As outdoor ambient air temperature decreases, what happens to a heat pump's COP?

As outdoor ambient air temperature decreases, a heat pump's coefficient of performance (COP) typically decreases. The COP is a measure of a heat pump's efficiency, defined as the ratio of heating output to electrical energy input.

At lower outdoor temperatures, the heat pump has to work harder to extract heat from the air, as the thermal energy available in the air decreases. This increased effort requires more energy consumption, which negatively impacts the COP. Essentially, as the ambient air gets colder, the system's ability to transfer heat diminishes, leading to a reduced efficiency in converting electrical energy into useful heat energy.

The correlation between temperature and COP is crucial for understanding heat pump performance across different climates, making it important for system design and energy efficiency considerations. Thus, the concept that the COP decreases with a drop in outdoor temperatures aligns with the fundamental principles of thermodynamics and heat transfer in the operation of heat pumps.