WIND ENERGY SYSTEMS - Cd3wd

Chapter 2—Wind Characteristics 2–11
than Kansas City. Dodge City had a mean wind speed for this year of 5.68 m/s while Kansas
City had a mean wind speed of 3.82 m/s. The anemometer height for both stations was 7 m
above the ground.
Figure 6: Speed-duration curves, 1970
Speed duration curves can be used to determine the number of hours of operation of a
specific wind turbine. A wind turbine that starts producing power at 4 m/s and reaches rated
power at 10 m/s would be operating 6500 hours per year at Dodge City (for the data shown
in Fig. 6 and would be producing rated output for 700 of these hours. The output would be
less than rated for the intermediate 5800 hours.
Speed duration curves do not lend themselves to many features of wind turbine design
or selection. It is difficult to determine the optimum rated wind speed or the average power
output from a speed duration curve, for example. For this reason, another type of curve has
been developed, the speed-frequency curve. The two speed-frequency curves corresponding to
the data of Fig. 6 are given in Fig. 7. These curves show the number of hours per year that
the wind speed is in a given 1 m/s interval. At Dodge City the wind speed of 4 m/s is seen to
occur 1240 hours per year. This actually means that we would expect wind speeds between
3.5 and 4.5 m/s for 1240 hours per year. The summation of the number of hours at each wind
speed over all the wind speed intervals should be the total number of hours in the year.
Speed-frequency curves have several important features. One is that the intercept on
the vertical axis is always greater than zero, due to the existence of calm spells at any site.
Another feature is that the most frequent speed (the wind speed at the peak of this curve) is
lower than the mean speed and varies with it. Still another feature is that the duration of the
most frequent speed decreases as the mean speed increases[10].
Wind Energy Systems by Dr. Gary L. Johnson November 20, 2001