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Topic 2
The Wind Resource
Alan Tham
November 2010
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PLANETARY WIND PATTERNS
Each hemisphere contains three wind belts(polar easterlies, prevailing westerlies, and tradewinds).
These wind belts arise due to the curvature of the earth.
The curvature of the earth results in(a) uneven heating of the earth
(b) the Coriolis effect.
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Uneven Heating of the Earth
Equatorial regions receive more sunlight than the poles.
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Uneven Heating of the Earth
This produces vertical air movement, called air current. When air is heated, it rises, producing low pressure areas.
Cold air sinks, producing high pressure areas.
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Uneven Heating of the Earth
Air pressuredifferences cause
air movement parallel
to the ground,
called wind. This results in
a circular motion
where cold air is
being replaced by
warm air.
On a global scale,
cells form.
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Coriolis Effect
The earth spins fastest at the equator, slowest at the poles. When air moves from the equator to the poles,
it travels east faster than the land beneath it.
Wind deflects to the RIGHT in the northern hemisphere,
and to the LEFT in the southern hemisphere.
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Wind Patterns
Trade Winds
Warm air moving from the
equator begins to cool and
sink near 30 north (or
south).
Air then moves towards
the equator : warm, steady
breezes that blow
continuously.
Coriolis Effect makesthese winds curve towards
the west.
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Wind Patterns
Prevailing Westerlies
These are winds moving
towards the poles.
Coriolis Effect makes
these winds curve towardsthe east.
Polar Easterlies
High pressure over the
poles causes the winds to
blow away from the poles.
Coriolis Effect makes
these winds curve towards
the west.
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Wind Patterns
InterTropicalConvergence Zone
(ITCZ)
This zone is also known
as the doldrums.
NE and SE trade winds
meet near the equator.
They are heated and rises.
Because air rises equally,
wind is not created.Winds here are very light.
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Wind Patterns
Jet Stream
Jet streams flow from
west to east.
They are currents in the
atmosphere just under thetroposphere (12 km up).
They take place in areas
where there is a major
difference in air
temperature. The air doesnot flow directly from hot
to cold but is deflected by
the Coriolis force.
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LOCAL CONDITIONS
The planetary wind pattern is modified by localconditions.
Differential heating produces
(a) sea and land breezes(b) mountain and valley breezes.
Topography creates local acceleration effects
(a) mountainous terrain
(b) rough terrain (vegetation and man-made structures).
Local winds have high variability in speed and direction.
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Sea & Land Breezes
During the day, the land is heated faster than the sea.Warm air rises. Cold air from the sea falls.
The circular motion produces sea breezes.
At night, the land cools faster than the sea.
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Valley & Mountain Breezes
During the day, the valley heats up.Warm air rises, producing a valley breeze.
At night, the mountain cools faster than the valley.
Cool dense air sinks, producing a mountain breeze.
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Mountainous Terrain
Wind speed increases when it flows(a) over hills and ridges
(b) through valleys, gorges and
breaks in mountain chains
(Venturi Effect).
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Rough Terrain
Wind slows down when the surface of the earth is rough. This roughness is caused by trees and buildings
(which block the flow).
Frictional drag makes the flow turbulent.
The energy in the flow is lost as eddies.
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WIND SHEAR
Implications for wind turbines Wind speeds at the top and bottom of the blades
are different, hence they are subjected to different loads.
In terrains where the power law exponent is small,
shorter and less expensive towers can be used.
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WIND SHEAR
The wind is slowed by objects at the ground. Lower layers of air in turn retard those higher than them.
This results in a
difference in meanwind speed between
two points at different
heights from the
ground.
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WIND SHEAR
Wind Shear Equation
where Vh = wind speed at height zh above the ground
Vref= reference speed taken at reference height hrefa = power law exponent dependent on roughness
a
ref
h
ref
h
z
z
V
V
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WIND SHEAR
Type of Terrain a
Smooth sea, sand 0.10
Short grass 0.13
Crops 0.143
Rural areas 0.16
High grass 0.19
Forest, woods 0.20
Suburban areas 0.23
Urban areas 0.32
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WIND MEASUREMENT
Measure wind speed & direction
Plot wind speed duration curve
Determine site characteristics
(a) long term characteristic
(b) extreme wind speed
(c) I15 turbulence
Select a wind turbine class
matching the site characteristics
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Measuring Wind Speed
Wind speed are classified using the Beaufort Scale. High speed winds are classified using the Fujita Scale.
Wind speeds are measured using a 3-cup anemometer.
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Measuring Wind Direction
Wind direction is obtained using a wind vane. The vane pointstowards the direction in which the wind
is blowing from.
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Wind Rose
Wind speed and direction can be reported usinga wind rose.
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Use of Standards
Wind speed is measured to IEC 61400-12 standards. Measurements are taken at a height of 10 m.
Method of bins
Wind speeds are measuredevery 3 seconds
over a 10-minute interval.
Compute the mean and standard
deviation for these 200 readings.
This gives one 10-minute bin.
Do a total of 36 bins.
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Windspeed(m/s)
Hours/year
Plot Wind Speed Duration Curve
The graph shows the number of hours in a year thatthe wind speed exceeds a given value.
(e.g. wind speed > 15 m/s for about 2000 hours).
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Determine Site Characteristics
Long Term Characteristic
The long term characteristic is defined by
the annual mean wind speed.
This can be calculated as the simple mean ofthe 36 bins.
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Determine Site Characteristics
Extreme Wind Speed
A wind gust is a short burst of high speed wind.
To determine the wind gust, in each of the 36 bins,
(a) calculate (highest wind speed
lowest wind speed)(b) discard any value < 5.28 m/s
(c) choose the largest value.
The maximum gust over a 50 year period is defined as
the extreme wind speed, designated as V50.
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Determine Site Characteristics
I15 Turbulence
Consider the bin with mean wind speed of 15 m/s.
The standard deviation of that bin is defined as
the I15 turbulence.
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Matching
The site characteristics are used to determine whichwind turbine is best suited for use there.
Procedure for matching:
(a) Fit the data statistically.(b) Select a wind turbine class from the IEC 61400
classification.
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Fit Data Statistically
Another way to present the data in a wind-speed durationcurve is to interchange the axes.
Time (h)
Wind speed (m/s)
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Fit Data Statistically
The best fit is described by a Weibull distribution. The shape factor describes how peaked the distribution is.
IEC 61400-12 uses a shape factor of 2 (Rayleigh).
Time (h)
Wind speed (m/s)
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Select Wind Turbine Class
WTG Class I II III IV
Average wind speed at hub-height (m/s) 10.0 8.5 7.5 6.0
V50 extreme 50-year gust (m/s) 70.0 59.5 52.5 42.0
I15 characteristic turbulence Class A 18%
I15 characteristic turbulence Class B 16%
Wind shear exponent 0.20
Designers base their selection on the average wind speed. They will modify only when the V50 extreme gust
exceeds the recommended value for the class selected.
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