Principle
General principles - the theory of refrigerating machines

If you want to transfer heat in the unnatural sense from a cold medium to a warm one, you have to devise and implement a special thermal system, and supply energy to it.

The particular thermodynamic system implemented is capable of effectively transferring heat from a lower-temperature medium where the heat is taken (cold source) to a higher-temperature medium where the heat is rejected (hot sink).

When the aim is to extract heat from a body, or a medium, in order to cool it or maintain it at a temperature below that of the ambient, the system that performs this operation is naturally called a refrigeration machine. The useful effect is the...