How heat pumps work
A modern heat pump is essentially an efficient heating appliance used to heat water, air or both. Basically, a heat pump moves thermal energy or ‘heat’ from one medium to another, in much the same way a refrigerator moves heat out of the fridge to cool it. The heat is moved in an opposite direction to its spontaneous flow, i.e. cold to a warm instead of warm to cold.
Of course some power is needed to move this thermal energy between the mediums but heat pumps are much more efficient than traditions hot water system which heat water directly from electricity or gas - this is explained in more detail in the performance section below.
Split-cycle or reverse cycle air-conditioning systems work on exactly the same principle but can operate in reverse and heat air instead of water.
How Heat pumps work
A heat pump works by employing a vapour-compression refrigeration 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).
How a heat pump works - Image credit Cromagen
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.
Performance
Heat pumps are particularly effective in heating systems as they use less electricity than is released in heat. For example, a simple electric (heating element) heater will take one unit of electricity and convert it into 1 unit of heat. However, most heat pumps will take one unit of electricity and transfer it to around 4 units of heat into the desired medium whether it’s water or air. Split-cycle or reverse cycle air-conditioning systems work on exactly the same principle but heat air instead of water. The measure of how well a heat pump works is called its coefficient of performance (COP).
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.
Some of the well-known and most popular heat pump hot water systems - Click to enlarge
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 6 to 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 much 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.
Credit: Luke Boyden. 2014
