All chillers use some form of external energy to move or ‘pump’ heat from a low temperature medium to a high temperature medium. Electric chillers use compressors to move heat to a higher temperature medium for heat rejection. Absorption chillers achieve this heat transfer by using an external heat supply such as hot water or steam to produce chilled water. An absorption chiller uses thermo-chemical ‘compressing’ where two different fluids are used: a refrigerant (usually water) and an absorbent (usually ammonia, potassium bromide or lithium bromide). The absorbent is such that it can easily dissolve the refrigerant.  The refrigerant can change phase easily between liquid and vapour and circulates through the system.

Heat from fuel combustion, solar or waste-heat source drives the process. The dissolved refrigerant in the absorbent causes it to boil at a lower temperature and pressure than it normally would. The vaporised refrigerant is condensed and is returned to the refrigerant circuit. In a two stage absorption unit, internal heat from this process is used to preheat the absorbent/refrigerant solution which makes the entire operation more efficient.

An absorption chiller is generally less efficient than an electric chiller. However, it can be appropriate  in applications where a source of waste or ‘free’ heat is available, even if it is low-grade heat. It also has fewer moving parts than an electric chiller so there is less wear and tear.