Geothermal Heating and Cooling
Geothermal heating is the direct use of geothermal energy for heating applications. Thermal efficiency is high since no energy conversion is needed, but capacity factors tend to be low since the heat is mostly needed in the winter.
Geothermal heating relies on an energy exchange between the air within the building being heated and the ground. Below 30 feet the earth's temperature is fairly constant (generally around 50ºF). During the summer when the ambient temperature of the building exceeds that of the ground heat pumps are used to pump heat from the building in to the transfer medium (typically water with small amounts of ethanol or glycol) and pumped through narrow pipes into the ground so that the heat can be dissipated in the earth. When the ambient temperature falls below the ground temperature the process works in reverse. Heat pumps extract heat from the ground and use it to heat the building.
Direct geothermal heating is far more efficient than geothermal electricity generation and has less demanding temperature requirements, so it is viable over a large geographical range. If the shallow ground is hot but dry, air or water may be circulated through earth tubes which act as heat exchangers with the ground.
Geothermal Heat Pumps
In areas where the shallow ground is too cold to provide comfort directly, it is still warmer than the winter air. The thermal inertia of the shallow ground retains solar energy accumulated in the summer. In many cases, this heat can be extracted with a geothermal heat pump more efficiently than it can be generated by conventional furnaces to provide space heating and air conditioning.
The heat pump forces the transfer of heat from the ground to the application (i.e. space heater or air conditioner). Ground-source heat pumps take advantage of moderate ground temperatures as opposed to air-source heat pumps that draw from the colder outside air and require more energy. Closed loop geothermal heat pumps circulate a carrier fluid (usually a water/antifreeze mix) through pipes buried in the ground. As the fluid circulates underground it absorbs heat from the ground. The now warmer fluid returns and passes through the heat pump which extracts the heat from the fluid. The re-chilled fluid is sent back through the ground thus continuing the cycle.
The heat extracted by the heat pump is used to heat the target location. By 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 same relatively cool soil (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 smaller temperature difference and this leads to higher efficiency and lower energy use.