2 the Wind Resource

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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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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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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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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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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.

2 the Wind Resource

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