Characteristics of the Wind - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 529

Anemometry

Anemometry for wind turbines is discussed in Wegley et al. [1980], Ramsdell and
Wetzel [1981], AWEA [1986], and ASME [1989]. Anemometer systems consist of three
major components: sensors, signal conditioners , and recorders. According to ASME
[1989], sensors can be classified into the following categories:

-- momentum transfer , cups, propellers, and pressure plates

-- pressure on stationary sensors: pitot tubes and drag spheres

-- heat transfer , hot wires and hot films

-- Doppler effects: acoustic and laser

-- special methods: ion displacement, vortex shedding, etc.

Cup and propeller anemometers are currently used in conjunction with wind turbines more
often than the other types. Two surveys of wind-measuring instruments that provide
information on principles of operation, specifications, and expected performance are those
by Stone and Bradley [1977] and Moroz and Brousaides [1980].
For accurate results, sensors should be mounted on free-standing, low-blockage masts or
towers at appropriate distances from interfering objects, including the mast itself. It is very
important that sequential wind data be retained. Most data loggers record the peak gust dur-
ing a certain interval and store information about turbulence over the same period.
Anemometers are subject to a variety of errors in the determination of true wind speed,
and equations which may be used to estimate the size of these errors are given in Justus
[1978]. When anemometers are calibrated in steady air flows in a wind tunnel, they may mea-
sure the true wind within ± 1 percent. In gusty winds, however, anemometers as a rule speed
up faster than they slow down, so that accuracies of ± 5 percent may be more realistic. Wind
speed measurements are the principal contributors to uncertainty in performance tests
[Vachon 1985], because of factors such as

-- anemometer instrument errors
-- measuring wind speed at a point rather than over an area
-- separation between the anemometer and the turbine
-- topography and turbulence effects

The number of anemometers required for measuring wind speed during a performance test
of a wind turbine is addressed in ASME [1989]. Duplicate anemometers at a single
elevation are recommended, with guidelines given for determining acceptable anemome-
ter elevations with respect to the swept area of the turbine and the profile of the terrain
between the anemometer and the turbine.
A wind rose is a convenient tool for displaying anemometer data for siting analysis.
A wind rose is defined as any one of a class of diagrams showing the temporal distribution
of wind direction and azimuthal distribution of wind speed at a given location. Figure 8-35
illustrates the most common form, which consists of several equally-spaced concentric
circles with 16 equally-spaced radial lines, each representing a compass point. Line length
is proportional to the frequency of the wind from the compass point, with the circles
forming a scale. The frequency of calm conditions is entered in the center. The longest
lines identify the prevailing wind directions. Average wind speeds are entered at the ends of
the lines. Table 8-10 lists the data contained in the sample wind rose. Wind roses may repre-
sent annual, seasonal, monthly, or time-of-day data.