The heat pump uses as an energy source the temperature level of the environment and the physical effect that a liquid evaporates when either the temperature is increased or the pressure is decreased. An example from everyday life is, for example, the cooking pot with water on a stove. A large amount of energy must be provided until the water evaporates and the aggregate state changes from liquid to gas. The heat pump uses the opposite process, called condensation (transition from gaseous to liquid), to produce heat. A heat pump uses a refrigerant that evaporates through the source (air, water, brine) at low pressure. A downstream compressor increases the pressure. This causes the liquid to condense, releasing energy that can be used for heating. Afterwards, a throttle reduces the pressure again. The refrigerant cools down even further and can then be reheated in the evaporator. Reversible heat pumps can reverse the cycle and provide cooling capacity.