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