Design of Wind Turbines - Wind Energy: Renewable Energy and the Environment - page 114

Pendulum mass
Blade
Flexbeam
Rotor shaft
FIGURE 6.15 Passive control with flexbeam and pendulum weights, unit was constant-rpm operation.
the blades toward stall. One problem was that over time, the flexbeam moved toward a different
set twist, which reduced the starting torque. The Proven wind turbine has a flexible hinge near the
root of the blade [37]. As rotor rpm increases, the blades are forced outwards, which changes the
pitch of the blade toward stall. So even in high winds, the rotor rpm is limited, and it can continue
to produce power.
6.8 CONSTRUCTION
6.8.1 B LADES
For years, small wind turbines blades were made of wood, carved from a single piece or from a
wood block glued together from several pieces. The material properties of wood are good: strength,
flexibility, and resistance to fatigue. Machines could carve up to four blades from a master blade.
However, for large blades, solid wood was not acceptable, as the weight became too large. For
larger blades, one construction was similar to an airplane wing, a spar and ribs with a covering.
The spar is the load-bearing part and the ribs form the airfoil shape. As noted earlier, fabrication of
blades depends on design, materials, and the construction processes, all of which are related. Wind
turbine blades have been made from a number of materials: aluminum cover, fabric cover, or metal
cover on rib and spar (like an airplane wing); a sail wing, which is fabric attached to a leading edge
spar; laminated wood composite (shell); fiberglass-reinforced plastics (FRPs), also carbon fibers;
pultruded FRPs; extruded aluminum (blades for vertical-axis wind turbines); and blades from injec-
tion molds for small wind turbine blades. Pultruded blades are where the fiberglass and other parts
are pulled through a dye and the epoxy is applied at the same time, and blades are cut to length.
Extruded blades are where the material is pushed through a dye, and for the Darrieus wind turbine,
the blades are bent to curvature afterwards. Cross sections of some different blades illustrating dif-
ferent manufacturing processes are presented in Figure 6.16 .