Flash steam 
Flash steam power plants use hot water (>360 ºF, or >182 ºC) from the geothermal reservoir.¹⁵ When the water is pumped to the generator, it is released from the pressure of the deep reservoir. The sudden drop in pressure causes some of the water to vaporize to steam, which spins a turbine to generate electricity. Both dry steam and flash steam power plants emit small amounts of carbon dioxide, nitric oxide, and sulfur, but generally 50 times less than traditional fossil-fuel power plants.¹⁶ Hot water not flashed into steam is returned to the geothermal reservoir through injection wells. Figure 3 is a schematic of a typical flash steam power plant.¹⁷

Binary-cycle
Binary-cycle power plants use moderate-temperature water (225 ºF–360 ºF, or 107 ºC–182 ºC) from the geothermal reservoir. In binary systems, hot geothermal fluids are passed through one side of a heat exchanger to heat a working fluid in a separate adjacent pipe. The working fluid, usually an organic compound with a low boiling point such as Iso-butane or Iso-pentane, is vaporized and passed through a turbine to generate electricity. An ammonia-water working fluid is also used in what is known as the Kalina Cycle. Makers claim that the Kalina Cycle system boosts geothermal plant efficiency by 20–40 percent and reduces plant construction costs by 20–30 percent, thereby lowering the cost of geothermal power generation.

The advantages of binary cycle systems are that the working fluid boils at a lower temperature than water does, so electricity can be generated from reservoirs with lower temperature, and the binary cycle system is self-contained and therefore, produces virtually no emissions. For these reasons, some geothermal experts believe binary cycle systems could be the dominant geothermal power plants of the future. Figure 4 is a schematic of a typical binary cycle power plant.¹⁸

Geothermal Power Generation

As of 2000, approximately 8,000 megawatts (MW) of geothermal electrical generating capacity was present in more than 20 countries, led by the United States, Philippines, Italy, Mexico, and Indonesia (see Table 2 below). This represents 0.25% of worldwide installed electrical generation capacity. In the United States, geothermal power capacity was 2,228 MW, or approximately 10% of non-hydro renewable generating capacity in 2001 (see Figure 5 below).¹⁹ This capacity would meet the electricity needs of approximately 1.7 million U.S. households.²⁰

Current geothermal use is only a fraction of the total potential of geothermal energy. U.S. geothermal resources alone are estimated at 70,000,000 quads²¹, equivalent to 750,000-years of total primary energy supply (TPES) for the entire nation at current rates of consumption. The geothermal energy potential in the uppermost 6 miles of the Earth’s crust amounts to 50,000 times the energy of all known oil and gas resources in the world.²² Not all of these resources are technologically or economically accessible, but tapping into even a fraction of this potential could provide significant renewable resources for years to come. The Geothermal Energy Association reports the potential for developing an additional 23,000 MW of generating capacity in the United States using conventional geothermal energy technology.²³

Table 2. Installed Geothermal Generating Capacities Worldwide²⁴

Country	1995 (MWe)	2000 (MWe)	Country	1995 (MWe)	2000 (MWe)
United States	2,817	2,228	Kenya	45	45
Philippines	1,227	1,909	Guatemala	33	33
Italy	632	785	China	29	29
Mexico	753	755	Russia	11	23
Indonesia	310	590	Turkey	20	20
Japan	414	547	Portugal	5	16
New Zealand	286	437	Ethiopia	0	8
Iceland	50	170	France	4	4
El Salvador	105	161	Thailand	0.3	0.3
Costa Rica	55	142	Australia	0.2	0.2
Nicaragua	70	70	Argentina	0.7	0
Total (MW)				6,833	7,974

Figure 5. U.S. Nonhydro Renewable Power Generating Capacity, 2001

Capacity Factor
The percentage of time a power plant runs is the plants capacity factor. Geothermal power plants typically produce electricity about 90% of the time, though can be run up to 98% of the time if the contract price of power is high enough to justify increased operational and maintenance costs. In comparison, coal-fired power plants are typically run 65–75% of the time, while nuclear plants in the United States have run at very high capacity factors (95–98%) in recent years due to lucrative market and regulatory conditions.

Endnotes
12. Energy & Geoscience Institute at the University of Utah. Geothermal Energy Brochure.
http://www.egi.utah.edu/geothermal/GeothermalBrochure.pdf; accessed Oct 1, 2002.
13. U.S. Department of Energy: Office of Energy Efficiency and Renewable Energy.
http://www.eren.doe.gov/geothermal/geopowerplants.html; accessed Sep 25, 2002.
14. National Renewable Energy Laboratory: Geothermal Technologies Program. http://www.nrel.gov/geothermal/geoelectricity.html; accessed June 5, 2003. http://www.eia.doe.gov/kids/renewable/geothermal.html.
15. Idaho National Engineering and Environmental Laboratory. http://geothermal.id.doe.gov/what-is.shtml; accessed Sep 25, 2002.
16. National Renewable Energy Laboratory. http://www.nrel.gov/geothermal; accessed Sep 25, 2002.
17. National Renewable Energy Laboratory: Geothermal Technologies Program. http://www.nrel.gov/geothermal/geoelectricity.html; accessed June 5, 2003.
18. National Renewable Energy Laboratory: Geothermal Technologies Program. http://www.nrel.gov/geothermal/geoelectricity.html; accessed June 5, 2003.
19. Energy Information Administration. Renewable Energy Annual 2001. http://www.eia.doe.gov/cneaf/solar.renewables/page/rea_data/table5.html; accessed Jan. 28, 2003.
20. This figure assumes a capacity factor of 90% and an annual average household electricity consumption of 10,200 kWh as reported by DOE’s EIA for 1997.
21. A quad is one quadrillion (1 x 10¹⁵) British Thermal Units (BTU). One BTU equals 0.293 watt-hours.
22. U.S. Department of Energy Office of Energy Efficiency and Renewable Energy. http://www.eren.doe.gov/state_energy/technology_overview.cfm?techid=5; accessed Sep 30, 2002.
23. Geothermal Energy Association. Geothermal Electric Production Potential. Based Upon U.S. Geologic Survey Testimony Before the Subcommittee on Energy and Mineral Resources of the House Resources Committee, U.S. House of Representatives, May 3, 2001, and other sources. http://www.geo-energy.org/UsResources.htm; accessed January 28, 2003.
24. International Geothermal Association. Installed Generating Capacity. http://iga.igg.cnr.it/electricitygeneration.php; accessed Jan 28, 2003.