Fundamentals of wind energy - Wind Power Generation and Wind Turbine Design - page 4

Wind energy represents a mainstream energy source of new power generation
and an important player in the world's energy market. As a leading energy technol-
ogy, wind power's technical maturity and speed of deployment is acknowledged,
along with the fact that there is no practical upper limit to the percentage of wind
that can be integrated into the electricity system [1]. It has been estimated that the
total solar power received by the earth is approximately 1.8 10 11 MW. Of this ×

10 9 MW) is converted into wind energy and about
35% of wind energy is dissipated within 1000 m of the earth's surface [2]. There-
fore, the available wind power that can be converted into other forms of energy is
approximately 1.26

solar input, only 2% (i.e. 3.6 ×

10 9 MW. Because this value represents 20 times the rate of
the present global energy consumption, wind energy in principle could meet entire
energy needs of the world.
Compared with traditional energy sources, wind energy has a number of bene-
fi ts and advantages. Unlike fossil fuels that emit harmful gases and nuclear power
that generates radioactive wastes, wind power is a clean and environmentally
friendly energy source. As an inexhaustible and free energy source, it is available
and plentiful in most regions of the earth. In addition, more extensive use of wind
power would help reduce the demands for fossil fuels, which may run out some-
time in this century, according to their present consumptions. Furthermore, the
cost per kWh of wind power is much lower than that of solar power [3].
Thus, as the most promising energy source, wind energy is believed to play a
critical role in global power supply in the 21st century.

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2 Wind generation

Wind results from the movement of air due to atmospheric pressure gradients.
Wind fl ows from regions of higher pressure to regions of lower pressure. The
larger the atmospheric pressure gradient, the higher the wind speed and thus, the
greater the wind power that can be captured from the wind by means of wind
energy-converting machinery.
The generation and movement of wind are complicated due to a number of fac-
tors. Among them, the most important factors are uneven solar heating, the Coriolis
effect due to the earth's self-rotation, and local geographical conditions.

2.1 Uneven solar heating

Among all factors affecting the wind generation, the uneven solar radiation on the
earth's surface is the most important and critical one. The unevenness of the solar
radiation can be attributed to four reasons.
First, the earth is a sphere revolving around the sun in the same plane as its
equator. Because the surface of the earth is perpendicular to the path of the sunrays
at the equator but parallel to the sunrays at the poles, the equator receives the great-
est amount of energy per unit area, with energy dropping off toward the poles. Due
to the spatial uneven heating on the earth, it forms a temperature gradient from the
equator to the poles and a pressure gradient from the poles to the equator. Thus, hot
air with lower air density at the equator rises up to the high atmosphere and moves