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REPP-CREST
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Suite 202
Washington, DC 20006
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Basics on Geothermal
What Is Geothermal Energy?
Geothermal energy is renewable heat energy from deep in the earth. Heat is brought to the near-surface by thermal conduction and by intrusion into the earth's crust of molten magma (defined below from Encyclopedia Britannica, britannica.com) originating from great depth. Ground water is heated to form hydrothermal resources -- naturally occurring hot water and steam. Use of hydrothermal energy is economic today at a number of high-grade sites.
Hydrothermal resources are tapped by existing well-drilling and energy-conversion technology to generate electricity or to produce hot water for direct use.
Earth energy is used by geothermal heat pumps. Hot dry rock, magma and geopressured geothermal energy have enormous potential, but cannot be economically developed without the benefit of further R&D.
For generation of electricity, hot water, at temperatures ranging from about 300deg.F to more than 700deg.F, is brought from the underground reservoir to the surface through production wells, and is flashed to steam in special vessels by release of pressure. The steam is separated from the liquid and fed to a turbine engine, which turns a generator.
Spent geothermal fluid is injected back into peripheral parts of the reservoir to help maintain reservoir pressure.
If the reservoir is to be used for direct-heat application, the geothermal water is usually fed to a heat exchanger before being injected back into the earth. Heated domestic water from the output side of the heat exchanger is used for home heating, greenhouse heating, vegetable drying and a wide variety of other uses.
Hot water and steam exist at many subsurface locations in the western U.S.
Non hydro renewable energy, like geothermal, supplied over 10% of California's electricity mix in 1999.
Many more hydrothermal resources could be used if better technology were available and if their development were actively promoted by the government and utilities.
From Britannica.com
magma
molten or partially molten rock from which igneous rocks form. It usually consists of silicate liquid, although carbonate and sulfide melts occur as well. Magma migrates either at depth or to the Earth's surface and is ejected as lava. Suspended crystals and fragments of unmelted rock may be transported in the magma; dissolved volatiles may separate as bubbles and some liquid may crystallize during movement. Several interrelated physical properties determine the characteristics of magma, including chemical composition, viscosity, dissolved gases, and temperature. As magma cools, crystals form in a systematic manner, which is most simply expressed in the form of Bowen's reaction series; early high-temperature crystals will tend to react with the liquid to form other minerals at lower temperatures. Two series are recognized: (1) a discontinuous reaction series, which from high to low temperatures is composed of olivine, orthopyroxene, clinopyroxene, amphibole, and biotite; and (2) a continuous reaction series, represented by high-temperature calcium-rich plagioclase to low-temperature sodium-rich plagioclase. Numerous variations can occur during crystallization to influence the resulting rock. Such variations include separation of early crystals from liquid, preventing a reaction; cooling of magma too rapidly for reactions to occur; and loss of volatiles, which may remove some components from the magma. Transport and emplacement of magma is strongly affected by its viscosity and by the fracture characteristics of rocks through which it moves. Viscosity is reduced by water and a lower silica content.
Bibliography:
CREST's geothermal renewable energy kiosk, 1994
photo credit:
NREL photo library
PIX number
05871
Alligators taking advantage of the warm spring water at the Colorado Alligator Farm in Mosca, Colorado. The Farm makes use of low-grade geothermal commercial applications in the mountains of Colorado.
Credit: Gretz, Warren
Index Date: 3/16/98
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