Understanding the Mechanisms of Aero-Structural Optimization of a Wind Turbine Blade Alejandro ...

156 Pages·2012·2.7 MB·English

Understanding the Mechanisms
of Aero-Structural Optimization
of a Wind Turbine Blade
Alejandro Velez Isaza
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DUWind

Understanding the Mechanisms of
Aero-Structural Optimization of a
Wind Turbine Blade
Master of Science Thesis
For the degree of Master of Science in at Delft University of Technology
Alejandro Velez Isaza
12 December 2012
Faculty of Aerospace Engineering Delft University of Technology
·
The work in this thesis was supported and conducted at ECN. Their cooperation is hereby
gratefully acknowledged.
Copyright c DUWIND
(cid:13)
All rights reserved.
Delft University Of Technology
Department Of
DUWIND
The undersigned hereby certify that they have read and recommend to the Faculty of
Aerospace Engineering for acceptance a thesis entitled “Understanding the Mecha-
nisms of Aero-Structural Optimization of a Wind Turbine Blade” by Alejan-
dro Velez Isaza in partial fulfillment of the requirements for the degree of Master
of Science.
Dated: 12 December 2012
Head of department:
prof. dr. Gerard van Bussel
Internal supervisor:
ir. W.A. Timmer
External supervisor:
Özlem Ceyhan, MSc.
Reader:
dr. Mostafa Abdalla, MSc.
ii
Alejandro Velez Isaza Master of Science Thesis
Abstract
The process of designing a wind turbine blade involves many disciplines and is of an itera-
tive character, making it cumbersome and sometimes inefficient. In this thesis an increased
understanding of the physical and modeling considerations required for a better aero-elastic
design of a blade is sought. To this end, an aero-structural model is developed and placed
in an optimization procedure, where the mechanisms driving the optimizer can be analyzed.
The model consists of a Blade Element Momentum analysis of the flow; and a process that
dimensions the internal structure of a blade for a set of aerodynamic loads and design re-
quirements. The optimizations use Genetic Algorithms and are done in a parametric way,
increasing the design space and changing the objective function step by step. The studied
parameters are the chord and twist distributions and the airfoil thickness of the outboard half
of the blade. The optimizations are carried out with the objective of improving the design of
the DOWEC reference turbine for power performance, blade mass and a combination of the
two. The results of the optimizations show the mechanisms that must be taken into account
when doing the preliminary aero-structural design of a wind turbine blade. In particular, one
of the results indicates that the mechanisms for an enhanced power performance design and
reducedblademassarenotalwayscounteractingandcanactually, withagoodunderstanding
of the physical phenomena, be used for an improved design in both aspects.
Master of Science Thesis Alejandro Velez Isaza
iv
Alejandro Velez Isaza Master of Science Thesis
Table of Contents
Acknowledgements xv
Symbols, conventions and frames of reference xvii
List of Acronyms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xvii
List of Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xvii
Coordinate systems and conventions . . . . . . . . . . . . . . . . . . . . . . . . . xx
Introduction 1
1 The aerodynamics of a wind turbine: two perspectives on the blade loads 3
1-1 The Blade Element Momentum Theory for a turbine in normal operation: power
performance and load prediction . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1-1-1 From momentum theory to induction at the rotor plane . . . . . . . . . . . 4
1-1-2 From actuator annulus to angle of attack . . . . . . . . . . . . . . . . . . 5
1-1-3 BEM validation with field measurements . . . . . . . . . . . . . . . . . . . 11
1-1-4 Loads and power performance of the DOWEC 6MW reference turbine . . . 12
1-2 The forces on a standing still turbine with extreme wind speeds . . . . . . . . . . 16
1-3 Loads on the DOWEC reference turbine . . . . . . . . . . . . . . . . . . . . . . . 18
1-4 Concluding remark: The aerodynamic modeling in the complete optimization project 18
2 Structural model: automatic dimensioning of the blade structure 21
2-1 Load cases: the forces during power production and standing still, in the event of
extreme wind speeds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2-2 Blade characterization and structural analysis: from load distribution to laminate
strain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2-2-1 Moments, shear forces and deflections . . . . . . . . . . . . . . . . . . . . 27
2-2-2 From cross section to structural component . . . . . . . . . . . . . . . . . 28
2-2-3 Material treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Master of Science Thesis Alejandro Velez Isaza
vi Table of Contents
2-2-4 Verification of the cross section properties calculation . . . . . . . . . . . . 30
2-3 Automatic dimensioning of the spar and shell . . . . . . . . . . . . . . . . . . . . 32
2-3-1 Epsilon-dimensioning: sizing the spar cap and the shell for a requirement
on strain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2-3-2 uTip-dimensioning: dimensioning for a requirement on tip deflection . . . . 34
2-4 Dimensioning of the DOWEC blade . . . . . . . . . . . . . . . . . . . . . . . . . 37
3 Laying the path for a better aero-structural design with Genetic Algorithms opti-
mization 43
3-1 The blade optimization problem . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3-1-1 The optimization method . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
3-1-2 The goal of optimization: choosing the objective function. . . . . . . . . . 45
3-1-3 The optimization variables: parameterization of the blade geometry . . . . 46
3-1-4 Including constraints in GA optimization . . . . . . . . . . . . . . . . . . . 48
3-2 Further considerations for the optimization . . . . . . . . . . . . . . . . . . . . . . 48
3-2-1 Exerting more control over parts of the distribution . . . . . . . . . . . . . 48
3-2-2 Convergence of an optimization . . . . . . . . . . . . . . . . . . . . . . . 49
3-3 Optimizing the optimization, the model or the physics? . . . . . . . . . . . . . . . 51
4 Characterizing the mechanisms of aero-structural optimization. 53
4-1 Optimization setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4-2 Methodology to interpret the results . . . . . . . . . . . . . . . . . . . . . . . . . 55
4-3 Power coefficient optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4-3-1 Identifyingthemechanisms: Comparisonoftheoptimizedandthereference
blade. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4-3-2 Assessment of the mechanisms: physical and modeling considerations . . . 58
4-4 Mass optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
4-4-1 Discovery of the mass reduction mechanisms: Optimization with LC 1.1+1.1 60
4-4-2 Optimization with LC 6.1+1.1 . . . . . . . . . . . . . . . . . . . . . . . . 66
4-5 Optimizing for C /m: a comparison on the objective function . . . . . . . . . . . 69
P
4-5-1 Aerodynamic performance. . . . . . . . . . . . . . . . . . . . . . . . . . . 69
4-5-2 Structural performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
4-6 Parametric study on the airfoil thickness . . . . . . . . . . . . . . . . . . . . . . . 73
4-6-1 C optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
P
4-6-2 Mass optimization LC 6.1+1.1 . . . . . . . . . . . . . . . . . . . . . . . . 76
4-6-3 C /m optimization: mechanism synthesis and airfoil thickness . . . . . . . 79
p
Conclusions 83
Recommendations 85
A The DOWEC turbine 87
Alejandro Velez Isaza Master of Science Thesis