Fluid Mechanics and Thermodynamics of Turbomachinery, 5e

364 Fluid Mechanics, Thermodynamics of Turbomachinery
Airfoil
S822
S823
NREL S822
Tip–region airfoil, 90% radius
Root–region airfoil, 40% radius
r/R
0.9
0.4
NREL S823
Design Specifications
Re (¥10 6 )
0.6
0.4
t/l
0.16
0.21
C Lmax
1.0
1.2
C D (min)
0.010
0.018
FIG. 10.22. Thick aerofoil family for HAWTs of diameter 2 to 11m (P = 2 to 20kW).
(Courtesy NREL.)
stall-regulated wind turbines with movable speed control tips may be replaced by variable-pitch
blades for more refined peak power control and reliability.
With the very large HAWTs (i.e. 104m diameter, refer to Figure 10.4a) being brought
into use, new blade section designs and materials will be needed. Mason (2004) has
described “lightweight” blades being made from a carbon/glass fibre composite for the
125m diameter, 5MW HAWT to be deployed in the North Sea as part of Germany’s
first deepwater off-shore project.
Control methods (starting, modulating and stopping)
The operation of a wind turbine involves starting the turbine from rest, regulating
the power while the system is running, and stopping the turbine if and when the
wind speed becomes excessive. Startup of most wind turbines usually means
operating the generator as a motor to overcome initial resistive torque until sufficient
power is generated at “cut-in” speed assuming, of course, that a source of power is
available.
Blade pitch control
The angle of the rotor blades is actively adjusted by the machine control system.
This, known as blade pitch control, has the advantage that the blades have built-in
braking which brings the blades to rest. Pitching the whole blade requires large actuators
and bearings, increasing the weight and expense of the system. One solution to this