Page 373 - Wind Energy Handbook

TEETERING 347

increased noise emission resulting from the faster rotation, but this would not be
an issue offshore. Another consideration is the reduced yield due to increased tip
loss. For example, a 40 m diameter machine consisting of a TR blade rotating at
48 r.p.m., with twist distribution reoptimized to give maximum energy yield, will
achieve the same maximum power output as the baseline design, but provide 12
percent less energy.
The single blade must be counterweighted to eliminate torque fluctuations and
any whirling tendency due to centrifugal loads. Furthermore, as a rigid hub would
expose the nacelle to very large nodding and yawing moments in comparison with
two- or three-bladed machines, it is customary to mount the rotor on a teeter hinge,
so that the unbalanced aerodynamic out-of-plane moment can be resisted by a
centrifugal couple, thereby reducing the hub moment. However, the teeter motion
of the blade is significantly greater than that of a two-bladed machine, so it is
normal to mount the rotor downwind. Morgan (1994) reports that particular
difficulties have been encountered in predicting teeter excursions after grid loss
and emergency stops, leading to excessive risk of teeter stop impacts.

6.6 Teetering

6.6.1 Load relief benefits

Two-bladed rotors are often mounted on a teeter hinge – with hinge axis perpendi-
cular to the shaft axis, but not necessarily perpendicular to the longitudinal axis of
the blades – in order to prevent differential blade root out-of-plane bending
moments arising during operation. Instead, differential aerodynamic loads on the
two-blades result in rotor angular acceleration about the teeter axis, with large
teeter excursions being prevented by the restoring moment generated by centrifugal
forces, as described in Section 5.8.8. However, when the machine is shut-down, the
centrifugal restoring moment is absent, so differential blade loading will cause the
rotor to teeter until it reaches the teeter end stops which need to be suitably
buffered. Consequently the teeter hinge is unlikely to provide any amelioration of
extreme blade root out-of-plane moments when the machine is shut-down.
The load relief afforded by the teeter hinge benefits the main structural elements
in the load path to the ground in varying degrees, as outlined below:

(a) Blade. The main benefit is the elimination of the cyclic variations in out-of-plane
bending moment due to yaw (Figure 5.10), shaft tilt, wind shear (Figure 5.11)
and tower shadow (Figure 5.14). By contrast, there is only a small reduction in
blade root out-of-plane bending moment due to stochastic loadings – see the
example in Section 5.8.8, where an 11 percent reduction is quoted. Thus, teeter-
ing results in a large overall reduction in out-of-plane fatigue loading, although
the significance of this will be tempered by the influence of the unaltered
edgewise gravity moment.

(b) Low-speed shaft. Low-speed shaft design is governed by fatigue loading, which