Ground source heat pump
A ground source heat pump (also geothermal heat pump) is a heating/cooling system for buildings that uses a type of heat pump to transfer heat to or from the ground, taking advantage of the relative constancy of temperatures of the earth through the seasons. Ground source heat pumps (GSHPs) – or geothermal heat pumps (GHP) as they are commonly termed in North America – are among the most energy-efficient technologies for providing HVAC and water heating, using far less energy than can be achieved by burning a fuel in a boiler/furnace or by use of resistive electric heaters. Efficiency is given as a coefficient of performance (CoP) which is typically in the range 3 – 6, meaning that the devices provide 3 – 6 units of heat for each unit of electricity used. Setup costs are higher than for other heating systems due to the requirement to install ground loops over large areas or drill bore holes, and for this reason air source heat pumps are often used instead. Ground-source heat pumps take advantage of the difference between the ambient temperature and the temperature at various depths in the ground. The thermal properties of the ground near the surface can be described as follows: In the surface layer to a depth of about 1 metre, the temperature is very sensitive to sunlight and weather, In the shallow layer to a depth of about 8–20 metres depending on soil type, the thermal mass of the ground causes temperature variation to decrease exponentially with depth until it is close to the local annual average air temperature; it also lags behind the surface temperature, so that the peak temperature is about 6 months after the surface peak temperature Below that, in the deeper layer, the temperature is effectively constant, rising about 0.025 °C per metre according to the geothermal gradient. The "penetration depth" is defined as the depth at which the temperature variable is less than 0.01 of the variation at the surface, and this depends on the type of soil: The heat pump was described by Lord Kelvin in 1853 and developed by Peter Ritter von Rittinger in 1855.
