05 July 2009

Geothermal power

The invention: Energy generated from the earth’s natural hot springs. The people behind the invention: Prince Piero Ginori Conti (1865-1939), an Italian nobleman and industrialist Sir Charles Parsons (1854-1931), an English engineer B. C. McCabe, an American businessman Developing a Practical System The first successful use of geothermal energy was at Larderello in northern Italy. The Larderello geothermal field, located near the city of Pisa about 240 kilometers northwest of Rome, contains many hot springs and fumaroles (steam vents). In 1777, these springs were found to be rich in boron, and in 1818, Francesco de Larderel began extracting the useful mineral borax from them. Shortly after 1900, Prince Piero Ginori Conti, director of the Larderello borax works, conceived the idea of using the steam for power production. An experimental electrical power plant was constructed at Larderello in 1904 to provide electric power to the borax plant. After this initial experiment proved successful, a 250-kilowatt generating station was installed in 1913 and commercial power production began. As the Larderello field grew, additional geothermal sites throughout the region were prospected and tapped for power. Power production grew steadily until the 1940’s, when production reached 130 megawatts; however, the Larderello power plants were destroyed late inWorldWar II (1939-1945). After the war, the generating plants were rebuilt, and they were producing more than 400 megawatts by 1980. The Larderello power plants encountered many of the technical problems that were later to concern other geothermal facilities. For example, hydrogen sulfide in the steam was highly corrosive to copper, so the Larderello power plant used aluminum for electrical connections much more than did conventional power plants of the time. Also, the low pressure of the steam in early wells at Larderello presented problems. The first generators simply used steam to drive a generator and vented the spent steam into the atmosphere. Asystem of this sort, called a “noncondensing system,” is useful for small generators but not efficient to produce large amounts of power. Most steam engines derive power not only from the pressure of the steam but also from the vacuum created when the steam is condensed back to water. Geothermal systems that generate power from condensation, as well as direct steam pressure, are called “condensing systems.” Most large geothermal generators are of this type. Condensation of geothermal steam presents special problems not present in ordinary steam engines: There are other gases present that do not condense. Instead of a vacuum, condensation of steam contaminated with other gases would result in only a limited drop in pressure and, consequently, very low efficiency. Initially, the operators of Larderello tried to use the steam to heat boilers that would, in turn, generate pure steam. Eventually, a device was developed that removed most of the contaminating gases from the steam. Although later wells at Larderello and other geothermal fields produced steam at greater pressure, these engineering innovations improved the efficiency of any geothermal power plant. Expanding the Idea In 1913, the English engineer Sir Charles Parsons proposed drilling an extremely deep (12-kilometer) hole to tap the earth’s deep heat. Power from such a deep hole would not come from natural steam as at Larderello but would be generated by pumping fluid into the hole and generating steam (as hot as 500 degrees Celsius) at the bottom. In modern terms, Parsons proposed tapping “hot dryrock” geothermal energy. (No such plant has been commercially operated yet, but research is being actively pursued in several countries.) The first use of geothermal energy in the United States was for direct heating. In 1890, the municipal water company of Boise, Idaho, began supplying hot water from a geothermal well. Water was piped from the well to homes and businesses along appropriately namedWarm Springs Avenue. At its peak, the system served more than four hundred customers, but as cheap natural gas became available, the number declined. Although Larderello was the first successful geothermal electric power plant, the modern era of geothermal electric power began with the opening of the Geysers Geothermal Field in California. Early attempts began in the 1920’s, but it was not until 1955 that B. C. McCabe, a Los Angeles businessman, leased 14.6 square kilometers in the Geysers area and founded the Magma Power Company. The first 12.5-megawatt generator was installed at the Geysers in 1960, and production increased steadily from then on. The Geysers surpassed Larderello as the largest producing geothermal field in the 1970’s, and more than 1,000 megawatts were being generated by 1980. By the end of 1980, geothermal plants had been installed in thirteen countries, with a total capacity of almost 2,600 megawatts, and projects with a total capacity of more than 15,000 megawatts were being planned in more than twenty countries. Impact Geothermal power has many attractive features. Because the steam is naturally heated and under pressure, generating equipment can be simple, inexpensive, and quickly installed. Equipment and installation costs are offset by savings in fuel. It is economically practical to install small generators, a fact that makes geothermal plants attractive in remote or underdeveloped areas. Most important to a world faced with a variety of technical and environmental problems connected with fossil fuels, geothermal power does not deplete fossil fuel reserves, produces little pollution, and contributes little to the greenhouse effect. Despite its attractive features, geothermal power has some limitations. Geologic settings suitable for easy geothermal power production are rare; there must be a hot rock or magma body close to the surface. Although it is technically possible to pump water from an external source into a geothermal well to generate steam, most geothermal sites require a plentiful supply of natural underground water that can be tapped as a source of steam. In contrast, fossil-fuel generating plants can be at any convenient location.