Page 202 - Wind Energy Handbook

176 WIND-TURBINE PERFORMANCE

4.1.3 The C Q – º curve

The torque coefficient is derived from the power coefficient simply by dividing by
the tip speed ratio and so it does not give any additional information about the
turbine’s performance. The principal use of the C Q –º curve is for torque assessment
purposes when the rotor is connected to a gear box and generator.
Figure 4.4 shows how the torque developed by a turbine rises with increasing
solidity. For modern high-speed turbines designed for electricity generation as low
a torque as possible is desirable in order to reduce gearbox costs. On the other hand
the multi-bladed, high-solidity turbine, developed in the nineteenth century for
water pumping, rotates slowly and has a very high starting torque coefficient
necessary for overcoming the torque required to start a positive displacement
pump.
The peak of the torque curve occurs at a lower tip speed ratio than the peak of the
power curve. For the highest solidity shown in Figure 4.4 the peak of the curve
occurs while the blade is stalled.

4.1.4 The C T – º curve

The thrust force on the rotor is directly applied to the tower on which the rotor is
supported and so considerably influences the structural design of the tower.
Generally, the thrust on the rotor increases with increasing solidity (Figure 4.5).

0.12

0.1

0.08
One blade
C
Q Two blades
0.06 Three blades
Four blades
Five blades
0.04

0.02

0
0 5 10 15
λ
Figure 4.4 The Effect of Solidity on Torque