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Each curve illustrated above in Fig.1 represents a blade operating at a fixed pitch angle (see legend). Consider the red curve A as a fixed pitch angle design choice. As wind speed (and Tip-Speed-Ratio, l) increase, the coefficient of power (Cp), moves up along the curve A following the red arrow . As the wind speed continues to increase, however, Cp peaks at the design wind speed and then starts to decrease again as it continues to move to the right, back down the curve. Stall and loss of efficiency ensue for that particular fixed ( 6 deg) blade angle choice .
In contrast, green curve B demonstrates the benefit of operating a blade with variable pitch blade angle. In this case, as the blade angle changes with increasing wind speed, Cp forms a continuous curve B, basically formed by following Cp max in a smooth continuum, from each fixed angle curve peak, to the next, to the next, etc. as wind speed increases. Note also that, at higher wind speeds, Cp exceeds that obtainable by a fixed pitch turbine blade set at 6 deg. (curve A). Simply, the variable pitch blade can easily adjust, as in this example, to a 2 deg. angle setting, a major advantage not possible with the fixed 6 deg. setting of curve A.
Thus it is clear that a fixed pitch angle can either be set for easy startup, which compromises high speed efficiency, or for high speed efficiency which compromises startup. You can’t have both.
You can have both and better with variable pitch turbine operation. ‘Better’ refers to a wider operating speed range and immunity to overspeed concerns.
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