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PO
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di M
Itecn
ico
lano
tecn
ico
lano
INTEGRATED ACTIVE AND PASSIVE LOAD CONTROL IN WIND TURBINES
C.L. Bottasso, F. Campagnolo, A. Croce, C. TibaldiPolitecnico di Milano, Italy
SAICA 20117-8 November 2011, Barcelona, Spain
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Need for Load MitigationTrends in wind energy:
Increasing wind turbine size ▶Off-shore wind ▼
To decrease cost of energy:• Reduce extreme loads• Reduce fatigue damage• Limit actuator duty cycle• Ensure high
reliability/availability
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Presentation Outline
Load mitigatio
n
Active control
Passive control
Blade design optimization
Passive blade design
Integrated active/passive
control Load mitigatio
n
Active control
Passive control
Blade design optimization
Passive blade design
Integrated active/passive
control
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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 ▼ Turbulent wind
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Active Load Mitigation: PredictiveLiDAR-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
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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)
(Credits: Risoe DTU)
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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
Sensor-enabled control solutions:• Complexity/availability/maintenance
Active Load Mitigation: Limits and Issues
Off-shore: need to prove reliability, availability, low maintenance in hostile environments
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Load mitigatio
n
Active control
Passive control
Blade design optimization
Passive blade design
Integrated active/passive
control Load mitigatio
n
Active control
Passive control
Blade design optimization
Passive blade design
Integrated active/passive
control
Presentation Outline
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Passive control: loaded structure deforms so as to reduce loadTwo main solutions:
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 (BTC): exploit anisotropy of composite materials
- Swept (scimitar) blades
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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
Objectives
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Load mitigatio
n
Active control
Passive control
Blade design optimization
Passive blade design
Integrated active/passive
control Load mitigatio
n
Active control
Passive control
Blade design optimization
Passive blade design
Integrated active/passive
control
Presentation Outline
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Optimizer • Local/global solvers (SQP, GA)• Functional approximators
Optimization-Based Multi-Level Blade Design
1. Aerodynamic Optimization
2. Structural Optimization
+ Controls
3. Combined Aero-Structural Optimization
Cp-Lambda aero-servo-elastic multibody simulator
2D ANBA cross sectional analyzer
3D FEM models
Parameters
Cost function & constraints
Cost: AEPAerodynamic 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: AEP/weigh (or cost model if available)Macro parameters: rotor radius, max chord, tapering, …
Controls:model-based (self-adjusting to changing design)
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“Fin
e”
level: 3
D F
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“Coars
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level: 2
D F
EM
sect
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beam
m
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- 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
- 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)
Automatic 3D CAD model generation by lofting of sectional geometry
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
SQP optimizer
min cost s.t. constraints
Constraint/model update heuristic (to repair constraint violations)
When SQP converged
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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)
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Load mitigatio
n
Active control
Passive control
Blade design optimization
Passive blade design
Integrated active/passive
control Load mitigatio
n
Active control
Passive control
Blade design optimization
Passive blade design
Integrated active/passive
control
Presentation Outline
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Fully Coupled Blades
1. Identify optimal section-wise fiber rotationConsider 6 candidate configurations
BTC coupling parameter:
Spar-cap angle
Skin angle
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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
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Fully Coupled Blades: Load Reduction Spar-cap/skin synergy:
good load reduction with small mass increase
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Fully Coupled Blades: Mechanism of Load Reduction
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Fully Coupled Blades: Effects on Duty Cycle
◀ Less pitching from active control because blade passively self-unloads
Much reduced life-time ADC ▶
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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
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Partially Coupled Blades: Effects on Mass
Too little coupling
Fully coupled blade
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Partially Coupled Blades:Effects on Loads F30: load reduction
close to fully coupled case
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Partially Coupled Blades:Effects on Duty Cycle
Best compromise: similar load and ADC reduction as fully coupled blade, decreased mass
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Load mitigatio
n
Active control
Passive control
Blade design optimization
Passive blade design
Integrated active/passive
control Load mitigatio
n
Active control
Passive control
Blade design optimization
Passive blade design
Integrated active/passive
control
Presentation Outline
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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 Baseline
controller:MIMO LQR
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Integrated Passive and Active Load Alleviation
Two IPC gain settings:1. Mild: some load reduction, limited ADC increase2. 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
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Integrated Passive/Active Control:Effects on Loads
▲ Synergistic effects of combined passive and
active control ▶
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Integrated Passive/Active Control:Effects on Duty Cycle
Not significant: ADC very small
here
Same ADC as baseline (but great load
reduction!)
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• 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
effectsReduction 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
Conclusions