WIND ENERGY SYSTEMS - Cd3wd

Chapter 4—Wind Turbine Power 4–27
for the curve of u F =2u R . There is no combination of practical values for cut-in, rated, and
furling speeds which will yield a capacity factor greater than 0.6 in a wind regime described
by the Weibull shape parameter k = 2. The average power will never be more than 0.6 of
the rated power in such a wind regime. Only if impractical values of rated and cut-in speeds
are selected can the capacity factor be raised above 0.6. For example, if we have a good wind
regime described by c = 10 m/s (mean speed ū = 9 m/s) we could have a capacity factor
approaching 0.9 if we pick a rated wind speed of 4 m/s (u R /c = 0.4) and if the turbine could
deliver rated power up to u F = 50 m/s. Even if this were technically possible, it would not
be economically practical. We shall see that economics will normally force us to a rated wind
speed greater than c, in which case a furling speed of approximately twice the value of c will
produce the same capacity factor as a larger furling speed. This is true because the wind
rarely blows at speeds greater than 2c, so wind speeds above 2c do not significantly affect the
average power.
Figure 18: Wind turbine capacity factor as a function of rated speed. u c =0.5u R and k =2.
The important point is that the capacity factor decreases rapidly with increasing values of
rated wind speed for practical values of u R . As the rated wind speed is increased, the turbine
will operate fewer hours at rated power and more hours at partial power or below cut-in. This
decrease in capacity factor must be balanced against an increase in total energy production
to obtain the desired economic optimum.
Example
Preliminary data suggest that the 50 m wind speeds at a potential wind farm site are characterized
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001