A heat pump is a device for transferring heat from a cold region by doing work (as required by the second law of thermodynamics). The working fluid or refrigerant is a condensible gas such as ammonia or Freon. A motor-driven compressor compresses the gas adiabatically (raising its temperature) and delivers it to a condenser coil or "radiator" in the space to be heated. As it loses heat it liquefies and passes through an expansion valve into the evaporator, a low-pressure region where the liquid evaporates, taking heat from its relatively cool environment. The gas then returns to the compressor to complete the cycle. Heat pumps are used in domestic refrigerators, the evaporator being inside the refrigerator, and also in air conditioning systems for space heating in winter and (by reversing the pump) cooling in summer.