Examples of how heat pumps are used
This page explains how heat pumps work and the different technologies and applications available.
- Inside a heat pump
- Air source heat pump (ASHP)
- Water source heat pump (WSHP)
- Heat pumps compared with traditional technologies
Inside a heat pumpA heat pump consists of three main elements; the evaporator, the compressor and the condenser. Unlike a fridge, a heat pump uses its ‘evaporator' to pump energy from outside to inside, raising the internal temperature through releasing heat via an internal ‘condenser'. The diagram below shows how this works.
Diagram showing how a heat pump works
There are three main heat pump technologies. This section explains them and indicates which applications are best suited for the various technologies.
Ground source heat pump (GSHP)
This harnesses the energy stored within the ground using a ‘ground loop' - a length of pipe buried beneath the earth's surface. The advantage of this technology is that temperatures beneath the surface remain fairly constant (8 to 12°C) even throughout the winter. This stability means that the heat output of the pump is fairly stable throughout the year.
If a ground source heat pump has the additional provision of a cooling system, then heat extracted from the internal space in the summer can be stored in the ground for extraction in the winter.
The ground loop can be installed in the following ways.
- Borehole - a vertical hole (15 to 120 meters) within which the ground loop is placed. These are more expensive to install but suit sites with limited access to land.
- Straight horizontal - a shallow trench (at least 1.5 to 2 meters) within which the ground loop is laid.
- Spiral horizontal - similar to straight horizontal however the trench is wider, with the ground loop laid out in a spiral pattern.
Laying ground source heat pump loop in spiral horizontal fashion at Greenway School in Hull.
Air source heat pump (ASHP)
This system extracts stored energy from within the air. Otherwise, it works on exactly the same principle as a ground source heat pump. It can extract useful heat from the air at temperatures as low as minus 15°C.
Unlike the subterranean ground source heat pump, the air source heat pump evaporator is contained within a box positioned in an external location, usually on an exterior wall. Here, a fan draws air into the unit and the air flows over the evaporator.
In the UK, these pumps will need to be fitted with a defrost cycle. This will keep the system running efficiently in cold temperatures (less than minus 7°C).
Since air temperature in the UK varies both daily and seasonally, the efficiency of an air source heat pump will fluctuate with these changes. This needs to be considered when installing a system.
Water source heat pump (WSHP)
A water source heat pump system extracts heat from a local water source and usually operates exactly like ground source heat pumps within a ‘closed loop' system. In a closed loop system, the pipe work will simply be sunk to the bottom of a water course.
However in some instances a water source heat pump can operate using an ‘open loop' system. This involves water being abstracted from a borehole and discharged via a heat exchanger to a river or sewer. These systems can be very efficient because of consistent water temperatures.
Heat pumps can be used for heating space and water. They can also be used for cooling space in some applications.
Heat pumps are best suited to low temperature heat distribution systems, such as under floor heating. This is largely due to traditional radiator systems demanding higher operational temperatures of up to 80°C.
Guidance within the UK advises that hot water stored in a tank should be kept above, or periodically heated to, 60°C. This is to avoid the danger of Legionella bacteria. It is possible for some heat pumps to operate at such high temperatures, however, this reduces the efficiency of the heat pump. For this reason most heat pump systems are fitted with an immersion heater which can be used to increase the water temperatures to the necessary levels.
An additional feature is that heat pumps can provide space cooling. Using heat pumps for cooling will result in increased electricity use as the pump will be running for a longer period of the year. Whether or not this is felt to be financially viable will depend on the size of cooling need.
Heat pumps have very few moving parts and so require minimal maintenance. Indeed only annual checks are recommended to ensure smooth operation.
Ground source heat pumps have a typical lifetime of 20 to 25 years, air source heat pumps of over 20 years and water source heat pumps of over 15 years.
However, if air or water source heat pumps are sited externally, they will have a shorter life expectancy of around 15 years.
Heat pumps compared with traditional technologies
Heat pumps are not deemed 100 per cent renewable, as energy is required to operate the pump. This can be overcome by using renewable electricity to drive the heat pump. The typical heat output is two to four times the electrical energy required by the heat pump.
To maximise efficiencies of a heat pump, it is important to have a low heating distribution temperature but also as high a source temperature as possible.
A heat pump installed in 2010 produced nine per cent less carbon dioxide than an average gas boiler and 28 per cent less than an average oil boiler per unit of heat. This is taking into account the efficiency factors of gas and oil boilers. The potential for carbon savings will only increase in future under the UK Government's plan to 'decarbonise' the electricity grid.
1 May 2012