Environmental Engineering Reference
In-Depth Information
variability of the wind climate, the length of the historical reference period, and the
correlation of the site data with the long-term reference. Referring again to the equation
from Chapter 12,
r
2
N
R
σ
1
−
r
2
N
T
σ
historical
=
2
2
T
R
+
σ
(15.1)
Here,
σ
T
are the standard deviations of the annual mean wind speeds (a
measure of the interannual variability of the wind climate) for the reference (R) and
target (T) sites, respectively, over a suitably long period;
N
R
and
N
T
are the number
of years of reference and overlapping reference-target data, respectively;
r
is the
Pearson correlation coefficient based on a suitable averaging period (such as daily
means); and
σ
R
and
σ
historical
is the uncertainty in the derived historical mean wind speed at
the target site.
As discussed in Chapter 12, this equation makes a number of assumptions, the
most important of which is that the reference data record has been consistent through
time, with no discontinuities or trends resulting from changing location, equipment,
surroundings, and other factors. Given the difficulty of confidently meeting these
requirements with reference data older than about 10-15 years, it is best not to allow
N
R
to exceed 15. In addition, because of seasonal effects, the equation should not be
used with much less than 1 year of overlapping target and reference data.
Even when these conditions are met, actual errors may depart substantially from
the equation. Figure 15-1 shows the results of an experiment to test the uncertainty of
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
0.2
0.4
0.6
0.8
1
Average
r
2
Figure 15-1.
The results of an experiment to determine the uncertainty of MCP for three
tall towers in different parts of the United States. Each point represents the standard error
of the predicted mean speeds based on 12-month unconstrained linear regressions between a
surface reference station and a tower. The smooth curve is the theoretical uncertainty based
on Equation 15.1, assuming a 3.5% interannual standard deviation, the average for the three
towers.
Source:
Taylor M, et al. An analysis of wind resource uncertainty in energy production
estimates. AWS Truepower; 2004.
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