428托福机经阅读拓展

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小马过河428托福机经，由于大多阅读都只是给个题目，我在google及其它软件上找到一些于托福阅读相似的文章，对于大家提前了解背景知识很有帮助，有的时候我想大家不是不会，就是觉得文章比较生，心里紧张……不一定就能完全压中，大家就当是课外阅读吧。
第一篇，20101113NA阅读 The development of waterpower
Waterpower, power derived from the fall of water from a higher to a lower level, and extracted by means of waterwheels or hydraulic turbines. Waterpower is a natural resource, available wherever a sufficient volume of steady water flow exists. The development of waterpower today requires extensive construction, including storage lakes, dams, bypass canals, and the installation of large turbines and electric generating equipment (see Electric Power Systems). Because the development of hydroelectric power requires a large capital investment, it is often uneconomical for a region where coal or oil is cheap, even though the cost of fuel for a steam-powered generating plant is higher than the cost of running a hydroelectric plant. However, increasing environmental concerns are focusing attention on renewable energy sources such as water (see World Energy Supply).

The use of waterpower dates from ancient Greece and Rome, where waterwheels were used for the milling of corn. The availability of cheap slave and animal labor, however, restricted the widespread application of waterpower until about the 12th century. During the Middle Ages, large wooden waterwheels were developed with a maximum power output of about 50 horsepower. Modern waterpower owes its development to the British civil engineer John Smeaton, who first built large waterwheels using cast-iron construction.

Waterpower played an important part in the development of early American towns. Starting with the grist mill and the sawmill of colonial times, waterpower gave impetus to the growth of the textile, leather, and machine-shop industries in the early 1800s. Although the steam engine had already been developed, coal was scarce and wood unsatisfactory as a fuel. Waterpower helped to develop the early industrial cities until the opening of the canals to the Midwest provided cheap coal by the middle of the 19th century. Most of the cities in the Midwest developed along the so-called fall line, where the upland mountain region meets the coastal plain. Combining the advantages of available waterpower with location at the upstream end of river navigation, the earliest waterworks that led to the growth of cities in the Northeast were located at what are now Pawtucket, Rhode Island (1790), Paterson, New Jersey (1791), and Fall River, Massachusetts (1813). The Mississippi River fall line at Saint Anthony Falls is largely responsible for the development of Minneapolis, Minnesota, as a grain-milling center.

Early American waterpower works were limited to drops of about 5 m (about 16 ft), and dams and canals were necessary for the installation of successive waterwheels when the drop was greater. Large storage-dam construction, however, was not feasible, and low water flows during summer and fall, coupled with icing during the winter, led to the replacement of nearly all waterwheels by steam when coal became readily available.


The rebirth of waterpower had to await the development of the electric generator, further improvement of the hydraulic turbine, and the growing demand for electricity by the turn of the 20th century (see Electric Motors and Generators). Commercial power companies began to install a large number of small hydroelectric plants in the mountain regions near the major population centers, and by 1920 hydroelectric plants accounted for 40 percent of the electric power produced in the United States.

	World’s Largest Dams

Hydroelectric power generation was accelerated by the establishment of the Federal Power Commission in 1920. Although additional hydroelectric plants were being built, the simultaneous development of larger and more cost-efficient steam-power plants made it obvious that only very large and costly hydroelectric installations could compete effectively, and that the federal government would have to assume a major share in their construction. Motivated by the search for the multiple use of water resources, including navigation, flood control, and irrigation, in addition to power production, the Tennessee Valley Authority, or TVA, started government participation in large-scale waterpower development in 1933.

	Electricity Generators

	Electricity Generators Generators at the Bonneville Dam in Oregon produce electricity as water flows through large turbines and drives the axles of the generators. The Bonneville Dam is located on the Columbia River between the states of Oregon and Washington. The Bonneville plant is one of many hydroelectric stations in the northwestern United States. Encarta Encyclopedia Porterfield-Chickering/Photo Researchers, Inc.
	Full Size

Most major installations depend on a large water-storage reservoir upstream of the dam where water flow can be controlled and a nearly constant water level can be assured. Water flows through conduits, called penstocks, which are controlled by valves or turbine gates to adjust the flow rate in line with the power demand. The water then enters the turbines and leaves them through the so-called tailrace. The power generators are mounted directly above the turbines on vertical shafts. The design of turbines depends on the available head of water, with so-called Francis-type turbines used for high heads and Kaplan, or propeller turbines, used for low heads.

In contrast to storage-type plants, which depend on the impounding of large amounts of water, a few examples exist where both the water drop and the steady flow rate are high enough to permit so-called run-of-the-river installations; one such is the joint American-Canadian Niagara Falls power project. See Niagara Falls (waterfall).

Small-scale hydroelectric plants, with capacities between 1 kilowatt and 1 megawatt, are being developed in some countries. In many of China's districts, for example, such dams are the main source of electric power. Other developing nations are also showing interest in such projects, which can make good use of available labor. In the United States, interest in small-scale plants increased following passage of the Public Utilities Regulatory Policies Act in 1978, because it states that large utilities must buy power fed into their lines by these small-scale plants.

IV.	GLOBAL HYDROELECTRIC POWER GENERATION

Worldwide, hydropower represented 17 percent of the total energy generated in 2003, the most recent year for which data are available. In many countries, hydroelectric power is the dominant source of electric power. In 2003 Norway derived 99 percent of its power from hydroelectric plants. The same year, hydroelectric power provided 100 percent of the electricity used in the Democratic Republic of the Congo (DRC, formerly Zaire) and 84 percent of the electricity used in Brazil.

The Itaipu hydroelectric plant on the Paraná River between Brazil and Paraguay, officially dedicated in 1982, has the greatest capacity in the world (12,600 megawatts when placed in full operation). The Grand Coulee Dam, the largest hydroelectric power plant in North America, provides about 6,480 megawatts.

Canada, the largest producer of hydroelectric power in the world, generated 332.5 billion kilowatt-hours (kwh) in 2003. This figure constituted 59 percent of the nation’s electric power. Hydroelectric-power generation in the United States increased from about 16 billion kwh in 1920 to 275.8 billion kwh in 2003. Although the United States runs a close second to Canada in the total amount of hydroelectric power produced, only 7 percent of the electric power used in the United States was generated by hydroelectric power plants in 2003.

ENVIRONMENTAL IMPACT

Hydroelectric power is regarded by some as a relatively clean source of energy because it emits fewer greenhouse gases than thermal power plants. Greenhouse gases contribute to global warming and climate change, and for many environmentalists, the buildup of greenhouse gases in the atmosphere is the most important environmental issue. However, the dams used to generate hydroelectricity can have other adverse environmental consequences.

Dams alter the water temperatures and microhabitats downstream. Water released from behind dams usually comes from close to the bottom of the reservoirs, where little sunlight penetrates. This frigid water significantly lowers the temperatures of sun-warmed shallows downstream, rendering them unfit for certain kinds of fish and other wildlife. Natural rivers surge and meander, creating small pools and sandbars that provide a place for young fish, insects, and other river-dwelling organisms to flourish. But dams alter the river flow, eliminating these microhabitats and, in some cases, their inhabitants.


Finally, dams also prevent nutrient-laden silt from flowing downstream and into river valleys. Water in a fast-moving river carries tiny particles of soil and organic material. When the water reaches a pool or a flat section of a river course, it slows down. As it slows, the organic matter it carries drops to the river bottom or accumulates along the banks. Following heavy rains or snowmelt, rivers spill over their banks and deposit organic matter on their floodplains, creating rich, fertile soil. Some of the organic matter makes it all the way to river mouths, where it settles into the rich mud of estuaries, ecosystems that nourish up to one-half of the living matter in the world’s oceans. Large dams artificially slow water to a near standstill, causing the organic matter to settle to the bottom of the reservoir. In such cases, downstream regions are deprived of nutrient-laden silt.

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一般来说ets都会节选一部分的，大家有选择的看