These heat pumps are used to supplement heating and air conditioning systems in buildings, both large and small. They only require an ambient ground temperature of around 55F rather than higher temperatures. They are perhaps the most under-appreciated energy-saving devices known.
They are variously known as geothermal heat pumps, geoexchange heat pumps, ground source heat pumps, and geosource heat pumps. Delta-Montrose Rural Electric Association is a leader in promoting the installation of these systems in Colorado. Although it is best to install the systems when a house is being built, a great advantage is that existing houses and buildings can be retrofitted with these systems.
Retrofitting the Governor’s Mansion with a geoexchange system lowered energy consumption by 70%. This success led to geoexchange being incorporated into the renovation of the Capitol. Users not only like the lower costs, but assert that it creates a more comfortable and enjoyable environment in their homes.
Heat pumps are systems that transfer heat and can transfer heat in the opposite direction to the temperature gradient, that is from a colder temperature to a hotter temperature. A common example of heat pump use is a refrigerator: heat is transferred from the freezer to the outside of the refrigerator, typically coils on the back or the underside.
Most building heat pumps are reversible and can operate in either direction. In summer they transfer heat out of the building. In winter they transfer heat into the building. Air-source heat pumps exchange heat on the outside with the air. This transfer is not very efficient because air has a low density and varies considerably in temperature throughout the day and throughout the year. A more efficient heat exchange medium is the ground. A few feet below the surface the daily and annual temperature changes are very small and earth materials have good thermal properties for heat exchange. In the 1970’s heat pumps that exchanged heat with the ground were called geothermal heat pumps. This name has been abandoned, however, as it implies that the heat pumps use the heat of the earth. The heat pumps only use the ground as a medium for exchanging heat: in winter they take heat from the ground; in summer they transfer heat into the ground. These heat pumps are now most commonly known as ground-source heat pumps, or geoexchange heat pumps.
Diagrams showing basic operations or ground-source heat pump in summer (upper) and winter (lower). (Modified from Geothermal Heat Pump Consortium; www.geoexchange.org)
Heat transfer with the ground occurs in loops of pipe through which fluid circulates. The fluid is usually water mixed with antifreeze to prevent freezing. The size of the loop is calculated on the basis of the heat exchange needs of the building: a small house typically requires one 300-foot loop; a very large house might require four or more loops. If adequate ground area is available, the loops are installed horizontally in 4- to 6-foot deep tranches. Without adequate ground area, the loops are installed in vertical boreholes. For efficient heat transfer the ground loops should have good thermal contact with the ground and should be thermally grouted. Heat transfer is most effective when the loop is placed below the water table or in wet or saturated ground.