Wind energy

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Windsock

Map of available wind power for the United States.

'Wind Energy' is the kinetic energy of air in motion; see also wind power.

Total wind energy flowing through an imaginary area A during the time t is:

$E = \frac{1}{2}mv^2 = \frac{1}{2}(Avt\rho)v^2 = \frac{1}{2}At\rho v^3$

where v is the wind speed; ρ is the air density; Avt is the volume of air passing through A (which is considered perpendicular to the direction of the wind); Avtρ is therefore the mass m passing per unit time. Note that ½ ρv² is the kinetic energy of the moving air per unit volume.

Power is energy per unit time, so the wind power incident on A (e.g. equal to the rotor area of a wind turbine) is:

$P = \frac{E}{t} = \frac{1}{2}A\rho v^3$

Wind power in an open air stream is thus proportional to the third power of the wind speed; the available power increases eightfold when the wind speed doubles. Wind turbines for grid electricity therefore need to be especially efficient at greater wind speeds.

History of usage by humans

Since ancient times man has harnessed the power of the wind, the earliest known wind mills were vertical axis type, developed by the Persians in 500-900 A.D.^{[citation needed]} One of the most scenic and historic applications of wind power can still be found on the Island of Crete; electric motors of pre-industrial Europe

The first windmills in Europe were developed in 1270 A.D. and were of the horizontal axis type.^{[citation needed]} By 1390 A.D, the Dutch set out to refine the tower mill design and after 500 years of incremental developments the wind sails had all the major features recognized by modern designers. Wind mills were the electric motors of pre-industrial Europe.^{[citation needed]}

Water pumping systems were perfected in 1854, beginning with the Halladay, and continuing with the Dempster Design wind mills.^{[citation needed]} By the end of the 19th Century, the first use of a windmill to generate electricity was a system built in Cleveland, Ohio, in 1888 by Charles F. Brush.^{[citation needed]}

In 1891, Dane Poul La Cour developed the first electrical output wind machine to incorporate aerodynamic design principles.^{[citation needed]} In the century to follow, the integration of composite materials allowed for increased efficiencies and performance.^{[citation needed]} Starting in the early 1990’s increasing electricity costs spawned wind farms all over the world.^{[citation needed]}

Wind mills have evolved from being used only for grinding grain and water pumping purposes to a renewable and reliable source of electricity used in various applications such as the following:^{[citation needed]}

• Water pumping
• Monitoring sites
• Cathodic protection
• Offshore platforms
• Telecommunications
• Commercial/retail
• Sailboats
• Military
• Grid-tied electrical systems
• Remote homes and cabins

Electric energy

Main articles: Wind power, Wind turbine, and Electric energy

Theoretical power captured by a wind turbine

Total wind power could be captured only if the wind velocity is reduced to zero. In a realistic wind turbine this is impossible, as the captured air must also leave the turbine. A relation between the input and output wind velocity must be considered. Using the concept of stream tube, the maximal achievable extraction of wind power by a wind turbine is 59% of the total theoretical wind power (see: Betz' law).

Practical wind turbine power

Further insufficiencies, such as rotor blade friction and drag, gearbox losses, generator and converter losses, reduce the power delivered by a wind turbine. The basic relation that the turbine power is (approximately) proportional to the third power of velocity remains.

Example of a horizontal axis machine Wind Turbine

See also

• Wind power

External links

• - Wind Energy Resource Atlas of the United States
• - American Wind Energy Association