WIND ENERGY SYSTEMS - Cd3wd

Chapter 2—Wind Characteristics 2–1
WIND CHARACTERISTICS
The wind blows to the south and goes round to the north:, round and round goes the wind,
and on its circuits the wind returns. Ecclesiastes 1:6
The earth’s atmosphere can be modeled as a gigantic heat engine. It extracts energy from
one reservoir (the sun) and delivers heat to another reservoir at a lower temperature (space).
In the process, work is done on the gases in the atmosphere and upon the earth-atmosphere
boundary. There will be regions where the air pressure is temporarily higher or lower than
average. This difference in air pressure causes atmospheric gases or wind to flow from the
region of higher pressure to that of lower pressure. These regions are typically hundreds of
kilometers in diameter.
Solar radiation, evaporation of water, cloud cover, and surface roughness all play important
roles in determining the conditions of the atmosphere. The study of the interactions between
these effects is a complex subject called meteorology, which is covered by many excellent
textbooks.[4, 8, 20] Therefore only a brief introduction to that part of meteorology concerning
the flow of wind will be given in this text.
1 METEOROLOGY OF WIND
The basic driving force of air movement is a difference in air pressure between two regions.
This air pressure is described by several physical laws. One of these is Boyle’s law, which
states that the product of pressure and volume of a gas at a constant temperature must be a
constant, or
p 1 V 1 = p 2 V 2 (1)
Another law is Charles’ law, which states that, for constant pressure, the volume of a gas
varies directly with absolute temperature.
V 1
T 1
= V 2
T 2
(2)
If a graph of volume versus temperature is made from measurements, it will be noticed
that a zero volume state is predicted at −273.15 o Cor0K.
The laws of Charles and Boyle can be combined into the ideal gas law
pV = nRT (3)
Wind Energy Systems by Dr. Gary L. Johnson November 20, 2001