All geothermal power plants use steam to turn large turbines, which run electrical generators. In the Geysers Geothermal area, dry steam from below ground is used directly in the steam turbines. In other areas of the state, super-hot water is "flashed" into steam within the power plant, and that steam turns the turbine.
Direct Dry Steam
Steam plants use hydrothermal fluids that are primarily steam. The steam goes directly to a turbine, which drives a generator that produces electricity. The steam eliminates the need to burn fossil fuels to run the turbine. (Also eliminating the need to transport and store fuels!)
This is the oldest type of geothermal power plant. It was first used at Lardarello in Italy in 1904. Steam technology is used today at The Geysers in northern California, the world's largest single source of geothermal electricity. These plants emit only excess steam and very minor amounts of gases.
Flash and Double Flash Cycle
Hydrothermal fluids above 360°F (182°C) can be used in flash plants to make electricity.
Fluid is sprayed into a tank held at a much lower pressure than the fluid, causing some of the fluid to rapidly vaporize, or "flash." The vapor then drives a turbine, which drives a generator.
If any liquid remains in the tank, it can be flashed again in a second tank (double flash) to extract even more energy.
Most geothermal areas contain moderate-temperature water (below 400°F). Energy is extracted from these fluids in binary-cycle power plants.
Hot geothermal fluid and a secondary (hence, "binary") fluid with a much lower boiling point than water pass through a heat exchanger. Heat from the geothermal fluid causes the secondary fluid to flash to vapor, which then drives the turbines.
Because this is a closed-loop system, virtually nothing is emitted to the atmosphere. Moderate-temperature water is by far the more common geothermal resource, and most geothermal power plants in the future will be binary-cycle plants.
Text and graphics from U.S. Department of Energy's Energy Efficiency and Renewable Energy Program.