Civil Engineering Reference
In-Depth Information
FIGURE 4-6
Optical wind speed sensor construction. (Source: Georgia Institute of Technology, Atlanta, GA.
With permission.)
Each of the two tiny detectors monitors a spot on the target inside the laser
beam and picks up shadowy waves or fringes moving across the laser beam.
The waves are
visible on the target itself. The fringes look much like the
shadows of waves seen at the bottom of a swimming pool on a sunny day.
Something akin to these shadows can be seen if a turbulent wind is viewed
with a laser beam. Each detector registers the moment when a dark fringe
passes its view. By digitizing the detected points, a computer can measure
time and separation, and, therefore, the average wind speed.
The major advantage
of the optical sensor is that it can measure the wind
speed over a wide range from a faint wind to a wild wind over a large area.
4.6
Wind Speed Distribution
Having the cubic relation with the power, the wind speed is the most critical
data needed to appraise the power potential of a candidate site. The wind
is never steady at any site. It is influenced by the weather system, the local
land terrain, and the height above the ground surface. The wind speed varies
by the minute, hour, day, season, and year. Therefore, the annual mean speed
needs to be averaged over 10 or more years. Such a long term average raises
the confidence in assessing the energy-capture potential of a site. However,
long-term measurements are expensive, and most projects cannot wait that
long. In such situations, the short term, say one year, data is compared with
a nearby site having a long term data to predict the long term annual wind
speed at the site under consideration. This is known as the “measure, cor-
relate and predict (mcp)” technique.
