Power output from a wind generator is equal to the cube of the windspeed. For every double in windspeed, the capacity of wind generators increases eightfold. For a general rule, a wind generators are practical where the average windspeed is greater than 12 mph (19 kph).
For a given survivable wind speed, the mass of a propeller (calculated from volume) is proportional to approximately the cube of its length. Wind intercepted by the propeller is proportional to the square of its length. The maximum length of a propeller is limited by both the strength and stiffness of its material. Labor and maintenance costs increase slowly with increasing propeller size, so given all these factors, to minimize costs, wind farm propellers are basically limited by their material strength. One of the best construction material available in 2001 is graphite. Graphite enables propellers of a radius of about sixty meters to be built, enough to tap a few megawatts of power. Smaller propellers for generators with a limited purpose are often made of lightweight fiberglass.
Gyroscopic inertia is proportional to the velocity change in the masses of the rotating structure when pivoted. What this means is that for each blade on a propeller of a pivoting wind generator, its gyroscopic force is at a minimum when the blade is horizontal and at a maximum when the blade is vertical. So, in the case of a one or two bladed propeller, as the wind changes direction it tends to start pivoting as the blade is horizontal, stop pivoting when the blade is vertical, the stop and go action creating extra wear and tear on the machine. When there are three or more blades, there are always at least two blades that are not quite horizontal, so the gyroscopic inertia changes less, which makes for a smoother pivot.
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 propeller 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 propeller is usually placed in front. It is pointed into the wind with a fin on small machines and usually a servo on large machines. The propeller 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 the first windmills. Unfortunately they have a short service life. Also they have a relatively low lift to drag ratio which means that they turn the generator slowly, waste much of the avaliable 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 propeller is spun by the wind, it adds a rotation to the wind, wasting some of the wind's power. Counter rotating propellers can be used to reduce this loss.
When the counter rotating propellers 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 one propeller is connected to the rotor and the other propeller is connected to the field windings. Counter rotating propellers geared together to the same generator need a complicated gear train which therefore has additional gearing losses. Counter rotating propellers connected to the rotor and stator are mecanically simpler; but, the field windings need slip rings which adds complexity, wastes some electricity and wastes some mecanical power.
Counter rotating propellers can be on opposite sides of the tower. In this case it is best that the one in back be smaller than the one in 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 propeller 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 propellers 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 propeller generator for a given tower. Putting helps pulls its side downwind. Since the rear propeller will be at a considerable distance behind the front propeller, it provides considerable leverage for a fin placed there, which along with the propeller which stalls at a higher wind speed being in back, means no servo is necessary to point the machine into the wind. To reduce sympathetic vibrations, the two propellers should have an irrational relative rate, the square root of two for instance. Overall, this is more complicated than the single propeller wind generator; but, taps more of the wind and can tap it at a wider speed range.
These devices were first used to pump water and mill grain, by the 1930's they were mainly used to generate electricity. The first propeller wind generators were placed atop brick towers, once high tensile steel was available they were placed atop open steel lattices, as aesthetics and durability became more important they were placed atop steel or reinforced concrete shells. Small ones are connected to the tower on the ground, then the tower is raised into position. For large ones, the generator is hoisted atop the tower and there is a ladder or staircase inside the tower to allow technicians to reach and maintain the generator.
Originally wind generators were built right next to where their power was needed. With the availability of long distance power transmission, wind generators are now often on wind farms in windy locations and huge ones are being built offshore. Since they're an inexpensive means of generating electricity they are being widely deployed, the main reason the world won't be covered with them is because they're an eyesore and a bit noisy.
For Further Reading
[Guided Tour on Wind Energy] [Air 403] [Residential Wind Power Q&A]
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