1. POLI di MI tecnicolanotecnicolanotecnicolano INTEGRATED ACTIVE AND PASSIVE LOAD CONTROL IN WIND TURBINES C.L. Bottasso, F. Campagnolo, A. Croce, C. Tibaldi Politecnico di Milano, Italy SAICA 2011 7-8 November 2011, Barcelona, Spain

2. Integrated Active/Passive Load Control POLITECNICO di MILANO POLI-Wind Research Lab Need for Load Mitigation Trends in wind energy Trends in wind energy: Increasing wind turbine size ▶ Off-shore wind ▼ To decrease cost of energy To decrease cost of energy: Reduce extreme loads Reduce fatigue damage Limit actuator duty cycle Ensure high reliability/availability

3. Integrated Active/Passive Load Control POLITECNICO di MILANO POLI-Wind Research Lab Presentation Outline

4. Integrated Active/Passive Load Control POLITECNICO di MILANO POLI-Wind Research Lab Active Load Mitigation: Pitch Control IndividualPitch Control (IPC) Individual blade Pitch Control (IPC) Inner loop Inner loop (collective pitch): regulation to set point and alleviation of gust loads Outer loopsreduction of Outer loops (individual pitch): reduction of -Deterministic (periodic) loads weight non-uniform inflow -Deterministic (periodic) loads due to blade weight and non-uniform inflow -Non-deterministic loads -Non-deterministic loads, caused by fast temporal and small spatial turbulent wind fluctuations ▼ Uniform wind ▼ Turbulent wind

5. Integrated Active/Passive Load Control POLITECNICO di MILANO POLI-Wind Research Lab Active Load Mitigation: Predictive LiDAR-Enabled Pitch Control Active Load Mitigation: Predictive LiDAR-Enabled Pitch Control LiDAR LiDAR: generic model, captures realistically wind filtering due to volumetric averaging Receding Horizon Control Receding Horizon Control: model predictive formulation with wind scheduled linear model, real-time implementation based on CVXGEN Non-Homogeneous LQR Control Non-Homogeneous LQR Control: approximation of RHC, extremely low computational cost LiDAR prediction span Reduced peak values Reduced peak values Reduced peak values Reduced peak values Reduced peak to peak oscillations

6. Integrated Active/Passive Load Control POLITECNICO di MILANO POLI-Wind Research Lab Flow control devices Flow control devices: TE flaps Microtabs Vortex generators Active jets (plasma, synthetic) Morphing airfoils … Active Load Mitigation: Distributed Control (Chow and van Dam 2007) (Credits: Smart Blade GmbH) (Credits: Risoe DTU)

7. Integrated Active/Passive Load Control POLITECNICO di MILANO POLI-Wind Research Lab Pitch control Pitch control: Limited temporal bandwidth (max pitch rate ≈ 7-9 deg/sec) Limited spatial bandwidth (pitching the whole blade is ineffective for spatially small wind fluctuations) Distributed control Distributed control: Alleviate temporal and spatial bandwidth issues Complexity/availability/maintenance Sensor-enabled control solutions Sensor-enabled control solutions: Complexity/availability/maintenance Active Load Mitigation: Limits and Issues Off-shore Off-shore: need to prove reliability, availability, low maintenance in hostile environments

8. Integrated Active/Passive Load Control POLITECNICO di MILANO POLI-Wind Research Lab Presentation Outline

9. Integrated Active/Passive Load Control POLITECNICO di MILANO POLI-Wind Research Lab Passive control Passive control: loaded structure deforms so as to reduce load Two main solutions Two main solutions: Potential advantages Potential advantages: no actuators, no moving parts, no sensors Other passive control technologies (not discussed here): -Tuned masses (e.g. on off-shore wind turbines to damp nacelle-tower motions) -Passive flaps/tabs -… Passive Load Mitigation Angle fibers in skin and/or spar caps -Bend-twist coupling -Bend-twist coupling (BTC): exploit anisotropy of composite materials - Sweptblades - Swept (scimitar) blades

10. Integrated Active/Passive Load Control POLITECNICO di MILANO POLI-Wind Research Lab Present study Present study: Designidentical design requirements Design BTC blades (all satisfying identical design requirements: max tip deflection, flap freq., stress/strain, fatigue, buckling) trade-offs Consider trade-offs (load reduction/weight increase/complexity) configuration Identify optimal BTC blade configuration Integrate Integrate passive BTC and active IPC synergies Exploit synergies between passive and active load control 45m Class IIIA 2MW HAWT Baseline uncoupled blade: 45m Class IIIA 2MW HAWT Objectives

11. Integrated Active/Passive Load Control POLITECNICO di MILANO POLI-Wind Research Lab Presentation Outline

12. Integrated Active/Passive Load Control POLITECNICO di MILANO POLI-Wind Research Lab Optimization-Based Multi-Level Blade Design Cost: AEP Aerodynamic parameters: chord, twist, airfoils Cost: AEP Aerodynamic parameters: chord, twist, airfoils Cost: Blade weight (or cost model if available) Structural parameters: thickness of shell and spar caps, width and location of shear webs Cost: Blade weight (or cost model if available) Structural parameters: thickness of shell and spar caps, width and location of shear webs Cost: AEP/weigh (or cost model if available) Macro parameters: rotor radius, max chord, tapering, … Cost: AEP/weigh (or cost model if available) Macro parameters: rotor radius, max chord, tapering, … Controls: model-based (self- adjusting to changing design) Controls: model-based (self- adjusting to changing design)

13. Integrated Active/Passive Load Control POLITECNICO di MILANO POLI-Wind Research Lab Preliminary design Design refinement Fully automated links

14. Integrated Active/Passive Load Control POLITECNICO di MILANO POLI-Wind Research Lab “Fine” level: 3D FEM “Coarse” level: 2D FEM section & beam models - Definition of complete HAWT Cp-Lambda multibody model - DLCs simulation - Campbell diagram DLC post-processing: load envelope, DELs, Markov, max tip deflection - Definition of complete HAWT Cp-Lambda multibody model - DLCs simulation - Campbell diagram DLC post-processing: load envelope, DELs, Markov, max tip deflection - Definition of geometrically exact beam model - Span-wise interpolation - Definition of geometrically exact beam model - Span-wise interpolation - ANBA 2D FEM sectional analysis - Computation of 6x6 stiffness matrices - ANBA 2D FEM sectional analysis - Computation of 6x6 stiffness matrices Definition of sectional design parameters Constraints: - Maximum tip deflection - 2D FEM ANBA analysis of maximum stresses/strains - 2D FEM ANBA fatigue analysis - Compute cost (mass) Constraints: - Maximum tip deflection - 2D FEM ANBA analysis of maximum stresses/strains - 2D FEM ANBA fatigue analysis - Compute cost (mass) Automatic 3D CAD model generation by lofting of sectional geometry Automatic 3D FEM meshing (shells and/or solid elements) Update of blade mass (cost) Automatic 3D FEM meshing (shells and/or solid elements) Update of blade mass (cost) Analyses: - Max tip deflection - Max stress/strain - Fatigue - Buckling Verification of design constraints Analyses: - Max tip deflection - Max stress/strain - Fatigue - Buckling Verification of design constraints SQP optimizer min cost s.t. constraints Constraint/model update heuristic (to repair constraint violations) When SQP converged

15. Integrated Active/Passive Load Control POLITECNICO di MILANO POLI-Wind Research Lab 2MW 45m Wind Turbine Blade Currently undergoing certification at TÜV SÜD CNC machined model ofaluminum alloy for visualinspection of blade shape Design developed in partnership with Gurit (UK)

16. Integrated Active/Passive Load Control POLITECNICO di MILANO POLI-Wind Research Lab Presentation Outline

17. Integrated Active/Passive Load Control POLITECNICO di MILANO POLI-Wind Research Lab Fully Coupled Blades Spar-cap angle Skin angle

18. Integrated Active/Passive Load Control POLITECNICO di MILANO POLI-Wind Research Lab Fully Coupled Blades: Effects on Weight Spar-caps: steep increase Skin: milder increase Spar-cap/skin synergy Stiffness driven design Stiffness driven design (flap freq. and max tip deflection constraints): increasing spar/skin thickness Need to restore stiffness by increasing spar/skin thickness

19. Integrated Active/Passive Load Control POLITECNICO di MILANO POLI-Wind Research Lab Fully Coupled Blades: Load Reduction Spar-cap/skin synergy: good load reduction with small mass increase

20. Integrated Active/Passive Load Control POLITECNICO di MILANO POLI-Wind Research Lab Fully Coupled Blades: Mechanism of Load Reduction

21. Integrated Active/Passive Load Control POLITECNICO di MILANO POLI-Wind Research Lab Fully Coupled Blades: Effects on Duty Cycle Less pitching ◀ Less pitching from active control because blade passively self- unloads Much reduced life-time ADC ▶

22. Integrated Active/Passive Load Control POLITECNICO di MILANO POLI-Wind Research Lab Partially Coupled Blades 2.Identify optimal span-wise fiber rotation 2.Identify optimal span-wise fiber rotation: 5 candidate configurations Reduce fatigue in max chord region Avoid thickness increase to satisfy stiffness-driven constraints

23. Integrated Active/Passive Load Control POLITECNICO di MILANO POLI-Wind Research Lab Partially Coupled Blades: Effects on Mass Too little coupling Fully coupled blade

24. Integrated Active/Passive Load Control POLITECNICO di MILANO POLI-Wind Research Lab Partially Coupled Blades: Effects on Loads F30 : load reduction close to fully coupled case

25. Integrated Active/Passive Load Control POLITECNICO di MILANO POLI-Wind Research Lab Partially Coupled Blades: Effects on Duty Cycle Best compromise Best compromise: similar load and ADC reduction as fully coupled blade, decreased mass

26. Integrated Active/Passive Load Control POLITECNICO di MILANO POLI-Wind Research Lab Presentation Outline

27. Integrated Active/Passive Load Control POLITECNICO di MILANO POLI-Wind Research Lab Integrated Passive and Active Load Alleviation Individual blade pitch Individual blade pitch controller (Bossaniy 2003): Coleman transform blade root loads PID control for transformed d-q loads Back-Coleman-transform to get pitch inputs Baseline controller: MIMO LQR Baseline controller: MIMO LQR

28. Integrated Active/Passive Load Control POLITECNICO di MILANO POLI-Wind Research Lab Integrated Passive and Active Load Alleviation Two IPC gain settings: 1.Mild 1.Mild: some load reduction, limited ADC increase 2.Aggressive 2.Aggressive: more load reduction, more ADC increase Five blade/controller combinations: BTC BTC: best coupled blade + collective LQR IPC1 IPC1: uncoupled blade + mild IPC BTC+IPC1 BTC+IPC1: best coupled blade + mild IPC IPC2 IPC2: uncoupled blade + aggressive IPC BTC+IPC2 BTC+IPC2: best coupled blade + aggressive IPC

29. Integrated Active/Passive Load Control POLITECNICO di MILANO POLI-Wind Research Lab Integrated Passive/Active Control: Effects on Loads ▲ Synergistic effects of combined passive and active control ▶

30. Integrated Active/Passive Load Control POLITECNICO di MILANO POLI-Wind Research Lab Integrated Passive/Active Control: Effects on Duty Cycle Not significant: ADC very small here Same ADC as baseline (but great load reduction!)

31. Integrated Active/Passive Load Control POLITECNICO di MILANO POLI-Wind Research Lab Optimization-based blade design tools: Optimization-based blade design tools: enable automated design of blades and satisfaction of all desired design requirements BTC passive load control BTC passive load control: -Skin fiber rotation helps limiting spar-cap fiber angle -Partial span-wise coupling limits fatigue and stiffness effects Reduction for all quality metrics: loads, ADC, weight Combined BTC/IPC passive/active control Combined BTC/IPC passive/active control: -Synergistic effects on load reduction -BTC helps limiting ADC increase due to IPC (e.g., could have same ADC as baseline blade with collective pitch control) Outlook Outlook: Manufacturing implications of BTC and partially coupled blades Passive distributed control and integration with blade design and active IPC control Conclusions