ECE 333 Green Electric Energy

1. ECE 333 Green Electric Energy Lecture 20 Intro to Wind Energy, continued… Professor Tim O’Connell Department of Electrical andComputer Engineering

2. Top 10 Countries - Installed Wind Capacity (as of the end of 2011) Global Wind Energy Council http://www.gwec.net/fileadmin/images/News/Press/GWEC_-_Global_Wind_Statistics_2011.pdf

3. Top 10 Countries - Installed Wind Capacity (as of the end of 2011) Global Wind Energy Council http://www.gwec.net/fileadmin/images/News/Press/GWEC_-_Global_Wind_Statistics_2011.pdf

4. US Wind Resources 50 meters http://www.windpoweringamerica.gov/pdfs/wind_maps/us_windmap.pdf

5. US Wind Resources 80 meters http://www.windpoweringamerica.gov/pdfs/wind_maps/us_windmap_80meters.pdf

6. Worldwide Wind Resource Map Source: www.ceoe.udel.edu/WindPower/ResourceMap/index-world.html

7. Types of Wind Turbines “Windmill”- used to grind grain into flour Many different names - “wind-driven generator”, “wind generator”, “wind turbine”, “wind-turbine generator (WTG)”, “wind energy conversion system (WECS)” One way to characterize wind turbines is in terms of the axis around which the turbine blades rotate Horizontal axis wind turbines (HAWT) Vertical axis wind turbines (VAWT) Groups of wind turbines are located in what is called either a “wind farm” or a “wind park”

8. Vertical Axis Wind Turbines Darrieus rotor - the only vertical axis machine with any commercial success Wind hitting the vertical blades, called airfoils, generates lift to create rotation No yaw (rotation about vertical axis) control needed to keep them facing into the wind Heavy machinery in the nacelle is located on the ground Blades are closer to ground where wind speeds are lower

9. Horizontal Axis Wind Turbines (HAWTs) “Downwind” HAWT – a turbine with the blades behind (downwind from) the tower No yaw control needed- they naturally orient themselves in line with the wind Shadowing effect – when a blade swings behind the tower, the wind it encounters is briefly reduced and the blade flexes

10. Horizontal Axis Wind Turbines (HAWTs) “Upwind” HAWT – blades are in front of (upwind of) the tower Almost all modern wind turbines are this type Blades are “upwind” of the tower Require somewhat complex yaw control to keep them facing into the wind Operate more smoothly and deliver more power (no shadowing)

11. Wind Energy Conversion System (WECS) Components

12. Wind Energy Conversion System (WECS) Components Vestas V52-850 kW

13. Number of Rotating Blades Windmills have multiple blades need to provide high starting torque to overcome weight of the pumping rod must be able to operate at low wind speeds to provide nearly continuous water pumping a larger area of the rotor faces the wind Turbines with many blades operate at much lower rotational speeds - as the speed increases, the turbulence caused by one blade impacts the other blades Almost all modern wind turbines have three blades now

14. Some Aerodynamics • We need to control our blades to vary their speed • Blades are like airplane wings (they are airfoils) • Airfoils use Bernoulli’s Principle to create lift Faster air on top www.energyeducation.tx.gov http://science.howstuffworks.com/environmental/green-science/wind-power3.htm

15. Some Aerodynamics • Wind turbine blades are carefully engineered devices softsolder.com cr4.globalspec.com

16. Some Aerodynamics A modern turbine blade

17. Some Aerodynamics • Wind turbine blades have one added complication over airplane wings: they create their own relative wind as they rotate • Blade is moving faster at its tip than at its hub, so the resulting wind is different along the blade • Blade is twisted along its axis to keep the angles right Fig. 7.7

18. Some Aerodynamics • Angle of Attack (AoA) is constantly adjusted to achieve the optimal efficiency or desired power output • Increasing AoA increases lift and drag, but eventually will cause the airfoil to stall (no more lift) Relative

19. Turbine Speed Control Methods • Passive stall-control • No moving parts • Blades carefully designed  They twist along their length to gradually reduce lift as wind speed increases • Simple and reliable • Sacrifices power at lower wind speeds • Used mostly on turbines below 1 MW in size

20. Turbine Speed Control Methods • Active pitch-control • Blade pitch is adjusted to shed wind as wind speed increases • AoA is reduced when winds are high • Pitch controlled with hydraulic actuation system • Used on most large turbines

21. Turbine Speed Control Methods • Active stall-control • Same as active pitch-control under normal wind speeds • But, when wind speed exceeds the turbine’s rated value, AoA is increased to induce stall

22. Induction Machines Large pump induction motor at LyondellBasell plant in Tuscola

23. Squirrel Cage Rotor The rotor of many induction generators has copper or aluminum bars shorted together at the ends, looks like a cage • Induction machine can be thought of as a pair of magnets spinning around a cage • Rotor current iR flows easily through the thick conductor bars Figure 7.10

24. Squirrel Cage Rotor • Instead of thinking of a rotating stator field, you can think of a stationary stator field and the rotor moving counterclockwise • Faraday’s Law induces a voltage in the rotor bar • Bar is shorted at ends, so current flows easily • The bar (conductor) experiences a clockwise force Figure 7.11

25. The Induction Machine as a Motor The rotating magnetic field in the stator causes the rotor to spin in the same direction (stator “pulls” the rotor along) As rotor approaches synchronous speed of the rotating magnetic field, the relative motion becomes less and less If the rotor could move at synchronous speed, there would be no relative motion, no current, and no force to keep the rotor going Thus, an induction machine as a motor always spins somewhat slower than synchronous speed

26. Slip The difference in speed between the stator and the rotor is called slip: Eqn 7.2 • s = rotor slip – positive for a motor, negative for a generator • NS = synchronous speed (rpm) • f = frequency (Hz) • p = number of poles • NR = rotor speed (rpm)

27. The Induction Machine as a Generator The stator requires excitation current from the grid if it is grid-connected or by incorporating external capacitors Wind speed forces generator shaft to exceed synchronous speed Slip is negative because the rotor spins faster than synchronous speed Slip is normally less than 0.01 for a grid-connected generator Ex: Typical rotor speed, 2-pole machine

28. Doubly-Fed Induction Generators (DFIGs) • A common design is a doubly-fed induction generator (DFIG) in which there is an electrical connection between the rotor and the grid through an ac-ac converter • No Squirrel-cage; uses a wound rotor • This allows operation over a wide-range of rotor speeds • 40% below to 20% above synchronous speed • Can control P and Q flows to grid independently of the DFIG speed Converter rated at about 30% of turbine’s rated power AC/AC Converter

29. Doubly-Fed Induction Generator Grid-synchronized ac currents Grid-synchronized ac currents Variable voltage and frequency ac currents Key feature

30. Variable-Speed Synchronous Generator • Operates over a wide speed range • Two main types: • Wound-rotor • Permanent magnet rotor PMSG • No slip rings or brushes needed • PMSG with many magnetic poles can operate with no gearbox Recall…

31. Variable-Speed Synchronous Generator • The PMSG is becoming the dominant design for offshore wind • When there is no gearbox, it is sometimes called a “direct drive system”, because the turbine is directly driven by the shaft connected to the blades

32. Variable-Speed Synchronous Generator Variable voltage and frequency ac currents Grid-synchronized ac currents Each converter rated at 100% of turbine’s rated power