Wind Characteristics - Wind Energy: Renewable Energy and the Environment - page 32

3 Wind Characteristics
3.1 GLOBAL CIRCULATION
The motion of the atmosphere can vary in distance and time from the very small to the very large
( Table 3.1 ) . There is an interaction between each of these scales and the flow of air is complex. The
global circulation encloses eddies, which enclose smaller eddies, which enclose smaller eddies, until
finally the microscale is reached.
The two main factors in global circulation are the solar radiation and the rotation of the earth
and the atmosphere. The seasonal variation is due to the tilt of the earth's axis to the plane of
the earth's movement around the sun. Since the solar radiation is greater per unit area when the
sun is directly overhead, there is a transport of heat from the regions near the equator toward the
poles. Because the earth is rotating on its axis and there is conservation of angular momentum,
the wind will be shifted as it moves along a longitudinal direction. The three-cell model explains
the predominant surface winds ( Figure 3.1 ) . Those regions in the trade winds are generally good
locations for the utilization of wind power; however, there are exceptions, as Jamaica is not nearly
as windy as Hawaii.
Superimposed on this circulation is the migration of cyclones and anticyclones across the mid-
latitudes, which disrupt the general flow. Also, the jet streams, the fast core of the central westerlies
at the upper levels, influence the surface winds.
Local winds are due to local pressure differences and are influenced by the topography, friction
of the surface due to mountains, valleys, etc. The diurnal (24 h) variation is due to temperature dif-
ferences between day and night. The temperature differences between the land and sea also cause
breezes; however, they do not penetrate very far inland ( Figure 3.2 ) .
3.2 EXTRACTABLE LIMITS OF WIND POWER
Solar energy drives the wind, which is then dissipated due to turbulence and friction at the earth's
surface. The earth's atmosphere can be considered a giant duct, and if energy is taken out at one
location, it is not available elsewhere. Therefore, it is important to distinguish between the kinetic
energy in the wind and the rate and limits of the extraction of that energy, the power in the wind,
and the maximum power extractable.
A comparison can be made on the basis of the kinetic energy of the winds per unit area of the earth's
surface. Of the solar input, only 2% is converted into wind power, and 35% of that is dissipated within 1
km of the earth's surface. This is the wind power available for conversion to other forms of energy.
The amount extracted would be limited by the criteria of not changing the climate; however, the
uncertainties are very large in determining such criterion. Man would be substituting wind turbines
for naturally occurring frictional features such as trees, mountains, etc. Gustavson [1] assumed the
extractable limit as 10% of the available wind power within 1 km of the surface. When these values
are applied to the contiguous forty-eight states of the United States, the limit would be 2 r 10 12 W
(2 TW), or 62 quads/year. A similar analysis can be made for the world. Therefore, wind energy
represents a very large energy source.
On a global scale, wind can be compared to other renewable sources ( Table 3.2 ) . In locations
with high wind speeds, wind power is comparable to, or better than, the amount of solar power.
The wind energy available represents approximately twenty times the rate of global energy
consumption.
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