A ground source heat pump (also geothermal heat pump) is a heating/cooling system for buildings that use a 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 (GHPs), as they are commonly termed in North America—are among the most energy-efficient technologies for providing HVAC and water heating, using less energy than that consumed by 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 of installing ground loops over large areas or of drilling bore holes. Air-source heat pumps have lower set-up costs but have a lower CoP in very cold or very hot weather.
Thermal properties of the ground 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 meter, the temperature is very sensitive to sunlight and weather. In the shallow layer to a depth of about 8–20 meters (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. This also depends on the type of soil:
History The heat pump was described by Lord Kelvin in 1853 and developed by Peter Ritter von Rittinger in 1855. Heinrich Zoelly had patented the idea of using it to draw heat from the ground in 1912. After experimentation with a freezer, Robert C. Webber built the first direct exchange ground source heat pump in the late 1940s; sources disagree, however, as to the exact timeline of his invention. The first successful commercial project was installed in the Commonwealth Building (Portland, Oregon) in 1948, and has been designated a National Historic Mechanical Engineering Landmark by ASME. Professor Carl Nielsen of Ohio State University built the first residential open loop version in his home in 1948. As a result of the 1973 oil crisis, ground source heat pumps became popular in Sweden and have since grown slowly in worldwide popularity as the technology has improved. Open loop systems dominated the market until the development of polybutylene pipe in 1979 made closed loop systems economically viable. As of 2004, there are over a million units installed worldwide, providing 12 GW of thermal capacity with a growth rate of 10% per year. Each year (as of 2011/2004, respectively), about 80,000 units are installed in the US and 27,000 in Sweden. In Finland, a geothermal heat pump was the most common heating system choice for new detached houses between 2006 and 2011 with market share exceeding 40%. In 2021, heat pumps accounted for 10% of global heating equipment sales. In the United Kingdom, the 2022 Boiler Upgrade Scheme has driven demand for ground source heat pumps. In 2023, 2,469 ground source heat pumps were installed in the UK. The scheme closes in 2027.
Arrangement
Internal arrangement
A heat pump is the central unit for the building's heating and cooling. It usually comes in two main variants: Liquid-to-water heat pumps (also called water-to-water) are hydronic systems that carry heating or cooling through the building through pipes to conventional radiators, underfloor heating, baseboard radiators and hot water tanks. These heat pumps are also preferred for pool heating. Heat pumps typically only heat water to about 55 °C (131 °F) efficiently, whereas boilers typically operate at 65–95 °C (149–203 °F) . The size of radiators designed for the higher temperatures achieved by boilers may be too small for use with heat pumps, requiring replacement with larger radiators when retrofitting a home from boiler to heat pump. When used for cooling, the temperature of the circulating water must normally be kept above the dew point to ensure that atmospheric humidity does not condense on the radiator. Liquid-to-air heat pumps (also called water-to-air) output forced air, and are most commonly used to replace legacy forced air furnaces and central air conditioning systems. There are variations that allow for split systems, high-velocity systems, and ductless systems. Heat pumps cannot achieve as high a fluid temperature as a conventional furnace, and require a higher volume flow rate of air to compensate.
Ground heat exchanger
Ground source heat pumps employ a ground heat exchanger in contact with the ground or groundwater to extract or dissipate heat. Pipework for the ground loop is typically made of high-density polyethylene pipe and contains a mixture of water and anti-freeze (propylene glycol, denatured alcohol or methanol). Monopropylene glycol has the least damaging potential when it might leak into the ground, and is, therefore, the only allowed anti-freeze in ground sources in an increasing number of European countries.
Horizontal A horizontal closed loop field is composed of pipes that are arrayed in a plane in the ground. A long trench, deeper than the frost line, is dug and U-shaped or slinky coils are spread out inside the same trench. Shallow 3–8-foot (0.91–2.44 m) horizontal heat exchangers experience seasonal temperature cycles due to solar gains and transmission losses to ambient air at ground level. These temperature cycles lag behind the seasons because of thermal inertia, so the heat exchanger will harvest heat deposited by the sun several months earlier, while being weighed down in late winter and spring, due to accumulated winter cold. Systems in wet ground or in water are generally more efficient than drier ground loops since water conducts and stores heat better than solids in sand or soil. If the ground is naturally dry, soaker hoses may be buried with the ground loop to keep it wet.
Vertical
… excerpt ends here. Continue reading the full article.
