Wind Power and Wind Turbines BJ Furman K
Wind Power and Wind Turbines BJ Furman K Youssefi 13 FEB 2008
Outline • Wind – causes? • Wind Power – example • Wind Turbine Design – Aerodynamics
Wind – what causes it? • Atmospheric pressure differences ~31 km (99% of mass) ’s rth Ea – Where does the pressure come from? su km 00 2, 8 ~1 ce rfa • Weight of air in atmosphere • 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 Space (zero pressure) (Definition) Edge of atmosphere Earth’s surface Check the formula by checking the units! Units multiply and divide like numbers! 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 Fluid density h PB PA Fluid height Acceleration due to gravity The higher pressure at B will cause fluid to flow out of the tank. So, what causes pressure variations in the atmosphere?
Prevailing Winds • Heating and cooling of the air http: //trampleasure. net/science/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 1 What causes wind? A. Air pressure B. Weight of the atmosphere C. Pressure difference in atmosphere D. Low pressure E. High pressure
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
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?
Wind Power - Example • Example: V = 10 m/s A = (2 m)2 = 4 m 2 r= 1. 2 kg/m 3 http: //enneagon. org/footprint/jpg/dvc 01 w. jpg http: //z. about. com/d/gonewengland/1/0/5/C/leaf 5. gif
Wind Power – Example, cont. Theoretical Maximum Betz Limit: 59. 3% of theoretical is the maximum amount extractable by a wind energy conversion device (WEC) Practical Maximum
Wind Turbine Size-Power Comparison
Wind Turbine Configurations HAWT VAWT Boyle, G. , Renewable Energy, 2 nd 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/newton 3. html
Two Types of Turbine Designs • Drag Designs –Savonius • Lift Designs http: //upload. wikimedia. org/wikipedia/commons/9/99/Savonius_Querschnitt. png –VAWT Darrieus –Most HAWT designs http: //upload. wikimedia. org/wikipedia/commons/9/9 e/Darrieus. jpg
Aerodynamics of HAWT Blade r[L(sinf) - D(cosf)] = Torque x rotational speed= Power Boyle, G. , Renewable Energy, 2 nd ed. , Oxford University Press, 2004
Aerodynamics of HAWT Blade, cont. Angle of attack, a (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 Blade size and shape 5 -station design as seen from the tip The blade twists to keep angle of attack constant
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 efficiently at lower tip speed ratios (ratio of tip speed to wind speed) http: //en. wikipedia. org/wiki/Wind_turbine_design
Review Question 3 The 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. Varies E. A and B
References • http: //www. bbc. co. uk/weather/ • http: //www. aos. wisc. edu/~hopkins/a os 100/sfc-anl. htm • http: //ww 2010. 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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