Cooling with Solar Heat: Growing Interest in Solar Air Conditioning

Sunny summer days are beautiful, yet in the office a hot day can be altogether stressful. Because productivity can suffer under such conditions, more and more buildings are being fitted with air-conditioning systems. This is where solar air conditioning comes in: The summer sun, which heats up offices, also delivers the energy to cool them. The thermal use of solar energy offers itself: Days that have the greatest need for cooling are also the very same days that offer the maximum possible solar energy gain. The demand for air conditioning in offices, hotels, laboratories or public buildings such as museums is considerable. This is true not only in southern Europe, but also in Germany and middle Europe. Under adequate conditions, solar and solar-assisted air conditioning systems can be reasonable alternatives to conventional air conditioning systems. Such systems have advantages over those that use problematic coolants (CFCs), not to mention the incidental CO2 emissions that are taking on increasingly critical values.

	Sorption-assisted air conditioning: collector system on the rooftop of Chamber of Commerce and Industry in Freiburg, Germany. Photo: Fraunhofer ISE.

The trend towards solar-assisted air conditioning is met by the organizers of the forum "Solar assisted Air-Conditioning of Buildings" at the convention Intersolar 2002: The German Association for Solar Energy (Die Deutsche Gesellschaft für Sonnenenergie (DGS)), the Fraunhofer Institute for Solar Energy Systems (Fraunhofer Institut für Solare Energiesysteme ISE), the Institute for Maintenance and Modernization of Buildings at the Technical University of Berlin (Institut für Erhaltung und Modernisierung von Bauwerken e.V. an der TU Berlin), and the Pforzheimer Solar Promotion Corporation (Pforzheimer Solarpromotion GmbH) are all offering a two-day international forum on the state of technology, the energy and economic aspects of solar cooling as well as the possible fields of application. Next to German companies, organizations from the entire world have registered including firms from Israel, Ghana, Spain, India, the Netherlands, Belgium, and Austria. This Solar-Report will briefly inform you over the possibilities and technology of solar air conditioning and will also cover economic aspects.

Basic structure of a solar air conditioning system

What is Solar Air Conditioning? Should buildings be cooled with the help of solar energy, then water-assisted air conditioning systems or ventilation systems can be powered with heat that is made available by solar collectors. No long-term intermediate storage is necessary in months of high solar energy gain or in southern lands. The sun can, at least seasonally at our latitudes, provide a substantial part of the energy needed for air conditioning. Combination water-assisted systems and ventilation systems are also possibilities. How does Solar Air conditioning Work? The basic principle behind (solar-) thermal driven cooling is the thermo-chemical process of sorption: a liquid or gaseous substance is either attached to a solid, porous material (adsorption) or is taken in by a liquid or solid material (absorption). The sorbent (i.e. silica gel, a substance with a large inner surface area) is provided with heat (i.e. from a solar heater) and is dehumidified. After this "drying", or desorption, the process can be repeated in the opposite direction. When providing water vapor or steam, it is stored in the porous storage medium (adsorption) and simultaneously heat is released. Processes are differentiated between closed refrigerant circulation systems (for producing cold water) and open systems according to the way in which the process is carried out: that is, whether or not the refrigerant comes into contact with the atmosphere. The latter is used for dehumidification and evaporative cooling. Both processes can further be classified according to either liquid or solid sorbents. In addition to the available refrigerating capacity, the relationship between drive heat and realized cold energy (coefficient of performance; COP) is also an essential performance figure of such systems (see Table 1 at end of article).

Absorption Refrigeration Machines In Germany, closed absorption refrigeration machines with liquid sorbent (water-lithium bromide) are most often operated in combination with heat and power generation (cogeneration) (i.e. with block unit heating power plants, district heating), but can also be assisted by vacuum tube solar collectors (operating temperature above 80 °C). With a single-step process the COP is 0.6-0.75, or up to 1.2 for a two-step process. A market overview is available from the Consortium for Economical and Environmentally Friendly Energy Use (Arbeitsgemeinschaft für sparsamen und umweltfreundlichen Energieverbrauch (ASUE)). Adsorption Refrigeration Machines Closed processes with solid sorbents work with so-called adsorption refrigeration machines (operating temperatures 60° - 95°; COP = 0.3 - 0.7). Solar energy can easily be used in the form of vacuum tube or flat plate collectors. A pilot system used for a laboratory's climate control at the University Clinic of Freiburg is fitted with tube collectors; the Fraunhofer ISE also took part in its scientific conception. The refrigerating machine is composed of two adsorbers, one an evaporator and the other a condenser. An adsorber chamber takes up the water vapor, which is transformed into the gas phase under low pressure and low temperatures (about 9°C) within the evaporator. Granulated silicate gel, well known as an environmentally friendly drying agent, then accumulates it (adsorbs the water vapor). In the other sorption chamber the water vapor is set free again (the chamber is regenerated or "charged") by the hot water from the solar collector (about 85°C). The pressure increases and at the temperature of the surroundings (30°C) the water vapor can be transformed once again into a fluid within a cooling tower (condensed). Through a butterfly valve the water is led back into the evaporator and the cycle begins from the beginning. Both the condensed water (low temperature) and the sorption heat (high temperature) are discharged.

Main components of the system at the University Clinic of Freiburg: Adsorption refrigeration machine (left) and solar thermal system (right).

The thermal operating power for this adsorption refrigeration machine is produced by vacuum tube collectors with a surface area of 170 m². Additionally, heat storage tanks improve the use of the solar heat. A cold storage tank functions as a buffer during short-term demand fluctuations. During colder times of the year, the solar energy heats the air inflow thereby reducing heating costs. Sorption-Assisted Air Conditioning Although the process of sorption-assisted air conditioning has been known for a long time, it has only been used in Europe for about 15 years. In principle, under middle European climate conditions, sorption-assisted air conditioning systems can be operated everywhere an air conditioner is wanted, for example in ventilation control centers. Their economical operation is then possible if cost-effective heat energy is available, i.e. from cogeneration plants, rather than from over loaded district heating systems. New heat sources, offering much promise, are solar thermal systems. Open sorption-assisted air conditioning systems are fresh air systems, that is they dry the outside air through sorption, pre-cool it with a heat reclamation rotor and finally cool it to room temperature through evaporation-humidification. The main principle of sorption-assisted air conditioning is shown in the graphic. The solar energy is used to dehumidify the sorbent.

Basic structure of the process of sorption-assisted air conditioning.

The most important steps of the process are: 1-2 Sorptive dehumidification of outside air with simultaneous rise in temperature through the freed adsorption heat 2-3 Cooling of the air in the heat reclamation rotor in the countercurrent to the exhaust air 3-4 further cooling of air through evaporation-humidification; the air inflow to the building has a lower temperature and less water vapor than the outside air 4-5 Heating of the air and if necessary addition of water vapor 5-6 Lowering of building's exhaust air temperature through evaporative cooling in the humidifier 6-7 Heating of exhaust air in the countercurrent to the air inflow in the heat reclamation rotor 7-8 Further heating of the exhaust air through external heat sources (i.e. solar thermal system) 8-9 Regeneration of the sorption rotor through the desorption of the bound water

At present, systems with rotating sorption wheels (sorption rotors) are mostly in use. The sorption wheel has small air channels that create a very large surface contact area, which has been treated with a material that easily takes up moisture, such as silica gel. The inflow air is dehumidified in one of the two sectors of the rotor and heated through the adsorption process (the exhaust air serves to dry the rotor). Finally, the inflowing air is cooled down in a heat reclamation rotor. The heat transfer here is made possible through the contact between the air and the rotor material. The last step in cooling the inflowing air is with conventional evaporation humidification. How well do Solar-Assisted Air Conditioning Systems Operate? Scientists of the Freiburg Fraunhofer Institute for Solar Energy Systems ISE (Freiburger Fraunhofer Instituts für Solare Energiesysteme ISE) tested solar assisted air conditioning systems for a study of the International Energy Agency (IEA) in the context of the TASK 25 "Solar-Assisted Air Conditioning of Buildings". Detailed descriptions and results of the compared systems can be gathered from the study's conclusion [1]. Year simulations of five variants of a solar-assisted system for air conditioning were conducted and compared to a conventional system for different climates (Trapani/Sicily; Freiburg and Coenhagen). Energy Balance Without the use of solar energy, thermally powered climate control raised the primary energy use (thermal and electrical) for all of the tested locations. The reason for this is the lower operating numbers of this process in comparison to electrically powered compression refrigeration machines. Whether absorption or adsorption refrigerating machines are used, a solar-covered share for cooling of 30 % (Freiburg) and almost 50 % (Trapani) is required to affect a primary energy savings. The solar-covered share for cooling is the portion used for cooling during the summer that comes from heat made available by the solar thermal system. With coverage shares of up to 85 %, the primary energy use can be decreased by over 50 % compared to the conventional reference system. The results were ascertained from an example reference office building and can therefore not simply be applied to other cases or buildings. In Trapani the sorption assisted air conditioning, in combination with a compression refrigeration machine, led to a small primary energy savings with a solar coverage share of 30 %. If the sun delivers 85 % of the heat for the air conditioning, then just about 50 % of the primary energy can be saved. In this case there are two apparent positive aspects: the sorption-assisted air conditioning can effectively be used for air dehumidification and additionally it can achieve relatively good overall efficiency.

	Photo: Sorption-assisted air conditioning system in Portugal.

Cost Effectiveness Although over 20 systems that use thermal solar energy to air condition buildings and that can be technically and economically assessed have been installed in Germany, there are still a number of obstacles to be overcome when it comes to the implementation of solar-assisted air conditioning. In the twelve countries taking part in the TASK 25 of the Solar and Heating Program of the IEA, experience with about 30 systems has been gained and currently 10 systems are being tested as a part of a demonstration program. Such pilot and demonstration programs are still necessary so that cost reductions become possible and so that relevant energy savings can be assured. Standardized programs, matured concepts and the development of components are starting points that can contribute to improved cost effectiveness and wide applicability of solar-assisted air conditioning. Because solar cooling is based on thermally driven processes instead of the normal electrical cold production, the costs for the used heat plays a central role: a fundamental problem arises from the inherently higher costs of solar heat compared to heat energy produced by fossil fuel systems or waste heat. Experts at the Fraunhofer ISE expect no economical advantages of the solar air conditioning in this respect. Their use becomes interesting if favorable requirements for a high output of solar heat are present and if the system also delivers energy for heating. The cost of electricity could also pose an argument for solar cooling: The thermally powered cooling process requires only a fourth (absorption/adsorption) or half (sorption-assisted air conditioning) of the electrical power required by the conventional reference system. The ISEs comparative testing showed that during the process the sorption-assisted air conditioning connected to a conventional machine (compression refrigeration machine) represents the most promising system combination, at least for a Mediterranean climate. The sorption-assisted air conditioning produced the lowest costs at all locations, while the adsorption machines were the most expensive solution. The scientists at the Fraunhofer ISE see a chance for sorption-assisted air conditioning in the cooperation between German facilities and companies that have gained experience with the operation of sorption processes for climate control and large solar thermal systems. Using this know-how, especially in Mediterranean regions, a gap could be found in the market.

Process	closed		open
Coolant circulation	closed refrigerant circulation systems		open refrigerant circulation systems (in contact with the atmosphere)
Process baic principle	cold water production		air dehumidification and evaporative cooling
Sorbent type	solid	liquid	solid	liquid
Typical material systems (refrigerant/sorbent)	water- silica gel ammonia- salt*	water-water-lithium bromide, ammonia- water	water-silica gel water- lithium chloride- cellulose	water- calcium chloride, waterlithium chloride
Marketable technoloy	adsorption refrigeration machine	absorption refrigeration machine	sorption assisted air conditioning	-
Marketable output [kW cooling]	adsorpzion refrigeration machine [50 - 430 kW]	absorption refrigeration machine: 35 kW - 5 MW	20 kW - 350 kW (per module)	-
Coefficient of Performance (COP)	0.3 - 0.7	0.6 - 0.75 (one step) <1.2 (two step)	0.5 - >1	>1
Typical operating temp.	60 - 95°C	80 - 110°C (one step) 130 - 160°C (two step)	45 - 95°C	45 - 95°C
Solar technology	vacuum tube collector, flat plate collector	vaccum tube collector	flat plate collector, solar air collector	flat plate collector, solar air collector

*still in development

Table 1: Overview of processes for thermally powered cooling and air conditioning

Material and Pictures: Fraunhofer ISE: Solarserver Editor: Rolf Hug. We thank Dr. Hans-Martin Henning and Diplom Engineer Carsten Hindenburg for their friendly support.