Coefficient of performance

The coefficient of performance or COP (sometimes CP or CoP) of a heat pump, refrigerator or air conditioning system is a ratio of the useful heating or cooling provided to the work (energy) required. Higher COPs equate to higher efficiency, lower energy (power) consumption and thus lower operating costs. While the coefficient of performance is a term commonly used with heat pumps, it is also applicable to any energy system that behaves in a thermodynamically open manner, receiving energy from the local environment, whether it be electromagnetic, electrostatic, or any other viable form.

Instead of converting work to heat (which has a maximum efficiency of 100% and a COP less than or equal to one), heat pumps, air conditioners and refrigeration systems use work to move existing heat from one place to another. Less work is required to move heat than to generate heat. Usually, more heat is moved than the amount of work put in so the COP usually exceeds 1, especially in heat pumps. The COP is highly dependent on operating conditions, especially absolute temperature and relative temperature between sink and system, and is often graphed or averaged against expected conditions. Most air conditioners have a COP of 3.5 to 5. Compared to compressor refrigerator chillers, the COP of absorption refrigerator chillers is typically much lower, as they do not rely on compression, but instead rely on chemical reactions driven by heat.

A difference between the dimensionless terms efficiency and COP is that the numerator in COP is the heat moved or generated, in contrast to the numerator in efficiency being only the heat generated. Additionally, efficiency cannot be used to evaluate the ability of a device to cool, because the useful output energy is undefined in this case.

As an example, if a heat pump has an internal compressor efficiency of 70% and the user supplies 1.2 kW of power to run the unit and 6.5 kW is drawn from the local thermal environment, then COP is 6.5/1.2 ≈ 5.417. The efficiency is already taken into account in the 6.5 kW; if the compressor efficiency is higher, it is expected to be higher than 6.5 kW (more heat moved) for the same input power.