NASA’s Wind Turbine Supporting Research and Technology Contributions

A very significant NASA Lewis contribution to wind turbine development involved the Center’s Supporting Research and Technology (SR&T) pro­gram. The primary objectives of this component of NASA’s overall wind energy program were to gather and report new experimental data on var­ious aspects of wind turbine operation and to provide more accurate ana­lytical methods for predicting wind turbine operation and performance. The research and technology activity covered the four following areas: (1) aerodynamics, (2) structural dynamics and aeroelasticity, (3) com­posite materials, and (4) multiple wind turbine system interaction. In the area of aerodynamics, NASA testing indicated that rounded blade tips improved rotor performance as compared with square rotor tips, result­ing in an increase in peak rotor efficiency by approximately 10 percent. Also in the aerodynamics area, significant improvements were made in the design and fabrication of the rotor blades. Early NASA rotor blades used standard airfoil shapes from the aircraft industry, but wind turbine rotors operated over a significantly wider range of angles of attack (angles between the centerline of the blade and incoming airstream). The rotor
blades also needed to be designed to last up to 20 or 30 years, which represented a challenging problem because of the extremely high num­ber of cyclic loads involved in operating wind turbines. To help solve these problems, NASA awarded development grants to the Ohio State University to design and wind tunnel test various blade models, and to the University of Wichita to wind tunnel test a rotor airfoil with ailerons.[1516]

In the structural dynamics area, NASA was presented with prob­lems related to wind loading conditions, including wind shear (vari­ation of wind velocity with altitude), nonuniform wind gusts over the swept rotor area, and directional changes in the wind velocity vec­tor field. NASA overcame this problem by developing a variable speed generator system that permitted the rotor speed to vary with the wind condition, thus producing constant power.

Development work on the blade component of the wind turbine systems, including selecting the material for fabrication of the blades, represents another example of supporting technology. As noted above, NASA Lewis brought considerable structural design expertise in this area to the wind energy program as a result of previous work on heli­copter rotor blades. Early in the program, NASA tested blades made of steel, aluminum, and wood. For the 2-megawatt Mod-1 phase of the program, however, NASA Lewis decided to contract with the Kaman Aerospace Corporation for the design, manufacture, and ground-test­ing of two 100-foot fiberglass composite blades. NASA provided the general design parameters, as well as the static and fatigue load infor­mation, required for Kaman to complete the structural design of the blades. As noted in Kaman’s report on the project, the use of fiberglass, which later became the preferred material for most wind turbine blades, had a number of advantages, including nearly unlimited design flexibil­ity in adopting optimum planform tapers, wall thickness taper, twist, and natural frequency control; resistance to corrosion and other envi­ronmental effects; low notch sensitivity with slow failure propagation rate; low television interference; and low cost potential because of adapt­ability to highly automated production methods.[1517]

The above efforts resulted in a significant number of technical reports, analytical tests and studies, and computer models based upon contributions of a number of NASA, university, and industry engineers and technicians. Many of the findings grew out of tests conducted on the Mod-0 testbed wind turbine at Plum Brook Station. One is work done by Larry A. Viterna, a senior NASA Lewis engineer working on the wind energy project, in aerodynamics. In studying wind turbine performance at high angles of attack, he developed a method (often referred to as the Viterna method or model) that is widely used throughout the wind tur­bine industry and is integrated into design codes that are available from the Department of Energy. The codes have been approved for worldwide certification of wind turbines. Tests with the Mod-0 and Gedser wind turbines formed the basis for his work on this analytical model, which, while not widely accepted at the time, later gained wide acceptance. Twenty-five years later, in 2006, NASA recognized Larry Viterna and Bob Corrigan, who assisted Viterna on data testing, with the Agency’s Space Act Award from the Inventions and Contributions Board.[1518]