Environmental Engineering Reference
In-Depth Information
v
i
v
N
3
v
i
v
N
2
P
P
N
=
c
p
c
p
,
N
·
T
N
=
c
p
/
c
p
,
N
T
n
N
=
λ
n
v
i
v
N
;
·
;
λ
N
·
(2.14)
λ
/
λ
N
The base quantities for rated condition are as follows:
n
N
=
60
π
P
N
=
2
Ac
p
,
N
v
N
Ω
N
=
2
A
D
P
N
2
c
T
,
N
v
N
;
T
N
=
;
λ
N
v
N
D
,
A
are as in (2.2), and
c
T
as in (2.4);
n
is in min
−
1
.
The example of Fig. 2.10 is based on the
c
p
(
where
ρ
) curve of Fig. 2.6, from which
λ
A
= 6
,
5 and c
p
,
A
= 0
,
52. The rated wind speed is chosen
v
N
= 12 m
/
s, and the
optimum
c
p
is assigned to
v
A
= 8m
/
s. According to (2.13) the tip speed ratio at rated
condition should be
λ
λ
N
=
λ
A
(
v
A
/
v
N
); it was chosen
λ
N
= 4
,
5, with
c
p
,
N
= 0
,
448.
2.4 Power Characteristics and Energy Yield
2.4.1 Control and Power Limitation
2.4.1.1 Pitching Mechanism
Variation of the blade angle is a means to control the rotor torque and power from
the wind side, and at the same time provide power and speed limitation at high
wind velocities. Normally the pitch mechanism is powered by a hydraulic or electric
drive. Pitch controlled rotors prevail in all larger systems.
In rotors equipped with a pitch mechanism the blade angle is adjusted subject to
a relevant controller output. Pitching is also used for power limitation at tip speed
ratios above a predesigned value by turning the blades out of the wind. With in-
creasing pitch angle the maximum power and torque coefficients are reduced, and
the maxima are shifted to lower
values. The no-load tip speed ratio is reduced,
while the torque coefficient shows increased values at starting. The drag coefficient
is significantly reduced. Principal curves of a design, based on the reference Fig. 2.6
are shown in Fig. 2.11.
λ
2.4.1.2 Stall Mechanism
For reasons of safety and to avoid overload, limitation of power above a preset rota-
tional speed is required. This can be accomplished by different methods. A simple
way is to turn the rotor out of the wind direction, which is done in the Western Mill.
For rotors without pitch angle variation the stall-effect can be utilized, where due
to a shift from laminar to turbulent air flow a braking effect is created. Figure 2.12
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