Correction: Although what I posted below may be true for some of the earlier models of large windmills, it is not true for the later models such as those installed in Spearville. Specifically, there are no longer electric motors used to start or maintain blade movement under low wind conditions, only to control yaw and blade pitch. The blades begin to turn under wind power at around 8 mph, but don't peak in energy generation until wind speed is 31 mph.
CPV has also claimed that the 'blade thump' problem has been eliminated by placing the rotor assembly forward of the mast, instead of to the rear. Although I have not been able to verify this, it would certainly prevent air turbulence created behind the mast from being 'chopped' by the blades passing through it. Although generator noise is substantial, it appears very well insulated within the tower. However, the higher frequency 'wooshing' noises do increase significantly as a function of wind speed. J.P.M.
Electric Windmill Magic Revealed
Why is it that people who tour wind energy installations with their developers always come away with the impression that their machines are quiet? At first I figured it was because standing underneath a windmill you don’t hear the same noise you would at some distance. While this is true, the whole truth is far more insidious and deceitful. The reason they are so quiet is that they are typically NOT GENERATING ANY POWER when the tour is brought through.
It’s all in the blade pitch
Apart from orienting the whole nacelle and blade assembly into the wind, electric motors also control the angle of the blades relative to their plane of rotation. There are basically three orientations that can be achieved, with a range of intermediate orientations possible.
Motorized ‘wind riding’: Feathering
With the blades angled with their edges pointing directly into the wind (zero degree pitch), air flows over the blades without turning them. No power is being generated, but blade resistance to wind flow is minimized and no aerodynamic noise is produced because air turbulence is minimal. This is called ‘feathering’ and is one orientation used to minimize power consumption when there is insufficient wind for power generation. If the blades are turning in this configuration they are actually UNDER POWER and consuming electricity. Because of the sheer size and mass of the blades, it is more economic to keep them turning with a small amount of electricity than it is to start them from rest.
Motorized ‘wind riding’: Slicing
With the blades angled at 90 degrees to the flow of wind, air resistance to blade rotation is minimized, so this is another configuration that can be used to minimize power consumption and keep blades turning when wind is insufficient for power generation. Once again, no power is being generated and no aerodynamic noise is produced, but if the blades are turning in this configuration they are once again UNDER POWER and drawing electricity out of the grid. This configuration is especially deadly for birds and bats because they feel no air turbulence as they fly through the plane of rotation to warn them of on-coming blades set in a ‘slicing’ orientation.
In order to generate power, the blades must be angled at 45 degrees to the wind. Any 10-year old with a hand-held pin wheel knows this. However, unlike the handheld pinwheel, the blades of large windmills are very narrow in order to reduce material fatigue and increase their elasticity under various unpredictable loading forces. Unfortunately, very narrow blades capture very little wind, although by virtue of their sheer mass, combined with mechanical resistance within the gearbox and generator (this can be varied by the operator), they require up to 500 hp just to start turning in ‘idle’ mode – without any power generation yet. This means the wind must blow at some minimum speed before idle rotation can be achieved (this varies among turbine models, but can be as high as 30 mph for the largest ones) and blow at speeds higher than that before actual power generation can occur. So any time you see a wind turbine rotating when the wind speed is low it is DRAWING POWER from the grid – not generating it.
When power generation begins, the gearbox is engaged to convert the low speed rotation into the higher speed rotation necessary for current to be generated by electromagnetic induction. This is typically accompanied by a loud droning noise, reminiscent of a 4-cylinder motorcycle engine. An absence of this noise is another sign that no power is being generated and that turning blades are being driven by electricity, not by wind.
Furthermore, it is only when power is being generated that substantial aerodynamic turbulence is created in the air passing over the blades. At a 45 degree blade pitch, turbulence now follows each blade through its revolution, like a wake following a boat. Every time a blade reaches the vertical position pointing straight down, the mast of the windmill is physically ‘slapped’ by this invisible ‘wake’, producing the low frequency noise and vibration known as ‘blade thump’. Neither blade thump nor any mechanical noise will be heard unless the turbine is actually generating power, which is usually only a small fraction of the time it is actually spinning.
The next time you see a wind turbine ask yourself these questions.
Are the blades angled 45 degrees into the wind ?
Is the wind blowing above 20 mph ?
Is any mechanical noise evident within the proximity of the turbine ?
If the answer to any of these questions is ‘no’, and the turbine blades are turning, they are actually DRAWING POWER FROM THE GRID – not generating it !
Consequently, it is a simple little ‘magic trick’ for wind developers conducting tours of their installations to ensure there is no power generation under way when visitors arrive – only motorized wind riding – and their machines seem serenely silent to all observers, all the while drawing power that must be produced by other forms of generation.
My sincere thanks to Eugene Kalwa, Ph.D. of Toronto, Ontario for sharing his technical analysis of what he refers to as “the bogus electric windmill scam”.
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