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The modern electricity-generating horizontal-axis wind turbine (now often labeled with the acronym HAWT) is an obvious descendant of the historic European windmill and the small, DC-generating wind turbines of the 1930s. The resemblance is somewhat deceptive, however, since the HAWT and its less-familiar vertical-axis cousin, the VAWT, have evolved as sophisticated products of current technology. Their high performance and reliability are the result of steady improvement in methods of aerodynamic and structural design, in new materials, and in mechanical and electrical engineering. The evolution of wind turbine technology in the United States and elsewhere since World War II is described in this chapter, along with some of the problems that have arisen and how these problems have been faced.
The world's largest wind power plant prior to the 1970s was the Smith-Putnam wind turbine (Fig. 1-22) erected in 1939 on Grandpa's Knob near Rutland, Vermont [Putnam 1948]. With a rotor diameter of 53.3 m and a power rating of 1.25 MW, this pioneering HAWT was a major work of mechanical engineering. The Smith-Putnam project was a milestone between the decline of fully-developed, stand-alone wind turbines generating DC power only for local use and the new growth of wind power plants connected to utility lines and producing AC power for distribution throughout the system.