A heat pump is a device that uses the refrigeration cycle for heat recovery. Normally thermal energy transfers from a hot body to a cold body. A heat pump uses external energy to transfer thermal energy in the opposite direction, that is to absorb heat and then reject it to ‘heat sink’. The simplest example of a heat pump is a window air conditioner that absorbs heat from the room and rejects it to the outside. If a window air conditioner is turned around, it can provide heating for the room the winter. Instead of flipping the orientation, most heat pumps have a reversing mechanism (reversing valve) that switches the direction of the refrigeration cycle. In this way a heat pump can provide cooling in the summer and heating in the winter. The source of heat for the heat pump can be the air, water or the ground.