<P> The number of blades is selected for aerodynamic efficiency, component costs, and system reliability . Noise emissions are affected by the location of the blades upwind or downwind of the tower and the speed of the rotor . Given that the noise emissions from the blades' trailing edges and tips vary by the 5th power of blade speed, a small increase in tip speed can make a large difference . </P> <P> Wind turbines developed over the last 50 years have almost universally used either two or three blades . However, there are patents that present designs with additional blades, such as Chan Shin's Multi-unit rotor blade system integrated wind turbine . Aerodynamic efficiency increases with number of blades but with diminishing return . Increasing the number of blades from one to two yields a six percent increase in aerodynamic efficiency, whereas increasing the blade count from two to three yields only an additional three percent in efficiency . Further increasing the blade count yields minimal improvements in aerodynamic efficiency and sacrifices too much in blade stiffness as the blades become thinner . </P> <P> Theoretically, an infinite number of blades of zero width is the most efficient, operating at a high value of the tip speed ratio . But other considerations lead to a compromise of only a few blades . </P> <P> Component costs that are affected by blade count are primarily for materials and manufacturing of the turbine rotor and drive train . Generally, the lower the number of blades, the lower the material and manufacturing costs will be . In addition, the lower the number of blades, the higher the rotational speed can be . This is because blade stiffness requirements to avoid interference with the tower limit how thin the blades can be manufactured, but only for upwind machines; deflection of blades in a downwind machine results in increased tower clearance . Fewer blades with higher rotational speeds reduce peak torques in the drive train, resulting in lower gearbox and generator costs . </P>

What is the length of a blade on a wind turbine