DATA VALIDATION - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 123

By correcting for these tower influences, a more accurate free-stream speed reading
can be obtained for an individual sensor. Currently, however, there are no commercial
tools available for doing this, so custom tools must be developed by the resource
analyst using information available in the literature (3). As an alternative, averaging
valid data from two sensors at the same height and oriented at the recommended
angular distance apart (depending on tower type) usually mitigates and can virtually
eliminate tower effects in the combined data record. Data averaging is discussed in
more detail in Section 9.4.2.
9.3.2 Turbulence
Cup anemometers are known to overestimate the wind speed in turbulent flow condi-
tions because of the tendency of the anemometers to respond more quickly to abrupt
increases in speed than to rapid slowdowns. The magnitude of the overspeeding
depends on the sensor type and degree of turbulence.
Research has shown that anemometers with a relatively large distance constant,
such as the NRG #40 and Second Wind C3, record greater wind speeds in turbulent
conditions than do IEC Class I anemometers used for turbine power performance
testing and certification. When turbulence is low, the opposite tendency can occur. By
applying a sensor-specific adjustment to account for these tendencies, a more accurate
energy production estimate can be obtained (4).
In contrast to cup anemometers, prop-vane anemometers tend to underestimate the
wind speed as a result of turbulence. This is because the wind direction changes so
quickly that the vane cannot keep the propeller aligned perfectly into the wind. A
propeller anemometer only measures the component of the wind speed that is parallel
to its rotation axis; the observed speed is reduced by a factor equal to the cosine of
the angle of deviation. Since greater turbulence produces larger direction shifts, the
magnitude of prop-vane underspeeding typically increases with increasing turbulence
intensity (5).
Sonic anemometers, lacking moving parts, are insensitive to turbulence. To bring
their measurements in line with those of Class I sensors, however, an adjustment for
turbulence may nonetheless be called for, although it is usually small.
9.3.3 Inclined Flow
Horizontal axis wind turbines generate power from the component of the wind that is
perpendicular to the turbine rotor's plane of rotation. To support an accurate energy
production estimate, anemometers should ideally measure only that component. How-
ever, cup anemometers, in particular, are sensitive to varying degrees of off-horizontal
winds depending on the geometry of the cups and instrument. Research has docu-
mented the impact of flow angle on wind speeds recorded by cup anemometers of
various types (6), but making use of this information requires knowledge of the flow
angle at the tower. This can be obtained from a sodar, a lidar, or a vertical anemome-
ter mounted on the mast. Without such a direct measurement, the flow angle can be
estimated from the terrain slope and from wind flow modeling. Inclined flow can also