Geothermal Energy

A

Flash Steam Plants

Most operating geothermal power plants are flash steam plants. In a flash steam plant, hot water from wells is piped into the plant, where, released from the high pressure of its underground location, some of the hot water boils (flashes) to steam. The force of the expanding steam is used to spin a turbine generator, which produces electricity. After turning the turbine, the geothermal water, along with the condensed steam, is piped back down into the reservoir to be reheated so it can be used again.

B

Dry Steam Plants

While most geothermal reservoirs produce hot water, a small number produce mostly steam. Steam from such a reservoir is used in a dry steam plant. In such a plant, the steam is piped directly through a turbine generator.

The first geothermal power plant, built at Larderello, Italy, in 1904, was a dry steam plant. The Larderello steam field is still producing electricity today. The largest producing dry steam geothermal reservoir in the world is located at The Geysers Geothermal Field in northern California. This dry steam geothermal reservoir, which supports 20 operating power plants, produces about 1,000 megawatts of electricity, enough to supply power to nearly 2 million people.

C

Binary Power Plants

In a binary power plant, heat from geothermal water is transferred through heat exchangers to a second liquid (called a working fluid, usually isobutane) contained in adjacent but separate pipes. Heat transferred from the geothermal water converts this low-boiling point working fluid into vapor, which powers a turbine generator. After turning the turbine generator, the working fluid is condensed back into liquid, which is repeatedly vaporized by the geothermal heat. After heating the working liquid, the geothermal water is piped back into the reservoir.

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The binary working fluid vaporizes at temperatures lower than is necessary to vaporize water. As a result, binary power plants can generate electricity using geothermal reservoirs of lower temperature, increasing the number of geothermal reservoirs in the world that can potentially be used for generating electricity. Binary power plants are generally more expensive to build and operate than flashed steam plants. However, binary power plants use geothermal heat and water more efficiently and have no emissions.

V

Direct Use of Geothermal Water

In addition to generating electricity, geothermal water is used directly in spas (balneology), to heat greenhouses (agriculture), and to speed the growth of fish and prawns (see aquaculture). The heat from geothermal water is used for industrial processes and for space heating in homes and other buildings. People in over 35 countries have developed geothermal water for such purposes.

A

Industry

Industrial uses of the heat from geothermal water include manufacturing paper, pasteurizing milk, and drying timber and other agricultural products. Geothermal waters are used in mining to speed the extraction of gold and silver from ore, and are also piped under sidewalks and roads to keep them from icing over in freezing weather.