Technical studies on toolbox components This publication has been produced with the �inancial assistance of the European Union under the ENPI CBC Mediterranean Sea Basin Programme Disclaimer
This document has been produced with the �inancial assistance of the European Union under the ENPI CBC Mediterranean Sea Basin Programme. The contents of this document are under the sole responsibility of STS-Med Consortium and can under no circumstances be regarded as re�lecting the position of the European Union or of the Programme’s management structures. The total budget of STS-Med project is 4.953.513 Euro and it is �inanced for an amount of 4.458.162 Euro by the European Union through the ENPI CBC Mediterranean Sea Basin TheProgramme Programme (www.enpicbcmed.eu).
The 2007-2013 ENPI CBC Mediterranean Sea Basin Programme is a multilateral Cross-Border Cooperation initiative funded by the European Neighbourhood and Partnership Instrument (ENPI). The Programme objective is to promote the sustainable and harmonious cooperation process at the Mediterranean Basin level by dealing with the common challenges and enhancing its endogenous potential. It �inances cooperation projects as a contribution to the economic, social, environmental and cultural development of the Mediterranean region. The following 14 countries participate in the Programme: Cyprus, Egypt, France, Greece, Israel, Italy, Jordan, Lebanon, Malta, Palestinian Authority, Portugal, Spain, Syria, Tunisia. The Joint Managing Authority (JMA) is the Autonomous Region of Sardinia (Italy). Of�icial Programme languages are Arabic, English, French and Greek. Launched in May 2011, the strategic call focused six topics chosen by the Joint Monitoring Committee based on their potential for the development of cooperation in the Mediterranean area. These are: agro-food industry, sustainable tourism, integrated coastal zone management, water management, waste treatment and recycling, solar energy. Out of 300 proposals presented, 19 projects were approved for funding. Total value of these operations is € 82.5 Themillion European (€ 74.1 millionUnion ENPI contribution).
The European Union is made up of 28 Member States who have decided to gradually link together their know-how, resources and destinies. Together, during a period of enlargement of 50 years, they have built a zone of stability, democracy and sustainable development whilst maintaining cultural diversity, tolerance and individual freedoms. The European Union is committed to sharing its achievements and its values with countries and peoples beyond its borders.
STS-Med Small scale thermal solar district units for Mediterranean communities Ref. I-A/2.3/174
Technical studies on toolbox components
issued by ENEA
December 5, 2015
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Table of contents
List of Figures and tables ...... 3 INTRODUCTION ...... 4 TOOLBOX COMPONENTS ...... 5 1.1 SOLAR CONCENTRATING TECHNOLOGIES ...... 5 1.1.1 Solar Steam Generation ...... 6 1.1.2 Concentrating Technologies ...... 7 1.1.3 Tracking CS ...... 8 1.1.4 Non-tracking CS ...... 14 1.1.5 Small scale CS System & Applications ...... 15 1.2 SOLAR THERMAL STORAGE ...... 18 1.2.1 TECHNICAL OPTIONS FOR THERMAL ENERGY STORAGE SYSTEM (TES) AT LOW-MEDIUM TEMPERATURE 18 1.3 POWER GENERATION SYSTEMS ...... 22 1.3.1 MINI AND MICRO CSP IN CONJUNCTION WITH ORGANIC RANKINE CYCLE (ORC) ...... 22 1.3.2 Description of technology ...... 23 1.3.3 Applications to solar energy ...... 29 1.4 SOLAR HEATING AND COOLING ...... 40 1.4.1 Absorption chillers ...... 40 1.4.2 Adsorption chillers ...... 50 1.4.3 Thermo chemical accumulator (TCA) ...... 54 1.4.4 Desiccant cooling for air-conditioning ...... 58 1.4.5 DEC-system with liquid sorbent materials ...... 69 1.5 DESALINATION SYSTEMS ...... 71 1.5.1 Water requirements ...... 72 1.5.2 Desalination technologies: a review ...... 73 1.5.3 Energy requirements: possible options ...... 79 1.5.4 Solar powered water desalination systems ...... 82 1.5.5 Expected developments ...... 87 2. SOME APPLICATION CASES ...... 88 3. SUMMARY and CONCLUSIONS ...... 98 4. REFERENCES ...... 99
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List of Figures and tables
Figure 1. Existing concentrating solar technologies as a function of operational working temperature (Source: Turboden) ...... 22 Figure 2. Comparison between organic liquids and water on Ts diagram ...... 23 Figure 3. General diagram and relative thermodynamic of an operational ORC cycle ...... 24 Figure 4. T -s diagram of a dry fluid ...... 27 Figure 5. Plant Technologies for the systems applicable as a function of both the temperature available and production of electrical energy ...... 27 Figure 6. Diagram of a Micro ORC system with Scroll type displacement rotary expander ...... 28 Figure 7. Layout of the solar thermodynamic plant with ORC Turboden ...... 30 Figure 8.High efficiency non-co generative Turboden solutions ...... 31 Figure 9. Turboden Solutions in cogeneration ...... 31 Figure 10. Schematic of the prototype plant of Helianthus Ltd...... 32 Figure 11. Diagram of the ORC SIP SOLAR SYSTEM FROM GROUP FLENCO ...... 33 Figure 12. Scheme and plant pictures of INGECO ...... 34 Figure 13. (a) Absorber Heat Exchange (AHE) (b) Generator-Absorber heat eXchange (GAX)...... 42 Figure 14. Schematic of a single unit thermo-chemical accumulator. (Source: ClimateWell AB)...... 55 Figure 15. Worldwide installed desalination capacity since first applications at the industrial scale...... 75 Figure 16. Desalination technologies market share (in % of capacity, source IDA 2008) ...... 76 Figure 17. Trend of the energy required to desalinate seawater using RO over the last three decades...... 79 Figure 18. Energy production cost by using renewable sources: actual and foreseen values ...... 82 Figure 19. Water cost as a function of plant capacity by a PV/RO system without and with incentives meant for solar section only ...... 85 Figure 20. Water cost as a function of plant capacity by a ST/MEE system without and with incentives meant for solar section only...... 86
Table 1. Organic fluids suitable to recover heat from heat sources at low temperature in ORC 26 Table 2. Organic fluids suitable to recover heat from heat sources at high temperature in ORC 26 Table 3. Non-exhaustive list of manufacturers of equipment and turbines coupled to ORC 28 Table 4. List of manufacturers of ORC with an indication of the models produced and data plate 29 Table 5. Distribution of population worldwide during the years (millions of inhabitants) 72 Table 6. Population (thousands of inhabitants) and yearly water availability (m3 per capita) in the South Mediterranean 73 Table 7. Main features of different desalination technologies 76 Table 8. 78 Table 9. Overall installed capacity and corresponding yearly pro capita production 80 Table 10. Annual worldwide energy production by most important renewable sources and overall demand ratio 81 Table 11. Annual horizontal solar energy available (kW h/m2) and relative peak value (W/m2) in the countries under investigation 83 Table 12. Possible options for coupling between solar energy and process of desalination 84
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INTRODUCTION
The overall objectives of STS-MED project are summarized in the following points:
§ to deploy technologies resulting from RTD activities, through case studies analysis and impact assessment, through technology optimization, integration and validation and through the creation of a network of “in scale” demonstrative units of poligenerative Concentrating Solar (CS) technologies. § to demonstrate the effectiveness of CS small scale integrated systems and promote their wide adoption in the Mediterranean basin, with a focus on a flexible toolbox of CS applications. § to create new opportunities for the commercial and industrial sectors concerned, by improving technical skills and enabling SMEs to impact local communities and set up a full supply chain in the solar energy sector.
In this framework, STS-MED Work Package 5, aims to develop a toolbox including the basic components for the construction of CS multi-generative power plants in the Mediterranean solar belt and, in particular, of small/medium size systems to be integrated in public buildings in rural and coastal communities. The goal is to combine and optimize innovative subsystems for the efficient capture of solar energy to generate electricity, produce heat and cold, drive thermal processes such as desalination or sterilization, alone or in combination with backups (e.g. gas or biomass).
The first step of Work Package 5, is a technical study of the different technologies and subsystems involved. In the present document, prepared by ENEA with significant contributions from other partners namely ALBUN-Al Balqa Applied University, CEA, Consorzio ARCA, Millennium Energy Industries, and the Cyprus institute, some evaluations about the technologies considered, are reported.
The starting point deals with the identification of the products to be transferred in the CS model (outcome from Subtask 5.1.1), with 6 technical studies on the 6 basic toolbox components:
1. solar concentrators, i.e. the collectors with all the tools to capture the solar radiation and convert it into “high temperature” heat
2. heat transfer fluids (HTF) used to transfer the thermal energy between the units
3. thermal energy storage (TES) systems
4. electric power generation systems (often called “power block”)
5. heating and cooling cogeneration systems for the building
6. desalination and water treatment units
The 6 technical studies primarily lead to the realization of this comprehensive technology report (Subtask 5.1.2) that will pave the way to the optimization and feasibility studies implemented in the following tasks (Tasks 5.2, 5.3 and 5.4) as reported in Deliverables D5.2, D5.3 and D5.4.
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TOOLBOX COMPONENTS
1.1 SOLAR CONCENTRATING TECHNOLOGIES
Concentrated solar power (also called concentrating solar power, concentrated solar thermal, and CSP) systems use mirrors or lenses to concentrate a large area of sunlight, or solar thermal energy, onto a small area. Electrical power is produced when the concentrated light is converted to heat, which drives a heat engine (usually a steam turbine) connected to an electrical power generator. Concentrating technologies exist in four common forms, namely parabolic trough, dish Stirlings, concentrating linear Fresnel reflector, and solar power tower.
Concentrating solar power system (CSP), a method of increasing solar power density, can provide electricity as well as thermal power. The thermal power can be efficiently used for cooking, water distillation and absorption refrigeration cycles. The drawback to these systems is that the most efficient solar thermal systems currently have an installation cost of $10,000/kW.
CSP has been theorized and contemplated by inventors for thousands of years. It is possible that as far back as ancient Mesopotamia, priestesses used polished golden vessels to ignite altar fires. The first documented use of concentrated power comes from the great Greek scientist Archimedes (287-212 B.C.). Seventeenth century, is considered to be the beginning of modern solar concentration. Solar concentrators then began being used as furnaces in chemical and metallurgical experiments.
Further applications opened for concentrated power when August Mouchot pioneered generating low-pressure steam to operate steam engines between 1864 and 1878. Abel Pifre made one of his solar engines operate a printing press in 1878 at the Paris Exhibition but after extensive testing he declared the system too expensive to be feasible. Pifre's and Mouchot's research began a burst of growth for solar concentrators.
The early twentieth century brought many new concentrating projects varying from solar pumps to steam power generators to water distillation. Mirrored troughs were used in a 1200 m2 collector field to provide the needed steam. In 1920 J.A. Harrington used a solar-powered steam engine to pump water up 5 m into a raised tank. This was the first documented use of solar storage. The water was stored for continual use as power for a turbine inside a small mine. Concentrating technology had made a huge leap from the nineteenth century but was halted by World War II and the resulting explosion of cheap fossil fuels. The advantages of solar power lost their luster and the technology would merely inch forward for nearly five decades.
Starting in the late seventies and early eighties, solar power came back to the forefront of researchers' agendas with oil and gas shortages. In 1977 in Shenandoah, GA, 1147-meter parabolic dishes were used to heat a silicon-based liquid for a steam Rankine cycle. The plan also supplied waste heat to a lithium bromide absorption chiller. The plants total thermal efficiency was 44%, making it one of the most efficient systems ever implemented.
More modern systems like the Department of Energy's Dish Engine Critical Components (DECC) project, which was built at the National Solar Thermal Test Facility. This system utilizes the high efficiency of the sterling engine to convert the heat generated into electricity. This efficiency is unmatched by any concentrator that utilizes a steam cycle, with one or two working fluids. 5
1.1.1 Solar Steam Generation
Solar thermal systems can utilize the Rankine cycle to produce electricity. This is done by creating steam using solar energy and passing it through a turbine. The most common modern technique to produce steam for electricity production or industrial needs has been to utilize a heat transfer fluid. Usually a salt or oil is pumped through a solar field to heat up the fluid. The hot oil is then passed through a heat exchanger with water to generate steam. The benefit of a heat transfer fluid is that it remains liquid at high temperatures, which increases heat transfer and ease of pumping. Oils have a liquid working range up to 400 °C, salts up to 600 °C and liquid metals can reach much higher temperatures.
The fluid is pumped to cavity receivers that are placed at the focal point of each dish. The collector efficiency of the concentrators is in the range of 70-78%. The heat exchangers are then utilized to preheat, boil and superheat water before it passes through a steam turbine. A portion of the steam generated can be used directly by the factory. The remaining hot water is used as a heat sink for a lithium bromide absorption chiller. The electricity generation efficiency could vary between 10-14%. Finally, the total heat work efficiency for all the processes could be calculated.
Even though using an oil or salt as a heat transfer fluid is the most popular choice, water is still used in some cases. The Johnson and Johnson Solar Process Heat System used pressurized water as the heat transfer fluid in a parabolic trough plant. The facility had 1070 m2 of parabolic troughs which heated water pressurized at 310 psi to 200 °C. The water was stored in a tank where fresh steam was created 4 times a day for industrial processes. To create steam, the pressurized water was throttled down to 125 psi where it vaporized additional feed water. The saturated steam that was produced was sent to do work. Thermal collection efficiency for the system was 30%.
Direct steam generation (DSG) can be a more efficient and economic way of producing steam from solar collectors. Eliminating storage and heat exchangers decreases losses, capital investment and maintenance. The most famous case of a solar facility producing direct steam was the Solar 1 plant built in Barstow, CA. Solar 1 was a solar power tower that produced 10 MW of electricity in 1982. The receiver sat nearly 100 m above ground and was powered by 1818 39 m2 collectors. The 13.7 m high and 7 m diameter receiver was made of 69 mm alloy tubes. The tubes were placed vertically, welded together and coated with an absorptive paint. The surface of the receiver reached temperatures up to 620 °C.
Water was pumped through the tubes where it was vaporized and superheated to 516 C. The steam was then passed through a turbine to produce electricity. The maximum net monthly electrical efficiency was 15%.
A more modern example of DSG comes from the INDITEP project built near Seville. The 5 MW parabolic trough power plant uses DSG to run steam turbines. The pilot plant is testing the efficiency of both saturated and superheated steam production. For the saturated case, water is pumped at 77 bar though a solar collector field. The steam/water mixture exits the collectors with a quality of 0.85 at 285 °C. A steam separator collects the steam and sends it to the turbine. The liquid water is recycled and passed back through the collector field. For the superheated case, the collected steam is passed through an additional set of collectors where it is superheated to 400 °C. Superheating the steam increases the turbine efficiency.
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Collection efficiency for the saturated steam configuration was 66.9% and 65.1% for the superheated case. The reduced efficiency of the superheated case can be attributed to the high temperature of the receiver line, which increases thermal losses.
1.1.2 Concentrating Technologies
Concentrating solar power plants produce electric power by converting the sun's energy into high- temperature heat using various mirror configurations. The heat is then channelled through a conventional generator. The plants consist of two parts: one that collects solar energy and converts it to heat, and another that converts heat energy to electricity.
Concentrating solar power systems can be sized for village power (10 kilowatts) or grid-connected applications (up to 100 megawatts). Some systems use thermal storage during cloudy periods or at night. Others can be combined with natural gas and the resulting hybrid power plants provide high- value, dispatchable power. There are four CSP technologies being promoted internationally. For each of these, there exists various design variations or different configurations. Like concentrating photovoltaic concentrators, those technologies use only direct-beam sunlight, rather than diffuse solar radiation. The CS technologies can be categorized to tracking technologies and non-tracking technology as the following:
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1.1.3 Tracking CS
There are four main tracking CS technology families can be classified by the way they focus the sun’s rays and the technology used to receive the sun’s energy, Which are parabolic trough, solar tower, linear Fresnel reflectors, and parabolic dish. Parabolic troughs are the most mature of the CS technologies and form the bulk of current commercial plants.
1.1.3.1 Parabolic trough
The sun's energy is concentrated by parabolically curved, trough-shaped reflectors onto a receiver pipe running along the inside of the curved surface. This energy heats oil flowing through the pipe, and the heat energy is then used to generate electricity in a conventional steam generator. A collector field comprises many troughs in parallel rows aligned on a north-south axis. This configuration enables the single-axis troughs to track the sun from east to west during the day to ensure that the sun is continuously focused on the receiver pipes. Individual trough systems currently can generate until about 80 megawatts of electricity.
Trough designs can incorporate thermal storage—setting aside the heat transfer fluid in its hot phase—allowing for electricity generation several hours into the evening. Currently, all parabolic trough plants found for this study, are "hybrids," meaning they use fossil fuel to supplement the solar output during periods of low solar radiation. Typically a natural gas-fired heat or a gas steam boiler/reheater is used; troughs also can be integrated with existing coal-fired plants.
The reflector follows the sun during the daylight hours by tracking along a single axis. A working fluid (e.g. molten salt) is heated to 150–350 °C as it flows through the receiver and is then used as a heat source for a power generation system. Trough systems are the most developed CSP technology.
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1.1.3.2 Enclosed trough
Enclosed trough systems are used to produce process heat. The design encapsulates the solar thermal system within a greenhouse-like glasshouse. The glasshouse creates a protected environment to withstand the elements that can negatively impact reliability and efficiency of the solar thermal system. Lightweight curved solar-reflecting mirrors are suspended from the ceiling of the glasshouse by wires. A single-axis tracking system positions the mirrors to retrieve the optimal amount of sunlight. The mirrors concentrate the sunlight and focus it on a network of stationary steel pipes, also suspended from the glasshouse structure.[21] Water is carried throughout the length of the pipe, which is boiled to generate steam when intense sun radiation is applied. Sheltering the mirrors from the wind allows them to achieve higher temperature rates and prevents dust from building up on the mirrors.
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1.1.3.3 Fresnel reflectors
Fresnel reflectors are made of many thin, flat mirror strips to concentrate sunlight onto tubes through which working fluid is pumped. Flat mirrors allow more reflective surface in the same amount of space as a parabolic reflector, thus capturing more of the available sunlight, and they are much cheaper than parabolic reflectors. Fresnel reflectors can be used in various size CSPs.
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1.1.3.4 Parabolic Dish Systems
Parabolic dish systems consist of a parabolic-shaped point focus concentrator in the form of a dish that reflects solar radiation onto a receiver mounted at the focal point. These concentrators are mounted on a structure with a two-axis tracking system to follow the sun. The collected heat is typically utilized directly by a heat engine mounted on the receiver moving with the dish structure. Stirling and Brayton cycle engines are currently favored for power conversion. Projects of modular systems have been realized with total capacities up to 5 MWe. The modules have maximum sizes of 50 kWe and have achieved peak efficiencies up to 30% net.
1.1.3.5 Dish Stirling
A dish Stirling or dish engine system consists of a stand-alone parabolic reflector that concentrates light onto a receiver positioned at the reflector's focal point. The reflector tracks the Sun along two axes. The working fluid in the receiver is heated to 250–700 °C and then used by a Stirling engine to generate power. Parabolic-dish systems provide high solar-to-electric efficiency (between 31– 32%), and their modular nature provides scalability. The Stirling Energy Systems (SES), United Sun Systems (USS) and Science Applications International Corporation (SAIC) dishes at UNLV, and Australian National University's Big Dish in Canberra, Australia are representative of this technology.
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1.1.3.6 Solar power tower
A solar power tower consists of an array of dual-axis tracking reflectors (heliostats) that concentrate sunlight on a central receiver atop a tower; the receiver contains a fluid deposit, which can consist of sea water. The working fluid in the receiver is heated to 500–1000 °C and then used as a heat source for a power generation or energy storage system.[18] Power-tower development is less advanced than trough systems, but they offer higher efficiency and better energy storage capability. The Solar Two in Daggett, California and the CESA-1 in Plataforma Solar de Almeria Almeria, Spain, are the most representative demonstration plants. The Planta Solar 10 (PS10) in Sanlucar la Mayor, Spain is the first commercial utility-scale solar power tower in the world.
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1.1.4 Non-tracking CS
Is a technology that concentrate large amount of solar radiation onto a small area using fixed or Compound Parabolic Concentrator (CPC) in order to achieve high temperatures.
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1.1.5 Small scale CS System & Applications
The selection of CS technology mainly depends on the applications and site requirements, the below table show the applicable applications for small scale CS systems.
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The below table shows the market player of small solar cooling chillers with some technical specifications.
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Finally, the below table shows small scale Thermal Electricity Generators with some technical specifications.
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1.2 SOLAR THERMAL STORAGE
1.2.1 TECHNICAL OPTIONS FOR THERMAL ENERGY STORAGE SYSTEM (TES) AT LOW-MEDIUM TEMPERATURE
In general Thermal Energy Storage (TES) technologies can be classified in terms of storage mechanism which can be based on sensible heat, latent heat, or chemical energy. In case of sensible heat storage systems, the storage medium can be either a solid or a liquid material. Differently, latent heat storage systems are based on the solid-liquid phase transition and for this reason these systems are often referred as “Phase Change Materials” (PCM) systems.
Considering the typical operational ranges of the small-medium scale solar collectors (i.e. maximum 350°C), and of the ORC system for power generation (typically 180-310°C), TES systems in the STS-Med toolbox should operate in the range of 200-350°C: it is not strictly necessary to identify TES systems exceeding 350°C because of limitations of small CSP units, while TES temperatures below 200°C are not suitable for the ORC operation, although the residual heat can be used for lower temperature thermal cogeneration duties like heating/cooling or desalination.
In the following Table is provided a list of solid and liquid TES media which can be applied in the 200-300°C range.
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Average Average Volumetric Average Average Temperature Average thermal heat heat specific cost per TES material (°C) density conductivity capacity capacity cost energy (kg/m3) Min. Max. (W/mK) (kJ/kgK) (kWht/m3) ($/kg) ($/kWht) Solid (or dual)
Sand-rock mineral oil 200 300 1,700 1.0 1.30 60 0.15 4.2
Reinforced concrete 200 400 2,200 1.5 0.85 100 0.05 1.0
NaCl (solid) 200 500 2,160 7.0 0.85 150 0.15 1.5
Cast iron 200 400 7,200 37.0 0.56 160 1.00 32.0
Cast steel 200 700 7,800 40.0 0.60 450 5.00 60.0
Silica free bricks 200 700 1,820 1.5 1.00 150 1.00 7.0
Magnesia fire bricks 200 1,200 3,000 5.0 1.15 600 2.00 6.0
Liquid
Mineral oil 200 300 770 0.12 2.6 55 0.30 4.2
HITEC Solar Salt a 141 450 b 1850 0.85 1.55 1j
HITEC Solar Salt a 141 450b
Na/K/Ca nitrate “eutectic”c 140 505d nae naf 1.7 na
Na/K/Li nitrate “eutectic”g 120 600 1850 naf 1.6 4j
Na/K/Ca/Li quaternaryh <95 nai 1850 0.45 1.6 na
Solid-Liquid (PCM)
40% KNO3, 60% NaNO3 220
k NaNO3 307 2,260 0.5 172 kJ/kg
k C(CH2OH)4 260 37 kJ/kg
Solid - chemical storage system
MgO/Mg(OH)2 250 400 a) Composition: NaNO3/KNO3/NaNO2 7/53/40 (wt%) b) Thermal stability under air, 538°C under nitrogen c) Composition: NaNO3/KNO3/Ca(NO3)2 16/42/42 (wt%) d) Obtained from: Bradshaw R.W., Meeker D.E., High-temperature stability of ternary nitrate molten salts for solar thermal energy systems, Solar Energy Materials 1990; 21: 51-60 e) Presumably the same of the lithium containing ternary and the quaternary mixtures f) Presumably the same of the quaternary mixtures g) Composition: NaNO3/KNO3/LiNO3 18/53/30 (wt%) h) Composition: NaNO3/KNO3/Ca(NO3)2/LiNO3 18/40/21/22 (wt%) i) Presumably the same (505 °C) of the calcium containing ternary, given calcium nitrate is the less stable compound j) Approx. in €/kg k) average heat of fusion.
The thermal conductivity of the material has a high impact on the required heat transfer surface, especially in the case of solid materials. Moreover, besides costs, in the perspective of application to small-medium scale plants, it is important to identify materials with high heat capacity per unit volume in order to maximize the compactness of the TES system.
It is also important to select TES materials with low toxicity and low vapor pressure. For example, superheated liquid water can be an interesting cost-effective option, but it requires pressurized systems up to 100 bar.
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In general, from the technical point of view, the most important requirements for a TES system are:
- high energy density in the storage material (storage capacity), expressed in kWht/m3;
- good heat transfer between heat transfer fluid (HTF), used in the CSP collectors and the power cycle (e.g. the ORC), and storage medium (efficiency);
- mechanical and chemical stability of storage material (must support several charging/discharging cycles);
- compatibility between HTF, heat exchanger and/or storage medium (safety);
- complete reversibility of a number of charging/discharging cycles (lifetime);
- low thermal losses;
- ease of control.
Thermal energy storage technologies can also be classified as “active” or “passive” systems. An active TES is generally characterized by a liquid storage medium which exchange its heat by forced convection with the hot fluid (from the solar field loop) or directly in the solar receiver, and the cold fluid (from the load loop). Active systems are subdivided into direct and indirect systems. In a direct system, the heat transfer fluid serves also as the storage medium, while in an indirect system, a second medium is used for storing the heat. In a passive TES the storage medium (solid, liquid, or PCM) is not pumped, but suitable heat transfer fluid(s) are circulated between the TES and the solar field or the heat load during the charge or discharge phases respectively.
The following Table reports a list of existing TES systems operating in the reference temperature range.
Temperature (°C) TES tank TES CSP plant TES Heat transfer Project TES concept volume capacity type medium fluid 3 Min. Max. (m ) (MWht)
Irrigation pump Parabolic Single Tank Coolidge, AZ Oil Oil 200 228 114 3 Trough Thermocline USA IEA-SSPS Parabolic Single Tank Almeria Oil Oil 225 295 200 5 Trough Thermocline Spain SEGS I Parabolic 4540 Daggett, CA Oil Oil 240 307 Two Tanks 120 Trough (hot tank) USA IEA-SSPS Parabolic Oil Single Dual Almeria Oil 225 295 100 4 Trough Cast Iron Medium Tank Spain Solar One, Oil Central Single Dual Barstow, CA Sand Steam 224 304 3460 182 receiver Medium Tank USA Rock CESA-1 Central 200 Almeria Liquid salt Steam 220 340 Two Tanks 12 receiver (hot tank) Spain
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1.2.1.1 Sensible heat storage systems
TES systems based on the release of sensible heat by the storage medium can be either active or passive systems.
Two tanks active TES systems are more efficient but also more expensive and space consuming than single tank systems.
When the fluid is a liquid but storage is made by the use of packed beds with solid materials featured by significant heat capacity, the system is called “dual storage system”: in this case packed beds favour thermal stratification, which has advantages in single tanks TES systems based on the thermocline effect (i.e. a thermal gradient along the tank). An advantage of a dual system is the use of inexpensive solids such as rock, sand or concrete for storage materials. Concrete, for example, is chosen because of its low cost, high specific heat, good mechanical properties, and high mechanical resistance to cyclic thermal loading. Steel reinforcement are sometimes added to the concrete to impede cracking and increase the thermal conductivity of this TES medium. On the other hand, rock is an inexpensive TES material from the standpoint of cost.
Liquid media (mainly molten salts, mineral oils and synthetic oils) maintain natural thermal stratification because of density differences between hot and cold fluid. The existence of a thermal gradient across storage is desirable. The requirements to use this characteristic are that the hot fluid is supplied to the upper part of storage during charging, and the cold fluid is extracted from the bottom part during discharging, or using another mechanism to ensure that the fluid enters the storage at the appropriate level in accordance with its temperature, in order to avoid mixing.
The main disadvantage of passive TES using sensible heat is that the temperature decreases during discharging as the storage material cools down: this results into lower efficiency and control
Issues. Additional drawback of passive TES (with respect to forced convection applied in active systems) are the low heat transfer rates when solid TES materials are used (usually there is no direct contact between the HTF and the storage material as the heat is transferred via a heat exchanger).
1.2.1.2 Latent heat storage systems
Thermal energy can be stored nearly isothermally in some substances called phase change materials (PCM) in the form of latent heat of phase change, usually as heat of fusion (solid–liquid transition). PCM materials allow large amounts of energy to be stored in relatively small volumes, resulting in some of the lowest storage media costs of any storage concepts.
On the other hand, design of PCM-based TES systems is not consolidated, yet, and performance assessment still need validation.
1.2.1.3 Chemical heat storage systems
In case of TES systems based on chemical reactions, it is necessary that the endothermic chemical reactions applied during the charging phase are completely reversible. The common advantages of chemical storage systems lies in the high storage energy densities. On the other hand, this concept is still at an early development stage.
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1.3 POWER GENERATION SYSTEMS
1.3.1 MINI AND MICRO CSP IN CONJUNCTION WITH ORGANIC RANKINE CYCLE (ORC)
To date, there are different solar technologies coupled with the ORC systems. Specifically, the technology for the production of thermal energy from the solar source more suitable for coupling with the ORC systems, undoubtedly is that of concentration. The concentrating solar technologies that best suit this application can be divided into two main categories:
• concentrating solar technology at high temperature ( over 250 °C)
• concentrating solar technologies at medium temperature (up to 250 °C)
The need of such co-generative systems arises to meet the demand of large amounts of heat for production processes (e.g. large sawmills, installations for the production of pellets, district heating plants, etc.) with thermal waste that can usefully be exploited in combination with these types of systems. Currently, the status of solar installations combined with ORC is very small if not almost non-existent.
The concentrating technologies which are best suited for this type of systems are:
• linear parabolic type concentration systems ( high and medium temperature)
• tower type concentration systems (high temperature)
• solar dish systems ( medium temperature )
• Fresnel linear mirrors concentrator systems (average temperature)
Figure 1. Existing concentrating solar technologies as a function of operational working temperature (Source: Turboden)
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1.3.2 Description of technology The organic fluid Rankine cycle (ORC - Organic Rankine Cycle) is a thermodynamic cycle essentially identical to traditional closed Rankine cycle, in which organic fluid is used as working fluid.
In conventional type Rankine cycle power systems, traditionally, water is used as working fluid. However, for the exploitation of heat at relatively medium temperatures (from 100 to 300 °C), the water losing much of its utility becomes little suitable. Under such circumstances, organic fluids with low-boiling temperature such as HFT or Halogenated hydrocarbons, are considered more appropriate.
Figure 2. Comparison between organic liquids and water on Ts diagram
Co-generative ORC systems, allowing the simultaneous production of electrical and thermal energy (hot water at 60 and 95 °C) can be divided into two broad categories:
• system generating thermal energy heat sources at low temperature (70 - 80 °C) and with limited thermal flow;
• system with outlet temperatures above 100-150 °C using cycles that involve use of so- called binary in which hot fluid exchanges heat with thermo- carrier having high boiling point (diathermic oil) and subsequently fluid thermo carrier exchanging the thermal energy (in heat exchangers evaporative ) with working organic fluid .
In this report will be considered essentially the ORC "binary" using oil as heat transfer fluid at temperatures between 100 - 150 °C and up to about 300 °C while working in co-generative structure.
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Figure 3. General diagram and relative thermodynamic of an operational ORC cycle
General scheme of operation of an ORC cycle and its relative thermodynamic is illustrated in Fig. 3. Specifically, the working fluid, in liquid phase and of low-pressure at the outlet of the condenser, is brought to the maximum cycle pressure by a pump (1-2). The high pressure fluid is then preheated and evaporated to exchange heat with a second fluid (typically diathermic oil) inside the evaporator (8-3-4) and made to expand in turbine (4-5) with production of mechanical energy and then electricity. The steam then passes through the regenerator (5-9) where it heats the organic liquid (2-8) thus recovering thermal energy before entering the evaporator. The steam is then condensed in the condenser (9-6-1) .
It is to be noted that ORC systems can be used in a more efficient way to convert thermal energy at low temperature into electrical energy. At such low temperatures, water to steam cycle would be very inefficient mainly because of huge volumes that would occur at reduced pressures (and temperatures) for which it would be necessary to condense the fluid, with unacceptable increases in volume and cost of the system themselves.
ORC systems are therefore extremely advantageous over traditional systems, especially, in water vapor :
• Applications of small size with power ranging from a few kW to several MW;
• Applications dedicated to the exploitation of heat sources at temperatures between 70 °C and 400 °C.
In this regard, development of such systems has gone towards modularization of machines able to be applied on small scale economies thus allowing general reduction of production costs .
The main advantages of using ORC technology can be summarized as follows:
§ lack of erosion of blades;
§ excellent turbine efficiency;
§ low mechanical stress of turbine due to low peripheral speed;
§ low number of revolutions of the turbine; 24
§ highly reliable and with minimum maintenance requirement due to the fact that the working fluid is non-corrosive and it keeps the parts lubricated;
§ simple start and stop procedures;
§ high safety of the plants resulting from the use of a generalized thermo- carrier fluid with high boiling point (diathermic oils);
§ low noise;
§ good performance even at partial load;
§ absence of super heaters;
§ high conversion efficiencies even for system of few kW power (generally varying between
§ 15 and 24%) ;
§ excellent ability to be adapted at partial loads while maintaining a good efficiency, up to about 30% of the load, but being able to operate in automatic mode up to 5-10% of the load.
Fluids, commonly used in the ORC cycles are characterized by sufficient thermal stability and belong to the family of so-called 'organic fluid', i.e. hydrocarbons and fluorocarbons with high molecular mass such as Freon, Coolants, Siloxane, Alcohol, Azeotropic mixtures and Zeotropic, etc..
The choice of one or other fluid depends on a number of factors, such as the toxicity, chemical and physical stability, environmental compatibility, as well as appropriate thermal properties, i.e. compatible with cold and hot source(to allow condensation and evaporation, respectively).
Some of the organic fluids used for heat recovery from low temperature sources, are shown in Table 1. Temperature and respective critical pressure as well as temperature and respective condensing pressure, are reported for each of the fluids shown. It can be observed from the table that it is possible to produce electricity using hot water at relatively low temperature (e.g. 150 °C in the case of integration with solar thermal) available from hot water springs, with use of fluids such as R134a ( in the vapor phase at 125 °C and 20 bar). Some of the organic fluids that can be used at high temperature applications, are reported in Table 2.
25
Table 1. Organic fluids suitable to recover heat from heat sources at low temperature in ORC
t p t P t p t P Substance c c cond cond Substance c c cond cond (°C) (bar) (°C) (bar) (°C) (bar) (°C) (bar) R125 66.18 36.30 30.00 15.64 RE245mc 133.68 28.87 54.50 2.42 R218 71.89 26.80 33.68 10.04 R600a 135.05 36.50 45.33 4.04 R143a 72.73 37.64 30.00 14.40 R236ea 139.22 34.12 53.92 2.44 R32 78.11 31.36 30.00 19.31 RE134 147.10 42.28 41.04 2.50 RE125 81.34 33.51 31.50 10.11 C F 148.85 20.40 72.76 1.04 5 12 RE12705 92.42 46.65 30.00 13.09 R600 152.05 38.00 48.43 2.85 R290 96.65 42.50 30.00 10.79 R245fa 154.05 36.40 50.70 1.80 R134a 101.03 40.56 30.00 7.72 R338mccq 158.80 27.26 63.08 1.12 R227ea 101.74 29.29 44.19 5.33 neo-C H 160.65 32.00 58.95 2.00 5 12 R152a 113.50 44.95 30.00 6.89 RE347mcc 164.55 24.76 66.98 0.96 RC318 115.23 27.78 54.72 3.68 RE245 170.88 30.48 58.47 1.04 CF I 123.29 39.53 30.00 5.65 R245ca 174.42 39.25 53.75 1.23 3 RC270 124.65 54.90 41.63 8.23 R601a 187.75 33.86 58.47 1.10 R236fa 125.55 32.00 48.61 3.24 R601 196.50 33.70 57.74 0.828 RE170 126.85 52.40 30.00 6.73 n-hexane 234.67 30.10 61.89 0.25
Another characteristic of the fluids used in the ORC is that to be considered "dry fluids", i.e. characterized by a positive slope of the curve of the saturated steam as shown in Figure 4 .
Table 2. Organic fluids suitable to recover heat from heat sources at high temperature in ORC
p t c0 Substance Symbol c cond p mol (bar) (°C) (J/mole K) benzene B 48.9 289.0 110.2 1-methyl-benzene MB 41.0 317.8 143.7 1,3- dimethyl-benzene DB 35.4 343.9 179.6 1,2,4- trimethyl-benzene TB 32.3 376.0 217.4 1,2,3,4- tetramethyl-benzene TEB 32.7 426.9 274.2 naphthalene N 40.5 475.2 226.4 2-methyl- naphthalene MN 35.0 487.9 266.9 quinoline Q 57.8 508.8 234.0 diphenile D 38.5 515.8 290.6
hexamethyl-cyclotrisiloxane D3 16.6 281.0 346.2
octamethyl-cyclotrisiloxane D4 13.6 313.3 498.9
octamethyl-cyclotrisiloxane D5 10.8 346.0 639.9 perfluoro-methyl-cyclohexane P1 20.2 212.7 283.6 perfluoro-2-butyltetrahydrofuran P2 16.0 227.0 403.1 perfluoro-1,3-dimethyl-cyclohexane P3 18.8 243.3 361.3 perfluoro-decalin P4 17.6 292.0 481.5 perfluoro-1-methyl-decalin P5 16.6 313.4 547.2 perfluoro-benzene PRFB 33.0 243.5 174.2 pentafluoro-benzene PFB 35.3 257.8 176.7 perfluoro-toluene PFRT 27.1 261.3 225.7 chloro-pentafluoro-benzene CPFB 32.4 297.6 194.1 hexamethyl-disiloxane MM 19.1 245.5 268.9 octamethyl-trisiloxane MDM 14.4 291.2 419.6
decamethyl-tetrasiloxane MD2M 12.3 326.2 577.5 26
Figure 4. T -s diagram of a dry fluid
This allows having a superheated steam at the end of expansion, even starting from saturated steam curve, with the advantage of the absence of condensation in the turbine and consequently preservation from erosion of the parts in contact with the fluid. To optimize the efficiency of the thermodynamic cycle, the choice of the organic fluid to be used depend upon the temperature of the heat source available.
From the analysis of the characteristics of organic fluids used and with regards to the temperature of the primary fluid, a wide range of potential for such technology, is outlined. The following diagram illustrates the sizes of plant available taken into account both the working temperature and prime mover used in the ORC cycle.
Figure 5. Plant Technologies for the systems applicable as a function of both the temperature available and production of electrical energy
The mini- or micro-ORC turbines are co-generative energy systems based on the thermodynamic cycle of the ORC, as described above. The size is of the order of a few kilowatts up to a maximum of a few hundred kilowatts.
27
The main components are similar to those of the motor of large size, i.e. a pump, an evaporator, an expander and a capacitor. In the case illustrated (Fig. 6) evaporator is directly facing the flame, and Scroll type displacement rotary expander, is used as an expander.
Cold water that reaches at the condenser is heated and used either as sanitary water or for space heating. Also, in this case, the fluid used is generally an organic fluid.
Figure 6. Diagram of a Micro ORC system with Scroll type displacement rotary expander
An advanced thermodynamic model that can be applied to design, analyse and optimize performance of an ORC using solar thermal energy for ultra-low grade heat recovery in the residential applications, developed by Davide Ziviani et al. was presented at ASME 2012 International Mechanical Engineering Congress and Exposition held at Houston (Texas) during Nov. 9-15, 2012. The adopted software AMESim has been demonstrated to be a powerful tool since it allows ORC simulation both for steady state and transient analysis. The complete ORC simulation model was validated using both numerical and experimental data. The agreement between the measured expander isotropic efficiency and model prediction was satisfactory the deviation being consistently lower than 4%. was Finally, the model has been used to evaluate ORC capability to meet a typical thermal and electricity loads for residential buildings.
The following table shows the main manufacturer producing system and turbines of small size suitable to be coupled with ORC. The table, in addition to reporting the electrical powers shows the relative temperatures at the evaporator which shows compatibility with those obtainable from solar plants at medium temperature.
Table 3. Non-exhaustive list of manufacturers of equipment and turbines coupled to ORC Manufacturer, Model Electric power Tev Technology Ormat 0.2 kW ai 20 MW - Macro Turbine Cogen Microsystems 2.5 kWel 20 kWel - displacement rotary expander Infinity Turbine 1-10 kWe Tev >70° C - 110° C Turbine Electratherm 30 – 65 kWel Tev > 88° C Espansore volumetrico scroll Calnetix 125 kWel Tev > 120° C Microturbine Eneftech 10 e 30 kWel (60 kWel) Tev > 145° C Microturbine Piglet 43 – 70 kWel Tev >80° C a 130° C Microturbine Renex 30 - 180 kWel Tev > 130° C Microturbine 28
Zuccato Energia 50 kWel 150 kWel Tev > 94° C e fino a 155° C Microturbine Turboden 200 kWel 15 MWel Tev > 270° C Turbine Ingeco Clean cycle 125 125 kWel Tev > 120° Turbine at high velocity Pratt & Whitney- Turboden 280 kWel Tev > 90° C e fino a 150° C Radial turbine Table 4. List of manufacturers of ORC with an indication of the models produced and data plate
1.3.3 Applications to solar energy
In the world there are very few systems using these types of technologies and, at the same time, almost all are hybrid systems that are matched to the recovery of thermal energy from energy systems from conventional thermal source. Below is a list, though not exclusively, of some plants built or under construction around the world. The list shows schematically, the main characteristics of the plants along with the layout process.
Few applications are available and practically running.
Turboden has a range of ORC dedicated to solar applications, discriminated between medium temperature (HR, up to 280°C) and high temperature (HRS up to 310°C). The gap between the two applications is almost negligible. A solar plant for 6 MWe production in Hawaii, using an HR ORC from Turboden, was announced in 2012.
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At a lower range of electricity production, Electratherm supplied 2 ORC for smaller solar plants:
In Louisiana, USA, a 50 kWe max ORC is connected to a 650 kWth parabolic solar plant (Figure 7).
In North Cyprus, Turkey, a 18 kWe (downsized of the 50 kWe Green Machine of Electratherm) is running with a parabolic solar plant of 200-400 kWth.
TURBODEN The company, Turboden, has realized in Morocco a plant/ facility to serve a cement company of the Group Italcementi.
Technical features of the system: • Application: solar thermal • Description: Cement + CSP • ORC turbine model: T1500 HR • Power: 2000 kW • Water temperature (in / out): 26-47 ° C The following figures show the layout of the plant next door with a photo of the installation.
Figure 7. Layout of the solar thermodynamic plant with ORC Turboden
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Figure 8.High efficiency non-co generative Turboden solutions
Figure 9. Turboden Solutions in cogeneration 31
SYSTEM ELIANTO
The company Helianthus srl has constructed in the province of Cagliari, a prototype plant of thermal power 1 MW using a solar field with Fresnel technology, coupled with ORC turbine cycle operating in co-generative structure and thermal oil as heat transfer fluid.
Technical features of the system:
− irradiance of reference (Southern Italy): 1.750 kWh / m²
− mirrors Area: 13,000 m²
− surface area occupied by the solar field: 2.5 to 3.0 acres
− fluid heat transfer: thermal oil
− heat storage: 4 hours
− Production technology thermoelectric ORC turbine
− Annual production of electricity: 2,200 Mwhe
− Production annual temperature (water 80-90 ° C): 8.800 MWhth
Figure 10. Schematic of the prototype plant of Helianthus Ltd.
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FLENCO GROUP
SIP SOLAR system: Solar power system using molten salts as independent and modular storage
Flenco, in collaboration with Biosolar, manufactures, ready to be installed plants with ORC modules of minimum power capacity of 120 kWe coupled to a parabolic trough concentrating solar systems that uses molten salts as heat storage fluid and secondary heat exchanger with thermal oil to feed ORC module.
Technical features of the system: Electric generator ORC FP120 :
• Total Power : 134 KW (120 kW net) . • Turbine : - 3 -stage radial . - working speed 30,000 rpm . - turbine inlet temperature of about 210 °C. - plain bearings.
• Heat transfer fluid : thermal oil 240 °C -280 °C
- inlet temperature : 225 °C - outlet temperature : 140 °C - duty: 742 kW
• Alternator : High Speed 4-pole permanent magnet rare earth
• Working fluid : Hydrocarbon .
• Cooling Condenser - capacity: 7,683 kg / s. - inlet temp. (Max ) 44 ° C - outlet temp. (MaxJ 68 ° C - duty: 578 kW
Figure 11. Diagram of the ORC SIP SOLAR SYSTEM FROM GROUP FLENCO 33
INGECO The system created by INGECO is located in the province of Siracusa at the Società Agricola Zasoli. Conceived as concentrating solar power system (CSP) using Linear Fresnel type panel , the thermal energy produced by the solar field is integrated with the energy produced by the combustion of biomass waste , produced locally in agriculture sector, to cover during the night, thermal requirements of the electricity generators.
The system CSP of type Linear Fresnel, is constituted by a series of concentration panels arranged in several parallel rows: Particular shape allows movement of mirrors from remote in accordance with different angles of solar radiation.
The solar rays are directed towards a linear collector disposed at a certain height from the mirrors where inside there is a flow of heat transfer fluid, in this case water and glycol . The fluid is heated to the temperature required for the process of generating downstream. A second reflective surface is disposed above the collector to avoid the dispersion of light and re-concentrate the solar rays .
To ensure, thermal requirements to the electricity generators for 24 h, both in case of bad weather and during night hours, CSP system is integrated with a system of biomass combustion consisting of a steam boiler.
− Technical characteristics of the plant
− CSP - concentrations panels of 800 kW − biomass boiler : 1 MW; Steam 20 bar − Steam Expander 60 kWe nominal − ORC system ( Unit No. 1 ) 120 kWe nominal
Figure 12. Scheme and plant pictures of INGECO
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SOPOGY
CYLINDRO PARABOLIQUE • Fluide : eau / huile 66 – 287 °C • Projets : • Electricité (1 projet : 2 MWe) • Chaleur (2 projets : 100 kW et 1 MW) • Froid (5 projets : 35-260 kW)
Imp 154 C - INES CONFIDENTIEL 14/11/2013| DTS/LETh|
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NEP SOLAR
CYLINDRO PARABOLIQUE • Positionnement : chaleur / froid / élec / desalement • Tmax = 220°C • Systèmes installés : • Chaleur : 115 m² à 630 m² / huile 200°C / laiterie • Froid : 350 m², chiller 230 kW • ORC : proto en évaporation directe • Dessalement : proto à CIEMAT
Imp 154 C - INES CONFIDENTIEL 14/11/2013| DTS/LETh|
CHROMASUN
MINI-FRESNEL • T < 204°C • Fluide : eau, huile, vapeur… • 3,5 m² par module • Projets • Froid : 27 modules / 57 kW • Chaleur : 205 °C / 120 kWth
Imp 154 C - INES CONFIDENTIEL 14/11/2013| DTS/LETh|
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CHROMASUN
MINI-FRESNEL • T < 204°C • Fluide : eau, huile, vapeur… • 3,5 m² par module • Projets • Froid : 27 modules / 57 kW • Chaleur : 205 °C / 120 kWth
Imp 154 C - INES CONFIDENTIEL 14/11/2013| DTS/LETh|
SOLTIGUA
CYLINDRO et FRESNEL
Imp 154 C - INES CONFIDENTIEL 14/11/2013| DTS/LETh|
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BBE-ENERGY
FRESNEL • Un prototype de qq m² / 150 kWth • Vapeur surchauffée à 40 bars et 250°C • Application minière pour Afrique du Sud • Projets de 600 à 2400 kW • Collaboration ESKOM
Imp 154 C - INES CONFIDENTIEL 14/11/2013| DTS/LETh|
SOLAR-ORC
ELECTRATHERM • Solar Thermal/ORC Application in the USA • Site Location: Crowley, LA, United States • Thermal Power Available: Up to 650 kWth • Green Machine Gross Power Output Avg: 15-50 kWe • Hot Water Input Range: 65-120°C • Water Cooling Tower: Up to 600 kWt http://electratherm.com/case_studies/solar_thermal_in_louisiana/
Imp 154 C - INES CONFIDENTIEL 14/11/2013| DTS/LETh|
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2.3.3.1 Considerations on Technology
This study was mainly concerned with the technological status of CSP systems combined with ORC cycle with main objective of combined production of electricity and heat at low enthalpy. An analysis though not exhaustive was conducted of some systems currently available in the market or in the marketing stage that can be coupled in such a way so as to ensure an acceptable electrical performance and able to provide heat energy from the cycle to be used for cogeneration.
The investigations carried out showed that the electrical efficiencies for these technologies in cogeneration varies from 15 % - 19 % and thermal efficiency on average around 78%. In cases where the aspect is less cogeneration , i.e. with thermal energy output from the cycle characterized by temperature to the condenser around 25 °C - 35 °C, electrical efficiency can reach values equal to 25 % .
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1.4 SOLAR HEATING AND COOLING
Commonly, conventional air conditioning systems are divided into two main categories:
§ Room Air Conditioners (RAC) characterized by an individual and autonomous appliance for household use, with a cooling capacity usually less than 12 kW. These systems include split, multi-split, single-duct and single-packaged units.
§ Central Air Conditioners (CAC) are instead systems with more than 12 kW of cooling capacity, having a central refrigerating unit that use a fluid (typically water or air) to transport the “cold”.
As illustrated in Figure given below, for “central air conditioning units” exist a large variety of systems and technical options, according to the type of refrigerant employed and/or the equipments used to distribute the air-conditioned.
1.4.1 Absorption chillers
1.4.1.1 Physical principles
Most thermally driven cooling systems and solar assisted air conditioning systems installed today are based on absorption chillers. Flat plate collectors, vacuum tubes, as well as concentrating collectors are used for the heat supply of solar assisted air conditioning systems. Absorption chillers are using heat to drive a refrigerant. A working pair, i.e. the refrigerant and a solvent are 40
used to drive the process in two loops. Absorption chillers work either in a continuous or intermittent mode. For solar driven absorption chillers continuously driven cycles are most suited.
The following figure shows schematic of the working principle of absorption chillers. Similar to a compression cooling machine, the main loop, shown in blue, is composed of an evaporator, a compressor, a condenser, and a throttle-valve. In the evaporator heat is extracted from the room to Q! be chilled ( cooling ) and transferred to the refrigerant on a low pressure level. In the thermal compressor the evaporated refrigerant is compressed by an absorption/desorption cycle, using ! heat e.g. from a solar collector. In the condenser heat is removed by a heat sink ( QA1 ), e.g. a cooling tower. Afterwards, the pressure of the refrigerant condensate is reduced in the expansion valve and flows back to the evaporator.
thermal compressor Qdrive e Q A1 r u s s e r generator
p condenser
throttle valve solution heat exchanger pump
throttle valve for the solvent
evaporator absorber
Qcooling QA2
temperature Absorption chillers, physical principle
Instead of a mechanical compressor used in compression cooling machines, a thermal compression cycle is used. It consists of an absorber, a pump, a heat exchanger, a generator, and a second throttle valve. A concentrated hygroscopic fluid absorbs the refrigerant vapour in the Q! absorber. Heat released during the exothermic absorption process ( A2 ) is transferred to a heat sink, i.e. a cooling tower.
The mixture of the two fluids, the diluted solution, is pumped from the absorber to the regenerator. In the regenerator the mixture is separated again by increasing the pressure due to heat supply, e.g. from a solar collector. Due to the fact that the boiling point of the mixture is higher than the boiling point of the pure hygroscopic solution, the refrigerant vapour can be released at high pressure to flow to the condenser.
The concentrated hygroscopic solution returns to the absorber. Heat recovery is applied from the hot concentrated solution to the diluted solution in a heat exchanger. The positions of the components shown in previous figure indicate the pressure and temperature levels of the fluids in the system.
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1.4.1.2 Advanced cycles
In order to increase the performance of absorption chillers, additional heat exchangers can be installed in the absorber and generator for heat recovery purposes. As shown in the following figure, the heat recovery can be realized either with internal loops inside both, the absorber and the generator, so-called Absorber Heat Exchange (AHE), or with an additional loop between the absorber and regenerator (Generator-Absorber heat eXchange, GAX).
CONDENSER Q A1 Qdrive
gene- RECTIFIER condenser rator
PRECOOLER
GENERATOR
EXPANSION VALVE ABSORBER EVAPORATOR evaporator ab- sorber SOLUTION P VAPOUR REFRIGERANT Q LIQUID REFRIGERANT cooling QA2 (b) (a)
Figure 13. (a) Absorber Heat Exchange (AHE) (b) Generator-Absorber heat eXchange (GAX).
1.4.1.3 Diffusion Absorption Chillers
In diffusion absorption chillers a gas bubble pump is used instead of the solution pump. During heating, gas bubbles are produced which carry the rising solution along to the regenerator. In opposite to the absorption chillers described in the last section, there is a uniform pressure all over the diffusion absorption chillers. The density difference in the absorber and evaporator is realized by a pressure equalizing auxiliary gas (usually Helium).
The main advantages of absorption chillers compared to conventional compression cooling machines are their low electrical energy consumption and maintenance needs. On the other hand, variable operating conditions (temperatures, etc.) have a large impact on the reliability of the systems.
1.4.1.4 Classification of absorption chillers
Absorption chillers can be classified by their working pairs, the number of stages the chillers consist of (single, double, triple effect), and according to developments obtainable from coupling the absorption chillers with solar collectors of advanced cycles.
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Working Pairs
The working pair consists of the refrigerant and a hygroscopic solvent. The fluid materials used depend on:
§ the chilling set temperature;
§ the temperature supplied in the generator;
§ the thermodynamic properties of the fluids relevant for the chilling process.
Suitable working pairs are chosen according to the absorption capability of the solvent and the capability to release as little heat as possible in the absorption process. Furthermore, the absorption capability of the solvent shall only marginally depend on temperature and the solution shall be liquid after the absorption of the refrigerant. In order to guarantee a sufficient separation of the refrigerant and the solvent in the generator, the solvent shall have a distinctly higher boiling temperature at generator pressure than the refrigerant.
So far, absorption chillers operating with the working pairs H2O/LiBr und NH3/H2O have been proven to be reliable. In order to achieve temperatures below 4°C the working pair NH3/H2O is used, whereas for air conditioning purposes mostly H2O/LiBr is used. Moreover, research and development projects are being carried out using NH3/H2O/He for diffusion absorption chillers. Prototypes of these systems are being developed.
The working pair H2O/LiBr (refrigerant/solvent) is used for air conditioning and water chilling down to a temperature of solely 4°C, in order to prevent the refrigerant to freeze. The operating pressure of the refrigerant is typically about 0.01 bar (evaporator, absorber) to 0.1 bar (generator, condenser). Crystallization of LiBr that occurs at high concentrations must be prevented.
With the working pair NH3/H2O (refrigerant/solvent) cooling temperatures down to about –20°C can be generated. In order to achieve these low temperatures, supply temperatures of 120 to 150°C are necessary in the regenerator. The working pressure of the refrigerant is typically about 2 bar (evaporator, absorber) to 10 bar (generator, condenser).
Number of cycles
Absorption chillers are built with either one, two or three cycles as shown in Figure given below. Additional generators and condensers are used on different temperature and pressure levels, in order to increase the amount of refrigerant to be evaporated in the regenerators and therewith the COP of the system. The heat removed from the condenser of the higher cycle is sufficient to be used in the lower-temperature regenerator. Double and triple effect absorption chillers are usually less suitable for solar energy assisted absorption chillers since higher working temperatures are needed. Parabolic trough collectors are applied for double effect absorption chillers recently, developed by the company Solitem (Austria).
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CONDE NSOR GENERATOR MIDDLE-TE MPERATUR MIDDLE-TEMPERATUR CONDENSOR GENERATOR
LOW-TEMPERATUR LOW-TEMPERATUR CONDE NSOR GENERATOR EVAPORATOR ABSOR BER
(a) EVAPORATOR ABSORBER
(b)
HIGH-TEMPERATUR HIGH-TEMPERATUR CONDENSOR GENERATOR
MIDDLE-TE MPERATUR MIDDLE-TEMPERATUR CONDENSOR GENERATOR
LOW-TEMPERATUR LOW-TEMPERATUR CONDE NSOR GENERATOR
EVAPORATOR ABSORBER
(c)
Single-effect (a), double-effect (b), and triple-effect (c) absorption chillers
Cycle Performance
The coefficient of performance for absorption chillers is defined as the useful cooling power related to the heat necessary to drive the process.
An overview of thermal COP values, the suitable driving temperature range, and the range of cooling capacity is given in the following table for absorption chillers operated with one and two cycles with the working pairs LiBr/H2O and H2O/NH3.
Number of cycles 1 2 1
Solvent LiBr LiBr H2O Refrigerent H2O H2O NH3 Driving temperature 80°C - 110°C 140°C - 160°C 80°C - 120°C hot water steam directly hot water steam Driven by hot water steam burned directly burned Thermal COP 0,6 - 0,8 0,9 - 1,2 0,3 - 0,7 Few producers Cooling capacity range few producers (20 to 100 kW) few producers (50 to 100 kW) 19 to 50 kW. market available many producers (> 100 kW more producers (> 100 kW) Custom producers for high cooling capacity Broad, Century, EAW, Ebara, LG Machinery, Sulzer-Escher Wyss, ABB, Colibri and Producer Dunham-Bush, Yazaki, York Mattes.
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Absorption chillers
The following picture, realized in the framework of the AIE SHC Task 48, gives an overview of serial thermally driven chillers market.
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If additional heat exchangers are included in both, the absorber and generator (Absorber Heat
Exchange, AHE cycle of single effect LiBr/H2O absorption chillers, higher driving temperatures Tdrive of at least 150°C are required compared to the basic cycle. Meanwhile, the thermal COP increases from 0.6 (for the basic cycle) to about 0.75. For GAX cycles (Generator-Absorber heat eXchange), a minimum driving temperature of 160°C is necessary, to generate a COP of about 0.75. The thermal COP increases rapidly to about 1, when the driving temperature approaches 200°C.
Ammonia/Water Advanced Cycles Parameter AHE GAX Typical size 10-90 KW 10-90 KW COP 0.7 – 0.75 0.8 – 0.9 Driving temperature 150 – 200°C 160 – 200°C Advanced cycles: Absorber Heat Exchange (AHE) and Generator-Absorber heat exchange (GAX). Typical sizes and COP.
Typical Sizes
Most single effect absorption chillers available have a cooling capacity of more than 100 kW. Only few products are on the market with a smaller cooling power. Companies offering absorption chillers are predominantly from USA and Asia (Japan, Korea, China, India). Among small absorption chillers the system WFC 10 by Yazaki (35 kW) has the largest market share. Moreover, small single effect LiBr systems are supplied by the companies Broad Air (20kW), Robur Corporation (17 kW – 88 kW) and by EAW (Westenfeld, 15 kW). Additionally, absorption chillers with a fairly low cooling power are being developed by small companies, for example Phönix (Germany) and SolarFrost (Austria), as well as by a number of research institutes (University of
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Applied Science Stuttgart, Germany), Joanneum Research (Austria), INETI (Portugal), UPC (Spain), CEA INES (France), etc.
There are few manufacturers of double effect absorption chillers. Most products are directly fired. Most two-stage absorption chillers are available with a cooling power of more than 100 kW.
An overview of 36 absorption chillers installed in Europe is available at http://www.ocp.tudelft.nl/ev/res/sace.htm).
Location, application, cooling capacity, and solar collector type are listed for each system. The mean cooling capacity of these systems is about 87 kW, and the mean collector area installed is about 250 m², which corresponds to a specific collector area of about 2.9 m² per kW cooling power. However, the data are not completely comparable, since the collector area is not defined identically, some of the plants are used as heating systems as well, and some of these absorption chillers are stand-alone, others are solar assisted systems.
Advantages and Drawbacks with Respect to Conventional Technologies
According to Henning et al. /Hen02/ the main problems and expected developments in order to achieve a further penetration of absorption chillers in solar-assisted air conditioning systems are the following:
§ The absorption chillers on the market are mainly intended for large-scale applications. Nevertheless, there is a demand for smaller solar-assisted air-conditioning systems.
§ For LiBr absorption chillers a cooling tower is needed, which leads to increasing investment costs.
§ For low driving temperatures only small efficiencies and capacities can be achieved.
§ More expensive collector types (e.g. vacuum tubes, CPCs) are required to guarantee sufficient efficiencies.
Nevertheless, it has been shown in the projects listed in table 4 that absorption chillers have been operating successfully and are established on the market.
Coupling of the Absorption Chillers with a Solar Heating System
Solar energy driven absorption chillers can be installed either with an auxiliary energy source (solar assisted system) or without a backup system (stand-alone system).
A schematic of absorption chillers coupled with a solar heating system and auxiliary energy supply is shown in figure given below. The solar heating system consists of solar collectors, a storage tank, hydraulics, and a back-up heating supply system.
The total coefficient of performance (COP) of solar assisted absorption chillers is defined as:
COPsol = COP chiller ⋅ηcoll
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A absorber ABC absorption chillers CT B thermal backup system SC C condenser CS cold storage ABC CT cooling tower B HS G C E evaporator G generator HS heat storage SC solar collector A E CS
Coupling of an absorption chillers with a solar heating system.
As shown below, the higher the supply temperature delivered by the solar collector, the lower the collector efficiency and the higher the COP of the absorption chillers. Thus, high efficiency collectors even at high temperatures are decisive for an economic and effective operation of absorption chillers. Collector types suitable for single and double-effect absorption chillers are shown in the following figure. In addition to the driving temperature the overall system performance depends on the reference conditions like solar irradiation available and desired cooling temperature and demand.
0,8 0,8 1 0,9 0,7 0,7 0,8 0,6 0,6 0,7 0,6 η 0,5 , 0,5 0,5
col lector 0,4 0,4 0,4 η 0,3 COP 0,2 col lector
0,3 0,3
η 0,1 0,2 0,2 0 0,1 0,1 0 0,05 0,1 0,15 0,2 0,25 0,3 0,35
0 0 (Tf -Tamb) / G 50 60 70 80 90 10 11 12
TG [°C] 0 0 0 CPC stationary compound parabolic concentrator EDF direct-contact evacuated tube collector EHP heat-pipe evacuated tube collector FPC flat-plate collector SAC solar air collector SYC evacuated tube collector (Sydney type)
Collector efficiency, COP and the resulting overall Collector efficiency curves of various collector types efficiency of the system over the temperature in the regenerator.
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Solar Collector
For stand-alone sorption chillers the specific collector area, defined as the collector area per nominal cooling capacity, was roughly chosen according to the following equation (rule of thumb):
1 Acoll,spec / m² = G⊥ ⋅ηcollector ⋅COP with:
G⊥ global irradiation;
ηcollector collector efficiency at design conditions; COP thermal coefficient of performance.
Thus, for G⊥ = 800 W/m², ηcollector = 50%, and COP = 0.7, the specific design collector area results to A coll_spec = 3.6 m² for 1 kW cooling capacity.
In general, the specific collector area for sorption chillers varies between about 1 and 6 m² per kW of installed cooling capacity.
Storage Tank
In order to store surplus energy in periods with high solar irradiation, storage tanks can be installed. They can either store surplus heat or cold, depending on the application and reference conditions, like additional solar heat supply (e.g. for domestic hot water, space heating), accepted temperature tolerances, solar fraction, performance of the absorption chillers, storage characteristics (size, insulation,..), supply temperatures, etc. The required storage volume per kWh stored cold is shown in the following figure over the useful temperature difference. Cold can be stored as Eutectic salts and water, ice, or chilled water.
0,20 ]
cold 0,18
/kWh 0,16 3 0,14 0,12 hot water 0,10 COP 0,5 chilled water 0,08 0,06 0,04 requiredstorage volume [m hot water 0,02 COP 0,8 0,00 5 10 15 20 25 30 35 40
useful temperature difference [K]
Required storage volume per kWh cold 49
The working principle, advantages and disadvantages of these three storage methods are listed in table given below.
Method Functioning Advantage Disadvantage Eutectic salts are a combination of inorganic salts. A mixture with water Storage and few other elements temperatures of 8 – Small stores, small Eutectic salts / H O make freezing possible at 10°C, only suitable 2 heat losses. the desired temperature for air conditioning,
(typically near 8°C). The the stores are still mixture is encapsulated being developed. and placed in a water store. Small stores, 10..20% of the size compared to a cold water store, 30..50% Low temperatures Ice store Ice is stored in the tank. compared to stores are needed. with eutectic salts; for direct air flow drying is not necessary. Cold water is stored with Inexpensive, no Cold water store typical temperatures of Large storage sizes. critical chemicals about 6°C.
Back-up System
In order to guarantee the functioning of absorption chillers also in times of low irradiation, back-up heating or cooling systems are installed.
For back-up cooling usually conventional compression cooling machines are used. These are connected to the cooling loop of the absorption chillers. Oil, gas, or pellet burners are usually used as back-up heating devices. The back-up heating system can either be connected to a heat store or it can be installed between the heat store and the absorption chillers. For both, cooling and heating back-up systems, modulation is decisive in order to insure a high overall efficiency of the cooling system.
1.4.2 Adsorption chillers
1.4.2.1 Physical principles
Adsorption means the binding of molecules or particles to a surface. Contrary to absorption the adsorption process represents a physical process in which the molecules of one substance are adsorbed on the internal surface of another substance. The binding to the surface is weak and reversible.
Suitable adsorbents are porous materials that are insoluble in water, which have enormous surface areas per unit weight where they can physically bind water or molecules. Typical substances used are silica gel, activated carbon or alumina.
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During the adsorption process condensation heat is released (exothermic reaction). The reverse process is called desorption, for which evaporation heat must be applied.
1.4.2.2 Classification of adsorption chillers
Concerning solar cooling it can be distinguished between closed and open sorption-chilling processes. Application areas for closed systems are the production of cold water which is either used in central ventilation stations (dehumidification) or for decentral air conditioning e.g. the cooling of building elements.
condenser cooling water 25…35 °C
firing cooling water water 50…90 °C
25…35 °C adsorber desorber chilled water
5…12 °C
evaporator
Schematic of the internal chambers of a adsorption chiller.
In the following the functional principle of adsorption chillers, which belong to the group of closed systems, is explained.
An adsorption chiller consists of two separate chambers, an evaporator and a condenser. Each of the chambers contains the adsorbent (e.g. silica-gel) and a heat exchanger. The main difference to a general chiller is that the solid sorbent cannot be circulated. The adsorption chiller works discontinuously. In one phase the adsorption process is linked with the evaporation and in another phase the desorption process is linked with condensation.
During one cooling cycle the following processes take place:
§ The refrigerant adsorbed is driven off through the use of firing water in the desorber (right chamber).
§ The refrigerant condenses in the condenser and heat of condensation is removed by cooling water.
§ The condensate is sprayed in the evaporator, and evaporates under low partial pressure. This step produces the useful cooling effect. Heat is driven off from the chilled water which is cooled to the required temperature.
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The refrigerant vapour is adsorbed in the adsorber (left chamber). Heat is removed by cooling water.
Once the adsorber is charged and the desorber regenerated, their functions are interchanged.
Solar energy is used for the regeneration of the absorbent: regeneration is done by passing hot water through the chamber that is heated by a solar system.
Typical cooling powers are between 8 – 200 kW.
A characteristic performance parameter of adsorption chillers is the coefficient of performance thermal (COP). This is the quotient of heat transferred in the evaporator to the heat required for regeneration, which is delivered by the solar collectors (electrical energy use is not considered). In general the thermal COP of adsorption chillers range from 0.5 to 0.7.
There are only two manufacturer of this kind of adsorption chillers worldwide. They are Japanese manufacturers: Mayekawa (adsorption chiller type: ADR), Mitsubishi, or Chinese manufacturer (Shuangliang) or German manufacturers (Sortech and Invensor).
In the following the adsorption chillers are classified according to their working pair (cooling agent/absorbent).
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Water/silica-gel
This adsorption chiller uses water as its cooling agent. Water evaporates in a vacuum at room temperature and because of that it extracts heat from the surroundings (evaporation energy). A cooling of the “chilled water” takes place because of this process.
Compared to open systems the evaporated water is not released as steam into the surroundings, but re-condensed within the chiller. The adsorption chiller is a closed system. A direct condensation of the evaporated water is energetically not possible for thermodynamic reasons (temperature of the evaporated water is lower than temperature level in the condenser). Therefore, first the water is adsorbed by a solid carrier material, the adsorbent. In this case silica-gel, a material related to quartz or sand, is used as adsorbent.
As the adsorption process is an exothermic reaction, condensation heat is released during the adsorption process. It has to be ensured that this heat is driven off the chamber through cooling water, as the maximum degree of possible adsorption depends on the temperature level of the adsorbent as well as on the pressure level in the chamber. The higher the temperature of the adsorbent, the lower is the relation between adsorbed water mass [kgH20] and mass of the adsorbent (silica-gel) [kgads]. That means that with higher adsorbent temperature less water vapour can be adsorbed until saturation is reached.
With the firing water the adsorbed water vapour on the silica-gel (carrier material) is desorbed again and the silica-gel is regenerated.
The desorbed water vapour can now directly be condensed in the condenser, as the resulting temperature level in the right chamber (desorption chamber) is higher than the temperature level in the condenser.
The following factors are essential for the process:
§ Silica-gel can easily take up water without causing a structural change or volume expansion.
§ It can release easily the stored water due to a temperature increase. This process is reversible and unlimitedly repeatable.
§ The evaporation process depends on temperature and pressure. Under common atmospheric pressure water evaporates at 100 °C. If the pressure drops, the evaporating temperature of the water also decreases.
§ By creation of a sufficient vacuum the water evaporates at a lower temperature. For the purpose in the adsorption chiller, a vacuum of 13 – 26 mbar is sufficient.
§ If water is sprayed or injected into a vessel under vacuum, it evaporates spontaneously and extracts energy from the surroundings.
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Ammonia/activated carbon
The disadvantage using water as cooling agent is that it is not a suitable refrigerant for sub-zero temperature application. For applications below 0 °C ammonia NH3 can be used as an appropriate cooling agent.
Activated carbon is produced by roasting organic material to decompose it to granules of carbon – coconut shell, wood, and bone are common sources. Spent activated carbon is regenerated by roasting but the thermal expansion and contraction eventually disintegrate the structure so some carbon is lost or oxidized.
But the ammonia/activated carbon pair requires high temperature (>120°C) input heat for regeneration.
Methanol/silica-gel
The disadvantage using water as cooling agent is that it is not a suitable refrigerant for sub-zero temperature application. For applications below 0°C methanol can be used as an appropriate cooling agent. Silica gel is a matrix of hydrated silicon dioxide.
The advantage of using methanol/silicagel as adsorbate/adsorbent pair is that it may get activated even at a temperature of 60 - 70 °C.
1.4.3 Thermo chemical accumulator (TCA)
1.4.3.1 Physical principles
The thermo-chemical accumulator (TCA) is an absorption process that uses a working pair, not only in the liquid, vapour and solution phases but also with solid sorbent /Ols00/, and was patented in 2000. This makes it significantly different from the traditional absorption processes in that it is a 54
three phase process (solid, solution and vapour). All other absorption processes are two phase processes with either solution + vapour or solid + vapour.
Below given Figure shows the schematic of a single TCA unit, where the solution is pumped over a heat exchanger via a spreader arm to increase the wetted area and improve heat transfer. The principle has been defined by the French Promes laboratory and the process has been developed by the Swedish company ClimateWell AB.
During desorption the solution comes closer and closer to saturation, and when it reaches saturation point further desorption at the heat exchanger results in the formation of solid crystals that fall under gravity into the reactor vessel. Here they are prevented from following the solution into the pump by a sieve, thus creating a form of slurry in the bottom of the vessel.
This gives the TCA the following characteristics:
§ High energy density storage in the solid crystals.
§ Good heat and mass transfer, as this occurs with solution.
§ Constant operating conditions, with constant temperature difference between reactor and condensor/evaporator.
Figure 14. Schematic of a single unit thermo-chemical accumulator. (Source: ClimateWell AB).
For discharging, where the process is reversed, saturated solution is pumped over the heat exchanger where it absorbs the vapour evaporated in the evaporator. The heat of evaporation is provided either by the building (cooling mode) or from the environment (heating mode). The solution becomes unsaturated on the heat exchanger, but when it falls into the vessel it has to pass through the slurry of crystals, where some of the crystals are dissolved to make the solution fully saturated again. In this way the solution is always saturated and the net result is a dissolving 55
of the crystals into saturated solution. The heat of condensation and binding energy release is transferred to the environment (cooling mode) or to the building (heating mode). Thus there is a flow of energy from the evaporator at low temperature to the reactor at moderate temperature.
The first TCA units have been built using water/LiCl as the active pair. The physical properties of the working pair have been summarised in the literature /Con04/ and empirical equations have been created for them based on data from a large number of studies over the last 100 years. ∆Tequ, the maximum theoretical temperature lift between the reactor and condensor/evaporator, is constant for a given set of boundary conditions resulting in constant operating conditions, such as solution temperature (Tsol), during charging / discharging. However, ∆Tequ does vary considerably over the operating range of the machine.
Figure given below shows the relationship of ∆Tequ to the temperature of the saturated solution based on Conde’s equations (solid line) and as measured by ClimateWell AB (filled squares). Note that ∆Tequ is the temperature difference between the evaporator and reactor and that the temperature difference between the liquids in the external circuits is greater than ∆Tequ for charging and smaller for discharging (both cooling and heating).
This technology has been developed to the demonstration phase with extensive lab and field testing during 2004.
Note that there is an inbuilt conflict between the energy density for heat storage and the COP for cooling. A higher binding energy for water to the salt will result in higher energy density storage, as well as higher thermal COP for heat recovery, but lower thermal COP for cooling.
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55 Conde 2004
50 ClimateWell 2004 45 ] C
° [ 40
u q
e
T 35 D 30
25
20
15 0 20 40 60 80 100 Tsol [°C]
Relationship of ΔTequ to the saturated solution temperature.
1.4.3.2 Classification of TCA machines
Two different types of prototype TCA machines have been developed and tested.
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In prototype 2 (Figure A) the main unit was used only for charging and the slave unit only for discharging. By pumping saturated solution and water from the main unit to the slave unit while pumping back dilute solution, semi-continuous operation was possible despite the fact that the process is by nature a batch process. This makes it similar to a standard single-effect LiBr absorption chiller. However, it has not been possible to include a solution heat exchanger as crystallisation occurs very easily there. This limits the potential thermal COP for the machine.
In prototype 7 (Figure B) there are two identical units that work in batch mode, with one undergoing charging while the other can provide cooling. When either the unit being charged is fully charged or the other unit is fully discharged, the units are swapped using a number of valves connected between the internal heat exchangers and the external circuits.
The machines that have so far been made are best viewed as prototypes. There is no commercial production in 2004, although prototype machines have been sold to a number of parties. The design thermal COP for cooling of the prototype 7 machine, called ClimateWell DB220, is 70% with no heat recovery. With 40% heat recovery during the swapping of units, thermal COP of 75% should be possible to achieve. The integral storage capability allows the machine to provide up to 50 kWh cooling if both units have been fully charged, although this figure is partially dependent on the cooling rate. The design thermal COP for recovery of heat from the machine is 87%, but in heat pump mode a minimum temperature of 5°C is required for the evaporation of water. Costs are not available.
TMC,in
Water to slave Vatten till Huvudkondensor/ slavevaporator TMCoute vMainaporato r evaporator Condenser Pump
TMR,in
TSE,out
Huvudreaktor SlaveSlave vEvaporatoraporator Main TMR,out Pump SubTstans til l Reactor slavreSE,aktoinr TSRout Substans från slavreaktor SlaveSlavr eReacaktotorr
TSR,in Pump Pump
Figure A: Schematic of TCA prototype 2.
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8 valves for controlling internal processes Tubes connected to the external circuits 180 mm Connection unit Condensor / evaporator – Heat exchanger
Reactor – Heat exchanger
Salt filter basket
Barrel 2000 mm
Internal pumps
700 mm 700 mm
Figure B: Schematic of TCA prototype 7, ClimateWell DB220.
The main advantage of the technology is the integral heat storage that makes it very suitable for intermittent heat sources such as solar. In addition, the regeneration temperature is relatively low, being roughly 50-65°C above ambient, depending on the charging rate and ambient temperature. This is possible to achieve with flat plate collectors, although other types might be more economically viable in certain cases.
The main disadvantages of the technology are the limited temperature lift during discharge. Here the performance is very dependent on the heat exchange with the ambient and with the air inside the building. It is not in general possible to provide dehumidification by having a low delivery temperature, and a delivery system with 12/15°C or even higher is preferable. Another disadvantage is the cost of the LiCl in the machine. Other, less expensive, salts are being investigated.
1.4.4 Desiccant cooling for air-conditioning
In general air-conditioning equipment has to fulfil following objectives:
§ Compensation of external loads (thermal transmission through the building envelope and solar gain through window) and of internal loads (latent and sensible heat of persons, machines and other thermal heat sources) 58
§ Dehumidification/ Humidification of supply air
§ Cooling/ Heating of supply air
§ Supply of fresh air according to hygienic needs
Traditional air-conditioning systems are more or less a combination of a ventilation system and a cooling device which is normally a conventional compression chiller. The ventilation system supplies the fresh air in accordance to the hygienic needs. The compression chiller provides chilled water to cool the supply air. The control of humidity and temperature of the supply air depends on the evaporator temperature of the compression cycle, i.e. dehumidification is realised by evaporator temperatures below the dew point of the supply air.
Desiccant cooling systems (DEC-systems) operate as well as a fully air-conditioning unit; supplying fresh air and controlling humidity and temperature of supply air. DEC-systems use sorption based air dehumidification with the help of liquid or solid sorption materials and the evaporative cooling effect. Consequently the air-treatment in DEC-systems is based on two physical principles: dehumidification and evaporation. Accordingly the technical equipment of DEC-systems abandons totally a use of refrigerant medium with high potential of global warming.
heat recovery heating coil desiccant return air wheel wheel humidifier
8 7 6 fan filter exhaust air return air
9 5
ambient air supply air 1 4
fan filter
2 3 supply air humidifier
General scheme of a desiccant cooling air-handling unit.
A standard desiccant cooling system consists on several different technical components which is shown in previous figure. This most common DEC-system is generally separated by a supply air and a return air stream. Furthermore it is composed on standard components of air-conditioning units such as filters, fans, heat recovery, heating or cooling coils and humidifiers. In comparison to standard air-handling units the desiccant wheel is additionally implemented which dehumidifies the supply air to enlarge the potential of evaporative cooling.
To guarantee a continuous operating air-conditioning process the desiccant material which is permanently charged with water molecules has to be discharged/ regenerated also constantly. The regeneration process of the desiccant material - whether liquid or solid – can be realised by providing regeneration heat. The required temperatures for an efficient regeneration of the desiccant wheel are in a range of 45°C up to 90°C. Due to this low driving temperatures economic 59
advantages arise particularly for DEC-systems when it is coupled with district heating or heat supplied from a combined heat and power (CHP) plant. Of particular interest is the coupling with thermal solar energy.
1.4.4.1 Physical principles
Contrary to thermally driven chillers producing chilled water which can be supplied to any type of air-conditioning equipment the open desiccant cooling cycle produce directly conditioned air. Therefore the open air-conditioning process of a DEC-system can be described and explained by a psychometric chart for moist air. Representative air states which appear in standard desiccant cooling cycles using a rotating desiccant wheel are shown in the following figure. DEC-Systems according to this scheme are typically applied in moderate climates.
ϕ = 5 % 80
8 ϕ = 10 70 %
60 ϕ = 20 % 9 50 2 ϕ = 30 %
40 7 ϕ = 50 1 %
30 5 ϕ = 100 3 % 20 6
4 10
0 0 2 4 6 8 10 12 14 16 18 20 22
Psychometric chart for moist air showing the state changes for a desiccant cooling process (Source: arsenal research/Austria)
The supply air fan sucks ambient air into the DEC-system passing primarily a filter unit. The filtered ambient air is dehumidified by the rotating desiccant wheel (2). The dehumidification is based on sorption which is an exothermal process effecting an air temperature increase. In the above figure, the psychometric chart for moist air the thermo dynamical process is approximately an adiabatic one. This warm and dry air is pre-cooled by the rotation heat recovery wheel (3).
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The cooling potential is enabled by using the return air humidification. That means the heat of the sorption process which causes a temperature decrease of the supply air is transferred to the return air. To provide the required supply air state, the supply air humidifier can be used to control humidity and temperature (4). Following the thermo dynamical principle the temperature and the humidity cannot be controlled separately. There is a strict dependency between temperature and humidity due to the adiabatic humidification process. In practise there is another temperature increase of about 1 Kelvin to the rejected heat of the electrical driven motor of the fan. The supply air fan sucks ambient air into the DEC-system passing primarily a filter unit. The filtered ambient air is dehumidified by the rotating desiccant wheel (2). The dehumidification is based on sorption which is an exothermal process effecting an air temperature increase. In Figure 2.5.1 this fact is not illustrated. A proper design of the fan is recommended in such a way that the heat added to the supply air is minimised.
The internal and external loads of the air-conditioned room normally cause an increase of temperature and humidity (5). The return air passes the second humidifier of the DEC-system which is almost humidified very close to the saturation point (6). This so called evaporative cooling of the exhaust air is an indirect method to pre-cool the supply air. The heat recovery wheel only transfers sensible heat between exhaust air and supply air. Regarding the overall performance of the DEC-system especially the heat recovery efficiency has to be as high as possible. The heat recovery from (6) to (7) leads to a temperature increase. In the following process step the exhaust air is heated by a heating coil up to the regeneration temperature (8). The provided regeneration heat effects the desorption process, i.e. the water molecules bound in the pores of the desiccant material of the sorption wheel is desorbed by means of the hot air (9). To enforce the required flow rate the exhaust air fan is finally implemented.
The DEC-process can be summarised as follows:
§ 1 → 2 sorptive dehumidification of supply air; the process is almost adiabatic and the air is heated by the adsorption heat and the warmed wheel matrix coming from the regeneration side
§ 2 → 3 pre-cooling of the supply air in counter-flow to the return air from the building
§ 3 → 4 evaporative cooling of the supply air to the desired supply air humidity by means of a humidifier
§ 4 → 5 supply air temperature and humidity are increased by means of internal an external loads
§ 5 → 6 return air from the building is cooled using evaporative cooling close to the saturation
§ 6 → 7 the return air is pre-heated in counter-flow to the supply air by means of a high efficient air-to-air heat exchanger, e.g. a heat recovery wheel
§ 7 → 8 regeneration heat is powered by a heating coil; this heating coil is driven by hot water; for instance by hot water generated by solar thermal collectors
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§ 8 → 9 regeneration process of the desiccant material; the water bound by pores of the desiccant material of the sorption wheel is desorbed by means of regeneration air
1.4.4.2 Coefficient of performance
The definition of the Coefficient of thermal Performance COP for open air-conditioning methods is based on a thermal energy balance on the system. The thermal COP - we consider - is generally spoken the quotient of the thermal cooling output and the driving heat input. In difference to the definition of efficiency the thermal COP can be greater than 1. The thermal cooling output or use is the enthalpy difference between ambient air and room inlet air. The driving heat input of the DEC- system corresponds to the enthalpy difference between outlet and inlet air of the regeneration heating coil. According to Figure 2.5.2 the thermodynamic term of the thermal COP is defined as:
Cooling output Coefficient of performance [-] COP = Driving heat input
m! Pr ocessAir (h4 − h1 ) COP = m h h ! RegenerationAir ( 8 − 7 )
The definition of refrigeration capacity and room cooling capacity are: Q! m h h Refrigeration capacity [kW] RC = ! Pr ocessAir ( 4 − 1 ) Q! m h h Room cooling capacity [kW] RCC = ! Air ( 5 − 4 )
The Room cooling capacity corresponds to sensible and latent cooling load of the room.
Regarding a total energy balance a thermal COP should also take into account all terms of energy consumptions, e.g. electrical and thermal. With open systems, the electrical energy powering the fans is of particular importance as a high number of additional components are usually installed compared to conventional ventilation systems. For such reason DEC-systems entail greater loss of pressure and therefore more electricity to move the air.
1.4.4.3 DEC-systems
For moderate climates (warm and humid; humidity values lower than 15 g/kg) DEC-systems are a capable application for air-conditioning. Especially in extreme dry climates air-conditioning can be based on evaporative cooling without any sorption wheel.
Assuming that the dehumidification performance of the sorption wheel is more or less ideal, i.e. dehumidification capacity of around 6 g/kg, the DEC-system provides supply air with a temperature of 15°C and a humidity of 9.5 g/kg. This consideration take into account the standard design conditions for air-conditioning systems in Central Europe (Tamb = 32°C; RHamb = 40%). Without any dehumidification by sorption the air-handling unit (supply air humidifier and adiabatic cooling of
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the exhaust air) only obtains a supply air temperature of around 20°C. The humidity values exceed already the required supply air humidity of maximal 11.6 g/kg.
DEC-systems in operation provide supply air temperatures in the range of 17°C and 19°C (standard design conditions). The humidifier efficiency is lower than 95% and the efficiency of the heat recovery wheel is in practice between 70% and 75%.
1.4.4.4 Configuration of Desiccant cooling systems
Analogue to the immense diversity of conventional air-conditioning system configuration DEC- system design are also manifold. DEC-systems consist on different components, e.g. heat recovery wheel, heating or cooling coil, desiccant wheel, humidifier - and their position in the system itself verify. Figure C illustrates promising DEC-system configuration designed for climates of Central Europe.
DEC-Systems I, II, VII and VIII in Figure C are classified as single-stage system configuration. Two heat recovery wheels in sequence are implemented in system configuration VII. In two steps the exhaust air is humidified in order to achieve an improved overall heat transfer performance of both heat recovery wheels, IV, V and VI in Figure D are classified as cascade system configuration. In such systems an additional heat pump contributes to transfer sensible heat from the supply air to the exhaust air. Only DEC-system configuration VI represents a combination between air- conditioning by ventilated air and a cooling ceiling which is provided by cold water of the heat pump. Cooling ceilings only cover sensible cooling loads.
Concerning the applicable solar collector (heat sources) for the regeneration process all DEC- system configurations - except for DEC-System VIII - are suitable configurations for solar collectors with liquid fluids. The solar heat provides the heating coil mounted in the exhaust air stream just in front of the desiccant wheel. The heating coil in the supply air stream can also be driven by heat from the solar collector. DEC-System configuration VIII is designed for a direct regeneration air heating. During the cooling period ambient air is heated by means of the solar air collector and is used to regenerate the desiccant wheel. The advantage of this DEC-system configuration is that there is no heat transfer from water to air. Water-air heat exchangers operate normally with a temperature level decrease which would be a disadvantage for the required high regeneration temperatures. A detailed description of such a DEC-system configuration takes places in chapter where examples for solar driven DEC-system are presented. The application of direct heated regeneration air results in improved collector performances but the installation and assembly of the air ducts is more complicate and extravagant.
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heating coil supplied by solar energy Figure C Configuration of DEC-systems considered as single-stage or cascade DEC- systems (source: Heinrich/ Franske »Solargestützte Klimatisierung«/ Germany)
For regions with a humid and warm climate DEC-system with cascade configuration are recommended. DEC-system cascades with integrated heat pumps and cooling ceilings are more or less the most sufficient system configurations to meet the required comfort demand. The overall DEC-system performance is improved when the design of the heat pump condensator covers the required regeneration heat. In this case the DEC-system performance should benefit from adiabatic evaporative cooling in the exhaust air stream because the design evaporator temperature of the heat pump can be higher. Consequently the design of the heat pump leads to smaller capacities which do have two positive impacts. On the one hand the primary energy consumption of the DEC-system is reduced and on the other hand the investment cost of the overall DEC- system is lower.
Finally DEC-systems consume less primary energy or operate more efficient than conventional air- conditioning systems.
1.4.4.5 Desiccant Wheel
In practise DEC-systems are commonly equipped with desiccant wheels. The permanent rotation of the desiccant wheel facilitates supply air dehumidification which results in a continuously operating air-conditioning process.
The desiccant wheel is quite similar constructed to a heat recovery wheel. As a basic principle a rotating matrix is passed through two air streams in counter-flow. Figure D shows the general
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construction. In comparison to heat recovery wheels the matrix of desiccant wheels is additionally coated with solid desiccant material. Typically applied solid materials are silica-gel or zoelite and other hygroscopic chemical compounds. The basic material which forms the supporting structure is a mix of different fibres, including glass, ceramic binders and heat resistant plastics.
Figure D: Schematic drawing of a desiccant wheel compound (left). The rotation is driven via a belt drive. The regeneration heat is supplied by a water-air heat exchanger (Source: University Hamburg Harburg / Germany). An Example of a desiccant wheel integrated into a cassette is on the right (Source: Klingenburg GmbH / Germany)
An example of a typical desiccant wheel performance under design conditions is shown in Figure E. This Figure represents performance data for a desiccant wheel which is applied in a DEC- system of the European Project DESODEC in Armenia (INCO-COPERNICUS Programme of Commission of the European Communities). The shown desiccant wheel is designed for a volume flow of 8 500 m³/h.
In such system the desiccant wheel is regenerated by a reduced air-flow. The regeneration air-flow is designed by 80% of the process air-flow. The regeneration heat is provided by flat-plate collectors which generate a regeneration air temperature of 85°C in the design point.
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ININ 85.0085.00 T [°C]T [°C] 15.0415.04 x [g/kg]x [g/kg] OUTOUT regenerationregeneration air air 3.913.91 r [%r [% ] ] T [°C]T [°C] 48.0548.05 78987898 v [m³/h]v [m³/h] x [g/kg]x [g/kg] 27.8727.87 71577157 m [kg/h]m [kg/h] r [%r [% ] ] 36.7336.73 v [m³/h]v [m³/h] 72267226 OUTOUT m [kg/h]m [kg/h] 71577157 59.9659.96 T [°C]T [°C] 5.575.57 x [g/kg]x [g/kg] 4.274.27 r [%r [% ] ] ININ 96499649 v [m³/h]v [m³/h] T [°C]T [°C] 32.0032.00 95439543 m [kg/h]m [kg/h] x [g/kg]x [g/kg] 15.1915.19 r [%r [% ] ] 48.0048.00 process air v [m³/h]v [m³/h] 89758975 process air cleaning section m [kg/h]m [kg/h] 95439543
Figure E: Performance data for a desiccant wheel which was proposed for a DEC-system in the frame of the European INCO-Copernicus Project DESODEC in Armenia (Source: Fraunhofer ISE / Freiburg / Germany)
The following table lists performance data of a desiccant wheel which is designed for an air-flow of 3000 m³/h. The temperature and humidity values of both process air and regeneration air differ, so that different dehumidification rates appear. The listed data are provided by Klingenburg.
Dehumidification Inlet air Outlet air rate [°C] [g/kg] [°C] [g/kg] [g/kg] Process air 32 12.0 53.2 6.9 A 5.1 Regeneration air 70 14.0 48.7 19.1 Process air 32 12.0 47.4 7.4 B 4.6 Regeneration air 60 12.0 44.5 18.6 Process air 35 15.0 50.9 9.4 C 5.6 Regeneration air 70 15.0 54.1 19.6 Process air 35 15.0 46.2 10.0 D 5.0 Regeneration air 60 15.0 48.8 18.9 Performance data of a desiccant wheel design for an air-flow of 3000 m³/h (Source: Klingenburg 1999)
In dependence of the rotation speed the desiccant wheel operates in two different modes – on one hand the dehumidification mode and on the other hand the enthalpy recovery mode. Using the desiccant wheel in the dehumidification mode the rotation speed is generally in the range of 6 - 12 rotations per hour. If the desiccant wheel rotates with 8-14 rotations per minute, it performs as an enthalpy recovery wheel.
Figure F illustrates both the dehumidification mode and the enthalpy mode. The supply air is dehumidified (state change 1 - 2) on the process side of the wheel. The return air is heated up to a sufficient regeneration temperature which flows through the regeneration side of the wheel (state change 8 - 9). The desorption of the water bounded in the desiccant material on the process side is activated since the vapour pressure of the water bound in it exceeds the partial pressure of the water vapour in the warm regeneration air. The energy associated with the sorption and desorption processes is equal to the latent heat of condensation plus a differential heat of sorption. It is 66
beneficial to have a low total heat of sorption. In addition, the state change is also affected by the heat stored in the rotor matrix on the regeneration side. As mentioned above at high rotation speeds the desiccant wheel performs as an enthalpy recovery wheel. Especially in wintertime heat and moisture recovery from the return stream is required thus the overall performance of the DEC- system benefits from the desiccant wheel only by increasing the rotation speed. The enthalpy recovery process is represented on the psychometric chart along a line which connects both inlet air points for the two streams – see again Fig. F.
The performance of desiccant wheels is mainly characterised by the dehumidification capacity which describes a number of bounded water per kilogram dry air. According to statements of desiccant wheel manufactures typical dehumidification capacities are in a range of 4 to 6 gH2O/ kg Dry Air for a regeneration temperature of 70°C.
To operate the desiccant wheel with an optimal performance the rotation speed is one of the important parameter. Depending on the values of humidity and regeneration temperature the dehumidification capacity is a function of rotation speed. Figure G illustrates some measurement results1 of a market available desiccant wheel. During the test the rotation speed was verified by values of 10, 15 and 20 rotations per hour. The maximal dehumidification capacity occurs with 15 rotations per hour. Providing a regeneration temperature of 70°C and rotating the wheel by 15 rotations per hour the dehumidification leads to values of round about 6 gH2O/ kg Dry Air.
ϕ = 5 % 80
8 ϕ = 10 % 70
60 dehumdification ϕ = 20 % 9 2 50 ϕ = 30 %
40 ϕ = 50 %
1 30 ϕ = 100 %
20
enthalpy recovery 10
0 0 2 4 6 8 10 12 14 16 18 20 22
Figure F: Psychometric chart for moist air showing the state changes for dehumidification of air in a desiccant wheel
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Dehumidification [g/kg] Dehumidification
Regeneration temperature [°C]
Figure G: Influence of rotation speed and
regeneration temperature on the dehumidification capacity of a desiccant wheel (Source: University of Applied Sciences Stuttgart/ Germany)
Some manufactures of desiccant wheels offer additionally a cleaning sector located between the supply air and exhaust air stream. This particular zone is passed by fresh ambient air and reduces the percentage of exhaust air transfer to the supply air, e.g. there is a hygienic benefit. The cleaning sector is used only if it is intended to prevent soiling or entrained olfactory impacts. In addition the heat transfer from the regeneration side to the supply air side is also minimised. Stored heat in the matrix on the regeneration side is pre-cooled by the cooler ambient air. Consequently the temperature increase of the supply air is lower which results in an improved performance of the overall desiccant system. Figure G is a schematic illustration of such a system with an additional fresh air zone.
4 000 3 500 3 000 2 500 2 000 1 500 1 000 matrix diameter [mm] matrix diameter 500 -
- 10 000 15 000 20 000 000 25 30 000 35 000 40 000 45 000 5 000 air-flow [m³/h]
Figure H: Influence of the required air-flow on the diameter of the desiccant wheel (Source: Klingenburg / Germany)
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Focusing on the general design of a DEC-system the desiccant wheel diameter strongly determines the geometric dimension of the air-handling unit itself.
Figure H shows the influence of the required air-flow on the diameter of the desiccant wheel. For example the DEC-system is designed for an air-flow of 12 500 m³ per hour the diameter of a sufficient desiccant wheel is around 2000 mm. The dimension of a DEC-system is an important issue during planning phase especially for an installation in existing buildings.
Country of Company Desiccant Wheel Size origin Munters USA USA SiGel, AlTi, Silicates, New Proprietary 0.25 – 4.5 m Munters AB Sweden SiGel, AlTi, Silicates, New Proprietary 0.25 – 4.5 m Seibu Giken Japan SiGel, Am, Silicates, New Proprietary 0.10 – 6.0 m Nichias Japan SiGel, Mol, Sieves 0.10 – 4.0 m DRI India SiGel, Mol, Sieves 0.30 – 4.0 m Klingenburg Germany Al oxide, LiCl 0.60 – 5.0 m PorFlute Sweden SiGel, Mol, Sieves 0.50 – 3.0 m Rotor Source US SiGel, Mol, Sieves 0.50 – 3.0 m NovelAire US SiGel, Mol, Sieves 0.50 – 3.0 m
1.4.5 DEC-system with liquid sorbent materials
Open desiccant air-conditioning systems with liquid sorbent materials work according to the same principle as all open processes: ambient air is dehumidified by means of sorption and cooled by water evaporation. In such liquid desiccant system water serves as the refrigerant. A large number of working fluid pairs are available for closed absorption refrigerating machines but there are only a small number of suitable materials for open liquid-based systems. According to the strict limitations for ventilation systems in which materials come in direct contact with the environment the used solutions should be non-toxic and environmentally friendly. They also should not contain any volatile material other than water. In practice, liquid sorbent agents which consist principally of salts dissolved in water are mainly used, e.g., lithium chloride or calcium chloride. These hygroscopic salts lower the vapour pressure of water in solution sufficiently to absorb humidity from the air. In contrast to the case of the solid sorbents, the water bonding mechanism is not adsorption, but absorption.
Figure I shows a DEC-system with liquid sorbent material optimised for solar operation. Ambient air is dehumidified in the absorber, where cooled contact surfaces are humidified with a concentrated liquid sorbent material using the falling film technique. The sorption heat is transferred to the exhaust air through a composite circuit system and an indirect evaporation cooler so that the ambient air is dehumidified and cooled at the same time. A downstream cooler cools the dry air below room temperature. The sorbent material is diluted during dehumidification of the air. In an air-flow regenerator it is heated up to 60-80°C and re-concentrated. Heat recovery from the air and the sorbent material increases the efficiency and saves collector area. Energy can be stored by storing diluted and concentrated sorbent separately. When using the usual aqueous lithium chloride solution as a sorbent, it is possible to achieve an energy storage density of up to 280 kWh/m² by using a special internally cooled absorber without diminishing the dehumidification potential of the concentrated solution.
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Figure I Schematic drawing of a desiccant cooling unit using liquid sorption and solar(source: Menerga/Germany) driven regeneration
(Source: ZAE Bayern - Germany)
In practice there are different ways to assemble such DEC-systems with liquid sorbent material. Figure I is a schematic drawing of the system with liquid sorption and solar regeneration which is developed by ZAE Bayern/ Germany.
Another concept is focused on a compact central evaporative cooling unit. In cooperation Menerga and the University of Essen/ Germany developed such compact DEC-system which is shown in Figure J. Since 2003 a pilot DEC-system design for a volume flow of 1500 m³/h is implemented at Fraunhofer Solar Building Innovation Centre (SOBIC) in Freiburg/Germany. The tested system performance allows an optimistic market perspective for such desiccant cooling system with liquid sorption material. Particularly with regard to the coupling with solar thermal energy the desiccant cooling is an attractive technical concept to reduce primary energy consumption for air- conditioning.
heat recovery and solution regenerator evaporative cooling cooler, heater
return air exhaust air
supply air ambient air
water pump solution pump absorber
Figure J: Schematic drawing of a central evaporative cooling unit operating on the principle of a liquid sorption and solar regeneration (Source: Menerga / Germany)
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Desiccant cooling systems with liquid sorbent materials are more complex than systems with rotors and are not yet available on the market. Some fundamental advantages such as potentially higher overall efficiency due to greater potential of heat and refrigeration recovery, lower possible regeneration temperature with the same dehumidification potential due to cooling of the sorption process, and the potential of efficient energy storage and, not least, the physical separation of supply and exhaust air-flows could help establish them in combination with solar systems.
1.5 DESALINATION SYSTEMS
The lack of pure potable water is a fact for approximately 1.1 billion people. Water scarcity affects all social and economic sectors and threatens the sustainability of the natural resources base. Countries in the Middle East and Africa have long dealt with water shortages but now the likes of China, India and the United States are grappling with the problem. It is projected that if will we continue with business as usual, a number of another countries such as China, Ethiopia, India, Kenya, Nigeria, Pakistan, Peru, etc., will also be pushed onto the list of countries likely to run short of water.
Moreover, since lot of areas, especially insular or coastal dry zones, with rather easily access to brackish or sea water, it is obvious to consider the use of salty water in order to resolve existing as well as future water shortage problems. Also, considering the fact that most serious problems of water supply generally occur in the isolated places (far remote areas, rural zones, small islands, etc.), practically having no access to the electric grid, in view of continuous cost reductions of solar energy (compared to increased fossil fuel price) efforts must be made to make use of solar energy for small to medium size PV/RO plant. Implementation of small to medium sized plants holds a number of benefits both economic (reduced capital outlay) and technical (minimum construction time, local availability of manpower and raw materials, simple operation and maintenance). In fact, most of the abovementioned countries are already engaged in desalination technologies, which are now well established and able to yield large amounts of good quality water.
However, considering the limited availability, high cost and, above all, the negative environmental impacts mainly due to use of conventional energy sources, it is imperative to search for new alternative sources to supplement or substitute to conventional fuels. In view of the problems mentioned above, it is not surprising that moves are afoot to use renewable energy resources, in addition to the traditional one for water desalination, but the fact remains that even at present, mainly because of very high cost (almost twice that of a conventional system), such solution has been practised on a low scale. In spite of the aforesaid economic restraints, it is worth exploring the potentialities of using solar energy, since its peculiar features appear to be most appropriate for this particular aim.
In the case of a medium capacity desalination plant, the production of desalted water using solar energy could be realized using both solar thermal and photovoltaic systems coupled with a reverse osmosis or multi-effect plant, respectively.
So, it is in the above context that the present work focuses its attention to cover various aspects of seawater desalination using both conventional and non-conventional energy sources together with market status, energy and economic assessment, possible technological development, etc.
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1.5.1 Water requirements
The data relevant to water requirements shows that around 40% of the total world population do not have an adequate fresh water supply, both for quality and quantity (source WHO). Although the water emergency concerns nearly 80 countries, the situation is certainly very alarming, especially in countries located within the Southern Mediterranean belt. Drought and desertification are increasing significantly, involving wider and wider areas of the planet. It is true that as of today, countries from Southern Europe are partially affected by the lack of drinking water, but to avoid serious negative impacts in the very near future, it is advisable to take appropriate actions as soon as possible.
Based upon the investigations conducted by the World Health Organization (WHO), it is to be noted that annual water availability of 1000 m3 per capita constitutes the limit below which it will not be possible to guarantee an acceptable living standard as well as economic development of a country. As shown in Table 5, the global picture will become more serious if the forecasts made by Food and Agriculture Organization (FAO) on the overall increase in world population are taken into consideration.
It is to be observed that the world population, actually a little more than 6 billions, should touch an unimaginable figure of nearly 8 and 9 billion during the year 2025 and 2050, respectively. It is estimated that the population increase over the next 20 years (2000–2020) will be around 50% in Africa, 25% in Asia, 14% in the USA and, surprisingly, 2% negative, in Europe. It is obvious from the above mentioned figures that a considerable increase in the world population (over the next decade or so) will be concentrated mainly in most of the developing countries and particularly in Africa, causing severe water shortages.
In a number of South Mediterranean countries (with vast desert or semi-desert areas), the water reserves per capita, already very limited, are destined to be reduced in the future. The data relevant to the population and available water per capita on an annual basis (for all uses) during the period 1960–1990 is presented in Table 6. Also, the predictions made by both the World Resources Institute (WRI) and World Bank for the year 2020 are quoted.
Table 5. Distribution of population worldwide during the years (millions of inhabitants) Year USA EU Africa Asia Total 1950 158 296 221 1377 2522 1960 186 316 277 1668 3022 1970 210 341 357 2101 3696 1980 230 356 467 2586 4440 1990 254 365 615 3114 5266 2000 278 376 784 3683 6055 2010 298 376 973 4136 6795 2020 317 371 1187 4545 7502 2030 333 362 1406 4877 8112 2040 343 349 1595 5118 8577 2050 349 332 1766 5268 8909
It is to be noted that due to the high population increase in the aforesaid period, the water resources have been strongly reduced. Moreover, the foreseen decrease over the following thirty years or so (−48%) will make water availability during the year 2020 even less than the minimum
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recommended by the WHO. The situation will certainly be worse, with an annual availability of less than 200 m3 per capita, at least in a majority of the countries listed in Table 6.
In order to face the current as well as future drastic water shortage problems, most of these countries have already engaged in alternative solutions, such as desalination. Considering the fact that nearly 97.5% of the total reserve of water on the planet is salty, while the remaining share is almost frozen or located in underground basins and, thus, practically inaccessible, seawater desalination appears to be very important. Resources of effective drinking water available correspond to around 0.06% of the water present on earth globally.
It is to be noted that so far as renewable water resources are concerned, thanks to the rain, nearly 42,655 billion m3 of water (annually), i.e. on average, more than 7000 m3 of fresh water available for each inhabitant on the planet, is simply poured into the rivers. Unfortunately, with the non- uniform distribution of such resources on the terrestrial surface, even using all different means available, it has not been possible to exploit more than one third of the above mentioned amount. Moreover, the increased water contamination caused due to progressive industrialization and urbanization will certainly reduce this amount further.
Table 6. Population (thousands of inhabitants) and yearly water availability (m3 per capita) in the South Mediterranean Country 1960 1990 2020
Saudi Arabia 4075 537 16,045 156 36,424 54 Libya 1349 538 4416 154 8103 59 Malta 312 100 354 75 427 76 Yemen 5247 481 11,590 214 34,190 82 Jordan 763 529 3306 224 8204 100 United Arab Emirates 90 3000 1921 189 3170 117 Syria 4561 1196 12,386 439 24,555 172 Israel 2114 1024 4660 467 7952 324 Tunisia 4221 1036 8156 532 12,254 334 Algeria 10,800 1704 24,936 737 43,853 376 Oman 558 4000 1785 1333 4719 477 Egypt 27,840 2251 56,333 1112 90,491 682 Morocco 11,626 2650 23,931 1185 36,742 685 Total 75,516 1751 171,809 760 313,104 393
In conclusion, the accessible drinkable water resources on earth are limited, whereas renewable ones will simply not be sufficient to cover a fairly distributed requirement. Since this picture is destined to get worse over the next decade or so, the sea represents the only reserve from which it is possible to draw new and huge resources of pure water.
1.5.2 Desalination technologies: a review
The first attempts to produce fresh water from salty water (based on the principle used for the renewal of natural water on the planet through rain, i.e. heating of water containing salt up to evaporation and subsequent condensation of vapor to get distilled water) started in the beginning of the 18th century. industrial processes appeared during 19th century for a significant market growth during 20th century. For example, a multiple effect distillation plant with an overall water production capacity of 75 m3/d was realized in Egypt during the year 1912. Anyway, it was only
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during the year 1957 that desalination technology was applied at the industrial scale, when the very first four stages multi-flash (MSF) desalination plant was built in Kuwait.
In the preceding years, however, rapid development of this technology was observed with the realization of the Shuwaikh plant in Kuwait (19 stages and 4550 m3/d) and the Guernsey plant in the Islands of the Channel (40 stages and 2775 m3/d). Since then, considerable progress has been made. MSF desalination plant with a capacity of 73,300 m3/d, in a single line, is in operation now. Especially, it was after the year 1970 that the reverse osmosis technology (thanks to the modular structure of plants) got a remarkable growth with the possibility to reach an installed capacity of more than 100,000 m3/d.
According to the data made available by the International Desalination Association (IDA) the overall worldwide installation, distributed over almost 120 countries (with production capacity of nearly 70 million m3/d and number of plants, actually operating or ready for production), amounts to more than 17,000 installations.
Fig. 15 presents the overall worldwide capacity since the beginning of desalination at industrial scale. After an interlocutory phase, especially after the year 1970, a noteworthy growth (almost constant) has been observed. According to recent predictions, presently, with annual market of about 7 million m3/d, there is an approximate investment of about 7 billion US$. As regards the type of resource treated, nearly 65% of the overall installed capacity has been realized through seawater desalination plants and the remaining 35% brackish water (with a percentage of total dissolved solids (TDS) inferior to 10,000 ppm) with other processes.
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Figure 15. Worldwide installed desalination capacity since first applications at the industrial scale.
The technologies used on the industrial scale are generally classified into the following categories:
Thermal processes
1. Multistage-Flash (MSF);
2. Multiple Effect Distillation (MED), with or without mechanical or thermal compression;
Membrane technologies
1. Reverse Osmosis (RO);
2. Electro Dialysis (ED).
3 Nano filtration (NF).
Fig. 16 illustrates the market share in 2008 for the different technologies. It can be observed that processes such as MSF and RO cover the most of overall worldwide production (32% and 51%, respectively). It is, however, to be noted that the progress of MSF technology over the last couple of years, is rather slow. Contrary, RO has gained significant market share.
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Figure 16. Desalination technologies market share (in % of capacity, source IDA 2008)
The contribution from the various technologies changes radically if only seawater desalination is taken into consideration. In this case, the relative percentages of the MSF and RO processes becomes 50% and 35%, respectively. Here, other thermal processes also earn importance.
The principal characteristics of the different desalination technologies are presented in Table 7. Here, extrapolation is made on the basis of the data available in literature. The analysis is limited to large scale seawater desalination plants that as seen above, in addition to the present specific interest, constitute the most fruitful future application of this technology. It is to be noted that ED, which is most appropriate for desalination of water with low salt concentration (up to 5000 ppm of TDS), could not be applied for seawater desalination with salinity concentration of around 35,000 ppm.
Table 7. Main features of different desalination technologies
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High capacities plants (above 30,000 m3/d) are always constructed by assembling of units operated in parallel. There is no theoretical limit; local needs drive plant capacity. Above about 100,000 m3/d, no scale effect on plant and water cost is observed. Capacity of Jubail MED-TC desalination plant reaches more than 800,000 m3/d (27 units of 29,630 m3/d).
* water cost depends on unit size, its location, energy source and water quality
** specific cost depends on unit size
*** Gain Output Ratio: production/steam (or thermal energy) consumption. GOR of MED can be 1 for a single effect machine (thermal energy consumption is maximum, according to GOR value)
It is worth noting that TDS contend of water, without any effect for thermal processes, has a remarkable effect for reverse osmosis where energy consumption (for a given quality of desalinated water) increases at a rate of more than (0.5 kWh/m3 per 10,000 ppm If, on the other hand, the operating pressures are left unchanged, the percentage of salts in the water produced could be intolerably high, due to the degradation of membranes selectivity with time. Normally, this value for the RO process is expected to be around 300 ppm. The value, though lying well within the limit of 500 ppm (fixed by the WHO for the drinking water), still results in being at least one order of magnitude higher than the salinity of water from thermal processes.
The equivalent electricity consumption has been estimated assuming simply that the required thermal energy corresponds to the electric energy obtainable from the expansion of the vapour in a turbine (with a fixed efficiency of 0.85) over the useful temperature range of 100 and 70 °C (MSF and MEE) down to a final value of 40 °C. This is merely an indicative value adopted in order to compare the overall efficiencies of the different technologies, as detailed comprehensive calculation is possible if and only if the effective conditions of each real plant are known.
MSF MED VC RO ED Specific thermal energy 60 - 90 50 - 700 40 (MED- 0 0 consumption kWh/m3 TVC) Electricity specific consumption 3 - 5 2 - 3 8 – 18 2 – 5 2 kWh/m3 (MVC) Product cost US$/m3* 0.7 - 2 0.7 - 2 1 - 4 0.5 – 2 (sea 0.3 – 1 (brackish water) water) 0.3 – 1 (brackish water) Specific investment cost US$/(m3/d) 1000 – 900 - 900 - 2500 850 – 1500 8000 - 1000 FOB** 1900 1700 GOR*** 8 – 12 1 - 15 17 (MED- - - TVC) The state of the desalination industry in the leading 11 nations (covering almost 73% of the world market) is shown in Table 8. It is evident from the above data that more than 64% of the world production is concentrated in the Persian Gulf Region (Saudi Arabia, United Arab Emirates, Kuwait, Qatar, Bahrain and Oman) with total population of less than 30 millions. Also, in the USA and, to less extent, in Japan, numerous plants have been realized. However, so far a European countries are concerned, only Spain and Italy have made significant efforts in this direction. It is to be noted that the Spanish industry (almost doubling its installed capacity over the period 1996– 2000 and especially providing drinking water to more than one million residents on the Canary Islands through desalination processes) has certainly made significant progress. Also, other 77
countries, such as Cyprus and Malta, have adopted this technology, though on a relatively modest scale.
Table 8.
So far as the technologies used in the Gulf region are concerned, the market is dominated by large scale MSF plants powered by thermoelectric power plants, covering nearly 62–97% of the desalted water production. Also, during the above mentioned period, i.e. 1996–2000, a quite sensible growth (nearly 10%) with the MED process (more complex but at the same time more energy efficient compared to MSF) was observed.
The Arabian Peninsula, with both severe operating conditions and water shortage, appears to be the most appropriate and potential site for large scale installation of desalination systems based upon processes such as MSF and MED. In fact, water from the Red Sea, characterised by both drastic temperature variations (12 to 35 °C from winter to summer) and, above all, the high salinity concentration (average 42,000 ppm and maximum 64,000 ppm), makes RO technology use very problematic and very costly.
In addition, the vast operating experience acquired over the years (since 1960), high degree of reliability and ever increasing water demand, certainly are strong bases for wide spread diffusion of the MSF process, at least in the near future. This will certainly be true in different countries and especially in the Persian Gulf. Contrary, in countries like the USA, South Europe and Japan with more advanced technology, scattered islands and low salinity water (brackish, river or ocean), RO appears to be the successful desalination process. Lastly, Italy, due to its particular conditions (certainly favorable but not like those of the Gulf region), lies within an intermediate level, using a mix of different technologies for seawater desalination.
It is, however, to be noted that since 1990, the installation of reverse osmosis plants worldwide has grown by more than 50%, whereas the increase in MSF processes is only 30%. In addition, 54% of the new contracts are of the RO type compared to only 32% for MSF plants. This is mainly due to progresses of energy recovery devices and enhanced membranes used in RO process. Lastly, nearly 13% of new contracts are drawn for MED systems with quite a large growth of this technology. 78
Based upon the above mentioned considerations, it can be stated that so far as the future of seawater desalination on a large scale is concerned, MSF and MED plants will continue to be preferred to deal with high salinity water. In order to enhance energy efficiency, such plants are coupled with thermoelectric power plants according to a dual purpose scheme (generation of both water and electricity). On the other hand, if operating conditions will not be excessively severe, RO plants will occupy the market of small to medium users or even big users in single purpose systems (water production only).
Table 9 shows the contribution of desalinated water to meet water requirements in MENA countries. Here, it is worth noting that the United Arab Emirates cover almost half of its multi- sectorial water needs using desalination. Malta and Saudi Arabia fulfil at least the household consumptions (about 70 m3 per capita yearly). Also, in Libya and Oman desalinatec water contributes significantly, while in countries like Jordan, Syria and Yemen, despite the situation being critical, the desalination yield seems to be practically negligible.
1.5.3 Energy requirements: possible options The well established seawater desalination technologies available worldwide, no doubt, can be employed to produce large amounts of good quality water at a cost that appears to be reasonably quite competitive. Unfortunately, it remains unaffordable for most of demanding countries. Two main drawbacks of all such processes still remaining to be resolved is the high energy consumption and their environmental impact: Greenhouse Gas emission of used energy, chemicals and corrosion products in brine. The specific energy consumption to desalinate 1 m3 of seawater using RO technology over the last three decades is presented in Fig. 17. It can be observed that continuous technological progress has reduced these values drastically. Thanks to the development of RO technology, the energy consumption of more than 20 kWh/m3 during the year 1970 has been reduced to less than 5 kWh/m3 today.
Figure 17. Trend of the energy required to desalinate seawater using RO over the last three decades.
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Table 9. Overall installed capacity and corresponding yearly pro capita production Country Capacity (m3/d) Production pro capita (m3/y) United Arab Emirates 2,164,500 357.5 Malta 146,900 143.0 Saudi Arabia 5,253,200 105.0 Libya 683,300 50.2 Oman 192,600 32.6 Israel 93,900 6.2 Algeria 211,700 2.8 Tunisia 53,700 2.2 Yemen 74,600 1.8 Egypt 129,000 0.8 Jordan 7000 0.6 Morocco 19,700 0.3 Syria 5600 0.1
Global specific energy consumption (including pre-treatment, post-treatment and process control) to a value of less than 3 kWh/m3 has been achieved through adoption of pressure exchangers. It is hoped that such systems yet under experimentation would be able to replace the turbines currently used in large scale RO plants to recover energy from the discharged brine that represents a huge quantity of water (generally 50% of input water) under high pressure conditions. It is, however, to be noted that, no doubt, the amount of energy needed to desalt seawater has been reduced considerably, but the fact remains that this value, still being high, constitutes the main reason for the limited diffusion of the technology. Particularly,
1. to reduce the production cost, large scale MSF and MEE plants need to be coupled with thermoelectric stations,
2. large scale RO and MVC plants need to be located at a site with cheap and sufficient electric power supply.
On site power availability is a major problem. In fact, one third of the total world population does not have any access to the electric grid, and the worst is that, unfortunately, this happens in most of the developing countries. In Southern Europe, for example, such percentage is a little less than 1%. Considering continuous population growth and expanding industrialization, the situation, however, is destined to become more serious.
Problems relevant to the use of fossil fuels, in part, could be resolved by considering possible utilization of renewable resources, such as biomass, solar, wind, geothermal energy etc. In fact, most developing countries, with vast areas but having no access to the electric grid, appear to be well versed in renewable energies. Such sources, able to be used directly even at far remote and isolated areas, could be exploited to power low to medium scale desalination plants.
A meaningful contribution from the above mentioned environmentally friendly energy resources would certainly be to extend the foreseen duration of fossil fuels store as well as attenuate the socially negative impacts caused by sudden increases in oil price. It is to be noted that nearly 3 kg of CO2 generated for each m3 of water produced (at an energy consumption rate of 5 kW h/m3
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with the best technology currently used on large scale) could be avoided if the conventional fuel is replaced by a renewable one.
It is, however, to be noted that in spite of the aforesaid favourable characteristics, the renewable energy contribution to cover energy demand worldwide, though increasing, is still marginal. Aside from hydroelectric energy (fully exploited and competitive economically) and biomass (used mainly for low-grade energy production; heating and cooking of the foods), the other principal resources (solar, wind, geothermal and tides) cover together a little more than 1% of the energy production worldwide, Table 10. Here, it is to be noted that the cost of renewable energy production still being high, systems of the types discussed above have not experienced widespread diffusion.
Table 10. Annual worldwide energy production by most important renewable sources and overall demand ratio Renewable source Annual production (TJ) ‰ Global demand Solar thermal 228,720 0.523 Solar thermal (electric) 1200 0.003 Photovoltaic 630 0.001 Geothermal 128,060 0.292 Geothermal (electric) 151,390 0.345 Wind 35,760 0.082 Tides 2160 0.005 Total 547,920 0.806
The cost of 1 kW h of renewable energy (in 1997) and future sceneries, according to evaluations of the US Department of Energy (DOE), is presented in Fig.18. As can be observed, as of today, the values for solar energy, when compared to the conventional one (0.03 $/kW h), are very high. It is, however, expected that the cost of 1 kW h of solar energy will certainly decrease significantly over the next few years (to nearly 0.1 $/kW h in the year 2010). An opposite trend is estimated as regards the cost of 1 kW h energy using conventional fuel. This may be attributed to both the foreseen price rise of fossil fuels and a plausible introduction of severe penalties, even fiscal, for systems causing environmental pollution. Instead, both geothermal and wind energy seem to be competitive, as 2020 situation doesn’t foreseen to be different significantly from 2010 one! Expected for small-scale PV but cost divided by 3 seems to be unrealistic.
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Figure 18. Energy production cost by using renewable sources: actual and foreseen values
1.5.4 Solar powered water desalination systems
Renewable Energy, especially the solar, OTEC and wind, is the most appropriate energy source for desalting water. It is sufficient to remember that it is simply due to solar radiation that the renewal of water on earth is made possible, by the cycle of evaporation and successive water condensation in the form of rain.
Thanks to this principle, ever since World War II can convert seawater into drinkable water through desalination. In such systems, so called solar stills, salt water is kept in a black bottom glazing basin. Solar heat makes the feed water evaporate. The vapors are condensed on the glazed surface and collected in a reservoir, using appropriate ducts. The system, though extremely simple, could not be applied on a large scale due to the enormous surface requirement due to low efficiency (GOR = 1), high specific investment cost and vulnerability to adverse meteorological conditions.
As a rule of thumb, a solar basin can produce around 4 l/d per m². Taking into consideration this value, it is clear that to satisfy the drinking water requirements of 1000 people, it is necessary to have a basin of area approximately 50,000 m². It is because of this reason that application of this technology is very limited. This is especially true in urban areas with scarce and, above all, extremely expensive unused land. Moreover, even if the system is quite simple to built and operate, driving the pumps, along with frequent and onerous maintenance, requires additional costs.
Solar stills could, however, be considered attractive for domestic purposes, especially in areas having no access to the electric grid and low labour cost. As the still is very small, its cost is low, despite of its high specific cost regarding its daily production rate. An improved version with enhanced efficiency needs more complex constructional, operational and maintenance standards and, as such, is not appropriate for installation in rural areas of developing countries with limited economic and technical resources.
It is to be noted that even today, a large number of low to medium capacity solar desalination plants are of this type. Generally, such systems have a very small capacity. From the literature 82
survey, it has been observed that nearly 100 plants (both solar and/or wind powered) with a mean capacity of around 20 m3/d and installed in more than 25 countries are still working satisfactorily. Such systems, being too small in size, and above all, built in far remote areas, make it really difficult to furnish detailed information. In view of this, it is credible that the data furnished by the IDA reports, abundantly underestimate the installations actually working worldwide.
Well then, it is true that solar energy has been used to desalt water for a long time, but the fact remains that even today, its application on a significant scale is very limited. Nevertheless, the present investigation aims at exploring fully the real potential of this technology with many favorable aspects such as:
As is evident from the data reported in Table 11, South Mediterranean countries with scarce water resources often have abundant solar radiation. The reported values have been calculated using the Surface Meteorology and Solar Energy Data (a software made available by NASA). Moreover, the problem certainly becomes critical, especially during the hot season, with abundant solar energy, scarce water availability but increased water consumption and, hence, demand (many times owed to the presence of tourists too).
Table 11. Annual horizontal solar energy available (kW h/m2) and relative peak value (W/m2) in the countries under investigation
Country Annual solar energy (kW h/m2) Peak radiation (W/m2) Yemen 2170 940 Saudi Arabia 2160 940 Oman 2140 930 Egypt 2050 1030 Jordan 2050 1020 Libya 2010 1040 United Arab Emirates 1980 910 Israel 1930 1010 Syria 1910 1040 Malta 1900 1040 Morocco 1860 960 Algeria 1840 950 Tunisia 1750 980
1. The use of solar energy for both heating and electricity production is fairly well diffused in countries lying within the South Mediterranean belt. It is, therefore, necessary that efforts must be made to take advantage of the large working experience acquired by a few leading nations (Israel, Jordan, Egypt and Morocco) with large solar installations, both thermal and photovoltaic.
2. Fresh water could be accumulated in a simple and economic way and successively supplied to users as and when requested. Economics is to be taken into consideration to determine if it is better to build a big unit (and so on very expensive) operated only during sunny hours, or a small one operated 24h/d with thermal storage. Here, it is worth to note that the relative cost of storage and desalination plant hold the key.
3. The most serious problems of water supply occur especially in isolated places (far remote areas, rural zones, small islands, big ships etc.) having practically no access to the electric
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grid. Under such conditions, solar energy could compete even economically when compared with the traditional alternative (Diesel engine powering RO).
4. In order to solve the shortage of fresh water, desalination plants of relatively small capacity, as discussed in the present paper, appear to be the most advantageous, both economically (reduced capital) and technically.
In order to reach a substantial exploitation of solar energy, it is crucial to investigate the possibility of powering medium to large capacity desalination plants. So far as the type of desalination technology to be coupled with solar energy is concerned, in principle, any process could be used. As shown in Table 12, obviously, solar thermal is the most appropriate energy source to feed thermal processes (MSF or MED), whereas photovoltaic ones can supply electric energy for processes such as RO.
Table 12. Possible options for coupling between solar energy and process of desalination Solar energy MSF MEE MVC RO Photovoltaic ● ● Though not cost effective Solar thermal ● ● Solar thermal (electric) ● ● ● ●
Solar thermal-electric stations, producing both electricity and eventually heat through a cogeneration arrangement, could feed all types of desalination processes, particularly the hybrid systems (for instance RO/MSF). It is quite unnecessary to emphasize that the techno-economic optimization of the plant requires, in this case, a rather high capacity of 10,000 m3/d or more. Taking into consideration the main objective of the present investigations, i.e. small to medium scale applications, the photovoltaic powered RO process (PV/RO coupling) and the solar thermal powered multiple effect system (ST/MED; using an advanced solar collector, such as an evacuated tubular collector) appear to be the best options.
It is to be noted that capacity has almost no effect on competitiveness of RO compared to MED at industrial scale. Difference is due to electricity and thermal energy cost. If thermal energy is free (thermal waste), MED is always cheaper than RO, whatever electricity cost is. If electricity is paid (i.e. 0.1 $/kWh), MED remains competitive up to marginal cost of thermal energy of 4 $/ton of steam (or equivalent. It is, therefore, to be noted that with the increase in capacity to 5000 m3/d, the production cost in both systems is around 2 $/m3.
It is, however, to be noted that even if the difference is reduced from 2 to 1.2 $/m3 (within the analyzed range), this value is still more than double the cost of water from the conventional system (about 0.75 $/m3).
So, an increase in the size of solar powered desalination plants, no doubt, lowers the water production cost, but also, it has negative repercussions on factors mentioned previously, i.e.
1. Land occupation by the solar field being proportional to the plant capacity, the PV/RO system will certainly be preferred compared to the ST/MED system (nearly 8 m2 compared to little less than 20 m2 per m3/d of installed capacity. Thermal processes are therefore effective at large scale for dual purpose plants only. It is because of this reason that the simple solar still, having 84
a colossal land request of about 250 m2 per m3/d of fresh water, cannot exceed certain size limits.
2. It is true that the increase in plant size cuts the specific plant cost significantly but the fact remains that it is still very high compared to that of a typical conventional plant. For example, a PV/RO system of capacity 5000 m3/d needs an initial investment of more than 22 million $ compared to about 6 million $ for a conventional RO system.
It can, therefore, be concluded that a capacity increase of solar powered plants, no doubt, reduces the water production cost but, unfortunately, increases significantly both the initial expenditure and the area required by the solar field. To this affect, It is certainly advantageous to make use of the funds provided by a number of Governments and International Institutions for renewable energy based productions thus reducing the water production cost in the future. Besides, depending upon the location, plants could also benefit of funding (made available by the World Bank) for installations of such systems in remote areas of least developed countries. Water production costs of PV/RO and ST/MED systems (with and without incentives of 30% and 50% for the solar section only) are shown in Fig. 19 and Fig. 20, respectively. As shown in the above mentioned figures, assuming 30% and 50% incentives, a net reduction in the production cost (to 1.7 and 1.4 $/m3 for the PV/RO system) and (1.6 and 1.3 $/m3 for the ST/MED system) could be possible.
Figure 19. Water cost as a function of plant capacity by a PV/RO system without and with incentives meant for solar section only
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Figure 20. Water cost as a function of plant capacity by a ST/MEE system without and with incentives meant for solar section only.
Anyway, such costs still remain high compared to those of a traditional system. It is important to note that the reasonable comparative value could be reached with cheap energy cost: dual purpose plants, cheap electricity, cheap oil, etc. Also, in view of the increased energy cost at isolated sites, solar energy may become competitive. With the prices of PV modules anticipated to decrease further, the future of photovoltaic technology certainly appears to be encouraging. It can be attributed to both the increased production and sale (especially in the USA and Japan, and technological development.
Ascertained from the favourable economic trend for photovoltaic technology, the cost estimation for each cubic meter of desalted water under reduced modules prices can be interpreted as possibly achievable goals over a short interval of time.
Solar energy could be competitive economically at places where the price of fossil fuels used to run a desalination unit is notably higher than their standard value. For example, in isolated areas having no access to the electric grid as well as suffering from a critical water supply shortage, a traditional desalination plant (reverse osmosis) is generally connected to a Diesel running electric generator. For such an application, Diesel, with its high transportation and storage cost, could certainly be very costly compared to a normal value of 0.2 $/kg more or less.
In conclusion, as of today, a solar system, in comparison to its conventional counterpart, does not appear to be competitive economically. Nevertheless, the possible intervention of some factors examined above could lead to a particularly favourable situation in the near future. For example with an amortization rate equal to that of conventional systems (as a result of increased reliability) and the possible reduction in the PV modules price, the water production cost using a PV/RO system (and that too without incentives) will be in line with that of a standard (conventional) system having no access to the electric grid.
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1.5.5 Expected developments
The results obtained in the previous paragraphs demonstrate that as of today, the difference in cost is certainly unfavourable for solar energy based systems compared to conventional ones. It is, however, important to note that the number of present and foreseen developments adopted/to be adopted at both the national and international levels would reduce this difference significantly, thus making renewable systems more favourable.
The most important foreseen developments are listed below, i.e.
1. Introduction of the so called carbon tax or similar measures will certainly make the further rise in the prices of traditional fuels almost unavoidable.
2. Prices of PV modules are destined to decrease in the near future. It will mainly be due to significant growth of the sold volume as well as continuous improvement of PV technology.
3. Reduction in the prices of solar thermal collectors, though a bit less compared to photovoltaic, will also be observed.
4. Availability of electricity at lower cost (from a PV system) to some extent may help PV-MVC coupling feasible.
5. Operation of the MED process at high working temperature could be possible using innovative pre-treatment methods (i.e. nano-filtration) at higher efficiency. However, here, the pollution due to chemicals begins to be critical. It is therefore hoped that low temperature MED (LT- MED) with very efficient heat transfer surfaces, may certainly prove to be a potential option.
6. Development of cheaper and less corrosive building materials for the MEE system (for example plastic), besides reducing the plant cost, would allow its operation at higher temperature with aforementioned advantageous consequences. Competitiveness of MED is strongly linked to dual purpose plants. Direct coupling of solar heat source with desalination MED remains possible but at higher cost. It may be the only way if water quality does not allow RO (salinity, fouling properties...).
7. Providing a MED plant with a heat absorption pump, a significant increase in efficiency (40% and 120% more for a single and double effect pump, respectively) with drastic reduction of the required solar field area will be achieved. Obviously, the associated pump cost and, above all, the constraint of resorting to more expensive solar devices able to achieve the high temperatures indispensable to feed the pump (100 and 170 °C, respectively) plant cost will increase as well.
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2. SOME APPLICATION CASES
OVERWIEW ON WORLD WIDE INSTALLED SOLAR COOLING SYSTEMS
In order to define the state of the art of the existing large scale solar cooling systems, relevant data have been collected.
The data about existing solar cooling plants have been collected through:
- direct contact to task 38 participants following single systems,
- contact of the institutions who own the systems,
- contact of installation firms and
- international projects like IEA-Task 25 [3], RoCoCo [4] and SACE
The list counts 81 installed large scale solar cooling systems, eventually including systems which are currently not in operation.73 installations are located in Europe, 7 in Asia, China in particular, and 1 in America (Mexico). 60% of these installations are dedicated to office buildings, 10% to factories, 15% to laboratories and education centers, 6% to hotels and the left percentage to buildings with different final use (hospitals, canteen, sport center, etc).
Within 56 installations absorption chillers are used, within 10 adsorption chillers and within 17 DEC (Desiccant Evaporative Cooling) systems. Among the DEC installations, only 2 systems use a liquid regenerator (DEC liquid). The overall cooling capacity of the solar thermally driven chillers amounts to 9 MW 31% of it is installed in Spain, 18% in Germany and 12% in Greece (Fig. 1).
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The overall solar cooling capacity is assisted by 23’720 m² solar thermal collectors. 53% of the total gross area is made off FPC (Flat Plate Collectors), 37% of VTC (Vacuum Tube Collectors), 7.3 % of CPC (Compound Parabolic Collectors) and 2.6% of AH (Air Heaters). Only one installation, a hotel in Dalaman (TR) applies PTC (Parabolic Through Collectors) technology. The main solar thermal surfaces installed for cooling purposes are located in Spain, Germany and Greece (respectively 24%, 20% and 18% of the total surface –Fig.2). The largest solar cooling plant is located in Viota (Greece). It has been realized for a cosmetic factory and it is made off 2 adsorption chillers (350 kWc each). 2’700 m² of flat plate collectors are installed delivering heat to the adsorption chillers as well as to the factory processes.
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SOLAR ASSISTED ABSORPTION PLANTS OPERATING IN EUROPE.
Kollektortyp Leistung Land Anlage / Ort Anwendung Kollektorfläche [kW] 2 [m ] Wolfferts / Köln Büroräume 70 ETC / 176 Ott & Spies / Langenau Büroräume 35 ETC / 22 Bundespresseamt / Berlin Büroräume 70 ETC / 244 Fraunhofer-Institut Umsicht / Büroräume 58 ETC / 108 Oberhausen Laborräume Deutschland Bundesverkehrsministerium / Berlin Kältenetz 70 FPC / 209 Büroräume und ZAE Bayern / Garching 7 ETC / 20 Laborräume M + W Zander / Stuttgart Büroräume 143 ETC / 260 Technologiezentrum / Köthen Büroräume 15 ETC / 79 American College I / Athens Ausbildungsräume 168 ETC / 615 Büroräume und Solar Lab Demokritos / Athen 35 FPC / 160 Grichenland Laborräume Rethymno Village Hotel / Rethymno Hotel 105 FPC / 450 Lentzakis Crete / Rethymno Hotel 105 FPC / 450 Social and Cultural Centre Clara Auditorium, 229 FPC / 150 Campoamor / Barakaldo Ausstellungsraum Education Department Regional Büroräume 252 ETC / 750 Goverment / Toledo Fabrica de Sol Building / Barcelona Büroräume 105 ETC / 120 Fundación Metrópoli Building / Madrid Büroräume 105 ETC / 72 Daoiz y Velarde Sports Centre / Madrid Sportcenter 170 ETC / 507 Head Offices of Inditex / Aretixo (La Büroräume 170 FPC / 1500 Coruna) Old Peoples Home / Fustinana (Navarra) verschiedenes 105 ETC / 102 University Rovira i Virgili / Tarragona Büroräume 35 ETC / 96 Head Offices of Viessmann Spain / Pinto FPC / 105 Büroräume 105 (Madrid) ETC / 6 Belroy Palace Hotel / Benidorm Hotel 125 ETC / 345 Spanien (Alicante) University of Sevilla (School of Engineers) Laborräume 35 FPC / 151 / Sevilla ÊTC / 50 University Carlos III / Leganés (Madrid) Laborräume 35 FPC / 50 Laia Hotel / Derio Hotel 105 FPC / 160 CARTIF, Boecillo Technology Park / Büroräume FPC / 37,5 35 Valladolid Laborräume ETC / 40 Siemens Contromatic / Corneliá del Büroräume 105 CPC / 214 Vallés National Institute of Airospecial FPC / 25 Laborräume 10 Techniques INTA / Huelva ETC / 18 FONTEDOSO / El Oso Industriegebäude 105 FPC / 504 Stella-Feuga Building, Santiago de Büroräume 115 FPC / 60 Compostela Büroräume und Portugal Verkehrsleitzentrale 70 CPC / 663 Leitzentrale Büroräume, Italien Baxter / Trento 108 FPC / 108 Ausstellungsfläche Österreich Weinbetrieb Peitler / Steiermark Weinlager 10 FPC / 100 CSTB-Gebäude/ Sophia Antipolis Laborräume 35 ETC / 63 Frankreich DIREN-Gebäude / Guadeloupe Büroräume 35 ETC / 61 Weinkeller / Banuyls Weinlager 52 ETC / 130 92
Energy storages
80% of the 20 installations have one or more tanks to storage thermal energy. The collected data shows that there is not a recurrent value for the storage capacity rated to the collector’s surface or to the installed cooling power, also when the same technologies are applied. In 10/17 cases a cold water storage is installed. A recurrent value for the storage capacity rated to the cooling power is not registered. At the University Rovira and Virgili in Tarragona (35 kW absorption machine) there is installed a cold storage tank of 5’000l, while at the Technologic Center in Valladolid (same chiller capacity), a storage tank of 1’000 l is installed , leading to 142.8 and 28.57 cold storage liters per cooling power unit respectively. In both installations no cold back up system is present.
Back-up systems
A heat back up system is present in 16/19 installations. Gas boilers, cogeneration units and district heating are the main systems used for heat backup purposes. On the cooling demand side, 10/16 installations have a chilled water back up systems. These are mainly compression chillers. Only one installation, the Ministry for Traffic, Building and Housing in Berlin (DE) use two compression chillers (65 kW and 180 kW ) and an ice storage tank of 7250 l to support a c c 70 kW absorption machine fed by 229 m² of flat plate collectors.
Heat rejection
9 plants based on absorption technology and 2 based on adsorption technology have installed a wet cooling tower as heat rejection system. 7 cases, of which 6 applying absorption technology, are driven by open cycle.
GREEK CSP PROJECTS [IASA]
• The 50-MW Minos CSP project, using heliostat mirrors and a tower system based on innovative superheated steam technology will be developed by UK-based Mediterranean region-focused solar power developer Nur Energie in southeastern Crete. A conventional diesel boiler will supply back-up energy. In 2009 NurEnergie, established a subsidiary, NUR-MOH HELIOTHRMAL SA (NUR-MOH) which is shared with Motor Oil Hellas. NUR-MOH works with leading CSP technology providers, to develop one of the most advanced tower systems in the world. Crete in an autonomous island, experiences high levels of solar radiation and Greece offers a feed in tariff regime for CSP with 25 years PPA. The project is fully licensed and ready for construction.
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• The 75.3-MW Maximus CSP project is a large-scale Stirling dish power plant, located in northwestern Greece's region of Florina. The project will comprise 37 modular small power plants to be grid-connected through one connection point. The plants will utilize 3-kW Stirling dish units, each inducing a cavity receiver to capture the irradiation from a parabolic-shaped reflector, a free piston Stirling engine to convert the solar energy to power and a closed loop air driven cooling system. The plant consists of 25160 Stirling dish units, each of the 3 kW rated power output. The plant is composed of 37 small power plants of modular design, built on different land plots, which will be connected to the grid via a single connection point. The Stirling dish unit consists of a cavity receiver that captures the concentrated solar irradiation from the parabolic-shaped reflector, a free-piston Stirling engine (FPSE) that converts the solar energy to electricity and a closed loop air driven cooling system. The concentrator is mounted on a structure with a two-axis tracking system to follow the sun. Infinia Corp announced in a press release its involvement as partners in NER300 project Maximus.
The table below shows different demonstrations initiatives in the field of simultaneous production of water, electricity and cooling from solar energy.
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Project name Type Status Technology Reference MATS FP7 Prototype building Cylindro + http://www.mats.enea.it/progetto.asp in progress stockage + turbine + MED Mu'Tah Industriel : Design in progress MicroCSP http://sopogy.com/projects/index.php?id=58 University Sopogy Parabolic 80 kWth
Trigeneration Industriel : NEP Prototype 130 kW Parabolic trough http://www.nep-solar.com/ PSA solar (230m²) Electricity (Plateform + heat / cooling + Solar of dessalination Almeria) Sana’a Solar Industriel Project stopped CSP : 1250 http://www.solardesalination.com.au/content/SolarD Water ACQUASOL MWth 1010 esalinationProjectsElsewhere.html Wh/year à 109m3/year desalination Port Augusta, ACQUASOL Project in Hybridation coal http://www.acquasol.com.au/content/projects.html South construction + solar Australia desalination. Ayla Oasis FP7 et Federal Theorical Study Fresnel, 120 000 http://www.trec-uk.org.uk/reports/AQUA-CSP-Full- Hotel resort Ministry for the m2 Report-Final.pdf Aqaba Environment, 10MWel MED-CSP et AQUA-CSP project Jordanie Nature 10000t/j water (F. Trieb and Müller-Steinhagen 2008) (Franz Trieb Conservation 40MW cooling et and Nuclear al. 2009) Safety Germany Namibie Gouvernement Faisability study Fresnel and http://www.reeei.org.na/admin/data/uploads/Pre- for solar parabolic trough Feasibility%20Study%20for%20the%20Establishm desalination, ent.pdf cooling and process heat Maroc Gouvernement CSP génération Parabolic trough http://www.iresen.org/download/communique_press IRESEN electricity, heat and ORC e_csp_orc.pdf cooling Sahara Forest Electricity and Parabolic trough http://saharaforestproject.com/sfp_qatar_folder.pdf water + PV http://saharaforestproject.com/ prototype in construction in Quatar MICROSOL French Electricity, Parabolic trough http//schneider-electric.com consortium dessalination
MULTI-EFFECT DISTILLATION FOR SEAWATER DESALINATION
Technology Description
The Multiple-Effect Distillation (MED) process is a thermal process for seawater desalination. A typical MED unit consists of several consecutive stages (or effects), maintained at decreasing levels of temperature (and pressure), leading from the hottest stage to the coolest. Each effect contains a heat exchanger; the newest MED units employ parallel plate falling film heat exchangers to maximize heat transfer area in a given volume.
Seawater and a heating fluid are introduced into the two sides of the heat exchanger. As heat is transferred to the seawater, part of it is evaporated and this vapor is allowed to fill the free space in the effect. The remaining (liquid) quantity is now further concentrated seawater, and collects at the bottom of the effect by gravity as brine. The heating fluid is typically steam, 95
which releases heat as it condenses within the heat exchanger to evaporate the seawater. The condensed steam exits the heat exchanger and the effect.
The process is repeated at the subsequent effect. The vapor raised in effect N-1 is ported as heating fluid to effect N. By condensing this heating fluid in effect N, the distillate product is obtained, which is fresh water. In the process, a new quantity of vapor is generated, driving the next effect. In order for this process to be maintained, the pressure in each effect must be decreased, so that evaporation occurs at lower temperatures.
Seawater can be fed to each effect from the source, which is referred to as parallel feed. Alternatively, the brine from effect N-1 can be used as seawater in effect N. This has the benefit of negating the need to preheat the seawater in each effect, thus saving some energy, but has the drawback of introducing seawater at higher salt concentrations in each effect, thus requiring more energy for evaporation. This arrangement is known as forward feed. Finally, the brine from the last effect can flow, via a pump, from effect N to N-1, known as backward feed. The benefit is that no preheating of the seawater is required since it is first introduced at the lowest temperature effect, however pumps are required to move the brine against the pressure gradient.
A schematic of a four-stage MED unit with parallel feed is depicted in Figure 1. Typical MED plants can contain as many as 20 effects.
Four-stage MED schematic with proposed operational conditions.
The first effect of the MED is where the process is initiated. No distillate is produced in this effect. Therefore, the heating fluid introduced here can be anything that can supply the required enthalpy to drive the process. For example, steam extracted from a turbine up stream can be supplied to the first effect and after releasing heat can be returned to the turbine loop, or the heat transfer fluid from a solar thermal process can be fed directly to the first effect.
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MED operates between ~80 °C to ~10 °C above ambient. Higher temperatures are not desired as heat exchanger scaling and fouling is promoted above this limit.
Technology Maturity
The MED technology is a mature process. Currently, MED and Reverse Osmosis (RO) are the two leading technologies for seawater desalination. However, the MED applications are for stand-alone processes; combination of MED with solar-thermal processes is still a very active research field.
In particular, water production can be used as a means of energy storage, as excess energy available from a solar thermal process can be utilized to generate water. This implies that the desalination cycle would operate at a variable production, and hence heat-input, rate. This, however, is in contrast with typical MED applications, which are steady state processes. Studying the time-dependent and variable load operation of MED units is of particular interest when the process is tied to solar thermal energy.
Current research efforts focus on developing models for the MED desalination process, for the cooling requirements of solar-thermal plants and for the integration of the two.
Commercial Suppliers
Applications of MED desalination in small scales exist, with applications in situations where there is scarcity of fresh water, typically on islands or remote locations, or in specialized applications such as cruise ships etc. There are some commercial suppliers active in this area:
• Gerindtec GmbH from Germany. They have some track record with installations in developing countries using MED and MEH (a variation of MED) using solar thermal energy. The input thermal power of their designs is in line with the project’s requirements. URL: http://www.gerindtec.com/index.html
• IDE Technologies from Israel. They specialize in desalination units of various sizes and various technologies, with a proven track record, although they do not seem to have equivalent experience with solar MED systems. URL: http://www.ide-tech.com/
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3. SUMMARY and CONCLUSIONS
This report represents the starting point of Work Package 5 of STS-Med project, providing a technical studies and overview of the different technologies and subsystems involved in the toolbox. Some evaluations about the considered technologies are reported too.
Particularly, the products to be transferred in the Concentrating Solar (CS) model have been identified (Subtask 5.1.1) with 6 corresponding technical studies on the 6 basic toolbox components: solar collectors (including all the devices to capture the solar radiation and convert it into “high temperature” heat), heat transfer fluids, thermal energy storage systems, power generation systems, building heating & cooling systems, and water treatment (desalination) units. The 6 technical studies primarily lead to the realization of this comprehensive technology report (Subtask 5.1.2) that will pave the way to the optimization and feasibility studies implemented in the successive tasks (Tasks 5.2, 5.3 and 5.4) as reported in Deliverables D5.2, D5.3 and D5.4.
It is noteworthy that this Deliverable D5.1 represents a first step for the production of a “handbook” for the small-medium CS technology, providing a sort of catalogue of technological and commercial options as a tool designers and installers. Therefore, this handbook will result from the combined outcome from WP5 (Task 5.1) and the components definition in WP6 (Task 6.1), and will represent a major WP5 outcome to be capitalized at the end of the STS-Med project.
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Project coordinator
Consorzio ARCA (Italy)
Partnership
• The Cyprus Institute (Cyprus) • Cyprus Chamber of Commerce and Industry (Cyprus) • Academy for Scienti�ic Research and Technology (Egypt) • New and Renewable Energy Authority (Egypt) • Elsewedy Electric (Egypt) • French Alternative Energies and Atomic Energy Commission (France) • CEEI Provence - Innovation business support (France) • University of Athens, Institute of Accelerating Systems and Application (Greece) • Al Balqa Applied University (Jordan) • Ministry of Energy and Mineral Resources (Jordan) • Millenium Energy Industries (Jordan) • Sicily Region - Department of Production Activities (Italy) • ENEA - National Agency for New Technologies, Energy and Sustainable Economic Development (Italy) For more information, please contacts:
www.stsmed.eu [email protected]
Consorzio ARCA Viale delle Scienze Edif. 16 - 90128 Palermo (Italy)
Project Leader Fabio Maria Montagnino +39 091 661 56 54
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