Advanced Wind Turbine Program Next Generation Turbine ... - NREL

damage equivalent flap moment, 25% reduction in edge moment, 20% reduction in yaw moment,
23% reduction in tilt moment, and 20% reduction in hub moment (My) in 17 m/s winds. The reduction
in edge moment corresponds approximately to the reduction in blade weight. There does
not appear to be any reduction or amplification due to the reduced natural frequency.
The only predicted increase is in tower-top thrust (7%). This is attributed to tower motion interaction
with the pitch control system and predicted that this increase could be reduced or eliminated
using nacelle acceleration feedback to the controller. It is interesting to note that the thrust
(m=4) and tower clearance are worse for configuration 2 than for configuration 4. The soft-soft
tower provides some mitigation of the negative aspects of the soft blade. Use of the soft blade
with the baseline tower (configuration 2) is not recommended. This configuration is predicted to
have less benefit and a greater reduction in tower clearance than configuration 4.
The blade tip/tower clearance is reduced by up to 25% for Configuration 4. This is the largest
challenge of the softer configurations. This problem can be reduced with little apparent negative
consequence by further increasing the precone angle. The energy yield is predicted to decrease
by 4%. Configuration 2 (with the same precone angle as configuration 4) has less energy loss at
11.3 m/s than configuration 4. This means that not all of the energy loss is due to the larger precone
angle of the softer blade. Based upon projected area alone, the expected energy loss due to
a 2° increase in preconing would be about 0.3%. Therefore, most of the loss is attributed to the
poorer speed regulation with the softer systems. Further “tuning” of the control system can
eliminate much of this loss.
This study demonstrated clear benefits in terms of reducing blade mass and loads in general, but
the study did not quantify in terms of impact on COE any of these benefits or costs such as energy
loss. The second study was similar, but in addition to examining the impact of rotor flexibility
on loads, this study also addressed several practical considerations in more detail:
• The relationship between mass and stiffness
• Tower clearance with more flexible rotor blades
• Impact on COE
• Engineering implementation issues, including scaling concerns.
In this study, the blade stiffness was halved, and the blade mass was reduced approximately
30%. A lifetime fatigue analysis and an extreme load calculation were performed. The extreme
loads analyses yields approximately equal maximum blade deflection. The extreme blade deflection
for the baseline design is 1.5 m. For the softened blade it is about 3.0 m. The principal
challenge of reducing blade stiffness with an upwind rotor configuration is to provide a safe
tower clearance to accommodate extreme blade deflections. The main options for increasing
tower clearance are:
• Increased rotor overhang
• Increased rotor tilt
• Increased blade coning
• Introduction of blade curvature (out-of-plane).
The introduction of a moderate amount of tilt (up to 10°) will add cyclic loads that are small in
comparison to the stochastic loads induced by turbulence. Simulations have been carried out with
8° of shaft uptilt and 3° of forward precone. This configuration provided for a maximum tip deflection
substantially equal to that of the baseline configuration. The results are summarized in
Figure 7, which can be compared directly to the results shown in Figure 6, which were obtained
for similar cases with slightly different values of uptilt and precone. Most of the fatigue reduction
is quite comparable, but the increases seen in the first study in a few components of fatigue
have been eliminated by careful tuning of the controller to avoid tower/blade resonance. Overall,
the results mentioned above are confirmed, in as much as substantial reductions of critical components
of fatigue can be realized through relaxing the rotor blade stiffness.
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