Page 47 - Fluid Power Engineering

CHAPTER3

Properties of Wind

Among the absolutes in man’s world are the stars and the weather, and
as inescapable as the existence of weather is the presence of winds.
—Goerge M. Hidy

Introduction
In this chapter, the basic properties of wind and air are discussed.
The treatment will be from the meteorological standpoint. The chap-
ter begins with a short exposition on how wind is created. The second
section deals with a popular classification scheme in the wind industry
that is based on annual average wind speed. A more rigorous method
to classify wind speed that is based on power density is described next.
The third section describes the statistical properties of wind speed
along with the probability density function and its impact on the av-
erage energy density. The next topic is wind shear, which describes
the variation in wind speed as a function of height. It is used to extrap-
olate measured wind speed to hub height and describes the variation
of wind speed in the plane of rotation of the rotor. The final section
describes air density and the impact of temperature, elevation, and hu-
midity on air density. This chapter, therefore, describes two of the three
termsinthewindpowerequation,namely,windspeedandairdensity.

How Is Wind Generated?
Uneven heating of the surface of the earth creates wind. Solar en-
ergy absorbed by land or water is transferred to the atmosphere. A
larger amount of solar radiation is received at the tropics compared
to the poles, which causes hot air to rise at the tropics and flow to-
ward the poles. This flow occurs 10 to 15 km above the earth’s surface.
The Coriolis force causes the flow of hot air in the upper atmosphere
◦
to turn right. This flow does not continue beyond 30 latitude. This
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