Page 98 - Fluid Power Engineering

76 Chapter Six

mechanism that aligns the normal to the plane of rotation
such that it is parallel to the direction of wind. For most tur-
bines, energy is derived from the wind velocity vector in the
horizontal plane.
Wind velocity in the vertical direction. The vertical component

of wind velocity is caused by convection, topography of land,
or other factors. This component of wind velocity can modify
the flow of wind over blades and degrade the performance of
a turbine.
Wind velocity at a point versus a volume. Because of the stochastic

nature of airflow, wind velocity vector at the particle level has
a significant random component. Therefore, the wind veloc-
ity vector is spatially averaged. A cup anemometer measures
wind speed in a small volume, whereas remote sensing de-
vices measure wind speed over larger volumes.
Wind velocity at a point of time versus over an interval of time.

Again, because of the stochastic nature of wind velocity, there
is a significant component of randomness to wind velocity
at a point in space and at a point in time. Temporal averag-
ing is, therefore, done to report wind speed. For wind energy
applications, wind speed is typically measured every 1–3 s
and 10-min statistics like average, minimum, maximum, and
standard deviation are computed and recorded, and the 1–
3-s sample data is discarded. In some meteorological appli-
cations, wind speed is measured only once an hour and the
quantity that is recorded is the wind speed at the end of the
hour. These single measurements are subject to significantly
large error compared to averaging of, say, 200 measurements
over 10 min.

As various methods of measurement and configurations of instru-
ments are described in sequel, references will be made to the above
facets of wind speed.

Configurations to Measure Wind
Broadly speaking, there are two types of measurements: In situ mea-
surement and remote sensing. In situ measurements are done with a
meteorological tower (met-tower) and remote sensing is done with
SODAR (based on sound waves) or LIDAR (based on light waves).
A met-tower is used to measure wind speed, wind direction, tem-
perature, barometric pressure, relative humidity, and few other atmo-
spheric conditions. Met-towers may be classified as:

Temporary or permanent. A met-tower installed for a period

of 1 to 3 years is considered a temporary structure, while