The Evolution of Rotor and Blade Design
James L. Tangler
National Renewable Energy Laboratory
1617 Cole Boulevard
Golden, CO 80401
The objective of this paper is to provide a historical perspective of the evolution of rotor and blade design during
the last 20 years. This evolution is a balanced integration of economic, aerodynamic, structural dynamic, noise,
and aesthetic considerations, which are known to be machine type and size dependent.
The resurgence of wind energy the last quarter of the past decade opened the door to a wide variety of rotor
designs and materials that through trial and error, has converged to the market-driven, three-bladed composite,
rotor configuration. The history behind this evolutionary process should be understood and documented to
minimize unproductive regressions for future rotor designs.
The design of a modern rotor includes choices of blade number, airfoils, chord and twist distributions, and
materials. The justification for each of these choices often includes conflicting considerations that need to be
prioritized. For example, thin airfoils are desirable for their high lift-to-drag ratios and are roughness tolerant,
whereas thick airfoils sacrifice some of these qualities to achieve the greater blade stiffness required for large
machines. The pros and cons of these considerations are explored to better understand the current state of rotor
and blade design. Some obvious blade design trends resulting from increased rotor size include lower blade
solidity, increased airfoil thickness, and maximum lift coefficient, along with incremental increases in tip speed.
Limits that govern these trends need to be understood in order to achieve a minimum cost-of-energy design.
WHERE WE STARTED
The rise of wind energy during the 1970s began with a trial-and-error process that included a wide variety of
rotor configurations. The taxonomy of this decade included a variety of horizontal-axis wind turbine (HAWT)
and vertical-axis wind turbine (VAWT) configurations. The HAWT’s included upwind and downwind
configurations with various performance enhancers, such as diffusers and concentrators. The VAWTs included
the lift-type Darrieus configuration and also the drag-type Savonius turbine. The evolution of wind turbines
during the 1980’s was driven largely by the cost of energy, which resulted in the demise of many of the early
concepts. Although VAWTs have the advantage of a drivetrain close to the ground for easy accessibility, their
cost-effectiveness does not equal that of HAWT’s for reasons not fully documented. Aerodynamically, VAWT’s
utilize less efficient symmetric airfoils than the higher lift-to-drag ratio, cambered airfoils used on HAWT’s. The
constant chord, VAWT blades adversely affected blade efficiency and self-start capability. Rotor wake-induced
losses of the VAWT’s are greater than those of HAWT’s since VAWT’s only operate at optimum lift-to-drag
ratio over a small azimuth of the rotation. This leads to excessive wind energy going into rotor thrust loads rather