Wind Speed and Energy Distributions - Wind and Solar Power Systems - page 48

TABLE 4-1

Comparison of Three Wind Farm Sites with the Same Mean Wind Speed

but Significantly Different Specific Power Density

Annual mean wind speed Annual average specific power

Site Meters/second Watts/m

2

Culebra, Puerto Rico 6.3 220

Tiana beach, New York 6.3 285

San Gorgonio, California 6.3 365

value by the total number of the hours in the
year gives the annual energy production potential of the site.
The importance of the rmc speed

Then, multiplying the P

rmc

It compares
the wind power density at three sites with the same annual average wind
speed of 6.3 m/s. The San Gorgonio site in California has 66 percent greater
power density than the Culebra site in Puerto Rico. The difference comes
from having the different shape factors k, and, hence, the different rmc
speeds, although all have the same annual mean speed.

is highlighted in Table 4-1 .

4.6.4 Mode, Mean, and rmc Speeds Compared

The important difference between the mode, mean, and the rmc speeds is
illustrated in Table 4-2 . The values of the three speeds are compiled for four
shape parameters (k = l.5, 2.0, 2.5, and 3.0) and three scale parameters (c =
10, 15, and 20 mph). The wind power densities are calculated using the
respective speeds in the wind power equation

P = 1/2 ρ · V

3

watts/m

2

using

the air mass density of 1.225 kg/m .

We observe the following from the c=15 rows:

3

1. For k = 1.5, the power density using the mode speed is 230 versus

the correct value of 4,134 watts/m using the rmc speed. The ratio

of the incorrect to correct value of the power density is 1 to 18 —
a huge difference.

2

2. For k = 2, the power densities using the mode and rmc speeds are

731 and 2,748 watts/m , respectively, in the ratio of 1 to 3.76. The

corresponding power densities with the mean and the rmc speeds
are 1,439 and 2,748 watts/m

2

2

in the ratio of 1 to 1.91.

3. For k = 3, the power densities using the mode and rmc speeds are

1,377 and 2,067 watts/m , respectively, in the ratio of 1 to 1.50. The

corresponding power densities with the mean and the rmc speeds
are 1,472 and 2,067 watts/m

2

2

, in the ratio of 1 to 1.40.

Thus, regardless of the shape and the scale parameters, use of the mode
or the mean speed in the power density equation would introduce a signif-
icant error in the annual energy estimate, sometimes of several folds, making
the estimates completely useless.