Geothermal Heat Pumps run on electricity and provide hot water for space and water heating. The clever thing about them is the way they use electricity to operate a compressor and refrigeration circuit (not dissimilar to what’s in a fridge) to move heat from a low temperature source to a high temperature source.
Heat can be sourced from the outside air or from the ground. Ground heat can be collected by horizontal collectors (pipes buried in trenches, typically 1 – 2 metres down), pipes in a vertical bore hole (100 – 200 m deep) or directly from ground water, e.g from a lake or well.
Heat pump technology allows appliances to achieve efficiencies greater than 100% as for every unit of energy (electricity) used typically 3 – 4 units are produced. The efficiency of a heat pump is given as it’s C.O.P (coefficient of performance) and is typically in the range 2.8 – 3.5 for air source units and 3.5 – 4.5 for ground source types. This figure represents the multiplier for electrical energy converted to heat energy. For example a heat pump quoted as having a COP of 4 would produce 4 times more energy than it consumes.
Air source pumps have lower efficiencies as it takes more energy to move required volumes of intake air than water. However it’s important to remember that for all heat pump systems one fundamental does not change: The greater the difference between the temperature of the source (air or water) and the output hot water temperature, the less efficient the unit becomes. So for example, a COP of 3.5 (350% efficient) at 35° might become 2.5 (250%) at 60°. For this reason a well designed heat pump system will use a low temperature heat distribution system such as under floor or oversized radiators to allow low temperature operation. This is easy to achieve in a new build or major renovation but more costly to implement in an existing dwelling.
Geothermal is particularly suited to under floor heating as this will typically allow all electricity consumed to be at night rate tariff (approx. 50% less than day rate). During the charging period ( 12 midnight to 8am ) the floors are heated and this heat is released slowly during the day. However, space permitting a solution can also be implemented for oversized radiator systems whereby a large volume of water (typically 1,000L – 2000L) in a buffer cylinder is heated at night and circulated by the radiator system during heating hours.