There are three types of heat pumps: ground, air and water source.
Ground source heat pumps extract heat from the ground using a buried vertical or horizontal ground loop which transfers heat from the ground into a building to provide heating and, in some cases, to pre-heat hot water.
The heat is collected through underground pipes laid about 1.5m below the surface, or from a borehole system. In both of these options, water is re-circulated in the loop and delivered to the heat pump, which in turn is delivered to the central heating system.
Efficiency of the system is measured by its Coefficient of Performance which shows how much heat is generated for every unit of electricity consumed through the pumping process. This is usually between 3 and 5, so for every unit of electricity consumed, between 3-5 units of heat are produced.
Air and water source heat pumps use air or water respectively. They do not rely on a collection system and simply extract the heat from the source at the point of use.
Air source heat pumps can be fitted outside a building or in the roof space and generally perform better at slightly warmer air temperatures.
Water source heat pumps can be used to provide heating in buildings near to rivers, streams and lakes for example.
While not strictly a renewable energy technology, heat pumps can provide significant efficiency savings over conventional systems. Using another renewable technology to provide power for the heat pump is a good way of maximising the benefits of both technologies.