The "efficiency" of a refrigerator is expressed in terms of the coefficient of performance, which is designated by the symbol β. For a refrigerator the objective is the energy sought QL, the heat transferred from the refrigerated space. The energy that costs is the workW. Then the coefficient of performance β is:
(Eq1)
β =
QL
W
=
QL
QH − QL
=
1
QH /QL − 1
A household refrigerator may have a coefficient of performance (often referred to as COP) of about 2.5, whereas a deep freeze unit will be closer to 1.0. Lower cold temperature space of higher warm temperature space will result in lower values of COP. For a heat pump operating over a moderate temperature range, a value of its COP can be around 4, with this value decreasing sharply as the heat pump's operating temperature range is broadened.
A refrigeration or heat pump cycle can be used with either of two objectives. It can be used as a refrigerator, in which case the primary objective is QL, the heat transferred to the refrigerant from the refrigerated space. It can also be used as a heating system (in which case it is usually referred to as a heat pump), the objective being QH, the heat transferred from the refrigerant to the high-temperature body, which is the space to be heated. QL is transferred to the refrigerant from the ground, the atmospheric air, or well water. The coefficient of performance for this case β' is: