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The Wind
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The origin of wind
The 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 (Troposphere)
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.
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
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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 Wind
winds 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
terrain
Source: www.windpower.org
Open
plain
At a sea
coast
Open sea
Hills and
ridges
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Global circulations
http://www.windpower.org/en/tour/wres/globwin.htm
Source: Wind Power Plants, Fundamentals, Design, Construction and Operation.
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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 V
F   
A A
k 1
e
V
 
A
k
Where:
F
k
A
V
Wind speed probability
Shape factor
Weibull parameter
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 wind
Synoptic variation
Source: Wind Power Plants, Fundamentals, Design, Construction and Operation.
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Sea land breeze
Diurnal variation
Source: Wind Power Plants, Fundamentals, Design, Construction and Operation.
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Sea land breeze
Diurnal variation
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Surface Boundary Layer
• The principal effects
governing the properties of
the boundary layer:
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Strength of the geostrophic wind
Surface roughness
Coriolis force
Thermal effects
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Vertical wind speed gradient
 h2 
ln  
z0 

v 2 (h 2 )  v1 
 h1 
ln  
 z0 
Type of terrain
Cities, forests
Suburbs, wooded countryside
Roughness
length, z0
[m]
0,7
0,3
Village, countryside with trees and hedges
0,1
Open farmland, few trees and buildings
0,03
Flat grassy plains
0,01
Flat desert, rough sea
0,001
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]
Source: Wind Power Plants, Fundamentals, Design, Construction and Operation. And Wind Energy Handbook
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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
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
Source: Wind Power Plants, Fundamentals, Design, Construction and Operation.
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Wind Spectrum
• Synoptic variations
– Passage of weather systems
• Diurnal variations
– Predictable daily variations
Source: Wind Energy Handbook
• Turbulence
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Turbulence
Turbulence refers to fluctuations in wind speed on a relatively fast
time-scale, typically less than 10 min.
V  V  v'
v' 
V
2
Where:
V Wind velocity
V Mean wind velocity
v’ Turbulent velocity
V
[m/s]
[m/s]
[m/s]
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The power and energy in the wind
V3
Power     A
2
Energy  Power  Time
Where:

V
A
Density
[kg/m3]
Wind velocity [m/s]
Area
[m2]
Energy [MWh]
Time [hours]
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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 Rose
To 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.