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INTEGRATED ACTIVE AND PASSIVE LOAD MITIGATION IN WIND TURBINES POLItecnico di MIlano C.L. Bottasso, F. Campagnolo, A. Croce, C. Tibaldi Politecnico di Milano, Italy Wind Turbine Control Symposium 28-29 November 2011, Ålborg, Denmark Integrated Active/Passive Load Control Need for Load Mitigation Trends in wind energy: Increasing wind turbine size ▶ Off-shore wind ▼ To • • • • POLITECNICO di MILANO decrease cost of energy: Reduce extreme loads Reduce fatigue damage Limit actuator duty cycle Ensure high reliability/availability POLI-Wind Research Lab Integrated Active/Passive Load Control Presentation Outline POLITECNICO di MILANO POLI-Wind Research Lab Integrated Active/Passive Load Control Active Load Mitigation: Pitch Control Individual blade Pitch Control (IPC) Inner loop (collective pitch): regulation to set point and alleviation of gust loads Outer loops (individual pitch): reduction of - Deterministic (periodic) loads due to blade weight and non-uniform inflow - Non-deterministic loads, caused by fast temporal and small spatial turbulent wind fluctuations ▼ Uniform wind POLITECNICO di MILANO ▼ Turbulent wind POLI-Wind Research Lab Integrated Active/Passive Load Control Active Load Mitigation: Predictive LiDAR-Enabled Pitch Control LiDAR: generic model, captures realistically wind filtering due to volumetric averaging Receding Horizon Control: model predictive formulation with wind scheduled linear model, real-time implementation based on CVXGEN Non-Homogeneous LQR Control: approximation of RHC, extremely low computational cost LiDAR prediction span Reduced peak values Reduced peak values Reduced peak to peak oscillations POLITECNICO di MILANO POLI-Wind Research Lab Integrated Active/Passive Load Control Active Load Mitigation: Distributed Control Flow control devices: • TE flaps • Microtabs • Vortex generators • Active jets (plasma, synthetic) • Morphing airfoils • … (Credits: Risoe DTU) (Credits: Risoe DTU) (Chow and van Dam 2007) (Credits: Smart Blade GmbH) POLITECNICO di MILANO POLI-Wind Research Lab Integrated Active/Passive Load Control Active Load Mitigation: Limits and Issues 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: • Alleviate temporal and spatial bandwidth issues • Complexity/availability/maintenance All sensor-enabled control solutions: • Complexity/availability/maintenance Off-shore: need to prove reliability, availability, low maintenance in harsh hostile environments POLITECNICO di MILANO POLI-Wind Research Lab Integrated Active/Passive Load Control Presentation Outline POLITECNICO di MILANO POLI-Wind Research Lab Integrated Active/Passive Load Control Passive Load Mitigation Passive control: loaded structure deforms so as to reduce load Two main solutions: - Bend-twist coupling (BTC): exploit anisotropy of composite materials - Swept (scimitar) blades Angle fibers in skin and/or spar caps Potential advantages: no actuators, no moving parts, no sensors (if you do not have them, you cannot break them!) Other passive control technologies (not discussed here): - Tuned masses (e.g. on off-shore wind turbines to damp nacelle-tower motions) Passive flaps/tabs … POLITECNICO di MILANO POLI-Wind Research Lab Integrated Active/Passive Load Control Objectives Present study: • Design BTC blades (all satisfying identical design requirements: max tip deflection, flap freq., stress/strain, fatigue, buckling) • Consider trade-offs (load reduction/weight increase/complexity) • Identify optimal BTC blade configuration • Integrate passive BTC and active IPC • Exploit synergies between passive and active load control Baseline uncoupled blade: 45m Class IIIA 2MW HAWT POLITECNICO di MILANO POLI-Wind Research Lab Integrated Active/Passive Load Control Presentation Outline POLITECNICO di MILANO POLI-Wind Research Lab Integrated Active/Passive Load Control Optimization-Based Multi-Level Blade Design Cost: AEP Aerodynamic parameters: chord, twist, airfoils Cost: AEP/weigh (or cost model if available) Macro parameters: rotor radius, max chord, tapering, … Cost: Blade weight (or cost model if available) Structural parameters: thickness of shell and spar caps, width and location of shear webs Controls: model-based (selfadjusting to changing design) POLITECNICO di MILANO POLI-Wind Research Lab min cost s.t. constraints When SQP converged - Definition of geometrically exact beam model - Span-wise interpolation Constraints: - Maximum tip deflection - 2D FEM ANBA analysis of maximum stresses/strains - 2D FEM ANBA fatigue analysis - Definition of complete HAWT Cp-Lambda multibody model - DLCs simulation - Campbell diagram - 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) DLC post-processing: load envelope, DELs, Markov, max tip deflection Analyses: - Max tip deflection - Max stress/strain - Fatigue - Buckling Verification of design constraints POLITECNICO di MILANO POLI-Wind Research Lab Constraint/model update heuristic (to repair constraint violations) “Coarse” level: 2D FEM section & beam models SQP optimizer - ANBA 2D FEM sectional analysis - Computation of 6x6 stiffness matrices “Fine” level: 3D FEM Integrated Active/Passive Load Control Definition of sectional design parameters Stress/strain/fatigue: Peak stress on initial model ITERATION 1 ITERATION 0 Fatigue damage index Buckling: Buckling constraint not satisfied at first iteration Update skin core thickness Update trailing edge reinforcement strip Converged at 2nd iteration Fatigue damage constraint satisfied ITERATION 1 ITERATION 0 POLITECNICO di MILANO Increased skin core thickness Increased trailing edge strip Trailing edge strip thickness - ITERATION 1 ITERATION 0 Normalized stress - Fatigue constraint not satisfied at first iteration on 3D FEM model - Modify constraint based on 3D FEM analysis - Converged at 2nd iteration Thickness Integrated Active/Passive Load Control The Importance of Multi-Level Blade Design ITERATION 1 ITERATION 0 POLI-Wind Research Lab Integrated Active/Passive Load Control 2MW 45m Wind Turbine Blade Currently undergoing certification at TÜV SÜD CNC machined model of aluminum alloy for visual inspection of blade shape Design developed in partnership with Gurit (UK) POLITECNICO di MILANO POLI-Wind Research Lab Integrated Active/Passive Load Control Presentation Outline POLITECNICO di MILANO POLI-Wind Research Lab Integrated Active/Passive Load Control Fully Coupled Blades 1. Identify optimal section-wise fiber rotation Consider 6 candidate configurations Skin angle Spar-cap angle BTC coupling parameter: 𝑲𝑩𝑻 𝜶= 𝑲𝑩 𝑲𝑻 POLITECNICO di MILANO POLI-Wind Research Lab Integrated Active/Passive Load Control Fully Coupled Blades: Effects on Weight Spar-caps: steep increase Skin: milder increase Spar-cap/skin synergy Stiffness driven design (flap freq. and max tip deflection constraints): Need to restore stiffness by increasing spar/skin thickness POLITECNICO di MILANO POLI-Wind Research Lab Integrated Active/Passive Load Control Fully Coupled Blades: Load Reduction Spar-cap/skin synergy: good load reduction with small mass increase POLITECNICO di MILANO POLI-Wind Research Lab Integrated Active/Passive Load Control Fully Coupled Blades: Mechanism of Load Reduction POLITECNICO di MILANO POLI-Wind Research Lab Integrated Active/Passive Load Control Fully Coupled Blades: Effects on Duty Cycle ◀ Less pitching from active control because blade passively selfunloads Much reduced life-time ADC ▶ POLITECNICO di MILANO POLI-Wind Research Lab Integrated Active/Passive Load Control Partially Coupled Blades 2. Identify optimal span-wise fiber rotation: 5 candidate configurations Reduce fatigue in max chord region Avoid thickness increase to satisfy stiffness-driven constraints POLITECNICO di MILANO POLI-Wind Research Lab Integrated Active/Passive Load Control Partially Coupled Blades: Effects on Mass Fully coupled blade Too little coupling POLITECNICO di MILANO POLI-Wind Research Lab Integrated Active/Passive Load Control Partially Coupled Blades: Effects on Loads F30: load reduction close to fully coupled case POLITECNICO di MILANO POLI-Wind Research Lab Integrated Active/Passive Load Control Partially Coupled Blades: Effects on Duty Cycle Best compromise: similar load and ADC reduction as fully coupled blade, decreased mass POLITECNICO di MILANO POLI-Wind Research Lab Integrated Active/Passive Load Control Presentation Outline POLITECNICO di MILANO POLI-Wind Research Lab Integrated Active/Passive Load Control Integrated Passive and Active Load Alleviation 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 POLITECNICO di MILANO Baseline controller: MIMO LQR POLI-Wind Research Lab Integrated Active/Passive Load Control Integrated Passive and Active Load Alleviation Two IPC gain settings: 1. Mild: some load reduction, limited ADC increase 2. Aggressive: more load reduction, more ADC increase Five blade/controller combinations: • BTC: best coupled blade + collective LQR • IPC1: uncoupled blade + mild IPC • BTC+IPC1: best coupled blade + mild IPC • IPC2: uncoupled blade + aggressive IPC • BTC+IPC2: best coupled blade + aggressive IPC POLITECNICO di MILANO POLI-Wind Research Lab Integrated Active/Passive Load Control Integrated Passive/Active Control: Effects on Loads ▲ Synergistic effects of combined passive and active control ▶ POLITECNICO di MILANO POLI-Wind Research Lab Integrated Active/Passive Load Control Integrated Passive/Active Control: Effects on Duty Cycle Not significant: ADC very small here Same ADC as baseline (but great load reduction!) POLITECNICO di MILANO POLI-Wind Research Lab Integrated Active/Passive Load Control Conclusions • Optimization-based blade design tools: enable automated design of blades and satisfaction of all desired design requirements • 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: - 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: • Manufacturing implications of BTC and partially coupled blades • Passive distributed control and integration with blade design and active IPC control POLITECNICO di MILANO POLI-Wind Research Lab Integrated Active/Passive Load Control Outlook: Testing with (WT)2, the Wind Turbine in the Wind Tunnel ▶ Aeroelastically-scaled wind tunnel model of the Vestas V90 wind turbine with individual blade pitch and torque control 13.8x3.8m, 14m/s, civil section: • Turbulence < 2% • With turbulence generators = 25% • 13m turntable Applications: • Testing of advanced control laws and supporting technologies • Testing of extreme operating conditions • Tuning of mathematical models • Aeroelasticity and system identification of wind turbines • Multiple wind turbine interactions • Off-shore wind turbines (moving platform actuated by hydro-structural model) 4x3.8m, 55m/s, aeronautical section: • Turbulence <0.1% • Open-closed test section Civil-Aeronautical Wind Tunnel of the Politecnico di Milano Aeroelastically scaled blades (70g, 1m) Pitch actuator: • Zero backlash gearhead • Built-in encoder Main shaft with torque meter Rotor sensor electronics POLITECNICO di MILANO Torque actuator: • Planetary gearhead • Torque and speed control Pitch actuator electronics Slip ring Conical spiral gears POLI-Wind Research Lab Integrated Active/Passive Load Control Outlook: Testing with (WT)2, the Wind Turbine in the Wind Tunnel Turbulence (boundary layer) generators Good aerodynamic performance even at low Reynolds ▶ Blockage correction verified by RANS CFD POLITECNICO di MILANO POLI-Wind Research Lab Integrated Active/Passive Load Control Outlook: Off-Shore Aero-Elastic Model ▶ Goal: aeroelastically-scaled wind tunnel model of off-shore wind turbine with individual blade pitch and torque control Applications: • Testing of control laws • Damping enhancement controllers • Load-reducing controllers • Floating platform effects on stability 6 DOF moving platform Platform motion WT response POLITECNICO di MILANO Proof of concept, 2 DOF hydraulic actuation (prescribed) Real-time PC running mathematical model of wet part of offshore machine (hydro-elastic model) POLI-Wind Research Lab Integrated Active/Passive Load Control Acknowledgements Thanks to the POLI-Wind team! Special thanks to M. Bassetti, P. Bettini, M. Biava, F. Campagnolo, S. Calovi, S. Cacciola, F. Cadei, G. Campanardi, M. Capponi, A. Croce, G. Galetto, L. Maffenini, P. Marrone, M. Mauri, S. Rota, G. Sala, A. Zasso of the Politecnico di Milano Funding provided by Vestas Wind Systems A/S, Clipper Windpower, Alstom Wind, Italian Ministry of Education, University and Research POLITECNICO di MILANO POLI-Wind Research Lab