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A generating plant at the largest geothermal field in the world, the Geysers, near San Francisco.  Photo courtesy of Pacific Gas & Electric.

Electricity from:
Geothermal Energy

The heat from the earth's own molten core can be converted into electricity. This core consists primarily of extremely high temperature liquid rock known as magma. This "geothermal" heat circulates within the rock or is transferred to underground reservoirs of water, which also circulate under the earth's crust. Because of the near limitless ability of the earth to produce magma, and the continuous transfer of heat between subsurface rock and water, geothermal energy is considered a renewable resource.

Geothermal resources have been harnessed as an energy source since the dawn of civilization, when natural hot springs were first used for cooking and bathing. The geothermal resources tapped to generate electricity are far more intense than those used for space heating and can reside as deep as 10,000 feet below the earth's surface. Capital costs for the construction of geothermal power plants are much higher than for large coal-fired plants or new natural gas turbine technologies. But geothermal plants have reasonable operation and maintenance costs and no fuel costs. Though more expensive than wind power in most cases, new geothermal electricity generation facilities are increasingly competitive with fossil options.

Geothermal plants can operate around-the-clock, which increases their value from a reliability point-of-view, unlike some intermittent renewable fuels such as solar and wind.

Geothermal electricity generation technologies consist of either "flash" technology or "binary" technology. With flash technology, water from 300 to 700 degrees Fahrenheit, but still in liquid form, is piped from its highly pressurized underground reservoir into a geothermal facility. Once this super-heated water is released, it flashes into steam that then drives a conventional turbine generator. With binary technology, underground reservoir waters of lower temperatures are used for flashing. Heat from geothermal water is transferred to a second (binary) liquid, which flashes into vapor upon heating, and that vapor is used to drive turbines. (With dry steam technologies - a much rarer fuel source but the one used in the world's largest geothermal power plant at The Geysers in northern California -- highly pressurized geothermal vapor is used directly to drive a turbine.)

What are the environmental impacts?

Flash technologies allow the geothermal fluid to expand and release gases into the atmosphere when the steam is created. Binary technology keeps the geothermal fluid contained, using heat exchangers to capture heat to provide steam. Though these air emissions represent tiny quantities and generally do not pose any serious environmental threat, the chemical characteristics of geothermal resources are highly site-specific. Dissolved gases usually include carbon dioxide (CO2), methane, hydrogen sulfide, ammonia, nitrogen and hydrogen.

Groundwater contamination, which can be easily prevented, is the principal pollution concern.

The disposal of water and wastewater may cause significant pollution of surface waters and ground water supplies. Still, used geothermal fluids are generally collected and re-injected. This maintains pressures in underground reservoirs, but also allows for recycling and reuse.

The best geothermal resources are sometimes located at remote sites that may have significant wilderness, scenic or recreation value. While requiring relatively little land itself, the siting of a geothermal plant - like remote wind farms -- may cause land impacts when new transmission lines are connected to power plants in these rural regions.


Additional Information:

U.S. Department of Energy - Geothermal Power http://www.eren.doe.gov/geothermal/

Idaho National Engineering and Environmental Laboratory (INEEL) - Geothermal Program http://geothermal.id.doe.gov/

Geothermal Resources Council http://www.geothermal.org/index.html

Geothermal Resource Information Clearinghouse http://rredc.nrel.gov/geothermal/

Union of Concerned Scientists - USA: Geothermal http://ucsusa.org/clean_energy/renewable_energy/page.cfm?pageID=81

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