Heat pump and refrigeration cycle
According to the second law of thermodynamics, heat cannot move from a colder location to a hotter area by itself. Work is required to achieve this. An air conditioner requires action to cool a living space by moving heat from the interior being cooled to the outdoors. A refrigerator moves heat from inside the cold interior area to the warm room temperature air of the kitchen. This idea of an ideal heat engine was explained using the Carnot cycle by Sadi Carnot in 1824. A refrigerator or heat pump is an example of a heat engine that is running in a reverse Carnot cycle.