As the wind energy industry
expands and larger-scale projects are developed, wind
turbines are becoming larger in rotor diameter as well as
in hub height. Accordingly it is becoming difficult to
accurately assess the wind resource with standard cup
anemometers mounted on standard height (50m or so)
meteorological (met) towers. For this reason, attention is
being directed to other methods of wind speed measurement,
such as SODAR (Sonic
Detection
and
Ranging).
Since the SODAR can measure wind speed as a function of
height the technology is very appealing.
The SODAR is able to measure wind speed by taking advantage
of the Doppler shift phenomenon, which refers to the
apparent change in frequency of an acoustic signal that is
perceived by a fixed observer relative to the moving
source. High frequency (typically 4500 Hz) acoustic signals
are emitted from the SODAR in three directions, one beam in
the vertical and two orthogonal beams tilted approximately
17 degrees from vertical. The acoustic waves are reflected
off of moving, turbulent layers of air in the atmosphere
thereby causing a portion of the signal to return to the
SODAR. The reflected signals are then measured by the SODAR
and an FFT (Fast Fourier Transform) is performed to analyze
the frequency content of the signal. The Doppler-shifted
frequency is calculated at a range of heights (up to 200m)
in each direction and the vector wind speed can then be
calculated.