6.2 Heat pumps

A large proportion of the world’s fossil-fuelled heating systems will need to be replaced by electric heat pumps. Small heat pumps are actually familiar objects. Every domestic refrigerator uses one. They use an electric compressor to ‘pump’ heat from an evaporator at a lower temperature (inside a fridge) to a condenser on the back of the fridge, where heat, now at a higher temperature, is released, warming one’s kitchen in the process.

In buildings a heat pump may be used for heating or cooling (more commonly known as air conditioning). When used for heating the evaporator, collecting low temperature heat, is located somewhere in the external environment. An air source heat pump is likely to have a fan coil unit, containing the evaporator, such as that shown in Figure 19.

A ground source heat pump uses pipes buried in the soil to gather low temperature heat. These may be laid in a shallow trench, or in a deep vertical borehole that may be 10 metres or more deep. This extracts heat indirectly from the outside air via the ground. Water source heat pumps can extract heat from rivers, the sea or outflowing sewage water. Heat is then pumped from the outside environment, via the evaporator, to a condenser inside the building, normally connected to a conventional central heating system. The temperature of the heat is sufficient to be useful for heating purposes.

Heat pumps can provide between 2 and 4 times the amount of heat for the same electrical consumption as direct electric heating. They are now widely used in Scandinavia and becoming more common right across Europe.