Environmental Engineering Reference
In-Depth Information
Table 3-1. Typical ranges of wind shear exponents for various types of terrain and
land cover
Approximate range
of annual mean wind
Terrain type
Land cover
shear exponent
Flat or rolling
Low to moderate vegetation
0.12-0.25
Flat or rolling
Patchy woods or forest
0.25-0.40
Complex, valley (sheltered)
Varied
0.25-0.60
Complex, valley (gap or thermal flow)
Varied
0.10-0.20
Complex, ridgeline
Low to moderate vegetation
0.15-0.25
Complex, ridgeline
Forest
0.20-0.35
Offshore, temperate
Water
0.10-0.15
Offshore, tropical
Water
0.07-0.10
Source: AWS Truepower.
the uncertainty in the shear exponent at a particular location is judged to be 0.10
(e.g.,
10), then the uncertainty in the mean speed extrapolated from 30
to 80 m would be 10%. (Note that this is the uncertainty associated with the shear
alone; it does not count the uncertainty associated with the speed measurement or the
instrument height.)
For most publicly available data sets, the wind shear exponent is not known and,
even if published, may not be accurate. Wind shear exponents vary widely depending
on vegetation cover, terrain, the characteristics of the atmosphere, and other fac-
tors. Table 3-1 presents typical ranges of annual mean shear exponent in different
regions and climates. It is important to note that these estimates assume the tower
in question is taller than the surrounding vegetation or obstacles. Where this is not
the case, it may be impossible to extrapolate the measurements with any assurance.
Where there is doubt, a wind resource analyst with experience in the region should be
consulted.
Ideally, data sets should span at least 1 year of measurement to reduce the effect
of seasonal and interannual variations and should provide consistent data for at least
90% of that period. A useful format is a time series of hourly or 10-min wind speed
and wind direction measurements, which can be analyzed for a number of wind
characteristics such as diurnal and seasonal patterns. In some instances, wind data
summaries may be available. Although convenient, such summaries should be used
with caution unless the analyst is familiar with the QC procedures and analytical
methods and is confident they were correctly applied. Otherwise, it is usually best to
perform one's own analysis from the original, raw data.
α =
0
.
20
±
0
.
3.3 FIELD SURVEYS
It is recommended that all candidate wind project sites be visited in person. Three main
goals of the visits are (i) to confirm the assumptions and data used in a GIS-based site
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