| Geothermal energy: | | | | for direct-use systems are widespread |
| Geothermal power uses the natural sources of | | | | throughout the western United States. |
| heat inside the Earth to produce heat or | | | | Geothermal power plants convert hydrothermal |
| electricity. Currently, most geothermal power is | | | | fluids (hot water or steam) to electricity. The |
| generated using steam or hot water from | | | | oldest type of geothermal power plant uses |
| underground. Geothermal power generation | | | | steam, accessed through deep wells, to directly |
| produces few emissions and the power source is | | | | drive a turbine to produce electricity. Flash steam |
| continuously available. | | | | plants are the most common type of geothermal |
| There are three geothermal technologies currently | | | | power plants in operation today. They use |
| in use in the United States: direct-use systems, | | | | extremely hot water (above 300 degrees F (149 |
| use of deep reservoirs to generate electricity, and | | | | degrees C)), which is pumped under high pressure |
| geothermal heat pumps. | | | | to the generation equipment at the surface. |
| In direct-use geothermal systems, a well is drilled | | | | Estimated subterranean temperatures at a depth |
| into a geothermal reservoir to provide a steady | | | | of 6 kilometers. The hot water is vaporized and |
| stream of hot water. The water is brought up | | | | the vapor in turn drives turbines to generate |
| through the well, and a mechanical system-piping, | | | | electricity. Binary-cycle geothermal power plants |
| a heat exchanger, and controls-delivers the heat | | | | use moderate-temperature water (100-300 |
| directly for its intended use. A disposal system | | | | degrees F (38-149 degrees C)). The water is |
| then either injects the cooled water underground | | | | used to vaporize a second fluid that has a much |
| or disposes of it in a surface storage pond. | | | | lower boiling point than water. The vapor from |
| Geothermal hot water is used for heating | | | | this second fluid is then used to drive the turbines |
| buildings, raising plants in greenhouses, drying | | | | to produce electricity. California, Hawaii, Nevada, |
| crops, heating water for fish farms, or for | | | | and Utah currently have operating geothermal |
| industrial processes, at hundreds of sites around | | | | power plants. |
| the country. Geothermal reservoirs appropriate | | | | |