In order to improve energy capture and reduce the cost of wind
energy, wind turbines have grown dramatically over time. Longer
blades are being developed to enable large, multi-MW wind turbines
capture more energy. The rated power of wind turbines has
grown almost linearly for the past 30 years [1,2]. At the time of
writing, a 73.5-m blade had been manufactured for a 6-MW
offshore wind turbine [3,4]. Blades are expected to grow even
larger in the future; current investigations include the development
of a 20-MW turbine [2] and the design of a 100-m blade [5].
However, the development of large blades at this scale is challenging.
For a simple geometric upscaling of a blade and assuming a
constant tip speed ratio, the bending stresses due to aerodynamic
forces are independent of blade length, and the bending stresses
due to gravitational forces increase linearly as blades get longer [5].
These bending loads are highest in the inboard region, near the root
of the blade. Hence, the inboard region needs an efficient structure
that can support the bending loads, provide favorable aerodynamic
performance, but also limit overall blade mass (and cost). The blade
cannot be too heavy, because excess blade mass increases inertial
fatigue loads, which decrease the blade lifetime. Heavier blades
also tend to cost more to manufacture and transport, which limits
the economic benefits of large wind turbines. This paper investigates
a new biplane structural design of the inboard region for
large wind turbine blades.
Recent research suggests that improving performance in the
inboard region of the blade may improve the overall blade performance.
Conventionally, the inboard region used thick airfoils to
support large flapwise bending loads [6e8]. While the aerodynamic
performance of thick airfoils is generally poor, this is the
standard compromise between structures and aerodynamics in
blade design. The inboard region is primarily designed for structures:
thick cross-sections (with a large second moment of area in
the flapwise direction) support the large flapwise bending loads.
(The inboard region design is also influenced by manufacturing
constraints related to the root attachment, as well as transportation
requirements that limit the maximum allowable dimensions.)