Environmental Engineering Reference
In-Depth Information
in the vicinity. In general, numerical models of wind flow will predict wind speeds to within
5% for relatively flat terrain and 10% for complex terrain, which means an error in energy of
15-30%. Therefore, a wind measurement program is imperative before a wind farm is installed.
However, if a number of wind farms are already in the region, then 1 year of data collection
might suffice.
For complex terrain, you may need one met station per three to five wind turbines. For wind tur-
bines of 500 kW to megawatts, you may need a met station per one or two wind turbines in complex
terrain. With more homogeneous terrain, as in the Plains, a primary tall met station and one to four
smaller met stations may suffice. The tallest met station should be a representative location on the
wind farm area, not the best point.
Contour maps are used for location of wind turbine pads and for roads. In general, the wind turbines
will be located on the higher elevations within the wind farm area. Topozone has interactive topogra-
phy maps (all different scales) online for the entire United States [19]. These maps are very useful in
selection of met tower locations, micrositing, roads, and other physical aspects of the wind farm.
The key factors for array siting for the Zond wind farms [20] in Tehachapi Pass were an extensive
anemometer data network, the addition of new stations during the planning period, a time frame
of 1 year to refine the array plans, a project team approach to evaluate the merits of different siting
strategies, and the use of initial operating results to refine the rest of the array. A large number of
met stations were needed because the spatial variation of the wind resource over short distances in
complex terrain was greater than expected. The energy output from 2 projects, 98 wind turbines
and 342 wind turbines, was within 3% of the predicted value. This experience shows it is possible
to estimate long-term production from a wind plant with acceptable accuracy for the financial com-
munity. One of the key factors is an extensive network of met towers.
In some older wind plants, the lowest producing wind turbines were relocated (these were small
wind turbines). The money spent on micrositing is a small fraction of the project cost, but the value
of the information gained is critical to accurately estimating the energy production. Many of the
problems with low energy production are because of poor siting.
Wind turbines have become larger, with rotor diameters from 60 to 100 m and hub heights of
60 to 100 m. There are very little data at or above these heights; however, NREL had a program for
tall tower data [21]. The problem is that any tall tower data collected by wind farm developers are
proprietary.
Because of wind shear, wind turbines are located on the higher elevations for rolling terrain, on
mesas, and on ridges in complex terrain. In the past, turbulence was considered a big problem for
siting at the edges of mesas and ridges. However, with the taller towers, wind turbines are placed
on the edges, which are perpendicular to the predominant wind direction. As an example, for wind
turbines on mesas in Texas, the north edge of the mesa would have increased winds from northern
storms in the winter due to the rise in elevation, and then in the summer with southern winds, there
is room for expansion of the wake. Data on turbulence for these sites are proprietary, primarily
because it affects operation and maintenance.
9.3 DIGITAL MAPS
Digital maps are useful as they give a general overview of the wind resource, confidence of the
data, and other data (land use, transmission lines, etc.), which can easily be displayed on the same
maps. NREL has created a higher-resolution digital wind map for the United States and is in the
process of updating the maps by state using terrain enhancement and geographic information sys-
tems (GIS).
A very useful interactive tool, windNavigator, based on GoogleMaps ® , is a wind resource map
and data for the continental United States [22]. The map (2.5 km resolution) provides wind speeds
at 60, 80, and 100 m and a pointer to give minimum and maximum mean annual wind speeds on a
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