Solar air conditioning
With solar air conditioning , a building, a room or, more abstractly, a volume is cooled and dried using solar energy . For this purpose, the drive energy source of a refrigeration machine is operated by solar radiation instead of electrical energy from the power grid.
Building air conditioning
Problems with electrically operated building air conditioning
The highest cooling requirement coincides with the midday peak in the load profile of the electricity consumption. She is therefore jointly responsible for the design of power plants and the power grid . Peak load electricity is particularly costly, both economically and ecologically.
Compression refrigeration machines work with refrigerants .
Correspondence between solar radiation and air conditioning needs of buildings
Buildings only need to be air-conditioned when and where they or their surroundings are heated by solar radiation. Solar energy is therefore always available where an air conditioning system is to be operated.
On closer inspection, however, the need for air conditioning follows the radiation with a delay, since the environment and the building are thermally inert due to their heat capacity . During the day, the highest irradiation is at astronomical noon (in Germany approx. 1:15 p.m. summer time ), but the highest temperature is in the afternoon around 3 p.m. In the course of the year, the most intense irradiation in the northern hemisphere is on June 21, the hottest days follow, depending on the local climate, from mid to late July. Nevertheless, a strong correlation between radiation and warming can be assumed in the dimensioning .
Building air conditioning with solar power
principle
An electric air conditioning system is powered by locally generated solar power. Normally, such a system is also connected to the power grid in order to use the electricity generated outside the summer months and to be able to operate the air conditioning even when the solar modules are not running .
Advantages and disadvantages
The advantage of such a system is that power generation and air conditioning can be planned, built and operated independently of one another; retrofitting of existing air conditioning systems is possible. Technically sophisticated devices are available. Solar modules and air conditioning can also be spatially separated.
The disadvantage is the conversion of the thermal solar radiation into thermodynamically valuable electricity in order to operate a thermal application again. Standard solar cells have efficiencies below 20%. Refrigerants are still required.
Building air conditioning with solar heat
In these processes, solar thermal energy is used to cool with heat. Since solar thermal collectors use the radiation up to 70%, a little less favorable ratio of heat removed to expended energy ( energy efficiency ratio , coefficient of performance , COP) are taken as in electric chillers in buying. The solar system can also be used for hot water preparation and heating .
Closed sorption
In closed processes, the air is cooled via cold water distribution systems with an absorption chiller or adsorption chiller , which does not come into contact with the sorbent. Common pairs of substances in absorption are mixtures of ammonia and water or salt solutions such as lithium bromide . During adsorption, solid sorbents such as silica gel or zeolite and the refrigerant water are used. As with compression refrigeration machines, dehumidification of the supply air is possible by cooling it below the dew point and then reheating it. The most recent developments in this area are sorption cooling media based on ionic liquids. In contrast to the previously mostly used lithium bromide-water mixture, these have a significantly higher activity compared to water and, moreover, cannot crystallize out because they have very low melting points.
Open sorption
With open sorption, the supply air is dried through contact with water-absorbing substances (e.g. silica gel , zeolite A ). Then water is sprayed that cools the air when it evaporates ( cooling through drying and evaporation ). The sorbent is then heat dried (regenerated).
Solid or liquid substances can be used for drying. The rotary dehumidifier is an example of a process with solid sorbents . Solutions with calcium chloride , lithium chloride or lithium bromide have so far been used as liquid sorbents . But sorbents based on ionic liquids are also attracting increasing interest.
Advantages and disadvantages
An advantage of sorption is that the thermodynamically unfavorable detour via the generation of electricity is avoided. In contrast to electricity, heat or water-drawing substances can be stored with little loss.
The disadvantage is that the solar system and air conditioning system have to be designed and built together, and subsequent changes are time-consuming. So far there are only a few manufacturers who offer such systems. Another disadvantage is the very large volume of the cooling unit compared to compact electrical air conditioning systems. In addition, the secondary energy demand for pumps and controls is in some cases not insignificant and must be included in the profitability calculation.
Application example
At the Bavarian Center for Applied Energy Research (ZAE Bayern) in Garching near Munich, an energy system for solar heating and cooling of the institute building has been installed and has been in operation for several years. The core of the system consists of an absorption chiller that draws its drive heat from solar flat collectors installed on the roof. The specialty of this solar air conditioning system is the special concept of the recooling of the chiller. On the one hand, the re-cooling takes place via a conventional dry re-cooling plant, which is relieved by a latent heat storage (PCM storage) at high outside temperatures by temporarily storing waste heat from the refrigeration machine during the day, which can then be efficiently released to the environment at night when the outside temperatures are lower .
Air conditioning of mobile rooms and containers
The procedures presented for building air conditioning do not take into account the weight and volume of the cooling unit. Cooling processes that are uneconomical for building air conditioning are also used here.
Air conditioning with solar power
principle
The solar power generated by sunlight is used to drive compression refrigeration machines or, for small volumes, Peltier elements .
Advantages and disadvantages
The advantage of such systems is that air conditioning is provided when air conditioning is required. In order to compensate for the disadvantage of the low efficiency and the slightly shifted cooling requirement compared to the generation of heat by sunlight, the insulation of the cooling volume from the environment is reinforced here. Examples are solar-powered cool boxes (cooling with Peltier elements), mobile homes with solar-powered air conditioning systems (cooling with compact compression refrigeration machines), etc. Another advantage is that you are largely independent of the stationary power grid and that the weight of the cooling unit is reduced by eliminating electrical storage units ( second car battery, accumulator).
The disadvantage is the higher cost factor.
Air conditioning with solar heat
In these processes, solar thermal energy is used to cool with heat. Both ad and absorption chillers and DEC units use time-tested chillers . The use of commercially available flat-plate collectors is only possible to a limited extent; special medium-temperature collectors with reinforced thermal insulation and double cover are preferred here. Vacuum tubes can also be used for smaller systems.
In addition to many rather small projects in research and development, the first commercial projects have also emerged in recent years.
There are also some exotic processes that use solar heat for cooling via a mechanical intermediate step. These include:
- Rankine process → compression
- Vuilleumier trial
Today (= 2005?) These processes are only used for "exotic" cooling applications. For commercial use, research projects are currently underway that are motivated by the successes in materials science.
Starting in 2003, various, increasingly larger projects are being built. Solar cooling for the Desert Outdoor Center near Phoenix , Arizona, USA, the logistics center for the Olympic Summer Games in Beijing , China, the bank building of the Caixa Geral de Depósitos in Lisbon , Portugal and the United World Colleges in Singapore . With collector areas of around 160 to 4000 m², commissioning from 2006 to 2011, four reference projects from SOLID, Graz, Austria.
literature
- Chapter 4: Solar Cooling. In: U. Eicker: Solar technologies for buildings , Vieweg-Teubner, 2nd edition from 2012, ISBN 978-3-8348-1281-0 , pp. 147–245
- Chapter 5.4 Solar cooling. In: Using regenerative energies in buildings , Springer-Vieweg, 2nd edition from 2016, ISBN 978-3-658-12404-5 , pp. 170–187
- Chapter 9: Solar Air Conditioning. In: R. Stieglitz, V. Heinzel: Thermal solar energy: Fundamentals, technology, applications , Springer-Vieweg, Berlin / Heidelberg 2012, ISBN 978-3-642-29474-7 , pp. 637-673
- Christian Stadler: Cooling with the sun , IHKS-Fachjournal: 2004/05 download
- Martin van Moll: Solar building air conditioning, IHKS-Fachjournal: 2005/06 download
- Hans-Martin Henning , Thorsten Urbaneck, Alexander Morgenstern, Tomas Núnez, Edo Wiemken, Egbert Thümmler, Ulf Uhlig: Cooling and air conditioning with heat , Solarpraxis, Berlin 2009, ISBN 978-3-410-17981-8 / ISBN 978-3- 934595-81-1 .
- Hans-Martin Henning, Mario Motta, Daniel Mugnier (Eds.): Solar Cooling Handbook: A Guide to Solar Assisted Cooling and Dehumidification Processes . 3rd, updated edition, Ambra 2013, ISBN 978-3-990434-38-3 (English, cooling and dehumidifying).
Individual evidence
- ↑ SOLID and ERI @ N - research cooperation to improve solar cooling in tropical areas Website of the Austrian Solid, accessed on June 8, 2014