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Solar Used for Solar - A Clean for a Big Problem

Stefan Bader

Editor Werner Lang Aurora McClain

csd Center for Sustainable Development II-Strategies

2 2.10 Solar Cooling for

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: Tube Collectors

Introduction taics convert the produced by solar into electrical power. This power can be “The global mission, these days, is an used to run a variety of devices which for extensive reduction in the consumption of example produce heat for domestic hot , fossil energy without any loss in comfort or lighting or indoor control. living standards. An important method to achieve this is the intelligent use of current and produce , which can be future solar . With this in mind, we used to power other devices, such as are developing and optimizing systems for compression for cooling buildings. architecture and industry to meet the high While using the heat of the sun to cool individual demands.” Philosophy of SolarNext buildings seems counter intuitive, a closer look AG, Germany.1 into solar cooling systems reveals that it might be an efficient way to use the energy received When is discussed, one of the from the sun. On the one hand, during the time first techniques mentioned is the use of solar that heat is needed the most - during the energy. There are many ways to utilize the winter months - there is a lack of . energy of the sun when designing a building. However, during the summer, when cooling is The primary and most efficient use of solar needed, there is a great surplus of solar energy is daylighting. In order to use natural energy. The best conclusion to draw from light effectively, the architect has to design these facts is that an efficient way might be to sensibly, allowing the light to enter the building utilize solar energy to additionally generate while avoiding excessive heat gain. Keeping cooling energy. this balance is the difficult part. In the following pages, the advantages of solar In addition to providing natural daylight, solar cooling will be explained by comparing solar energy can also be used through solar thermal cooling with existing compression cooling collectors or photovoltaic panels. Using either technology, while analyzing the efficiency and of these two devices requires a certain amount applicability of each system. 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, photovol-

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Solar cooling overview fields, some part of the gained energy needs to be used for re-cooling the large system and Historical review most of it will not be necessary in summer (and then might even lead to stagnation Solar cooling technology is actually not a problems), which seems counterproductive. recent invention. It had already been used in Instead, when it is used for cooling, the energy the late 19th century when a solar collector collected in the summer will produce at was used to produce ice blocks at the 1878 the time when it is most needed (Figure 5). World Exhibition in Paris by A. Mouchot - the cooling device (Figure 2). In 1892, a A similar situation can be found when looking solar hot water heater was advertised in the at processes which also benefit United States of America. Several years later, economically from running continuously. in 1910, one of the first private home solar Therefore, it is interesting to combine standard thermal applications was seen in Pomona cogeneration with thermal cooling: In summer, Valley, USA, where solar collectors were the heat produced can be converted to cold installed on the roof of a private house (Figure directly and thereby prolong the running time 3). MIT took a closer look into solar thermal of the engines (Figure 6). The same applies on a larger scale to . Here again, in Figure 2: World Exhibition 1878 in Paris - A. Mouchot collectors while building the MIT Solar House produced the first manufactured ice block using solar energy in 1939, a research building with an integrated summer it is hard to sell the heat directly, but solar collector roof (Figure 4). Absorption with the addition of a thermal cooling process chillers are not a new invention either. The first the heat can be used all year. applications were developed in the early 20th century for ships and there are some Conventional air conditioning that are still running after more than 70 years. Solar cooling has a big potential to restrict the Solar thermal cooling as a valuable add-on enormous amounts of electrical energy feature to conventional applications currently consumed for conventional compres- sion cooling. Electrically driven split-units have Up to now, has generally their peak loads at the same time during the been used only for domestic hot water and day, when a lot of other electrical consumers heating support. Because of the variation reach their maximum capacity as . In many through the seasons and the opposite demand countries energy providers have a hard time for cooling in Europe, it appears that it is not providing enough energy for these kinds of ideal to use solar energy for heating alone. air-conditioning machines during their peak The cooling loads of buildings display a times. In addition to electricity demands which parallel energy curve compared to the solar are predominantly met by burning fossil fuels, energy curve throughout the year. It seems these units use -damaging and ideal to benefit from solar input while combin- have a leaking range up to 5 - 15 % per year ing solar cooling with a standard solar heating which leads to an additional severe global technology. When solar collectors are used for warming potential (Figure 7). However, the Figure 3: Private home with Solar Thermal application Private(around Home with 1910), Solar PomonaThermal Appli Valley,cation USA (around 1910), Pamona Valley, USA solar heating, large installations are necessary. option to power these units with solar th © SolarNext AG / Hightex Group - Solar Air-Conditioning - Jan Cremers, September 24 , 2008 Because of theclean energylarge for you size of the collector photovoltaic panels could as an alternative to

G P P [W] [W] [W]

1 3

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Jan Dez 0 delta t 8760 [h]

1 Global Radiation 2 Cooling Load Thermal Heat Energy Demand 3 Heating Load Solar Surplus Supply Heat Energy Demand for Cooling in Summer

MIT-SolarFigure 4:House, MIT-Solar Research House, Building Research with Integrated Building Solar withCollector integrated Roof (1939), Figure USA 5: Relationship between Solar Radiation and Cooling Figure 6: Combined Heat, Cold and Power: Extension of ©solar SolarNext collector AG / Hightex Group roof - Solar (1939), Air-Conditioning USA - Jan Cremers, September 24th, 2008 Demand clean energy for you Operation Time

4 2.10 Solar Cooling for Solar Air Conditioning

solar thermal cooing help to reduce their environmental impact.

Competitive advantage of solar thermal Europe 8.1 21.5 cooling against conventional compression USA 17.0 cooling Middle Comparing solar cooling with traditionally East 3.7 powered compression cooling reveals some India 2.4 8.4 significant differences between the two systems, as can be seen in Figures 8 and 9. Solar cooling produces much lower CO 2 Central/South East Asia 7.1 emissions than compression cooling due to its America Africa 1.3 use of the sun as a CO2-free 3.9 sources. Although the Coefficient of Perfor- mance (COP), which is defined as the ratio of 0.9 the cooling output to the driving heat required, is lower in the case of solar cooling, the overall

CO2 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 Figure 7: Market Situation of Conventional Air-Conditioning Worldwide in 2007 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 Lost Heat: 1.67 kWhth that is brought into the process. The case described in Figures 8 and 9 compare small CO2 Emission: capacity applications (< 10kWh). 369 g/kWh

Status on installed solar cooling systems

60 % 0.67 kWh In Europe there are about 200 systems run- el 100 % Compression 1 kWh Cooling ning, which is about 2/3 of all built systems. Primary Energy th The reality that the combined capacity of 1.91 kWh Lost Energy: COP: 1.5 1.24 kWhth these units is about 15 MW for which they 29 % need a surface of only 30,000 m2 (322,917 ft2) proves the existence of a still small market. 11 % The following cooling technologies were used

(Figures 15 and 16): Figure 8: Standard Compression Chiller (< 10 kwth cooling)

• 60 % absorption cooling • 11 % adsorption cooling • 25 % DEC systems • 4 % sorption

Process of solar cooling Lost Heat: 2.67 kWhth

The solar thermal driven cooling technologies CO2 Emission: can be divided into four principal technologies: 83 g/kWh

• absorption chillers • adsorption chillers 80 % • open sorption cooling systems (DEC) 100 % chillii® - Technology 1 kWh Cooling • compression chillers ( when driven by Primary Energy 1.67 kWhth COP: 0.6 th 2.1 kWh PVs these are another option for “solar 0.15 kWhel cooling”. However, this is not a thermally 20 % Lost Energy: 0.28 kWh driven process.) th

In principle, there are three different tech- Figure 9: Solar Cooling Absorption Chiller (< 10 kwth cooling)

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

Figure 14: Application of Solar Cooling in a Building Figure 13: Fresnel Collectors Figure 12: Flat Plate Collectors

6 2.10 Solar Cooling for Solar Air Conditioning

If solar thermal collectors are used as the heat60 39,1 27,5 11 8,7 source for the solar cooling system they could25 24,7 also be used to provide hot water during the4 year (kitchen, shower etc.) and to support the 4% in winter time.1 Using the 25% re-cooling capacities for heating a pool is 25% especially appealing if the swimming pool is partially or even completely shaded. In order to 39% have agreeable temperatures in the pool, the 9% heat extracted out of the indoor environment will be guided directly into the pool. In the best 11% case scenario, solar collectors can be used to 60% supply four different kinds of systems: 28%

• Solar Cooling • Solar Heating Absorption Cooling Adsorption Cooling Germany Spain Greece Rest of Europe • Solar Domestic Hot Water (DHW) DEC Systems Liquid Sorption • Solar Pool Heating (by using re-cooling Figure 15: Solar Cooling Systems, Different Systems, 2007 Figure 16: Solar Cooling Systems, Main Countries, 2007 capacities)

Solar thermal collector technologies

• Vacuum Tube Collector: very well insulated absorber - suited to climates with very low temperatures (Figure 10) Heating Capacity QH (Solar Collector) • Parabolic Collector: focuses on absorber in the middle - a spot is heated up to temperatures of 300°C by using a special oil (Figure 11) Re-Cooling Capacity QC

Electrical Input Pel Re-Cooling Capacity QC () • Flat Plate Collector: dark metal and glass (Cooling Tower) covered absorbing plate which gets heated by solar radiation (Figure 12)

• Fresnel Collector: flat focusing on a specific and central absorber area (Figure 13)

Electrically driven compression chiller

Cooling Capacity QO Cooling Capacity QO The conventional cooling process - compres- (fan coils / cold ceilings) (fan coils / cold ceilings) sion technology - needs an electrical input to Figure 17: Schematic Cooling Processes - Electrically Driven Figure 18: Schematic Cooling Processes - Thermally Driven run the process (Figure 17). For this process Compression Chiller Absorption or Adsorption Chiller split units are commonly used - a and a separated which produce the cold energy (Figure 19). This system uses an electrically powered to pressurize the . When the is released, the gas becomes very cold, similar to a camping cartridge, which gets cold while the gas is released. Condenser Condenser Generator

Thermally driven absorption chiller High pressure level PH Solution The thermal process replaces the High pressure level PH by two other components - the generator and Electricity Low pressure level PL Solution Pump the absorber. These two components enable Throttle Throttle Throttle Compressor Low pressure level P the use of heat to drive the cooling process, L rather than electricity (Figures 18 and 20). Evaporator Evaporator Absorber What follows is a sample description of the technology of a small scale Absorption Chiller (here for an -Water chiller, 12 kW). Figure 19: Scheme of a Electrically Driven Compression Chiller Figure 20: Scheme of a Thermally Driven Absorption Chiller

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The process of a typical small scale absorption chiller contains four major components: Generator Condenser ammonia vapor ≤ 12 bar, e.g. 12 bar, • The Generator i.e. depending on point of operation: 30°C (86° F) • The Condenser approx. 70 - 110° C (158 - 230° F) • The Evaporator 85°C (185° F) 29°C (84° F) • The Absorber 78°C (172° 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 am- monia vapor. The left over solution becomes ammonia vapor 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 evapo- 6°C (43° F) rate out of the solution. The ammonia vapor is led to the condenser. To be able to with 12°C (54° F) the ammonia vapor the pressure has to be e.g. 4 bar, adjusted depending on the point of operation 5°C (41° F)

ammonia water solution ≤ 4 bar, (Figures 21 and 22). i.e. depending on point of operation: approx. 28 - 39° C (82 - 102° F) Condenser Absorber Evaporator 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 Figure 21: Small Scale Absorption Chiller (Ammonia-Water, 12 kW) Ammonia is created.

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 5 kW 5 kW where it reacts with water and becomes the

Figure 22: Principle: The Generator Principle:Figure The 23:Condenser Principle: The Condenser rich ammonia water solution again, which is © SolarNext AG / Hightex Group - Solar Air-Conditioning - Jan Cremers, September 24th, 2008 clean energy for you then used for the first step of the whole Principle: The Generator © SolarNext AG / Hightex Group - Solar Air-Conditioning - Jan Cremers, September 24th, 2008 clean energy for you 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 appropri- ate re-cooling processes: the “wet cooling tower” exposes the water directly to the air so 5 kW 5 kW that it can be cooled through evaporation, Figure 24: Principle: The Evaporator Figure 25: Principle: The Absorber

Principle: The Evaporator © SolarNext AG / Hightex Group - Solar Air-Conditioning - Jan Cremers, September 24th, 2008 Principle: Theclean Absorber energy for you © SolarNext AG / Hightex Group - Solar Air-Conditioning - Jan Cremers, September 24th, 2008 clean energy for you 8 2.10 Solar Cooling for Solar Air Conditioning

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

Absorption Technology Single-Effect Double-Effect Single-Effect Adsorption DEC water water ammonia water - Sorbent bromide water silicia gel silicia gel or lithium chloride Cooling Medium water water water glycol water air Cooling Temperature 6° - 20° C 6° - 20° C -20° - +20° C 6° - 20° C 16° - 20° C (42°-70° F) (42°-70° F) (-4° - +70° F) (42°-70° F) (61° - 70° F) Heating Temperature 75° - 100° C 130° - 160° C 80° - 160° C 55° - 100° C 55° - 100° C (167° - 212° F) (266° - 320° F) (176° - 320° F) (130° - 212° F) (130° - 212° F) Cooling Water Temperature 30° - 50° C 30° - 50° C 30° - 50° C 25° - 35° C not required (86° - 122° F) (86° - 122° F) (86° - 122° F) (77° - 95°F) Cooling Capacity Range (per Unit) 5 - 20,500 kW 170 - 23,300 kW 5 - 1,000 kW 5 - 350 kW 6 - 300 kW Coefficient of Performance (COP) 0.6 - 0.7 1.1 - 1.4 0.5 - 0.6 0.6 - 0.7 0.5 - 1.0

Figure 26: Overview of Thermal Driven Cooling and Air Conditioning Applications

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water in the solution into vapor. The vapor is Notes Technology. München: Birkhäuser, 2005. removed from the rest of the solution and is Hausladen, Gerhard. Climateskin - Building- condensed while the now concentrated solu- 1 - 2 http://www.solarnext.eu/pdf/eng/solarn- Skin Concepts that Can Do More with Less tion is either fed into a second evaporator or is ext Energy. München: Birkhäuser, 2006. removed. _brochure.pdf

Flat Plate Collector: Consists of a thin Further Reading absorber sheet (of thermally stable , Figures aluminum, steel or copper, to which a or Henning, Hans-Martin. Solar Assisted Air- selective coating is applied) backed by a grid Figure 1: http://www.pocosolar.de/abbildungen/ Conditioning in Buildings - A Handbook for or coil of fluid tubing and placed in an insulated IMG_3029.jpg Planners. New York: Springer Verlag, 2004. casing with a glass or polycarbonate cover. Figures 2-4: Hegger, Manfred. Energy Manual Hegger, Manfred. Energy Manual - Sustain- Fresnel Collector: Uses a series of long, Sustainable Architecture. Munich: Birkhäuser, able Architecture. Munich: Birkhäuser, 2008. narrow, shallow-curvature (or even flat) mirrors 2008. p. 111/112 to focus light onto one or more linear receiv- Jakob, Uli. Cool climate from the scorching ers positioned above the mirrors. On top of Figures 5-6: SolarNext AG, Rimsting, Ger- sun. Sun & Wind Energy. No. 2, pp 64-72. the receiver a small parabolic can be many. Modified by Stefan Bader ISSN 1861-2741, 2008. attached for focusing the light further. These systems aim to offer lower overall costs by Figures 7-9: Jarn / SolarNext AG, Rimsting, Zimmermann, Mark. Case Studies of Low sharing a receiver between several mirrors Germany. Energy Cooling Technologies. Coventry: British (as compared with trough and dish concepts), Crown, 1998. while still using the simple line-focus geometry Figures 8-9: SolarNext AG, Rimsting, Ger- with one axis for tracking. many.

Parabolic Collector: Functions due to the Figure 10: http://images.google.com/ Biography geometric properties of the paraboloid shape: imgres?imgurl=http://www.pressebox.de/at- if the angle of incidence to the inner surface tachment/94251/Referenzanlage%2BNr.%2B1. Jan Cremers is the Director of Envelope of the collector equals the angle of reflection, jpg&imgrefurl=http://www.pressebox.de/ Technology ofat Solarnext AG / and Hightex then any incoming ray that is parallel to the pressemeldungen/oertlirohleder-waermetech- Group, Rimsting (Germany). axis of the dish will be reflected to a central nik-gmbh/boxid-150226. point, or “focus”. Because many types of He studied at the University of Karlsruhe from energy can be reflected in this way, parabolic Figure 11: http://www.dlr.de/en/Portaldata/1/ 1991-1999, at which time he received the 1st reflectors can be used to collect and con- Resources/energie/galerie/parabolrinnen.jpg prize in the building network competition for centrate energy entering the reflector at a the Diploma of the Year. He has also studied particular angle. Figure 12: http://www.junkers.com/de/media/ Architecture and management at Westminster bilder/presse/6064_Flachkollektor_Indachmon- University, , UK. Primary Energy: Energy that has not been tage.jpg subjected to any conversion or transformation In 2006 he received awards for his outstand- process. It is contained in raw fuels and any Figure 13: http://www.dlr.de/en/Portaldata/1/ ing doctoral thesis: “Applications of Vacuum other forms of energy received by a system as Resources/portal_news/newsarchiv2007/ Insulation Systems in the ” input to the system. fresnel.JPG from both the Alliance of Friends of the Techni- cal University in Munich and the Marshall Thermal Collector: Takes up the heat of Figure 14: SolarNext AG, Rimsting, Germany. Foundation. the solar radiation through a medium (water + ). This is heated and circulates Figures 15 - 16: Source IEA - SHC Task 38 So- Jan Cremers has lectured frequently at the between the collector and the storage tank. A lar Air-Conditioning and Refrigeration, 2007 Technical University of Munich School of high degree of efficiency is achieved by using Architecture on topics concerning membranes black absorbers or, even better, through selec- Figures 17 - 26: SolarNext AG, Rimsting, and facade construction. He is a regular tive coating. Germany. reviewer for the referenced international magazine Solar Energy, official journal of the Vacuum Tube Collector: Made of a series Figure 27: Werner Pink / SolarNext International Solar Energy Society. Since 2008 of modular tubes, mounted in parallel, whose he is a full professor of Building Technology number can be added to or reduced as hot and Integrated Architecture at the University water delivery needs to be changed. This type of Applied Sciences Hochschule für Technik in of collector consists of rows of parallel trans- Reference Stuttgart, Germany. parent glass tubes, each of which contains an absorber tube (in place of the absorber plate http://www.solarnext.eu/eng/home/home_eng. to which metal tubes are attached in a flat- shtml plate collector). The tubes are covered with a special light-modulating coating. In an evacu- http://www.hightexworld.com/page/index.html ated tube collector, sunlight passing through an outer glass tube the absorber tube Hausladen, Gerhard. ClimateDesign - Solu- contained within it. tions for Buildings that Can Do More with Less

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