Page 103 - Fluid Power Engineering

W i nd Measurement 81

anemometers are higher cost and, therefore, are not routinely used
for prospecting. The uncertainty associated with the anemometers is
one of the components in the computation of the overall uncertainty of
the wind resources and eventually the uncertainty of the wind project
itself. Higher uncertainty leads to lower valuation of the project. (See
section “Uncertainty in Wind Speed Measurement with Anemome-
ters” in this chapter and Chapter 7 for a more detailed exposition of
uncertainty.)

Calibration of Anemometers
A cup anemometer measures wind speed by using the following pro-
cess: Rotation of cups is proportional to wind speed, angular speed of
the rotating cups is proportional to the frequency of the AC current
that is produced, and the frequency of the AC current is measured and
converted to wind speed using a transfer function. As an example, the
mean consensus transfer function for a NRG #40 is: 4

Wind speed in m/s = (Hz.0.765) + 0.35
(6-2)
Wind speed in miles per hour = (Hz.1.711) + 0.78

where Hz is the measured frequency of the AC current. In the above
equation, the slope and intercept are the two parameters of the transfer
function, with values of 0.765 and 0.35. In order to reduce the impact
of variability between anemometers, most manufacturers calibrate in-
dividual anemometers and provide instrument-specific transfer func-
tion parameters.
For wind resource assessments, only calibrated anemometers
should be used. It removes any bias introduced because of the manu-
facturing process. There are several agencies that calibrate anemome-
ters, the most prominent is MEASNET (www.measnet.org), which is
a collection of international institutes that calibrate anemometers.

Wind Vane
Wind vane measures the direction of wind in the horizontal plane.
The output of the sensor is an analog voltage that is proportional to
the direction. Depending on the type of wind vane, there is a “north”
marking on the vane. During installation, the north marking must be
in the true north direction. The difference between magnetic and true
north must be considered when aligning the vane. In most cases, this
compensation can be set in the data logger, if the vane’s north marker
is not pointing toward true north.
Wind vanes have a dead band around the zero degree direction.
For instance, if the dead band is 8 , then measurements between 356
◦
(−4 ) and 4 are not accurate. There are two methods to deal with this:
◦
◦
(i) Dead band compensation, which converts all readings in the range