Environmental Engineering Reference
In-Depth Information
Once the air has been set into motion, secondary forces (velocity-dependent
forces) act. These secondary forces are caused by Earth's rotation (Coriolis force)
and contact with the rotating Earth (friction). The
Coriolis force
, named after its
discoverer, French mathematician Gaspard Coriolis (1772-1843), is the effect of
rotation on the atmosphere and on all objects on the Earth's surface. In the Northern
Hemisphere, it causes moving objects and currents to be deflected to the right; in the
Southern Hemisphere, it causes deflection to the left, because of the Earth's rota-
tion. Air, in large-scale north or south movements, appears to be deflected from its
expected path. That is, air moving poleward in the Northern Hemisphere appears to
be deflected toward the east; air moving southward appears to be deflected toward
the west.
Friction
(drag) can also cause the deflection of air movements. This friction
(resistance) is both internal and external. The friction of molecules generates internal
friction, and external friction is caused by contact with terrestrial surfaces. The mag-
nitude of the frictional force along a surface is dependent on the air's magnitude and
speed, and the opposing frictional force is in the opposite direction of the air motion.
WIND ENERGY
*
Wind energy is power produced by the movement of air. Since early recorded his-
tory, people have been harnessing the energy of the wind to, for example, mill grain
and pump water. Wind energy propelled boats along the Nile River as early as 5000
BC. By 200 BC, simple windmills in China were pumping water, and windmills
with woven reed sails were grinding grain in Persia and the Middle East.
The use of wind energy spread around the world, and by the 11th century people
in the Middle East were using windmills extensively for food production; returning
merchants and crusaders carried this idea back to Europe. The Dutch refined the
windmill and adapted it for draining lakes and marshes in the Rhine River delta.
When settlers took this technology to the New World in the later 19th century,
they used windmills to pump water for farms and ranches and, later, to generate
electricity for homes and industry. The first known wind turbine designed to pro-
duce electricity was built in 1888 by Charles F. Brush, in Cleveland, Ohio; it was a
12-kW unit that charged batteries in the cellar of a mansion. The first wind turbine
used to generate electricity outside of the United States was built in Denmark in
1891 by Poul la Cour, who used electricity from his wind turbines to electrolyze
water to make hydrogen for the gas lights at the local schoolhouse. By the 1930s
and 1940s, hundreds of thousands of wind turbines were being used in rural areas
of the United States that were not yet being served by the grid. The oil crisis in
the 1970s created a renewed interest in wind energy, until the U.S. government
stopped giving tax credits.
*
Adapted from EERE,
History of Wind Energy
, Energy Efficiency and Renewable Energy, U.S.
Department of Energy, Washington, DC, 2005 (http://www1.eere.energy.gov/windandhydro/wind_
history.html).
