Geothermal Energy
The discovery that the temperature in deep mines exceeded the surface temperature implied the existence of a source of deep geothermal energy. Within the continental crusts, the temperature differential gradient averages about one micro calorie per square centimeter (equivalent to an increase of about 95°F per mile or 33°C per kilometer of increasing depth).
In some areas geothermal energy is a viable economic alternative to conventional energy generation. Commercially viable geothermal fields have the same basic structure. The source of heat is generally a magmatic intrusion into Earth's crust. The magma intrusion generally measures 1110–1650°F (600–900°C), at a depth of 4.3–9.3 mi (7–15 km). The bedrock containing the intrusion conducts heat to overlying aquifers (i.e., layers of porous rock such as sandstone that contain significant amounts of water) covered by a dome-shaped layer of impermeable rock such as shale or by an over-lying fault thrust that contains the heated water and/or steam. A productive geothermal generally produces about 20 tons (18.1 metric tons) of steam, or several hundred tons of hot water, per hour. Historically, some heavily exploited geothermal fields have had decreasing yields due to a lack of replenishing water in the aquifer, rather than to cooling of the bedrock.
There are three general types of geothermal fields: hot water, wet steam, and dry steam. Hot water fields contain reservoirs of water with temperatures between 140–212°F (60–100°C), and are most suitable for space heating and agricultural applications. For hot water fields to be commercially viable, they must contain a large amount of water with a temperature of at least 140°F (60°C) and lie within 2,000 meters of the surface.
Wet steam fields contain water under pressure and usually measure 212°F (100°C). These are the most common commercially exploitable fields. When the water is brought to the surface, some of the water flashes into steam, and the steam may drive turbines that can produce electrical power.
Dry steam fields are geologically similar to wet steam fields, except that superheated steam is extracted from the aquifer. Dry steam fields are relatively uncommon.
Because superheated water explosively transforms into steam when exposed to the atmosphere, it is much safer and generally more economical to use geothermal energy to generate electricity, which is much more easily transported. Because of the relatively low temperature of the steam/water, geothermal energy may be converted into electricity with an efficiency of 10–15%, as opposed to 20–25% for coal or oil fired generated electricity.
To be commercially viable, geothermal electrical generation plants must be located near a large source of easily accessible geothermal energy. A further complication in the practical utilization of geothermal energy derives from the corrosive properties of most groundwater and steam. In fact, prior to 1950, metallurgy was not advanced enough to enable the manufacture of steam turbine blades resistant to corrosion. Geothermal energy sources for space heating and agriculture have been used extensively in Iceland, and to some degree Japan, New Zealand, and the former Soviet Union. Other applications include paper manufacturing and water desalination.
While geothermal energy is generally presented as nonpolluting energy source, water from geothermal fields often contains large amounts of hydrogen sulfide and dissolved metals, making its disposal difficult.
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