In absorption systems, compression of the working fluid is achieved thermally in a solution circuit which consists of an absorber, a solution pump, a generator and an expansion valve as shown in Figure 3 . Low-pressure vapour from the evaporator is absorbed in the absorbent. This process generates heat. The solution is pumped to high pressure and then enters the generator, where the working fluid is boiled off with an external heat supply at a high temperature. Heat is extracted from the heat source in the evaporator.
Useful heat is given off at medium temperature in the condenser and in the absorber.
In the generator high-temperature heat is supplied to run the process. A small amount of electricity may be needed to operate the solution pump. For heat transformers, which through the same absorption processes can upgrade waste heat without requiring an external heat source, refer to the section on Heat pumps in industry .