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

power of the turbine does not change, the mean loads on the system are not increased.
However, every wind gust that strikes the turbine when it is operating at rated power is
converted to a dynamic load on the turbine without producing any additional power. This
can result in a substantial increase in fatigue on mechanical and structural components.
Therefore, unless the rotor diameter increase is affected in an intelligent manner, it is
possible that increasing the rotor diameter will impact the cost of fatigue-driven components
in such a way that the turbine cost is increased more than the energy capture.
• Average wind speed and turbulence intensity strongly affect the importance of increased
rotor diameter. For sites with strong winds or high turbulence, the dynamic loads experienced
at and above rated wind speed can become a bigger design driver than additional
energy capture below rated. The use of larger rotors is largely a consideration for wind
farm sites with relatively low mean speeds and relatively low turbulence, where the turbine
operates less often at and above rated wind speeds and is less likely to experience
the strong wind gusts which induce fatigue damage.
The subsequent parts of this section address several alternative concepts investigated by GE for
significantly increasing energy capture without proportionately increasing the loads or turbine
cost. GE also investigated several alternative concepts that attempt to reduce rotor cost without
impacting energy capture. These concepts include:
• Rotor and other turbine structural flexibility
• Concurrent aerodynamic and structural design optimization
• Carbon composite rotor blades
• Aeroelastic tailoring of rotor blades
• Rotor blade aerodynamic boundary layer control
• Variable diameter rotor.
2.3.1 Rotor and Structural Flexibility
The main aim in exploring the implications of rotor structural flexibility is that, while some
steady state loads cannot be altered, dynamic loads on the wind turbine system are often design
drivers and may be reduced with the energy content of the input wind loading in various frequency
ranges partly dissipated by aerodynamic damping of blade motions. Hence, reduced loading
is passed further into the wind turbine system. The other aspect of structural flexibility that
is specific to a rotor system, as opposed a static cantilevered beam, is the effect of centrifugal
force in providing relief of blade out-of-plane bending moments, both steady and dynamic. Reducing
the stiffness of a tubular tower may also allow for a reduction in tower material, and introducing
damped compliance into the machine carrier yaw and pitch (i.e., nodding) degrees of
freedom may similarly reduce the absorption by the bedplate, yaw deck, or tower top of dynamic
loading imparted by the rotor. It is likely that tower and blade flexibility may be cheaper and easier
to engineer than yaw deck compliance and may even be a source of mass and cost reduction
of blades and tower rather than an added cost to be paid for by other benefits. All concepts are
investigated here.
The reduction in loads that might result from increased structural flexibility could result in benefits
to COE either as benefits to component designs or through increased energy capture by enabling
the use of a larger rotor with loads no higher than would be produced by a smaller, stiffer
rotor.
21