Wind resource and site assessment - Wind Power Generation and Wind Turbine Design - page 59

Figure 4: Example top mounted anemometers [ 2 ].
such strict calibration procedures are only in place for cup anemometers. Highest
quality calibrations are ensured when calibrating in wind tunnels that have been
accredited by MEASNET. MEASNET members participate regularly in a round robin
test to guarantee interchangeability of the results, which has increased the quality of
the calibration signifi cantly. It should be kept in mind that even calibrations according
to highest standard bear an uncertainty of around 1-2%.
Currently there are no calibration standards available for sonics and remote
sensing devices.
2.4 Mounting
Accurate wind speed measurements are only possible with appropriate mounting
of the cup anemometry on the meteorological mast. In particular, the anemometer
shall be located such that the fl ow distortion due to the mast and the side booms is
minimised. The least fl ow distortion is found by mounting the anemometer on top of
the mast at a suffi cient distance to the structure. Other instruments, aviation lighting,
and the lightning protection should be mounted in such a way that interference with
the anemometer is avoided. Figure 4 shows a possible top mounting arrangement.
Boom-mounted anemometers are infl uenced by fl ow distortion of both the mast
and the boom. Flow distortion due to the mounting boom should be kept below
0.5% and fl ow distortion due to the mast should be kept below 1%. If the anemom-
eter is mounted on a tubular side boom, this can be achieved by mounting the
anemometer 15 times the boom diameter above the boom. The level of fl ow distor-
tion due the mast depends on the type of mast and the direction the anemometer is
facing with respect to the mast geometry and the main wind direction.