Thermodynamic Cycle

      According to the second law of thermodynamics heat cannot spontaneously flow from a colder location to a hotter area ; work is required to achieve this. Work is applied to cool a living space or storage volume by pumping heat from a lower temperature heat source into a higher temperature heat sink. The operating principles of the refrigeration cycle was described mathematically by sadi carnot  in 1824 as a heat engine. A heat pump can be thought of as a heat engine which is operating in reverse.
      Heat pump and refrigeration cycles can be classified as vapor compression , vapor absorption , gas cycle or stirling cycle types.