Wind Power and Wind TurbinesBJ FurmanK Youssefi13FEB2008

Outline• Wind – causes?• Wind Power – example• Wind Turbine Design

– Aerodynamics

Wind – what causes it?•Atmospheric pressure differences

– Where does the pressure come from?

• Weight of air in atmosphere

AreaForce Pressure

~12,8

00 km

~31 km(99% of mass)

Earth’s surface

• 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)

Pressure

depthg Density Pressure

AreaForce Pressure (Definition)

Earth’s surface

depthgravity todue onaccelerati Pressure volumemass

ms

m Pressure 23

mkg

Check the formula by checking the units!

Units multiply and divide like numbers!

Space (zero pressure)

Edge of atmosphere

22

2

22s

mkgms

1 PressuremN

mmkg

Okay!

Wind – what causes it?, cont.• Pressure differences cause the flow of fluids (gases and liquids)

– pressure is always measured relative to some reference pressure

• 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 Phg P

Fluid density

Acceleration due to gravity

Fluid height

Prevailing Winds• Heating and cooling of the air

http://trampleasure.net/science/coriolis/coriolis.png

Wind – what causes it?, cont.•Pressure maps

– Contours of constant pressure (usually 4 mb between contours– Close spacing means stronger winds–In N.H., winds circulate around a low pressure region in CCW direction

Weather Processor Symbols•WXP Legend

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

Review Question 1What causes wind?

A. Air pressureB. Weight of the atmosphereC. Pressure difference in

atmosphereD. Low pressureE. High pressure

Review Question 2What are the units of pressure?

A. Force/AreaB. Pascals (Pa)C. Pounds per square inch (psi)D. MilliradsE. B and C

Wind Energy and Power• Atmospheric pressure differences

accelerate and impart kinetic energy into the air

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

• How much power can we extract?

time)()(

timeK.E. Power

22

1 velocitymass

velocityareadensitytimemass

2)()( Power

33

21 AVvelocityareadensity

Wind Power - Example•Example:

V = 10 m/sA = (2 m)2 = 4 m2

= 1.2 kg/m3

http://z.about.com/d/gonewengland/1/0/5/C/leaf5.gifhttp://enneagon.org/footprint/jpg/dvc01w.jpg

2)()( Power

33

21

AVvelocityareadensity

WsmNP

smN

sm

smkg

smkg

smmmkgP

24002400

240024002400

21042.1

23

2

323

Wind Power – Example, 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

Wind Turbine Size-Power Comparison

Wind Turbine Configurations

HAWT

VAWT

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

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?

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

Aerodynamics of Wind Turbine Blades•Forces are transmitted from a moving fluid to an object in the flow stream

–Lift = 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

Two Types of Turbine Designs•Drag 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

Aerodynamics of HAWT Blade

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

r[L(sinD(cos)] = Torque

Torque x rotational speed= Power

Aerodynamics of HAWT Blade, cont.

Angle of attack, (blade angle between chord and relative wind direction)

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

Design of HAWT Turbine Blade5-station design as seen from the tip

The blade twists to keep angle of attack constant

Blade size and shape

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

Review Question 3The lift force on a wing or turbine blade is:

A. In line with the relative wind direction

B. Perpendicular to the relative wind direction

C. Somewhere between in line and perpendicular to the relative wind direction

D. VariesE. A and B

References• http://www.bbc.co.uk/weather/• http://www.aos.wisc.edu/~hopkins/

aos100/sfc-anl.htm• http://ww2010.atmos.uiuc.edu/(Gh)/

guides/mtr/prs/hghdef.rxml• http://en.wikipedia.org/wiki/

Wind_turbine_design• http://www.grc.nasa.gov/WWW/K-

12/airplane/short.html