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About Geothermal

General information


Is the use of geothermal heat to generate electricity. It is often referred to as a form of renewable energy, but because the heat at any location can eventually be depleted it is by definition not strictly renewable. Geothermal comes from the Greek words geo, meaning earth, and therme, meaning heat. Geothermal literally means 'earth heat'. Geothermal-generated electricity was first produced at Larderello, Italy, in 1904


Geothermal energy is a form of renewable energy derived from heat deep in the earth’s crust. This heat is brought to the near-surface by thermal conduction and by intrusion into the earth's crust of molten magma originating from great depth. As groundwater is heated, geothermal energy is produced in the form of hot water and steam. The heated groundwater can be used for direct heating of homes and greenhouses, for vegetable drying, and for a number of other uses. These are known as direct uses of geothermal energy.

Geothermal energy is also used for electricity production. Geothermal power generation is used today throughout the world where good geothermal resources exist, including many locations in the western United States. The REPP Geothermal Issue Brief on this page provides a general background on the fundamentals of geothermal power, including discussions of the technical, economic, and policy dimensions of geothermal power worldwide. Economic data and current U.S. geothermal policy help illustrate the concepts of the issue brief.

Calculations show that the earth, originating from a completely molten state, would have cooled and become completely solid many thousands of years ago without an energy input in addition to that of the sun. It is believed that the ultimate source of geothermal energy is radioactive decay occurring deep within the earth (Burkland, 1973).
In most areas, this heat reaches the surface in a very diffuse state. However, due to a variety of geological processes, some areas, including substantial portions of many western states, are underlain by relatively shallow geothermal resources.

These resources can be classified as low temperature (less than 90°C or 194°F), moderate temperature (90°C - 150°C or 194 - 302°F), and high temperature (greater than 150°C or 302°F). The uses to which these resources are applied are also influenced by temperature. The highest temperature resources are generally used only for electric power generation. Current U.S. geothermal electric power generation totals approximately 2200 MW or about the same as four large nuclear power plants. Uses for low and moderate temperature resources can be divided into two categories: direct use and ground-source heat pumps.

Direct use, as the name implies, involves using the heat in the water directly (without a heat pump or power plant) for such things as heating of buildings, industrial processes, greenhouses, aquaculture (growing of fish) and resorts. Direct use projects generally use resource temperatures between 38°C (100°F) to 149°C (300°F). Current U.S. installed capacity of direct use systems totals 470 MW or enough to heat 40,000 average-sized houses.

Ground-source heat pumps use the earth or groundwater as a heat source in winter and a heat sink in summer. Using resource temperatures of 4°C (40°F) to 38°C (100°F), the heat pump, a device which moves heat from one place to another, transfers heat from the soil to the house in winter and from the house to the soil in summer. Accurate data is not available on the current number of these systems; however, the rate of installation is thought to be between 10,000 and 40,000 per year.

The current production of geothermal energy from all uses places third among renewables, following hydroelectricity and biomass, and ahead of solar and wind. Despite these impressive statistics, the current level of geothermal use pales in comparison to its potential. The key to wider geothermal use is greater public awareness and technical support.