1.1. Technology

The mainly used technologies for solar cooling systems with small-scale cooling capacity (< 30 kW) are absorption and adsorption cooling. Single-effect absorption chillers with the working pair water/lithium bromide and ammonia/water respectively are generating cold over a closed, continuous cycle. The ammonia/water absorption chillers could generate evaporator temperatures down to -60°C, which are useful for industrial cold processes. Using water as refrigerant the evaporator temperature is limited to temperatures above the freezing point. In absorption chillers the refrigerant (water or ammonia) is absorbed by a liquid sorbent (lithium bromide or water) as shown in Fig. 1. In the directly or indirectly solar powered generator with high heating temperatures, the refrigerant is desorbed from the solution. This generates a high refrigerant vapour pressure, which is sufficient to condense the refrigerant in the condenser. After evaporation, the refrigerant vapour is absorbed in the solution which is cooled in the absorber. The solution is pumped to the generator by a solution pump where it is regenerated and throttled back to the absorber. The heating temperatures for desorption are between 70 and 120°C according to the technology. Basically absorption chillers are used as central air-conditioning systems with decentralised fan coils or cooled ceilings.

At adsorption chillers the refrigerant water is adsorbed on a solid sorbent like silica gel among disposal of latent heat on the surface (Fig. 1.). The latent heat decreases to zero with increasing addition of water molecules, then only evaporation heat has to be dissipated. The desorption of the stored water and the pressure generation for the condensation is already caused by low heating temperatures of 55 to 70°C, so that this technology is especially appropriate to the application of solar energy. The closed adsorption chillers are generating cold water of minimal 5 to 6°C through the periodical cycle. These chillers can also be used as central air conditioning systems with decentralised fan coils or cooled ceilings.