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  3. How Wind Energy Works Harnessing the wind is one of the cleanest, most sustainable ways to generate electricity. Wind power

How Wind Energy Works Harnessing the wind is one of the cleanest, most sustainable ways to generate electricity. Wind power

admin2010-04-30  30
问题                                          How Wind Energy Works
    Harnessing the wind is one of the cleanest, most sustainable ways to generate electricity. Wind power produces no toxic emissions and none of the heat trapping emissions that contribute to global warming. This, and the fact that wind power is one of the most abundant and increasingly cost-competitive energy resources, makes it a viable alternative to the fossil fuels that harm our health and threaten the environment.
The History of Wind Power
    Wind power is both old and new. From the sailing ships of the ancient Greeks, to the grain mills of pre-industrial Holland, to the latest high-tech wind turbines rising over the Minnesota prairie, humans have used the power of the wind for thousands of years.
    In the United States, the original heyday of wind was between 1870 and 1930, when thousands of farmers across the country used wind to pump water. Small electric wind turbines (叶轮机) were used in rural areas as far back as the 1920s, and prototypes of larger machines were built in the 1940s. When the New Deal brought grid-connected electricity to the countryside, however, windmills lost out.
    Interest in wind power was reborn during the energy crises of the 1970s. Research by the U.S. Department of Energy (DOE) in the 1970s focused on large turbine designs. While these 2- and 3-MW machines proved mostly unsuccessful at the time, they did provide basic research on blade design and engineering principles. In the early 1990s, improvements in technology resulting in increased turbine reliability and lower costs of production provided another boost for wind development.
    In other parts of the world, particularly in Europe, wind has had more consistent, long-term support. As a result, European countries are currently capable of meeting more of their electricity demands through wind power. Denmark, for example, already meets about 20 percent of its electricity demand from wind power. Wind generation also accounts for about six percent of the national power needs in Spain, and five percent in Germany. Serious commitments to reducing global warming emissions, local development, and the determination to avoid fuel imports have been the primary drivers of wind power development in Europe.
The Wind Resource
    The wind resource how fast it blows, how often, and when plays a significant role in its power generation cost. The power output from a wind turbine rises as a cube of wind speed. In other words, if wind speed doubles, the power output increases eight times. Therefore, higherspeed winds are more easily and inexpensively captured.
    Wind speeds are divided into seven classes with class one being the lowest, and class seven being the highest. A wind resource assessment evaluates the average wind speeds above a section of land (usually 50 meters high), and assigns that area a wind class. Wind turbines operate over a limited range of wind speeds. If the wind is too slow, they won’t be able to turn, and if too fast, they shut down to avoid being damaged. Wind speeds in classes three (6.7 - 7.4 meters per second (m/s)) and above are typically needed to economically generate power. Ideally, a wind turbine should be matched to the speed and frequency of the resource to maximize power production.
    Several factors can affect wind speed, and the ability of a turbine to generate more power. For example, wind speed increases as the height from the ground increases. If wind speed at 10 meters off the ground is 6 m/s, it will be about 7.5 m/s at a height of 50 meters. The rotors (旋翼) of the newest wind turbines can now reach heights up to 70 meters. In addition to height, the power in the wind varies with temperature and altitude, both of which affect the air density.
    The more the wind blows, the more power will be produced by wind turbines. But, of course, the wind does not blow consistently all the time. The term used to describe this is "capacity factor", which is simply the amount of power a turbine actually produces over a period of time if it had run at its full rated capacity over that time period.
    A more precise measurement of output is the "specific yield". This measures the annual energy output per square meter of area swept by the turbine blades as they rotate. Overall, wind turbines capture between 20 and 40 percent of the energy in the wind. So at a site with average wind speeds of 7 m/s, a typical turbine will produce about 1,100 kilowatt-hours (kWh) per square meter of area per year. If the turbine has blades that are 40 meters long, for a total swept area of 5,029 square meters, the power output will be about 5.5 million kWh for the year. An increase in blade length, which in turn increases the swept area, can have a significant effect on the amount of power output from a wind turbine.
    The Mechanics of Wind Turbines
    Modern electric wind turbines come in a few different styles and many different sizes, depending on their use. The most common style, large or small, is the "horizontal axis design" (with the axis of the blades horizontal to the ground). On this turbine, two or three blades spin upwind of the tower that it sits on.
    From the outside, horizontal axis wind turbines consist of three big parts: the tower, the blades, and a box behind the blades, called the nacelle. Inside the nacelle is where most of the action takes place, where motion is turned into electricity. Large turbines don’t have tail fans. Instead they have hydraulic controls that orient the blades into the wind.
   In the most typical design, the blades are attached to an axle that runs into a gearbox. The gearbox, or transmission, steps up the speed of the rotation, from about 50 rpm up to 1,800 rpm. The faster spinning shaft spins inside the generator, producing AC electricity. Electricity must be produced at just the right frequency and voltage to be compatible with a utility grid. Since the wind speed varies, the speed of the generator could vary, producing fluctuations in the electricity. One solution to this problem is to have constant speed turbines, where the blades adjust, by turning slightly to the side, to slow down when wind speeds gust. Another solution is to use variable-speed turbines, where the blades and generator change speeds with the wind, and sophisticated power controls fix the fluctuations of the electrical output.
The Market for Wind
    The cost of electricity from the wind has dropped from about 25 cents/kWh in 1981 to as low as 4-6 cents/kWh in recent years. Though wind turbine prices have increased some since 2005, in areas with the best resources, wind power is cost-competitive with new generation from coal and natural gas plants.
    As wind power costs become more competitive, demand is growing exponentially all over the world. Global wind power capacity rose from just over 6,000 MW in 1996 to more than 59,000 MW by the end of 2005 almost a ten-fold increase. Growth has recently been most significant in Northern Europe, Spain, and India, but markets in Asia and the Pacific region are emerging as well.
    At the end of 2005, the U.S. wind power market reached more than 9,100 MW providing enough power to serve the needs of 2.3 million homes. The majority of this capacity is located in California, Texas, Iowa, and Minnesota, but there are wind power projects either in operation or under development in at least 36 states.
The Future of Wind Power
    With increasingly competitive prices, growing environmental concerns, and the call to reduce dependence on foreign energy sources, a strong future for wind power seems certain. The global wind capacity will double in size to over 120,000 MW by 2010, with much of the growth happening in the United States, India, and China. Turbines are getting larger and more sophisticated. The next frontiers for the wind industry are deep-water offshore, and land-based systems capable of operating at lower wind speeds. Both technological advances will provide large areas for new development.
    As with any industry that experiences rapid growth, there will be occasional challenges along the way. For example, beginning in 2005, high demand, increased steel costs (the primary material used in turbine construction), increased profit margins, and certain warranty issues have led to turbine shortages and higher prices. There are also concerns about collisions with bird and bat species in a few locations. And the not-in-my-backyard (NIMBY) issue continues to slow development in some regions. But new manufacturing facilities, careful siting and management practices, and increased public understanding of the significant and diverse benefits of wind energy will help overcome these obstacles.
The term used to describe the amount of power a wind turbine actually produces over a period of time is______.
选项
答案"capacity factor"
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