Fig.1 illustrates a group of operating curves for a fixed pitch wind turbine whose blades are set at various fixed angle settings.
[power coefficient (cp) vs tip-speed-ratio (l), ]
Using each curve individually, we will investigate the shortcomings of fixed pitch turbine design.
Later, we will use green curve B to demonstrate the concept of variable pitch blade control.
Fixed pitch blade shortcomings
From the legend, first consider red curve A (6 degrees). As wind speed and TSR increase, the power coefficient moves up, peaks at the design wind speed and then starts to decrease again, as stall and loss of efficiency ensue. This curve was picked for a reason. It might be as good a choice as possible for fixed pitch. Letís see why.
Now choose the 15 degree fixed pitch curve (a good startup angle). It is the small black curve 3rd from the left (see legend). Unfortunately the power coefficient hardly reaches a value of 0.2 before peaking and heading back down with onset of stall. This situation shows the same limitations as the low speed transmission analogy. The action is finished almost before it starts. The pitch angle is too high.
So, rather, letís consider a 2 degree fixed pitch blade (a good pitch angle for high speed, but it doesnít start well), just like the high speed transmission analogy earlier. That means both 15 and 2 degree choices are problematic. Clearly, of the three angle choices, the 6 degree curve is clearly a better compromise, though still not close to perfect.
In the end, we see that the major shortcoming of all fixed pitch turbines is that no single blade angle will perform well at all wind speeds. Letís investigate variable pitch control.
Variable pitch blade advantages
Now, we will illustrate the advantages of variable pitch blade control. It provides optimum blade efficiency at all wind speeds.
Follow the path up green curve B starting at the bottom left. As the output of the variable pitch blade changes pitch angle with increasing wind speed, the variable pitch blades seek the maximum point of whatever curve the pitch angle at the moment corresponds to (dictated by mechanical feedback).
Notice, on the way up the blended green curve, the blade first behaves like the 15 degree pitch angle blade at its maximum performance point. As speed increases, the blade continues to change pitch causing curve B to blend into the 10 degree peak performance point. Continuing further up, the curve blends into the 8 degree peak point, then the 6, and the 4, and so on to the 2 degree peak point, forming a continuum of peak point maximums which defines curve B. Utilizing this process, once the control speed is reached, rotor speed is held constant (controlled) by blade angle according to green curve B.
For practical reasons we started at 15 degrees eliminating the 25 and 35 degree curves where the blades act more as paddles than airfoils. Similarly we stopped at a practical 2 degrees since control issues take precedence as we approach Ď0í degrees. We can now appreciate the reasons behind the smooth action of variable pitch control when comparing fixed pitch and variable pitch output curves. Note again variable pitch blades continuously act at maximum possible efficiency.
From the above explanation we can easily see why the 6 degree angle is a single limited solution for a fixed pitch blade and how the variable pitch blade control is superior over the vast majority of operating wind conditions.
We can also appreciate more clearly the shortcomings of fixed pitch operation which either offers easy startup, which compromises high speed efficiency, or high speed efficiency which compromises startup. You canít have both.
Finally we can better appreciate the overall virtues of variable pitch turbine operation in providing wider operating speed range, high efficiency and elimination of overspeed problems.