Page 219 - Wind Energy Handbook

WIND-TURBINE PERFORMANCE MEASUREMENT 193

consistency and if any of the variables have been found to be misread the sample
should be discarded. Data collected while the anemometer is in the wake of the
wind turbine or in the wake of the anemometer tower, or other obstacle, should also
be discarded.

4.7.2 Wind-speed measurement

Wind speed is the most critical parameter to be measured so considerable emphasis
should be placed on its accuracy. According to the IEA (1982) the anemometer
should have an accuracy of 5 percent or better over the range of relevant wind
speeds and according to the revised IEA recommendation (1990) it should be
accurate to 0:1m=s or less for wind speeds between 4 and 25 m/s. Finally the IEC
have opted to eschew a stated precision, and require instead calibration against a
traceable instrument. The instrument should be calibrated, before and after the test,
so as to establish that its accuracy has been maintained throughout the test
(MEASNET have documented a specified calibration procedure). To avoid pro-
blems, it is advisable to run in a new anemometer for a period of about 2 months
before use, to allow the bearings to ease. Another characteristic of an anemometer is
its distance constant, which the IEC states, should be 5 m or less. The distance
constant is an indication of the response of the anemometer and is defined as the
length of wind run which must pass the anemometer for its output to reach
(1 1=e) ¼ 0:63 of its final value. Large distance constants can give rise to a signifi-
cant over speeding effect because the cup anemometer responds more quickly to
increases in wind speed than decreases, and this is the reason for the 5 m cut off in
the standard. Some believe that, even with a distance constant of 5 m, the instru-
ment should be assessed to evaluate the likely over-speeding error, and a correction
applied if necessary. The IEC allow this as the accuracy can be shown to be
improved.
The wind speed that is measured should be as representative as possible of wind
which would have been present in the plane of the rotor in the absence of the wind
turbine. The desired velocity never exists and so a suitable upstream velocity is
selected instead. The anemometer location is generally chosen so as to minimize
any interference from the wind turbine rotor itself whilst maintaining a reasonable
correlation between the measured wind speed and the output from the wind
turbine. Between 2 and 4 rotor diameters from the wind turbine is stated in the IEC
standard, which recommends 2.5 diameters as the optimum. This compares with
the 2 and 6 diameters recommended by the IEA. If the anemometer were placed
significantly nearer than 2 diameters, correction for the velocity deficit caused by
the rotor would have to be made.
Although corrections can in theory be made to take account of wind shear for
anemometers not at hub height, this is discouraged and whenever possible the
wind speed should be measured at the same height (relative to ground level) as
the hub of the wind turbine rotor. The IEC specify a height within 2.5 percent of the
turbine hub height.
As already mentioned, the anemometer must not be located in the wake of the
turbine, or other significant obstacles on the site, including of course other wind