WIND ENERGY SYSTEMS - Cd3wd

Chapter 2—Wind Characteristics 2–25
according to the equation
α = a − b log 10 u(z 1 ) (14)
The coefficients a and b can be determined by a linear regression program. Typical values
of a and b are 0.11 and 0.061 in the daytime and 0.38 and 0.209 at night.
Figure 14: Variation of wind profile exponent α with reference wind speed u(z 1 ).
Several sets of this figure can be generated at each site if necessary. One figure can be
developed for each season of the year, for example. Temperature, wind direction, and height
effects can also be accommodated by separate figures. Such figures would be valid at only the
site where they were measured. They could be used at other sites with similar terrain with
some caution, of course.
Theaveragevalueofα has been determined by many measurements around the world to
be about one-seventh. This has led to the common reference to Eq. 13 as the one-seventh
power law equation. This average value should be used only if site specific data is not available,
because of the wide range of values that α can assume.
Wind Energy Systems by Dr. Gary L. Johnson November 20, 2001