Heat energy continuously flows to the Earth’s surface from its interior, where central temperatures of about 6 000°C exist. The predominant source of the Earth’s heat is the gradual decay of long-lived radioactive isotopes (40K, 232Th, 235U and 238U). The outward transfer of heat occurs by means of conductive heat flow and convective flows of molten mantle beneath the Earth’s crust. This results in a mean heat flux at the Earth’s surface of 80kW/km2 approximately. This heat flux, however, is not distributed uniformly over the Earth’s surface; rather, it is concentrated along active tectonic plate boundaries where volcanic activity transports high temperature molten material to the near surface.
Although volcanoes erupt small portions of this molten rock that feeds them, the vast majority of it remains at depths of 5 to 20 km, where it is in the form of liquid or solidifying magma bodies that release heat to surrounding rock. Under the right conditions, water can penetrate into these hot rock zones, resulting in the formation of high temperature geothermal systems containing hot water, water and steam, or steam, at depths of 500 m to >3,000 m.
Characteristics and Applications of Geothermal Energy
Geothermal energy is an enormous, underused heat and power resource that is clean (emits little or no greenhouse gases), reliable (average system availability of 95%), and homegrown (making us less dependent on foreign oil). Geothermal resources range from shallow ground to hot water and rock several miles below the Earth's surface, and even farther down to the extremely hot molten rock called magma. Mile-or-more-deep wells can be drilled into underground reservoirs to tap steam and very hot water that can be brought to the surface for use in a variety of applications.
The general characteristics of geothermal energy that make it of significant importance for both electricity production and direct use include:
Extensive global distribution; it is accessible to both developed and developing countries.
Environmentally friendly nature; it has low emission of sulphur, CO2 and other greenhouse gases.
Indigenous nature; it is independent of external supply and demand effects and fluctuations in exchange rates.
Independence of weather and season.
Contribution to the development of diversified power sources.
Geothermal energy can be used very effectively in both on- and off-grid developments, and is especially useful in rural electrification schemes. Its use spans a large range from power generation to direct heat uses, the latter possible using both low temperature resources and “cascade” methods. Cascade methods utilise the hot water remaining from higher temperature applications (e.g., electricity generation) in successively lower temperature processes, which may include binary systems to generate further power and direct heat uses (bathing and swimming; space heating, including district heating; greenhouse and open ground heating; industrial process heat; aquaculture pond and raceway heating; agricultural drying; etc.)
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