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How heat pumps work

Martin Newkirk

Luke Boyden

A heat pump is a device that moves thermal energy (“heat”) from one area to another. The heat is moved in an opposite direction to its spontaneous flow, i.e. cold to a warm instead of warm to cold. Some additional amount of power is needed to move this thermal energy between areas.

Heat pumps in the context of heating systems can be thought of as fridges in reverse – heat is released into the desired space, instead of removed. Heat pumps are also available as dual use machines, with air conditioning (cooling) functionality. Heat pumps are best employed to deliver a lower temperature, for most if not all of the day, to well insulated buildings.  They are often most cost effective when integrated into a new build, but can be installed later and work with ducted air systems and hydronic (central) heating.

Performance

Heat pumps are particularly effective in heating systems as they use less electricity than is released in heat. For example, an electrical resistance space heater will take one unit of electricity and convert it into 1 unit of heat. However, a good heat pump will take one unit of electricity and transfer around 4 units of heat into the desired space. The measure of how well a heat pump works is called its coefficient of performance (COP).

How it works

A heat pump works by employing a vapour-compression cycle. A working fluid (refrigerant) is pumped through a closed circuit. The pump moves and compresses the refrigerant, which raises its temperature on the side to be warmed. The fluid then passes through an expansion valve, which reduces the pressure of the fluid cooling it down to a low temperature, cooler than the environment in which it then absorbs heat (then repeating the cycle).

The power used to drive the compressor (pump) is provided from electricity. The greater the temperature difference, the higher the pressure difference needed, which increases the energy needed to drive the compressor (pump). Consequently, the coefficient of performance (efficiency measure) decreases as the temperature difference increases between the cold and warm areas.

Types of refrigerant

A refrigerant is the working fluid used in a heat exchanger, which absorbs, carries, and releases the heat energy to where it is needed. It undergoes phase changes (changes between liquid and gas states) at different temperatures, and this is the main property exploited when used for heat pumps.

Traditional refrigerants used in heat pumps have caused damage to the ozone layer and also emitted significant amounts of greenhouse gases. A number of refrigerants are now being used that have less effect on greenhouse gases. In fact carbon dioxide is now widely used in heat pumps, which is a less potent greenhouse gas than other refrigerants whilst still maintaining a high COP.

Heat sources 

The environment around a building is used as the source for thermal energy for heating and can come from the air or ground. At about 9 metres under the earth’s surface the temperature hardly varies through seasons, therefore ground source heat pumps are usually the most efficient. However, they have greater capital costs due to the requirement to install an underground pipe network or drill bore holes. Some systems may be hybrid and switch between air and ground sources in order to lower the temperature difference seen and maximise efficiency.