INSTALLATION OF MONITORING STATIONS - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 71

the NRG Systems 60-m TallTower, the measurement heights might typically be as
follows:

•

57.2 m. Sufficiently far below the top of the tower to avoid 3D flow effects; the
tower is 203 mm (8 in) wide at this point, so this satisfies the 10-tower-width
condition.

47.4 m. Just above the second highest guy ring. Placing the anemometers above
the guy ring assures that the wind speed measurements are not affected by the
guy wires.

•

32.0 m. Just above the guy ring at the joint where the tower diameter changes
from 254 (10 in) to 203 mm (8 in). This ensures the tower diameter is
the same as for the anemometers mounted above, which reduces errors in
wind shear.

•

Lattice Towers. Lattice tower designs vary widely. In addition to the height,
important characteristics to know include the face width, compass orientations of the
three or four faces (which determine the possible boom directions), heights of guy
rings, the diameter of the lattice tubing (or rod), and for preexisting towers, the heights
and approximate sizes of other instruments already mounted on the tower. Although it
is impossible to define suitable anemometer heights for every situation, typical heights
for new, dedicated lattice towers are listed below.

80 m. This height represents the approximate hub height of typical utility-scale
wind turbines. The tower top should be at least 10 face widths above this height
so that wind flow over the top of the tower does not affect the speed readings.

•

•

60 m. This intermediate measurement level is included to provide redundancy
and to help define the shear profile. The height chosen should be such that
measurements are not affected by guy wires. The effect of guy wires can persist
for surprisingly long distances—as far as 40 wire diameters (about 0.25 m for a
6-mm wire) downstream.

40 m. This is approximately the minimum height reached by the blade tip on a
large wind turbine.

•

Mounting. Most anemometers are mounted on booms extending horizontally
from the side of the tower. The booms should be made of hollow tubing rather
than solid stock, and care should be taken that sensor drainage holes are not
blocked.
To produce the most accurate estimate of wind shear, it is strongly recommended
that the booms at different heights point in the same direction so that the influence of
the tower on the speed readings at each height will be similar. For the same reason, the
topmost anemometers should be at least 10 tower widths below the top of the tower;
placing them too close to the top can result in a significant error in the estimated wind
shear.