Geothermal energy is the heat from the Earth. It's clean and
sustainable. Resources of geothermal energy range from the shallow
ground to hot water and hot rock found a few miles beneath the Earth's
surface, and down even deeper to the extremely high temperatures of
molten rock called magma.
Almost everywhere, the shallow ground or upper 10 feet of the Earth's
surface maintains a nearly constant temperature between 50° and 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 (ductwork), and a heat exchanger-a system
of pipes buried in the shallow 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 a free source of 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 a
turbine/generator, while others use the hot water to boil a working
fluid that vaporizes and then turns a turbine. Hot water near the
surface of Earth 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 at depths of 3 to 5 miles everywhere
beneath the Earth's surface 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 from another well. Currently, there are no commercial applications
of this technology. Existing technology also does not yet allow
recovery of heat directly from magma, the very deep and most powerful
resource of geothermal energy.
Many technologies have been developed to take advantage of geothermal
energy - the heat from the earth. NREL performs research to develop and
advance technologies for the following geothermal applications:
Geothermal Energy Technologies:
- 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.