Environmental Engineering Reference
In-Depth Information
WIND POWER BASICS
A
ir
in
M
otion
*
In all dynamic situations, forces are necessary to produce motion and changes in
motion—winds and air currents. The air (made up of various gases) of the atmo-
sphere is subject to two primary forces: gravity and pressure differences from tem-
perature variations.
Gravity
(gravitational forces) holds the atmosphere close to the
Earth's surface. Newton's law of universal gravitation states that every body in the
universe attracts another body with a force equal to
FG
mm
r
12
2
=
(2.1)
where
F
is the magnitude of the gravitational force between the two bodies,
G
is the
gravitational constant ≈ 6.67 × 10
-11
N (m
2
/kg
2
),
m
1
and
m
2
are the masses of the two
bodies, and
r
is the distance between the two bodies.
The force of gravity decreases as an inverse square of the distance between the
two bodies. Thermal conditions affect density, which in turn affects vertical air
motion and planetary air circulation (and how air pollution is naturally removed
from the atmosphere). Although forces acting in other directions can overrule gravi-
tational force, gravity constantly acts vertically downward, on every gas molecule,
which accounts for the greater density of air near the Earth.
Atmospheric air is a mixture of gases, so the gas laws and other physical prin-
ciples govern its behavior. The pressure of a gas is directly proportional to its
temperature. Pressure is force per unit area (pressure = force/area), so a tempera-
ture variation in air generally gives rise to a difference in pressure of force. This
difference in pressure resulting from temperature differences in the atmosphere
creates air movement—on both large and local scales. This pressure difference
corresponds to an unbalanced force, and when a pressure difference occurs the air
moves from a high- to a low-pressure region. In other words, horizontal air move-
ments (called
advective winds
) result from temperature gradients, which give rise
to density gradients and subsequently pressure gradients. The force associated with
these pressure variations (
pressure gradient force
) is directed at right angles to
(perpendicular to) lines of equal pressure (called
isobars
) and is directed from high
to low pressure.
Localized air circulation gives rise to
thermal circulation
(a result of the relation-
ship based on a law of physics whereby the pressure and volume of a gas are directly
related to its temperature). A change in temperature causes a change in the pressure
and volume of a gas. With a change in volume comes a change in density (density
= mass/volume), so regions of the atmosphere with different temperatures may have
different air pressures and densities. As a result, localized heating sets up air motion
and gives rise to thermal circulation.
*
Adapted from Spellman, F.R. and Bieber, R.,
The Science of Renewable Energy
, CRC Press, Boca
Raton, FL, 2011.
