Wind Energy Question? Why do the wind mills have only 3 small blades?

Question

I know that this is an older model but why 3 small blades. Why not 5? 8? I have a theory and I hoping to prove my dad wrong so let me hear your answers.

Here's a picture of what I'm talking about.
http://www.unisa.edu.au/researcher/issue/2005may/images/winds.jpg
http://www.kidcyber.com.au/IMAGES/windfarm.jpg

Answer 1

Windmills with a high number of blades move at high velocities, and low amount of torque. The high velocity of the blades creates a lot of turbulence and drag, which is inefficient. In moderately high winds, a large number of blades can cause cavitation, which is a situation where turbulence ends up stopping the flow of air across the blade. The large, 3-blade models have very little drag, they have high torque needed to turn electrical generators, and they can operate in extreme winds without causing cavitation.

Answer 2

Depending on the design of the blade and the speed at wich it turns, the blades cause turbulence that would reduce the efficiency of the following blades of the turbine.

By haveing fewer blades (I know some that have only 2) the turbulence factor can be reduced.

Some older multi blade windmills were not designed to turn as fast and therefore did not have as big of a turbulence problem. Also they were not designed with modern wind tunnels to determine the most efficient design.

Answer 3

The blades are more like aircraft wings (not aircraft propellers). When the wind blows across them to create "lift" -- that is, the force to turn the shaft, it also gets turbulant. Turbulance is bad -- it causes unstable rotation, and loss of power.

This turbulance is especially bad when the blade crosses the pole.

It takes time for the turbulance to dissipate (blow away), and if there were 4, 5, or 8 blades rotating that turbulance would not be "gone" by the time the next blade crossed the pole. With 3 blades, there is enough time for the turbulance to dissipate.

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Answer 4

More blades would obviously turn the turbine faster, generating more electricity. Which one would think would be beneficial. However, as in all things controlled by the powerful energy commissions, you are limited by how much energy you can produce.

Wind farms are allowed to sell back excess energy to the power companies, but only a certain ammount. Anything produced above that limit is considered wasted energy.

The reasons for the three blades was a scientific calculation designed to produce the right ammount of energy to fullfill the needs of the user and also provide just enough energy for sell back to the power company.

More blades = more power = more waste.

Less blades = less power = profit loss for the wind tower owners.

Cheers!