Environmental Engineering Reference
In-Depth Information
9.4.2 Data Averaging
When anemometers are mounted in pairs at each height on the tower, the question
arises, should both sets of measurements be used in characterizing the wind resource,
and if so, how should they be combined?
A popular approach is to designate one of each pair of anemometers as the primary
sensor and the other as the secondary sensor. The primary anemometer's data are
used exclusively for the analysis except when they have been flagged as suspect or
invalid. In those periods, the flagged data are replaced by valid data from the secondary
sensor in the manner described in the previous section. (If no valid redundant data
are available, a gap is left in the data record.)
The underlying assumption of this approach is that the primary sensor is the more
accurate of the two. That may be a reasonable assumption in some cases—for example,
when the secondary sensor is a heated cup anemometer (heated cup anemometers being
generally less accurate than unheated cup anemometers, except, of course, in freezing
conditions); when the primary sensor is of superior quality; or when the secondary
sensor is in the tower shadow far more often than the primary sensor.
Very often, however, there is no reason to expect either sensor to be more accurate
than the other most of the time, so the choice of primary sensor is arbitrary. The
preferred method is then to average the data from the two anemometers. Assuming
the measurement errors of the two sensors are uncorrelated and of roughly the same
m ag nitude, this method reduces the uncertainty in the observed speed by a factor of
2, or 1.414, compared to relying on the data from one sensor alone. Averaging can
also help mitigate secondary tower effects.
Averaging can be used only when the data from both sensors are valid; whenever
one is shadowed or experiences some other problem, only the other's data should be
counted. In those periods, the uncertainty reverts back to the uncertainty of the solitary
sensor. (Uncertainty in resource assessment is covered in Chapter 15.)
9.5 QUESTIONS FOR DISCUSSION
1. Why is it sensible for an automated screening routine to produce more false
positives than false negatives?
2. Why is boom orientation accuracy important? Name two possible impacts on
the accuracy of energy production estimates if the assumed boom orientation
is wrong. What are some ways one can independently verify the anemometer
boom orientations given in the site documentation?
3. Describe a scenario in which a meteorological mast could artificially increase
the average wind speed recorded by an anemometer. In what scenario might it
be decreased? (Ignore tower shadow.)
4. Suppose the wind speed values for a given period of time equal the offset in the
conversion equation and the standard deviation is zero. What might be causing
this phenomenon, and what are some ways to determine the cause?
5. Why do you think wind vanes tend to freeze before anemometers?
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