Page 371 - Wind Energy Handbook

NUMBER OF BLADES 345

Table 6.5 Comparison of Two-bladed Design Variants Utilizing the Same Blades with 40 m
Diameter, 500 kW Three-bladed Baseline Machine
Variant Rotational Rated Annual Reduction in Tower cost Reduction Increase/
speed power energy annual energy governed in overall reduction
(r.p.m.) (kW) yield yield compared by machine in cost of
(MWh) with baseline cost energy
machine
(a) 30 331 1054 19% Fatigue 16% þ4%
loading
Extreme 20% 1%
loading
(b) 35.4 500 1256 4% Fatigue 1% þ3%
loading
Extreme 7% 3%
loading

with consequent potential cost benefits. The hinge eliminates the transfer of out-of-
plane aerodynamic moments from the rotor to the low-speed shaft, resulting in
large reductions in the operational loadings on the shaft, nacelle and yaw drive.
The dependence of these loads on rotational speed is also largely removed, with the
result that the optimum rotational speed for a two-bladed machine in energy cost
terms is increased, approaching the value giving maximum energy yield.
Although teetering provides scope for significant cost savings on the shaft,
nacelle and yaw drive (which account for nearly 20 percent of the baseline machine
cost), these savings are offset by the additional costs associated with the teeter hinge
and teeter restraint system.

6.5.4 Effect of number of blades on loads

Moment loadings on the low speed shaft and nacelle structure from three-bladed
and rigid-hub two-bladed machines were examined in Sections 5.10 and 5.11, and
are compared in Table 6.6 below for machines of the same diameter and rotational
speed. The stochastic loading comparison is based on a turbulence length scale to
rotor diameter ratio of 1.84.
It is seen that loadings from a rigid-hub two-bladed rotor are significantly larger
than from a three-bladed rotor. However, in most two-bladed machine designs, the
rotor is allowed to teeter instead of being rigidly mounted, with the result that
aerodynamic moments on the shaft and nacelle structure quoted in Table 6.6 are
eliminated, and the blade out-of-plane root bending moments are reduced. The
benefits and drawbacks of teetering the rotor are examined in Section 6.6.
The rotor thrust variations at blade passing frequency due to stochastic loading,
which are a dominant factor in tower fatigue design, are very similar for two- and
three-bladed machines rotating at the same speed. However, two-bladed machines
usually rotate faster than three-bladed machines of the same diameter, so the cyclic
rotor thrust variations are higher.