SITING A WIND PROJECT - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 30

towers off the ridgeline as well. Careful attention to tower placement can substantially
reduce the wind flow modeling uncertainty and consequently the uncertainty in the
predicted plant energy production (Chapter 13).
Siting a tower near significant obstructions such as buildings, rock outcroppings,
or isolated stands of trees can adversely affect the analysis of the site's wind
characteristics (unless the proposed turbines would experience similar obstructions).
Figure 3-2 illustrates the effects of an obstruction, which include reduced wind speed
and increased turbulence. The zone of increased turbulence can extend up to 2 times
the obstacle height in the upwind direction, 10-20 times the obstacle height in the
downwind direction, and 2-3 times the obstacle height in the vertical direction. As
a guideline, if sensors must be placed near an obstruction, they should be located at
a horizontal distance of no less than 20 times the height of the obstruction in the
prevailing wind direction.
When placing wind monitoring systems near or within forests or extensive tree
stands, it is important to consider whether the vegetation is typical of where turbines
are likely to be located. If so, then so long as the necessary clearances (e.g., for
mast installation or sodar operation) are respected, there is no reason to avoid them.
However, the lowest speed sensors on the tower should be placed well above the
tree canopy to ensure an accurate measurement of wind shear. Tower instrumentation
guidelines are provided in Chapter 4.
Although highly recommended for large projects and in areas of substantial shear
uncertainty, the use of very tall towers (i.e., hub height and greater) can create chal-
lenges for tower placement. They often require a larger cleared area to erect than
shorter towers. In forested areas, clearing requirements may limit either the tower
height or the locations where towers can be placed; the same goes for steep terrain.
Taller towers are also more exposed to severe weather than shorter towers and must
be designed to withstand expected occurrences of icing, high winds, lightning, and
Figure 3-2. Obstruction effects on airflow. Source : AWS Truepower.