Geothermal energy is heat from the Earth. It’s clean and sustainable. Geothermal energy resources can range from the temperature effects found in the first ten feet underground to the hot water and the hot rock found a few miles beneath the Earth’s surface, or even deeper to the extremely high temperatures of molten rock, or magma.
In most locations the upper 10 feet of the Earth’s surface maintains a nearly constant temperature of between 50° to 60°F (10° and 16°C). Geothermal heat pumps can tap into this resource to heat and cool buildings. A geothermal heat pump system consists of a heat pump, an air delivery system, and a heat exchanger, (a system of pipes buried in the ground near the building). In the winter, the heat pump removes heat from the heat exchanger and pumps it into the indoor air delivery system. In the summer, the process is reversed, and the heat pump moves heat from the indoor air into the heat exchanger. The heat removed from the indoor air during the summer can also be used to provide hot water.
In the United States, most geothermal reservoirs of hot water are located in the western states, Alaska, and Hawaii. Wells can be drilled into underground reservoirs for the generation of electricity. Some geothermal power plants use the steam from a reservoir to power turbines or generators, while others use the hot water to boil a working fluid that vaporizes and then turns a turbine. Hot water near the Earth’s surface can be used directly for heat. Direct-use applications include heating buildings, growing plants in greenhouses, drying crops, heating water at fish farms, and several industrial processes such as pasteurizing milk.
Hot dry rock resources occur beneath the Earth’s surface at depths of 3 to 5 miles and at lesser depths in certain areas. Access to these resources involves injecting cold water down one well, circulating it through hot fractured rock, and drawing off the heated water through another well. Currently, there are no commercial applications of this technology. As well, existing technology does not yet allow for the recovery of heat directly from magma, the deepest and most powerful geothermal energy resource.
Many technologies have been developed to take advantage of geothermal energy. NREL currently performs research to develop and advance technologies for the following geothermal applications:
Geothermal Electricity Production
Generating electricity from the earth’s heat.
Geothermal Direct Use
Producing heat directly from hot water within the earth.
Geothermal Heat Pumps
Using the shallow ground to heat and cool buildings.
