Geothermal energy (from the Greek roots geo, meaning earth, and from thermos, meaning heat) is energy made by heat inside the Earth's crust.

Although the Sun does heat the surface of the Earth, the heat from inside the Earth is not caused by the Sun. The geothermal energy of the Earth's crust comes 20% from the original formation of the planet, and 80% from the radioactive decay of minerals.  The Earth is hottest at its core and, from the core to the surface, the temperature gets gradually cooler.

Resources of geothermal energy range from the shallow ground to hot water and hot rock found a few miles or kilometers beneath the Earth's surface, and down even deeper to the extremely high temperatures of molten rock called magma. It has been used for bathing from Paleolithic times, but is now better known for making electricity.

All over the world, geothermal energy was used to make about 10 gigawatts of electricity in the year 2007, about 0.3% of the electricity used around the world.  When used to generate electricity, geothermal power plants typically offer constant output.

Geothermal energy is also used directly for district heating, or in other heating and cooling applications. Buildings in Iceland are heated in this way from the country's many geothermal sites.

Power plants and thermal applications of geothermal energy are mature technologies, whereas enhanced geothermal systems (EGS) projects are a new type of application.

Almost everywhere, the ground 10 feet (3m) below 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 uses power to remove 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 uses power again to move 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. The power to run the heat pump comes from another source.

Geothermal energy escapes as hot water at many hot springs or as steam at geysers.

In the United States, most geothermal reservoirs of hot water are 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 (5–8 km) 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 rocks, and drawing off the heated water from another well. Currently, nobody uses this method commercially. Existing technology also does not yet allow recovery of heat directly from magma, the very deep and most powerful resource of geothermal energy.

References

Other websites 

 Alliant Geothermal Energy
 Bassfeld Technology Transfer - Introduction to Geothermal Power Generation (3.6 MB PDF file) 
 Energy Efficiency and Renewable Energy - Geothermal Technologies Program
 Geothermal Energy Association
 International Energy Agency Geothermal Energy Homepage
 MIT-led panel backs geothermal energy source 
 MIT - The Future of Geothermal Energy (14 MB PDF file) 

 
Alternative energy