As wind turbine designers try to capture greater energy with each turbine, blade length, size and weight have increased dramatically. Further, blade tip speed is limited by noise ordinances, so very large turbines typically spin at less than 20 rpm. Yet hub rotation is the driver for the generator. For that reason, Selsam Innovations (Fullerton, Calif., U.S.A.) is testing a highly unusual configuration on its 3-kW Superturbine at a Windtesting.com (Tehachapi, Calif., U.S.A.) windfarm. The turbine features seven 2.1m/7-ft rotors with composite blades fixed on a 21.3m/70 ft carbon-fiber reinforced horizontal driveshaft. The rotors, three upwind and four downwind of an 18.3m/60-ft tower, can rotate fast enough to capture a wind volume equivalent to that captured by a much larger single-rotor turbine, an arrangement that inventor and patent-holder Doug Selsam says could overcome challenges in wind turbine design, rotor weight and rotational speed.
The driveshaft is inclined about 25°, with the lower end pointing into the wind to allow each rotor to catch "fresh" wind, and the rotors are designed to shed wind downward, below the subsequent rotors.
The carbon fiber shaft allows the rotors to be positioned some distance out from the tower, without excessive added weight and, Selsam points out, "It can go through a lot of fatigue cycles with no problem." The driveshaft was fabricated from three filament wound tubes of nominal 4 mm/0.16 inch wall thickness. (Pultruded tubes provided inadequate hoop strength and straightness, Selsam reports.) The center section, with uniform 76-mm/3-inch inside diameter (ID), was wound by ACPT (Huntington Beach, Calif., U.S.A.). On each end are tapered tubes (76 mm to 50 mm/3 inch to 2 inch ID) wound by Advanced Composites (Salt Lake City, Utah, U.S.A.). Standard commercial epoxy, 149°C/300°F cure, is applied to high-modulus 12K PAN tow in ACPT's wet filament winding, reports ACPT engineer Lee Truong.
Prototypes funded by the California Energy Commission have achieved promising results during tests and Selsam sees commercialization on the horizon, especially in areas with much lower wind speed than is required for utility-size turbines. The multi-blade design also is a candidate for placement atop tall buildings.