Page 41 - Wind Energy Handbook

ANNUAL AND SEASONAL VARIATIONS 15

p ffiffiffi
ˆ(1 þ 1=k) has the value ð=2 ¼ 0:8862. A higher value of k, such as 2.5 or 3,
indicates a site where the variation of hourly mean wind speed about the annual
mean is small, as is sometimes the case in the trade wind belts for instance. A lower
value of k, such as 1.5 or 1.2, indicates greater variability about the mean. A few
examples are shown in Figure 2.2. The value of ˆ(1 þ 1=k) varies little, between
about 1.0 and 0.885 see (Figure 2.3).

k = 1.25 1.5 2.0 2.5 3.0
0.15

Probability density 0.05
0.1

0
0 5 10 15 20 25
Wind speed (m/s)
Figure 2.2 Example Weibull Distributions

1.00

0.98

0.96

0.94

0.92

0.90

0.88
1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0
Shape factor, k
Figure 2.3 The Factor ˆ(1 þ 1=k)