Siting - Wind Energy: Renewable Energy and the Environment - page 183

E exposure, m
z o roughness length; crop land 0.03 m, crop land/mixed woodland 0.10.3 m, forest 0.81.0 m
Care must be taken on use of P a . Do you use the bottom or the middle of the wind class? Do you
limit the number of wind class changes to one, especially for mountainous terrain?

9.5 WIND RESOURCE SCREENING
As an example, wind resource screening for the Texas Panhandle is presented [25, 26]. The DEM
data (3 arc seconds resolution) along with DLG data were used. The original DEM data were
in blocks of 1° by 1°. Data for utility transmission lines (69 kW and higher) were input by hand.
Two GIS systems, IDRISI and PC ARC INFO, for personal computers were used. IDRISI has
built-in functions that enhance its use for wind resource screening: slope, hill shading, aspect,
and orthographic projection. A data sheet accompanies these functions, which shows bin size,
max, min, etc.
The Panhandle of Texas is part of the Southern High Plains, with rolling hills in the East and
above the Caprock, flat plains. The elevation rises from 450 m in the Southeast to 1,460 m in the
Northwest. The Canadian River goes from west to east across the Panhandle. The other notable
feature is Palo Duro Canyon. The graphs can be viewed in color or gray scale, with a number select-
able up to 256. At 256 colors, a DEM map for all of the Texas Panhandle would display contours
4 m apart. The base map ( Figure 9.7 ) is the DEM data for the Panhandle. Most of the images were
created using sixteen values. The elevation data of the base map can be analyzed by the different
commands in IDRISI. Instead of the whole area, subsets of the data can be analyzed in the same
manner for more detail. The limitations on resolution are the cell size of the original data.
The Panhandle has a large wind energy potential since it has class 3 and 4 winds over the whole
area. On the flat open plains, which describe much of the Panhandle, close to 100% of the area will be
in the same wind power class. In this region, wind speed increases with height; therefore, modest relief
may increase the wind power dramatically. Terrain exposure selects those areas, which are above and
below the average elevation. A 15 km radius was used to determine an average elevation, then the maxi-
mum change from this average was 190 m ( Figure 9.8 ). An orthographic projection with the overlay of
terrain elevation shows more clearly the areas of higher elevation. On the basis of terrain exposure, a
revised wind map was calculated. Some of the regions with positive exposure have been changed to a
higher wind class by this process, and low areas have been changed to a lower wind class.
GIS was used to screen the wind resource in terms of the following criteria: wind power class,
terrain type, vicinity of transmission line, slope, and aspect. Within the criteria, classes or levels
can be selected to exclude or limit the area for wind plants. A map was generated for each of the
following screening parameters:

r Wind class 3 and above

r Slope of 0-3°

r Aspect from 155° to 245° for area where slope is greater than 1°

r Multiples of 8 km from transmission line (69 kV and above)

r Excluded lands: parks, roads, urban, lakes, wildlife refugees

Then the maps are combined to show a map of the possible areas for wind farms by wind class. Within
8 km of transmission lines, the total area was 28,600 km 2 , around 37% of the land in the Panhandle.

9.5.1 E STIMATED W IND P OWER FOR T EXAS ,P ACIFIC N ORTHWEST L ABS
Pacific Northwest Labs (PNL) estimated the capturable wind power for Texas at 50 m height as
134,000 MW from class 3 and above winds, with 28,000 MW for class 4 winds. Class 4 winds are