Geothermal

Posted: 27 November 2007

Geothermal energy is the thermal energy contained in the rock and the fluid within the rock in the earth's crust. It is believed that the ultimate source of geothermal energy is radioactive decay occurring deep within the earth.

Geothermal resources come in five forms: hydrothermal fluids, hot dry rock, geopressured brines, magma, and ambient ground heat. Of these five, only hydrothermal fluids have been developed commercially for power generation. Three technologies can be used to convert hydrothermal fluids to electricity. The type of conversion used depends on the state of the fluid (whether steam or water) and its temperature.

Geysers Geothermal Power Plant California
Geysers Geothermal Power Plant California
Twenty plants are still operating at the Geysers steam field in Northern California, the largest single source of geothermal power in the world. Wastewater from nearby cities is injected into the field, providing environmentally safe disposal and increased steam to power plants. Photo © 2000 Geothermal Education Office.
Steam resources are the easiest to use, but they are rare. One plant, The Geysers, located in Northern California, began producing electricity in 1960. It is now the largest single source of geothermal power in the world. Hot water plants, using high- or moderate-temperature geothermal fluids, are a relatively recent development. However, hot water resources are much more common than steam, and hot water plants are now the major source of geothermal power world-wide.

Uses for low and moderate temperature resources can be divided into two categories: direct use and ground-source heat pumps. Direct use 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. 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 to 38°C, the heat pump transfers heat from the soil to the house in winter and from the house to the soil in summer.

  • The current production of geothermal energy from all uses places it third among renewables, following hydroelectricity and biomass, and ahead of solar and wind.

  • Geothermal energy has by far and away the highest technical potential of all renewable energy sources - 5,000 exajoules (EJ) per year (100 times more than hydro power) out of a total of 7,600 EJ for all renewables.
  • Since the first geothermally-generated electricity in the world was produced at Larderello, Italy, in 1904, the use of geothermal energy for electricity has grown worldwide to over 9,000 MW in 25 countries around the world. The United States alone produces 2,540 MW of electricity from geothermal energy, electricity comparable to burning 60 million barrels of oil each year.

    Geothermal power station, Fang, Thailand
    Geothermal power station, Fang, Thailand
    Geothermal power station, Fang, Thailand. Photo © 2000 Geothermal Education Office
  • The Philippines is the world's second largest geothermal energy producer and the largest geothermal energy user. Costa Rica, El Salvador, Kenya, and Nicaragua generate 10 to 20 per cent of their electricity from geothermal resources.

  • Iceland generates nearly 40 per cent of its energy supplies from geothermal sources.

  • Hot dry rock offers enormous potential for electricity production, by circulating water through man-made fractures in the hot rock.

  • Geopressured brines - hot, pressurised, methane-rich waters found in sedimentary basins 10,000 to 20,000 feet below the surface - and magma - molten or partially molten rock within the Earth's crust - may also someday provide electricity.