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Geothermal energy is derived from heat within the earth. Earth’s internal heat is due to the residual heat that was produced when Earth formed, in addition to heat generated by radioactive decay. Earth’s temperature increases with depth below the surface. The inner core has a temperature of about 4,000 degrees C. The increase in temperature with depth is referred to as the geothermal gradient. A normal geothermal gradient is 15 to 30 degrees Cl kilometer. The geothermal gradient is much higher, double or triple, in areas of recent volcanic activity. Generally, the higher the geothermal gradient, the higher the heat flow to the surface.
Generally, geothermal energy is tapped either by drilling wells into bedrock and allowing hot water and steam to flows up to turn a turbine on or near the surface. Usually the water and steam extracted is routed back into the subsurface to close the circuit and add pressure for extraction.
The world’s major geothermal energy fields are associated with areas of active or recent volcanism. The Pacific Rim of Fire has many developed geothermal fields associated with subduction zone volcanism associated with the convergence of tectonic plates. Some of the popular geothermal areas include New Zealand, Indonesia, the Philippines, Japan, northern California, Mexico, and several countries in Central America.
Iceland is located along the Mid-Atlantic Ridge, a divergent plate boundary. Icelanders use geothermal energy to the extent that, together with hydropower, they are able to supply electricity and heat to the entire island. Because of this, Iceland is independent of fossil fuels except as an automobile fuel.
The geothermal fields at Lardarello, Italy, are the oldest in the world. They were developed in the early 1900s utilizing dry steam. This geyser and hot spring area is associated with the recently active volcanism north of Rome. The geothermal fluids have a high content of boric acid, which is utilized along with the heat.
In areas of high geothermal gradient, the geothermal resources occur as hot or dry steam, or hot water that circulates through a permeable zone, such as Yellowstone National Park. The hot water or steam can be used as a direct source of heat or as a source of mechanical energy to turn turbines and generate electricity. Circulation is usually deep, but more recently the circulation of shallow groundwater in areas of high heat flow has been utilized.
Hot dry rocks occur in areas where magma or recently solidified magma is isolated from groundwater. Hot dry rocks require that water be pumped into the ground and recycled to extract the power. Temperatures can reach as high as 1,200 degrees C, depending on the type of magma.
Geopressurized systems are associated with areas of deep burial such as along the Gulf Coast of the United States, where the normal heat flow is trapped by insulating layers of sediment. Along the Gulf Coast, temperatures reach over 270 degrees C at depths of 4 to 7 kilometers.
Areas of normal geothermal gradient are extremely common but have a low level of energy. Geothermal heat pumps utilize lower geothermal gradient, and may therefore be useful in most areas, whether or not they are close to volcanic centers.
Geothermal energy is a relatively clean and environmentally friendly source of energy. Adverse effects occur mainly in the form of gas emissions, and thermal and chemical pollution from the wastewater. Geothermal energy is considered a renewable resource because there is no practical limit to the supply of this energy.
- Wendell A. Duffield and John Sass, Geothermal Energy: Clean Power from the Earth’s Heat (U.S. Geological Survey, 2003);
- S. Department of Energy, Geothermal Division, Geothermal Energy, the Environmentally Responsible Energy Technology for the Nineties (U.S. Department of Energy, 1993).