Environmental Engineering Reference
In-Depth Information
Real wind generators do not reach this theoretical optimum; however,
good systems have power coefficients
c
P
between 0.4 and 0.5. The ratio of the
used power
P
T
of the turbine to the ideal usable power
P
id
defines the efficiency
η
for the power utilization of the wind:
(5.20)
Drag devices
If an object is set up perpendicularly to the wind, the wind exerts a force
F
D
on the object. The wind speed
v
, the effective object area
A
and the
drag
coefficient c
D
, which depends on the object shape, define the drag force:
(5.21)
Figure 5.5 shows drag coefficients for various shapes. With
P
D
=
F
D
·
v
, the
power to counteract the force becomes:
(5.22)
If an object moves with speed
u
by the influence of the wind in the same
direction as the wind, the drag force is:
(5.23)
and the power used is:
(5.24)
Plate
Cylinder
Sphere
Hemisphere (front)
Cone with hemisphere
Hemisphere with cone
Streamlined form
Hemisphere (front)
Source:
Hering et al, 1992
Figure 5.5
Drag Coefficients for Various Shapes
