Environmental Engineering Reference
In-Depth Information
Lift coefficient
c
L
and drag coefficient
c
D
vary significantly with the angle
of attack
α
in °. For
α
< 10°, the following approximation can be used (Gasch
and Twele, 2002):
(5.39)
Pitching the rotor blade, i.e. changing the pitch angle
ϑ
that is shown in Figure
5.8, influences the angle of attack
and therefore the power coefficient
c
P
.
The maximum of the power coefficient decreases significantly at high pitch
angles and occurs at lower tip speed ratios. Pitch-controlled turbines use this
effect: during starting of the wind turbine, high pitch angles are chosen.
Pitching of the rotor blades can also limit the power at very high wind speeds.
The drag coefficient
c
D
can be neglected at angles of attack lower than 15°. A
lift device takes power
P
T
from the wind. With the power coefficient
c
P
and
the power content
P
0
, the taken power is given by:
α
(5.40)
The resultant force causes torque
M
. With
(5.41)
the
torque
becomes:
(5.42)
The torque
M
with the associated
moment coefficient
(5.43)
Source:
data from Vestas, 1997
Figure 5.9
Power Coefficient
c
P
as a Function of the Tip Speed Ratio
λ
for
the Vestas V44-600-kW Wind Generator