An Air-Breathing, Portable Thermoelectric Power Generator Based on a Microfabricated Silicon Combustor by Christopher Henry Marton B.A.Sc. Chemical Engineering, University of Waterloo, 2005 M.S. Chemical Engineering Practice, Massachusetts Institute of Technology 2008 Submitted to the Department of Chemical Engineering in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY IN CHEMICAL ENGINEERING AT THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY FEBRUARY 2011 © 2010 Massachusetts Institute of Technology. All rights reserved. Signature of Author: ____________________________________________ Department of Chemical Engineering October 22, 2010 Certified by: ___________________________________________________ Klavs F. Jensen Warren K. Lewis Professor of Chemical Engineering Professor of Materials Science and Engineering Thesis Supervisor Accepted by: __________________________________________________ William M. Deen Carbon P. Dubbs Professor of Chemical Engineering Chairman, Committee for Graduate Students An Air-Breathing, Portable Thermoelectric Power Generator Based on a Microfabricated Silicon Combustor by Christopher Henry Marton Submitted to the Department of Chemical Engineering on October 22, 2010 in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemical Engineering ABSTRACT The global consumer demand for portable electronic devices is increasing. The emphasis on reducing size and weight has put increased pressure on the power density of available power storage and generation options, which have been dominated by batteries. The energy densities of many hydrocarbon fuels exceed those of conventional batteries by several orders of magnitude, and this gap motivates research efforts into alternative portable power generation devices based on hydrocarbon fuels. Combustion-based power generation strategies have the potential to achieve significant advances in the energy density of a generator, and thermoelectric power generation is particularly attractive due to the moderate temperatures which are required. In this work, a portable-scale thermoelectric power generator was designed, fabricated, and tested. The basis of the system was a mesoscale silicon reactor for the combustion of butane over an alumina-supported platinum catalyst. The system was integrated with commercial bismuth telluride thermoelectric modules to produce 5.8 W of electrical power with a chemical-to-electrical conversion efficiency of 2.5% (based on lower heating value). The energy and power densities of the demonstrated system were 321 Wh/kg and 17 W/kg, respectively. The pressure drop through the system was 258 Pa for a flow of 15 liters per minute of air, and so the parasitic power requirement for air- pressurization was very low. The demonstration represents an order-of-magnitude improvement in portable-scale electrical power from thermoelectrics and hydrocarbon fuels, and a notable increase in the conversion efficiency compared with other published works. The system was also integrated with thermoelectric-mimicking heat sinks, which imitated the performance of high-heat-flux modules. The combustor provided a heat source of 206 to 362 W to the heat sinks at conditions suitable for a portable, air- breathing TE power generator. The combustor efficiency when integrated with the heat sinks was as high as 76%. Assuming a TE power conversion efficiency of 5%, the design point operation would result in thermoelectric power generation of 14 W, with an overall chemical-to-electrical conversion efficiency of 3.8%. Thesis Supervisor: Klavs F. Jensen Title: Department Head, Chemical Engineering Warren K. Lewis Professor of Chemical Engineering Professor of Materials Science and Engineering - 3 - Acknowledgments I would first like to thank Prof. Klavs Jensen for his tremendous support throughout my thesis research. During my first meeting with Klavs to discuss the possibility of my joining his research group, I described the type of project I was looking for as one that incorporated both my interest in systems engineering and my core chemical engineering fundamentals, hopefully with an energy application. His responded that he thought he had just the project I was looking for, and he was absolutely right. But it is important to find an advisor whose leadership style suits your personality, and in that way the fit was also ideal. I very much appreciated Klavs’s pragmatism, his dedication to rational research, and his tremendous knowledge base, as well as his willingness to give me freedom to maneuver. I would like to thank my thesis committee of Prof. Paul I. Barton and Prof. William H. Green for their guidance. I would also like to thank Prof. Martin Schmidt and Dr. Hanqing Li for their assistance with the development and trouble-shooting of my silicon microfabrication process. And I am grateful to the staff of Microsystems Technology Laboratories and the Center for Materials Science and Engineering for assistance with fabrication and characterization. Special thanks to Alina Haverty and Christine Preston for all of their help and kindness. I thank MIT Lincoln Laboratories for funding. Several collaborators from Lincoln Laboratories were instrumental in this work, particularly Dr. George Haldeman, Dr. Todd Mower, Dr. Christopher Vineis (now at Wakonda Technologies), Dr. Robert Reeder, Jon Howell, and Keith Patterson (now at Caltech). I would very much like to thank my collaborators at MIT’s Institute for Soldier Nanotechnology, with particular thanks to Walker Chan, Dr. Ivan Celanovic, Dr. Peter Bermel, and Professor John Joannopoulos. I am deeply indebted to the students and postdoctoral researchers in the Jensen group. Having an informed, collaborative research group to rely on was essential for the accomplishments in this thesis research. Particular thanks go to Jerry Keybl for his seemingly-endless support in the design, set-up, operation, maintenance, and trouble- shooting of my experimental system and to Dr. Nikolay Zaborenko for his guidance in all things silicon. - 4 - I am grateful to the David H. Koch School of Chemical Engineering Practice for providing me with exceptional experiences as a student and as a Station Director. I would particularly like to thank my Station Directors, Dr. Robert Fisher and Dr. William Dalzell, for their guidance and encouragement. I owe great thanks to Prof. Alan Hatton for placing his trust in me as a Director, allowing me to represent the Department and guide the development of eight students. I would also like to thank those students (Amrit, Christian, Eric, Jason, Jen, Juhyun, Sashikant, and Vivian) for their hard work and positive attitudes. And I thank Prof. Emeritus Gregory McRae, and the rest of the Morgan Stanley sponsors, for a tremendous opportunity to see the application of chemical engineering in a unique and nontraditional setting. My achievements at MIT would not have been possible if not for the assistance of the Faculty in the Chemical Engineering Department at the University of Waterloo. I would especially like to thank Prof. Ali Elkamel for his guidance, friendship, and the faith that he had in my capabilities. His impact on my professional development cannot be overstated. This work would not have been possible, and certainly the journey would not have been worthwhile, if not for my friends. An attempt at complete enumeration would be foolish. However, I would like to thank Dr. Jonathan McMullen for being a great friend, as well as for always being a half-step ahead of me so that he could teach me what I needed to learn to be an effective graduate student. I would also like to thank Kristen Farn for her immeasurable impact on my life, and for being so supportive through the challenges as well as the good times. To all my friends, at MIT and elsewhere, thank you for making life more enjoyable. And finally, I thank my family for all of their affection and support through this process, and for keeping me grounded. To my Grandparents, Parents, Brother, Sisters, Aunt, Uncle, and Cousins: thank you! From my parents, I learned to work hard and to work until the job is finished. And from my Brother, my first role model, I continue to learn a host of lessons that make me a better person. I dedicate this thesis to the man who first saw the engineer in me – Henry Skinner. - 5 - Table of Contents 1. Introduction ............................................................................................................... 13 1.1. Motivation ......................................................................................................... 13 1.2. Portable Power Generator Metrics and Definitions .......................................... 14 1.3. Limitations of Electrochemical Cells................................................................ 15 1.4. Fuel-based Portable Power Generation Strategies and Demonstrations ........... 17 1.4.1. Combustion-based Power Generation Strategies ...................................... 18 1.4.1.1. Thermoelectric Power Generation ........................................................ 19 1.4.1.2. Thermophotovoltaic Power Generation ................................................ 25 1.4.1.3. Heat Engines ......................................................................................... 27 1.4.2. Fuel Cell Power Generation Strategies ..................................................... 28 1.4.2.1. Hydrogen-Fueled Proton Exchange Membrane Fuel Cells .................. 29 1.4.2.2. Reformed Hydrogen Fuel Cells