Siting - Wind Energy: Renewable Energy and the Environment - page 185

located primarily in the Panhandle. The PNL estimate was made in the following manner. The total
power intercepted over a given land area is a function of the number of wind turbines, the rotor
swept area, and the available power in the wind. Environmentally sensitive land, urban areas, and
terrain that is in valleys and canyons were excluded. The following formula is used to calculate the
power intercepted by the rotor area of the wind turbines:

P i P a A t N (9.2)

where P a average wind power potential, W/m 2 ; A t rotor area, . D 2 /4; D rotor diameter, m ; and
N number of wind turbines.
The number of turbines that can be placed on the land area is

A
SS

i

N (9.3)

rc

where A i land area; S r spacing between turbine rows, D; and S c spacing within turbine row,
D m 2 . Note that S r S c is the land area devoted to one turbine. In general, wind plants only remove
3-10% of the land, primarily for roads, from other productive uses. At some wind farms the roads
are only 5 m wide, while at another wind farm with 3 MW wind turbines, the roads are over 10 m
wide.
If the cost of land is high, then the land area for one wind turbine is smaller; however, the out-
put from the wind plant will be reduced due to array effects. In California, some wind plants have
turbine spacing of 2D within the rows and 5D to 7D to the next row. As a general rule, in the Plains
area, 5-12 MW can be installed per square kilometer (spacing of 4D by 8D), and for the edge of
bluffs and on ridges, 6-15 MW can be installed per linear kilometer (spacing of 2D to 3D, one row
only). With closer array spacing the MW/km 2 would be larger; however, the array losses would also
be larger.
The average intercepted power can be calculated from Equation 9.2, or the intercepted power per
unit land area can be calculated from

P
A

.
4

P
SS

i a

(9.4)

t rc

Remember, this is the intercepted power, and capacity factors of 0.30 to 0.35 are used to estimate
the capturable wind power.

9.5.2 E STIMATED W IND P OWER FOR T EXAS ,A LTERNATIVE E NERGY I NSTITUTE
The same procedures of terrain enhancement and GIS were used to estimate the capturable wind
power for Texas [27]. The selection criteria were wind class 3 or higher from revised wind map using
terrain exposure, slope of 0-3°, excluded lands (urban, highways, federal and state parks, lakes,
wildlife refuges, and federal wetlands), and within 15 km of transmission lines (115 kV and above).
The capturable annual power was calculated for the following conditions for the wind turbines:
50 m hub height, 10D by 10D spacing, 30% capacity factor, and no array losses (reasonable since the
spacing is large). With these assumptions, the estimated annual capturable wind power was 157,000
MW (525,000 MW of wind turbines at 30% efficiency) with an annual energy production of 1,300
TWh. These results are somewhat larger than the estimates determined by PNL.
The estimates were further revised with data (at 40 and 50 m) from Alternative Energy Institute (AEI)
met sites and private sites [28], which were then used to update the wind map (1 km pixel size) for Texas
( Figure 9.9 ) . The amount of class 3 and 5 lands was reduced from the previous estimate, while class 4