Environmental Engineering Reference
In-Depth Information
Of course, all booms should be as level as possible for an accurate horizontal speed
reading. Equally important, they should be long enough so that the influence of the
tower on the measured speeds (aside from when the anemometer is directly shadowed
by the tower) is small. A boom that is too short can result in significant errors in wind
speed measurements (for example, a slowdown when the anemometer is upwind of the
tower or a speedup when it is off to the side). According to specifications published
by the IEC for wind turbine power performance testing,
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anemometers should be
separated by at least 7 tower diameters from the tower. For example, if the tower
diameter is 203 mm (8 in), the minimum recommended distance is 1.4 m (4.7 ft). For
lattice towers of relatively low porosity, the distance can be reduced to perhaps 3.75
tower widths (for triangular lattice towers, the tower width is the width of one face).
Anemometers should be mounted well above the booms to minimize their influ-
ence on the measured speed. According to the same IEC specifications, the minimum
vertical separation between the boom and anemometer is 7 boom diameters, although
a much larger separation (12 or 15 diameters) is the norm (for rectangular stock, the
diameter is the height of the vertical face).
At two or more heights, and usually for the top two heights, the anemometers
should be deployed in pairs on separate booms. This redundancy reduces data losses
caused by sensor failures and tower shadow. A typical configuration on tubular towers
is for the boom pairs to be oriented 90
◦
from one another and 45
◦
on either side of, and
facing into, the prevailing wind direction. On triangular lattice towers, the booms are
usually mounted on two tower faces 120
◦
apart and 60
◦
on either side of the prevailing
wind direction. However, it may be advisable to depart from these guidelines if there
are strong secondary wind directions. For example, if the wind commonly comes from
both the east and west, it may be best to mount the anemometers toward the north and
south, 180
◦
apart, if possible. The charts in Figure 5-7, which show typical patterns
of wind flow disturbance around tubular and triangular lattice towers, may be used
for guidance in boom placement.
Mounting an anemometer on a vertical boom at least 7 tower diameters above the
tower top is a good configuration for obtaining accurate speed measurements largely
unaffected by the tower in all directions. A “goal post” configuration with two such
anemometers providing redundancy is also in common use, especially for wind tur-
bine power performance testing. Readings from such vertically mounted anemometers
should not be combined with readings from horizontally mounted anemometers to esti-
mate shear, for the familiar reason that the differing effects of the tower on the wind
flow could cause significant errors. For this reason, a vertical top-mounted anemome-
ter does not eliminate the need for a pair of horizontal side-mounted anemometers
near the top of the tower.
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The authors thank the IEC for permission to reproduce information from its International Publication
IEC 61 400-12-1 ed.1.0 (2005). All such extracts are copyright of IEC, Geneva, Switzerland. All rights
reserved. Further information on the IEC is available from www.iec.ch. The IEC has no responsibility for
the placement and context in which the extracts and contents are reproduced by the author, nor is IEC in
any way responsible for the other content or accuracy therein. IEC 61 400-12-1 ed.1.0 Copyright © 2005
IEC Geneva, Switzerland. www.iec.ch.
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