350
10 Wind Turbines
where u T is the blade speed at the rotor tip. A typical value with optimum operation
is about 6. The triangle drawn is thus representative for a section in the inner part of
the rotor. With an axial machine, the rotor blade drag force causes energy dissipation. This drag is small compared to the lift.
Figure 10.3 represents a front view of a three-blade horizontal-axis wind turbine
for electricity generation. The rotor diameter is 127 m. It is one of the biggest machines existing nowadays (manufacturer Enercon). The rated power is 7.58 MW for
locations with a high average wind speed (10 m/s at hub height).
Fig. 10.3 Horizontal-axis
wind turbine: E126. (Courtesy
Enercon)