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Wind Turbine DesignWind Turbine DesignA wind turbine strongly resembles a propeller, but has subtle differences. The turbine is perpendicular to the wind, mounted on a tower. With small wind generators the tower height is usually at least twenty meters. In the case of large generators, the tower height is about twice as great as the propeller radius. Power output from a wind generator is proportional to the cube of the wind speed. As wind speed doubles, the capacity of wind generators increases eightfold. There is usually a means of stalling the turbine's blades to reduce its wind resistance when the wind is extremely strong. For a given survivable wind speed, the mass of a turbine (calculated from volume) is approximately proportional to the cube of its blade-length. Wind intercepted by the turbine is proportional to the square of its blade-length. The maximum blade-length of a turbine is limited by both the strength and stiffness of its material. Labor and maintenance costs increase only slowly with increasing turbine size, so given all these factors, to minimize costs, wind farm turbines are basically limited by the strength of materials, and siting. One of the best construction materials available (in 2001) is graphite-fiber in epoxy. Graphite composites enable turbines of sixty meters radius to be built, enough to tap a few megawatts of power. Smaller turbines can be made of lightweight fiberglass, aluminum, or sometimes laminated wood. Small machines are pointed into the wind by a vane. Large machines have a wind-sensor driving a servomotor. When it turns to face the wind, the turbine acts like a gyroscope. When the turbine pivots to face the wind, precession tries to twist the turbine into a forward or backward somersault. For each blade on a wind generator's turbine, precessive force is at a minimum when the blade is horizontal and at a maximum when the blade is vertical. This cyclic twisting can quickly fatigue and crack the blade roots, hub and axle of the turbine. To reduce the precessive stresses, modern turbines have three blades, only one of which is in a maximum stress position (vertical) at a time. The major historic design defect is to have an even number of blades, so that two blades are vertical at the same time. Two-bladed turbines have the highest cyclic stresses. Home-made wind turbines often have two blades, e.g. something a person can easily carve from one long piece of wood, and use to turn an automobile alternator. Two-bladed turbines thus often avoid the need for using a hub with linkages to individual blades. Many commercially made wind turbines, such as the Whisper 175, still use 2 blades, because such turbines are easy to construct, and the blade(s) can be shipped easily in one long package. Three-bladed turbines, which are much more efficient, and more quiet, must usually be assembled onsite. When there are four or more blades, the blades of a high-speed, high efficiency turbine start stalling in the disturbed air from the previous blade. There are a number of vibrations that decrease in peak intensity as the number of blades increases. Some of the vibrations, besides wearing out the machine, are also audible. However, fewer, larger blades operate at a higher Reynolds number and are therefore more efficient. Also, the cost of the turbine increases with the number of blades, so the optimum number of blades turns out to be three. Since a tower produces turbulence behind it, the turbine is usually placed in front. The turbine has to be placed a considerable distance in front and sometimes tilted up a small amount to ensure that the lower blade doesn't impact the tower. Downwind machines are occasionally built despite the problem of turbulence because they don't need an additional pointing device and in high winds, the blades can be allowed to bend which reduces their wind resistance. Sails were originally used on early windmills. Unfortunately they have a short service life. Also, they have a relatively high drag for the force they capture. They turn the generator slowly, waste much of the available wind power and have a large wind resistance for their power output, requiring a strong wind tower. For these reasons they were superseded with solid airfoils. When a turbine is spun by the wind, it adds a rotation to the wind, increasing the apparent wind on the blade. Since blades are really designed to work like an airplane wing, this increases the torque produced by the turbine. But this also increases the force in the wind direction on the blade and therefore on the tower. The mechanical stress is significantly higher when the turbine rotates. That's why wind turbines are stopped during high wind. Counter rotating turbines can be used to increase the rotation speed of the electrical generator. When the counter rotating turbines are on the same side of the tower, the one in front is angled inwards slightly so as to never hit the rear one. They are either both geared to the same generator or, more often, one is connected to the rotor and the other to the field windings. Counter rotating turbines geared to the same generator have additional gearing losses. Counter rotating turbines connected to the rotor and stator are mechanically simpler; but, the field windings need slip rings which adds complexity, wastes some electricity and wastes some mechanical power. Counter rotating turbines can be on opposite sides of the tower. In this case it is best that the one at the back be smaller than the one at the front and set to stall at a higher wind speed. This way, at low wind speeds, both turn and the generator taps the maximum proportion of the wind's power. At intermediate speeds, the front turbine stalls; but, the rear one keeps turning, so the wind generator has a smaller wind resistance and the tower can still support the generator. At high wind speeds both turbines stall, the wind resistance is at a minimum and the tower can still support the generator. This allows the generator to function at a wider wind speed range than a single-turbine generator for a given tower. Putting a turbine at the back helps pull its side downwind. Since the rear turbine will be at a considerable distance behind the front, it provides considerable leverage for a fin placed there, which means no servo is necessary to point the machine into the wind. To reduce sympathetic vibrations, the two turbines should have an irrational relative rate, (e.g. the square root of two). Overall, this is more complicated than the single-turbine wind generator, but taps more of the wind at a wider range of wind speeds. Go to... |