Wind Energy

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3

Generators

The generator converts the mechanical energy of the spinning rotor into electricity (see Electric Motors and Generators). Most wind turbines use a generator and transmission in combination. Many of these wind turbines use two generators, a small generator for light winds and a large generator for strong winds. Other wind turbines use a single generator that contains dual electric windings. These dual electric windings accomplish the same task as the combination of a small and a large generator. Some wind turbines use another type of specially designed generator that is driven directly by the rotor without a transmission.

B

Wind Turbine Size

Wind turbines can be arbitrarily divided into three classes: small, medium, and large. Small wind turbines are capable of generating between 50 watts and 60 kilowatts of power, and use rotors ranging in diameter from less than 1 to 15 m (3 to 50 ft). Small wind turbines are installed primarily in remote areas where power is needed but access to conventional sources of electricity is either too expensive or too unreliable. Some small turbines, known as micro-turbines, are so compact they can be carried to remote locations on horseback.

Most commercial wind machines are medium-size turbines. Medium-size turbines use rotors spanning diameters between 15 and 60 m (50 and 200 ft), and have a generating capacity ranging from 50-1,500 kilowatts. Most medium-size commercial turbines have a generating capacity in the range of 500 kilowatts to 750 kilowatts.

Large wind turbines are behemoths with rotors spanning diameters between 60 and 100 m (200 to 330 ft), and are capable of generating 2 to 3 megawatts of power. Because the cost-effectiveness of conventional coal-fired and oil-fired power plants increases with the size of the plants, it was originally thought that giant wind turbines would be more economical than smaller turbines. Various countries have attempted to develop commercial multi-megawatt wind turbines, but these machines have proved less economical and less reliable than medium-size turbines.

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IV

The Modern Wind Energy Industry

Many countries began exploring alternative sources of energy during the oil shortages of the 1970s. As improvements in wind energy technology have evolved, the modern wind energy industry has emerged. Concern about global warming and adoption of the Kyōto Protocol in 1997 also spurred interest in wind energy as an alternative to burning fossil fuels, which release greenhouse gases into the atmosphere. Increasingly, modern wind turbines produce electric power as efficiently as other power generation technologies. One of the major obstacles for developing wind energy is finding suitable terrain and wind conditions.

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Placement of Wind Turbines

Location is critical for maximizing the electricity wind turbines can produce. The amount of kinetic energy available in the wind is a cubic function of wind speed—that is, for every doubling of wind speed, there is a corresponding eight-fold increase in available energy. This exponential relationship between wind speed and wind energy makes location extremely important. A site with high average wind speeds can provide considerably more wind energy than a site with only slightly lower wind speeds.

For most continental locations, winds are strongest during winter and spring and weakest during summer and fall. Regional weather patterns and local topographic conditions can also cause wind patterns to vary. For example, wind speeds in California’s Altamont Pass are highest during summer months when temperature differences between the hot Central Valley and the cold waters of the Pacific Ocean are greatest. These high winds are created by cold Pacific Ocean air rushing in to fill the vacuum created by hot, rising air in the Central Valley.

B

Wind Power Plants

Wind turbines can be installed in single units, in clusters of two to ten turbines, and in large arrays, called wind power plants or wind farms. Wind power plants can contain large numbers of wind turbines. California's Tehachapi Pass contains several wind power plants, each with more than 1,000 wind turbines.

Wind turbines aggregated into wind power plants are thought to generate electricity more economically than single turbines or those in clusters. It can be more cost-effective to operate and maintain large arrays of wind turbines; however, concentrating wind turbines can reduce individual turbine production when turbines upwind disrupt the airflow of turbines downwind.

The arrangement of wind turbines in a wind power plant is often determined by local geography. Wind power plants on flat terrain are often assembled in long parallel rows. One of the world's most visually pleasing wind power plants is Denmark’s Tændpibe-Velling Mærsk, a geometric array assembled in marching-band formation on the country’s Jutland Peninsula.

In hilly or mountainous terrain running perpendicular to prevailing winds, designers often line the ridgetops with long rows of wind turbines. This formation is used in several wind power plants in California's Altamont Pass. Wind turbines may also be placed in long single rows along other windy, exposed land features. Throughout the Netherlands, linear arrays are placed parallel to many of the country's dikes and drainage canals. Long rows of wind turbines are also located along harbor breakwaters at Ebeltoft in Denmark, Zeebrugge in Belgium, and Blyth Harbor in England.

Like conventional power plants, wind farms are an assembly of multiple independent generators—in this case, wind turbines. Although each wind turbine in a wind power plant operates independently, the turbines are typically connected to a central monitoring system. Whether power is produced from two turbines or from two thousand turbines, the power is aggregated and delivered to an electric utility network.

California is home to some of the largest arrays of wind turbines in the world. Wind power plants in the Altamont Pass contain a total of 6,000 wind turbines, and wind farms in the Tehachapi Pass comprise nearly 5,000 wind turbines. Wind farms near Palm Springs include some 3,000 wind turbines.