Wind Resource - Wind Energy: Renewable Energy and the Environment - page 61

is the cost; however, the cost for met towers over 60 m is substantial and the cost for a met tower of
150 m, a height to the top of the rotor for large turbines, is quite expensive. A short-term study [25]
compared the relative accuracy of high-resolution pulsed Doppler LIDAR with a mid-range Doppler
SODAR and direct measurements from a 116 m met tower that had four levels of sonic anemom-
eters. The primary objective was to characterize the turbulent structures associated with the Great
Plains low-level nocturnal jet. The actual measuring volumes associated with each of the three mea-
surement systems vary by several orders of magnitude, and that contributed to the observed levels
of uncertainty. The mean differences were around 0.14 m/s.
There are three general types of instrumentation for wind measurements: (1) instruments used
by national meteorological services, (2) instruments designed specifically for determining the wind
resource, and (3) instruments for high sampling rates in determining gusts, turbulence, and inflow
winds for measuring power curves, stress, fatigue, etc., for wind turbines.
The data collection by meteorological services is the most comprehensive and long term; how-
ever, in much of the world, the data are almost worthless for determining wind power potential. The
reasons are the following: few stations; most locations are in cities and airports, which are generally
less windy areas; sensors are mounted on buildings and control towers; the quantity of data actually
recorded is small (one data point per day, or sometimes monthly averages); and lack of calibration
after installation. As an example of the problem of using meteorological data, the annual mean wind
speed for Brownsville, Texas, is 5.4 m/s, compared to 2.8 m/s for Matamoros, Mexico, which is just
across the Rio Grande River.
There are several types of instruments at varying costs for measuring wind speed: handheld
anemometers, $400; data loggers, $1,500; and data loggers with cell phones, $3,000. Companies
sell instruments that sample at rates of 0.1 to 1 Hz and with the output displayed on analog devices
(meters and recorders) or digital devices (stored on tape or chips). Instruments will record and
analyze time sequence data, as not only wind speeds and direction can be stored for selected time
intervals, but the power can be calculated and selected events such as maximums, gusts, and time
of occurrence are also available. Companies that sell instrumentation specifically for wind mea-
surements also sell digital readers and provide software for analyzing the data. Pole towers are
available specifically for wind measurements from 10 m, $500, to 60 m (with gin pole), $10,000.
Guyed lattice towers can be obtained for higher heights. Pole towers of 50 and 60 m are normally
used for the following reasons: tower can be raised and lowered with gin pole, tall enough to obtain
the higher nighttime wind speeds, and tower is below height (61 m, 200 ft) for required lights per
U.S. Federal Aviation Administration.
In many countries, mechanical anemometers were the norm; however, they require more main-
tenance and more frequent calibration. The power from the cup anemometers drove the strip chart
recorder or a counter. Because of the small number of data points, the Weibull distribution was
widely used to estimate wind power potential. As an example of the problem, wind run data were
collected three times a day from an anemometer at less than 2 m height at a national meteoro-
logical station in Jujuy, Argentina ( Figure 4.8 ) , to determine daily average wind speed. Due to
height and of course blockage of trees and buildings, the wind power potential would be vastly
underestimated.
Data from Mexicali Airport, Mexico [26], provide an example of a trend in wind speed data over
time ( Figure 4.9 ) . The number of observations, 1 h values, was fairly consistent from 1973 to 1999,
when the airport was operating. The downward trend indicates degradation in the anemometer (not
maintained or recalibrated) or less exposure due to increased vegetation or other obstructions. The
wind power changes from 170 W/m 2 at the beginning to 25 W/m 2 at the end, a factor of 7.
4.5.1 C UP AND P ROPELLER A NEMOMETERS
A widely used anemometer for wind resource measurements has a circular magnet (four poles) in
the cup housing, and then one or two coils for pickup of the signal ( Figure 4.10 ) , which approximates