Harvesting the Wind

1. Harvesting the Wind the physics of wind turbines Kira Grogg Carleton College February 23, 2005

2. Why Wind-power? • Wind-power is clean – wind turbines emits no pollutants and create no other types of waste • Wind is renewable – the fuel replenishes itself at a rate comparable to the extracted rate • Modern wind turbines can run as efficiently as conventional power plants (~30-40%)

3. What is a wind turbine? http://www.canren.gc.ca/tech_appl/index.asp?CaID=6&PgID=219

4. History Origins of wind Power from the wind Aerodynamics of the blades Loads, stress, and fatigue Generators and Electricity Current issues Future developments Outline

5. History • First windmills about 1000 B.C.E. • 12th century – horizontal wind mills appear in Europe • Water pumping windmills in America • 1888 – first experiments with electricity generating wind turbines • 1891-1918 – Poul La Cour builds over 100 small wind turbines • Increase in production in 1970s due to oil difficulties

6. The Wind • Earth receives 1.74 x 1017 watts from the sun • Each year this is 160 times the total energy in the world’s reserves of fossil fuels. • 30% radiated out, 47% warming, 23% absorbed by evaporation of water, the remaining goes to plants, wind, and waves • 1-2% of the sun’s energy becomes wind energy—100 times the energy in biomass

7. Forming the Wind • Wind begins as the sun heats the air in the atmosphere • Uneven heating combined with the Coriolis force lead to geostrophic winds

8. Finding a Site • Landscape • Roughness • 10-4 over water to 1 m in cities • Height vs. wind speed: • Sea breezes • Mountains desired height roughness length known velocity at height zref reference height

9. Wind Speed Distribution

10. Power in the Wind • The power is proportional to the cube of the wind speed: • Wind speed data can be misleading : • Average wind speed data is too: < U>3 ≠ <U3>

11. Extracting Power from the Wind • Not all of the power in the wind can be turned into useable energy • The upper limit on power extracted for a HAWT is ~ 59% (Betz’ Law) Cp = power to rotor / power in wind

12. Power Curves • No system that converts between types of energy can be 100% efficient • Actual power output (of electricity) is about 30% of the power in the wind

13. Wake Rotation • Energy and momentum must be conserved • Angular velocity added from the turning of the blades, Ω, implies acompensating angular velocity in the wake of the turbine, ω www.windpower.org

14. Tip Speed Ratio λ • λ= ratio of rotor speed to wind speed • The tip speed ratio can range from 5 to about 10 for electricity generating applications • Equating the thrust equations: • Tip speed ratio: λ = ΩR/U where R is the length of the blade

15. Torque and Power Cp Manwell, et. al (2002)

16. Induction Factors Tip speed ratio λ= 7.5 Manwell, et. al (2002)

17. The Blades • Lift vs. Drag Lift Thrust Drag Weight Early Persian drag windmill Manwell, et. al (2002)

18. Airfoils • Types of airfoils: • Airfoil geometry: Manwell, et. al (2002)

19. Angles and Relative Winds • Angle of attack, α, is usually between 3 - 10 degrees during normal operation The angle of the relative wind is the sum of the angle of attack and the section pitch angle: Hansen (2000)

20. Manwell, et. al (2002)

21. Lift and Drag • Lift and drag coefficients: • Wind tunnel tests of airfoils for lift and drag data Manwell, et. al (2002)

22. Blade Element Momentum Theory (BEM) • BEM uses conservation of momentum and forces on individual elements

23. Maximizing Power • Computational algorithms are employed to determine the most effective blade shape, in terms of chord length and twist (λ = 7, R = 5, Cl=1, α = 70, B =3) • A new power coefficient with lift and drag:

24. Cp -λ Curve Manwell, et. al (2002)

25. Blade Control • Stall Control • Pitch Control • Active Stall Manwell, et. al (2002)

26. Yaw Control • Wind Rose data • No yaw – only VAWTs • Tail – water pumping windmills • Free/Damped yaw – only downwind HAWTs • Active yaw – upwind HAWTs • Red section is the power x frequency, • Middle section is the wind speed x frequency • Outer section is the wind frequency distribution

27. Loads, Stress, and Fatigue • Types of loads: static, steady, cyclic, transient, impulsive, stochastic, and resonance induced • Certain loads will occur over 109 times during a 20 year lifetime • Testing for fatigue – dynamic and static www.windpower.org

28. Some Loads • Bending Moments: • Flapwise • Edgewise • Coning to reduce flapwise bending: Hansen (2000)

29. Blade Construction • Metal does not work, composites do • Glass reinforced plastic (GRP) • Carbon fibers • Wooded frames • Measuring strains

30. The rotor is about 80m across: • This is what excessive loads can do to a WT Hansen (2000)

31. Inside the Nacelle

32. Gearbox • As the speed increases, torque decreases, so power remains constant • The ratio of the speeds is equal to the inverse ratio of the number of teeth • The gear ratio of Carleton’s 1.65 MW wind turbine is 1:84.3, so that when the rotor is operating at its rated speed of 14.4 rpm, the generator shaft is turning at about 1214 rpm

33. Gears vs. Direct Connection • Advantages of gearless connection: • Cheaper • Quieter • Fewer losses • Disadvantages: • Special low-speed generator is costly

34. Converting Energy • Electrical to kinetic conversion in an electrical motor is about 90% efficient, • Heat to kinetic conversion of an internal combustion engine is about 10-20% efficient • Coal fired power station—chemical to electrical—is about 35-40% efficient • Newer wind turbines are 30-40% efficient

35. Creating Electricity • Wind turbines use generators to create electricity • Synchronous generators • Asynchronous (induction) generators • Permanent magnet • Compatibility with The Grid

36. Faraday’s Law of Induction • Use a moving part and a magnetic field to create a current • Magnetic flux through a loop of wire in a magnetic field: • Induced emf: Magnetic field Angle between A and B Area of loop Rotational velocity of loop ω = θt

37. Magnetic fields and currents • Three phase current • Coil connections • Number of poles N Manwell, et. al (2002) S S N N S

38. Asynchronous/Induction Generators • Usually 4-pole • Rotor uses converted DC from grid to generate a magnetic field • Stator consists of six coils creating 3-phase current • Slip—ratio of the rotating magnetic field speed to the rotor speed

39. The Parts of the Generator • Cage wound rotor • Layered stator

40. Generator Manwell, et. al (2002)

41. Electronics • Every part of the turbine has a sensor monitoring and/or controlling it • Every sensor has a duplicate to make sure they are working correctly

42. Power Quality and Grid Connection • Power quality is checked 7680 times per second • The current must be in phase with the grid current before connection • Thyristors (semi-conducting transistor type controlling devices) allow a ‘soft’ start of a wind turbine

43. Current Issues • Bird and bat deaths • Electromagnetic interference • Noise • Visual appearance

44. Off-shore Wind Farms • Off shore winds are stronger and less turbulent • Larger turbines, ~4 MW, are feasible • Foundations and transmission are the major obstacles

45. What Now? • Optimization of • Blade shape • Gearbox • Generator • Tower height • Siting • More testing • More wind turbines

46. Acknowledgements • To: • my advisor, Steve Parker • the Carleton physics faculty • my fellow physics majors • my friends and family • the audience