Wind Power and Wind Turbines

1. Wind Power and Wind Turbines www.assignmentpoint.com

2. Outline • Wind – causes? • Wind Power – example • Wind Turbine Design • Aerodynamics www.assignmentpoint.com

3. Wind – what causes it? ~31 km (99% of mass) Earth’s surface ~12,800 km • Atmospheric pressure differences • Where does the pressure come from? • 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) www.assignmentpoint.com

4. Pressure (Definition) Earth’s surface Space (zero pressure) Edge of atmosphere Check the formula by checking the units! Units multiply and divide like numbers! Okay! www.assignmentpoint.com

5. Wind – what causes it?, cont. Fluid density Fluid height h PA PB Acceleration due to gravity • 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 The higher pressure at B will cause fluid to flow out of the tank. So, what causes pressure variations in the atmosphere? www.assignmentpoint.com

6. Prevailing Winds • Heating and cooling of the air http://trampleasure.net/science/coriolis/coriolis.png www.assignmentpoint.com

7. 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 www.assignmentpoint.com

8. Weather Processor Symbols • WXP Legend http://virga.sfsu.edu/inline/legend.gif www.assignmentpoint.com

9. Review Question 1 What causes wind? • Air pressure • Weight of the atmosphere • Pressure difference in atmosphere • Low pressure • High pressure www.assignmentpoint.com

10. Review Question 2 What are the units of pressure? • Force/Area • Pascals (Pa) • Pounds per square inch (psi) • Millirads • B and C www.assignmentpoint.com

11. 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? www.assignmentpoint.com

12. Wind Power - Example http://enneagon.org/footprint/jpg/dvc01w.jpg http://z.about.com/d/gonewengland/1/0/5/C/leaf5.gif • Example: • V = 10 m/s • A = (2 m)2 = 4 m2 • = 1.2 kg/m3 www.assignmentpoint.com

13. Wind Power – Example, cont. Practical Maximum Theoretical Maximum Betz Limit: 59.3% of the theoretical is the maximum amount extractable by a wind energy conversion device (WEC) www.assignmentpoint.com

14. Wind Turbine Size-Power Comparison www.assignmentpoint.com

15. Wind Turbine Configurations HAWT VAWT Boyle, G., Renewable Energy, 2nd ed., Oxford University Press, 2004 www.assignmentpoint.com

16. 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? www.assignmentpoint.com

17. 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 www.assignmentpoint.com

18. 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 www.assignmentpoint.com

19. Two Types of Turbine Designs • Drag Designs • Savonius • Lift Designs • VAWT Darrieus • Most HAWT designs http://upload.wikimedia.org/wikipedia/commons/9/99/Savonius_Querschnitt.png http://upload.wikimedia.org/wikipedia/commons/9/9e/Darrieus.jpg www.assignmentpoint.com

20. Aerodynamics of HAWT Blade r[L(sinf) - D(cosf)] = Torque Torque x rotational speed= Power Boyle, G., Renewable Energy, 2nd ed., Oxford University Press, 2004 www.assignmentpoint.com

21. 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?) www.assignmentpoint.com

22. Design of HAWT Turbine Blade Blade size and shape 5-station design as seen from the tip The blade twists to keep angle of attack constant www.assignmentpoint.com

23. 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 www.assignmentpoint.com