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Heat pumps 
Ground source heat pumps(or geothermal as they are commonly called), extract low grade heat from the ground or a body of water via a large heat exchanger, compress it into high grade heat and deliver to your property's heating system. The heat exchanger (or ground array) consists of a long loop of plastic pipe that is filled with an antifreeze fluid- this can be buried in trenches, boreholes or a body of water.
A popular misconception is that ground source heat pumps collect geothermal energy from the earth's core. This is not the case, ground source heat pumps, like most other forms of renewable energy collect solar energy. The top few metres of the earth's surface is kept at a relatively stable temperature year round by the sun and ground water- it is this heat that a ground source heat pump captures.
An antifreeze fluid is pumped into the ground array at around -1 degree C, as it travels around the ground array it absorbs heat from the ground, so when the fluid returns to the heat pump, it has gained approximately 10 degree C. The fluid is then passed over a heat exchanger and the heat that has been gained from the ground is transferred to a refrigerant gas. This gas is then put into a compressor and pressurised, causing it to liquefy and gain heat. The refrigerant (now as a liquid) is passed over a second heat exchanger and allowing high grade heat to be transferred to the wet central heating system in the property. The refrigerant is then allowed to expand, causing it to evaporate and turn back into a gas. The whole cycle then begins again.