The main technical parameter determining the economic success of a wind turbine system is its annual energy output, which in turn is determined by parameters such as average wind speed, statistical wind speed distribution, distribution of occurring wind directions, turbulence intensities, and roughness of the surrounding terrain. Of these, the most important and sensitive parameter is the wind speed, which increases exponentially with height above the ground; the power in the wind is proportional to the third power of the momentary wind speed. As accurate meteorological measurements and wind energy maps (as shown in Figure G-12) become more commonly available, wind project developers can more reliably assess the long-term economic performance of wind farms.
Estimates show that U.S. wind resources could provide more than 10 trillion kWh (Deyette et al., 2003; Elliott and Schwartz, 1993), which includes land areas with wind class 3 or above (corresponds to wind speeds greater than 7 meters per second [m/s] [15.7 mph] at a height of 50 m), within 20 miles of existing transmission lines, and excludes all urban and environmentally sensitive areas. This is over 4 times the total electricity currently generated in the United States. In the DOE’s Hydrogen Posture Plan (DOE, 2003a), wind availability is estimated to be 3250 GW, equivalent to the above value for a capacity factor of 35 percent. In 2002, installed wind capacity was about 5 GW generating 12.16 billion kWh, corresponding to a capacity factor of 29 percent (EIA, 2003).
There has been a gradual growth of the unit size of commercial machines since the mid-1970s. In the mid-1970s the typical size of a wind turbine was 30 kW. By 1998, the largest units installed had a capacity of 1.65 MW, while turbines with an installed power of 2 MW have now been introduced into the market with over 3 MW machines being developed. The trend toward larger machines is driven by the demand side of the market to utilize economies of scale and to reduce visual impact on the landscape per unit of installed power, and by the expectation that the offshore potential will be growing.
Recent technical advances have made wind turbines more controllable and grid-compatible and have reduced the number of components, making them more reliable and robust. The technology is likely to continue to improve. Such im-