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Hydrothermal Power Systems

Photo of geothermal power plant.

Geothermal power plant in the Imperial Valley, California.

There are three geothermal power plant technologies being used to convert hydrothermal fluids to electricity. The conversion technologies are dry steam, flash, and binary cycle. The type of conversion used depends on the state of the fluid (whether steam or water) and its temperature. Dry steam power plants systems were the first type of geothermal power generation plants built. They use the steam from the geothermal reservoir as it comes from wells, and route it directly through turbine/generator units to produce electricity. Flash steam plants are the most common type of geothermal power generation plants in operation today. They use water at temperatures greater than 360°F (182°C) that is pumped under high pressure to the generation equipment at the surface. 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.

U.S. Geothermal Power Plants

Power plant photographs


Dry Steam Power Plants

Photo of dry steam power plants.

Dry steam power plants at The Geysers in California.

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, and is still very effective. Steam technology is used today at The Geysers in northern California, the world's largest single source of geothermal power. These plants emit only excess steam and very minor amounts of gases.

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 Plants

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 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 Plants

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.

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.

The Future of Geothermal Electricity

Steam and hot water reservoirs are just a small part of the geothermal resource. The Earth's magma and hot dry rock will provide cheap, clean, and almost unlimited energy as soon as we develop the technology to use them. In the meantime, because they're so abundant, moderate-temperature sites running binary-cycle power plants will be the most common electricity producers.

Before geothermal electricity can be considered a key element of the U.S. energy infrastructure, it must become cost-competitive with traditional forms of energy. The U.S. Department of Energy is working with the geothermal industry to achieve $0.03 to $0.05 per kilowatt-hour. We believe the result will be about 15,000 megawatts of new capacity within the next decade.

DOE Support

The U.S. Department of Energy recognizes the strategic value of geothermal electricity, and supports its development in several ways through its Geothermal Technology Development Program. First, it works with Congress to ensure support for geothermal energy and renewables in general. Second, it sponsors millions of dollars in research and development at national laboratories and universities. Investigators are working on issues in exploration, geochemistry, drilling, resource usage, and equipment operation. Third, through its GeoPowering the West initiative, it works with state and local officials and other stakeholders to identify and overcome regulatory and institutional barriers to geothermal power development.