Siting - Wind Energy: Renewable Energy and the Environment - page 175

9 Siting
The crucial factor is the annual energy production from the wind turbine or wind farm (also called
wind park or wind plant), and how the value of that energy compares to other sources of energy.
Much of the data from meteorological stations in the world are of little use in predicting wind power
potential and expected energy production from wind turbines.
9.1 SMALL WIND TURBINES
For small wind turbines, a measuring program may cost more than the wind turbine; therefore, other
types of information are needed. As wind maps are developed for potential wind farms by countries,
these maps can be used as guides to determine regions with enough wind for small wind turbines.
Also, wind maps for countries and large regions obtained from numerical models have sufficient
resolution for siting of small wind turbines. Since small wind turbines will be located close to the load,
local topography will influence the decision on estimating wind speeds and siting. If the location is
on exposed terrain, hills, or ridges, then the wind speeds would be higher than those in the valley. In
complex terrain, some sites will be adequate for small wind turbines and other sites will be sheltered.
One of the factors in the settlement of the Great Plains of the United States was the farm windmill,
which provided water for people and livestock. Therefore, if farm windmills are used or were used in
the past in a region, then there is enough wind for small wind turbines in that region. Another possi-
bility is to install met towers for reference data for a region. Generally, this would be done by regional
or state institutions or governments, not by individuals interested in siting of small wind turbines.
Small wind turbines can be cost-effective for stand-alone systems using the general rule that the
average wind speed for the lowest wind month should be 3 to 4 m/s. Also, general maps of wind
power or wind energy potential for small wind turbines have been developed for large regions
(Figure 9.1) [1]. These gross wind maps will be supplanted by national wind maps developed for
determining wind energy potential for wind farms. Finally, if there are wind farms in the area, there
is definitely enough wind for small wind turbines.
It is obvious that a small wind turbine should be located above (10 m if possible) obstructions
and away from buildings and trees [2]. Towers for small wind turbines should be a minimum of 10
m and preferably 20 m, as higher towers generally capture more energy ( Figure 9.2 ) . Again, the
trade-off is the extra energy versus the cost of a taller tower. Even towers of 35 m are sometimes
used. As a general rule for avoiding most of the adverse effects of building wakes, the turbine
should be located (1) upwind a distance of more than two times the height of the building, (2)
downwind a minimum distance of ten times the building height, or (3) at least twice the building
height aboveground if the turbine is immediately downwind of the building. The above rule is not
foolproof because the size of the wake also depends upon the building's shape and orientation
to the wind ( Figure 9.3 ) . Downwind from the building, power losses become small at a distance
equal to fifteen times the building height. However, a small wind turbine cannot be located too far
away from the load, as the cost of wiring will become prohibitive. Also, there will be more losses
in the wires if you have DC rather than AC from the wind turbine to the load. In general, small
wind turbines should not be mounted on occupied buildings because of possible problems of noise,
vibration, and even turbulence. For the very small wind turbines, tower heights vary from stub
poles on sailboats to short, 3 to 5 m towers, and some are even mounted on buildings. Paul Gipe
has written numerous articles on all aspects of wind energy [3], and two of his topics are for small
wind systems [4, 5].
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