Efficiency is important for windmills, because the entire cost is in the technology, not the source of energy, which is wind.
The most significant factor determining the efficiency of windmills is the number of blades. This is because more blades capture more energy.
Large windmills must use few blades due to stress on the metal. Yet the concept of windfarms require large windmills. Over sea coasts, they must be very large; but most places, they could be smaller and use more blades. For homeowners, they are small, yet they are not using the large number of blades which would create high efficiency.
A large government study measured two blade systems only and showed moderate efficiency. Smaller, multi blades systems would have been much more efficient.
The overall efficiency of a windmill has to be directly measured, it cannot be calculated, because there are too many interacting factors. A good guess at design efficiency can be made by direct observation. Only measurements under operating conditions can improve upon direct observation.
For this reason, the US government spent a large amount of money (probably more than a billion dollars) during the late seventies and eighties creating experimental windmills for testing their practicality. But researchers only studied two blade systems. The results were worthless, in spite of having tested dozens of windmills. None of the experimental windmills were designed for good overall efficiency, and the most important questions were not studied. The test designs were so large that the structures made them impractical, and such designs are never used. Three blade systems are used on wind farms.
Overall efficiency means the amount of electricity that can be generated over time on a cost basis. In other words, will a kilowatt hour cost ten cents, twenty cents, or what?
Two factors are important in determining overall efficiency of a windmill. One is its ability to use low velocity wind, and the other is its conversion efficiency.
The ability to use low velocity wind determines whether the windmill is working or doing nothing while wind velocities are low, which is a large part of the time.
For example, an locale might have wind of 15 miles per hour (mph) or greater 20% of the time, and 10 mph or greater 40% of the time. A windmill that can use 10 mph wind is operating 40% of the time, while one requiring 15 mph wind is only operating 20% of the time.
The windmills tested by the experimenters would not use low velocity wind. The minimum wind speed required by their "best" designs required more than 15 mph wind speed. There is no reason why 5-10 mph winds could not be utilized to create highly efficient windmills for generating electricity.
To utilize low velocity wind effectively, there has to be more blades. The researchers studied two blade systems. They never even made comparisons to systems with more blades.
Conversion efficiency also requires more than two blades. More blades allows lower rpms, which results in less turbulence, so conversion is more efficient.
A quagmire of technicalities does not change these overall facts. Arguers will talk about torque and velocity trying to prove something, but it proves nothing, because there are infinite options in integrating all of those factors.
For example, they repeatedly say high rpm rotors are needed for generating electricity. That's mindlessness, because the rpms (revolutions per minute) are determined by diameterthe larger the slower.
The tip speed of the blade increases as the circular diameter of motion increases. In the government experiments, two blade systems were used, with rotors up to 300 ft long. Tip speeds were in excess of 200 mph, which created problems with noise and vibrations. Normal rotation was 18 rpm. This means more than 3 seconds per revolution.
Notice that the rpms have to decrease as the diameter of motion increases. Otherwise tip speeds would be excessive. So the rpms are determined by the diameter of motion, not the number of blades.
Arguers say high rpm at low torque is needed for generating electricity, instead of low rpm and high torque. Supposedly, they are saying, use fewer blades for higher rpms. But the diameter has more influence on rpms than the number of blades, and no one is suggesting small diameters.
They need a gear box regardless, and they make low velocity winds unusable with fewer blades. A gear box makes the torque-speed question irrelevant. And they contradict themselves to an extreme by using large diameters of motion, which is what really requires low rpms.
They could get the same rpms by using a smaller diameter of motion with more blades. They would then need more rotors per unit output, but more small ones are cheaper and more efficient than fewer large ones. Throwing away the low velocity wind and creating turbulence result in loss of efficiency.
Also, metal stress becomes the limiting factor with large size. Smaller windmills reduce that problem. One of the most significant facts about size is that steel gets like rubber when large. Excessive steel and heavy mass create the overriding expenses for large windmills.
The government experimenters said they could produce electricity at the usual price by using their large two blade systems, which required wind speeds in excess of 15 mph. This means that if systems were designed to use lower velocity winds with less turbulence, they would have produced electricity cheaper than other sources.
The design of the government windmills was extremely expensive due to the large size. Supposedly, the larger the size, the greater the efficiency. That point was not tested, but general appearance indicates that the technical difficulties of getting control of forces that high off the ground was not as cost-effective as smaller designs would be.
Let me say this in plain English. Anyone can look at an old-fashioned farm windmill, which had about 18 blades, and compare it to an experimental windmill, which had 2 blades, and see that the farm windmill was extremely efficient, and the experimental windmill is extremely inefficient. It is quite visible that unused wind goes between the blades with two and three blade systems. Propagandists use a lot of numbers and jargon to deny the obvious, but their numbers are not fact.
Modern wind farms use three blade systems, which is a major improvement over the two blade systems which were studied. Diameter is reduced from about 300 ft to about 120 ft, which reduces tip speeds and cumbersome stress forces. Whether four or five blades would be an improvement depends upon mechanical factors and reliability; but if smaller diameter and multi-blade systems were designed, they would be more practical for small users and home owners.
The point is that if commercial windmills used five blades with a diameter of 60 ft. they would produce much more energy per pound of steel than the present ones which tend to be three blades and 120 ft. diameter.
If homeowners were using sheet metal blades with a diameter of 6 ft and close to the ground, they would produce much more energy at a fraction of the cost compared to the three blade systems 50 ft high. Trying to get every bit of wind through height is ridiculous when they throw most of it away through speed and turbulence.
What has occurred since early studies is that energy companies build ever larger windmills with less and less efficiency, because they get paid on a cost-plus basis. The more wasteful they are, the more money they make. All the while, the government makes up fake numbers which rationalize any absurdity without explanations in contradiction to obvious logic.