Environmental Engineering Reference
In-Depth Information
The optimal power coefficient
c
P,opt,D
of a drag device can be calculated
using
(5.30)
Using
u
/
v
= 1/3 as well as the maximum drag coefficient of
c
D,max
= 1.3, this
gives:
(5.31)
This value is also much below the ideal value of 0.593. Therefore, modern
wind turbines are lift devices, rather than drag devices, and these achieve much
better power coefficients.
Lift devices
If wind, which circulates around a body, develops higher flow speeds along the
upper surface than along the lower, an overpressure emerges at the upper
surface and an underpressure at the lower. The result is a
buoyancy force
,
according to Bernoulli:
(5.32)
The buoyancy force is calculated using the
lift coefficient c
L
, the air density
,
the apparent wind speed
v
A
and the projected body area
A
P
. Rotor blades of
modern wind generators usually make use of the buoyancy force. The
projected area
ρ
(5.33)
of a rotor blade is defined by the chord
t
and span that is approximately equal
to the rotor radius
r
.
Sense
of
rotation
Section B-C
Figure 5.7
Apparent Wind Speed
v
A
Resulting from the Real Wind Speed
v
W
and Rotor Motion
