All heat pumps make use of the vapour compression cycle. When a liquid boils, it absorbs heat in order to change into a vapour, the absorbed heat being known as the latent heat of vaporisation. When the vapour is condensed back to a liquid, this latent heat is given up to the surroundings. This is the basis of the vapour compression cycle, which can be used for either heating or cooling, or a combination of both.
The two units used for heating are a Ground Source Heat Pump (GSHP) and an Air Source Heat Pump (ASHP), with the diagram on the left showing a GSHP. GSHPs make use of the fact that at a depth of 1 to 1.5 metres ground temperature is relatively constant all year round at about 8 to 10șC. ASHPs take heat from external air, for which air temperature varies depending on the season.
The efficiency of a heat pump is measured by the coefficient of performance (CoP), the higher the CoP the more efficient the heat pump is. For a CoP of 4, for every 4kW of heat produced only 1kW of electricity is consumed. CoPs for GSHPs are higher than those for ASHPs because the ground temperature is constant throughout the year, whereas for ASHPs air temperature varies, with the CoP falling as air temperature falls. However, GSHPs require a large area of ground for the ground loop to be located, whereas an ASHP does not need a ground loop. Hence, there are benefits with both systems, depending on location.
For both systems to maintain good CoPs water delivery temperature to the heating system ideally needs to be 40șC or below, as above this figure efficiency falls rapidly. Hence, heat pumps are ideally suited for highly insulated new build properties where underfloor heating can be installed.
Where correct electricity tariffs are applied GSHPs can operate at a similar cost to gas fired condensing boilers, but with annual savings in carbon emissions of 10 to 20% depending on application. Against other fuels, with the exception of biomass for carbon savings, cost and carbon savings increase further.