3.14.2 Pitching to stall
Figure 3.58 shows 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.
3.14.3 Pitching to feather
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 3.59 is 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.
The advantages of the pitching to feather method are that the flow around the blade remains attached, and so well understood, and provides good, positive damping. Feathered blade parking and assisted starting are also available.
Pitching to feather has been the preferred pitch control option mainly because the blade loads can be predicted with more confidence than for stalled blades.
3.15 Comparison of measured with theoretical performance
The turbine considered in this section is stall regulated and is run at constant rotational speed. More detail about this method of operation will be discussed in the next section, but the main feature is that there is, theoretically, a unique power output for a given wind speed.
When the turbine was under test, the chosen rotational speed was 44 rpm. Energy output and wind speed were measured over one‐minute time intervals and the average power and wind speed determined. The test was continued until a sufficient range of wind speeds had been covered. The one‐minute average results were then sorted in ‘bins’ 0.5 m/s of wind speed wide, and a fairly smooth power vs wind speed curve was obtained, as shown in Figure 3.60.
The turbine has a diameter of 17 m and would be expected to produce rather more power than shown above if operated at a higher rotational speed.
From the data in Figure 3.60, the CP ‐ λ curve can be derived. The tip speed of the blades is (44π)/30 rad/s × 8.5 m = 39.2 m/s, the swept area is 8.52.π = 227 m2, and the air density was measured (from air pressure and temperature readings) at 1.19 kg/m3.
Figure 3.60 Power vs wind speed curve from the binned measurements of a three blade stall‐regulated turbine.
Figure 3.61 Comparison of measured and theoretical performance curves.
Therefore,
(3.96)
The mechanical and electrical losses were estimated at 5.62 kW, and this value was used to adjust the theoretical values of CP. The resulting comparison of measured and theoretical results is shown in Figure 3.61.
This comparison looks reasonable and shows that the theory is reliable, but the quality of the theoretical predictions really relies upon the quality of the aerofoil data. The blade and aerofoil design are the same as given in Section 3.11.
Figure 3.62 Measured raw results of a three blade wind turbine.
One last point should be made before classifying the theory as complete: it would be as well to look at the raw, one‐minute average data before it was reduced down by a binning process; this is shown in Figure 3.62. In the post‐stall region, there seems to be a much more complex process taking place than the simple theory predicts, and this could be caused by unsteady aerodynamic effects or a bistable separation condition.
3.16 Estimation of energy capture
The quantity of energy that can be captured by a wind turbine depends upon the power vs wind speed characteristic of the turbine and the wind speed distribution at the turbine site.
Wind speed distribution is discussed in Section 2.4. The distribution at a given site is described by a probability density function, Eq. (2.3), with parameters specified for the site.
A performance curve is shown in Figure