Environmental Engineering Reference
In-Depth Information
Table 11-1.
Equivalence of
z
0
and
α
for neutrally buoyant conditions and
heights of 40 and 60 m
α
z
0
0.08
0.0002
0.14
0.039
0.2
0.33
0.35
2.8
Overall, the differences between the power law and logarithmic approaches are
small, and which one is used is largely a matter of the resource analyst's preference.
For the remainder of this chapter, only the power law method is discussed.
11.2 TIME-VARYING SPEEDS AND SPEED DISTRIBUTIONS
While the time-averaged shear exponent is a convenient parameter for characterizing
the wind resource at a site, it is not appropriate for scaling a time series of wind
speed data or a speed frequency distribution. The reason is that it overlooks the wide
variation in shear, especially its dependence on direction, time of day, and time of year.
The plot in Figure 11-3 illustrates a common pattern of variation of shear with time of
day, showing a minimum during daytime hours when the atmospheric boundary layer
is well mixed and a maximum at night under thermally stable conditions. Relying on
an average shear can introduce errors in the speed distribution at hub height, with an
impact on energy production estimates.
One approach to this problem is to calculate a shear exponent for each time
interval (e.g., 10 min) and to use that exponent to extrapolate the top anemometer
speed to hub height. A potential drawback of this “instantaneous shear” method is
that extreme (albeit valid) shear values can occasionally occur in the record, which
may not persist to hub height. These extreme values can produce unrealistically high
or low hub-height speeds, but they are generally rare. A more common problem is
that shear values are available only for those records for which both the upper and
lower sensors have valid data. The method should not be applied to substituted data.
Whether this is a serious problem depends on the amount of data substitution that has
occurred.
As an alternative, many analysts choose to bin the speeds by direction, time of
day, or time of year, or a combination of these, and to calculate the mean speed
and average shear for each bin. The 10-min wind speeds can then be extrapolated to
hub height using the shear for the appropriate bin for each record. This resolves the
problem of extrapolating substituted data, while preserving at least part of the full
variation of shear. Care must be taken to handle bins with few or no points in an
appropriate way, such as averaging speeds and shears from adjacent bins. When using
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