Wind power technology - Wind Power Integration: Connection and System Operational Aspects - page 62

Both C l and C d change with the angle of incidence of the blade in the air
stream. The lift coefficient is approximately linear up to its maximum and the
airfoil then stalls and the lift coefficient reduces (Figure 3.7). Stalling is caused by
the boundary layer on the upper surface of the airfoil separating and a circular wake
forming. The drag coefficient increases more slowly with a .
Figure 3.8 illustrates how a wind turbine operates and how the two usual
mechanisms of controlling the power - stall and pitch regulation - work. Figure 3.8a

Cl
Cd

1.4

Stall due to
flow separation

1.2

1

0.8

0.6

0.4

0.2

15°

0

-20 0 20 40 60 80

-0.2

a

Figure 3.7 Airfoil characteristics

wR

U U

a

F l

F r

F q

V V

F d

(a) (b)

Figure 3.8 Operation of wind turbine