Changes

Economically significant concentrations of geothermal energy occur now where high temperatures (40° to more than 380°C; 104° to more than 716°F) are found in [[Porosity|porous]] and [[Permeability|permeable]] rocks at shallow depths (less than 3000 m; about 10,000 ft). The geothermal energy is stored in both the solid rock and the water or steam-filling pores and [[fracture]]s. The steam or hot water are used mainly as the fuel for the operation of electricity-generating turbines or for space heating.

Economically significant concentrations of geothermal energy occur now where high temperatures (40° to more than 380°C; 104° to more than 716°F) are found in [[Porosity|porous]] and [[Permeability|permeable]] rocks at shallow depths (less than 3000 m; about 10,000 ft). The geothermal energy is stored in both the solid rock and the water or steam-filling pores and [[fracture]]s. The steam or hot water are used mainly as the fuel for the operation of electricity-generating turbines or for space heating.

[[file:St54Figure48.JPG|thumb|200px|World geothermal electricity generation.<ref name=Salvador_2005>Salvador, A., 2005, Energy-A historical perspective and 21st century forecast: AAPG Studies in Geology 54, 208 p.</ref>]]

[[file:St54Figure48.JPG|thumb|200px|World geothermal electricity generation.<ref name=Salvador_2005>Salvador, A., 2005, Energy—A historical perspective and 21st century forecast: [http://store.aapg.org/detail.aspx?id=ADD-732 AAPG Studies in Geology 54], 208 p.</ref>]]

There are, therefore, three main requirements for the commercial development of geothermal resources: shallow high temperatures, rocks with good permeability, and sufficient volumes of water.

There are, therefore, three main requirements for the commercial development of geothermal resources: shallow high temperatures, rocks with good permeability, and sufficient volumes of water.