Geothermal Basics

Photo of hot springs with steam rising.

Hot Springs in Steamboat Springs, Nevada.


Photo of The Geysers.

Several geothermal power plants at The Geysers.

Geothermal Overview

Heat from the Earth, or geothermal — Geo (Earth) + thermal (heat) — energy can be and already is accessed by drilling water or steam wells in a process similar to drilling for oil. Geothermal energy is an enormous, underused heat and power resource that is clean (emits little or no greenhouse gases), reliable (average system availability of 95%), and homegrown (making us less dependent on foreign oil).

Geothermal resources range from shallow ground to hot water and rock several miles below the Earth's surface, and even farther down to the extremely hot molten rock called magma. Mile-or-more-deep wells can be drilled into underground reservoirs to tap steam and very hot water that can be brought to the surface for use in a variety of applications. In the U.S., most geothermal reservoirs are located in the western states, Alaska, and Hawaii.

Hydrothermal Power Systems

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 360F (182C) 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.

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.

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.

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.

Dry Steam Power Plants

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.

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.

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.

Direct-Use Piped Hot Water Warms Greenhouses and Melts Sidewalk Snow

In the U.S., most geothermal reservoirs are located in the western states, Alaska, and Hawaii. Hot water near Earth's surface can be piped directly into facilities and used to heat buildings, grow plants in greenhouses, dehydrate onions and garlic, heat water for fish farming, and pasteurize milk. Some cities pipe the hot water under roads and sidewalks to melt snow. District heating applications use networks of piped hot water to heat buildings in whole communities.

Photo of a diamond-studded drill bit.

A diamond-studded drill bit developed at Sandia National Laboratories.


Photo snow melting on sidewalks in Klamath Falls, OR.

Snow melting on sidewalks in Klamath Falls, OR.

Geothermal Heat Pumps (GHPs) Use Shallow Ground Energy to Heat and Cool Buildings

Almost everywhere, the upper 10 feet of Earth's surface maintains a nearly constant temperature between 50 and 60°F (10 and 16°C). A geothermal heat pump system consists of pipes buried in the shallow ground near the building, a heat exchanger, and ductwork into the building. In winter, heat from the relatively warmer ground goes through the heat exchanger into the house. In summer, hot air from the house is pulled through the heat exchanger into the relatively cooler ground. Heat removed during the summer can be used as no-cost energy to heat water.

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. Many believe the result will be about 15,000 megawatts of new capacity within the next decade.

Photo of Buildings in Louisville, Kentucky.

World's Largest Heat Pump System in Louisville, KY.

Photo of a home in Oklahoma City.

This 3,000 sq. ft. house in Oklahoma City has a verified average electric bill of $60 per month - using a geothermal heat pump.


U.S. Geothermal Power Plants

Casa Diablo Geothermal Area

The Mammoth-Pacific geothermal power plants at Casa Diablo on the eastern front of the Sierra Nevada Range generate enough power for approximately 40,000 homes. The power is sold to Southern California Edison under long-term contracts.

Photo of the Casa Diablo Geothermal area.

Mammoth-Pacific Geothermal Power Plant.

The Navy 1 Geothermal Project is located on the test and evaluation ranges of the Naval Air Weapons Station, China Lake. At its peak, the project produced more than 273 megawatts of electricity that was sold into the local utility grid under a long-term power sales agreement.

Photo of the Coso Geothermal Area.

Navy 1 Geothermal Power Plants in Coso Junction, CA.

The Geysers Geothermal area, north of San Francisco, California, is the world's largest dry-steam geothermal steam field. Power production at the Geysers reached peak production in 1987, at that time serving 1.8 million people.

Photo of The Geysers power plant.

Geothermal Power Plant at The Geysers.

The Hawaii geothermal area includes the Puna Geothermal Venture, which is located about 21 miles south of Hilo on the Big Island of Hawaii. The facility is situated along the Lower East Rift Zone of the Kilauea Volcano. At the Puna Geothermal Venture, geothermal fluid is brought to the surface through production wells, which tap into the resource at a depth of almost a mile. The steam, along with its non-condensable gases, is routed to the power plant and used to produce electricity for the Big Island of Hawaii.

Photo of Hawaii's Big Island (Puna) power plant.

This geothermal power plant provides about 30% of electricity demand on the Big Island (Puna) of Hawaii.

The Honey Lake geothermal area is located in Lassen County, California and Washoe County, Nevada. There are three geothermal projects actively producing electrical power. They are located at Wendel, Wineagle, and Amedee.

Photo of Amedee Geothermal Venture power plant in Amadee, CA.

Amedee Geothermal Venture power plant in Amadee, CA.

The Imperial Valley Geothermal project consists of 10 generating plants in the Salton Sea Known Geothermal Resource Area in Southern California's Imperial Valley. The combined capacity at Imperial Valley is approximately 327 net megawatts.

Photo of the Leathers geothermal power plant

The Leathers geothermal power plant in Calipatria, CA.

The extensive Steamboat Springs geothermal area contains three geothermal power-generating plants. The plants provide approximately 30% of the total Nevada geothermal power output.

Photo of Nevada power plant.

Normal water-vapor emissions from the Steamboat 1 geothermal power plant in Washoe, NV.

Utah has two geothermal electric plants: the 23-megawatt Roosevelt Hot Springs facility near Milford run by Utah Power and CalEnergy Corp., and the Utah Municipal Power Association's Cove Fort Station, which is located north of Beaver, Utah.

Photo of the Bud L. Bonnett Geothermal Plant in Cove Fort Sulphurdale, UT

The Bud L. Bonnett Geothermal Plant in Cove Fort Sulphurdale, UT.

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