Wind Power and Wind Turbines
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Transcript of Wind Power and Wind Turbines
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