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.
Wind power is the conversion wind energy into a useful energy source such as electricity. Wind turbines use an electrical generator grid to convert the electrical current produced by the rotations of the wind turbine blades then transferred into gear box, from gear box it will turn the generator and produce DC electricity, which can be further converted into an alternating current(AC).
Wind energy is actually produced by the sun. the energy from the sun hitting the earth’s surface will converted into wind energy. This conversion happens because of a phenomena called “differential heating” which is a result of the many different types of landscapes and water ways, heating and cooling at different rates causing a global atmospheric convection system.
in this earth, wind rises from the equator and moves north and south in the higher layers of the atmosphere, The region of Earth receiving the Sun's direct rays is the equator. Here, air is heated and rises, leaving low pressure areas behind. Moving to about thirty degrees north and south of the equator, the warm air from the equator begins to cool and sink. Between thirty degrees latitude and the equator, most of the cooling sinking air moves back to the equator. The rest of the air flows toward the poles
to learn more about this see at http://winds-energy.blogspot.com/2007/03/geographical-variation-in-wind-resource.html
picture below also give good visualization of global wind direction in earth 
Wind Distributions in our World
before calculating the exact number or think that your want to builds wind turbine in your area, first let's look same fact date circulation of global wind, and wind speed distributions in our worlds.
United States Annual Average Wind Turbine 
Maps of mean 80-m wind speeds for year 2000
from this map we can get detail information of wind power in each countries.
Europe
North America
South America
Asia
Australia
Africa
Conclusion and Summary
1. Approximately 13% of all stations worldwide belong to class 3 or greater (i.e., annual mean wind speed ≥ 6.9 m/s at 80 m) and are therefore suitable for wind power generation.
2. Sub tropical Countries have more good potential wind energy than tropical countries.
3.Offshore stations experience mean wind speeds at 80 m that are ~90% greater than over land on average.
References
http://www.etopiamedia.net/emeenn/pages/wpw/wpw2-5551212.html
http://www.windstuffnow.com/main/wind_charts.htm
http://www.clemson.edu/scies/wind/Poster-Schmidt.pdf
Vertical-axis wind turbines (VAWT) are a type of wind turbine where the main rotor shaft runs vertically. VAWTs work somewhat like a classical water wheel in which water arrives at a right angle (perpendicular) to the rotational axis (shaft) of the water wheel. Vertical-axis wind turbines fall into two major categories: Darrieus turbines and Savonius turbines.
Savonius Turbine
invented in Finland, the Savonius turbine is S-shaped if viewed from above. This drag-type VAWT turns relatively slowly, but can produce a high torque. It is useful for grinding grain, pumping water and other works that need high torque, but if this kind of turbine connected with electric generator, it only can produce a little electricity
picture below shown schematics Savonius Turbine
The commonest VAWT is a Savonius VAWT which is an extended version of an anemometer (wind speed measuring tool). VAWTs can offer up to 15% efficiency and they work equally well no matter which direction the wind is coming from.
Darrieus wind turbine
A Darrieus machine, a VAWT, is a vertical axis wind turbine. It acts much like a windmill except that a windmill has a horizontal axis
Darrieus wind turbines are not self-starting. Therefore a small powered motor is required to start off the rotation, and then when it has enough speed the wind passing across the aerofoils starts to generate torque and the rotor is driven around by the wind.
In overall comparison, while there are some advantages in Darrieus design there are many more disadvantages, especially with bigger machines in MW class. The Darrieus design uses much more expensive material in blades while most of the blade is too near of ground to give any real power.
others type that derivaties from Darrieus wind turbine are
The giromill was included in Darrieus's 1927 patent for vertical aerofoil powered vertical axis wind turbines, however development is now starting again on new giromills which take advantage of modern ultra strong light materials to produce turbine blades robust enough to cope with the stresses they are put under.
Another variation of the Giromill is the Cycloturbine, in which the blades are mounted so they can rotate around their vertical axis
below is interesting concept for callobaration between Darrieus and Savonius 
