ABSTRACT
Heat ›ows from hot regions to cold regions, driven by temperature difference and nature’s desire to “level” energy differences (the Third Law of Thermodynamics). To reverse this process in a system and move heat from a lowertemperature region to a higher-temperature region requires that “work” be done on the system. Thus, we use refrigeration machines to provide work to move heat from a cooled area and reject it to a hot area. The performance of these machines is usually characterized by a quantity known as the coefˆcient of performance (COP), deŒned as
COP
removed heat input work
=
Therefore, COP is a dimensionless ratio of how much heat is transferred out of the cooled space to the amount of work that is used to accomplish this task. Unlike typical deŒnitions of “efŒciency,” the COP can be larger than unity. Higher values are better, indicating that more heat is removed for a given amount of work. COP is usually dependent on operating conditions, such as the temperatures of the cooled space and the hot space to which heat is to be rejected, and the type of refrigeration cycle utilized.
