Page 108 - Fluid Power Engineering

86 Chapter Six

FIGURE 6-9 Comparison of performance of a Theis First Class Advanced for
different inflow angles and different wind speed from a calibration service. cos
is the theoretical cosine response. (From WindGuard, Deutsche. Summary of
Cup Anemometer Classification [Online] 12 2008. http://www.skypowerinter
national.com/pdf/News%20and%20Media/new/First Class Advanced
Testing Results. pdf.)
measurements provide a better estimate. Systematic error or bias, on
the other hand, is a repeatable flaw and repeated measurements do
not reduce the systematic error. Calibration error is an example of
systematic error.
The most common uncertainties in measurement of wind speed
and estimation of related quantities (wind speed at hub height) are: 5
1. Sensor calibration uncertainty. This refers to uncertainty in
the calibration process and differences between the test
anemometer and production anemometer. Depending on
anemometer, this uncertainty can be 0.1–2%
2. Dynamic overspeeding. Cup anemometers are susceptible to
overspeeding in the presence of turbulence. Overspeeding is
a phenomenon by which the anemometer speeds up more
rapidly when faced with higher wind speed; it does not slow
down as rapidly when faced with lower wind speed. The
uncertainty has been determined to be about 0.3%.
3. Tower shadow, boom, and mounting effects. As explained in the
section on Placement of Sensors above, tower shadow causes
a negative bias; an estimate of the bias is −1.5%. Longer booms
can reduce this bias; however, the booms themselves disturb
the airflow. Long booms can also cause the anemometer to
deviate from vertical position resulting in measurement error;
an estimate of uncertainty is 0.5%.
4. Wind shear can be a large component of uncertainty in pre-
diction of wind speed at hub height. There are several com-
ponents to this uncertainty: Anemometer quality, measure-