BET'Z Limit: -

The Betz limit is the theoretical maximum efficiency for a wind turbine, conjectured by German physicist Albert Betz in 1919. Betz concluded that this value is 59.3%, meaning that at most only 59.3% of the kinetic energy from wind can be used to spin the turbine and generate electricity. In reality, turbines cannot reach the Betz limit, and common efficiencies are in the 35-45% range.

Wind turbines work by slowing down passing wind in order to extract energy. If a wind turbine was 100% efficient, then all of the wind would have to stop completely upon contact with the turbine—which isn't possible by looking at a wind turbine (figure 1). In order to stop the wind completely, the air wouldn't move out of the way to the back of the turbine, which would prevent further air from coming in—causing the turbine to stop spinning.

Proof of Betz's Law: -

http://mstudioblackboard.tudelft.nl/duwind/Wind%20energy%20online%20reader/Static_pages/betz_law.htm

Why wind turbines have three blades?

By and large, most wind turbines operate with three blades as standard. The decision to design turbines with three blades was actually something of a compromise.

Because of the decreased drag, one blade would be the optimum number when it comes to energy yield. However, one blade could cause the turbine to become unbalanced, and this is not a practical choice for the stability of the turbine.

Similarly, two blades would offer greater energy yield than three, but would come with its own issues. Two-bladed wind turbines are more prone to a phenomenon known as gyroscopic precession, resulting in a wobbling. Naturally, this wobbling would create further stability issues for the turbine as a whole. This would also place stress on the component parts of the turbine, causing it to wear down over time and become steadily less effective.

Any number of blades greater than three would create greater wind resistance, slowing the generation of electricity and thus becoming less efficient than a three blade turbine.

For these reasons, turbines designed with three blades are the ideal compromise between high energy yield and greater stability and durability of the turbine itself.

Why are wind turbines white?