Environmental Engineering Reference
In-Depth Information
Figure 5.14
Stall Effect at Higher Wind Speeds
Stall control
takes advantage of the stall effect that occurs at high angles of
attack (see Figure 5.14). This effect destroys the buoyancy force and thus limits
the power that the wind transfers to the rotor blade. The rotor's rotational
speed
n
and the circumferential speed
u
remain constant for stall-controlled
wind generators. Stall control is achieved by increasing the angle of attack at
higher wind speeds
v
W
. The rotor blades do not pitch, i.e. the pitch angle
remains constant; stall control can be realized by construction measures
without advanced technical requirements. The disadvantage of stall control is
the low possibility to influence operation because stall control is purely
passive. The maximum power of a newly designed rotor blade is not easy to
estimate because a mathematical description of the stall effect is rather
difficult. After reaching the maximum power, the power output of stall-
controlled wind turbines decreases again and does not remain on a constant
level as shown in Figure 5.13.
Many manufacturers of wind turbines prefer
pitch control
, although the
technical effort is much higher than for stall control. However, since pitch
control is an active control, it can be adjusted to suit the conditions, in contrast
to stall-controlled systems. Pitch control directly increases or lowers the pitch
angle of the rotor and therefore the angle of attack. The rotor blade is turned
into the wind at higher wind speeds (see Figure 5.15), lowering the angle of
attack and actively decreasing the power input of the rotor blade. Pitch-
controlled wind turbines are more difficult to manufacture because the rotor
blades must twist inside the rotor hub. Small systems often use mechanically
controlled pitch mechanisms using centrifugal forces. An electric motor moves
the rotor blade to the desired position in large systems.
If the wind generator is stopped due to storm protection, the pitch control
can pitch the rotor blade towards the feather position. This reduces the power
input and avoids damage to the wind turbine. Stall-controlled systems often
have additional aerodynamic brakes. For instance, the rotor tip can bend.
During storms, the tip bends by 90° and slows the wind turbine.
