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Figure 3.55 Non‐dimensional performance curves for constant‐speed operation.

      3.13.3 Stall regulation

      An important feature of this K_P –1/λ curve is that the power, initially, falls off once stall has occurred and then gradually increases with wind speed. This feature provides an element of passive power output regulation, ensuring that the generator is not overloaded as the wind speed increases. Ideally, the power should rise with wind speed to the maximum value and then remain constant regardless of the increase in wind speed: this is called perfect stall regulation. However, stall‐regulated turbines do not exhibit the ideal, passive stall behaviour.

Stall regulation provides the simplest means of controlling the maximum power generated by a turbine to suit the sizes of the installed generator and gearbox. The principal advantage of stall control is simplicity, but there are significant disadvantages. The power vs wind speed curve is fixed by the aerodynamic characteristics of the blades, in particular the stalling behaviour. The post‐stall power output of a turbine varies very unsteadily and in a manner that, so far, defies prediction (see Figure 3.62, for example). The stalled blade also exhibits low vibration damping because the flow about the blade is unattached to the low‐pressure surface, and blade vibration velocity has little effect on the aerodynamic forces. The low damping can give rise to large vibration displacement amplitudes, which will inevitably be accompanied by large bending moments and stresses, causing fatigue damage. When parked in high, turbulent winds, the fixed‐pitch, stationary blade may well be subject to large aerodynamic loads that cannot be alleviated by adjusting (feathering) the blade pitch angle. Consequently, the blades of a fixed‐pitch, stall‐regulated turbine must be very strong, involving an appropriate cost penalty.

      3.13.4 Effect of rotational speed change

The power output of a turbine running at constant speed is strongly governed by the chosen, operational rotational speed. If a low rotation speed is used, the power reaches a maximum at a low wind speed, and consequently it is very low. To extract energy at wind speeds higher than the stall peak, the turbine must operate in a stalled condition and so is very inefficient. Conversely, a turbine operating efficiently at a high speed will extract a great deal of power at high wind speeds, but at moderate wind speeds it will be operating inefficiently because of the high drag losses. Figure 3.56 demonstrates the sensitivity to rotation speed of the power output – a 33% increase in rpm from 45 to 60 results in a 150% increase in peak power, reflecting the increased wind speed at which peak power occurs at 60 rpm.

At low wind speeds, however, there is a marked fall in power with increasing rotational speed, as shown in Figure 3.57. In fact, the higher power available at low wind speeds if a lower rotational speed is adopted has led to two speed turbines being built. Operating at one fixed speed that maximises energy capture at wind speeds at or above the average level will result in a rather high cut‐in wind speed, the lowest wind speed at which generation is possible. Employing a lower rotational speed at low wind speeds reduces the cut‐in wind speed and increases energy capture. The increased energy capture is, of course, offset by the cost of the extra machinery.

      3.13.5 Effect of blade pitch angle change

Another parameter that affects the power output is the pitch setting angle of the blades β_s. Blade designs almost always involve twist, but the blade can be set at the root with an overall pitch angle. The effects of a few degrees of pitch are shown in Figure 3.58.

Figure 3.56 Effect on extracted power of rotational speed.

Figure 3.57 Effect on extracted power of rotational speed at low wind speeds.

Figure 3.58 Effect on extracted power of blade pitch set angle.

      Small changes in pitch setting angle can have a dramatic effect on the power output. Positive pitch angle settings increase the design pitch angle and so decrease the angle of attack. Conversely, negative pitch angle settings increase the angle of attack and may cause stalling to occur, as shown in Figure 3.58. A turbine rotor designed to operate optimally at a given set of wind conditions can be suited to other conditions by appropriate adjustments of blade pitch angle and rotational speed.

3.14 Pitch regulation

      3.14.1 Introduction

      Many of the shortcomings of fixed‐pitch/passive stall regulation can be overcome by providing active pitch angle control. Figure 3.58 shows the sensitivity of power output to pitch angle changes.

      The most important application of pitch control is for power regulation, but pitch control has other advantages. By adopting a large positive pitch angle, a large starting torque can be generated as a rotor begins to turn. A 90° pitch angle is usually used when the rotor is stationary because this will minimise forces on the blades such that they will not sustain damage in high winds. At 90° of positive pitch the blade is said to be ‘feathered’. The blades need not be as strong, therefore, as for a stall‐regulated turbine, which reduces blade costs. Only a small change of pitch angle is needed to provide an assisted start‐up.

      The principal disadvantages of pitch control are lower reliability and cost, but the latter is offset by lower blade costs.

      Power regulation

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