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

2.3 Rotor and Structural Design Concepts
The rotor arguably represents the most important component of a wind turbine. It is the component
responsible for extracting energy from the wind, but it is also the component responsible for
extracting loads from the wind and transmitting them to the rest of the turbine. Furthermore, the
energy in the wind is unsteady and stochastic, and the rotor also becomes the principal conduit of
dynamic loads experienced by the turbine. These loads induce fatigue and drive the design of
most wind turbine components.
Therefore, a significant portion of GE’s concept studies have focused on rotor design concepts
that can significantly improve energy capture with a less than proportionate increase in dynamic
loads and turbine cost. The challenge inherent in achieving this goal is that trying to extract more
energy from the wind generally entails extracting more of the dynamic loads.
The most obvious and simplest design change that can affect energy capture is increasing the rotor
size for a given drive train. There is no wind industry standard for the diameter of a rotor to
be coupled with a generator of a given size. In recent years, however, all of the manufacturers of
utility-scale turbines of roughly 500–1,650 kW rating have offered them with rotors corresponding
to a loading of between 380 and 450 W/m 2 . For reference purposes, this corresponds to rotors
with a diameter between 38 and 41 m for a 500-kW turbine, between 46 and 50 m for a 750
kW turbine, and between 65 and 71 m for a 1,500-kW turbine.
Early in the concept studies, GE personnel began examining the possibility of reducing the rotor
loading (i.e., increasing the diameter) to as low as 320 W/m 2 , corresponding to a 55-m rotor on
the baseline 750-kW turbine or a 77-m rotor on a 1.5-MW turbine. Such a design change is not
trivial, however, for several reasons:
• For a pitch-regulated, variable speed wind turbine, increasing the rotor diameter will only
increase the energy capture at wind speeds below rated. Therefore, a 20% increase in rotor
swept area may result in a 20% increase in power production at wind speeds below
rated; but it will result in less than a 20% increase in annual energy production, probably
nearer a 8%–12% increase, depending upon the wind characteristics at the site.
• Acoustic emissions considerations may require that the rated shaft speed be reduced proportionately
to the rotor diameter increase, in order to avoid increasing the rotor tip
speed, which strongly affects aerodynamic noise. This will cause a proportionate increase
in the low-speed shaft torque with concomitant effects on several major components,
including the main shaft, gearbox, and bedplate.
• Increasing the rotor diameter increases the loads more than the energy capture. The
power production is obviously proportional to the swept area, which increases as the
square of the rotor diameter. If the shaft speed-vs-wind speed schedule remains unchanged
up to rated power, however, then the tip speed increases proportionately with the
rotor diameter, and the aerodynamic pressure at the tip increases roughly as the square of
the tip speed. Assuming a roughly constant-chord section is added to the tip, the area
over which the pressure acts is also proportional to the diameter, so the overall loading
increases as the cube of the rotor diameter. If the entire blade is scaled, including the
chord length, the loading scales as the fourth power of the diameter. The thrust loading
and in-plane loading induce strong moments on the tower base and drive the design of the
tower. Furthermore, the moment arm from the aerodynamic loading to the blade root is
also proportional to the increase in diameter, so the increase in blade root bending moment
scales as the fourth to fifth power of the rotor diameter. These loads drive the design
of the blades, hub, and drive train mechanical and structural components. If the rated
20