What Is Geothermal Energy?
Geothermal energy is the natural, internal heat of the earth trapped in rock formations deep within the ground. Only a fraction of this vast storehouse of energy can be extracted, usually where there are large fractures in the earth's crust. Hot springs, geysers, and fumaroles (holes in or near volcanoes from which vapor escapes) are the most easily exploitable sources of geothermal energy. (See Figure 10.8.) Geothermal reservoirs provide hot water or steam that can be used for heating buildings, processing food, and generating electricity.
To produce power from a geothermal energy source, pressurized steam or hot water is extracted from the earth and directed toward turbines. The electricity produced by turbines is then fed into a utility grid and distributed to residential and commercial customers. By the late 1990s electricity from this source accounted for almost two-thirds of the world's geothermal energy use.
Types of Geothermal Energy
Like most natural energy sources, geothermal energy is usable only when it is concentrated in one spot, in what is called a "thermal reservoir." The four basic categories of thermal reservoirs are hydrothermal (dry steam and hot, or wet, steam), dry rock, geopressurized, and magma (rock so hot it has liquefied) reservoirs. Most of the known areas for geothermal power in the United States are located west of the Mississippi River. (See Figure 10.9.)
Hydrothermal reservoirs consist of a heat source covered by a permeable formation through which water circulates. Dry steam is produced when hot water boils underground and some of the steam escapes to the surface under pressure. Once at the surface impurities and tiny rock particles are removed and the steam is then piped directly to the electrical generating station. These systems are the cheapest and simplest form of geothermal energy.
Hot steam systems are created when underground water is heated to more than 700 degrees Fahrenheit by the surrounding hot rock or magma, but the water remains liquid because of intense pressure. When the water is brought to the surface and the pressure is reduced, a small amount of water becomes steam that is then separated and used to power an electrical generating plant.
Dry rock formations are the most common geothermal source, especially in the west. To tap this source of energy, water is injected into naturally hot rock formations to produce steam or water for collection.
FIGURE 10.8
Cross-section of the Earth showing source and transport of geothermal energy
Geopressurized reservoirs are sedimentary formations containing hot water and methane gas. Supplies of geo-pressurized energy remain uncertain, however, and drilling is expensive. Scientists hope that advanced technology will eventually permit the commercial exploitation of the methane content in these reservoirs.
Magma resources are found where molten or partially liquefied rock is located from 10,000 to 33,000 feet below the earth's surface. Because magma is so hot, ranging from 1,650 degrees to 2,200 degrees Fahrenheit, it is a good geothermal resource. The process for extracting energy from magma is still in the experimental stages.
Disadvantages of Geothermal Energy
Geothermal plants are expensive because they must be built near the source. Other drawbacks include low efficiency, bad odors from sulfur released in processing, noise, lack of access for most states, and potentially harmful pollutants (hydrogen sulfide, ammonia, and radon). Also, poisonous arsenic or boron are often found in geothermal waters. Serious environmental concerns have been raised over the release of chemical compounds, potential water contamination, the collapse of land surface around the area from which the water is being drained, and potential water shortages resulting from massive withdrawals of water.
American Production of Geothermal Energy
Geothermal energy ranked fourth in U.S. renewable energy production in 2003 after hydroelectric, wood, and waste energy. Geothermal energy accounted for 0.3 quadrillion Btu in 2003, down from the early 1990s when it approached 0.4 quadrillion Btu. In 2003 it represented only 5 percent of renewable energy consumed in the United States.
FIGURE 10.9
Geothermal resources map
Public-sector involvement in the geothermal industry began with the passage of the Geothermal Steam Act of 1970 (PL 91-581), which authorized the U.S. Department of the Interior to lease geothermal resources on federal lands. Although the United States is the greatest producer of geothermal power with 28 percent of the world's capacity, the geothermal industry in the United States is losing steam. Oil prices have remained relatively low, and most of the easily exploited geothermal reserves have already been developed. In addition, utility companies and independent power producers argue over who should build additional generating capacity and what prices should be paid for the power. As a result, U.S. geothermal capacity has dropped. Growth in the American market depends on the regulatory environment, oil price trends, and the success of unproven technologies for economically exploiting some of the presently inaccessible geothermal reserves.
World Production of Geothermal Energy
During the oil crisis of the 1970s, when energy was at the forefront of the international agenda, governments scrambled to find domestic alternatives to imported oil. As public interest grew, research dollars became available and a large number of geothermal energy plants were built. Although interest has since faded, geothermal power's commercial development worldwide has continued at a slow but steady pace.
Since 1979 worldwide geothermal electrical generating capacity has nearly tripled. According to the International Geothermal Association, there were nearly 8,000 megawatts of installed capacity in 2000. The U.S. accounted for 28 percent of the total, followed by the Philippines (24 percent), Italy (10 percent), Mexico (9 percent), and Indonesia and Japan (7 percent each). Together these countries accounted for 85 percent of total capacity.
Worldwide geothermal capacity is still less than that of a handful of average-sized coal-fired power plants. World geothermal reserves are immense but unevenly distributed. These reserves fall mostly in seismically active areas at the margins or borders of Earth's nine major tectonic plates. Exploited reserves represent only a small fraction of the overall potential—many countries are believed to have in excess of 100,000 megawatts of geo-thermal energy available.
A few nations in the developing world—El Salvador, Kenya, Bolivia, Costa Rica, Ethiopia, India, and Thailand—have considerable steam reserves available for power generation. Debt-ridden developing nations that have substantial unexploited geothermal reserves are especially eager to use them instead of relying on costly fossil fuel imports for their energy needs.
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