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Title-Wind as alternative energy source
  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.
    Rotor Blades
    Changing the Wind Speed Changes Wind Direction Relative to the Rotor Blade
    In this next picture we have taken one rotor blade from the previous page off its hub, and we look from the hub towards the tip, at the back side (the lee side) of the rotor blade. The wind in the landscape blows between, say 8 m/s and 16 m/s (from the bottom of the picture), while the tip of the blade rotates towards the left side of the picture. In the picture you can see how the angle of attack of the wind changes much more dramatically at the root of the blade (yellow line) than at the tip of the blade (red line), as the wind changes. If the wind becomes powerful enough to make the blade stall , it will start stalling at the root of the blade.
    Lift Direction
    Lift vector imageNow, let us cut the rotor blade at the point with the yellow line. In the next picture the grey arrow shows the direction of the lift at this point. The lift is perpendicular to the direction of the wind. As you can see, the lift pulls the blade partly in the direction we want, i.e. to the left. It also bends the rotor blade somewhat, however.
    Rotor Blade Profiles (Cross Sections)
    As you can see, wind turbine rotor blades look a lot like the wings of an aircraft. In fact, rotor blade designers often use classical aircraft wing profiles as cross sections in the outermost part of the blade. The thick profiles in the innermost part of the blade, however, are usually designed specifically for wind turbines. Choosing profiles for rotor blades involves a number of compromises including reliable lift and stall characteristics, and the profile's ability to perform well even if there is some dirt on the surface (which may be a problem in areas where there is little rain).
    Rotor Blade Materials
    Most modern rotor blades on large wind turbines are made of glass fibre reinforced plastics, (GRP), i.e. glass fibre reinforced polyester or epoxy. Using carbon fibre or aramid (Kevlar) as reinforcing material is another possibility, but usually such blades are uneconomic for large turbines. Wood, wood-epoxy, or wood-fibre-epoxy composites have not penetrated the market for rotor blades, although there is still development going on in this area. Steel and aluminium alloys have problems of weight and metal fatigue respectively. They are currently only used for very small wind turbines.
    Rotor Aerodynamics
To study how the wind moves relative to the rotor blades of a wind turbine, we have fixed red ribbons to the tip of the rotor blades of our model wind turbine, and yellow ribbons some 1/4 out the length of the blade from the hub.
Side view cartoonFront view cartoon
We then let the ribbons float freely in the air (in the cartoon we abstract from the air currents created by the blades themselves, and the centrifugal force). The two images on this page give you one view from the side of the turbine, and another view from the front of the turbine. Since most wind turbines have constant rotational speed, the speed with which the tip of the rotor blade moves through the air (the tip speed) is typically some 64 m/s, while at the centre of the hub it is zero. 1/4 out the length of the blade, the speed will then be some 16 m/s. The yellow ribbons close to the hub of the rotor will be blown more towards the back of the turbine than the red ribbons at the tips of the blades. This is obviously because at the tip of the blades the speed is some 8 times higher than the speed of the wind hitting the front of the turbine.
    Why are Rotor Blades Twisted?
    Rotor blades for large wind turbines are always twisted. Seen from the rotor blade, the wind will be coming from a much steeper angle (more from the general wind direction in the landscape), as you move towards the root of the blade, and the centre of the rotor. As you know, a rotor blade will stop giving lift, if the blade is hit at an angle of attack which is too steep. Therefore, the rotor blade has to be twisted, so as to acheive an optimal angle of attack throughout the length of the blade. However, in the case of stall controlled wind turbines in particular, it is important that the blade is built so that it will stall gradually from the blade root and outwards at high wind speeds.
    Microsoft® Encarta® Encyclopedia 2002. © 1993-2001 Microsoft Corporation. All rights reserved.