WIND ENERGY SYSTEMS - Cd3wd

Chapter 2—Wind Characteristics 2–55
Figure 23: Yearly mean wind speeds (knots) at Russell, Kansas, versus simultaneous speeds
at Dodge City, Kansas.
year following the first two is totally random. Based on these results, Justus concluded that
clusters of bad wind months or years are not highly probable. In fact, the variation of speed
from one period to the next is almost totally random, and therefore such clusters should not
be of major concern.
We have seen that the 90 % confidence interval for the long term annual mean speed at a
site is between 0.9ū and 1.1ū when only one year’s data is collected. If this is not adequate,
then additional data need to be collected. Corotis[7] reports that the 90 % confidence interval
is between 0.93ū and 1.07ū for two year’s data, and between 0.94ū and 1.06ū for three year’s
data. The confidence interval is basically inversely proportional to the square root of the time
period, so additional years of data reduce the confidence interval at a slower and slower rate.
10 DISTRIBUTION OF EXTREME WINDS
Two important wind speeds which affect turbine cost are the design wind speed which the
rotor can withstand in a parked rotor configuration, and the maximum operating wind speed.
A typical wind turbine may start producing power at 5 to 7 m/s (11 to 16 mi/h), reach rated
powerat12to16m/s(27to36mi/h),andbeshutdownatamaximumoperatingspeedof
20 to 25 m/s (45 to 56 mi/h).
Wind Energy Systems by Dr. Gary L. Johnson November 20, 2001