Figure 16. a) SOLAN the portable stove with a chimney, oven and place for two pots b) portable stove IDA with chimney and pot skirt c) portable one-pot stove with an integrated fan, ULLA.

The two stationary stoves named KARIN and MAYA can be seen in Figure 17. They are more traditionally designed and inspired by the Malena cook stove [62]. Both of them have submerged pot holders to preserve the heat and also a chimney to carry away the gases. They are also both integrated with a combined ash trap and air intake. The first version KARIN seen in Figure 17 a) have perpendicular gas flow compared to the pots and the second stationary stove, MAYA seen in Figure 17 b) have a parallel flue gas flow to the pots.

Figure 17. a) Stationary stove KARIN with a chimney, ash trap and two settled pot holders b) Stationary stove MAYA with two settled pot holders, chimney and ash trap 3.4 Concept Evaluation A Decision Matrix was formulated to evaluate the generated concepts accordingly. The method scores each concepts relative in order it meets the given criteria [104]. Each concept from the concept generation phase was evaluated and assimilated to an ideal concept. The highest scoring concept was the most interesting concept for further development and the complete matrix can be seen in Appendix 5.

The highest scoring overall design concept in the Decision Matrix, with respect to the requirement specification, was MAYA seen in Figure 17 b. This is the traditionally and stationary designed stove with two pots.

What was decided to keep out of MAYA in the further development was two sunken down pots, keep it stationary, a chimney and that the stove should be made out of metal. This because it is advisable to use thin low density material in the stove construction due to reduced heat transfer through the walls [106]. For the surface area not being too warm and wall insulation should be used. Further the design with a high technical Stirling engine will look inhomogeneous if a metal material were not chosen. Furthermore, since reduction of materials contributes to lower material costs it was decided to only cover the necessary parts of the stove body. The further work regarding MAYA contained additional development of the stove design, material parameters, inner design and components.

3.5 Further Design and Design Approach for MAYA A number of different factors affect the fuel efficiency of which the key factors found are described below.

1. Combustion Efficiency, formulates how much energy in the fuel that releases as heat. 2. Heat Transfer Efficiency, percentage of generated heat that transfers, through conductive, convective and radiative heat transfer processes, to the contents of the pot. 3. Control Efficiency: adjustment so only as much heat as needed for cooking is generated.

The thermal efficiency is the combustion and heat transfer efficiency together. These two with the control efficiency is named the stove efficiency [108].

However the actual combustion process taking place in the stove is too complicated, highly interdependent and variable to be modeled within this project. Because the complex processes a physical model must be tested to determine the inner design parameters which correlates to the stove efficiency. The complexity in the combustion process formulated a design approach where successfully improved stove designs were analyzed and adapted to the final concept. Some successfully tested stoves will be displayed together with important data and how these can be adapted in the MAYA stove.

InStove This institutionally used stove adapted for pot sizes 60 or 100 liters has a high thermal efficiency [109]. The MAYA concept is inspired of its high performance combustion chamber.

InStove has the best grading regarding efficiency and indoor emission and second best grading when it comes to emission and safety in Global Alliance for Clean Cookstoves third part testing [109]. A principle sketch of the stove can be seen in Figure 18.

Figure 18. A principle sketch of InStove, an institutional cook stove The combustion chamber is made out of high-temperature 310 stainless steel and 601 nickel alloys. The operating temperature rises to 1100 °C which is why the stoves emission grading is good. The stove can be bought for 850 - 995 USD depending on the size [110].

Uganda 2-pot The two pot stove with a chimney showed in Figure 19 boils water fast and is considered fuel efficient. Compared to open fire the carbon monoxide and the particulate matter are low, 2 respective 3 %. Tests regarding the Uganda 2-pot stove declares that the first pot in the flue gas passage needs to be smaller than 25 cm in diameter if the second one is 23 cm in diameter if both of the pots should be able to boil [111].

Figure 19. A drawing of Uganda 2-pot with outer dimensions [111] The combustion chamber is of rocket type made with insulated firebricks. Hot flue gases are forced to pass in a narrow insulated channel around the sunken pots before exiting the chimney. In the sheet metal tops are the pots fitted tightly to prevent smoke in the kitchen. This stove is available for 40 USD [111].

Rocket Stove For the rocket stove, combustion chamber diameter and area are given together with different gap dimensions for respective pot diameters. These dimensions can be seen in Table 4 for different pot diameters.

Table 4. Given gap sizes, combustion chamber area and diameter for pot diameters [106]

Pot Combustion Chamber K=D/2 H=K+D Pot Skirt Diameter Chamber Area Gap Diameter (L) (D)

[cm] [cm] [cm2] [cm] [cm] [cm] < 21 12.0 113 6.00 18.00 1.6 21 - 27 14.0 154 7.00 21.00 1.5 28 - 30 16.0 201 8.00 24.00 2.0 31 - 35 16.0 201 8.00 24.00 1.5

The description of where these gaps are located in the rocket stove can be seen in Figure 20.

Figure 20. The described gaps inside the Rocketstove [106]

To understand Uganda 2-pot and Instove more deeply regarding material, usage and looks can pictures of them be seen in Appendix 6.

3.6 Stove Components After studying the improved cook stoves together with their dimensions and materials the information was adapted to the MAYA concept. Respective component will be analyzed and discussed accordingly. Potential materials are presented for each part, final decision should be considered in further work where the price aspect of the materials should be considered as well. In further work technical resistant, exact dimensions, assembly, installation and a secure solution for the stove not tipping also should be taking in account.

Combustion Chamber, Grate and Ash Trap The pot diameter and combustion chamber size are highly connected to each other. Therefore to determine the combustion chamber size, the pot diameter needed to be identified. Uganda 2-pot stated that the pot diameters for guaranteed boiling was 25 cm for the first pot and 23 cm for the second. These dimensions were guidelines when the pot dimensions should be determined for MAYA. Suggested pot size for MAYA is two pots with 24 cm in diameter [112]. The pot bottom area for two 24 cm in diameter pots are roughly the same, with a difference in area of 1.4 cm2, as two pots with diameter 25 and 23 cm respective. This means, if the pot height is the same, the

30 total pot volume is roughly the same for the two options. Boiling in both pots can therefore be assumed. Pots with 24 cm in diameter and the height of 16 cm fit 5 liters [112]. Two of these pots should correspond to the cooking need in a family of five, which is required in the requirement specification.

When the pot sizes were determined a dimensioning suggestion for the combustion chamber could be made. The suggested combustion chamber area is 154 cm2 and the diameter is determined to be 14 cm according to Table 4. The combustion chamber is shaped in a cylinder form because smaller mantel area leads to smaller heat losses through the walls.

It is crucial in the means of create complete combustion to have high temperatures in the combustion chamber. The high temperature formulates strict material requirements. In the combustion chamber the temperatures can rise to over 1100 °C which is why the material needs to function in these temperature rates. Analyzing the InStove materials their solution for the high temperature problem in the combustion chamber was to integrate 301 stainless steel and nickel alloy [110].

Type 301 is an austenitic chromium nickel stainless steel which is often for example applied in aircrafts trailer bodies and sinks [114]. The nickel alloy covering material around the stainless steel which makes it more heat resistant [115].

It was advisable to have a pot to grate distance of no less than 0.4 times the pot diameter [107]. Because of the combustion chamber diameter and the stove insulation this distance was set to 145 mm.

For air to pass under the burning wood sticks and make the combustion more complete a grate should be placed in the combustion chamber. It is even better if the air passes through the fuel first to get preheated [60]. Since the grate will be exposed to high temperatures it needs to be made in a heat resisting material. This is why the suggested grate material is the same as in the combustion chamber, 301 stainless steel and nickel alloy. Ash will fall down from the grate to the bottom of the combustion chamber which constitutes an ash trap. Minimizing components in this way is also advisable regarding DFM [104].

Insulation 14 - 42 % of total energy input goes into the stove body though conduction which accounts for the biggest energy losses [106]. This is why it is essential to select proper material for the insulation. Insulation will also reduce the surface temperature which leads to safer use. Relevant

31 material properties in stove design are thermal conductivity, specific heat and density. Specific heat (cp) is a measure of a material's ability to store thermal energy. Thermal conductivity (k) is a measure of a material's ability to conduct heat where a higher value equals faster conduction [116].

It was advisable to use thin, low density material in stove construction. A thick and dense stove body material with low thermal conductivity and high specific heat will conduct less heat from a fire than a thin material under steady state conditions but this will most likely not compensate for the heat required to warm the body during transient conditions, which implies when the stove is getting warm [106].

Double metal walls with an insulated material in between have been proven a good solution for the improved cook stove [107]. This insulation material needed to consider specific factors such as melting temperatures, thermal conductivity, strength and also costs of materials [117]. The lower the thermal conductivity, the lower heat transfer and therefore a good insulating material

[118]. Different insulating materials suitable for the stove construction will be presented below.

Fiberglass can be a good alternative. It is one of the most used insulation materials in modern times because of its reduction of heat transfer but also because of its low cost [119]. Fiberglass with a thickness of 25.4 mm, a density of 64.0 kg/m3 and with a thermal conductivity of 0.036 W/mK are available on the market [120].

Vermiculite is a lightweight, cheap and fireproof material and another alternative for the insulation [113]. It is for example used in open fireplaces, high temperature insulation and fireproofing of structural steel. Vermiculite is a hydrated magnesium, aluminum and silicate mineral [121]. To formulate this into an insulated material vermiculite should be mixed with water and clay. It is possible to find vermiculite in various parts of the world, closest producing country to Bolivia is Brazil [122]. A vermiculite mix with 85 % vermiculite and 15 % clay has a thermal conductivity of 0.120 W/mK and a density of 559 kg/m3 [106].

Perlite is an amorphous volcanic glass that has high water content [123]. The perlite is light weight, fireproof and insulating and is used for example as insulation and in constructions. This is why perlite was jet another alternative for insulation material [124]. Perlite is available both inexpensive and plentiful in countries close to where it is produced. Closest producing country to Bolivia is Mexico [123]. The perlite needs, for best insulated property, be made into a graded mix

32 before combined with clay and formed into bricks [113]. A perlite mix of 85 % perlite mix and 15 % clay has a thermal conductivity of 0.128 W/mK and a density of 439 kg/m3 [106].

Stove Body As mentioned it is advisable to use thin, low density material in the stove construction. These ideas also adapts to the stove body. Sheet metal is a flat and thin metal processed industrial though roll slitters. The metals are possible to find in different thicknesses and metals such as aluminum, brass, copper, steel and tin. Sheet metal is used for car bodies, airplane wings and medical tables and is easy to form into desired geometrics [125]. Different possible stove body materials will be presented below.

Stainless steel is iron alloys with minimum 10.5 % chromium where other elements can be added to formulate the desired material properties but stainless steel is commonly corrosion resistant [126]. The chromium formulates a film surrounding the steel and is good for high temperatures since it protects the steel [127]. High temperature stainless steel are available at different qualities and often used in high temperature tubes. In the stove construction Sanicro 31HT and Sandvik 253MA could be suitable because they both have a good resistance for combustion gases and are structurally stable at high temperatures [128][129][130].

Aluminized steel is often used in water heaters, ovens and heat exchanger and is therefore good applicant for the high temperatures inside the stove. The outside level consists of aluminum oxide and the inside is a mixture of aluminum, silicone and steel [131]. ASTM A463 is a low cost aluminized steel that is possible to formulate into steel sheet. The type of metal is used in ovens, firewalls and heaters and is therefore working in high temperatures [132]. ASTM A463 reflects up to 80 % of the radiant heat below temperatures of 427 °C [133]. Therefore, the suggested aluminized steel material was ASTM A463.

Flue Gas Passage and Chimney The cross sectional areal where the flue gases pass with hot air are suggested to be the same size throughout the entire stove. This helps to keep good draft throughout the stove which further keeps the fire hot and helps the air transfer its heat to the pot [60]. It is also stated that a constant cross sectional area will not slow down the draft inside the stove. The goal is to keep gases as hot as possible and flowing as quick as possible [134]. Since the combustion chamber diameter was determined to be at least 14 cm the cross sectional area turned out to be minimum 154 cm2 which also includes the chimney. To avoid rainwater in the chimney and to make cleaning easier a

33 detachable hood was advisable. This type of chimney can typically be constructed of metallic steel [62].

Outer Dimensions Women are often in charge of cooking in Bolivia as around the world [6]. The average height of a Bolivian woman is 142 cm [135]. Standing height is roughly half of the full length why around 70 cm is a good stove height. This is also confirmed by the Malena stove which has an average height from 70 - 80 cm [62].

A scaffold was needed in the construction to assembly the entire stove. This formulated the base for the construction and gave the stove its height of 70 cm. To minimize the cost, it was determined to construct the stove in a steel profile, and to further minimize the cost and weight it was determined to work with a square hollow steel profile. These profiles are also known to be firm construction wise. The suggested dimensions for the steel profiles in MAYA were determined to 40 x 40 mm with a 2 mm thickness and can either be made in aluminum or iron [137][138]. The density for aluminum is 2.7 kg/m3 and costs about 0.66 USD/ kg [139]. Iron on the other hand costs 0.036 USD/kg and has a density of 7860 kg/m3 [140].

3.7 Stirling Engine Placement To get realistic dimension when determine possible placements for the Stirling Engine in the stove, a prototype was chosen. Technical data for the engine in the stove has been taken from Sunpower’s prototype free piston Stirling engine EE-80-H as shown in Figure 21. This prototype does not fulfill all requirements for the wanted Stirling engine but it was the only free piston

Stirling engine prototype with technical information found. The power output of 95 We is slightly less than the minimum of 100 We, stated in the requirement specification, and the engine is sun and not biomass driven. However the size of the prototype will give a good indication of the size of the required engine.

Figure 21. Free Piston Stirling engine from Sunpower [82]

The technical information for the prototype can be seen in Table 5. The temperature ratio displays the difference in temperatures between the hot and the cold side.

Table 5. Technical data for free piston Stirling engine [82]

Temperature Mass Dimensions Ratio Diameter x Length (nominal)

[Th/Tc] [kg] [mm] 3.0 1.0 65 x 186

To display forbidden placements of the engine red areas have been marked in stove pictures. The Stirling engine will be damaged if it is placed in direct contact with flue gases why the flue gas channel is red shown in Figure 22. The top of the stove is another forbidden area to place the Stirling engine when this area is intended for pots and cooking. As other improved stove reports has presented, the combustion chamber temperature can rise above 1100 °C [110]. The warm side of the engine is for the chosen prototype three times warmer than the cold side (Th/Tc = 3) why the combustion chamber most likely will be too hot. It is crucial to have the combustion chamber well insulated, which is why it was not preferable to place the engine inside.

Figure 22. Inside of the stove with forbidden areas for the Stirling Engine

Dimensional good placements for the engine seen in Figure 23. The temperatures for these placements have to be further investigated when the final stove design and testing is made. As seen in Figure 23 the cold side of the red engine (placed to the right) might disturb the user depending on the stove placement in the house. Weather the blue or green engine is best positioned must be determined when the temperatures in the stove are known. Sealing between flue gas path and the stove body where the engine will be placed must be further investigated to secure the lifetime of the engine.

Figure 23. Three different Stirling engine placements in the stove 3.8 Pricing for the stove Two types of pricings can be made where the first option is market based pricing where the markets assets determine the price and what the costumers are willing to pay. The second option is cost based pricing, which is the price of the produced product correlated to the company’s expenses and profit margins. To determine a final price of the stove, the list price, regarding cost base pricing one needs to know the direct and indirect cost. The direct cost is the cost that can be directly linked to a specific component, assembly or product. The indirect cost is every other cost regarding production of a product such as, for example, selling expenses and profit marginal. To evaluate the cost of each component, and determine the expenses of the machined component one needs to know [104]:

- Material cost for the raw material and the value of the scrap produced - Type of machine used - Component dimensions since it determines the size of the needed machine - Surfaced areas, the more the longer time will it take to produce - Number of components made - Tolerance and surface finish - Labor rate of the used machine Therefore, regarding the price of the stove a similar approach was made where the already analyzed stoves got compared to each other so that a price estimation could be determined for the MAYA stove. Since the price is an estimation will it be presented within a range. Firstly the price for the stove itself will be analyzed and afterwards the cost together with the Stirling engine and also different batteries.

As seen in Appendix 2 and in Section 3.5, the price of the improved cook stoves can vary between 40 - 995 USD. The cheapest of the once compared was the Uganda 2-pot and the most expensive one was the InStove. The InStove will not be included in pricing of MAYA since it is institutional and also far away from the other stoves in price.

The MAYA stove was similar to Uganda 2-pot in its outer design, a simple metal construction with two pots and a chimney but with brick insulation. MAYA was designed with a different combustion chamber material and insulation technics meaning the price of the stove would increase. This indicates that the price range more likely will reach something similar to GAMMA11411 with the price 60 - 80 USD or Philips HD4012 priced 89 USD resulting in a price range for MAYA of 60 - 89 USD, Stirling engine excluded.

The purchasing power of the people living in rural areas needed to be investigated. Since the stoves used today were cheaply built in the villages the budget for the rural people to invest in a new stove was limited [6]. The price of the Malena stove was 45 USD, which was showed a possible amount to overcome for many rural people, even if some needed economic assistance [62][151]. Using the MAYA stove leads to decreased costs regarding monthly electricity use and also saved grid connection expenses that could vary between 22 - 726 USD [6].

As seen the assets of the possible consumers are very limited why some form of financial support is needed for the stove implementation. Since the governmental plan to electrify all the citizens in Bolivia by 2025 seem to be difficult for the government economic support to projects like this can be a solution [37][38].

Household scaled Stirling engines driven on biomass fuel was only produced as prototypes and the price could therefore not be determined. Prices for larger output Stirling engines of 1 kW are about 16 500 USD and therefore too expensive to integrate in rural households.

The price of any technology product decreases with time because inter alia technology advancement and competition. Nothing indicated these economical decreases would not happen to the Stirling engine. Stirling engines of the wanted size will hopefully be accessible for a reasonable price in a 5 – 10 years period from now when the CHP will be further developed [136].

Lithium-ion batteries could be found at the wanted size for circa 350 USD [79], lithium iron batteries could be found in the same size for 203 USD [88][89]. Another solution could be batteries known as power banks. A 25 000 mAh and 12 V lithiumion rechargeable power bank could be found for 130 USD [90]. Car batteries of the wanted size were accessible and could be bought for about 92 USD [91]. To mention is that these prices are not wholesale prices why the prices can be expected to be lower.

4 Results and Discussion Here the full design of MAYA will be displayed together with dimensions, material and the flue gas passage. The stove construction, as already discussed, is a draft that has to be tested and further developed before production. Physical resistance must be calculated and the stove material and design needs further development and optimization as well the assembly and manufactory analysis. The dimensions displayed are suggestions as well as the materials.

4.1 Dimensions of MAYA The design of the new stove, MAYA, and its inside can be seen in Figure 24. As seen in the figure the pots are tightly fit in the stove body making the only possible escape for the flue gases through the chimney.

Figure 24. Design for MAYA where also the inside is displayed. The dimension of the flue gas path and combustion chamber was based on the dimensions of the pots. In Table 6 one can see the pot dimensions together with its material.

Table 6. Dimensions for the pot [112] Pot diameter Pot height Total volume Material [cm] [cm] [liter] [-] 24 16 5 Stainless steel

The MAYA pot sizes was inspired from the Uganda 2-pot where total bottom pot area for both pots to boil was < 906.4 cm2 [113]. The total pot area for MAYA was defined as 904.8 cm2, which results in an area difference between the stoves of 1.6 cm2. Since the total pot area in MAYA is slightly smaller both pots was assumed to boil. The pot diameter gave information about the combustion chamber. The combustion chamber can be seen in Figure 25.

Figure 25. The combustion chamber The dimensions for the combustion chamber can be seen in Table 7 together with the material suggestion.

Table 7. Combustion chamber dimensions [106]

Area Diameter Material [cm2] [cm] [-] 154 14 301 stainless steel and nickel alloy

Even though the combustion chamber dimensions was fetched from the rocket stove, which has one pot instead of two, the size will probably be sufficient. As seen in Table 4 the combustion chamber size does not increase drastically with bigger pot diameters. Uganda 2-pot has a smaller combustion chamber size of 12 cm in diameter and two pots that both could boil for the diameters given earlier [113].

To reduce wall losses through the stove body it was discovered that a thin and low density material was preferable in stove construction. Advised for the stove wall construction was to use two metal sheets with insulation between to reduce heat losses [107]. The suggested materials for the insulation which will be located in between the walls, can be seen in Table 8.

Table 8. Material and material properties for the insulation material [120][106]

Materials Thickness Thermal conductivity Density [-] [mm] [W/mK] [kg/m3] Fiber glass 25.4 0.036 64.0 Vermiculite - 0.120 559 Perlite - 0.128 439

In this application, fiber glass is considered to be the best material and in an isolative perspective but it is also indicated in Table 8 by its light weight and good thermal conductivity which is desirable [107]. Fiberglass with a thickness of 20 mm in between metal walls could provide up to 50% more radiant heat flux to the pot than a bare metal wall [107]. Additional, a well insulated stove body will reduce the risk of the stove surface being unsafely warm. This is highly weighted in the requirement specification and crucial for safety. The reason why both vermiculite and perlite is not excluded from further investigations is because of the economic advantages which also is an important perspective regarding a successful improved cook stove [54].

As seen in Figure 26 the sheet metal and insulation material of the stove body was built with layers on each other. First layer of sheet metal (A in figure) had a thickness of 6 mm, the insulation material (B) has a thickness of 25.4 mm and the last layer of sheet metal (C) has a thickness of 3 mm.

Figure 26. The walls with steel sheet and insulation in between. A ductile, cheap, thin and flat metal is suitable to use in the stove body which is why sheet metal is advisable [106]. The suggested materials for the stove body, which implicate the walls of the stove, can be seen in Table 9.

Table 9. Material suggestion for the stove body

Stainless steel Aluminized steel [-] [-] Sanicro 31HT Sandvik 253MA ASTM A463

Both these materials could function in the walls. Both of them are good heat resistors, for example used in heat exchangers [128][132].

A highly weighted quality in the QFD was that the construction should be made in a standing height which, for an average Bolivian woman, was determined to be 70 cm [135]. The scaffold in the construction constitutes the base for the assembly and gives the stove its correct height. MAYA is designed with a 40 x 40 mm square profile with a 2 mm thickness. The suggested materials and material qualities can be seen in Table 10.

Table 10. Suggested materials for the scaffold construction together with material qualities [139] [140]

Material Density Price [-] [kg/m3] [USD/ kg] Iron 7860 0.036 Aluminum 2.7 0.66

In order to create a light stove construction aluminum seemed to be the best solution for the scaffold material. On the other hand to create a cheaper solution iron might be interesting. This is why the materials need to be further investigated in a later stage. The physical resistance calculations are of highest importance for the scaffold.

The flue gas passage can be seen in Figure 27. Air enters the combustion chamber, passes through the flue gas path, where the pots are placed, and leaves through the chimney. To create a good draft throughout the construction the aim was to maintain a constant cross sectional area [113]. This means the area should be the same throughout the combustion chamber, flue gas passage and chimney, independent on the geometrical form. For MAYA this cross sectional area was determined to be is 154 cm2 [106].

Figure 27. The inner design of MAYA with combustion chamber, flue gas passages, pot holes and chimney Maintaining constant cross sectional area throughout the entire flue gas passage resulted in the gap size, pot to pot hole, to be 34 mm. This can be seen in Figure 28 and also applies on the

43 distance between the pot bottom to bottom of the pot hole. Further the passages between the two pot holes and pot hole to chimney also needed to be included in the constant cross sectional area which formulated this to a rectangular passage with the dimensions 109 x 141 mm. The aim, as said before, was to maintain this area constant. In MAYA the cross sectional area is in the interval of 153.7 - 155.7 cm2 which makes the assumption of a good draft valid.

Figure 28. The dimensions of the flue gas path and gap size to maintain the cross sectional area constant It was shown that Bolivia had a good posibility to manufacture a stove because their well equiped workshops, for example the existing UMSS workshop visited by the authors March 2016. The production volume capacity and their possibilities to assembly should be investigated in further work.

4.2 Pricing The suggested price of the MAYA stove without the SE could possibly be around 60 - 89 USD when looking at other improved cook stoves. Comparing this with the price for Malena of 45 USD the price could be considered as possible to overcome for at least some rural people since the grid connection cost as well as monthly electricity cost will be excluded. The produce and sale the product for this price the production volumes has to be optimized and Design for Assembly (DFA) have to be seriously regarded.

The big problem with the price is connected to the Stirling engine since the wanted one is not on the market and bigger ones are in a price ranges far away from the price reasonably to pay the given electricity outcome for a rural family. Due to this the development of the Stirling engine is fundamental for how the realization of the stove is possible. 44

4.3 Requirement specification The MAYA is a design needed to be tested and can therefore not be truthfully evaluated to the requirement specification. The restrictive requirements need to be further investigated but the functional requirements can be analyzed and the following requirements are fulfilled with the current MAYA design:

The size of the stove shall be customized for a family of five. The stove shall have a solution for easy removal of ashes. The fire shall not be in direct contact with the pot. The stove shall have minimum two pots. The airflow inside the stove shall be ideal. An electricity generating engine shall be integrated in the stove. The stove shall be adapted for burning fuel wood. The stove shall satisfy the rural culture and cooking habits. The stove shall be designed for a standing position for an average woman in Bolivia Dimensions of the stove shall not be bigger than 3 x 1 m. The QFD showed that the safety for the user is a highly weighted and then important requirement. For the stove surface to not exceed 52 °C testing and modeling whit insulation thickness is necessary. With the right insulation material and thickness the construction can fulfill this requirement. Another highly weighted quality regarding the QFD, is the stove having the right height for a Bolivian woman to use it. As seen in the list above this is fulfilled. Regarding that the Stirling engine should have the capacity of 100 W and the chosen engine has a capacity of 95 W is one of the restrictive requirements MAYA does not achieve. Finding the right engine for the stove depends on the development and commercialization of free piston Stirling engines with electrical output around 100 – 200 We.

The thermal- and overall efficiency cannot be evaluated without a physical model and are further crucial for the stove to be successful or not, both regarding the QFD but also WHO and Global Alliance for Clean Cookstoves. Due to this the efficiencies shall play a central role in further development.

5 Conclusion and Further work This final chapter presents conclusion and further work.

Conclusion Cooking with traditional stoves occupy a large amount of time in families of the developing world’s everyday life. Not only time spending on cooking food because of the stoves bad thermal efficiency but also the time it takes to collect the fuel. These factors together with the pollution the stoves emit that contributes to health issues and even premature deaths creates an obstacle for the families to be integrated in the developed world. Electricity access is another crucial aspect in order for a country to develop. Therefore existing improved cook stoves with integrated electricity source can be seen successfully implemented in developing countries, most frequently in Africa. These cook stoves usually use TEG technology and are good indicator that a market exist for a biomass stove integrated with a Stirling engine.

The difficulties in designing an improved cook stove are the complex calculations regarding combustion processes, flue gas passages and the economic aspects. These factors determine the complexity of cook stove design and must be tested through a physical model via trial and error. This does, understandably, formulate difficulties in the early design process since it is only possible to formulate qualified guesses. Therefore the specific objective number 2, combustion process in the developed stove shall be complete and the objective number 5, to investigate the economical limitation for the stove in order to make it accessible, needs to be investigated in further work after the model testing has been done.

If the MAYA stove should be able to fulfill its purpose the Stirling engine must be available to a reasonable price. The price limitation also applies to the stove construction and the battery.

5.1 Further Work MAYA is a design draft, which is why further work and most central physical model testing needs to be done before the stove can become reality. Vital parts of the further work are described in a list below. - Experimental testing of the flue gases path inside the stove together with a full evaluation of the heat losses - Formulate exact dimensions for the stove components - Decide exact placement of the Stirling engine after thermal testing is done - Investigate the need and possibilities to integrate an oven in the stove - Investigate the possibility to integrate a fan to improve the combustion

- Optimize material thicknesses and investigate the most suitable material including insulation materials, scaffold materials and stove body materials - Analyze the stove price - Investigate the information and integration program for the stove - Optimizing the construction assembly of the stove and investigate the manufacturing possibilities

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Appendix 1 Sustainable Development Goals Goal 1. End poverty in all its forms everywhere Goal 2. End hunger, achieve food security and improved nutrition and promote sustainable agriculture Goal 3. Ensure healthy lives and promote well-being for all at all ages Goal 4. Ensure inclusive and equitable quality education and promote lifelong learning opportunities for all Goal 5. Achieve gender equality and empower all women and girls Goal 6. Ensure availability and sustainable management of water and sanitation for all Goal 7 Ensure access to affordable, reliable, sustainable and modern energy for all Goal 8. Promote sustained, inclusive and sustainable economic growth, full and productive employment and decent work for all Goal 9. Build resilient infrastructure, promote inclusive and sustainable industrialization and foster innovation Goal 10. Reduce inequality within and among countries Goal 11. Make cities and human settlements inclusive, safe, resilient and sustainable Goal 12. Ensure sustainable consumption and production patterns Goal 13. Take urgent action to combat climate change and its impacts* Goal 14. Conserve and sustainably use the oceans, seas and marine resources for sustainable development Goal 15. Protect, restore and promote sustainable use of terrestrial ecosystems, sustainably manage forests, combat desertification, and halt and reverse land degradation and halt biodiversity loss Goal 16. Promote peaceful and inclusive societies for sustainable development, provide access to justice for all and build effective, accountable and inclusive institutions at all levels Goal 17. Strengthen the means of implementation and revitalize the Global Partnership for Sustainable Development *Acknowledging that the United Nations Framework Convention on Climate Change is the primary international, intergovernmental forum for negotiating the global [11].

Appendix 2 Stove Testing Center (CPC) La Paz, 22 March 2016

A comparison of the successfully tested stoves in CPC. Some of the qualities is marked as “-” which means information could not be found.

Name: Philips HD4012

Type: Portable

Fuel type: wood Hot start thermal efficiency: 40.5 %

Cold start thermal efficiency: 36.2 %

Air flow: Fan

Pot: 1 pot placed on top

Cost: USD 89 [141]

Name: Gamma11411

Type: Portable

Fuel type: Wood Hot start thermal efficiency: 41 %

Cold start thermal efficiency: 29 %

Air flow: Open ashtray

Pot: 2 pots placed on top

Cost: 60-80 USD [142]

Name: Jikokoa

Type: Portable

Fuel type: wood Hot start thermal efficiency: 43 %

Cold start thermal efficiency: -

Air flow: Open trap

Pot: 1 pot placed on top

Cost: 40 USD [144]

Figure: [143]

Name: Prakti Double Pot Woodstove

Type: Portable

Fuel type: wood Average thermal efficiency: 30.26 %

Air flow: Open ash trap

Pot: 2 pots placed on top

Cost: - [145]

Name: Malena

Type: Non portable, traditional

Fuel type: wood Hot start thermal efficiency: 28 %

Cold start thermal efficiency: 22 %

Air flow: Open ash trap + chimney

Pot: 2 pots sunken in

Cost: 45 USD [147] [62]

Name: 60 liter institutional cookstove

Type: Non portable, institutional

Fuel type: Wood Hot start thermal efficiency: 48.69 %

Cold start thermal efficiency: 48.09 %

Air flow: Open ash trap + chimney

Pot: 1 pot, sunken in [109]

Figure: [110]

Appendix 3 Cook stoves integrated with electricity producing source Pictures of improved cook stoves integrated with electricity source

µPower Cook stove

[92]

TEGSTOVE

[95]

BioLite

[96]

Score stove

[146]

Appendix 4 Requirement Specification Version 5, April 2016

Time frame This project is limited from 18th of January to 1th of June 2016.

Functional Requirements Requirements The size of the stove shall be customized for a family of five persons [62]

The combustion in the stove must be complete [5]

The stove shall have a solution for easy removal of ashes

The air flow inside the stove shall be ideal [60]

The fire shall not be in direct contact with the pot [60]

The stove shall have minimum two pots

The stove shall be designed for a standing position for an average woman in Bolivia [2]

An electricity generating engine shall be integrated in the stove

The engine shall be integrated with a battery

The stove shall satisfy the rural culture and cooking habits [54]

The stove shall be adapted for burning fuel wood [35]

Aspirations The stove can handle different biomass fuel

Restrictive Requirements Requirements The lifetime of the entire stove concept shall be minimum of 10 years

The stove shall boil 5 liter of water in maximum 30 minutes [58]

The minimum thermal efficiency shall be 25 % [58]

Stirling engine shall maintenance interval of minimum 5000 hours [77]

Dimensions of the stove shall not be bigger than 3x1 m [62]

3 Not emit more than 0.035 µg/m per minute of PM10 to the indoor air [149]

The Stirling Engine shall have the capacity of minimum 100 W

The stove will need maximum 20 000 kJ to boil 5 l of water [58]

Emission rates from fuel combustion should not exceed 0.80 mg/min of PM2.5 (fine particles) [148]

Emission rates from fuel combustion should not exceed 0.59 g/min of CO [148]

The outer temperature of the stove body shall not exceed 52 °C [150]

Appendix 5 Decision Matrix

Appendix 6 Pictures of improved cook stoves

[110] Instove

Uganda2pot [111]

Appendix 7 Quality Function Deployment

┼ Title: House of Quality, Stove

Author: Julia Kuylenstierna, Petra Nasstrom

Date: 2016-03-18

Notes:

Legend Θ Strong Relationship 9 Ο Moderate Relationship 3 ▲ Weak Relationship 1 ┼┼ Strong Positive Correlation ┼ Positive Correlation ▬ Negative Correlation ▼ Strong Negative Correlation ▼ Objective Is To Minimize ┼ ▲ Objective Is To Maximize x Objective Is To Hit Target ┼

▼

┼ ┼┼ ┼┼ ┼ ┼┼ ┼

Column # 1 2 3 4 5 6 Direction of Improvement: Minimize (▼), Maximize (▲), or Target (x) ▼ ▼ ▼ ▲ ▼ ▼

Quality Characteristics (a.k.a. "Functional Requirements" or "Hows")

Max Relat ions Number Number hip Relativ Weigh Demanded Quality of of Boiling Surface Valu e t / (a.k.a. "Customer screwes remova time of area Ro e in Weigh Import Requirements" or in the ble Total Number 5 liter temerat w # Row t ance "Whats") stove parts volume of pots water ure 1 9 5,5 9,0 Easy to remove pots ▲ ▲ Θ Θ

2 9 1,8 3,0 Easy to install Θ Θ Θ

3 9 5,5 9,0 Easy to clean Θ Θ Ο ▲ ▲

4 9 5,5 9,0 Easy to maintain Ο Θ Ο ▲ ▲ 5 9 5,5 9,0 Inexpencive to own Θ

6 9 5,5 9,0 Inexpencive to buy Ο Θ Θ Ο

7 9 5,5 9,0 Efficient use of fuel wood Θ Θ

8 9 5,5 9,0 Satisfy rural cooking habits ▲ ▲ Θ

9 9 0,6 1,0 An oven ▲ Ο

10 9 1,8 3,0 Room for two pots Θ Θ Θ Ο

11 9 5,5 9,0 The stove will not tip ▲

12 9 5,5 9,0 Minimisze indoor pollution Ο Ο

13 9 5,5 9,0 Surface area will not be too hot Θ Θ

14 9 5,5 9,0 Complete combustion Θ Ο

15 9 1,8 3,0 Standing cooking height Ο

16 9 1,8 3,0 Ergonomic cooking position Ο Θ Ο

17 9 1,8 3,0 Easy to take out ashes ▲ Θ Ο

18 9 1,8 3,0 Enough electricity output Ο

19 9 5,5 9,0 Reliable electricity ▲

20 9 5,5 9,0 Easy to refuel firewood Θ Ο Ο

21 9 5,5 9,0 Safe and accessable to stir in pots ▲ Θ Ο Θ

22 9 5,5 9,0 Easy to light the fire ▲ ▲

23 9 5,5 9,0 Accessable to buy for rural population Θ ▲

4 cubic 30 52 Target or Limit Value 500 4 200% meters minutes Celcius

Difficulty 1 2 2 3 6 7 (0=Easy to Accomplish, 10=Extremely Difficult) Max Relationship Value in Column 9 9 9 9 9 9 Weight / Importance 99,4 262,0 283,4 182,2 226,4 270,6 Relative Weight 2,1 5,6 6,1 3,9 4,9 5,8 ┼

▬

┼ ┼ ▬ ┼┼ ▬ ┼ ┼┼ ┼┼ ▼ ┼┼ ┼┼ ┼┼ ▬ ┼ ┼┼ ┼┼ ▼ ┼┼ ┼ ┼┼ ┼┼ ┼┼ ┼ ┼┼ ┼┼ ┼ ┼ ┼┼ ┼┼ ┼┼ ┼┼ ┼┼ ┼┼ ┼ ┼┼ ┼ ┼┼ ┼┼ ┼┼ ┼ ┼┼ ┼┼ ┼┼ ┼┼ ┼┼

6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 ▼ ▲ ▲ x ▲ ▲ ▼ ▼ ▲ ▲ ▲ x x ▼ ▼ ▲

Mass Constan centrum t cross Stirling Surface in sectiona engine area CO centere Hight of Size of Output Thermal Overall l area PM2.5 PM10 mainten temerat Stove emissio d the Size of the from the efficienc efficienc Battery for flue emissio emissio ance ure lifetime n rate position stove the pots Price engine engine y y capacity gases n rate n rate interval Θ Ο Θ Θ ▲ Ο Θ ▲ ▲ Ο Θ Ο Ο ▲ Ο Ο ▲ ▲ Ο Ο Θ Θ Ο Ο Θ Ο Θ Θ Θ Θ Θ Ο Ο ▲ Θ ▲ ▲ Θ Θ Θ Ο Ο Θ ▲ Θ Θ Θ Θ Θ Θ Θ Θ Θ Θ ▲ Θ Ο Θ Ο ▲ Θ ▲ ▲ ▲ ▲ Θ Ο Ο Ο Θ Θ ▲ Ο ▲ Ο Θ Θ Θ Θ Θ Θ Θ Θ Θ Ο Θ Θ Θ Θ Θ Θ Θ Ο Θ Θ Ο ▲ ▲ ▲ ▲ ▲ Ο Θ Θ Θ Θ Ο ▲ Θ Θ Ο Ο Ο Θ Θ Ο Ο Θ Θ Θ ▲ Ο Ο Θ ▲ ▲ Ο Θ Θ Ο Θ ▲

52 10 0.59 200 25 000 0.80 0.035µg TRUE 70 cm 3 liters 20 dm3 100 W 25% 35% 154 cm2 50000h Celcius years g/min USD mAh mg/min /m3min

7 6 6 7 2 2 9 3 3 4 4 2 3 6 6 2

9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 270,6 110,4 198,8 128,8 308,0 176,7 215,3 265,0 198,8 322,7 322,7 254,0 314,7 198,8 198,8 116,0 5,8 2,4 4,3 2,8 6,6 3,8 4,6 5,7 4,3 6,9 6,9 5,5 6,8 4,3 4,3 2,5 21 22 23 24 25 Competitive Analysis 1,2 ▲ (0=Worst, 5=Best)

1 Stirling engine mainten ance interval 0 1 2 3 4 5

0,8 Θ ▲ ▲

Serie1 Serie2

0,6 Serie3 Serie4 Serie5 Serie6

0,4

Θ 0,2

▲

0 0 1 2

50000h