Geothermal Heating - Geothermal Heat Pumps

Geothermal Heat Pumps

Even in regions without large high temperature geothermal resources, a geothermal heat pump can still provide space heating and air conditioning. Like a refrigerator or air conditioner, these systems use a heat pump to force the transfer of heat from the ground to the application. In theory, heat can be extracted from any source, no matter how cold, but a warmer source allows higher efficiency. A ground-source heat pump uses the shallow ground or ground water (typically starting at 10–12 °C, 50–54 °F) as a source of heat, thus taking advantage of its seasonally moderate temperatures. In contrast, an air-source heat pump draws heat from the air (colder outside air) and thus requires more energy.

Closed loop geothermal heat pumps circulate a carrier fluid (usually a water/antifreeze mix) through pipes buried in the ground. Single-home systems can be "vertical loop field" systems with bore holes 50-400 feet deep or, if adequate land is available for extensive trenches, a "horizontal loop field" is installed approximately six feet subsurface. As the fluid circulates underground it absorbs heat from the ground and, on its return, the now warmer fluid passes through the heat pump which uses electricity to extract the heat from the fluid. The re-chilled fluid is sent back into the ground thus continuing the cycle. The heat extracted and that generated by the heat pump appliance as a byproduct is used to heat the house. The addition of the ground heating loop in the energy equation means that more heat is generated than if electricity alone had been used directly for heating.

Switching the direction of heat flow, the same system can be used to circulate the cooled water through the house for cooling in the summer months. The heat is exhausted to the relatively cooler ground (or groundwater) rather than delivering it to the hot outside air as an air conditioner does. As a result, the heat is pumped across a larger temperature difference and this leads to higher efficiency and lower energy use.

This technology makes geothermal heating economically viable in any geographical location. In 2004, an estimated million ground source heat pumps with a total capacity of 15 GW extracted 88 PJ of geothermal energy for space heating. Global geothermal heat pump capacity is growing by 10% annually.