Wind Power Wind Turbines

7/27/2019 Wind Power Wind Turbines

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Wind Power and WindTurbinesBJ Furman

K Youssefi13FEB2008


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Outline

Wind causes?

Wind Power example

Wind Turbine Design

Aerodynamics


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Windwhat causes it?Atmospheric

pressure differences Where does the

pressure come

from?

Weight of air inatmosphere

Area

ForcePressure

~31 km(99% of mass)

Avg. pressure at sea level

101325 Pa (Pascal)

1013.25 mb (millibar)

29.92 in. Hg (inches of mercury

1 atm (atmosphere)

14.7 psi (pound per square inch)


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Pressure

depthgDensityPressure Area

ForcePressure (Definition)

Earths surface

depthgravitytodueonacceleratiPressure volumemass

ms

mPressure 23

mkg

Check the formula by checking the units!

Units multiply and divide like numbers!

Space (zero pressure)

Edge of atmosphere

22

2

22s

mkg

ms

1Pressurem

N

mmkg

Okay!


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Windwhat causes it?, cont. Pressure differences cause the flow off lu ids(gasesand liquids)

pressure is always measured relative to somereferencepressure Sometimes relative to vacuum absolute Sometimes relative to atmospheric pressure

hPB PA

The higher pressure at B will cause fluid to flow out of the tank.

So, what causes pressure variations in the atmosphere?

AB PhgP

Fluid density

Acceleration due to gravity

Fluid height


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Prevailing Winds Heating and cooling of the air

https://reader009.{domain}/reader009/html5/0404/5ac3d01cbfd0d/5ac3d020c


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Windwhat causes it?, cont.Pressure maps

Contours of

constant pressure

(usually 4 mb

between contours

Close spacingmeans stronger

winds

In N.H., winds

circulate around a

low pressure region

in CCW direction


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Weather Processor SymbolsWXP Legend

http://virga.sfsu.edu/inline/legend.gif


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Review Question 1

What causes wind?

A. Air pressure

B. Weight of the atmosphere

C. Pressure difference inatmosphere

D. Low pressure

E. High pressure


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Review Question 2

What are the units of pressure?

A. Force/Area

B. Pascals (Pa)

C. Pounds per square inch (psi)

D. Millirads

E. B and C


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Wind Energy and Power Atmospheric pressure differences

accelerate and impart kinetic energy intothe air

Wind energy conversion machines (WEC)convert wind energy into electrical or

mechanical forms How much powercan we extract?

time

)()(

time

K.E.Power

22

1 velocitymass

velocityareadensitytimemass

2)()(Power

33

21

AVvelocityareadensity


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Wind Power - Example

Example:

V = 10 m/s

A = (2 m)2 = 4 m2

= 1.2 kg/m3

http://z.about.com/d/gonewengland/1/0/5/C/leaf5.gif

https://reader009.{domain}/reader009/html5/0404/5ac3d01cbfd0d/5ac3d02475b36

2

)()(Power3

32

1AV

velocityareadensity


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Ws

mNP

s

mN

s

m

s

mkg

s

mkg

smmmkgP

24002400

240024002400

2

1042.1

23

2

323

Wind PowerExample, cont.

Theoretical Maximum

Betz Limit: 59.3% of the theoretical is the maximum amount

extractable by a wind energy conversion device (WEC)

WWPBetz 2.1423)2400(593.0 Practical Maximum


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Wind Turbine Size-Power Comparison


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Wind Turbine Configurations

HAWT

VAWT

Boyle, G., Renewable Energy, 2nd ed., Oxford

University Press, 2004


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Configuration Tradeoffs

Factors

Efficiency Power produced per unit cost

Directionality

Support configuration Speed of rotation

Reliability

Cost Maintainability

Which type is best, HAWT or VAWT?


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Common HAWT Construction

Rotor

Blades are connected to a hub, which is connected to a shaft

Rotational speed will depend on blade geometry, number of blades,

and wind speed (40 to 400 revolutions per minute typical speed

range)

Gear box needed to increase speed to 1200-1800 RPM for generator


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Aerodynamics of Wind Turbine BladesForces are transmitted from a moving

fluid to an object in the flow streamLift = the force component perpendicular

to the original flow direction

Drag = the force component in line with

the original flow direction

Lift

Drag

http://www.grc.nasa.gov/WWW/K-12/airplane/newton3.html


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Two Types of Turbine DesignsDrag Designs

Savonius

Lift Designs

VAWT Darrieus

Most HAWT

designs

http://upload.wikimedia.org/wikipedia/commons/9/9e/Darrieus.jpg

http://upload.wikimedia.org/wikipedia/commons/9/99/Savonius_Querschnitt.png


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Aerodynamics of HAWT Blade

Boyle, G., Renewable Energy, 2nd ed., Oxford University Press, 2004

r[L(sinf - D(cosf)] = TorqueTorque x rotational speed= Power


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Aerodynamics of HAWT Blade, cont.Angle of attack, a (blade angle between chord and relative winddirection)

Has a large effect on the lift and dragTypical values between 1 and 15 degrees (what is optimum?)

D i f HAWT T bi Bl d


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Design of HAWT Turbine Blade

5-station design as seen from the tip

The blade twists to keep angle of attack constant

Blade size and shape

D i f HAWT T bi Bl d


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Design of HAWT Turbine Blade, cont.Number of blades

Increasing the number

of blades tends to

increase the

aerodynamic efficiency

Increasing the number

of blades increases the

cost (material and

manufacturing

Turbines with fewer

blades

tend to run most

efficientlyat lower tip speed ratios

(ratio of tip speed to wind

speed)

http://en.wikipedia.org/wiki/Wind_turbine_design


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R f


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References

http://www.bbc.co.uk/weather/

http://www.aos.wisc.edu/~hopkins/aos100/sfc-anl.htm

http://ww2010.atmos.uiuc.edu/(Gh)/g

uides/mtr/prs/hghdef.rxml http://en.wikipedia.org/wiki/Wind_tur

bine_design

http://www.grc.nasa.gov/WWW/K-12/airplane/short.html

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Wind Power Wind Turbines