Page 131 - Fluid Power Engineering

108 Chapter Six

validation of remote sensing equipment increases, it is ex-
pected that, in the near future, data from these equipment
will be accepted by the wind project financing community.
SODAR dataset is usually not as complete as cup anemometer

data. The reason is that data depends on backscatter of small
fluctuations of the thermal refractive index in atmosphere.
SODAR performs poorly when the atmosphere is thermally
well mixed (afternoon), in the presence of precipitation and
in the presence of ambient sound. Therefore, signal-to-noise
filters are crucial; setting the filter too high will cause useful
data to be rejected; setting the filter too low will cause noisy
11
data to be included in the dataset. Operator experience can
be crucial in setting filters for accepting data with smaller
signal-to-noise ratio. Raw SODAR data also undergoes com-
plex data processing algorithms that are proprietary to the
manufacturer.
Wind speeds are measured and averaged over a larger vol-

ume, whereas, cup anemometer measures almost at a point.
SODAR measurements at 100 m typically use 15,000 to 20,000
times the volume of air compared to cup anemometer.
In a complex terrain, the measurements may not correlate well

with met-tower data.
Onsite calibration of SODAR is required. LIDAR usually does

not require onsite calibration.
SODAR created audio signal is a chirping noise, therefore it

must be placed with an appropriate setback from residences
and other inhabited structures. The chirp audio signal is also
susceptible to echoes because of interactions with structures
and trees.

SODAR and LIDAR can yield very useful wind speed and direction
data for prospecting, even with the above limitations.

References
1. International Electrotechnical Commission (IEC). Wind Turbines—Part
12-1: Power Performance Measurements of Electricity Producing Wind
Turbines, 2005. IEC 61400-12-1.
2. Dahlberg, J. A., Pedersen, T. F., and Busche, P. ACCUWIND—Methods for
Classification of Cup Anemometers, Riso National Laboratory, Roskilde, 2006.
Riso-R-1555(EN).
3. Young, M., Met Towers & Sensors—Science & Equipment Considerations, DNV
Global Energy Concepts, AWEA Resource and Project Energy Assessment
Workshop, Minneapolis, MN, 2009.