U.S. Department of Energy - Energy Efficiency and R enewable 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 thr ee elements naturally. In addition to conventional hydrothermal resources, po wer can be generated from low temperature, coproduced and other permeable sedimen tary resources.

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

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### Power Plants Power plants use steam produced from geothermal res ervoirs 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) depend s 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 t he 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 Califor nia, 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. 

### Page 1 of 3 Geothermal Technologies Program: Electricity Generation

6/22/2012 http://www1.eere.energy.gov/geothermal/printable_versions/powerplants.htmlFlash Steam Power Plant Flash steam plants are the most common type of geot hermal power generation plants in operation today. Fluid at temperatures gr eater than 360°F (182°C) is pumped under high pressure into a tank at the surfa ce held at a much lower pressure, causing some of the fluid to rapidly vapo rize, 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 m ore energy.

Binary Cycle Power Plant Binary cycle geothermal power generation plants dif fer from Dry Steam and Flash Steam systems in that the water or steam from the g eothermal reservoir never comes in contact with the turbine/generator units. Low to moderately heated (below 400°F) geothermal fluid and a secondary (hence, "bi nary") fluid with a much lower boiling point that water pass through a heat exchan ger. 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 a nd virtually nothing (except water vapor) is emitted to the atmosphere. Resource s below 400°F are the most common geothermal resource, suggesting binary-cycle power plants in the future will be binary-cycle plants. 

### Page 2 of 3 Geothermal Technologies Program: Electricity Generation

6/22/2012 http://www1.eere.energy.gov/geothermal/printable_versions/powerplants.htmlContacts | Web Site Policies | U.S. Department of Energy | USA.gov Content Last Updated: 02/22/2012 Resources

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

U.S. geothermal power plant examples with photograp hs: • Casa Diablo - California/Nevada ◦ Navy 1 - California ◦ The Geysers - California ◦ Hawaii ◦ Honey Lake - California/Nevada ◦ Imperial Valley - California ◦ Nevada ◦ Utah ◦ 

### Page 3 of 3 Geothermal Technologies Program: Electricity Generation

6/22/2012 http://www1.eere.energy.gov/geothermal/printable_versions/powerplants.htmlU.S. Department of Energy - Energy Efficiency and R enewable Energy



### Geothermal Technologies Program



## Geothermal FAQs Read our frequently asked questions and their answe rs to learn more about the use of geothermal energy.

What are the benefits of using geothermal energy? 1. Why is geothermal energy a renewable resource? 2. Where is geothermal energy available? 3. What are the environmental impacts of using geother mal energy? 4. What is the visual impact of geothermal technologie s? 5. Is it possible to deplete geothermal reservoirs? 6. How much does geothermal energy cost per kilowatt-h our (kWh)? 7. What does it cost to develop a geothermal power pla nt? 8. What makes a site good for geothermal electric deve lopment? 9. How much water does a plant require? 10.



### 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 a bout 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 essentia lly 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 depende nce 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 depen dent 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 h ot 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 geother mal resources - hot dry rock and magma, for example - are awaiting further techn ology development.

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### 4. What are the environmental impacts of using geot hermal energy?
Answer: Geothermal technologies offer many environmental a dvantages over conventional power generation: 

### Page 1 of 1 Geothermal Technologies Program: Geothermal FAQs

6/22/2012 http://www1.eere.energy.gov/geothermal/printable_versions/faqs.html