Absorption Chillers

Absorption chillers are the most distributed thermally driven chillers worldwide. A thermal compression of the refrigerant is achieved by using a liquid refrigerant/sorbent solution and a heat source, which is replacing the electric power consumption of a mechanical compressor. For chilled water above 0 C, as it is used in air conditioning, typically a liquid H2O/LiBr solution is applied with water as refrigerant. Nevertheless, other liquid solutions can be used like H2O/LiCl or NH3/H2O which permits to produce chiled water at temperatures below 0 C.

The main components of an absorption chiller are the generator, the condenser, the evaporator and the absorber (see figure on the right).

The cooling effect is based on the evaporation of the refrigerant (water) in the evaporator at very low pressure. The vaporised refrigerant is absorbed in the absorber, thereby diluting the H2O/LiBr solution. To make the absorption process efficient, the process has to be cooled. The solution is continuously pumped into the generator, where the regeneration of the solution is achieved by applying the driving heat such as from hot water supplied by a solar collector. The refrigerant leaving the generator by this process condenses through the application of cooling water in the condenser and circulates by means of an expansion valve again into the evaporator.

Many of these products are available in the market, however typical chilling capacities of absorption chillers are several hundred kW. For several years, the smallest machine available was a Japanese product with 35 kW capacity. Mainly, they are supplied with district heat, waste heat or heat from co-generation. The required heat source temperature is usually above 80 C for single-effect machines and the coefficient of performance (COP) is in the range from 0.6 to 0.8. Double-effect machines with two generator stages require driving temperature of above 140 C, but the COPs may achieve values up to 1.2.

Since a few years, several single-effect absorption chillers with capacities below 50 kW are available. In SAC systems with absorption chillers, these small units are now often implemented. A chiller model, newly developed for small capacities, enables part-load operation with reduced chilling power at a heat source temperature of 65 C and a COP of still approximately 0.7, which is very promising in combination with a solar heat source. This shows that there is a high potential for performance improvements of absorption chillers.The new medium-size and small-size developments have been designed recently by European and Asian manufacturers, convenient to cover the cooling loads for small areas such as from 200 m to 500 m. The European manuacturers are located in Germany, Austria, Spain, Sweden, Italy and Portugal. Some of the developments are still being tested in pilot installations.

Schematic drawing of an absorption chiller (click to enlarge)