WIND ENERGY SYSTEMS - Cd3wd

Chapter 2—Wind Characteristics 2–12
Figure 7: Speed-frequency curves, 1970
Speed frequency curves, or similar mathematical functions, can be used in studies to
develop estimates of the seasonal and annual available wind power density in the United
States and elsewhere. This is the power density in the wind in W/m 2 of area perpendicular
to the flow of air. It is always substantially more than the power density that can actually
be extracted from the wind, as we shall see in Chapter 4. The result of one such study[9] is
shown in Fig. 8. This shows the estimated annual average power density available in the wind
in W/m 2 at an elevation of 50 m above ground. Few wind data are recorded at that height, so
the wind speeds at the actual anemometer heights have been extrapolated to 50 m by using
the one-seventh power law, which will be discussed later in this chapter.
The shaded areas indicate mountainous terrain in which the wind power density estimates
represent lower limits for exposed ridges and mountain tops. (From [9])
The map shows that the good wind regions are the High Plains (a north-south strip about
500 km wide on the east side of the Rocky Mountains), and mountain tops and exposed ridges
throughout the country. The coastal regions in both the northeastern and northwestern United
States are also good. There is a definite trend toward higher wind power densities at higher
latitudes, as would be expected from Fig. 5. The southeastern United States is seen to be
quite low compared with the remainder of the country.
There are selected sites, of course, which cannot be shown on this map scale, but which
Wind Energy Systems by Dr. Gary L. Johnson November 20, 2001