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January 1997
No. 4
Wind Energy Resources
ny plan to develop wind energy must begin by understanding the wind resource. Where are the best potential wind sites located? How much energy could be extracted from the wind at those sites? Will wind turbine performance be affected by turbulence or other wind resource characteristics? These are just a few of the questions that must be answered. This briefing paper describes the U.S. wind energy potential, discusses some basic characteristics of wind resources, and summarizes the key elements of a wind resource assessment program
Wind energy potential of the United States
The United States is fortunate to possess one of the largest wind energy resources in the world. The amount of energy theoretically available for use has been estimated at as much as 40 times the current U.S. energy consumption. Of course, only a small fraction of this potential could be used because of constraints on available land for wind power plants, limits on the efficiency of energy extraction, cost, siting issues and other factors. Even after taking these factors into account, various studies suggest nonetheless that wind has the potential to supply anywhere from 10 percent to 40 percent of U.S. electricity needs.
Wind Classes
To simplify the characterization of wind potential, it is common to assign areas to one of seven wind classes, each representing a range of wind power densities or wind speeds at a specified height above the ground. The standard wind class definitions are shown in Table 1. (The meaning of wind power density is discussed below.)
By and large, the areas being developed today using large wind turbines are ranked as class 5 and above. Class 3 and 4 areas may be developed in the near future as wind turbines are adapted to run more efficiently at lower wind speeds. Class l and 2 areas are not deemed suitable for large wind machines, although smaller wind turbines may be economical in areas (such as remote or off-grid communities) where the value of the energy produced is high.
Geographic distribution
Wind energy resources are distributed unevenly across the United States (see map). Most of the class 4 and higher areas are found near the east and west coasts, along ridges in the Rocky and Appalachian mountain systems, and in a wide belt stretching across the Great Plains. The southeastern United States is characterized by class 1 and 2 winds.
Constraints on wind potential
Wind resource classification is only one of the factors that must be considered in estimating wind energy potential. For one thing, not all windy sites are suitable for wind power development. Some are on steep, rocky or inaccessible terrain; others are in scenic or protected forests and parks; still others are in densely populated areas.
One U.S. Department of Energy study that took these factors into account grouped areas of the country into five categories of land use: environmental (such as national parks), urban, forest, cropland and range land. The researchers then constructed four scenarios that considered varying degrees and types of possible land use exclusions to arrive at a range of estimates of the wind energy potential in each wind class.
They found that the windiest areas (class 5 and above) could support enough wind power capacity to provide 18 percent to 53 percent of the electricity consumed in 1993. The lower figure represents the most severe assumptions of land use exclusion, while the upper figure represents no exclusions at all. Most of the prospective sites in these classes are concentrated in the Great Plains states. In contrast, class 3 and 4 areas are distributed much more widely around the country and, according to the study could supply from 1.7 to 6 times the current U.S. electricity demand.
Other constraints may reduce these figures considerably. Two factors not considered in the study were possible conflicts between wind power plants and bird habitats and migration routes and constraints on the transmission capacity needed to carry wind power to population centers. (A recent study by the Energy Information Administration indicates that many windy sites are located near existing transmission lines.) The intermittent nature of wind energy also may impose limitations on its use, although probably not until it begins to generate more than 10 percent of a utility's power needs.
Characteristics of wind
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http://www.nationalwind.org/publications/wes/wes04.htm
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