Environmental Engineering Reference
In-Depth Information
The torque connected with the power according to (1.2) is then
T
=
c
T
D
2
2
Av
1
2
(2.5)
Note that the torque varies with the square (
v
1
2
) and the power varies with the
third power (
v
1
3
) of the wind speed.
Figure 2.2 shows typical characteristics
c
p
(
) for different types of rotor. Besides
the constant maximum value according to Betz the figure indicates a revised curve
c
p
(Schmitz) which takes the downstream deviation from axial air flow direction into
account. The difference is notable in the region of lower tip speed ratios, as calcu-
lated by Schmitz and, before, Glauert. Together with Fig. 2.3 indicating associated
characteristics
c
T
(
λ
), the current preference for three-blade rotors with horizontal
shaft is understood. The so-called fast-running turbines with 3, 2 or one blades dis-
play the larger values of
c
p
, while the curves
c
T
indicate the poor starting torque
capability of the fast-running types. Since one and two blade rotors are also prob-
lematic with respect to torque variations and noise, the three-blade rotors are cur-
rently predominant in all modern wind energy systems. The rotors are normally
designed to values
λ
λ
A
= 5
...
8.
Fig. 2.2
Typical power coefficients of different rotor types over tip-speed ratio
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