Design of Wind Turbines - Wind Energy: Renewable Energy and the Environment - page 109

29

28

27

26

25

24

-5 -4 -3 -2 -1 0 1 2 3 4 5

Pitch Angle, degrees

FIGURE 6.9 PROP93: Prediction of rotor power output for different pitch angles at 10 m/s. The Carter 25
wind turbine is a fixed-pitch, constant-rpm machine.

would be obtained by using twenty data stations. These blades had large twist and larger chord
toward the root, and then the same chord and twist from the midpoint, which produced an aerody-
namic efficiency close to the theoretical limit. Notice the twist is to obtain the correct angle of attack
due to the different inflow wind due to the contribution of the blade speed, which is slowest at the
root. Also, twist on the inward part of the blade increases the torque for starting rotation. Note that
for constant-pitch blades with little twist, there is not enough starting torque and the rotor needs
to be motored for start-up. For variable-pitch blades, the blades are in the feather position, which
produces enough torque for start-up. Notice that for constant tip speed ratio, the power continues to
increase with wind speed (Figure 6.10).
Tangler and Kocurek [23] provided guidelines for input of post-stall airfoil data for the predic-
tion of peak and post-peak rotor power for performance programs using blade element momentum
theory. A steady-state data set from the rotor test in the Unsteady Aerodynamics Experiment was

45

40

35

30

25

20

15

10

5

0

3 456 789 10 11 12 13 14 15 16

Wind Speed, m/s

FIGURE 6.10 PROP93: Theoretical power curve for Carter 25 rotor, tip speed ratio 6.1.