The use of solar heat for heating and cooling applications offers a wide field of possibilities. The supply of solar radiation all over the year with the peak in the summertime forces to extend the existing installations for heating support in the heating season with separate installations for cooling purposes.

The most applied technologies for solar assisted air conditioning systems are classified into closed and open cycle systems.

The main component of closed cycle systems is the thermally driven chiller which provides chilled water. The cold water can be used either in air handling units for cooling and de- humidification processes or is distributed with a chilled water network within the building to the rooms to operate decentralized room installations, like fan coils and chilled ceilings. Chillers with absorption as well as adsorption technology are available on the market. The most part of them (about 88 %) use absorption technology, but only about 12 % use adsorption technology (Source: TECSOL, Internal report for IEA SHC Task 38).

Open cycle systems

Open cycle systems provide cooled and dehumidified air according to the comfort room air conditions. Water is used as refrigerant and it is in direct contact with the atmosphere. Therefore the systems are called open. Two different technologies of desiccant cooling systems exist: systems with solid and with liquid sorbents. Open cycle systems can be applied in buildings with ventilation systems or parts of buildings in which the cooling loads can be covered by controlled ventilation.

Working Scheme:

• Closed cycle systems

         Closed cycle systems with absorption as well as adsorption chillers can consist of the main components shown in fig. 1:

· Solar thermal collector field (types: flate plate, evacuated tubes, cpc, …)

· Hot water storage tank (types: stratified storage, buffer storage, …)

· Heat backup (fossil fuel)

· Thermally driven chiller (absorption or adsorption)

· Hot water distribution network


· Heat rejection unit (cooling tower dry/wet, ground tube)

· Cold water distribution network

• Open cycle systems
  

         Concerning the open cycle systems there has to be differentiated between systems with solid sorbents and liquid sorbents. In both systems the processes ofdehumidification and cooling are separated. Because of the coupling of these two processes it is called Desiccant Evaporative Cooling (DEC). In normal configurations for moderate climates the supply air is dehumidified in the first step. In a next step the supply air is cooled by heat transfer to the return air stream and additional by controlled humidifying. The solar thermal heat is the driving energy for the dehumidification process. The heat is used for the regeneration of the rotating desiccant wheel (adsorption wheel) and the salt solution in liquid systems respectively.

• Solid sorption system

         In solid sorption systems the supply air comes in contact with the solid adsorption material and becomes dehumidified. Main components are:
· Desiccant wheel with silica gel or lithium chloride
· Heat recovery wheel
· Humidifier
· Solar thermal collector field to provide the driving heat

• Liquid sorption system

         In liquid sorption systems a salt solution is in contact with the supply air. The ability of concentrated salt solutions to absorb water is used for the dehumidification process (see fig. 3). Main components:
· Absorber for dehumidification of supply air by use of concentrated salt solution (LiCl mainly)
· Regenerator for thermal regeneration of diluted salt solution by enrichment
· Storage tanks for the diluted and concentrated salt solution


        One of the main advantages of liquid sorption systems is the possibility to store the concentrated solution and therefore to keep the system running while no solar radiation is available.

CASES:

Four cases come to enlighten the application of these technologies in buildings cooling & heating systems. Full description of the cases is available HERE respectively at page:

Closed Systems:

3.a. Fraunhofer ISE - Freiburg, Germany (closed system)   p.20
3.b. Résidence du Lac - Maclas, France  (closed system)   p.23

Open Systems:

3.c. Chamber of Commerce “Südlicher Oberrhein” – Freiburg, Germany  p.26
3.d. Solar Info Center SIC – Freiburg, Germany  p.29