What is a heat pump?
In essence a heat pump is a refrigeration system operating in reverse.
A compressor circulates refrigerant aorund a sealed system. This collects heat from one place and pumps it to another.
The system uses electrical energy to power the compressor. This allows a heat pump to produce up to four units of heat , or more, from one unit of electricity.
For more information see the wikipedia article on heat pumps
The diagram above shows how a typical heat pump works
1) Heat is collected from the air or ground by the evaporator which uses this heat to evaporate low temperature, low pressure liquid refrigerant into a low temperature, low pressure gas.
2) The compressor uses electrical energy to compress the gas, increasing both the temperature and pressure
3) The usable heat is collected at the condensor and this process causes the gas to condense into a liquid
4) The refrigerant flow is controlled by an expansion valve