WIND ENERGY SYSTEMS - Cd3wd

Chapter 2—Wind Characteristics 2–56
Figure 24: Yearly mean wind speeds (knots) at Russell, Kansas, versus simultaneous speeds
at Wichita, Kansas.
The design wind speed is prescribed in a ANSI Standard[1], as modified by height and
exposure. This varies from 31 m/s (70 mi/h) to 49 m/s (110 mi/h) in various parts of the
United States. It is not uncommon for wind turbines to be designed for 55 m/s (125 mi/h)
or more. This high design speed may be necessary at mountainous or coastal sites, but may
be unnecessary and uneconomical in several Great Plains states where the highest wind speed
ever measured by the National Weather Service is less than 45 m/s (100 mi/h).
Also of interest is the number of times during a year that the wind reaches the maximum
turbine operating wind speed so the turbine must be shut down. This affects the turbine
design and has an impact on the utility when the power output suddenly drops from rated to
zero. For example, if a wind farm had to be shut down during the morning load pickup, other
generators on the utility system must ramp up at a greater rate, perhaps causing operational
problems.
Wind turbine designers and electric utility operators therefore need accurate models of
extreme winds, especially the number of times per year the maximum operating speed is
exceeded. Variation of extreme winds with height is also needed. The ANSI Standard[1] gives
the extreme wind that can be expected once in 50 years, but daily and monthly extremes are
also of interest.
The extreme wind data which are available in the United States are the monthly or yearly
fastest mile of wind at a standard anemometer height. Anemometers will be discussed in
detail in the next chapter, but the electrical contact type used by the National Weather
Wind Energy Systems by Dr. Gary L. Johnson November 20, 2001