WIND ENERGY SYSTEMS - Cd3wd

Chapter 4—Wind Turbine Power 4–33
Figure 19: MOD-0 power output for three high-speed shaft configurations.
Darrieus at a rotational speed of 50.6 r/min and at wind speeds of 9.8, 15.2, and 19.7 m/s.
These are measurements of the actual torque caused by the wind, obtained by a clever use
of accelerometers on the blades[6]. The shaft torque measured by torque sensors is much
smoother. As expected, the two bladed machine has two distinct torque cycles per rotor
revolution. At a wind speed of 9.8 m/s, the aerodynamic torque peaks at a rotor angle just
below 90 o , as defined in Fig. 21, at which point the plane of the rotor is parallel to the wind.
The torque variation at this wind speed is nearly symmetric with changes in angular position
and goes slightly negative when the plane of the rotor is perpendicular to the direction of the
wind.
As the wind speed increases the torque pattern becomes more complex. We saw in Fig. 15
that the power output of this Darrieus does not increase above a certain point, even though
the power in the wind continues to increase with wind speed. We now see in Fig. 20 that
the average torque at two wind speeds may be about the same, but that the instantaneous
torque of the higher wind speed may oscillate more widely. This is due to complex interactions
between the blades, the supporting tower, and the air flow, which we shall not try to explain.
The important point to note is that there is a cyclic torque variation in both the horizontal and
vertical axis turbines and that the drive train needs to be designed with this torque variation
in mind.
7 DRIVE TRAIN OSCILLATIONS
When torque is applied to a shaft, it will twist. This is illustrated in Fig. 22 where the line
AB on a shaft of length L has been twisted to position AC. The total twist is the angle θ.
The twist will be directly proportional to the torque as long as the material remains in its
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001