21.5 : Refrigerators and Heat Pumps

Refrigerators or heat pumps are heat engines operating in a reverse direction. For a refrigerator, the focus is on removing heat from a specific area, whereas, for a heat pump, the focus is on dumping heat into one particular area. A refrigerator (or heat pump) absorbs heat Q_c from the cold reservoir at Kelvin temperature T_c and discards heat Q_h to the hot reservoir at Kelvin temperature T_h, while work W is done on the engine’s working substance.

A household refrigerator removes heat from the food while exhausting heat to the surrounding air. The required work is performed by the motor using electricity, which moves a coolant through the coils. A coolant with a boiling temperature below the freezing point of water is sent through the cycle. The coolant extracts heat from the refrigerator at the evaporator, causing the coolant to vaporize. It is then compressed and sent through the condenser, where it exhausts heat to the outside.

The effectiveness or coefficient of performance K_R of a refrigerator is measured by the heat removed from the cold reservoir divided by the work done by the working substance cycle by cycle. Conversely, the coefficient of performance K_P of a heat pump is measured by the heat dumped to the hot reservoir divided by the work done to the engine on the working substance cycle by cycle.