MONITORING STATION INSTRUMENTATION AND MEASUREMENTS - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 40

sonic anemometers provide a direct measure of the horizontal wind components,
whereas propeller anemometers, like wind turbines, are sensitive only to the
horizontal wind. Some cup anemometers (known as 2D anemometers ) behave
rather like propeller anemometers and have little or no sensitivity to the vertical
component of the wind. Others (known as 3D anemometers ) respond to vertical
winds and thus can produce a misleading estimate of the horizontal speed. Cor-
rections can be made for these anemometers if the vertical wind speed can be
measured or estimated and the anemometer's sensitivity to the inclination angle
is known (1).

Sensor Calibration. The transfer function (slope and offset) for cup and propeller
anemometers either can be a default (or consensus) function previously estab-
lished by testing a large number of sensors of the same model or can be measured
specifically for the sensor that was purchased. In the latter case, the sensor is
said to be calibrated. Either approach may be acceptable depending on the cir-
cumstances. There is evidence that for NRG #40 cup anemometers, in particular,
the accepted consensus function produces results that match anemometers used
for power performance testing more closely than do measured transfer functions
(2, 3). An advantage of using calibrated sensors, whether the measured trans-
fer function is used or not, is that there is greater assurance that “bad” sensors
will be discovered before they are installed in the field. In addition, with cal-
ibrated sensors, it is possible to determine the change in sensor response over
the course of the monitoring period by removing it from the field and testing
it repeatedly. High quality, undamaged anemometers should exhibit very little
change. Calibrated anemometers are sometimes assigned a lower uncertainty than
uncalibrated ones.

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There are many wind-industry-accepted cup anemometers on the market. A list is
provided in Appendix A. Although each of the sensor models meets wind industry
standards, it may be desirable to deploy more than one model on each mast. This
strategy can reduce the risk of data losses or measurement errors caused by problems
affecting just one model. Some anemometers have been classified according to the
standards outlined by institutions such as MEASNET or the International Electrotech-
nical Commission (IEC). 1 The performance of these anemometers complies with the
institution's specifications for high accuracy applications such as power curve test-
ing. However, it is usually unnecessary for every anemometer on a mast to meet this
standard.

4.1.2 Wind Direction
Wind direction measurements are a necessary ingredient for modeling the spatial
distribution of the wind resource across a project area and for optimizing the layout

1 The IEC standard on power performance measurements (IEC 61400-12-1) classifies cup anemometers
based on sensor accuracy. It should be noted that this document requires that turbine performance tests be
carried out with calibrated Class I anemometers and that the measured calibration constants be used.