Hydrothermal Features

Hydrothermal resources are categorized as dry steam (vapor dominated) or hot water resources, depending on the predominant phase of the fluid in the reservoir. Although the technology is similar for both, dry steam technology i s not included in this Technology Characterization because dry steam resources are relatively rare. Hot water resources are further categorized as being high temperature (>200oC/392oF), moderate temperature (between 100oC/212oF and 200oC/392oF), and low temperature (<100oC/212oF). Only the high and moderate temperature resources are adequate for commercial power generation.

Two separate power generation technologies, flash and binary, are characterized. The boiling temperature of wate r depends on its pressure, so as the pressure of the high temperature geothermal fluid is lowered in the plant, a portio n (about 10 to 20% of it, depending on temperature and pressure) "flashes" to steam, which is used to drive a turbine to produce electricity. For moderate temperature resources, binary technology is more efficient. It is termed "binary " because the heat is transferred from the geothermal fluid to a secondary working fluid with a lower boiling temperature than water. The secondary fluid, vaporized by the heat, drives the turbine.

Beginning commercially in the 1950s, hydrothermal electric power generation has grown into an active and healthy , albeit not large, industry. About 7,000 MW of electric generation capacity have been developed worldwide, including about 2,800 MW in the U.S. [3]. Supply and demand forces and anticipated restructuring in the U.S. electric markets have resulted in very low demand for new geothermal capacity since 1990. However, geothermal energy is competing very well in markets outside the U.S., especially in Indonesia and the Philippines, where demand is high, geotherma l resources are plentiful, and government policy is favorable. Approximately 2,000 additional MW will likely b e developed worldwide in 1996 through 2000, with the majority of this being in Asia.

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