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The Wind

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The origin of windThe earth is unevenly heated by the sun

resulting in the poles receiving less energy from the sun than the equator does. Also the dry land heats up (and cools down)

more quickly than the seas do. The differential heating powers a global

atmospheric convection system reaching from the earth's surface to the stratosphere

which acts as a virtual ceiling.

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The atmosphere around the globe is a very thin layer. The globe has a diameter of

12,000 km. The troposphere, which extends to about 11 km altitude, is where all of our

weather, and the greenhouse effect occurs.

On the picture you can see a stretch of islands 300 km across, and the approximate height of the troposphere. To look at it at a

different scale: If the globe were a ball with a diameter of 1.2 meters the atmosphere

would only be 1 mm thick.

The Atmosphere (Troposphere)

Source: www.windpower.org

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An estimated 1 to 3 % of energy from the Sun that hits the earth is converted into wind energy.

This is about 50 to 100 times more energy than is converted into biomass by all the plants on earth through photosynthesis. Most of this wind energy can be found at high altitudes where continuous wind speeds of over 160 km/h occur. Eventually, the wind energy is converted through friction into diffuse heat all through the earth's surface and

atmosphere.

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The wind resource

• The Geostrophic Wind • Geographical variations• Global circulations• Annual and seasonal variations• Synoptic and diurnal variations• The surface boundary layer• Turbulence• The energy in the wind

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The Geostrophic Windwinds balanced by the Coriolis and Pressure Gradient forces

An air parcel initially at rest will move from high pressure to low pressure because of the pressure gradient force . However, as that air parcel begins

to move, it is deflected by the Coriolis force to the right in the northern hemisphere (to the left on the southern hemisphere). As the wind gains

speed, the deflection increases until the Coriolis force equals the pressure gradient force. At this point, the wind will be blowing parallel to the isobars.

When this happens, the wind is referred to as geostrophic.

The geostrophic wind is found at altitudes above 1000 meters above ground level.

Source: http://ww2010.atmos.uiuc.edu

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Geographical variations

• Different surface heating from the sun– High surface heating close to equator

– Day and night differences

• The non-uniformity of the earth’s surface– Land masses and ocean

– Hills an mountains

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Source: www.bergey.com

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Wind Resources at 50 (45) m Above Ground Level

Color Sheltered Open At a sea Open sea Hills and terrain plain coast ridges

Source: www.windpower.org

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Global circulations

Source: Wind Power Plants, Fundamentals, Design, Construction and Operation.

http://www.windpower.org/en/tour/wres/globwin.htm

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Global circulations• Rossby circulation

– Northern and southern hemisphere– Moves warm air to the poles and cold air to the subtropical areas

• Hadley circulation – Equatorial regions (30oS – 30oN)– Produces the constant wind systems of the north-east and south-east

trade winds

• Monsoons– Large scale motion due to differences in temperature

• Indian ocean• Atlantic ocean• Africa

• Tropical cyclones– Rising hot humid air at the equatorial regions

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Annual and seasonal variations• Year to year variation in annual wind speeds is hard to predict• Characterized by probably distribution

– Wind speed probability is calculated as a Weibull curve

k

A

V1k

eA

V

A

kF

Where:

F Wind speed probability [ - ]k Shape factor [ - ]A Weibull parameter [ - ]V Wind velocity [m/s]

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Synoptic and diurnal variations

• Synoptic variations– Passage of weather systems

• Catabatic wind

• Diurnal variations– Predictable daily variations

• Sea land breeze• Mountain top – mountain valley wind

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Catabatic windSynoptic variation

Source: Wind Power Plants, Fundamentals, Design, Construction and Operation.

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Sea land breezeDiurnal variation

Source: Wind Power Plants, Fundamentals, Design, Construction and Operation.

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Sea land breezeDiurnal variation

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Surface Boundary Layer

• The principal effects governing the properties of the boundary layer:– Strength of the geostrophic wind

– Surface roughness

– Coriolis force

– Thermal effects

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Vertical wind speed gradient

0

1

0

2

122

zh

ln

zh

ln

v)h(v0,001Flat desert, rough sea

0,01Flat grassy plains

0,03Open farmland, few trees and buildings

0,1Village, countryside with trees and hedges

0,3Suburbs, wooded countryside

0,7Cities, forests

Roughness length, z0

[m]

Type of terrain

0,001Flat desert, rough sea

0,01Flat grassy plains

0,03Open farmland, few trees and buildings

0,1Village, countryside with trees and hedges

0,3Suburbs, wooded countryside

0,7Cities, forests

Roughness length, z0

[m]

Type of terrain

Source: Wind Power Plants, Fundamentals, Design, Construction and Operation. And Wind Energy Handbook

Where:h1 Reference height [m]h2 Height [m]z0 Roughness length [m]v1 Wind velocity at the reference height [m/s]v2 Wind velocity [m/s]

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Obstacles on the ground

Source: Wind Power Plants, Fundamentals, Design, Construction and Operation.

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The wind stream over hills

Source: Wind Power Plants, Fundamentals, Design, Construction and Operation.

The wind stream over hills with a gradient lower than 10% is accelerated at the hill top, but without disturbing stalls and turbulence. This is an excellent opportunity for utilizing the power in the wind

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Wind Spectrum

• Synoptic variations– Passage of weather systems

• Diurnal variations– Predictable daily variations

• TurbulenceSource: Wind Energy Handbook

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Turbulence

Turbulence refers to fluctuations in wind speed on a relatively fast time-scale, typically less than 10 min.

'vVV

2'v V V Where:

V Wind velocity [m/s] V Mean wind velocity [m/s] v’ Turbulent velocity [m/s]

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The power and energy in the wind

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2

VPower A

TimePowerEnergy

Where:

Density [kg/m3]V Wind velocity [m/s] A Area [m2]

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Tim

e [h

ours

]

En

erg

y [M

Wh

]

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Wind measurements

• The cup anemometer

• The vane anemometer

• The ultrasonic anemometer

• The hot-wire anemometer

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The wind RoseTo show the information about the distributions of

wind speeds, and the frequency of the varying wind directions, one may draw a so-called wind rose on the

basis of meteorological observations of wind speeds

and wind directions.