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Windmill Efficiency; NASA comments of wind turbines

These are the opinions of Gary Novak (proclaimed wind energy expert) and are contradicted by other wind energy experts.

One can look at two or three blade windmills and see that most of the wind goes between the blades without effect.

One of the reasons is that the mis-definition of energy creates the assumption that higher velocity blades deliver more power, since power is proportional to force times velocity with the erroneous definition of energy. But when definitions are corrected, power equals force only. (See Energy Mis-defined.) Therefore, two more blades could be added, which would increase output by two thirds at almost the same cost.

Windmills for generating electricity would be much more practical than assumed, except that their efficiency is not being properly utilized.

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.

So 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. 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.

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 area 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, which is half as much.

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 primarily 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.

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 by 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. Fewer blades is how to do that. But they need a gear box regardless, and they make low velocity winds unusable with that.

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