Environmental Engineering Reference
In-Depth Information
10
0.80
9
0.70
8
0.60
7
0.50
6
5
0.40
4
0.30
3
0.20
2
Hourly speed
Hourly shear
0.10
1
0
0.00
02468 0 2
Hour
14
16
18
20
22
Figure 11-3.
A typical pattern of diurnal variation of mean wind speed and shear. In this
example, the speed is observed at 60 m, and the shear is calculated between 40 m and 60 m.
Source:
AWS Truepower.
this method, it is almost always important to bin by time of day to capture the effect
of diurnal variations of atmospheric stability.
Where the shear is believed to change with height, using instantaneous or binned
shear exponents can introduce an additional challenge since each shear value must
in principle be adjusted separately, and there is rarely enough information to do this
with confidence. An alternative is to rely on the time-averaged shear, adjusted if
necessary for displacement and other effects, to establish the mean wind speed at
hub height. Any difference between this mean and that derived from the extrapolated
time series or speed distribution can be resolved by rescaling the extrapolated data so
that the means match. The rescaling is done by multiplying each speed value by the
ratio of the expected hub-height mean speed and the mean of the extrapolated data.
This method, while not ideal, represents a commonsense compromise between the
simplicity and transparency of using a time-averaged shear and the greater accuracy
of speed distributions derived from binned or time-varying shears.
11.3 OTHER PARAMETERS
Three other wind resource characteristics, the wind direction, air density, and TI, must
also be projected to hub height for estimating turbine and plant power production.
11.3.1 Wind Direction
It is generally assumed that the wind direction is constant with height above the top
anemometer. This is not strictly true even in principle, as the interaction of the earth's
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