comparison of coefficient of efficiency between two blade and three blade. three, six, and twelve blade system.
The major factors involved in deciding the number of blades includes:
1. the effect on power coefficient;
2. the design TSR (tip-speed ratio);
3. The means of yawing rate to reduce the gyroscopic fatigue.

Effect of blade number on aerodynamic performance
Various experiments results was publish in internet or book what the exact number of blade that have good aerodynamic performance? Solidity of material, kind of material, coefficient of friction on the blade surface, chord (width) of blade turbine and much others.
One that very interesting to make conclusion and discussion is relation between number of blade and coefficient of performances of wind turbine machines.
The best blades number from 3 until 12
When designing number of blade, the number of blade that we choose influence the aerodynamic performance like coefficient of performances.
Modern wind turbines are neither built with many rotor blades nor with very wide blades even though turbines with high solidity (defined as the ratio between the actual blade areas to the swept area of a rotor) have the advantage of enabling the rotor to start rotating easily because more rotor area interacts with the wind initially.
Since, our current goal is to convert the wind energy into electricity, rotors will not benefit with high solidity because it is neither cost effective nor efficient. The number of the blades of a turbine has great impact on its performance.
Picture below shown coefficient of performances between 3,6, and 12 blades with same solidity and same speed 5 m/s, from this picture we can concludes that 3 blade have the most efficient number of blades, as we know almost 70% modern wind turbine use 3 blades.

Cp calculate from equation

One, two or three blades ?
The next question is condition for three blades, two blades and one blade
Single-bladed wind turbine has the advantage of saving the cost of one rotor blade and its weight; it also runs much faster. However, it is not widespread commercially due to the difficulties of balancing the rotor, It is also likely to generate a supersonic tip speed and a highly pulsating torque and causing excessive vibrations.
Besides of the higher rotational speed, the noise, and visual intrusion problems, these turbines also require a counterweight to be placed on the other side of the hub from the blade in order to balance the rotor. This obviously negates the savings on weight compared to a two-bladed design.

Picture above shown that three blades turbine is more efficient to produce energy, two blades machine turbine need more speed to produce energy that same with three blades machine turbine, another way to increase efficiency is increase chord of blade, two bladed machine with 50 % increased chord have similar efficiency with three blade system although not same.
The maximum output power drops by almost exactly one third due to the
reduction in the number of blades, but the reduction in energy yield is less
severe at 19 percent. This is because, although the coefficient of performance
(CP) for the two-bladed machine is very nearly two thirds that of the threebladed
machine at the low tip speed ratio (62:8=16 ¼ 3:9) corresponding to
peak power output, the maximum value of CP is almost as large as that of the
three-bladed machine.
The reduced number of blades and reduced rated power lead to an overall
cost reduction of 16 percent (made up of 6 percent on the blades, 1 percent on
the foundation and 9 percent on the gearbox, brake, generator and grid
connection), leading to an increase in energy cost of 4 percent compared with
the baseline three-bladed machine.


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