ASSESSMENT OF RESEARCH NEEDS FOR WIND TURBINE ROTOR MATERIALS TECHNOLOGY

Committee on Assessment of Research Needs for Wind Turbine Rotor Materials Technology

Energy Engineering Board

Commission on Engineering and Technical Systems

National Research Council

NATIONAL ACADEMY PRESS
Washington, D.C.
1991



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Assessment of Research Needs for Wind Turbine Rotor Materials Technology ASSESSMENT OF RESEARCH NEEDS FOR WIND TURBINE ROTOR MATERIALS TECHNOLOGY Committee on Assessment of Research Needs for Wind Turbine Rotor Materials Technology Energy Engineering Board Commission on Engineering and Technical Systems National Research Council NATIONAL ACADEMY PRESS Washington, D.C. 1991

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Assessment of Research Needs for Wind Turbine Rotor Materials Technology NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance. This report has been reviewed by a group other than the authors according to procedures approved by a Report Review Committee consisting of members of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Frank Press is president of the National Academy of Sciences. The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. Robert M. White is president of the National Academy of Engineering. The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal government and, upon its own initiative, to identify issues of medical care, research, and education. Dr. Samuel O. Thier is the president of the Institute of Medicine. The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy’s purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Frank Press and Dr. Robert M. White are chairman and vice chairman, respectively, of the National Research Council. This report and the study on which it is based were supported by Contract No. DE-AC04-89AL58181 between the U.S. Department of Energy and the National Academy of Sciences-National Research Council. Library of Congress Catalog Card No. 91-60990 International Standard Book Number 0-309-04479-0 NAT S-324 Additional copies of this report are available from: National Academy Press 2101 Constitution Avenue, N.W. Washington, D.C. 20418 Printed in the United States of America

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Assessment of Research Needs for Wind Turbine Rotor Materials Technology COMMITTEE ON ASSESSMENT OF RESEARCH NEEDS FOR WIND TURBINE ROTOR MATERIALS TECHNOLOGY GEORGE E. DIETER (Chairman), Dean, College of Engineering, University of Maryland, College Park, Maryland JAMIE CHAPMAN, Power Systems Consultant, Boston, Massachusetts H. THOMAS HAHN, Department of Engineering Science and Mechanics, Pennsylvania State University, University Park, Pennsylvania DEWEY H. HODGES, School of Aerospace Engineering, Georgia Institute of Technology, Atlanta, Georgia CHARLES W. ROGERS, Bell Helicopter Textron, Inc., Fort Worth, Texas LENA VALAVANI, Department of Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge, Massachusetts MICHAEL D. ZUTECK, Consultant, Kemah, Texas National Research Council Staff KAMAL J. ARAJ, Study Director, Energy Engineering Board THERESA A. FISHER, Study Assistant (to April 1990) JAN C. KRONENBURG, Study Assistant (to February 1991) ENERGY ENGINEERING BOARD JOHN A. TILLINGHAST (Chairman), Tiltec, Portsmouth, New Hampshire DONALD B. ANTHONY, Bechtel Corporation, Houston, Texas RICHARD E. BALZHISER, Electric Power Research Institute, Palo Alto, California BARBARA R. BARKOVICH, Barkovich and Yap, Consultants, San Rafael, California JOHN A. CASAZZA, CSA Energy Consultants, Arlington, Virginia RALPH C. CAVANAGH, Natural Resources Defense Council, San Francisco, California DAVID E. COLE, University of Michigan, Ann Arbor, Michigan H. M. (HUB) HUBBARD, Midwest Research Institute (retired), Golden, Colorado ARTHUR E. HUMPHREY, Lehigh University, Bethlehem, Pennsylvania (to February 1991) CHARLES IMBRECHT, California Energy Commission, Sacramento, California CHARLES D. KOLSTAD, University of Illinois, Urbana, Illinois HENRY R. LINDEN, Gas Research Institute, Chicago, Illinois JAMES J. MARKOWSKY, American Electric Power Service Corporation, Columbus, Ohio (to February 1991) SEYMOUR L. MEISEL, Mobile R&D Corporation (retired), Princeton, New Jersey DAVID L. MORRISON, The MITRE Corporation, McLean, Virginia MARC H. ROSS, University of Michigan, Ann Arbor, Michigan MAXINE L. SAVITZ, Garrett Ceramic Component Division, Torrance, California HAROLD H. SCHOBERT, Pennsylvania State University, University Park, Pennsylvania GLENN A. SCHURMAN, Chevron Corporation, San Francisco, California JON M. VEIGEL, Oak Ridge Associated Universities, Oak Ridge, Tennessee BERTRAM WOLFE, GE Nuclear Energy, San Jose, California

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Assessment of Research Needs for Wind Turbine Rotor Materials Technology Staff ARCHIE L. WOOD, Executive Director, Commission on Engineering and Technical Systems, and Director, Energy Engineering Board (to January 1991) MAHADEVAN (DEV) MANI, Director, Energy Engineering Board KAMAL J. ARAJ, Senior Program Officer ROBERT COHEN, Senior Program Officer (retired) GEORGE LALOS, Senior Program Officer JAMES J. ZUCCHETTO, Senior Program Officer JUDITH A. AMRI, Administrative/Financial Assistant THERESA M. FISHER, Administrative Secretary JAN C. KRONENBURG, Administrative Secretary PHILOMINA MAMMEN, Administrative Secretary NANCY WHITNEY, Administrative Secretary

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Assessment of Research Needs for Wind Turbine Rotor Materials Technology PREFACE Wind-driven power systems represent a renewable energy technology that is still in the early stages of development. These wind power plants installed in the early 1980s suffered structural failures chiefly because of incomplete understanding of the wind forces (especially the turbulence component) acting on these large structures and in some cases because of poor quality in manufacture. Failure of the rotor blades was one of the principal and most serious structural failures. Failures from these causes are now somewhat better understood. Another limitation to economical achievement of the potential of wind energy is uncertainty about the long-term response of wind turbine rotor materials to the turbulent stochastic loadings to which they are subjected. These structures can be subjected to as many as a billion stress cycles. In accordance with its assessment of its long-term research responsibilities, the Department of Energy requested the National Research Council to assess the research needs for wind turbine rotor technology. Such a study would assist in organizing the information about the current status of wind turbine rotor materials, their manufacture into blades, and their operation and life performance in service. Of special importance was an assessment of current materials technology and design methodologies to provide perspective for future investigations. The Committee on Assessment of Research Needs for Wind Turbine Rotor Materials Technology was formed by the Energy Engineering Board to specifically evaluate the following issues (see Appendix A for Statement of Task): the adequacy of existing models to predict dynamic stress patterns; the properties of wind turbine materials in dynamic and fatigue failure; understanding of the performance of joints, fasteners, and critical sections in relation to failure modes and fracture; the tools needed to study these phenomena, such as computer design tools, and materials databases; the need for special laboratory facilities, models, and prototypes to improve the design and operation of wind energy systems; the opportunities for new materials to improve wind turbine life; the potential for improved design methods, advanced control techniques, and better manufacturing processes and advanced materials for better performance and longevity. In addition, the committee took as its responsibility the development, in broad outline, of a research and development program that would place U.S. wind power technology in a preeminent world position. In carrying out its assignment the committee examined an extensive literature on wind machines and wind turbine rotor materials. The emphasis of the study was on wind machines suitable for utility applications. Many U.S. experts on these subjects briefed the committee in two meetings of two-day duration (see Appendix B). The committee wishes to acknowledge with gratitude the assistance of the following individuals for their time and knowledge:

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Assessment of Research Needs for Wind Turbine Rotor Materials Technology Holt Ashley, Stanford University; Charles Carlson and Marilyn W. Wardle, E.I. duPont de Nemours & Co.; John C. Doyle, California Institute of Technology; Brant Goldsworthy, ALCOA/Goldsworthy Engineering; Richard H. Hilt, Gas Research Institute; William Holley, U.S. Windpower; Donald Hunston, National Institute for Standards and Technology; Edwin T.C. Ing; John Mandell, Montana State University; Robert C. Monroe, Hudson Products; Robert H. Monroe, Gougeon Brothers; Peter Ogle, Dow United Technologies Composite Products; Donald Pederson and Fred J. Policelli, Hercules, Inc.; Lawrence Rehfield, University of California-Davis; Forrest S. Stoddard, Alternative Energy Institute; Robin Whitehead, Northrop; Daniel F. Ancona III, Leonard J. Rogers and Jeffrey H. Rumbaugh, U.S. Department of Energy; James Tangler and Robert Thresher, Solar Energy Research Institute; and Herbert Sutherland, Sandia National Laboratories. George E. Dieter, Chairman Committee on Assessment of Research Needs for Wind Turbine Rotor Materials Technology

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Assessment of Research Needs for Wind Turbine Rotor Materials Technology CONTENTS     LIST OF FIGURES   ix     LIST OF TABLES   x     EXECUTIVE SUMMARY   1 1   INTRODUCTION   5     Scope and Content,   5     Wind-Driven Power Plants,   5     Why Materials Knowledge Is Critical,   8     The Evolution of Wind-Driven Power Plants,   9     The Principal Component: Wind Turbines,   12     Power Conversion Equations,   16     The Wind Environment,   19     Fatigue Cycle Accumulation,   20     References and Bibliography,   23 2   STRUCTURAL LOADING CHARACTERISTICS   25     Load Characterization,   26     Blade Failure Experience,   27     Lessons from Helicopter Experience,   27     Recommendations,   32     References and Bibliography,   33 3   MATERIALS PROPERTIES AND LIFE PREDICTION   35     Fibers,   35     Matrix Materials,   39     E-Glass/Plastic Composites,   43     Fatigue Life Prediction,   48     Toughness Considerations,   49     Wood/Epoxy Composites,   50     Recommendations,   60     References and Bibliography,   61 4   WIND TURBINE ROTOR DESIGN   67     Airfoil Evolution,   67     Aerodynamic Tip Brakes,   69     Blade Root Retention,   71     Glass-Reinforced Plastic (GRP) Blade Roots,   71     Wood/Epoxy Blade Roots,   75     Blade Joining,   75     Blade Design Considerations,   76     Fiberglass Blades,   76     Wood/Epoxy Blades,   78     Recommendations,   79     GRP,   79     Wood/Epoxy,   79     Generic,   79     References and Bibliography,   80

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Assessment of Research Needs for Wind Turbine Rotor Materials Technology 5   MANUFACTURING PROCESSES FOR ROTOR BLADES   81     Current Manufacturing Processes,   81     Blade Root End Concepts,   82     Manufacturing Methods Influence Blade Life,   82     Matrix Stress Versus Fatigue,   83     Helicopter Rotor Blade Design and Processing,   84     Manufacturing Processes Applicable to Wind Turbine Blades,   84     Resin Transfer Molding (RTM),   84     Pultrusion,   86     Fiber Placement,   87     Root End Design for Producibility,   87     Manufacturing Recommendations,   87     References and Bibliography,   89 6   ACTIVE CONTROL IN WIND TURBINES   91     The Control Problem for Wind Turbines,   91     Recent Trends in Control System Theory,   93     Existing Control Technology for Wind Turbines,   94     Role of Control Technology in the Wind Power Industry,   95     References and Bibliography,   98 7   CONCLUSIONS AND RECOMMENDATIONS   101     Conclusions,   101     Research Recommendations,   102 APPENDIX A:   STATEMENT OF TASK   105 APPENDIX B:   COMMITTEE MEETINGS AND ACTIVITIES   107

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Assessment of Research Needs for Wind Turbine Rotor Materials Technology LIST OF FIGURES Figure 1-1   Wind power plant in Altamont Pass, California   6 Figure 1-2   World energy generation by wind power plants during 1989   10 Figure 1-3   Growth of generating capacity for California wind power plants   10 Figure 1-4   Growth of annual energy production for California wind power plants   12 Figure 1-5   Wind turbine subsystems   13 Figure 1-6   Wind turbine power curve   15 Figure 1-7   Wind flow field and turbine loads   16 Figure 1-8   Comparison of logarithmic power in the wind with a wind turbine power curve   17 Figure 1-9   Wind turbine blade control methods   19 Figure 1-10   Accumulation of fatigue cycles   22 Figure 2-1   Process necessary to analyze composite blades   29 Figure 3-1   Trends of longitudinal tensile fatigue S-N data for unidirectional composites with various fibers   38 Figure 3-2   Schematics of various fiber preforms   39 Figure 3-3   Stress-strain relationships of glass/epoxy laminates under uniaxial tension   45 Figure 3-4   Modes of damage growth in composite laminate under fatigue   47 Figure 3-5   Laminate directional properties and shear directional nomenclature   52 Figure 3-6   Wood strength change due to temperature at two moisture conditions   55 Figure 3-7   Wood moisture content versus atmospheric relative humidity   55 Figure 3-8   Effect of moisture content on laminate mechanical properties   56 Figure 3-9   Typical tensile fatigue strength of wood/epoxy laminate   56 Figure 3-10   Typical compression fatigue strength of wood/epoxy laminate   57 Figure 3-11   Typical reversed stress fatigue strength of wood/epoxy laminate   57

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Assessment of Research Needs for Wind Turbine Rotor Materials Technology Figure 4-1   Commonly used early HAWT airfoils   68 Figure 4-2   SERI advanced wind turbine airfoils   68 Figure 4-3   Nine-meter GRP wind turbine blade   70 Figure 4-4   Eleven-meter wood/epoxy blade airfoil thickness distribution and planform   70 Figure 4-5   Flanged GRP blade root design   71 Figure 4-6   Flanged root design limitation due to steel/GRP strain incompatibility   73 Figure 4-7a   Bonded steel root tube GRP hub design   74 Figure 4-7b   Wood/epoxy blade root   74 Figure 4-8   All-wood/epoxy composite hub   76 Figure 5-1   UH-1H composite main rotor blade   85 Figure 5-2   RTM cooling tower blade   86 Figure 5-3   4BW hingeless bearingless rotor   88 Figure 6-1   Wind turbine drive train   92 LIST OF TABLES Table 2-1   Blade Sectional Analysis Codes   30 Table 3-1   Typical Properties of Fibers   36 Table 3-2   Typical Properties of Unidirectional Composites   37 Table 3-3   Properties of Case Resins   41 Table 3-4   High-Performance Thermoplastics Used as Matrix Resins   43 Table 3-5   Trade-offs Between Thermosets and Thermoplastics as Matrices   44 Table 3-6   Typical Static Strength, Type 110 Laminate   53 Table 6-1   Control Effectors and Sensors for a Pitch Controlled Wind Turbine   92