Wind Turbine Blades

Given a specific size (diameter) and shape of a lift-type blade, there are three other variables that have a significant impact on the maximum power output of a wind turbine.

1)The blade angle of attack can either increase (or decrease) the amount of drag that is created as air passes across a turbine blade. The angle of attack is the angle between the chord of the blade and the direction of the air flow (assuming that the air flows approximately parallel to the ground). Basically, with a given wind speed, the higher the attack angle, the greater the lift–up to a certain extent. Also, at higher wind speeds, a lower angle of attack will produce the same lift as a higher angle of attack at lesser speeds. On a rotating propeller, air moves faster towards the tip of a propeller blade. Therefore, the angle of attack of the blade can be lessened towards the tip of the blade. This is the reason why some propeller blades have a slightly twisted shape. Less sophisticated blades do not have this twist and are merely set at a fixed angle (or “pitch”) with respect to the plane of rotation. (Whether “twisted” or not, most blades still have varying blade widths and curvatures across the length of each blade.)

2)The weight of the propeller assembly (hub and blades) affects the rotational speed of the blade at a given wind speed. Lighter propellers are easier to turn (by slower moving wind) but once the wind stops blowing, the lighter propellers loose their speed faster than the heavier propellers (the effect of momentum). Propeller weight depends upon the material from which it was made and from the inner structure of the blades and hub. Also, denser (and heavier) propellers are generally less prone to flexing and bending, which wastes the energy of the wind. Newer blades are light but stiff enough to resist flexing.

3)The number of the blades: generally, the more the blades, the greater the surface area that is present to “catch” the wind. But the trade-off with more blades is that they need to be bunched closer together which results in putting each blade closer to the backwash of the preceding blade. Propellers with more blades (>3 or more) are applicable when wind speeds are low. These propellers are efficient at low speeds and turn more slowly but because of the greater surface area that catches the wind, these propellers also deliver more torque.

There are other variables that impact on blade performance. Additional information is available on my website.