第6课-未来新能源

Section A GEOTHERMAL POWER The earth contains a great deal of heat, most of it left over from its early history, some continually generated by decay of radioactive elements in the earth. Slowly, this heat is radiating away and the earth is cooling down, but under normal circumstances, the rate of heat escape at the earth's surface is so slow that we do not even notice it and certainly cannot use it. If the heat escaping at the earth's surface were collected over an average square meter for a year, it would be sufficient only to heat about 2 gallons of water to the boiling point, though local heat flow can be substantially higher, for example in young volcanic areas. The Geothermal Resource Magma rising into the crust from the mantle brings unusually hot material nearer the surface. Heat from the cooling magma heats any ground water circulating nearby (figure 14.20). This is the basis for generating geothermal energy. The magma-warmed waters may escape at the surface in geysers and hot springs, signalling the existence of the shallow heat source below (figure 14.21). More subtle evidence of the presence of hot rock at depth comes from sensitive measurements of heat flow at the surface, the rate at which heat is being conducted out of the ever-cooling earth: High heat flow signals unusually high temperatures at shallow depths. High heat flow and recent (or even current) magmatic activity go together and, in turn, are most often associated with plate boundaries. Therefore, most areas in which geothermal energy is being tapped extensively are along or near plate boundaries (figure 14.22). Applications of Geothermal Energy Exactly how the. geothermal energy is used depends largely on how hot the system is. In some places, the ground water is warmed, but not enough to turn to steam. Still, the water temperatures may be comparable to what a home heating unit produces (50 to 90°C, or about 120 to 180°F). Such warm waters can be circulated directly through homes to heat them. This is being done in Iceland and in parts of the former Soviet Union. Other geothermal areas may be so hot that the water is turned to steam. The steam can be used, like any boiler-generated steam produced using conventional fuels, to run electric generators. The largest U.S. geothermal-electricity operation is The Geysers, in California (figure 14.23), which has operated since 1960 and now has a generating capacity of close to 2 billion watts. In 1989, The Geysers and six smaller geothermal areas together generated close to 10 billion kilowatt-hours of electricity (0.3% of total energy consumed) in this country. By 1995, utilities were generating only about 5 billion kilowatt-hours of geothermal electricity, while non-utility power producers generated about twice that much. Other steam systems are being used in Larderello, Italy, and in Japan, Mexico, the Philippines, and elsewhere. Altogether, there are about forty sites; worldwide where geothermal electricity is actively being developed. Environmental Considerations of Geothermal Power Where most feasible, geothermal power is quite competitive economically with conventional methods of generating electricity. The use of geothermal steam is also largely pollution-free. Some sulfur gases derived from the magmatic heat source may be mixed with the steam, but these certainly pose no more serious a pollution problem than sulfur from coal burning. Moreover, there are no ash, radioactive-waste, or carbon-dioxide problems as with other fuels. Warm geothermal waters may be a somewhat larger problem. They frequently contain large quantities of dissolved chemicals that can clog or corrode pipes (a potentially significant problem that will increase operational costs) or may pollute local ground or surface waters if allowed to run off freely (figure 14.24). Sometimes, there are surface subsidence problems, as at Wairakei (New Zealand), where subsidence of up to 0.4 meters per year has been measured. Subsidence problems may be addressed by reinjecting water, but the long-term success of this strategy is unproven. Limitations on Geothermal Power While the environmental difficulties associated with geothermal power are relatively small, three other limitations severely restrict its potential. First, each geothermal field can only be used for a period of time—a few decades, on average—before the rate of heat extraction is seriously reduced. This is because rocks conduct heat very poorly. That characteristic can easily be demonstrated by turning over a flat, sunbaked rock on a bright but cool day. The surface exposed to the sun may be hot to the touch, but the heat will not have traveled far into the rock; the underside stays cool for some time. Conversely, as hot water or steam is withdrawn from a geothermal field, it is replaced by cooler water that must be heated before use. Initially, the heating can be rapid, but in time, the permeable rocks become chilled to such an extent that water circulating through them heats too slowly or too little to be useful (figure 14.25). The heat of the magma has not been exhausted but its transmittal into the permeable rocks is slow. Some time must then elapse before the permeable rocks are sufficiently reheated to resume normal operations. Steam pressure at The Geysers has declined rapidly in recent years forcing the idling of some of its generating capacity: By 1991, despite a capacity of 2 billion watts, electricity production was only 1/2 billion watts, and power generation had declined to half its 1987 peak within a decade. A second limitation of geothermal power is that not only are geothermal power plants stationary, but so is the resource itself. Oil, coal, or other fuels can be moved to power-hungry population centers. Geothermal power plants must be put where the hot rocks are and long-distance transmission of the power they generate is not technically practical, or, at best, inefficient. Most large cities are far removed from major geothermal resources. Also, of course, geothermal power can not contribute to such energy uses as transportation. The total number of sites suitable for geothermal power generation is the third limitation. Clearly, plate boundaries cover only a small part of the earth's surface and many of them are inaccessible (seafloor spreading ridges, for instance). Not all have abundant circulating subsurface water in the area, either. Even accessible regions that do have adequate subsurface water may not be exploited. Yellowstone National Park has the highest concentration of thermal features of any single geothermal area in the world, but because of its scenic value and uniqueness, the decision was made years ago not to build geothermal power plants there. Alternative Geothermal Sources Many areas away from plate boundaries have heat flow somewhat above the normal level, and rocks in which temperatures increase with depth more rapidly than in the average continental crust. Even in ordinary crust, rock temperature increases with depth at a rate of 30° C/kilometer (about 85° F/mile). Where thermal gradients are at least 40° C/kilometer, even in the absence of much subsurface water, the region can be regarded as a potential geothermal resource of the hot-dry-rock type. Deep drilling to reach usefully high temperatures must be combined with induced circulation of water pumped in from the surface to make use of these hot rocks. The amount of heat extractable from hot-dry-rock fields is estimated at more than ten times that of natural hot-water and steam geothermal fields, just because the former are much more extensive. Most of the regions identified as possible hot-dry-rock geothermal fields in the United States are in thinly populated western states with restricted water supplies. There is, therefore, a large degree of uncertainty about how much of an energy contribution they could ultimately make. Certainly, hot-dry-rock geo-thermal energy will be less economical than that of the hot-water or steam fields where circulating water is al-ready present. Although some experimentation with hot-dry-rock fields is underway—Los Alamos National Laboratory has an experimental project in operation in the Jemez Mountains of New Mexico—commercial development in such areas is not expected in the immediate future, particularly if oil prices remain low. The geopressurized natural gas zones described in chapter 13 represent a third possible type of geothermal resource. As hot, gas-filled fluids are extracted for the gas, the heat from the hot water might, in theory, also be used to generate electricity. Substantial technical problems probably will be associated with developing this resource, as was noted. Moreover, it probably would be economically infeasible to pump the spent water back to such depths for recycling. No geopressurized zones are presently being developed for geothermal power, even on an experimental basis. Summary of Geothermal Potential For the near term and well into this century, geothermal energy probably will be a major energy source only in the handful of areas best suited to its production, supplying them with heat and/or electricity. Its overall contribution to either U.S. or world energy consumption in the foreseeable future is likely to be minor. Summary The basis for generating geothermal energy is the heat from the cooling magma warms any ground water circulating nearby. Though geothermal power has economical competition, it has limited use for three reasons: short period to use, not uniform contribution and little sites suitable for using geothermal power. As a result, we need to search for some alternative geothermal sources. Questions for Review 1 Explain the nature of geothermal energy. 2 How is geothermal energy extracted? 3 What factors restrict the use of geothermal energy in time and in space? 4 How do hot-dry-rock geothermal areas explain its potential? For Further Thought 1 Find out some places in China which have potential geothermal resources. Terms to Remember geothermal energy 地热能 geyser 间歇泉 hot spring 热泉 thermal gradient 地温梯度 Other Terms utility: a useful article or device 有用的物体或器械 clog: to obstruct movement on or in: block up 堵塞，阻碍 corrode: to destroy a metal or alloy gradually 腐蚀 stationary: not capable of being moved; fixed 不能被移动的，固定的