Variable pitch blade control explained

Background

A good approach to understanding variable pitch blade control is to have the following wind turbine blade performance chart explained. We will start in a general way with the aerodynamic term ‘tip-speed-ratio’ TSR. It is the ratio of the speed of the rotating blade tip to the speed of the oncoming wind.  For example, if the tip of the blade were moving at 60 km/hr in a 10 km/hr wind, the TSR would be 6:1.  Similarly, if the tip were moving at 120 km/hr in a 20 km/hr wind the TSR would be the same, 6:1. 

Not surprisingly, blades that can deliver high TSR values are aerodynamically characterized by a high lift-to-drag ratio and high lift coefficient. Typical curves are shown in the chart below (fig. 1).  Note that pitch angle has a large effect. As an aside, be aware that no stamped or cupped sheet metal blades can ever provide high performance.

Concisely, all the best turbines have blades which deliver a TSR of 6:1 or better. In fact, the TSR is the most reliable figure of merit for any horizontal axis wind turbine (HAWT).

It is significant at this point, to note that fixed pitch turbine blades which can also exhibit a TSR of 6:1 at a specific design speed and pitch, cannot adjust to deliver high performance over the same  large wind speed range that a variable pitch turbine can.(discussed later).

Let us now consider the suite of high performance ‘secrets’ for large turbines: 1) all large wind turbines have an optimal 3 blade configuration 2) all use high performance blade airfoils, and 3) all use variable pitch blade control to deliver the highest efficiency possible over the widest operating range of wind speeds.

Yet there is one other consideration. It is important to be aware that variable pitch control of high performance airfoils is even more advantageous as turbines are scaled down in size (based on decreased Reynolds No. values as the turbine is scaled down). It happens that airfoils perform increasingly poorly as size goes down, making higher performance airfoil choices much more critical as turbines become smaller.

In overview, all variable pitch turbines,  upon reaching control speed, automatically and continuously deliver high efficiency output at constant speed, well past what is possible with a fixed pitch turbine with the same blade airfoil performance and over the same range. Note that in Fig. 1 below, the TSR range shown is broadened slightly to better illustrate the variable pitch concept in the more detailed explanation that follows.

By way of a wider comparison, an old multi-blade farm wind turbine will deliver a TSR of just under 1:1, as will the majority of helical vertical axis (VAWT) wind turbines.  None of these utilize high performance airfoils.   Their poor efficiency is mainly related to high drag and no lift. Conversely, high efficiency turbine blade performance is solely related to high lift and very low drag

Fig. 1

Explanation

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 (c_p) 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. Let’s 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, let’s consider a 2 degree fixed pitch blade (a good pitch angle for high speed, but it doesn’t 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.  Let’s 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 ‘0’ 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.

Conclusions

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 can’t 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.
 

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