In the effort to capture more energy from the wind, the blades of wind turbines have become bigger and bigger to the point where the diameter of the rotors can be over 100 m (328 ft). Although larger blades cover a larger area, they are also heavier, which means more wind is needed to turn the rotor. The ideal combination would be blades that are not only bigger, but also lighter and more durable. A researcher at Case Western Reserve University has built a prototype blade from materials that could provide just such a winning combination.

The new blade developed by Marcio Loos, a post-doctoral researcher in the Department of Macromolecular Science and Engineering, is the world's first polyurethane blade reinforced with carbon nanotubes. Using a small commercial blade as a template, Loos manufactured a 29-inch (73.6 cm) blade that is substantially lighter, more rigid and tougher than conventional blades. Rigidity is important because as a blade flexes in the wind it loses the optimal shape for catching air, so less energy is captured.

Working with colleagues at Case Western Reserve, and investigators from Bayer Material Science in Pittsburgh, and Molded Fiber Glass Co. in Ashtabula, Ohio, Loos compared the properties of the new materials with that of conventional blades manufactured using fiberglass resin.

"Results of mechanical testing for the carbon nanotube reinforced polyurethane show that this material outperforms the currently used resins for wind blades applications," said Ica Manas-Zloczower, professor of macromolecular science and engineering and associate dean in the Case School of Engineering.

Comparing reinforcing materials, the researchers found that the carbon nanotubes are lighter per unit of volume than carbon fiber and aluminum and had five times the tensile strength of carbon fiber and more than 60 times that of aluminum.

Meanwhile, fatigue testing showed the reinforced polyurethane composite lasts about eight times longer than epoxy reinforced with fiberglass, while delamination fracture tests showed it was also about eight times tougher. The performance of the material was even better when compared against vinyl ester reinforced with fiberglass, another material used to make wind turbine blades. Fracture growth rates were also a fraction of that found for traditional epoxy and vinyl ester composites.

Loos and her team are now working to determine the optimal conditions for the dispersion of the nanotubes, the ideal distribution within the polyurethane and the ways to achieve both.

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