Page 372 - Wind Energy Handbook

346 CONCEPTUAL DESIGN OF HORIZONTAL-AXIS TURBINES

Table 6.6 Comparison of Loads on Shaft and Nacelle for Three-bladed and Rigid-hub Two-
bladed Machines
Deterministic loading arising from wind Stochastic loading
shear and/or yaw misalignment, in terms of
blade root out-of-plane bending moment
amplitude, M o
Location of moment Three-bladed Rigid-hub two-bladed % increase for rigid-hub
loading machine machine two-bladed machine
compared with three-
bladed machine
Shaft bending 1.5 M o 2 M o 22%
moment
amplitude
Nacelle nodding 1.5 M o M o (1 þ cos 2ł) 22%
moment
Nacelle yaw Zero M o sin 2ł 22%
moment

6.5.5 Noise constraint on rotational speed

As noted in Section 6.5.3, there may be significant cost benefits to be gained from a
two-bladed design with increased rotational speeds, because, in addition to the
blade saving, the cost of the whole of the drive train is reduced because of the
reduced torque. However, as noted in Section 6.4.2, it is normal to restrict tip speed
to about 65 m/s in order to limit aerodynamic noise emission. At 62.8 m/s, the tip
speed of the baseline machine discussed in Section 6.5.3 is within this limit, but the
tip speed of option (b) of 74 m/s would be less likely to be acceptable, except at
remote sites or offshore. This subject is considered further in Section 6.9.

6.5.6 Visual appearance

Although the assessment of visual appearance is essentially subjective, there is an
emerging consensus that three-bladed machines are more restful to look at than
two-bladed ones. One possible reason for this is that the apparent ‘bulk’ of a three-
bladed machine changes only slightly over time, whereas a two-bladed machine
appears to contract down to a one-dimensional line element, when the rotor is
vertical, twice per revolution. A secondary factor is that two-bladed machines
generally rotate faster, which an observer can also find more disturbing.

6.5.7 Single-bladed turbines

Apart from the saving in rotor cost itself, the single-bladed turbine concept is an
attractive one because of the reduction in drive train cost realizable through
increased rotational speed (Section 6.5.2). An obvious disadvantage is the resulting