U.S. Department of Energy - Energy Efficiency and Renewable Energy

Geothermal Technologies Program

Electricity Generation

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 

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. 

Geothermal power plant in the 

Imperial Valley, California.

Dry steam power plants at The 

Geysers in California.

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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.

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. 

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Content Last Updated: 02/22/2012 

Resources

• U.S. geothermal power plants, Geothermal Energy Association 

• U.S. geothermal power plant examples with photographs: 

◦ Casa Diablo - California/Nevada 

◦ Navy 1 - California 

◦ The Geysers - California 

◦ Hawaii

◦ Honey Lake - California/Nevada 

◦ Imperial Valley - California 

◦ Nevada

◦ Utah

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U.S. Department of Energy - Energy Efficiency and Renewable Energy

Geothermal Technologies Program

Geothermal FAQs

Read our frequently asked questions and their answers to learn more about the use 

of geothermal energy.

1. What are the benefits of using geothermal energy?

2. Why is geothermal energy a renewable resource?

3. Where is geothermal energy available?

4. What are the environmental impacts of using geothermal energy?

5. What is the visual impact of geothermal technologies?

6. Is it possible to deplete geothermal reservoirs?

7. How much does geothermal energy cost per kilowatt-hour (kWh)?

8. What does it cost to develop a geothermal power plant?

9. What makes a site good for geothermal electric development?

10. How much water does a plant require?

 

1. What are the benefits of using geothermal energy?

Answer: Several attributes make it a good source of energy. 

First, it's clean. Energy can be extracted without burning a fossil fuel such as 

coal, gas, or oil. Geothermal fields produce only about one-sixth of the carbon 

dioxide that a relatively clean natural-gas-fueled power plant produces, and 

very little if any, of the nitrous oxide or sulfur-bearing gases. Binary plants, 

which are closed cycle operations, release essentially no emissions.

•

Geothermal energy is available 24 hours a day, 365 days a year. 

Geothermal power plants have average availabilities of 90% or higher, 

compared to about 75% for coal plants.

•

• Geothermal power is homegrown, reducing our dependence on foreign oil.

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2. Why is geothermal energy a renewable resource?

Answer: Because its source is the almost unlimited amount of heat generated 

by the Earth's core. Even in geothermal areas dependent on a reservoir of hot water, 

the volume taken out can be reinjected, making it a sustainable energy source.

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3. Where is geothermal energy available?

Answer: Hydrothermal resources - reservoirs of steam or hot water - are available 

primarily in the western states, Alaska, and Hawaii. However, Earth energy can 

be tapped almost anywhere with geothermal heat pumps and direct-use 

applications. Other enormous and world-wide geothermal resources - hot dry rock 

and magma, for example - are awaiting further technology development.

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4. What are the environmental impacts of using geothermal 

energy?

Answer: Geothermal technologies offer many environmental advantages over 

conventional power generation:

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