Page 93 - Fluid Power Engineering

Advanced Aerodynamics of W ind T urbine Blades 71

1. Cut-in speed, v ci . A turbine starts producing energy at this
wind speed. At low wind speeds, the torque is small and not
enough to overcome the inertia of the entire system. v ci for
most turbines is in the range of 3 to 5 m/s. Although turbine
manufacturers like to trumpet lower cut-in speed, it must be
remembered that very little amount of energy is produced at
low wind speed.
2. Rated speed, v rs . This is the wind speed at which the rated
power is produced. For most turbines, v rs is in the range of
11.5 to 15 m/s. This is a crucial number because it defines the
shape of the power curve. Power curves with lower v rs , will
produce more energy overall because it will produce more en-
ergy at wind speeds between cut-in and rated. When evaluat-
ing the power production capabilities of turbines for a given
wind condition, it is common to compare the capacity factor,
which is the average annual energy production divided by the
annual energy production at the rated power. Power curves
with lower v rs will yield a higher capacity factor. Larger and
more efficient blades lead to lowering of v rs . A deceptive way
to lower v rs is to take a turbine designed for, say, 1.65 MW and
rate it at 1.5 MW.
3. Cut-outspeed,v co .Aturbinestopsoperatingatthiswindspeed.
v co for most turbines is 25 m/s. The primary reason for stop-
ping is safety. Components of a turbine are not designed to
handle the loads created by wind speeds above the cut-out
speed. There are various methods to stop a turbine.
Modernpitch-controlledturbinesincreasethepitchtocom-

plete stall position. The control algorithm has a delay pe-
riod that defines the wait time for restart of the turbine after
wind speed has dropped below v co .
The second method is a spring-loaded mechanism in the

blade that turns the tip section of the blade in a feather
position.
Thethirdmethodistoturntheturbine90 aboutthevertical
◦
axis and thereby change the axis of rotation. This is done
in a few small turbines.
Note, most stall-regulated turbines will start to slow down
because of the sharp rise in drag force. This is a result of higher
angle of attack, which leads to separation of flow and high
drag.
4. Survival wind speed. Although this is not part of the power
curve, it is an important wind speed, which specifies the de-
sign wind speed of the entire turbine structure including the
tower. This is in the range of 50 to 60 m/s.