A typical wind turbine consists of a number of components: rotor, controls, drive-train (gearbox, generator, and power converter), tower, and balance of system.¹ Each of these components has undergone significant development in the last 10 years, with improvements integrated into the latest turbine designs. In addition, improved understanding and better modeling capabilities have contributed to the rapid introduction of technical improvements. What were initially small clusters of 100 kW turbines in the early 1980s have grown to clusters of hundreds of machines, including machines of 1.5 MW or more.

In general, wind speed increases with height, and the energy capture capability depends on the rotor diameter. Figure 3.1 shows the change in rotor diameter and rated capacity over time. In 2006 the most common installed machine had hub heights of 275 ft (84 m) and a rotor diameter of 220 ft (67 m). Turbines as big as 5 MW have been installed in offshore locations; these have 505 ft (154 m) hub height and 420 ft (128 m) rotor diameter (IEEE, 2007a).² As noted in Chapter 1, the U.S. wind energy industry installed almost 14,000 MW of capacity during 2007 and 2008. The U.S. wind power capacity is now more than 25 GW and spans 34 states; the world’s largest wind power plant, Horse Hollow Wind Energy center with a capacity of 750 MW, was recently commissioned in Texas (SECO, 2008). U.S. wind farms will generate an estimated 52,000 GWh of electricity in 2008, about 1.2 percent of the U.S. electricity supply. As discussed in Chapter 1, the installed wind power generating capacity worldwide at the end of 2006 was 75,000 MW.