Environmental Engineering Reference
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FIGURE 3.5 Example of vertical wind shear.
Measurements taken at heights of 10, 20, and 50 m for the northwest Texas region [3] for 12 h
periods (6-18 h, day-night) showed a large difference between 10-20 m and 50 m levels. Data for
sixteen sites in Texas and one site in New Mexico show the same results, a change in diurnal wind
speed pattern at around 40 m [4]. Wind speeds were sampled at 1 Hz and averaging time was 1 h.
The data were averaged by hour over a month, and then those were averaged over a year to obtain
an annual average day ( Figure 3.7 ) . This same pattern is noted for data taken at heights above 50 m
( Figure 3.8 ) . The wind speed is still increasing with height, so the issue for wind farms is the trade-
off between increased output with wind turbine height and increased cost for taller towers. These
results clearly show that wind speed data need to be taken at least at a height of 40 m or higher to
find the shift in pattern between day and night wind speeds. Once there are data at 10 m and 40-50
m, the wind shear can be used to predict wind speeds at higher levels. The higher night wind speeds
means there is more power; however, those hours are also when there is less demand, so if the wind
farm is selling at the market price, that energy may be worth less.
The wind shear exponent changes from low values during the day to high values at night over a 2
h period ( Figure 3.9 ) . Time of day data were averaged over each month. So the low values occur for
more hours in the summer. There are locations where there is little wind shear, primarily mountain
passes ( Figure 3.10 ) . In this case taller towers for wind turbines would not be needed.
The world standard height is 10 m for meteorology measurements for weather; however, using
10 m data and the 0.14 wind shear exponent to estimate wind power potential for 50 m for many
Height
m
Wind Speed, m/s
12.6
50
12.2
40
11.7
30
11
20
10
8.8
10
5
0
FIGURE 3.6 Wind shear, change in wind speed with height. Calculations are for given wind speed of 10 m/s
at 10 m, ] 1/7.
 
 
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