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Electricity Generation

Photo of geothermal power plant.

Geothermal power plant in the Imperial Valley, California.

A geothermal resource requires fluid, heat and permeability in order to generate electricity:

Fluid—Sufficient fluid must exist naturally or be pumped into the reservoir.

Heat—The earth's temperature naturally increases with depth and varies based on geographic location.

Permeability—In order to access heat, the fluid must come into contact with the heated rock, either via natural fractures or through stimulating the rock.

Conventional hydrothermal resources contain all three elements naturally. In addition to conventional hydrothermal resources, power can be generated from low temperature, coproduced and other permeable sedimentary resources.

Geothermal systems can also be engineered or enhanced to access the earth's heat where subsurface fluid and permeability are lacking. These systems are called Enhanced Geothermal Systems.

Power Plants

Power plants use steam produced from geothermal reservoirs to generate electricity. There are three geothermal power plant technologies being used to convert hydrothermal fluids to electricity—dry steam, flash steam and binary cycle. The type of conversion used (selected in development) depends on the state of the fluid (steam or water) and its temperature.

Dry Steam Power Plant

Photo of dry steam power plants.

Dry steam power plants at The Geysers in California.

Dry steam plants use hydrothermal fluids that are primarily steam. The steam travels 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). These plants emit only excess steam and very minor amounts of gases.

Dry steam power plants systems were the first type of geothermal power generation plants built (they were first used at Lardarello in Italy in 1904). Steam technology is still effective today at currently in use at The Geysers in northern California, the world's largest single source of geothermal power.

 Illustration of a Dry Steam Power Plant - Geothermal steam comes up from the reservoir through a production well.  The steam spins a turbine, which in turn spins a generator that creates electricity.  Excess steam condenses to water, which is put back into the reservoir via an injection well.

Flash Steam Power Plant

Flash steam plants are the most common type of geothermal power generation plants in operation today. Fluid at temperatures greater than 360°F (182°C) is pumped under high pressure into a tank at the surface held at a much lower pressure, 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 to extract even more energy.

Illustration of a Flash Steam Power Plant - Pressurized geothermal hot water comes up from the reservoir through a production well.  The water enters a flash tank where it depressurizes and flashes to steam.  The steam then spins the turbine, which in turn spins a geneator that creates electricity.  Excess steam condenses to water, which is put back into the reservoir via an injection well.

Binary Cycle Power Plant

Binary cycle geothermal power generation plants differ from Dry Steam and Flash Steam systems in that the water or steam from the geothermal reservoir never comes in contact with the turbine/generator units. Low to moderately heated (below 400°F) geothermal fluid and a secondary (hence, "binary") fluid with a much lower boiling point that water pass through a heat exchanger. Heat from the geothermal fluid causes the secondary fluid to flash to vapor, which then drives the turbines and subsequently, the generators.

Binary cycle power plants are closed-loop systems and virtually nothing (except water vapor) is emitted to the atmosphere. Resources below 400°F are the most common geothermal resource, suggesting binary-cycle power plants in the future will be binary-cycle plants.

Illustration of a Binary Cycle Power Plant - Illustration of a binary-cycle power plant.  Geothermal hot water comes up from the reservoir through a production well.  The hot water passes by a heat exchanger that is connected to a tank containing a secondary hydrocarbon fluid.  The hot water heats the fluid, which turns to vapor.  The vapor spins a turbine, which in turn spins a generator that creates electricity.  The hot water continues back into the reservoir via an injection well.  This closed-loop system produces no emissions.

 

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Content Last Updated: 02/22/2012