Absorption chillers use low-grade waste heat to do the chilling. They lack the motorized compressor used by other chiller types. Absorption chillers use a special solution to absorb low-pressure refrigerant vapor, which is then sent to a high-pressure generator where the refrigerant is desorbed and vaporized by the waste heat. The refrigerant is then condensed to a high-pressure liquid, and passed through an expansion valve and into an evaporator, which cools the evaporator. The vapors leave the evaporator and go back to be recombined with the absorbent, and the process starts all over again. Commonly used refrigerants include water (used with lithium bromide) and ammonia (used with water).
Absorption chillers are useful in situations where waste heat (or a cheap supply of fuel) is available, or where cost or other limitations prevent the use of a mechanical compressor.