Page 370 - Wind Energy Handbook

344 CONCEPTUAL DESIGN OF HORIZONTAL-AXIS TURBINES

0.5
Three bladed machine Two bladed machine with same
0.45 (baseline) blade geometry
0.4
Two bladed machine with 50%
Coefficient of performance (C P ) 0.25 three-bladed machine
0.35
increased chord
0.3
2/3 times C P of

0.2

0.15
0.1

0.05
0
0 2 4 6 8 10 12 14
Tip speed ratio (λ)
Figure 6.5 Comparison of C P Curves for Two- and Three-bladed Machines

reduction in gearbox cost due to an 18 percent increase in rotational speed
would yield a 2 percent reduction in total machine cost.
As the blade skin thickness is assumed to increase in proportion to rotational
speed, the saving associated with eliminating one blade will be offset by an 18
percent increase in the weight of the remaining two, resulting in a 21 percent
reduction in rotor cost, and a 4 percent reduction in overall cost. The cost
savings on the blades, drive train and foundations are offset by cost increases
on the hub, shaft, nacelle, yaw drive and tower due to increased rotational
speed, resulting in an overall cost saving of only 1 percent. Hence the energy
cost is 3 percent higher than for the baseline machine.

It is apparent that, with the tower design assumed dependent on fatigue loads,
the two-bladed variants (a) and (b) considered above result in a small increase in
the cost of energy relative to the three-bladed machine. However, if the tower
design is governed by extreme loads rather than fatigue loads, the situation is
reversed; see Table 6.5, in which it is assumed that the reduction of extreme load
due to the reduced number of blades results in a 20 percent reduction in tower cost.
The results shown in Table 6.5 indicate that two-bladed, rigid-hub machines are
unlikely to yield significant cost benefits vis-a `-vis three-bladed machines, even if the
tower design is determined by extreme loading. However, the results should be
treated with caution, because the cost changes are based on a simplistic treatment
of blade loadings, and of their knock-on effects on other components. (Loads on
rigid-hub two-bladed machines are compared with those on three-bladed machines
in more detail in the next section.)
The loadings on the nacelle of a two-bladed machine can be reduced significantly
by the introduction of a teeter hinge between the rotor and the low-speed shaft,