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

2.3.3 Aeroelastic Tailoring of Rotor Blades
GE was motivated to determine whether composite rotor blades can be designed and costeffectively
manufactured in such a way that they provide an aeroelastic response tailored to ameliorate
fatigue-inducing transient loads and concomitant extreme blade deflections. Traditionally,
aeroelastic response–that is, the tendency of a structure to exhibit coupling between aerodynamic
loading and structural behavior was something the designer, whether aerospace, civil, or mechanical,
had to accept with design considerations. In the past three decades, the advent of composite
materials have allowed for tailoring of aeroelastic properties through deliberate design of
desired flap-twist coupling. Off-axis (e.g., 20°) orientation of reinforcement fibers (e.g., glass,
carbon, Kevlar, etc.,) along a supporting spar will cause the spar to twist under sufficient bending
strain. This can be used to tailor the bend-twist coupling of an aircraft wing or a wind turbine
blade.
GE’s pitch-regulated wind turbines maintain constant power above rated wind speed by constantly
pitching the blades in response to wind speed gustiness. When the baseline turbine is operating
at wind speeds just less than or at rated wind speed. it is most vulnerable to rapidly rising
wind gusts in terms of their ability to induce damaging transient loads and blade deflections.
The blade pitch is at a fine setting to optimize the power factor, and so any gust that hits the turbine
is seen as a sudden and significant increase in angle of attack along the entire blade span.
Since for optimal power factor, the blade design and pitch are set so each station is operating
near the angle of attack for maximum lift-to-drag ratio, any increase in angle of attack results in
increased lift. The change in lift combined with the change in inflow angle results in a sudden
increase in both in-plane and out-of-plane loading on the blade. Such transient loads induced by
wind gusts combined with the response of the turbine are the most significant contributions to
structural fatigue on wind turbines.
Presently, the pitch control system responds to increased rotor torque by pitching the blades to
smaller angles of attack, thereby reducing the blade loading and torque. As the wind subsides,
the pitch is returned to normal. Nevertheless, the pitch system can respond only so quickly, particularly
when it must transition from Region 2 to Region 3 of the power curve. As a result, the
wind turbine will still realize significant transient increases in loads before the pitch system can
counteract them.
An aeroelastically tailored blade could reduce such transient loads. As the gust would induce
flapwise deflections, the tailored bend-twist coupling of the blade would passively induce a
twisting of the blade towards reduced angles of attack, thereby relieving the loads and reducing
pitch activity. This will prevent the loads and hence the deflection from rising to the levels they
would reach if the blade were not twist-bend coupled. Although it might be desirable to have an
entirely passively controlled blade, GE has never believed this was achievable, nor that it was the
goal of research for the concept. The reason is because of a simple but unavoidable quandary–
the blade can only reduce loads by twisting into the wind, and it will only twist more if it flaps
more, and it only flaps more if it is subjected to higher loading. It is simply not possible to shed
all transient loads passively. But trimming the amplitude of transient loads and reducing pitch
activity are both extremely desirable goals for twist-bend coupling.
Introduction of twist-bend coupling is not without cost, however. First, twist-bend coupling can
cost some energy capture if not implemented carefully. If a turbine is operating at a mean wind
speed near rated, the blades will be subject to some relatively high mean loading that will pro­
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