INSTALLATION OF MONITORING STATIONS - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 68

In addition, it is recommended that each design adhere to the sensor mounting and
exposure specifications presented in Section 5.7. For example, to minimize the effect of
especially wide lattice towers on speed measurements, much longer mounting booms
fabricated from heavier stock may be required. Where possible, anemometry should
be mounted at heights where there is minimal wind flow disturbance caused by the
equipment already mounted on the tower (e.g., dishes, antennas, and lightning masts).
Unlike tilt-up towers, fixed towers must be climbed for the equipment to be
installed, repaired, or replaced. Before climbing is permitted, qualified personnel
should evaluate the structural integrity of the tower, especially the climbing pegs,
ladder, climbing safety cable, and guy wires (if present). Tower climbers must be
properly trained and equipped. Since the work will be performed aloft, the weather
must be given close attention. Strong wind can make it difficult to raise mounting
hardware. In cold, windy weather, the danger of frostbite may be high, and tasks
involving manual dexterity can become very difficult.
Note that adding support booms for anemometers and other instruments can create
wind or ice loads exceeding the tower's design specifications. The implications of
adding equipment to an existing tower should be reviewed by a qualified engineer.
5.7 SENSOR AND EQUIPMENT INSTALLATION
Sensors should be mounted on towers in a way that provides the desired resource
information while minimizing the influence of the tower, mounting hardware, other
equipment, and other sensors on the measurements. This can be achieved by adhering
to the following guidelines, consulting manufacturers' instructions, and referring to
the example installation configurations shown in Figures 5-5 and 5-6.
5.7.1 Anemometers
The number of heights at which the speed is measured depends partly on the height
of the tower. For a 50- or 60-m tower, anemometers are typically installed at three
heights, and taller towers may have four. The following general guidelines govern the
selection of heights and are applicable to most towers.
• One of the heights should be as close as possible to the expected turbine hub
height, consistent with other requirements.
• The topmost anemometers, if mounted on horizontal booms rather than above
the tower, should be at least 10 tower diameters below the top of the tower to
avoid effects of flow over the top (known as 3D flow ).
• The lowest height should be near or below the bottom of the turbine rotor plane
and well above the direct influence of trees, buildings, and other obstacles; 30 m
is typical.
• The heights should be as widely separated as possible to minimize uncertainty in
shear, consistent with other requirements. A height ratio of at least 1.6 between