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Modern wind energy systems consist of three basic components: a tower on which the wind turbine is mounted; a rotor that is turned by the wind; and the nacelle, which houses the equipment, including the generator, that converts the mechanical energy in the spinning rotor into electricity. The tower supporting the rotor and generator must be strong. Rotor blades need to be light and strong in order to be aerodynamically efficient and to withstand prolonged use in high winds

Improvements in structural design and construction materials have led to the construction of taller towers, allowing rotors to be mounted farther off the ground, where winds are typically stronger. Small wind turbines (less than 1 kilowatt) are installed on simple guyed (cable-anchored) poles ranging in height from 10 to 20 m (30 to 65 ft). Turbines from 1 to 30 kilowatts are installed on lattice or tubular towers ranging in height from 20 to 40 m (65 to 130 ft). Medium-size wind turbines are typically installed on tubular steel towers ranging in height from 25 to 50 m (80 to 165 ft). Wind turbine towers, which house the cables that conduct electricity from the generator through the base of the tower, can be constructed from metal, reinforced plastics, and concrete.

The rotor, which spins when driven by the wind, supports blades that are designed to capture kinetic energy in the wind. Nearly all modern wind turbines have rotors that spin about an axis parallel to the ground. The spinning rotor turns a shaft which converts the wind’s energy into mechanical power. In turn, the shaft drives the generator, which converts mechanical energy into electricity. Although some modern wind turbines have rotor blades made of composite wood, most modern wind turbine blades are made of fiberglass, a lightweight, strong material typically composed of polyester resins and glass fibers. Unlike the American farm windmill, contemporary wind turbines do not use blades made from aluminum or steel; aluminum is unable to withstand continuous stress from flexing in strong winds, and steel is too heavy. Small wind turbines (see Wind Turbine Size section below) typically use a tail vane to keep the rotor pointing into the wind. Most medium-size wind turbines use an electric motor to mechanically aim the rotor into the wind.

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.

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.

Although wind energy is a relatively clean means of generating electricity, there are associated impacts. One of these is the potential of an array of turbines to alter the visual quality of the landscape, especially when located in a scenic area. In addition to aesthetic concerns, noise associated with spinning wind turbine rotors has generated complaints from the public. Another environmental concern associated with wind energy is the impact on wildlife. Wind turbines in some areas are responsible for killing birds—including some protected species—that are accidentally caught in the rotor blades.

Some of these environmental concerns are being addressed by more careful siting of wind turbines. Other concerns, such as the noise of spinning rotor blades, may be partially solved by design refinements that successfully reduce aerodynamic and mechanical noise.