l l J The Tvindmill Marshal F. Merriam7 College of Engineering University of California, Berkeley, CA 94720, USA A most remarkable endeavor in the effort to use renewable energies passed a critical milestone recently in Denmark. At the Tvind Schools!, assembly of the world's largest windmill has been completed. The last of three giant blades was bolted into place on 17 November 1977. After trials the machine is scheduled to go into regular service early in 1978. The Tvind endeavor is remarkable in several aspects. First, consider the technological features of the machine. It is by far the largest windmill standing in the world today, and by a small margin the largest ever built. A horizontal axis, 3blade, downwind propeller, the blades sweep a circle of diameter 54 meters. The hub is 50 m. above the ground. For comparison, the rotor diameter and hub height of the twoblade Smith-Putnam wind turbine, Grandpa's Knob, Vermont. U.S.A. (1941-45) were 53 and 37 meters, respectively. The NASA-ERDA windmill which has operated at Plum Brook (near Sandusky), Ohio, U.S.A., since late 197 5 has rotor diameter and hub height both 38 m. The tip of a vertically upright blade of the Tvind machine stands higher than the roof of a 20-story office building. In addition to sheer size, and all the engineering that implies, the machine includes a number of technological features which are imaginative, novel and interesting. The blades, for example, are constructed of fiberglass and plastic foam and weigh only five tons each. (The Smith-Putnam blades, Student-built T~ind mill: ·2 MW rated output at 33 mph (15 m/s) windspeed, 177 ft. blade diameter, 174 ft. tower, 3-blade downwind rotor, cost: $660,000; construction time: 2½ years. January 1978 RAIN Page 13 which were metal, weighed about 8 tons each). Along with lower mass, the use of fiberglass composite should have advantages from a fatigue point of view. The Tvind blades were laid up and constructed mostly by hand techniques. A large number of volunteer workers accomplished the task without heavy machinery or modern automation. Fiberglass blades have been used before (though rarely), but not with the same construction technique or on the same scale. The emergency speed control system, a matter of great importance on a big machine, has novel aspects. In addition to more or less conventional shaft brakes and pitch controls, there are parachutes! Parachutes of the type used to slow high-speed airplanes when landing are stowed in the wing tips of the windmill, and deploy under emergency overspeed conditions when centrifugal forces overcome magnetic latches. The drag from even one of the three parachutes is calculated to be sufficient to slow the rotor to a safe speed. The operational speeq control system is also unusual. The Tvind windmill is the first large aerogenerator to be designed to operate at variable rpm. It is intended that it will operate at that rpm which maximizes efficiency, which means faster rotation at higher wind speeds, up to a maximum of 42 rpm at the rated wind speed (15 m/s). Up to rated wind speed the rpm is controlled by controlling the load on the generator; above rated wind speed hydraulic blade pitch control is used to keep rpm constant. Schachle & Sons production prototype WECS: 140 kw rated output at 26 mph windspeed, 72 ft. blade diameter, 80 ft. tower, 3-blade upwind rotor, located at airport, Moses Lake, Washington (note circular concrete pad on which entire tower turns to orient wind-turbine into wind).
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