Wind Speed and Energy Distributions - Wind and Solar Power Systems - page 53

FIGURE 4-15
Wind speeds variation with height over different terrain. Smooth terrain has lower friction,
developing a thin layer above.

TABLE 4-3
Friction Coefficient of Various Terrain

Friction
Coefficient
α Terrain Type

Lake, ocean and smooth hard ground 0.10

Foot high grass on level ground 0.15

Tall crops, hedges, and shrubs 0.20

Wooded country with many trees 0.25

Small town with some trees and shrubs 0.30

City area with tall buildings 0.40

4.6.8 Importance of Reliable Data

Some of the old wind-speed data around the world may have been primarily
collected for meteorological use with rough instruments and relatively poor
exposure to the wind. This is highlighted by the recent wind resource study
of Mexico. 1 Significant differences in the old and the new data have been
found, as listed in Table 4-4 . The 1983 OLADE Atlas of Mexico indicates very
low energy potential, whereas the 1995 NREL data reports several times more
energy potential. The values from the OLADE Atlas are from a few urban
locations where anemometers could be poorly exposed to the prevailing