Environmental Engineering Reference
In-Depth Information
of 1.5), an error of 1.5% in the speed ratio—a reasonable deviation under field
conditions—results in an error of 0.037 in the shear exponent. This, in turn, produces
an error of 1.1% in the predicted speed at 80 m. For heights of 50 and 60 m (height
ratio 1.2), the same relative speed error produces an error of 0.082 in the exponent
and 2.4% in the speed at 80 m.
Given the sensitivity of the calculated shear to small speed errors, three rules
should be followed to produce a reasonably accurate shear estimate: First, the speed
ratio should only be calculated using concurrent, valid speed records at both heights.
This avoids errors caused by mixing data from different periods or with different
rates of DR. Second, the two heights in the shear calculation should be separated by
a ratio of at least 1.5 (e.g., 33 and 50 m or 40 and 60 m). Third, the speed data
should originate from anemometers mounted on horizontal booms of the same length
and with the same directional orientation relative to the tower, so that the effects of
the tower on the speed observations will be similar. One implication of this last rule
is that, in general, data that have been substituted from other sensors should not be
used in shear calculations. Instead, only data originally collected from two identically
oriented anemometers are appropriate for this purpose.
Just one average shear value for each pair of heights is usually provided in wind
resource reports. This shear is calculated as noted above, by averaging the concurrent
speeds from each anemometer, then taking the ratio and calculating the exponent.
Some analysts choose to exclude speeds below 3 or 4 m/s in this calculation, as
shear tends to be more variable in light winds, and low speeds do not contribute
significantly to energy production. In the following chapter, the use of instantaneous or
binned shear exponents to extrapolate a time series of wind speed data to hub height,
along with possible adjustments to the shear above the top anemometer height, is
discussed.
10.1.4 Turbulence Intensity
Wind turbulence, defined as rapid fluctuations in wind speed and direction, can have
a significant impact on turbine performance and loading. The most common indicator
of turbulence is the standard deviation (
) of the wind speed calculated from 1- or
2-s samples over a 10-min recording interval. Dividing this value by the mean wind
speed for the same interval gives the turbulence intensity (TI):
σ
=
v
TI
(10.8)
where
σ
=
the standard deviation of wind speed for the recording interval;
v
=
the mean wind speed for the recording interval.
The mean TI generally decreases with increasing wind speed up to about 7-10 m/s,
above which it is relatively constant. TI values above 10 m/s typically range from less
than 0.10 in relatively flat terrain with few trees or other obstacles to more than 0.25
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