THERMOCHEMICAL CYCLE OF A MIXED METAL OXIDE FOR AUGMENTATION OF SOLAR THERMAL ENERGY STORAGE USING SOLID PARTICLES by BRIAN DAVID EHRHART B.S., Rensselaer Polytechnic Institute, 2010 A thesis submitted to the Faculty of the Graduate School of the University of Colorado in partial fulfillment of the requirement for the degree of Master of Science Department of Chemical and Biological Engineering 2013 This thesis entitled: Thermochemical Cycle of a Mixed Metal Oxide for Augmentation of Solar Thermal Energy Storage Using Solid Particles written by Brian David Ehrhart has been approved for the Department of Chemical and Biological Engineering _____________________________________________ Alan W. Weimer, Committee Chair _____________________________________________ David E. Clough, Committee Member Date _______________ The final copy of this thesis has been examined by the signatories, and we Find that both the content and the form meet acceptable presentation standards Of scholarly work in the above mentioned discipline. Ehrhart, Brian David (M.S., Chemical and Biological Engineering) Thermochemical Cycle of a Mixed Metal Oxide for Augmentation of Solar Thermal Energy Storage Using Solid Particles Thesis directed by Professor Alan W. Weimer An exploration was done on the feasibility of storing both sensible and thermochemical energy at high temperatures for concentrated solar power in order to mitigate issues with each type of energy storage alone. Two potential processes were suggested and discussed for use with a solid oxide reaction: an augmented solid particle receiver and a dish system with a gaseous heat transfer fluid and solid blocks of active material. Thermochemical energy storage using the “hercynite cycle” has been explored using the FACTSageTM Gibbs free energy minimization software, which predicted material compositions and enthalpy changes at conditions of interest. Calculations predict that the hercynite cycle material reduces above 1000°C and <2% O2. The hercynite cycle reduces with a reaction enthalpy of 264.8 kJ/kg 1400°C and <0.5% O2; this is 18.5% of the total sensible energy in the same material from 23°C to 1400°C. The reduction enthalpies were compared to a more limited exergy, which was calculated from 900°C instead of 23°C. The highest fraction of this enthalpy comparison was 66.1% at 950°C at 0% O2, despite the fact that the reduction reaction had less conversion. The thermochemical enthalpy compared favorably to this smaller exergy, indicating that it is useful to match the reaction temperature changes to the temperature range of the process. The isothermal thermochemical enthalpy was predicted to be up to 131.3 kJ/kg, which is 9.2% of the full sensible energy at 1400°C. iii Various material formulations were cycled in a TGA/DSC at temperatures between 900°C and 1500°C using argon and air during reduction and oxidation. The observed oxidation enthalpies spanned an order of magnitude, from 10 – 100 kJ/kg. Isothermal energy storage was demonstrated at 1200°C, resulting in enthalpy values of 32.6 kJ/kg. Mixtures with excess Al2O3 tended to have lower observed specific heats of reaction due to the additional inert material. The heats of reaction obtained for the oxidation exotherms were lower than equilibrium predictions and it is suggested that side reactions not predicted by well-mixed thermodynamic equilibrium are occurring and contributing to changes the total reaction enthalpy; data from XRD and Raman Spectroscopy indicate that this may be occurring. iv To my parents, David and Julie, who have always supported and believed in me, And To Sarah, who has stuck with me through it all v ACKNOWLEDGMENTS There are many, many people who have helped me in the past few years make my time here possible and enjoyable. First, I would like to thank my adviser, Professor Al Weimer. He agreed to take me on in somewhat unconventional circumstances, and has always been very supportive of my ideas and abilities. His “hands-off” management style seems at time to give me just enough rope to hang myself, but also provides invaluable experience at independent thought and action, and I wouldn’t want it any other way. Next I would like to thank Professor Nate Siegel, now at Bucknell University. He helped get me started at Sandia, and taught me much and more about the importance of thermal energy storage. Aside from lots of advice about research in general, he really helped get me excited about solar energy by teaching me to melt metal with the sun; definitely an experience worth having. He helped get me started on this project, suggesting this as an area of exploration when I was floundering for ideas. He has also contributed many very helpful (and understanding) discussions throughout my career. I would also like to thank two of my mentors at Sandia: David Gill and Brian Iverson. Dave is a researcher at Sandia and the de-facto “thermal energy storage guy” at Sandia now. Brian Iverson is now a professor at Brigham Young University, and will do very well. Both have given me plenty of advice on all sorts of topics from the etiquette of peer review to proposal writing, and both have given me a lot of support which is very much appreciated. Experimental results are not always easy to get, and so I would also like to thank the various people who helped obtain them. Eric Coker at Sandia National Laboratories graciously let me use his TGA, and helped collect and analyze the results. He also contributed a (large) vi number of very helpful and insightful discussions regarding the chemistry of high temperature solid materials, which were very valuable. I would like to thank Mark Rodriguez, also at Sandia, for running a sample on his High-Temperature XRD and for helping with analysis of the results. Thank you to Kristin Meyer, who ran XRD samples for me at the AML while an intern at Sandia. Thanks to you Kalvis Terauds, a graduate student in the Department of Mechanical Engineering here at the University of Colorado, for running samples in the Raman Spectrometer and for help figuring out the results. I’d like to thank Kim Zimmer at CU for running samples and answering any questions I have. I’d also like to thank Darwin Arifin and Torrie Aston for helpful discussions and help with preparing and running samples. I also want to thank the rest of “Team Weimer” for many helpful discussions and lots of support. It is invaluable to have people to bounce ideas off of, and very interesting to discuss research topics ranging from catalysis to solar hydrogen production to poop pyrolysis. My sincere thanks also to Dom De Vangel in the ChBE department at CU for answering a multitude of questions over the years and for helping to get all programmatic and departmental issues figured out. I would also like to thank many different people at Sandia National Laboratories for their support. I would like to thank my first manager, Joe Tillerson, who hired me on under the Critical Skills Master’s Program; this experience was a fantastic opportunity for me and my career, and a very humbling vote of confidence in my abilities. I would like to thank my next manager Bill Kolb, who always had time for me despite being absurdly busy with big changes in the department. I would also like to thank my current manager, Subhash Shinde, who continues to support me while I try to further my education and experience. Thank you also to Rich Diver, who retired from Sandia before I went to grad school and now consults on solar energy vii applications and research, for many valuable lessons and discussions. Many thanks to all the rest of Org 6123 at Sandia for always being helpful and supportive. I am very grateful to be able to participate in the CSMP; it is a great benefit and opportunity. I would like to thank Suzanne Moya, who was program manager of the CSMP when I started, and helped me get off to grad school. My thanks also to Rick Alexander, the current program manager of the CSMP, who continues to keep me on the right track and is very supportive of my future plans. Finally, I would very much like to thank Camille Valdez, who provides invaluable help and support in navigating the CSMP and Sandia in general, and who has always had answers to all of my many, many questions. I would like to make some personal acknowledgments. My love and thanks to my parents and family, who are always very supportive of everything that I do. We are definitely a scientific family, and I love you all. Lastly, I would very much like to give a heartfelt thank you to Sarah; her love and support have been invaluable, and her willingness to listen to late-night rants about the intricacies of beer brewing and thermochemical energy storage is very kind. I love you very much, and look forward to many more years together. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04- 94AL85000. viii CONTENTS CHAPTER I. INTRODUCTION ............................................................................................................ 1 1.1 The Need for Renewable Solar Energy ............................................................ 1 1.2 Concentrated Solar Power................................................................................. 5 1.3 Thermal Energy Storage ................................................................................. 11 1.4 Thermochemical Energy Storage