Solar Cooling Used for Solar Air Conditioning - A Clean Solution for a Big Problem

Transcription

1 Solar Cooling Used for Solar Air Conditioning - A Clean Solution for a Big Problem Stefan Bader Editor Werner Lang Aurora McClain csd Center for Sustainable Development

2 II-Strategies Technology 2

3 2.10 Solar Cooling for Solar Air Conditioning Solar Cooling Used for Solar Air Conditioning - A Clean Solution for a Big Problem Stefan Bader Based on a presentation by Dr. Jan Cremers Figure 1: Vacuum Tube Collectors Introduction The global mission, these days, is an extensive reduction in the consumption of fossil energy without any loss in comfort or living standards. An important method to achieve this is the intelligent use of current and future solar technologies. With this in mind, we are developing and optimizing systems for architecture and industry to meet the high individual demands. Philosophy of SolarNext AG, Germany. 1 When sustainability is discussed, one of the first techniques mentioned is the use of solar energy. There are many ways to utilize the energy of the sun when designing a building. The primary and most efficient use of solar energy is daylighting. In order to use natural light effectively, the architect has to design sensibly, allowing the light to enter the building while avoiding excessive heat gain. Keeping this balance is the difficult part. In addition to providing natural daylight, solar energy can also be used through solar thermal collectors or photovoltaic panels. Using either of these two devices requires a certain amount of technical equipment. Solar thermal collectors gather solar energy and transfer it to a medium, normally water, that can then be used to heat a certain space. While solar thermal collectors transfer the solar energy directly from one medium to another, photovoltaics convert the heat produced by solar energy into electrical power. This power can be used to run a variety of devices which for example produce heat for domestic hot water, lighting or indoor temperature control. Photovoltaics produce electricity, which can be used to power other devices, such as compression chillers for cooling buildings. While using the heat of the sun to cool buildings seems counter intuitive, a closer look into solar cooling systems reveals that it might be an efficient way to use the energy received from the sun. On the one hand, during the time that heat is needed the most - during the winter months - there is a lack of solar energy. However, during the summer, when cooling is needed, there is a great surplus of solar energy. The best conclusion to draw from these facts is that an efficient way might be to utilize solar energy to additionally generate cooling energy. In the following pages, the advantages of solar cooling will be explained by comparing solar cooling with existing compression cooling technology, while analyzing the efficiency and applicability of each system. 3

4 II-Strategies Technology Figure 2: World Exhibition 1878 in Paris - A. Mouchot produced the first manufactured ice block using solar energy Figure 3: Private home with Solar Thermal application (around 1910), Pomona Valley, USA Solar cooling overview Historical review Solar cooling technology is actually not a recent invention. It had already been used in the late 19th century when a solar collector was used to produce ice blocks at the 1878 World Exhibition in Paris by A. Mouchot - the first solar cooling device (Figure 2). In 1892, a solar hot water heater was advertised in the United States of America. Several years later, in 1910, one of the first private home solar thermal applications was seen in Pomona Valley, USA, where solar collectors were installed on the roof of a private house (Figure 3). MIT took a closer look into solar thermal collectors while building the MIT Solar House in 1939, a research building with an integrated solar collector roof (Figure 4). Absorption chillers are not a new invention either. The first applications were developed in the early 20th century for ships and there are some machines that are still running after more than 70 years. Solar thermal cooling as a valuable add-on feature to conventional applications Up to now, solar thermal energy has generally been used only for domestic hot water and heating support. Because of the variation through the seasons and the opposite demand for cooling in Europe, it appears that it is not ideal to use solar energy for heating alone. The cooling loads of buildings display a parallel energy curve compared to the solar energy curve throughout the year. It seems ideal to benefit from solar input while combining solar cooling with a standard solar heating technology. When solar collectors are used for solar heating, large installations are necessary. Because of the large size of the collector fields, some part of the gained energy needs to be used for re-cooling the large system and most of it will not be necessary in summer (and then might even lead to stagnation problems), which seems counterproductive. Instead, when it is used for cooling, the energy collected in the summer will produce cold at the time when it is most needed (Figure 5). A similar situation can be found when looking at cogeneration processes which also benefit economically from running continuously. Therefore, it is interesting to combine standard cogeneration with thermal cooling: In summer, the heat produced can be converted to cold directly and thereby prolong the running time of the engines (Figure 6). The same applies on a larger scale to district heating. Here again, in summer it is hard to sell the heat directly, but with the addition of a thermal cooling process the heat can be used all year. Conventional air conditioning Solar cooling has a big potential to restrict the enormous amounts of electrical energy currently consumed for conventional compression cooling. Electrically driven split-units have their peak loads at the same time during the day, when a lot of other electrical consumers reach their maximum capacity as well. In many countries energy providers have a hard time providing enough energy for these kinds of air-conditioning machines during their peak times. In addition to electricity demands which are predominantly met by burning fossil fuels, these units use ozone-damaging gases and have a leaking range up to 5-15 % per year which leads to an additional severe global warming potential (Figure 7). However, the option to power these units with solar photovoltaic panels could as an alternative to G [W] P [W] P [W] Jan Dez 0 delta t 8760 [h] Figure 4: MIT-Solar House, Research Building with integrated solar collector roof (1939), USA 1 Global Radiation 2 Cooling Load 3 Heating Load Solar Surplus Supply in Summer Figure 5: Relationship between Solar Radiation and Cooling Demand Thermal Heat Energy Demand Heat Energy Demand for Cooling Figure 6: Combined Heat, Cold and Power: Extension of Operation Time 4

5 2.10 Solar Cooling for Solar Air Conditioning solar thermal cooing help to reduce their environmental impact. Competitive advantage of solar thermal cooling against conventional compression cooling Comparing solar cooling with traditionally powered compression cooling reveals some significant differences between the two systems, as can be seen in Figures 8 and 9. Solar cooling produces much lower CO 2 emissions than compression cooling due to its use of the sun as a CO 2 -free renewable energy sources. Although the Coefficient of Performance (COP), which is defined as the ratio of the cooling output to the driving heat required, is lower in the case of solar cooling, the overall CO 2 emissions (primary energy related) are significantly lower as it is not the standard national energy mix which has to be taken into account but nearly only the sun: 80% of the primary energy used for solar cooling comes from the sun rather than from external sources such as fossil fuels, unlike compression cooling, which primarily uses these sources. The higher re-cooling capacities for the solar cooling process are due to the lower COP and the additional heat energy from the collectors that is brought into the process. The case described in Figures 8 and 9 compare small capacity applications (< 10kWh). Status on installed solar cooling systems USA 17.0 Central/South America 3.9 Europe 8.1 Middle East 3.7 Africa 1.3 Figure 7: Market Situation of Conventional Air-Conditioning Worldwide in 2007 CO 2 Emission: 369 g/kwh Lost Heat: 1.67 kwh th India 2.4 China 21.5 Japan 8.4 East Asia 7.1 Australia 0.9 In Europe there are about 200 systems running, which is about 2/3 of all built systems. The reality that the combined capacity of these units is about 15 MW for which they need a surface of only 30,000 m 2 (322,917 ft 2 ) proves the existence of a still small market. The following cooling technologies were used (Figures 15 and 16): 60 % 29 % 11 % 100 % Primary Energy 1.91 kwh 0.67 kwh el Lost Energy: 1.24 kwh th Figure 8: Standard Compression Chiller (< 10 kw th cooling) Compression Chiller COP: kwh th Cooling 60 % absorption cooling 11 % adsorption cooling 25 % DEC systems 4 % liquid sorption Process of solar cooling Lost Heat: 2.67 kwh th The solar thermal driven cooling technologies can be divided into four principal technologies: CO 2 Emission: 83 g/kwh absorption chillers adsorption chillers open sorption cooling systems (DEC) compression chillers ( when driven by PVs these are another option for solar cooling. However, this is not a thermally driven process.) 80 % 100 % Primary Energy 2.1 kwh 1.67 kwhth 0.15 kwh el 20 % Lost Energy: 0.28 kwh th chillii - Technology COP: kwh th Cooling In principle, there are three different tech- Figure 9: Solar Cooling Absorption Chiller (< 10 kw th cooling) 5

6 II-Strategies Technology niques used for thermal cooling. Absorption and Adsorption chillers use heat produced by solar thermal collectors or other heat sources to run the process, with a minimum amount of additional electrical energy needed for solution pumps, controller, etc. The thermal collector provides water heated to between 70 C (158 F) and 120 C (248 F), which is guided into a storage system. This hot water will be used to drive the chiller, which will then produce cold water that is kept in another storage system from which it can be distributed to different cooling devices. Open sorption and desiccant cooling systems use heat gathered from solar collectors to dry out a desiccant that is then used to absorb moisture from hot air so that it can be cooled using evaporation. Compression chillers use photovoltaic elements to drive an electrical device that produces cold by using a compression process. This process is not as attractive as the use of systems powered by solar thermal collectors, due to the currently still high cost of installing the volume of photovoltaic elements needed for producing enough electricity to power the compression system. Most solar cooling systems use absorption chillers (60% in Europe, Source: IEA SHC Task 38, Solar Air-conditioning and Refrigeration). Driving heat sources for thermal cooling Instead of using electricity, the Absorption and Adsorption Chillers are driven by heat sources. This system has a lower efficiency, which leads to a higher need for driving energy. When using heat as a driving energy, the system not only has to get rid of the Cooling Capacity Q O (heat inside the room) but also of the heat running the system (Heating Capacity Q H ) which leads to a higher Re-Cooling Capacity Q C. These chillers use the hot water generated by solar collectors or other heat sources to provide cold water at temperatures between 6 C (42.8 F) and 18 C (64.4 F). They can therefore be used for central air conditioners as well as cooling systems with decentralised air treatment, such as fan coils, cooling ceilings, or concrete slabs. Thermal cooling systems can utilize different heat sources for driving cooling and heating systems, for example: solar energy harvested by a solar collector like a vacuum tube, flat plate collector or even concentrating systems. district heating energy (a central system producing heat). waste heat from power stations (normally, energy is used to get rid of this heat; with this technology the heat coming from power stations or centralized solar power plants can actually be used for cooling in summer). heat from cogeneration processes (Cogeneration Heat and Power or CHP-Units use both heat and energy produced by an engine to reach a high level of efficiency. By using a CHP-Unit in an office building heat can either be used for producing heat and hot water or it can be converted into cold. Smaller units can even be used in residential buildings. Figure 10: Vacuum Tube Collectors Figure 11: Parabolic Collectors Figure 14: Application of Solar Cooling in a Building Figure 13: Fresnel Collectors Figure 12: Flat Plate Collectors 6

7 2.10 Solar Cooling for Solar Air Conditioning If solar thermal collectors are used as the heat source for the solar cooling system they could also be used to provide hot water during the year (kitchen, shower etc.) and to support the heating system in winter time. 1 Using the re-cooling capacities for heating a pool is especially appealing if the swimming pool is partially or even completely shaded. In order to have agreeable temperatures in the pool, the heat extracted out of the indoor environment will be guided directly into the pool. In the best case scenario, solar collectors can be used to supply four different kinds of systems: Solar Cooling Solar Heating Solar Domestic Hot Water (DHW) Solar Pool Heating (by using re-cooling capacities) Solar thermal collector technologies 4% 25% 11% 60% Absorption Cooling Adsorption Cooling DEC Systems Liquid Sorption Figure 15: Solar Cooling Systems, Different Systems, % 39% 9% 28% Germany Spain Greece Rest of Europe Figure 16: Solar Cooling Systems, Main Countries, 2007 Vacuum Tube Collector: very well insulated absorber - suited to climates with very low temperatures (Figure 10) Parabolic Collector: focuses sunlight on absorber in the middle - a spot is heated up to temperatures of 300 C by using a special oil (Figure 11) Flat Plate Collector: dark metal and glass covered absorbing plate which gets heated by solar radiation (Figure 12) Electrical Input P el Re-Cooling Capacity Q C (Cooling Tower) Heating Capacity Q H (Solar Collector) Re-Cooling Capacity Q C (Cooling Tower) Fresnel Collector: flat mirrors focusing on a specific and central absorber area (Figure 13) Electrically driven compression chiller The conventional cooling process - compression technology - needs an electrical input to run the process (Figure 17). For this process split units are commonly used - a condenser and a separated evaporator which produce the cold energy (Figure 19). This system uses an electrically powered pump to pressurize the gas. When the pressure is released, the gas becomes very cold, similar to a camping cartridge, which gets cold while the gas is released. Thermally driven absorption chiller The thermal process replaces the compressor by two other components - the generator and the absorber. These two components enable the use of heat to drive the cooling process, rather than electricity (Figures 18 and 20). What follows is a sample description of the technology of a small scale Absorption Chiller (here for an Ammonia-Water chiller, 12 kw). Condenser Throttle Evaporator High pressure level P H Low pressure level P L Cooling Capacity Q O (fan coils / cold ceilings) Figure 17: Schematic Cooling Processes - Electrically Driven Compression Chiller Compressor Electricity Figure 19: Scheme of a Electrically Driven Compression Chiller Condenser Throttle Evaporator High pressure level P H Solution Heat Exchanger Solution Pump Low pressure level P L Cooling Capacity Q O (fan coils / cold ceilings) Figure 18: Schematic Cooling Processes - Thermally Driven Absorption or Adsorption Chiller Generator Absorber Throttle Figure 20: Scheme of a Thermally Driven Absorption Chiller 7

8 II-Strategies Technology The process of a typical small scale absorption chiller contains four major components: Generator ammonia vapor Condenser The Generator The Condenser The Evaporator The Absorber 85 C (185 F) 78 C (172 F) 12 bar, i.e. depending on point of operation: approx C ( F) e.g. 12 bar, 30 C (86 F) 29 C (84 F) 24 C (75 F) Generator To run the generator, temperatures of about 85 C (185 F) are needed. This could come from the sun, or any other appropriate heat source. The heat expels ammonia out of a rich ammonia water solution to generate ammonia vapor. The left over solution becomes a weak solution, which means that it has less ammonia in the water. At a pressure of 12 bars, the heat causes the ammonia to evaporate out of the solution. The ammonia vapor is led to the condenser. To be able to work with the ammonia vapor the pressure has to be adjusted depending on the point of operation (Figures 21 and 22). Condenser The re-cooling device causes the ammonia vapor to condense at a temperature of around 24 C (75 F) in the condenser (Figure 23). Pure Ammonia is created. ammonia water solution Absorber ammonia vapor 4 bar, i.e. depending on point of operation: approx C ( F) Figure 21: Small Scale Absorption Chiller (Ammonia-Water, 12 kw) Evaporator e.g. 4 bar, 5 C (41 F) 6 C (43 F) 12 C (54 F) Evaporator The liquid ammonia is guided through a throttle in order to release the pressure. After that step, the actual cold is produced through heat supply by the external cold water circuit. When the pressure is reduced the ammonia is not only turned back from the liquid phase into vapor but also cooled down to the desired temperature (5 C or 41 F) (Figure 24). Absorber The ammonia vapor reaches the absorber where it reacts with water and becomes the rich ammonia water solution again, which is then used for the first step of the whole process by reducing the pressure and the high temperatures that are received by the cooling process. The rich ammonia water solution goes back to the condenser and the heat is lost in the re-cooling process, closing the loop (Figure 25). Re-cooling There are several options for rejecting the heat produced by the system. One option is to use a recooler. There are three types of appropriate re-cooling processes: the wet cooling tower exposes the water directly to the air so that it can be cooled through evaporation, 5 kw Figure 22: Principle: The Generator Figure 24: Principle: The Evaporator 5 kw Figure 23: Principle: The Condenser Figure 25: Principle: The Absorber 8

9 2.10 Solar Cooling for Solar Air Conditioning while the dry recooler uses a heat exchanger to transfer heat through an intervening medium so that the water does not come into contact with the air. A hybrid recooler combines the two processes, passing hot water through tubes that are sprayed with a fluid that evaporates to cool them and the water within. The excess heat can also be rejected into a body of water, such as a swimming pool, lake, or groundwater. Each of these options has benefits and drawbacks, the most important of which are the hazards of excess humidity produced by wet cooling towers and the danger of altering the ecosystem in a body of water by substantially altering the temperature. High outdoor temperatures ( 100 C or 212 F) might require additional sources of lower temperatures like geothermal or evaporative cooling, depending on the climate conditions, or alternative re-cooling options. Back-up heat sources One can imagine situations in which cooling would be needed but the levels of solar radiation needed to produce temperatures that could power the chiller might not be available, e.g. a hot but cloudy day or generally hot nights. In this cases the energy from the heat storage system will be taken but in cases this is not enough or the storage s dimensions are not sufficient the chiller can be driven by a back-up heat source such as a burner to produce the temperatures that it needs to operate. Ideally, the back-up burner is also run by renewable energy sources (e.g. biomass). Controller The controller is essential to the stability and efficiency of the system. It manages and coordinates the function of the chiller, heat source, pumps, fans, storage(s), back-up, re-cooling and heat and cold distribution systems. It can be set to preferentially use renewable energy sources and to respond quickly and efficiently to cooling demands. The controller manages the flow of energy through these complex systems in order to ensure that no energy will be wasted and that the system runs in an economical way which is a very complex task. A well designed controller is necessary to maximize the potential of the cooling system. Conclusion Using solar thermal energy to produce cold seems to be a complicated process requiring numerous steps. It is certainly true that using very high temperatures to produce low temperatures requires the integration of many diverse components, which need to function together perfectly in order to be efficient and sustainable. However, every new technology seems complicated before it becomes widely used, and solar cooling systems build on technologies and components that have been in use for decades. With the use of electronic control systems, all of the different processes can be coordinated to create a very efficient system with multiple benefits. Glossary Chiller: A machine that removes heat from a liquid via a vapor-compression or ad-/or absorption refrigeration cycle. Compressor: Mechanical device that compresses a gas (e.g. air or natural gas). Condenser: Device or unit used to condense vapor into liquid. COP: Coefficient of Performance. The COP is defined as the ratio of the energy output (e.g. cold) and the driving energy (e.g. solar thermal energy) required for this. Evaporator: A solution containing the desired product is fed into the evaporator and passes a heat source. The applied heat converts the Absorber Condenser Evaporator Internal Controller Mechanical Solution Mixer Solution Heat Exchanger Generator Figure 27: Example of a small scale absorption chiller Solution Pump Technology Absorption Single-Effect Double-Effect Single-Effect Adsorption Refrigerant water water ammonia water - Sorbent lithium bromide lithium bromide water silicia gel silicia gel or lithium chloride Cooling Medium water water water glycol water air Cooling Temperature Heating Temperature Cooling Water Temperature 6-20 C (42-70 F) C ( F) C ( F) 6-20 C (42-70 F) C ( F) C ( F) C ( F) C ( F) C ( F) 6-20 C (42-70 F) C ( F) C (77-95 F) DEC C (61-70 F) C ( F) not required Cooling Capacity Range (per Unit) 5-20,500 kw ,300 kw 5-1,000 kw kw kw Coefficient of Performance (COP) Figure 26: Overview of Thermal Driven Cooling and Air Conditioning Applications 9

10 II-Strategies Technology water in the solution into vapor. The vapor is removed from the rest of the solution and is condensed while the now concentrated solution is either fed into a second evaporator or is removed. Flat Plate Collector: Consists of a thin absorber sheet (of thermally stable polymers, aluminum, steel or copper, to which a black or selective coating is applied) backed by a grid or coil of fluid tubing and placed in an insulated casing with a glass or polycarbonate cover. Fresnel Collector: Uses a series of long, narrow, shallow-curvature (or even flat) mirrors to focus light onto one or more linear receivers positioned above the mirrors. On top of the receiver a small parabolic mirror can be attached for focusing the light further. These systems aim to offer lower overall costs by sharing a receiver between several mirrors (as compared with trough and dish concepts), while still using the simple line-focus geometry with one axis for tracking. Parabolic Collector: Functions due to the geometric properties of the paraboloid shape: if the angle of incidence to the inner surface of the collector equals the angle of reflection, then any incoming ray that is parallel to the axis of the dish will be reflected to a central point, or focus. Because many types of energy can be reflected in this way, parabolic reflectors can be used to collect and concentrate energy entering the reflector at a particular angle. Primary Energy: Energy that has not been subjected to any conversion or transformation process. It is contained in raw fuels and any other forms of energy received by a system as input to the system. Thermal Collector: Takes up the heat of the solar radiation through a medium (water + antifreeze). This is heated and circulates between the collector and the storage tank. A high degree of efficiency is achieved by using black absorbers or, even better, through selective coating. Vacuum Tube Collector: Made of a series of modular tubes, mounted in parallel, whose number can be added to or reduced as hot water delivery needs to be changed. This type of collector consists of rows of parallel transparent glass tubes, each of which contains an absorber tube (in place of the absorber plate to which metal tubes are attached in a flatplate collector). The tubes are covered with a special light-modulating coating. In an evacuated tube collector, sunlight passing through an outer glass tube heats the absorber tube contained within it. Notes _brochure.pdf Figures Figure 1: IMG_3029.jpg Figures 2-4: Hegger, Manfred. Energy Manual Sustainable Architecture. Munich: Birkhäuser, p. 111/112 Figures 5-6: SolarNext AG, Rimsting, Germany. Modified by Stefan Bader Figures 7-9: Jarn / SolarNext AG, Rimsting, Germany. Figures 8-9: SolarNext AG, Rimsting, Germany. Figure 10: imgres?imgurl= jpg&imgrefurl= pressemeldungen/oertlirohleder-waermetechnik-gmbh/boxid Figure 11: Resources/energie/galerie/parabolrinnen.jpg Figure 12: bilder/presse/6064_flachkollektor_indachmontage.jpg Figure 13: Resources/portal_news/newsarchiv2007/ fresnel.jpg Figure 14: SolarNext AG, Rimsting, Germany. Figures 15-16: Source IEA - SHC Task 38 Solar Air-Conditioning and Refrigeration, 2007 Figures 17-26: SolarNext AG, Rimsting, Germany. Figure 27: Werner Pink / SolarNext Reference shtml Hausladen, Gerhard. ClimateDesign - Solutions for Buildings that Can Do More with Less Technology. München: Birkhäuser, Hausladen, Gerhard. Climateskin - Building- Skin Concepts that Can Do More with Less Energy. München: Birkhäuser, Further Reading Henning, Hans-Martin. Solar Assisted Air- Conditioning in Buildings - A Handbook for Planners. New York: Springer Verlag, Hegger, Manfred. Energy Manual - Sustainable Architecture. Munich: Birkhäuser, Jakob, Uli. Cool climate from the scorching sun. Sun & Wind Energy. No. 2, pp ISSN , Zimmermann, Mark. Case Studies of Low Energy Cooling Technologies. Coventry: British Crown, Biography Jan Cremers is the Director of Envelope Technology ofat Solarnext AG / and Hightex Group, Rimsting (Germany). He studied at the University of Karlsruhe from , at which time he received the 1st prize in the building network competition for the Diploma of the Year. He has also studied Architecture and management at Westminster University, London, UK. In 2006 he received awards for his outstanding doctoral thesis: Applications of Vacuum Insulation Systems in the Building Envelope from both the Alliance of Friends of the Technical University in Munich and the Marshall Foundation. Jan Cremers has lectured frequently at the Technical University of Munich School of Architecture on topics concerning membranes and facade construction. He is a regular reviewer for the referenced international magazine Solar Energy, official journal of the International Solar Energy Society. Since 2008 he is a full professor of Building Technology and Integrated Architecture at the University of Applied Sciences Hochschule für Technik in Stuttgart, Germany. 10

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