Wind turbine aerodynamics, continued (Part 4/4)

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Wind turbine aerodynamics, continued (Part 4/4) Ene-47.

1 Wind turbine aerodynamics, continued (Part 4/4) Ene Wind Energy Ville Lehtomäki, VTT Wind 2 Content Recap: lift & drag and their coefficients Blade & rotor terminology Rotor aerodynamics: BEM-method Blade and rotor design Different power control concepts Summary 1

2 3 Content Recap: lift & drag and their coefficients Blade & rotor terminology Rotor aerodynamics: BEM-method Blade and rotor design Different power control concepts Summary 4 Recap: lift & drag 2

3 5 Drag coefficients of different cross sections 6 Max Cp for a drag turbine λ=tip speed ratio 3

4 7 Max Cp for a drag turbine Betz limit far away 8 Content Recap: lift & drag and their coefficients Blade & rotor terminology Rotor aerodynamics: BEM-method Blade and rotor design Different power control concepts Summary 4

5 9 The blade & important terminology Trailing edge Span Leading edge Chord Pic:

6 11 Aerodynamic profiles What keeps the airplanes flying? And why do wind turbine blades rotate? 12 U, Pressure decreases Profile Pressure increases Pressure coefficient p p 1 2 U 2 C pres 6

7 Kerroin [-] 13 angle of attack twist rotational speed U wind speed U (1-a) r(1+a ) V rel =- Aerodynamic forces -Lift and drag forces- L F T F N 90- D Plane of rotation 1 V cc rel L 2 1 V cc rel D 2 L 2 D 2 Lift to drag ratio C L / C D F N F T Lcos Dsin L sin Dcos V rel The flow, that the blade segment feels, is a combination of the axial (wind) and tangential (rotor rotation) flows! 14 Aerodynamic profiles Angle of attack (AoA) effect on lift and drag coefficiets NACA64XXX Nostovoimakerroin Cl Vastusvoimakerroin Cd Kohtauskulma α [ ] 7

8 15 Aerodynamic profiles 16 Aerodynamic profiles Cl coefficients of different profiles WT operating range r/r= Definition of a 5-MW Reference Wind Turbine of Offshore System Development, NREL,

9 Cl/Cd [-] Liitoluku Cl/Cd max max Paksu Ohut Kohtauskulma[ ] Liitoluku = the ability of an object to glid the Cl/Cd unit amount forward and 1 unit down towards the ground Typical glider airplane Cl/Cd = Content Recap: lift & drag and their coefficients Blade & rotor terminology Rotor aerodynamics: BEM-method Blade and rotor design Different power control concepts Summary 9

10 19 What is BEM? U (1-a) r(1+a ) V rel =- F N L = BEM D P M r M m2ra' r A U Momentum theory d (1 a)2a' r 2 F T Blade Element theory 20 BEM as a tool for WT blade design A simple way to design a WT rotor Blade geometry included: Number of blades Twist distribution Chord distribution Assumptions: Only 2D flow effects (no flow between elements) Constant flow in each annular tube Blade tip and root losses with empirical formula A B C Eric Hau, Wind Turbines Fundamentals, Technologies, Applications, Economics 10

11 21 Content Recap: lift & drag and their coefficients Blade & rotor terminology Rotor aerodynamics: BEM-method Blade and rotor design Different power control concepts Summary 22 Blade shape in > Previously (1980->) a lot of aviation blade profiles were used Today, most profiles designed from wind energy point of view (robust against dust, structural stiffness at root) WT blade is not flat-straight but with a thick root and narrow tip cross section and the entire blade is twisted along the longitudal axis ( corkscrew ) Today, Cp numbers are higher on a broder scale (no stall turbines) 11

12 23 Chord distribution, lenght Basic info on blade geometry Blade twist and profile distribution Relative wind speed [m/s] Pics: Delft University 24 The blade profile feels the relative wind speed differently acc. to spanwise location 12

13 25 An optimal AoA can be assigned to different blade segments -> blade twist! 26 Assumed Max Cl/Cd at AoA =

14 Teho [kw] 27 Content Recap: lift & drag and their coefficients Blade & rotor terminology Rotor aerodynamics: BEM-method Blade and rotor design Different power control concepts Summary 28 P v 3 Modern WT and aerodynamic power control -Why power control/limitation?- E.g. if wind increases 20% -> theoretical power increases 50% 5000 REpower 3.4MW REpower 3.4MW/104m Tuulennopeus [m/s] 14

15 29 Modern WT and aerodynamic power control -Consept 1- Passive stall Blades rigidly attached to hub Passive Constant tip speed + simple asynchronous generator Power control: hi wind AoA increase flow stalls Thick profiles, soft stall characteristics Popular between s r = radius [m] ω = rotational speed [1/s] = wind speed [m/s] = relative wind [m/s] = pitch angle Eric Hau, Wind Turbines Fundamentals, Technologies, Applications, Economics 30 Modern WT and aerodynamic power control -Consept 2- Active stall Blade pitch angle adjusted to stall Nose up i.e increasing α Why was it done? Tighter grid codes A lot of gearbox failures 2 different concepts: Option A: rpm control -> Enercon E- 16/55kW in 1980 Option B: pitch-to-stall -> Aerodyn GmbH 600kW A B Eric Hau, Wind Turbines Fundamentals, Technologies, Applications, Economics 15

16 Teho [kw] 31 Modern WT and aerodynamic power control -Consept 3- Blade pitch control (pitch-to-feather) Wind increases, blade pitch nose down decrease lift force! Avoiding stall Pitch control only at hi winds Works as a power and safety control unit REpower 3.4MW Gen. tq control Pitch control REpower 3.4MW/104m Tuulennopeus [m/s] Eric Hau, Wind Turbines Fundamentals, Technologies, Applications, Economics 32 Modern WT and aerodynamic power control -Comparing different concepts- Passive stall + Simple and cheap - Large loads need heavy/stiff construct - Blade design hard (e.g. tip brake), nonconstant power at hi winds Not used any more Active stall + Better power control than passive stall + Small pitch settings enough - Very complex phenomenon to master - Noise at hi winds Rarely used Pitch control + Optimal AoA + No stall, smaller loads - Complicated, expensive pitch mechanism Most common and used concept THE WINNER! 16

17 33 Summary Betz limit 59 %, remember, understand With BEM, wind turbine performance, loads and even blade design is quite accurate, fast and simple! Pitch-to-feather: the winner in power control/limitation by decreasing lift and avoiding stall! 34 VTT luo teknologiasta liiketoimintaa 17

EE 742 Chap. 7: Wind Power Generation. Y. Baghzouz

EE 742 Chap. 7: Wind Power Generation Y. Baghzouz Wind Energy 101: See Video Link Below http://energy.gov/eere/videos/energy-101- wind-turbines-2014-update Wind Power Inland and Offshore Growth in Wind