Environmental Engineering Reference
In-Depth Information
2.4.3 System Power Characteristics
Most important for any wind energy system capability is the power curve. Measured
curves of the delivered power over wind speed are, together with the knowledge of
average wind velocity and distribution properties (e.g. Rayleigh), indispensable for
predicting the annual energy yield. Figure 2.14 shows typical power curves of pitch-
controlled and stall-controlled systems. Below a predesigned wind speed, normally
the rated wind speed, the power curve is intended to follow a
v
3
function using
optimum
c
p
(
). Note that useful power generation starts only at the cut-in wind
speed, normally at
v
between 3 and 4 m/s.
Power limitation at wind speeds above the rated value is effected by either one
of the control systems:
- pitch control, where the power is controlled to rated power above a preset thresh-
old wind speed (mostly the rated speed),
- stall control, where a transient phenomenon with power overshoot is observed
for wind speeds above rated value.
A number of characteristic wind velocity values are specified with the design for
each wind turbine:
•
λ
Average velocity
v
av
(in e.g. 10 m or 30 m above ground, or measured in hub
height).
•
Optimum velocity
v
opt
at
λ
opt
(best point).
•
Velocity at maximum energy yield.
•
Value of velocity at which the power limitation begins to work. This point is
mostly called the rated wind speed.
•
Cut-in velocity, at which the turbine starts to supply power.
•
Cut-off velocity, at which the turbine is brought to standstill for safety reasons.
•
Survival-velocity, in view of an assumed “once-in-a-century storm”.
Regulations require a wind energy converter to have two independent braking sys-
tems, where the first serves as the main brake and the second as fixing brake. Shut
down must be tripped at a maximum wind speed of usually 25 m/s.
Fig. 2.14
Typical power curves for pitch-controlled and stall-controlled systems
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