Heat pumps transfer heat from a lower temperature source to one of a higher temperature. This is the opposite of the natural flow of heat.
A refrigerant fluid is run through the lower temperature source. The fluid ‘absorbs’ heat and boils, even at temperatures below 0° C (although the coefficient of performance (COP) decreases with lower temperature). The resulting gas is then compressed, which further increases its temperature. The gas is passed into heat exchanger coils, where it condenses, releasing its latent heat. The process then repeats.
This is the same process that is used to extract heat from a fridge.
Heat pumps can be used domestically or commercially to provide hot water, space heating (either by providing hot water for under-floor heating or radiators, or supplying hot air) or other applications such as heating swimming pools.
In the UK, heat pumps are most commonly air-source or ground-source heat pumps. However, the temperature of ground water sources in the UK tends to be fairly constant for most of the year, at between 8 and 12°c, and so water source heat pumps can be more efficient.
- Air source heat pumps absorb heat from the outside air. Air-to-water systems provide hot water for direct use or to supply radiators or underfloor heating systems. Air-to-air systems provide hot air, either directly into an internal space, or to be distributed by fans throughout a building. See Air source heat pumps for more information.
- Ground source heat pumps use lengths of plastic pipe buried in the ground, either in a borehole or a horizontal trench. See Air source heat pumps for more information.
- Water source heat pumps absorb heat from a suitable local water source, such as a lake, river, well, borehole and so on. They can be ‘open-loop’ or ‘closed-loop’ systems. See Air source heat pumps for more information.
Heat pumps may be eligible for payments under the government’s Renewable Heat Incentive(RHI) scheme. Domestic schemes may also be eligible for financing through the green deal, however, there has been very poor uptake of the Green Deal, and it is generally considered that better financial deals are available elsewhere.
Refrigeration units also use heat pumps, as do chiller units that provide chilled water (CHW) for space cooling, for example in air handling units, fan coil units, chilled beams and so on. In this case, the system is reversed, absorbing heat from the interior and releasing it to the outside.
Reversible heat pumps can operate in either heating or cooling mode, using a reversing valve to change the direction of flow.
Heat pumps generally use electricity to power the compressor. However, in absorption heat pumps, the refrigerant (typically ammonia) is absorbed into water, then is released again by being heated. The heat source is generally gas fuelled (Gas Absorption Heat Pumps GAHP) but other heat sources can be used, such as solar heated water.