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Geothermal Energy

Geothermal energy is the heat from the Earth.

The Geysers, Calistoga, CA
Photo credit: National Renewable Energy Laboratory
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.

Geothermal energy has tremendous potential for producing electricity. About 8,000 megawatts (MW) of geothermal electricity are currently produced around the world, including about 2,800 MW of capacity in the United States. Today's technology produces electricity from hydrothermal (hot water/steam) resources. In the future, it may be possible to use the heat of the deep, hot, dry rock formations of Earth's crust, and possibly the even deeper, almost unlimited energy in Earth's molten rock, or magma.

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.

Economics and Future Prospects

For every dollar invested on geothermal energy, the resulting growth of output to the U.S. economy is $2.50. The United States is the world's biggest producer of commercial geothermal energy. To date there are 18 natural geothermal resources in the United States in California, Nevada, Hawaii and Utah. In total, they supply around 2,700 megawatts (MW) of electricity. In California, geothermal energy supplies 5% of total electricity demand and 7% of that in Northern California, which boasts the site of the largest geothermal plant (the Geysers, 620 MW) in the world. Apart from these 18 geothermal plants, the USA has more than 1 million geothermal heat pumps, supplying an additional 3700 MW for heating and cooling.

Real levelized costs for geothermal electricity generation are $0.045-$0.07/KWh, which is competitive with some fossil fuel facilities, without the pollution. The lowest cost of geothermal electricity is approximately $0.015/KWh. At the Geysers, the largest complex of geothermal power plants in the world located in Northern California, power is sold at $0.03 to $0.035/KWh. Some geothermal power plants can charge more per KWh during some time periods, because of incentives related to reliability of generation and power provided during peak demand. The cost of generating power from geothermal resources has decreased about 25% over the past two decades.

The goal of the geothermal industry and the U.S. Department of Energy is to achieve a geothermal energy life-cycle cost of electricity of $0.03/KWh. It is anticipated that costs in this range will result in about 10,000 MW of new capacity installed by U.S. firms within the next decade.

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