How does a heat pump work?

To implement a heat pump cycle the following items are required: a high temperature heat exchanger, a low temperature heat exchanger and a heat transfer medium that transports heat from the low temperature source to the high temperature sink. Currently, the most common design involves four main components: evaporator, condenser, compressor and expansion valve. The heat transfer medium is called refrigerant.


The evaporator is a low-temperature heat exchanger where the refrigerant enters as a low-temperature liquid, absorbs heat from the heat source by evaporation at a low pressure and leaves as a low-temperature vapour.


In the compressor the low pressure of the low-temperature refrigerant from the evaporator is raised to a pressure that is sufficiently high to match the desired condensing temperature in the condenser. During compression not only the pressure but also the temperature of the refrigerant will increase.


The condenser is a high-temperature heat exchanger where the refrigerant enters as a high-temperature vapour, rejects heat to the heat sink by condensation at a high pressure and leaves as a high-temperature liquid.

Expansion valve

On its return to the evaporator from the condenser the high-temperature, high-pressure liquid refrigerant must be changed to the low-temperature, low-pressure liquid that enters the evaporator. This is usually achieved by a throttling device known as the expansion valve. When the hot liquid passes through this valve, not only will its pressure be reduced but at the same time its temperature will drop. As the pressure drops, refrigerant starts to evaporate in the valve and the heat of evaporation is taken from the refrigerant itself which causes its temperature to drop and the result is a low-temperature, low-pressure mix of liquid and vapour.