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Thermionics Quo Vadis?: An Assessment of the DTRA’s Advanced Thermionics Research and Development Program Thermionics Quo Vadis? An Assessment of the DTRA’s Advanced Thermionics Research and Development Program Committee on Thermionic Research and Technology Aeronautics and Space Engineering Board Division on Engineering and Physical Sciences National Research Council NATIONAL ACADEMY PRESS Washington, D.C.
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Thermionics Quo Vadis?: An Assessment of the DTRA’s Advanced Thermionics Research and Development Program NATIONAL ACADEMY PRESS 2101 Constitution Avenue, N.W. Washington, DC 20418 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 study was supported by Contract No. DTRA01-00-C-0001 between the National Academy of Sciences and the Defense Threat Reduction Agency. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the organizations or agencies that provided support for the project. International Standard Book Number: 0-309-08282-X Available in limited supply from: Aeronautics and Space Engineering Board, HA 292, 2101 Constitution Avenue, N.W., Washington, DC 20418, (202) 334–2855 Additional copies available for sale from: National Academy Press, 2101 Constitution Avenue, N.W., Box 285, Washington, DC 20055, (800) 624–6242 or (202) 334–3313 (in the Washington metropolitan area), <http://www.nap.edu> Copyright 2001 by the National Academy of Sciences. All rights reserved. Printed in the United States of America
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Thermionics Quo Vadis?: An Assessment of the DTRA’s Advanced Thermionics Research and Development Program THE NATIONAL ACADEMIES National Academy of Sciences National Academy of Engineering Institute of Medicine National Research Council 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. Bruce M.Alberts 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. Wm.A. Wulf 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. Kenneth I. Shine is 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. Bruce M.Alberts and Dr. Wm.A. Wulf are chairman and vice chairman, respectively, of the National Research Council.
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Thermionics Quo Vadis?: An Assessment of the DTRA’s Advanced Thermionics Research and Development Program This page in the original is blank.
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Thermionics Quo Vadis?: An Assessment of the DTRA’s Advanced Thermionics Research and Development Program COMMITTEE ON THERMIONIC RESEARCH AND TECHNOLOGY TOM MAHEFKEY, Chair, Consultant, Atlanta, Georgia DOUGLAS M. ALLEN,* Schafer Corporation, Dayton, Ohio JUDITH H. AMBRUS, Space Technology Management Services, Bridgewater, New Jersey LEONARD H. CAVENY, Aerospace Consultant, Fort Washington, Maryland HAROLD B. FINGER, Consultant, Chevy Chase, Maryland GEORGE N. HATSOPOULOS, Thermo Electron Corporation, Waltham, Massachusetts THOMAS K. HUNT, Advanced Modular Power Systems, Inc., Ann Arbor, Michigan DEAN JACOBSON, Arizona State University, Tempe, Arizona ELLIOT B. KENNEL, Applied Sciences, Inc., Cedarville, Ohio ROBERT J. PINKERTON, Spectrum Astro Corporation, Gilbert, Arizona GEORGE W. SUTTON, NAE, ANSER Corporation, Arlington, Virginia Staff DOUGLAS H. BENNETT, Study Director, Aeronautics and Space Engineering Board GEORGE LEVIN, Director, Aeronautics and Space Engineering Board ALAN ANGLEMAN, Senior Program Officer ANNA L. FARRAR, Administrative Associate BRIDGET EDMONDS (July 2, 2001, until December 27, 2001), Senior Project Assistant MARY LOU AQUILO (June 12, 2000, until July 2, 2001), Senior Project Assistant JAN BERGER (September 1, 2001 until October 26, 2001), Project Assistant VIKTORIA HERSON (January 28, 2000, until June 12, 2000), Project Assistant * The full committee served from April 19, 2000 until December 27, 2001. Mr. Allen served on the com mittee from April 19, 2000, until June 20, 2001.
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Thermionics Quo Vadis?: An Assessment of the DTRA’s Advanced Thermionics Research and Development Program AERONAUTICS AND SPACE ENGINEERING BOARD WILLIAM W.HOOVER, Chair, United States Air Force (retired), Williamsburg, Virginia A.DWIGHT ABBOTT, Aerospace Corporation (retired), Los Angeles, California RUZENA K.BAJSCY, NAE, IOM, National Science Foundation, Arlington, Virginia WILLIAM F.BALLHAUS, JR., NAE, Aerospace Corporation, Los Angeles, California JAMES BLACKWELL, Lockheed Martin Corporation (retired), Marietta, Georgia ANTHONY J.BRODERICK, Aviation Safety Consultant, Catlett, Virginia DONALD L.CROMER, United States Air Force (retired), Lompoc, California ROBERT A.DAVIS, The Boeing Company (retired), Seattle, Washington JOSEPH FULLER, JR., Futron Corporation, Bethesda, Maryland RICHARD GOLASZEWSKI, GRA Inc., Jenkintown, Pennsylvania JAMES M.GUYETTE, Rolls-Royce North America, Reston, Virginia FREDERICK H.HAUCK, AXA Space, Bethesda, Maryland JOHN L.JUNKINS, NAE, Texas A&M University, College Station JOHN K.LAUBER, Airbus Industrie of North America, Washington, D.C. GEORGE K.MUELLNER, The Boeing Company, Seal Beach, California DAVA J.NEWMAN, Massachusetts Institute of Technology, Cambridge JAMES G.O’CONNOR, NAE, Pratt & Whitney (retired), Coventry, Connecticut MALCOLM R.O’NEILL, Lockheed Martin Corporation, Bethesda, Maryland CYNTHIA SAMUELSON, Opsis Technologies, Springfield, Virginia WINSTON E.SCOTT, Florida State University, Tallahassee KATHRYN C.THORNTON, University of Virginia, Charlottesville ROBERT E.WHITEHEAD, NASA (retired), Henrico, North Carolina DIANNE S.WILEY, The Boeing Company, Long Beach, California THOMAS L.WILLIAMS, Northrop Grumman, El Segundo, California Staff GEORGE LEVIN, Director
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Thermionics Quo Vadis?: An Assessment of the DTRA’s Advanced Thermionics Research and Development Program Preface Generating electricity from a heat source using no moving mechanical parts is the ultimate goal of the Defense Threat Reduction Agency’s thermionics program. However, developing thermionic energy conversion devices has proven difficult, although much progress has been made. In spite of initial success during the late 1960s and intermittent funding since that time, for a variety of reasons no thermionic system has yet been developed in the United States that can be used today on Earth or in space. The ability of human-kind to reach farther and farther into the solar system and beyond is determined, in part, by our ability to generate power in space for spacecraft use. Thermionic energy conversion has been pursued since the advent of the space age by virtue of its intrinsic attributes as a compact, high performance space power system candidate. While the revolutionary missions that spawned interest in thermionics 40 years ago have yielded to an evolutionary approach to space utilization and exploration, potential future revolutionary missions prompt interest in maintaining and supporting development and examination of this potential technology option today. Progress in the technology was substantial during the 1960s but waned in the early 1970s due to a shift in space technology funding priorities. The advent of the Strategic Defense Initiative (SDI) and space exploration initiatives in the late 1970s rekindled interest and investment in thermionics. However, that investment diminished again in the mid 1990s, not as a result of lack of progress, but because of changes in national technology investment priorities. Today, the thermionic technology base and infrastructure stand close to extinction. Only a modest $1.5 million to $3 million per year is directed toward sustaining the technology. Two complete 5 kilowatt-electric nuclear reactor thermionic systems have been developed and flown in space by the former Soviet Union for experimental purposes, but no follow-up Russian or U.S. development on a high power thermionic system has taken place for a variety of reasons. Among them, the political nature of funding priorities involves decisions based on technology considerations, specifically concerning competing technologies that might accomplish the same system-level mission goals as thermionic systems. The Committee on Thermionic Research and Technology started by asking a difficult question: In light of past efforts and the lack of apparent success in developing a fully functioning system and uncertain requirements, why do thermionics at all? This report is written to answer that question in view of potential future needs and applications while recognizing the existing technological risks as well as the currently available alternative power conversion technologies, in the context of the present, congressionally mandated, DTRA thermionics technology program (see Appendix A for the statement of task). This study was sponsored by DTRA and was conducted by the Committee on Thermionic Research and Technology appointed by the National Research Council (see Appendix B). This report has been reviewed by individuals chosen for their diverse perspectives and technical expertise, in accordance with procedures approved by the National Research Council’s Report Review Committee. The purpose of this independent review is to provide candid and critical comments that will assist the authors and the National Research Council in making the published report as sound as possible and to ensure that
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Thermionics Quo Vadis?: An Assessment of the DTRA’s Advanced Thermionics Research and Development Program the report meets institutional standards for objectivity, evidence, and responsiveness to the study charge. The review comments and draft manuscript remain confidential to protect the integrity of the deliberative process. The committee wishes to thank the following individuals for their participation in the review of this report: Henry W.Brandhorst, Jr., Space Power Institute, Auburn University, Lee S.Mason, NASA Glenn Research Center, Gerald D.Mahan, NAS, Applied Physical Sciences, and Mohamed S.El-Genk, University of New Mexico, Institute for Space and and Nuclear Power Studies. Although the reviewers listed above have provided many constructive comments and suggestions, they were not asked to endorse the conclusions or recommendations, nor did they see the final draft of the report before its release. The review of this report was overseen by Simon Ostrach, Case Western Reserve University. Appointed by the National Research Council, he was responsible for making certain that an independent examination of this report was carried out in accordance with institutional procedures and that all review comments were carefully considered. Responsibility for the final content of this report rests entirely with the authoring committee and the institution. The committee also wishes to thank others whose efforts supported this study, especially those who took the time to participate in committee meetings and the thermionics workshop held in La Jolla, California. Tom Mahefkey, Chair Committee on Thermionic Research and Technology
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Thermionics Quo Vadis?: An Assessment of the DTRA’s Advanced Thermionics Research and Development Program Contents EXECUTIVE SUMMARY 1 1 INTRODUCTION 6 Background, 6 Approach, 6 Organization of This Report, 7 References, 9 2 CONCLUSIONS REGARDING THE CURRENT DTRA PROGRAM 10 The Mission of the Defense Threat Reduction Agency, 10 Work Conducted Under the DTRA Program, 11 Knowledge Capture, 13 Future Thermionic Work with Russia, 13 References, 14 3 OVERVIEW OF THE TECHNOLOGY 15 Device Physics, 15 Potential Applications and Competing Technologies, 18 History of Thermionic Systems and Development, 26 References, 32 4 SOLAR THERMIONICS 33 Potential Solar Thermionic Missions, 33 High-Power, Advanced, Low-Mass Concept, 35 Solar Orbital Transfer Vehicle Program, 39 String Thermionic Assembly Research Testbed Tests at the New Mexico Engineering Research Institute, 40 References, 42 5 NUCLEAR THERMIONICS 43 Lessons Learned from TOPAZ, 43 Nuclear Thermionic Technology Development, 45 Potential Space Nuclear Thermionic Missions, 47 Bibliography, 49
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Thermionics Quo Vadis?: An Assessment of the DTRA’s Advanced Thermionics Research and Development Program 6 TERRESTRIAL APPLICATIONS 50 Commercial Power Production, 50 Special Purpose Military Applications, 50 7 ASSESSMENT OF PROGRESS 52 Materials and Device Research, 52 Close-Spaced Vacuum Converter, 56 Theory and Theory Validation, 57 Microminiature Thermionic Converter, 57 References, 60 Bibliography, 60 APPENDIXES A Statement of Task, 63 B Biographical Sketches of Committee Members, 64 C Electric Propulsion Considerations, 67 D Acronyms, 71
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Thermionics Quo Vadis?: An Assessment of the DTRA’s Advanced Thermionics Research and Development Program Tables, Figures, and Boxes TABLES ES-1 Major Elements of the DTRA Thermionics Program, 2 2–1 Major Elements of the DTRA Thermionics Program, 12 3–1 Potential Missions for Solar and Nuclear Thermionic Power Systems, 19 3–2 Comparison of Flight Demonstrated Power Conversion Technologies, 25 3–3 Comparison of Ground Demonstrated Power Conversion Technologies, 27 3–4 Comparison of Projected Power Conversion Technology Capabilities, 28 C-1 Performance of Chemical and Electrical Propulsion Systems, 69 FIGURES 3–1 Basic thermionic converter schematic, 15 3–2 A cross sectional view of a thermionic fuel element (TFE), 16 3–3 Solar thermionic output voltage based on emitter-collector spacing, 17 3–4 The current-voltage curve of a typical thermionic converter, 18 3–5 Power system options for specific mission durations, 20 3–6 Inverse specific mass versus electrical power output, 21 3–7 Increase in power density of a nuclear thermionic system as a function of temperature, 23 4–1 Artist’s rendition of the HPALM solar thermionic concept, 36 4–2 Artist’s rendition of a solar orbital transfer vehicle, 40 5–1 Cylindrical inverted multicell cross section, 47 5–2 Solar energy flux as a function of distance from the Sun, 48 7–1 Effect of emitter bare work function on performance, using computer code TECMDL, 54 7–2 Cesiated work function versus bare work function, 55 7–3 Effects of cesium oxide vapor on converter performance, 56 BOXES 3–1 Alkali Metal Thermal to Electric Converter, 26 4–1 The Solar Energy Technology Thermionic Program, 41
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