Characteristics of the Wind - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 476

(commonly referred to as the doldrums ) moves north and south within the tropics, following
the annual march of the sun. The trades or monsoons on either side of the ITCZ are also
affected. Therefore, large seasonal time variations of the wind regime occur at most
latitudes in the tropics, and they are most pronounced over continents. During a year in the
tropics, it is reasonable to expect three or four months of low-to-mediocre winds and eight
or nine months of good-to-excellent wind energy conditions.
Skill in reliable forecasting of wind variability on the time scale of a day or two has
been shown to be valuable in the operation of a wind power station [Goldenblatt et al.
1982]. However, there is little evidence that the appropriate level of reliability in wind
forecasting is available through public or private organizations.
Diurnal Variations
In both tropical and temperate latitudes, large wind variations can occur on the diurnal
(daily) time scale. In the tropics, these variations are most pronounced over land areas and
during dry seasons, when the humidity content of the air is very low and the skies are often
cloudless or almost so. Variations in radiation flux during the day enhance momentum
transfer in the vertical direction during daytime and inhibit such transfer during night. As
a result, wind speeds are a maximum during the afternoon and a minimum during the early
morning. In extreme cases this diurnal range of wind speed may approach 10 m/s. Such
wind variations could not be detected from monthly and seasonal averages, but require
sampling with time resolutions of one hour or less.
Daily variations in solar radiation are also responsible for diurnal wind variations in
temperate latitudes over relatively flat land areas. The largest diurnal changes generally
occur in spring and summer, and the smallest occur in winter. Low-level jets may be
induced at night in some parts of the Great Plains of the United States [Mahrt and Heald
1979]. In some cases they extend downward to low enough elevations (30 m) to benefit
large-scale HAWTs, but they disappear during the day.
Finally, wind speed and direction variations with periods on the order of a few minutes
are important to turbines with active pitch and/or yaw controls. Wind variability on this
time scale is evidence of turbulence in the flow that may have been generated by upstream
topographic features, surface roughness elements, or thermal stratification. Modeling studies
have shown that operational strategies involving wind measurements and the logic of turbine
start-up, shut-down, and yaw control actions can play an important role in optimizing
performance and reducing control-generated fatigue loads.
Wind Resources in the United States and Around the World
U.S. Wind Resources
The National Renewable Energy Laboratory (NREL) maintains a current wind energy
resource atlas of the U.S. This atlas can be accessed on-line at http://www.nrel.gov/gis/wind_
maps.html.
A preliminary wind resource assessment of the U.S. and its territories was carried out
under the Federal Wind Energy Program described in Chapter 3, producing twelve regional
wind energy atlases , which are listed in Table 8-2. These atlases map annual and seasonal
average wind resources on regional and state levels, based on data collected before 1979.
They include certainty ratings of the estimated resource and areal distributions ( i.e.
percentages of land area suitable for wind energy development). The complexity of the