Environmental Engineering Reference
In-Depth Information
systems. One approach is to apply an adjustment to the shear observed at the mast
based on the change in the shear observed in the sodar data:
α
(
m
)
h
2
→
h
h
=
α
(
m
)
h
1
→
h
2
+
(α
(
s
)
h
2
→
h
h
−
α
(
s
)
Difference method :
h
1
→
h
2
)
(11.2)
The superscripts (m) and (s) refer to the shears measured by the mast and sodar (or
lidar), respectively. The subscripts
h
1
→
h
h
refer to the shears measured
between the bottom height (1) and top height (2) of the anemometers on the mast and
between the top height and the hub height. Though the approach is reasonable, care
must be taken to avoid unrealistic or unreasonable results.
To see how such an adjustment might be applied in practice, consider the following
examples.
h
2
and
h
2
→
1.
Simple Case.
Suppose a 60-m tower and a sodar unit are deployed in a relatively
open and flat project area. Also suppose that the shear exponent from 40 to 60 m
measured at the mast is 0.18 and that the exponent measured by the sodar is
0.20 from 40 to 60 m, decreasing to 0.16 from 60 to 80 m (the presumed hub
height). It would be reasonable to assume in this case that the shear at the mast
also decreases with height. The difference method would produce a reduction
of 0.04 in the exponent at each tower.
2.
Challenging Case.
Suppose a 60-m tower and a sodar unit are deployed in
complex, forested terrain. The mast is near the edge of a steep drop-off, whereas
the sodar is deployed 100 m back from the edge, in a clearing within deep forest.
The shear exponent between 40 and 60 m observed at the mast is 0.25, while
the exponent measured by the sodar is 0.40 between 40 and 60 m and 0.20
between 60 and 80 m. Using the difference method, the inferred shear from 60
to 80 m at the mast would be 0
05. This result seems
unrealistically low, however. It is clear that the sodar is not in a good location
for interpreting the mast profile. In this case, the reasonable course might be to
assume a slight reduction, to perhaps 0.20, in the mast shear to hub height.
.
25
+
(
0
.
20
−
0
.
40
)
=
0
.
As the second example shows, direct measurements of the shear to hub height are
not always easy to interpret, nor do they remove all the uncertainty in extrapolating the
observed anemometer speeds to hub height. As sodar and lidar technology improves
and becomes more widely used, it is likely that the uncertainties in the process will
diminish.
When relying on much less than a year of direct measurements to hub height, it is
important to consider seasonal variations in the wind shear as well. Where possible,
only concurrent data should be compared between towers or between a tower and
remote sensing system. For example, the wind shear measured by sodar over a 1-month
period should be compared to that measured at a tower over the same period. Because
wind shear can vary widely depending on atmospheric conditions, if concurrent data
are not available, it may not be possible to use the remotely sensed data at all. In
general, the most accurate adjustments requires either a full year, or a statistically
representative sample of a full year, of direct measurements from a hub-height tower
or remote sensing system.
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