Commercial Wind Turbine Systems and Applications - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 216

and 2) as well as in manufacturing. However, without significant and yet-to-be-developed
changes in materials or manufacturing processes, further cost increases with rotor diameter
may follow closer to a scaling exponent of 3.0 [Williams 2008].
Blades are commonly made of composite materials, predominantly epoxy and fiber-
glass, and remain the most labor intense components of wind turbines. As shown in Figure
4-8, blade fabrication typically involves hand layup, open-mold wet processing. Critical pa-
rameters that must be closely controlled for strength and stiffness are fiber direction, resin
content, and void content. The use of carbon fiber and other high-strength materials is in-
creasing but remains limited to small, local applications in the structure. Vestas and Gamesa
are among the manufacturers of large-scale wind turbines that are using carbon fiber compos-
ites in some blades to reduce weight.
Figure 4-8. Typical hand layup processing in the manufacture of a large-scale wind
turbine blade.
Number of Blades
The great majority of today's large-scale wind turbines have three blades, an evolution
of successful early commercial designs in Denmark. Two blades are becoming more com-
mon in small- and medium-scale turbines, such as the Vergnet turbine shown in Figure 4-4.
Vergnet is also one of the new entries into the large-scale class with a -MW two-bladed
machine.
One-blade rotors have been developed for wind turbines but none are commercially
available. The addition of a second blade increases the rotor's aerodynamic efficiency by
about 7 percent. Further increases in the number of blades yield minimal improvements in
rotor aerodynamic efficiency.