Geothermal energy is the heat deep inside Earth heat that is thought to be generated by the natural decay of

Transcription

1 Geothermal Energy Geothermal energy is the heat deep inside Earth heat that is thought to be generated by the natural decay of radioactive material. It is virtually inexhaustible a vast potential source of power. But can it be tapped economically and in large enough quantities to be useful? Theoretically, geothermal energy could be obtained at any point on Earth by drilling deeply enough and providing some method for extracting the heat. In most parts of the world, the hot interior mass is too deep to reach with existing drilling technology. In some areas, however, such as portions of the western United States, New Zealand, Iceland, and Italy, large heat sources are very close to the surface. Hot springs, geysers, and recent volcanic activity frequently indicate the presence of thermal reserves close to the surface. Forms of Geothermal Energy Three basic forms of geothermal heat have been used for human purposes: natural steam; hot water; and hot, dry rock. So far, geothermal steam has been used mainly to generate electricity. Hot water with and without steam has been employed largely for heating purposes in a few locations, but its application to power generation is only now being seriously studied. Dry, hot rock, which is the largest heat resource, has not yet been tapped commercially, but new methods for using it are being tested. Steam to Electricity

2 Historically, the commercial application of geothermal energy for generating electricity dates back to 1904 in Larderello, Italy. Today the this area in central Tuscany produces 10 percent of the world's geothermal energy. Electrical power is also produced from geothermal sources in the United States, New Zealand, Japan, Iceland, Mexico, Philippines, and Russia. The Geysers, a natural steam field in northern California, is the world's largest producer of renewable geothermal power. There, steam is collected from a number of wells, filtered, and passed through turbines that drive electric generators. However, because of the lower pressures and temperatures of the steam at The Geysers, the amount of available heat that is converted to electrical energy is less than that achieved by conventional fossil-fuel plants. Nevertheless, this type of geothermal installation costs less and is easier to build than a conventional steam plant of the same capacity. And, once constructed and put into operation, the "fuel" that runs the generators is virtually free. These factors usually compensate for the lower efficiency of geothermal heating. The overall cost of natural-steam power is less than that of power generated by fossil-fuel or nuclear-power plants. The technology is limited in its application, however, because additional "dry-steam" (steam not mixed with hot water) fields are unlikely to be discovered. But The Geysers has successfully produced a steady supply of natural-steam power since the 1960s. Today, it continues to generate enough energy to meet the electricity needs of both San Francisco and Oakland. Hot Water to Electricity The geothermal wells in The Geysers region of California and at Larderello in Italy produce steam without hot water. It is more common for geothermal wells to yield a mixture of steam and hot water. The steam is produced because a portion of the hot water spontaneously boils as it comes to the surface. The combination of steam and hot water is not as economically useful as dry steam. Much greater quantities of fluid must be produced by the geothermal well for a given plant's generating capacity, and more water must therefore be disposed of, usually by reinjection into Earth. Also, since, in most cases, only the steam is used to generate power, as much as half of the available heat may be discarded with the water. The overall efficiency in using the original thermal energy is therefore correspondingly less than for dry steam. There are many places where hot-water wells without steam are located, but only limited use has been made of these resources. In one method of harnessing this energy, known as the "vapor-turbine cycle," the hot water is passed under pressure through a heat exchanger. The water causes a sealed-in secondary liquid with a lower boiling point such as isobutane or freon to vaporize. This vapor expands through a turbine, which drives an

3 electric generator. It is then condensed to a liquid and returned to the heat exchanger to start the cycle all over again. The original well water, which does not boil because it is kept under considerable pressure, is returned to the ground. The most important feature of the vapor-turbine concept is that it permits the generation of electricity from geothermal waters at temperatures that would not be practical or efficient for steam turbines. An installation of this type is in operation in Russia, and several small prototype units are also currently operating in the United States. Another method, formulated at the U.S. Energy Research and Development Administration's (ERDA's) Lawrence Livermore Laboratory, is operating in Cerro Prieto, Mexico, as part of a demonstration project originally sponsored by the U.S. Department of Energy (DOE). This system, which is referred to as the "total-flow" concept, is designed to convert a portion of the thermal energy of the pressurized steam and hot-water mixture directly into kinetic energy. This is done through the use of a converging-diverging nozzle, roughly comparable to a jet engine. The resulting high-velocity output is used to drive a modified hydraulic-impulse turbine and, in turn, an electric generator. This system produces 25 to 40 percent more power than other methods designed to extract energy from geothermal waters at the temperatures and pressures involved. Hot Water for Heating Hot water from underground sources has been used extensively for many years in Iceland, not only for generating electricity, but also for heating purposes. Today, nearly all of the nation's buildings are heated by hot water piped through an elaborate network from geothermal wells and hot springs. In fact, only 0.1 percent of Iceland's electricity production comes via fossil fuels. Residential and commercial buildings are also heated with geothermal water in parts of Japan, China, Sweden, Romania, Russia, Hungary, Italy, France, and Belgium. In the western United States, hot-water wells provide heat for more than 270 communities, including Boise, Idaho, and Klamath Falls, Oregon. In Japan, other applications have included soil heating and experimental fish-farming projects. In Russia and New Zealand, geothermal water is employed for industrial purposes, such as in air-conditioning and refrigeration systems, and to provide heat for greenhouses, fish ponds, and livestock farms.

4 Hot, Dry Rock Although the practical uses of geothermal energy have been associated with naturally occurring steam or a mixture of steam and hot water, evidence indicates that by far the largest potential geothermal-energy resource is hot, dry rock, which reaches temperatures of about 464 F (240 C) at a depth of about 2.5 miles (4 kilometers). One promising method of extracting heat from deposits of dry rock employs a technique similar to that used in petroleum recovery. In this scheme, two deep wells, forming a closed loop, are drilled into solid rock, such as granite, which overlays an area of high-heat flow from Earth's interior. Cold water under very high pressure is forced into the well, creating a network of cracks in the hot rock. The pressurized water is heated by the hot rock, and rises through the second well to the surface, where it enters a heat exchanger. This process has been used to power electric generators at an experimental site near Los Alamos, New Mexico. Another method of using these sources of dry heat is to create large artificial cracks by means of relatively powerful explosives, and then to circulate water from the surface through the cracks in order to extract the heat. This technique requires extensive investigation because there are several major problems involved, including the effect of blast waves on surface facilities, the economics of creating sufficient fresh rock surface to extract heat in useful quantities, and the speed with which the rock will conduct heat to the cracks from which it is being withdrawn. Locating Geothermal Resources Sources of geothermal energy must be located and explored before they can be used. One of the most important first steps is to identify and outline broad regions where the outward flow of heat near Earth's surface is significantly greater than average. For various reasons, temperatures at the surface can be misleading, and it is therefore more reliable to make measurements at depths of about 60 to 300 feet (20 to 90 meters). Geothermal exploration has in the past been largely confined to locations in the vicinity of hot springs, and has used methods developed by the petroleum industry. Such techniques are not necessarily well suited to discover and evaluate possible new geothermal fields, and new specialized methods are now being developed. Measurements of how well rock masses at various depths conduct electricity as well as magnetic, electromagnetic, and gravity readings are analyzed to determine if the rock structure is such that geothermal

5 energy is likely. Seismic methods are also used. In addition, chemical analyses of waters can give further information about the nature of related geothermal deposits. It is also vital that the technology of deep drilling and drilling at high temperatures be improved. At depths of more than 19,800 feet (6,000 meters), drilling costs increase sharply. Furthermore, at temperatures of approximately 390 F (200 C) and above, the reliability of instruments used to obtain information about conditions at the bottom of a well deteriorates rapidly. Particularly needed are higher-performance drills and drilling fluids, as well as instruments that will operate more reliably in geothermal wells. Environmental Effects From an environmental standpoint, putting geothermal energy to work has many advantages. An important one is that almost all the activities related to the production of geothermal power are in the immediate vicinity of the plant. The kind of distant support operations required for fossil-fuel or nuclear plants such as mining, fuel processing, transportation, storage, and other activities are not involved. On the negative side, there are certain undesirable environmental effects that could extend for some distance from a geothermal-plant location. For example, the proper disposal of wastewater from steam or hot-water wells can become an important problem, particularly when the water has a substantial mineral content. Gaseous discharge is a significant factor for those geothermal wells that produce noxious gases. Other difficulties, which are similar to those experienced by fossil-fuel or nuclear-power plants, include waste-heat disposal and the atmospheric effect of cooling towers. In some instances, undesirable environmental impacts may be produced by seismic effects and the sinking of land surfaces that lie above reservoirs from which geothermal fluids are withdrawn. In most cases, however, the environmental problems encountered through the use of geothermal energy are not as serious as those connected with steam-electric plants burning fossil or nuclear fuel. Furthermore, geothermal problems can usually be satisfactorily handled by known technology. Prospects for the Future The inherent advantages of geothermal energy make it an important potential supplement to nuclear and fossil fuels for many purposes. The most important characteristic of geothermal energy is its potential availability in truly enormous quantities possibly on an inexhaustible basis wherever it can be located, tapped, and utilized. The most

6 readily accessible resources for geothermal power in the United States are located in Alaska, Hawaii, and several western states. Researchers studying the feasibility of tapping the heat contained in magma subterranean molten rock believe that this virtually inexhaustible source of energy could be successfully utilized. Most magma is located more than 20 miles (32 kilometers) below the surface of Earth, but it is much closer to the surface in some places, particularly in volcanic craters. Technological advances may make magma a viable energy source, but some experts think that hot, dry rocks are the best option. The key is determining just the right location and well depth to provide a geothermal facility with heated water for up to 20 years. Several technological advances have provided improvements in both drilling methods and equipment, and as a result, energy from geothermal sources is produced more efficiently than it was in the past. Over the past decade, geothermal energy production has increased 50 percent, serving some 47 million people worldwide. Geothermal plants account for 0.3 percent of total U.S. electricity production. The U.S. Department of Energy predicts a modest increase (to 0.8 percent) by Geothermal Field 1 (of 1) How to cite this article: MLA (Modern Language Association) style: "Geothermal Energy." The New Book of Popular Science Grolier Online. 23 Apr < APA (American Psychological Association) style: Geothermal Energy. (2009). The New Book of Popular Science. Retrieved April 23, 2009, from Grolier Online Chicago Manual of Style: "Geothermal Energy." The New Book of Popular Science. Grolier Online (accessed April 23, 2009). Source: The New Book of Popular Science