A geothermal heat pump system is a heating and/or an air conditioning system that uses the Earth's ability to store heat in the ground and water thermal masses. These systems operate based on the stability of underground temperatures; the ground a few feet below surface has a very stable temperature throughout the year, depending upon location's annual climate. A geothermal heat pump uses that available heat in the winter and puts heat back into the ground in the summer. A geothermal system differs from a conventional furnace or boiler by its ability to transfer heat versus the standard method of producing heat. As energy costs continue to rise and pollution concerns continue to be a hot topic, geothermal systems may hold a solution to both of these concerns.
Geothermal heat pumps are also known as "geo-exchange" systems (a term created by an industry association) and "ground-source heat pumps." The latter term is useful as it clearly distinguishes the technology from air-source heat pumps. Geothermal heat pumps, which can be used in almost any region, should also be distinguished from geothermal heating. Geothermal heating is used in areas where exceptionally high underground temperatures, such as those at hot springs and steam vents, are used to heat indoor spaces without the use of a heat pump.
A geothermal heat pump is a heat pump that uses the Earth as either a heat source, when operating in heating mode, or a heat sink, when operating in cooling mode.
Geothermal heat pumps can be characterised as having one or two loops. The heat pump itself, explained more fully in the article on heat pumps, consists of a loop containing refrigerant. The refrigerant is pumped through a vapor-compression refrigeration cycle that moves heat from a cooler area to a warmer one.
In a single loop system, the copper tubing refrigerant loop actually leaves the heat pump appliance cabinet and goes out of the house and under the ground and directly exchanges heat with the ground before returning to the appliance. Hence the name "direct exchange" or DX. In a double loop system, the refrigerant loop exchanges heat with a secondary loop made of plastic pipe containing water and anti-freeze (propylene glycol, denatured alcohol or methanol). After leaving the heat exchanger, the plastic pipe then leaves the appliance cabinet, and goes out of the house and under the ground before returning, so the water is exchanging heat with the ground. This is known as a water-source system. Secondary loops are popular for ground use because they are not pressurized, so cheap plastic tubing can be used, and because they reduce the amount of expensive refrigerant required. Copper loop DX systems are gaining acceptance due to their increased efficiency and lower installation costs.