Michael Graham - Wind Energy Handbook

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.

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 3.56demonstrates 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.

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.

Many of the shortcomings of fixed‐pitch/passive stall regulation can be overcome by providing active pitch angle control. Figure 3.58shows 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 can be achieved either by pitching to promote stalling or pitching to feather, which reduces the lift force on the blades by reducing the angle of attack.

Figure 3.58shows the power curves for a turbine rated at 60 kW, which is achieved at 12 m/s. At wind speeds below the rated level, the blade pitch angle is kept at zero degrees. As rated power is reached, only a small negative pitch angle, initially of about 2°, is necessary to promote stalling and so to limit the power to the rated level. As the wind speed increases, small adjustments in both the positive and negative directions are all that are needed to maintain constant power.

The small size of the pitch angle adjustments make pitching to stall very attractive to designers, but the blades have the same damping and fatigue problems as fixed‐pitch turbines.

By increasing the pitch angle as rated power is reached, the angle of attack can be reduced. A reduced angle of attack will reduce the lift force and the torque. The flow around the blade remains attached. Figure 3.59is for the same turbine as Figure 3.58, but only the zero degree power curve is relevant below the rated level. Above the rated level, fragments of power curves for higher‐pitch angles are shown as they cross the rated power line: the crossing points give the necessary pitch angles to maintain rated power at the corresponding wind speeds. As can be seen in Figure 3.59, the required pitch angles increase progressively with wind speed and are generally much larger than is needed for the pitching to stall method. In gusty conditions, large pitch excursions are needed to maintain constant power, and the inertia of the blades will limit the speed of the control system's response.

Figure 3.59 Pitching to feather power regulation requires large changes of pitch angle.

Because the blades remain unstalled if large gusts occur at wind speeds above the rated level, large changes of angle of attack will take place with associated large changes in lift. Gust loads on the blades can therefore be more severe than for stalled blades.