A heat pump is a device that moves heat from one location (the ‘source’) to another location (the ‘sink’). Most heat pumps move heat from a low temperature source to a higher temperature sink. Common examples are fridges and freezers, where heat is extracted to achieve a cooling effect, or when operating in reverse, to heat water or space. Heat pumps usually draw heat from the air or the ground, but can extract heat from water.
Heat pump systems are widely used in Europe and the USA, where continuous refinement has led to major increases in both efficiency and reliability, meaning heat pumps now offer a proven, cost-effective, lower carbon alternative to fossil fuels.
When comparing the performance of heat pumps the term Coefficient of Performance (CoP) is used to describe the ratio of energy output to energy input. A CoP of 1.0 means that the input of one unit of electrical energy will result in the production of one unit of heat.
The CoP of a ground source heat pump is always around 3.5-4.0 as the ground temperature is surprisingly constant at around 10°C, predominantly resulting from stored solar heat, rather than geothermal heat. When used for cooling, a heat pump's operating performance is described as its Energy Efficiency Ratio (EER) or Seasonal Energy Efficiency Ratio (SEER) - the greater the EER the better the performance.
On a mild day, a typical air-source heat pump will have a CoP of 3.0-4.0, which compares favourably to a conventional electric resistance heater’s CoP of 1.0. On a very cold day, the efficiency of an air-source heat pump drops significantly, as it has to work harder to move heat from the air source and into the building interior (the sink). The heat pump's performance will continue to drop until it reaches an external temperature of -18°C, when the CoP will be reduced to around 1.0. Therefore, in extremely cold weather, heating is better provided by an electric-resistance heater, and most heat pump systems include a back up heating element housed within a storage cylinder to allow for this eventuality.
Heat pump systems running costs are significantly lower than heating systems utilising LPG, oil or electricity and are usually lower than those of a conventional gas boiler-heating system. Although initial set-up cost of a heat pump is higher than for conventional heating systems, comparison of the whole-life cost (combining capital and running costs) proves that heat pumps compare extremely favourably to fossil fuel systems - especially in comparison to electric heating.
In the UK around 40% of CO2 emissions are derived from the heating of buildings. Energy in heat pump systems is consumed by the movement of heat from one place to another. UK testing has shown that where space and domestic hot water heating is provided by a heat pump system, only around 55% of the greenhouse gas emissions of energy efficient ‘A’ rated domestic gas heating system is generated. As most electricity suppliers now offer 'green' electricity from renewable sources it is possible to achieve zero carbon emissions by using this to power a heat pump.
Heat pumps provide one of the most cost and energy-effective forms of exploiting renewable energy as can be seen from the following figures based on 2008 installations:
System Type | Capital cost (installed) | Total output (MWh) / p.a. | Renewable output (MWh) / p.a. | Renewable energy per year per £1000 spent (MWh) |
8 kW air source heat pump | £4,500 | 14 | 9.3 | 2.0 |
2 kW solar PV system with 16 m2 of panels | £9,500 | 1.6 | 1.6 | 0.17 |
Solar thermal system with 2x12 evacuated tube collector arrays | £3,950 | 1.6 | 1.6 | 0.40 |
A ground-source heat pump - Horizontal collectors | £8,500 | 14 | 9.3 | 1.01 |
A ground-source heat pump - Borehole collectors | £10,500 | 14 | 9.3 | 0.88 |
A gas-fired condensing boiler plus solar thermal system | £6,250 | 14 | 1.6 | 0.25 |
