Thermochemical Cycle of a Mixed Metal Oxide for Augmentation of Solar Thermal Energy Storage Using Solid Particles

THERMOCHEMICAL CYCLE OF A MIXED METAL OXIDE FOR AUGMENTATION OF SOLAR THERMAL ENERGY STORAGE USING SOLID PARTICLES

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.

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

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)

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 ...... 16

1.5 Power Cycles ...... 22

1.6 High Temperature Operation ...... 25

1.7 Scope of Thesis ...... 31

References ...... 34

II. THERMOCHEMICAL AUGMENTATION ENERGY STORAGE CONCEPT ...... 43

2.1 Use of Sensible and Thermochemical Energy Storage ...... 43

2.2 Gas-Solid Reactions ...... 46

2.3 Hercynite Thermochemical Cycle ...... 49

2.4 Potential System Concepts ...... 52

2.4.1 Chemically Augmented Solid Particle Receiver Concept ...... 52

2.4.2 Dish with Chemically Augmented Solid Storage System Concept . 56

2.5 Oxygen Separation ...... 61

2.6 Conclusions ...... 64

References ...... 68

III. THEORETICAL PREDICTIONS ...... 71

3.1 Introduction ...... 71

3.2 Methods...... 72

3.3 Results ...... 77

3.4 Discussion ...... 83

3.5 Conclusions ...... 95

References ...... 100

IV. EXPERIMENTAL EXAMINATION OF HERCYNITE CYCLE ...... 102

4.1 Introduction ...... 102

4.2 Experimental Setup and Methods ...... 104

4.3 Results ...... 109

4.4 Discussion ...... 116

4.5 Conclusions ...... 127

4.6 Future Work ...... 131

References ...... 132

V. CONCLUSIONS ...... 133

5.1 Thermochemical Augmentation of Sensible Energy Storage Concept ...... 133

5.2 Theoretical Exploration of Hercynite Cycle for Thermochemical

Augmentation of Thermal Energy Storage ...... 136

5.3 Experimental Investigation of Hercynite Cycle for Thermochemical Energy

Storage Potential ...... 141

5.4 Future Work ...... 144

BIBLIOGRAPHY ...... 146

APPENDIX A: OUTPUT FROM THERMODYNAMIC EQUILIBRIUM PREDICTIONS ... 151

A.1: Calcination Composition Comparison Prediction Results ...... 151

A.2: Composition Predictions for Hercynite Cycle Material with Inert Atmosphere .... 155

A.3: Isothermal Reduction-Oxidation Equilibrium Prediction Results for Hercynite Cycle

...... 157

A.4: Estimation of Thermochemical Heat of Reaction for Stoichiometric Hercynite Cycle

at Various Temperatures and O2 Concentrations ...... 158

A.5: Sensible Energy Calculations for Stoichiometric Hercynite Material ...... 165

A.6: Equilibrium Composition Predictions of Cobalt Oxide ...... 167

A.7: Comparison of Fe3O4 and Fe2O3 Synthesis Using Equilibrium Compositions of

Solid Material...... 168

APPENDIX B: EXPERIMENTAL DATA ...... 170

B.1: X-Ray Diffraction Data ...... 170

B.2: High-Temperature in-situ X-Ray Diffraction Data ...... 172

B.3: Raman Spectra ...... 173

B.4: SEM/EDX Data of Base Powder ...... 175

B.5: TGA/DSC Results ...... 177

B.5.1: Summarized Results ...... 177

B.5.2: Base Powder – Thermal Reduction and Isothermal Redox ...... 178

B.5.3: T-Oxidation Variance...... 179

B.5.4: Base Powder 1400-1000 Cycling ...... 182

B.5.5: 1400-1000 Cycle Test #2 ...... 183

B.5.6: Thermal Reduction 1500 Variable Ramp Rate ...... 184

B.5.7: Alumina-6 – Thermal Reduction and Isothermal Redox ...... 185

xii

B.5.8: Alumina-9 – Thermal Reduction and Isothermal Redox ...... 186

xiii

TABLES

TABLE 2-1: LIST OF POTENTIAL GAS-SOLID REACTION CYCLES ...... 46

TABLE 2-2: LIST OF VARIOUS THERMOCHEMICAL REACTIONS STUDIED FOR SPLITTING H2O AND

CO2 ...... 48

TABLE 2-3: CALCULATION OF EXERGETIC EFFICIENCY OF “HERCYNITE” CYCLE FOR VARIOUS

TEMPERATURES AND ENERGETIC EFFICIENCY LEVELS, SHOWING THE EFFECTS OF DIFFERENT

OPERATIONAL TEMPERATURES AND ENERGY EFFICIENCY VALUES ...... 52

TABLE 3-1: CALCULATION INPUTS AND METHODOLOGY FOR ISOTHERMAL REDOX AT 1200°C .... 76

TABLE 3-2: COMPARISON OF ISOTHERMAL THERMOCHEMICAL HEAT STORAGE TO SENSIBLE HEAT

STORAGE AT VARIOUS TEMPERATURES...... 90

TABLE 3-3: PREDICTIONS OF REACTION ENTHALPIES FOR OTHER SOLID OXIDE THERMOCHEMICAL

REDUCTION REACTIONS ...... 91

TABLE 4-1: THEORETICAL PREDICTION OF BASE POWDER 1400-1000 CYCLES EXPERIMENTAL RUN

AND COMPARISON TO DATA ...... 115

TABLE 4-2: THEORETICAL PREDICTION OF ALUMINA-6 VARIABLE THERMAL REDUCTION AND

ISOTHERMAL REDOX EXPERIMENTAL RUN AND COMPARISON TO DATA ...... 115

TABLE 4-3: THEORETICAL PREDICTION OF ALUMINA-9 VARIABLE THERMAL REDUCTION AND

ISOTHERMAL REDOX EXPERIMENTAL RUN AND COMPARISON TO DATA ...... 116

TABLE 4-4: BASIC STATISTICS FOR BASE POWDER 1400°C-1000°C CYCLES ...... 118

xiv

FIGURES

FIGURE 1-1: CONCENTRATING SOLAR POWER PROSPECTS OF THE SOUTHWESTERN UNITED STATES,

CORRECTED FOR POTENTIALLY ENVIRONMENTAL SENSITIVE LANDS, MAJOR URBAN AREAS,

WATER FEATURES, LAND AREA WITH SLOPE >1%, AND CONTINUOUS AREAS OF LESS THAN 1

SQUARE KILOMETER (FROM [8])...... 4

FIGURE 1-2: SCHEMATIC (NOT TO SCALE) OF A PARABOLIC TROUGH PLANT WITH TWO-TANK,

MOLTEN SALT THERMAL ENERGY STORAGE AND SYNTHETIC OIL HEAT TRANSFER FLUID

(FROM [15]) ...... 7

FIGURE 1-3: SYSTEM SCHEMATIC OF A DIRECT STORAGE MOLTEN SALT CENTRAL RECEIVER

SYSTEM (FROM [23]) ...... 9

FIGURE 1-4: SCHEMATIC OF PARABOLIC DISH-STIRLING SYSTEM SHOWING REFLECTIVE MIRRORS,

SUPPORTING STRUCTURE, SUPPORTING PODIUM, AND STIRLING ENGINE WITH SUPPORTING

BOOM ARM (FROM [15]) ...... 10

FIGURE 1-5: SCHEMATIC DIAGRAM OF THERMOCHEMICAL ENERGY STORAGE USING AMMONIA

(FROM [42]) ...... 17

FIGURE 1-6: CALCULATION OF THE DIFFERENCE BETWEEN THIGH AND TCOOL FOR INDICATED THIGH

VALUE IN ORDER TO ACHIEVE 95% EXERGETIC EFFICIENCY WHILE ASSUMING 100%

ENERGETIC EFFICIENCY ...... 19

FIGURE 1-7: SULFUR THERMAL ENERGY STORAGE SYSTEM, BASED UPON A COMBINED-CYCLE

POWER PLANT WITH SO2 CONVERSION (FROM [46]) ...... 20

FIGURE 1-8: ILLUSTRATION OF HEAT EXCHANGER TEMPERATURE PROFILES FOR (A) A SENSIBLE

HEAT TRANSFER FLUID WITH AN ISOTHERMAL WORKING FLUID, (B) A SENSIBLE HEAT

TRANSFER FLUID EXCHANGING HEAT WITH A SENSIBLE WORKING FLUID, (C) AN

ISOTHERMAL HEAT TRANSFER FLUID WITH AN ISOTHERMAL WORKING FLUID, AND (D) AN

ISOTHERMAL HEAT TRANSFER FLUID WITH A SENSIBLE WORKING FLUID ...... 25

FIGURE 1-9: (A) CONCEPTUAL DESIGN OF A SOLID PARTICLE RECEIVER (B) SCHEMATIC DIAGRAM

OF SOLID PARTICLE RECEIVER SYSTEM (BOTH FROM [63]) ...... 29

FIGURE 2-1: THEORETICAL AND EXPERIMENTAL EVOLUTION OF OXYGEN VIA “HERCYNITE” CYCLE

AT VARIOUS TEMPERATURES (FROM [1])...... 50

FIGURE 2-2: CYCLING OF “HERCYNITE” MATERIAL UNDER CONSTANT FLOW OF OXYGEN (FROM

[7]) ...... 51

FIGURE 2-3: SCHEMATIC DIAGRAM OF DISH CONCEPT (NOT TO SCALE) ...... 57

FIGURE 2-4: SCHEMATIC DIAGRAM OF SOLID REDOX STORAGE UNIT CONCEPT (NOT TO SCALE) . 60

FIGURE 2-5: SCHEMATIC OPERATION OF A SINGLE ELECTROCHEMICAL CELL IN AN ITM SEOS

DEVICE (FROM [20]) ...... 62

FIGURE 2-6: O2 PRODUCTION OF A THREE-CELL STACK TEST (FROM [20]) ...... 63

FIGURE 3-1: PREDICTED COMPOSITION OF SOLID MATERIAL AT THERMODYNAMIC EQUILIBRIUM

AT VARIOUS TEMPERATURES WITH INERT DILUTANT ...... 78

FIGURE 3-2: ISOTHERMAL THERMOCHEMICAL ENERGY STORAGE FOR HERCYNITE CYCLE USING

CHANGES IN EQUILIBRIUM COMPOSITIONS AND ENTHALPY ...... 80

FIGURE 3-3: CHANGES IN AMOUNT OF SOLID MASS PRESENT DURING FOUR ISOTHERMAL

REDUCTION/OXIDATION CYCLES AT VARIOUS TEMPERATURES FROM 1000°C TO 1400°C .... 80

FIGURE 3-4: THERMOCHEMICAL REACTION ENTHALPY CHANGE FOR STOICHIOMETRIC HERCYNITE

CYCLE MATERIAL BASED ON THERMODYNAMIC EQUILIBRIUM CALCULATIONS AT

TEMPERATURES BETWEEN 900°C AND 1400°C AND OXYGEN CONCENTRATIONS BETWEEN 0%

AND 10% WITH BALANCE INERT ARGON ...... 81

xvi

FIGURE 3-5: CALCULATED SPECIFIC MOLAR-WEIGHTED HEAT CAPACITY OF REACTIVE SOLID

MATERIAL ...... 82

FIGURE 3-6: CALCULATED CUMULATIVE EXERGY OF REACTIVE SOLID MATERIAL ...... 83

FIGURE 3-7: REMAINING SOLID MASS OF REDUCTION REACTION AS A PERCENTAGE OF ORIGINAL

SOLID MATERIAL PRESENT FOR VARIOUS TEMPERATURES AND O2 CONCENTRATIONS ...... 84

FIGURE 3-8: COMPARISON OF THERMOCHEMICAL HEAT OF REACTION TO TOTAL SENSIBLE

EXERGY AT INDICATED TEMPERATURE ...... 86

FIGURE 3-9: COMPARISON OF THERMOCHEMICAL HEAT OF REACTION TO SENSIBLE EXERGY AT

INDICATED TEMPERATURE FROM 900°C...... 87

FIGURE 3-10: PREDICTED COMPOSITION OF SOLID COMPONENTS AT THERMODYNAMIC

EQUILIBRIUM FOR COBALT OXIDE REACTION AT TRANSITION TEMPERATURES AND WITH

INERT ATMOSPHERE ...... 94

FIGURE 4-1: COMPARISON OF PREDICTED EQUILIBRIUM COMPOSITIONS OF SOLID MIXTURES OF

COO+FE2O3+3AL2O3 AND COO+FE3O4+3AL2O3...... 106

FIGURE 4-2: SEM IMAGE OF BASE POWDER ...... 110

FIGURE 4-3: X-RAY DIFFRACTION PATTERNS FOR “BASE” SAMPLE POWDER FOR XRD SCANS

DONE IN-SITU AT THE TEMPERATURES INDICATED ...... 111

FIGURE 4-4: HIGH TEMPERATURE IN-SITU X-RAY DIFFRACTION PATTERNS FOR “BASE” SAMPLE

POWDER FOR SCANS DONE ON THE INITIAL SAMPLE, UNDER REDUCING CONDITIONS, AND

AFTER RE-OXIDATION ...... 112

FIGURE 4-5: RAMAN SPECTRA PATTERNS FOR “BASE” SAMPLE POWDER FOR SCANS DONE WITH

UNALTERED MIXED POWDER, CALCINED POWDER, AND SAMPLE POWDER THAT HAD BEEN

CYCLED IN THE TGA/DSC...... 113

xvii

FIGURE 4-6: HEAT OF OXIDATION FOR BASE, ALUMINA-6, AND ALUMINA-9 MATERIAL

FORMULATIONS AT INDICATED REDUCTION AND OXIDATION TEMPERATURES ...... 114

FIGURE 4-7: COMPARISON OF FRACTIONAL MASS CHANGES FROM EXPERIMENTAL RESULTS FOR

BASE POWDER 1400-1000 CYCLES TO THEORETICAL PREDICTIONS ...... 119

FIGURE 4-8: COMPARISON OF REACTION ENTHALPIES FROM EXPERIMENTAL RESULTS FOR BASE

POWDER 1400-1000 CYCLES TO THEORETICAL PREDICTIONS ...... 121

FIGURE 4-9: COMPARISON OF REACTION ENTHALPIES FROM EXPERIMENTAL RESULTS FOR

ALUMINA-6 VARIABLE THERMAL REDUCTION AND ISOTHERMAL REDOX TO THEORETICAL

PREDICTIONS ...... 122

FIGURE 4-10: COMPARISON OF REACTION ENTHALPIES FROM EXPERIMENTAL RESULTS FOR

ALUMINA-9 VARIABLE THERMAL REDUCTION AND ISOTHERMAL REDOX TO THEORETICAL

PREDICTIONS ...... 123

FIGURE 4-11: RAMAN SPECTRA OF ACTIVE MATERIAL MADE VIA ATOMIC LAYER DEPOSITION

WITH COMPARISON TO LITERATURE SPECTRA (FROM [2]) ...... 127

xviii

CHAPTER I

INTRODUCTION

1.1 The Need for Renewable Solar Energy

There is a fundamental limit on the amount of energy that can be obtained from fossil fuels. Eventually, these limited resources will run out, necessitating a shift to more sustainable energy sources. Efficiency gains in energy usage are important, but global electricity demand is growing exponentially and this trend is expected to continue for the foreseeable future; this trend is especially true for emerging markets such as China, India, and the Middle East [1, 2]. While it is becoming more widely understood that any long-term energy solution will need to rely almost exclusively on renewable energy sources, it is important to note that renewable energy sources will be important even in the near and medium term [1]. This is due to the fact that the transition to renewable energy sources must be a gradual one, in order to ensure that the infrastructure will be able to support and accept different (and intermittent) energy sources [2].

Another important aspect to the use of alternative energy sources is the problem of greenhouse gas emissions. Carbon emissions from energy consumption and generation have nearly tripled in the past 60 years, despite some recent reductions [3], and emissions are expected to more than double again by 2035 [2]. Coal is one of the largest producers of carbon dioxide, sulfur dioxide and nitrogen oxide emissions, emitting more than other energy generation sources combined in 2010 [3]. These emissions are especially concerning for developing economies, which typically have much faster growing populations and less stringent environmental controls, and these emissions are expected to grow the fastest in the near future. Emissions from developing economies such as India and China currently exceed emissions from developed

economies by 24%, and are expected to exceed emissions from developed countries by more than 100% by 2035 [2].

Renewable electricity generation has a strong potential to drastically reduce harmful emissions. The majority of the emissions come from coal, and coal generates 46% of the electric power in the United States [3]. Based on the criteria that long term clean, sustainable energy should be completely renewable and carbon-free, solar and wind energy are two of the most promising options which can provide completely renewable and carbon-free electricity without radioactive waste. There is also the possibility of wide distribution of these technologies, with strong solar resources in the Southwest United States, and strong wind resources in the

Midwestern United States [3]. Solar power has very limited distribution due to high cost, and thus has a large growth potential once efficiencies are increased and costs are reduced [2]. Wind power has grown rapidly in the past decade, from 18 GW net installed capacity in 2000 to 121

GW in 2008 [2]. This growth is due to lower costs, wider acceptance, and government subsidies, and this growth is expected to continue into the future [2].

The sun radiates approximately 1.7 × 105 TW of power to the earth [4]. It is obvious that much of this energy would not be realistic to use due to the fact that it is very diffuse and impacts a lot of the earth’s surface that is unrealistic for development. However, if only 0.01% (of the 1.5

9 × 10 TWhth) of the sun’s thermal energy could be utilized at a very conservative 15% thermal to electric conversion, it could more than meet the world’s demand for electricity, which is about

4 1.8 × 10 TWhe [4]. This is especially true in the United States, which has some of the best solar resource in the world [4, 5]. Of the total amount of solar energy that irradiates the U.S. every day, even if land not suitable for concentrated solar power development (such as urban areas, environmentally sensitive lands, and lands with a grade of >1%) are removed, the amount of

solar energy available in the Southwestern United States is still substantial (see Figure 1-1) . It was found that when these filters were applied, that the Southwest United States still has the potential to produce several thousand GW of electric power [6], which is more than four times the total electricity demand of the entire contiguous United States (759.6 GW in 2011) [7]. Very few other places on earth have this large amount of high quality solar resource over large areas, most notably some parts of Africa and Australia [4, 5]. While it would be difficult and unrealistic to power the entire U.S. from the Southwest alone, this does serve to illustrate the very large and mostly untapped potential for solar electricity generation in the Southwest.

Figure 1-1: Concentrating Solar Power Prospects of the Southwestern United States, Corrected for Potentially Environmental Sensitive Lands, Major urban Areas, Water Features, Land Area with Slope >1%, and Continuous areas of Less than 1 Square Kilometer (from [8])

Solar and wind energy do have the theoretical potential to provide 100% of energy to the world, but there is a concern with the transient nature of these energy sources. This is because both wind and solar energy can vary widely throughout the year, month, day, and even throughout a single hour. This wide variability and rapidly changing power level makes it difficult to use these power sources on a utility scale. It has been estimated that only 50% of the total energy supply of the U.S. can come from intermittent renewable sources such as wind and solar without putting undue demands on the electric infrastructure [9]. Therefore, a way to store

energy in order to even out supply in order to meet demand will be required for a reasonable long-term energy solution.

1.2 Concentrated Solar Power

Concentrated solar power (CSP) uses mirrors to concentrate the sun’s rays onto a receiving material, heating it to a higher temperature. This thermal energy is then used to drive a power cycle for the production of electric power. This overcomes the diffuse nature of sunlight be concentrating it into a higher quality energy source. There are different configurations of CSP technologies that broadly fall into one of two categories: line-focus and point-focus systems.

Line-focus systems use mirrors to focus the concentrated sunlight onto a linear target, such as a parabolic trough or linear Fresnel system [10]. As the name suggests, these types of systems focus sunlight onto a line which typically a pipe which contains the heat transfer fluid. By contrast, a point-focus system directs the concentrated sunlight onto a single point, as in a central receiver or dish system [10]. These systems typically have much higher concentration ratios, since the concentration of sunlight occurs in two axes, rather than a single axis as in line-focus systems [10].

Parabolic troughs are by far the most mature of the CSP technologies; current parabolic trough systems have been operating since 1985 [11]. These systems have been deployed all over the world, such as the Solar Electric Generation Systems (SEGS) in California [11] and the

Archimede plant in Sicily, Italy [12]. These systems pump a heat transfer fluid through tubular receivers at the focus of a curved mirror that is in the shape of a parabola to focus the sunlight onto the receiver tube (see Figure 1-2). These troughs are aligned north-south and track the sun east-west during the day. Parabolic trough systems are beneficial because they are a mature

technology and relatively simple to implement. However, parabolic troughs suffer temperature and thus efficiency limitations. This is due to the fact that troughs cannot achieve high enough solar concentration ratios to produce high temperatures [10]. Additionally, the heat collection elements are spread out over a very large area throughout the solar field; some receiver tubes can be as long as 150 meters [13]. This means that there is a very large amount of area that will lose heat to the ambient. The receiver tubes are typically vacuum insulated [10], which greatly reduces thermal losses, but the receiver tube length still provides large amounts of surface area for thermal losses. Another effect of this large solar collection field is the issues associated with moving heat transfer fluids through a large field: large pumping requirements and clogging of the lines. Moving a large amount of liquid through very long pipes requires a large pumping load

[14]. Heat transfer fluids have typically been synthetic oils [15], though recently molten nitrate salts have begun to be used [16]. The previously mentioned Archimede solar plant is the first commercial parabolic trough plant which uses a molten salt heat transfer fluid [12]. When molten salts are used, they can freeze in the long lines of the solar field due to the high freezing point of the salts, typically around 250°C. This means that the entire solar system must be heat traced with expensive electric heating in order to prevent salt freezing [15, 16].

Figure 1-2: Schematic (not to scale) of a Parabolic Trough Plant with Two-Tank, Molten Salt Thermal Energy Storage and Synthetic Oil Heat Transfer Fluid (from [15])

Another line focus system is the Linear Fresnel configuration. In this technology, a field of individually tracking flat mirrors focuses the solar radiation onto a tubular receiver, which is raised up off the ground above the mirrors. Similar to a Fresnel lens, the Fresnel reflectors are able to achieve concentration with many individually aligned flat mirrors instead of a single parabolic mirror. This allows for the solar concentration to be done with smaller and lower profile mirrors; instead of the mirrors being a continuous sheet which can lead to very high wind loads, the smaller mirrors are low to the ground, making control and support much easier [10].

However, this technology does run into some issues due to shading from the raised tubular receiver [10] and the fact that this technology cannot achieve very high concentration ratios, limiting the maximum operating temperature and associated system efficiency.

Central receiver systems, also called “power towers”, are widely regarded as a promising technology due to the high temperatures that can be achieved. In these systems, a field of mirrors

(called heliostats) tracks the sun in two axes and reflects the image onto a receiver on top of a

tower (see Figure 1-3). Many past research projects have demonstrated the operation of power towers, including the Solar One (operated from 1982 to 1988) and Solar Two (operated 1996-

1999) projects by Sandia National Laboratories near Barstow, CA [17]. These projects demonstrated operation of a 10 MWe direct steam generation system (Solar One) and demonstration of a 10 MWe molten nitrate salt system with thermal storage (Solar Two); both systems used a conventional Rankine cycle to generate electricity [17]. Many commercial systems are in operation and under construction around the world, such as Gemasolar in Fuentes de Andalucía (Seville, Spain), a 19.9 MWe power tower with 15 hours of thermal energy storage. Owned by Torresol Energy, this plant came online in May 2011 [18]. Additionally,

BrightSource began construction of a three tower, 377 MWe capacity Ivanpah Solar Electric

Generating system in Ivanpah Dry Lake, California [19], and SolarReserve began constructing a

110 MWe molten salt power tower near Tonopah, Nevada, in September 2011 [20]. However, a downside of these systems is that the systems must be very large scale (100 MWe or more) in order to be more cost effective [21]. This very high capital cost can be prohibitive for some projects, making widespread deployment difficult. Additionally, the current heat transfer fluid being used in these systems, a molten nitrate salt eutectic, is limited to a maximum operational temperature of around 565°C due to thermal stability and degradation issues [22].

Figure 1-3: System Schematic of a Direct Storage Molten Salt Central Receiver System (from [23])

Dish systems use mirrors arranged in a 3-dimensional parabolic shape to reflect the sun to a central point at the focus. These systems track the sun in two axes and can achieve very high solar concentration ratios. Sunlight concentration is expressed in suns, where a single sun (~1000

W/m2) is the approximate average incident solar flux at the earth’s surface [10]. While most parabolic troughs can achieve 15-45 suns and current power tower configurations can achieve

150-1500 suns, dish systems can easily achieve 100-3000 suns concentration [10, 24]. Research and commercial systems in the past have run with Stirling engines in the dish focus to produce the electric power [10], and these Dish-Stirling systems currently hold the world record for the highest efficiency in converting natural sunlight to grid-ready power [25, 26]. Commercial dish systems have been installed at both small- and utility-scale deployments [27, 28].

Figure 1-4: Schematic of Parabolic Dish-Stirling System Showing Reflective Mirrors, Supporting Structure, Supporting Podium, and Stirling Engine with Supporting Boom Arm (from [15])

The high concentration ratios that dish systems can achieve means that dishes can utilize the solar resource in the most efficient manner possible. This can be done by using the concentrated sunlight to produce high temperatures for the heat engine to convert to electricity.

Higher temperatures will lead to more efficient heat engine operation [15]. This high concentration and the high temperatures that dishes can achieve mean that they can achieve higher temperatures than other CSP technologies [23, 29]; this means that dishes could have a high probability for achieving SunShot performance targets [24].

Many large scale parabolic trough and power tower systems have large-scale centralized power generation, meaning that the entire large-scale system must be built at once for power generation, leading to very high initial capital cost [30]. Additionally, once the system is built for a particular power level, the system cannot adjust its power output level without a major system retrofit, due to the fact that the power cycle is sized for a particular power production rating; the power cycle would need to be completely re-done along with the receiver, storage, and solar

field subsystems. By contrast, dish systems are very modular, in that each individual dish produces electric power. That means that as few as a single dish can be installed in a new location at a time, drastically lowering the initial capital cost barrier to power production.

Additionally, if it is desired to expand the scale of an existing installation, more dishes are simply added instead of a major redesign and retrofit of all existing systems. This makes dish technology much more flexible and attractive for small scale distributed power generation.

However, a major issue with current dish systems is the difficulty of adding thermal storage. This is due to the fact that the heat engine for electricity production is at the focus of the dish. Thermal storage takes up space, and when this added volume is attached to the heat engine, it optically blocks area below the heat engine that could be used for the most efficient mirrors, in addition to putting unrealistic weight requirements on the boom arm that supports the receiver

[24]. The alternate way that storage can be put on the ground near the dish, but this would require high temperature rotary joints around the dish tracking pivots [24]. As such, the receiver in the focus of the dish should be made as small as possible, as any excluded area that is blocked from reflecting and concentrating sunlight will be blocking the most efficient reflective area.

This has made it very difficult to add significant thermal energy storage to dish systems.

1.3 Thermal Energy Storage

As stated previously, concentrated solar power uses mirrors to concentrate the sun’s rays onto a receiving material, heating it to a higher temperature. This thermal energy can then be stored and later used to drive a power cycle for the production of electric power. This thermal storage is a key benefit of CSP, as it mitigates the issue of solar transience by decoupling power

production and solar irradiance [31]. This means that power production can continue when the sun is not shining, such as passing clouds or at night.

Thermal storage increases the time during which the electricity generation subsystem can be run, meaning that more power can be produced. This is reflected in the capacity factor, which is a fraction of amount of electricity produced during a set amount of time compared to the amount of electricity that would be produced if the electricity production system had been running and full capacity during the entire time [32]. Coal fired power plants typically have capacity factors on the order of ~70%, due to the fact that electricity production continues more or less constantly [32]. CSP systems can have a wide range of capacity factors, depending on the amount of thermal storage the system has. Past systems have demonstrated had capacity factors of between 14% and 19%. This includes the ANDASOL-1 plant (capacity factor of 14.70%),

Solar Two (19%), and SOLAR TRES-PSA (13.81%) [33]. Future system studies indicate that higher capacity factors (40%-70%) are more economically favorable, thus further stressing the importance of thermal storage [30]. The importance of these metrics is the fact that electric production systems typically take some amount of time to start up and shut down, which will always incur some sort of energy penalty. Therefore, if the electricity production system can run for longer, it will have fewer start-ups and shut-downs, meaning that less of the thermal energy is wasted. From an economic perspective, thermal storage also means that the electric power can be produced for longer time periods than just when the sun is shining, increasing revenues and thus the economic viability of CSP [34]. Additionally, thermal storage allows for electric power to be dispatched to the grid when demand for electricity is highest, maximizing the value for the electric power produced [10]. This resource transience is a major issue for most renewable energy sources.

There are various types of thermal energy storage. The simplest is sensible energy storage, where thermal energy is stored in a material by raising its temperature; the higher the temperature, the more thermal energy is stored in the material. This type of energy storage uses the heat capacity of the material and a temperature difference between the heat storage media to store and control temperature. This straightforward relationship is shown in Equation 1-1, in which Qsensible is the amount of heat stored via sensible thermal energy storage, m is the mass of storage material, CP is the specific sensible heat capacity of the material, and Thot and Tcool are the high and low temperatures of the system, respectively. As can be seen in this relationship, the two ways in which to store additional energy are to increase the amount of storage material used or to increase the temperature difference between the high and low system temperatures.

( ) ∫ ( ) Equation 1-1

Sensible energy storage can be achieved in a number of ways. Direct thermal energy storage is done by storing the heat transfer media once it has been sensibly heated in the solar receiver in an insulated tank [35]. The most common method of this type of storage is the state- of-the-art molten salt two-tank system [15]. In this system configuration, a molten nitrate salt eutectic is used as both the heat transfer fluid and storage media; the molten salt is pumped through a solar receiver, where it is heated to the hot operating temperature (~565°C), and then is sent to a hot storage tank. The salt is then drawn from the hot storage tank and set through a heat exchanger in order to produce steam and drive the power cycle. The cooled salt, which is still molten at about 290°C, is then sent to a cold storage tank until it is sent through the solar receiver once again (see Figure 1-3). The benefit of this type of direct storage system is the fact

that there is no intermediate storage material, necessitating an additional heat exchanger which can lead to more efficiency losses.

Alternatively, a thermocline energy storage system can be used, where both the hot and cold heat transfer fluid is stored in a single tank [35, 36]. This is done by establishing a buffer region, or “thermocline” in the middle of the tank, which is a region of heat transfer fluid which has a temperature gradient from the hot to cold temperatures. When the system is charging, the hot salt is pumped into one end of the tank while cold salt is drawn out the other side; this moves the thermocline along the tank in order to accommodate the changing volumes. Discharging the thermal energy storage system takes place in the opposite manner. A solid filler material can used in the tank in order to slow mixing and thermal diffusion [37]. This type of system is beneficial because it only needs a single storage tank instead of two (or more), but can suffer degradation of the thermocline, where the “buffer” between the hot and cold fluids is lost and mixing occurs. This leads to a warming of the cool fluid and a cooling of the hot fluid, which makes the heat transfer fluids less efficiently useable in either case. The thermocline can become unstable under certain conditions, such as when the flow rates are two high for the materials in use, leading to a mixing effect and degradation of the thermocline [38]. Additionally, even for stable thermoclines, the output temperature of the thermocline will degrade over significant periods of time [39].

However, it may be of interest to use a heat transfer fluid to absorb the thermal energy from the solar receiver but store the thermal energy using a different material. This avoids problems of having pressurized thermal energy storage (in the case of gaseous heat transfer fluid) or of having to store a high volume of high temperature liquid heat transfer fluid. This is especially important for expensive heat transfer fluids; if the thermal energy storage can be done

in a much cheaper material, this leads to very high cost savings [35]. This indirect storage has primarily been studied in two different ways. First, some parabolic trough CSP plants use synthetic oil as the heat transfer fluid in the field. However, this oil is very expensive, so the hot oil runs through a heat exchanger in order to store its heat in a molten salt storage system [36].

The thermal energy storage is discharged on the opposite fashion. Another indirect thermal energy storage method has been running a heat transfer fluid through a solid storage media, such as sand or rock [37]. This is done because the solid storage media used is typically very cheap, especially relative to the heat transfer fluid [37]. These indirect storage systems do suffer efficiency losses from the heat exchanger when transferring heat into or out of the storage system, but the cost savings can potentially outweigh the efficiency losses.

Another way to store energy in a material is through latent heat, or the energy transfer that occurs when a material changes phase. This is done at a single temperature, and the energy density is often much higher than sensible energy storage [35]. This isothermal heat storage is calculated via Equation 1-2, were Qlatent is the amount of heat stored, m is the mass of the energy storage media, and L is the specific latent heat at a particular temperature.

Equation 1-2

This storage density can be very important for thermal energy storage; if more energy can be stored per unit mass or per unit volume of storage material, less material must be used to store the same amount of energy. This generates cost savings both from the fact that less potentially expensive storage media is needed and the storage volume itself can be much smaller, leading to less heat losses from storage and expensive containment [35, 40]. Much like sensible energy storage, this type of system is controlled by changing the temperatures near the storage media.

However, the system must be designed to accommodate the volume change and changes in

materials properties that typically accompany phase change, which can sometimes be quite drastic [35]. Most research done in this area has attempted to encapsulate the phase change material in some kind of protective material in order to ensure good contact between the heat transfer fluid and the phase change material as it undergoes phase change; otherwise, the volume changes associated with phase can greatly reduce contact between the two [35, 40].

1.4 Thermochemical Energy Storage

A third method of storing energy is thermochemical energy storage. In this energy storage method, a reversible chemical reaction is used to store thermal energy: energy is absorbed via an endothermic reaction, and then released via an associated exothermic reaction.

This method offers some of the highest energy density values of any of the thermal energy storage methods, since the energy is stored in chemical bonds instead of the spaces between molecules [15, 35]. Energy storage in chemical bonds is beneficial for a number of reasons aside form high energy storage density; if the energy storage reaction is complete and the products are stable (under particular conditions), then the energy can be stored for long periods of time. This has been studied for seasonal energy storage applications, where additional heat is collected during especially hot and sunny months of the year and used during colder and darker months

[35]. Finally, another important aspect of thermochemical energy storage is the fact that the chemical reaction (or at least each half reaction of a thermochemical cycle) can be done at a single temperature. However, the conditions to control the chemical reactions may introduce additional parasitic losses to the process [15, 35].

There is currently no commercial technology that takes advantage of thermochemical cycles to store thermal energy at high temperature. Thermochemical energy storage using an

ammonia synthesis reaction has been studied extensively in the past [41]. This technology uses the ammonia dissociation/synthesis reaction [41]: NH3 + ΔH ↔ 1/2 N2 + 3/2 H2

In that system, ammonia dissociation was performed at 700°C in a solar reactor to produce H2 and N2 gas, then these two gases were stored in pressurized tanks. The two reactants could then be recombined to produce ammonia and heat on demand, thus boiling water to produce steam [41]. A system at Australian National University (ANU) has demonstrated 24- hour thermochemical energy storage on a 15 kW solar receiver and a 10 kW ammonia synthesis reactor [41]. However, this technology has some major drawbacks: the N2 and H2 gases must be stored at high pressure (~100 - 300 atmospheres), making storage of these gases difficult and costly [41]. Additionally, the ammonia synthesis component is constrained to very slow ramp rates, limiting this technology’s ability to provide power to match varying loads [41].

Figure 1-5: Schematic Diagram of thermochemical Energy Storage using Ammonia (from [42])

Another major concern with the ammonia synthesis technology is the exergetic efficiency loss of the system. The high-temperature ammonia-splitting reaction occurs at ~700°C [41], whereas the low-temperature synthesis reaction occurs at ~500°C [43]. This means that the high- grade heat in the solar reactor loses 11.5% of its exergy, or useful heat. This is calculated using

the equation of exergy (see Equation 1-3), and taking a ratio of the exergy of an amount of heat

Q at Thot of 700°C to an equivalent amount of heat Q at a temperature Tcool of 500°C (see

Equation 1-4). In this calculation, T0 is the ambient temperature, taken here to be 22°C. The exergy loss calculated here is a best-case scenario, as it assumes that there is no energy loss

(since the amount of energy, Q, for both cases is the same). Obviously any energy loss will directly affect the exergetic efficiency as well. It can also be seen that if both the Thot and Tcool temperatures are raised to higher values, the exergetic efficiency loss would be less (see Figure

1-6).

( ) Equation 1-3

( ) ( ) Equation 1-4

( ) ( )

Maximum Temperature Drop to Acheive 95% Exergetic Efficiecy 400

350

300

250

200 Del-T, K Del-T, 150

100

0 200 400 600 800 1000 1200 1400 1600 1800 T-high, K

Figure 1-6: Calculation of the Difference between Thigh and Tcool for Indicated Thigh Value in Order to Achieve 95% Exergetic Efficiency while Assuming 100% Energetic Efficiency

Another water splitting process has also been studied which uses a chemical cycle based upon the decomposition of sulfuric acid to split water into hydrogen and oxygen [44-46]. The sulfur-iodine process reacts with the SO2 from the sulfuric acid decomposition with iodine and water to form HI along with reforming the H2SO4. The HI is then decomposed to regenerate the iodine and produce hydrogen gas as a product of the system [44]. However, the reactive environments are highly corrosive, meaning special materials will be needed to protect against system damage; this is even more of a concern at high temperatures [47]. Additionally, the concentrated sunlight heats ambient air, which then transfers heat to the chemical cycle [46]; this indirect heating can be a major concern when efficiency is so important to CSP [48]. A slightly different version of this process is being studied to directly store thermochemical energy for CSP

(see Figure 1-7), where the decomposed H2SO4 produces SO2, which is sent to a disproportionation reaction to produce pure sulfur. The sulfur is then combusted to produce heat to produce electricity [46]. However, both of these cycles feature indirect heating of the reactants and a corrosive environment that make the cycle difficult to operate efficiently.

Figure 1-7: Sulfur Thermal energy Storage System, Based Upon a Combined-Cycle Power Plant with SO2 Conversion (from [46])

Work is also being done with solid oxides for thermochemical energy storage using an open system concept [49, 50]. In this system, the concentrated sunlight heats ambient air which flows over a bed of particles (moving or fixed) to heat them. The oxides reduce, storing heat.

Then later, when there is no sun, ambient air is flowed over the particles; the particles re-oxidize, releasing the stored energy to heat the air. In both cases, the hot air is used to drive a power cycle after it passes through the particle bed [49]. The reaction of cobalt oxide (Co3O4 ↔ 3CoO + ½

O2) is the considered in these studies, which is expected to have a high heat of reaction and can

reduce at temperatures above 900°C in air [50]. However, this reaction cycle does have some issues: it was found that there was lower than expected reactivity for the material, probably due to poor mixing [50]. Additionally, because air is used as the heat transfer fluid and it is not stored, the air must be reheated every time it passes through the cycle, thus lowering the overall efficiency. Furthermore, this cycle also suffers from exergetic losses due to temperature changes between reduction and oxidation. Much of the published work in this area has focused on packed beds, which have a high pressure drop and must be indirectly heated with air [51].

There have also been studies into thermochemical energy storage using calcium reactions. Specifically, calcium carbonate can be thermally reduced using solar energy at temperatures between 600°C and 900°C (CaCO3 ↔ CaO + CO2) [52]. The calcium solids and the carbon dioxide could then be stored separately and then recombined to produce electric power. Modeling efforts have predicted that a plant operating this type of cycle could achieve overall plant efficiencies at or above 40%, though it was noted that this is depended on an active material activity that may be unrealistic [52]. Furthermore, high temperature reaction chambers for the active material would need to be developed [52]. Additionally, this cycle would operate at elevated pressures for the reaction chamber (between 2 – 10 bar) and would store the carbon dioxide at 60 bar [52]; as seen previously, operating thermochemical cycles at high temperature and pressure can be difficult, and storing large amounts of gaseous reactants at high pressure can also be problematic.

Another calcium-based thermochemical cycle considered for thermochemical energy storage is calcium hydroxide. This reaction (Ca(OH)2 ↔ CaO + H2O) is predicted to react at somewhat lower temperatures of around 500°C [53]. This reaction is considered for a packed bed configuration, and analysis was done to determine if pressure drops or diffusional limitations

would affect the possibility of thermochemical energy storage for various particle size, and it was found through numerical predictions of a 1-D bed that particle diameters between 1 and 40 mm (depending on the system size) are required, though heat transfer limitations can be an issue for larger particles [53]. Experimentation has reported a reaction enthalpy of 104.4 kJ/mol, and stability has been tested over 100 reaction cycles [54]. Kinetics were found to be dependent on the partial pressure of water, and elevated pressures were found to be desirable [54]. The reaction with water does present some challenges to operation however; as stated before, high operating pressures can make efficient operation difficult. The high latent value of steam formation of water is also an issue, as this heat must be effectively recovered to avoid large efficiency losses

[53]. This can mean that the heat from steam condensation must be used whenever the forward reaction is run, rather than storing this heat for later. This imposes operational restrictions on the effectiveness of the thermal energy storage system, since a significant fraction of the heat must be used immediately (or stored separately).

1.5 Power Cycles

The end goal of concentrated solar power plant is electricity. While not the focus of this thesis, the way in which the electricity is generated does put constraints on the heat transfer fluid and thermal storage media, and so must always be included in any sort of design considerations.

As such, the main ways in which thermal energy from concentrated solar receiver or thermal storage subsystem is turned into electric power is through some kind of power cycle or heat engine. The most widely used method is through a conventional subcritical steam Rankine cycle

[10, 15]. In this cycle, liquid water is sent to a boiler where it is turned into superheated steam, which is then sent to a turbine to generate electricity. The near-saturated steam is then sent to a

condenser before being pumped back to the boiler to begin the cycle again. The Rankine cycle has been primarily used in parabolic trough and power tower systems in the past; indeed, for all power generation technologies, the subcritical Rankine cycle is the mostly widely used power generation cycle in the world [55]. However, the Rankine cycle is limited by the materials compatibility of supercritical water, which can only realistically operate at temperatures up to

610°C without significant corrosion [55].

Another power cycle of interest of CSP is the Stirling cycle. This cycle operates by utilizing the thermal volumetric changes of a working fluid in a closed system. A Stirling heat engine utilizes two pistons and a gaseous working fluid which cycles between them. As the working fluid heats up the volume increases, driving the primary piston to turn the shaft to produce work. The return stroke of the primary piston shifts the hot gas into the cold section, where a second piston is expanding to allow for the working fluid to enter. The gas is then cooled, decreasing the pressure. The return stroke of the secondary piston then compresses the gas and drives it into the hot section once again to complete the cycle. Stirling engines have primarily been used on dish systems for concentrated solar power production, due to the fact that

Stirling engines have high reported thermal-to-electric efficiency, which have been reported to be greater than 40% [56]. Furthermore, the Stirling engine is a closed system, meaning it has the potential for long-term, low-maintenance operation [56]. Stirling engines utilize isothermal heat, and typically operate at temperatures between 650°C to 800°C [57]. The Stirling cycle is limited by the cost of the working fluid and need to reject isothermal heat. The typically used helium gas is expensive for large scale power output, and hydrogen or air working fluids are not as efficient and suffer from the possibility of dangerous explosions at high temperatures [57]. Isothermal

heat rejection can lead to large radiators for heat rejection, which are not always feasible in small systems.

There are different configurations for each of the above cycles, and only a basic description of a simple implementation of the underlying cycle is provided here. There are also other thermodynamic cycles that could be considered, but these are the most commonly used for

CSP applications. An important concept for the electricity generation cycle is that the type of heat that is used in the system should be matched to the power cycle to be used. Parabolic troughs and power towers heat a working fluid between high and low operating temperatures, and use this sensible heat to run a Rankine cycle. Dish Stirling systems directly heat the working fluid of a Stirling engine, utilizing the isothermal heat this provides. If sensible heat is used to heat an isothermal power cycle, this results in large exergy losses, since the sensibly heated working fluid must operate a higher temperature in order to transfer heat to the power cycle, but the power cycle would still only operate at the single lower temperature [24]. This means that the exergy of the working fluid at all temperatures above this minimum temperature is lost.

Conversely, if an isothermal heat source or heat transfer fluid is transferring heat to a sensible power cycle, the isothermal heat must be at a higher temperature than the maximum temperature of the power cycle. This leads to a waste of exergy, since the high temperature isothermal heat is wasted on the lower temperature parts of the sensible power cycle (see Figure 1-8). This same logic applies to thermal energy storage, which is intimately related to the heat transfer fluid that will transfer heat to the power cycle. Thermal energy storage that is purely sensible would be inefficient for a Stirling cycle, since the temperature of the Stirling cycle would need to be lower than the lowest temperature in the heat exchanger (see Figure 1-8a). Similarly, a purely isothermal thermal energy storage system would be inefficient for something like a Rankine or

Brayton cycle (which will be explained below), since isothermal heat transfer fluid/thermal storage subsystem would need to be consistently at a higher temperature than the hottest temperature of the sensible working fluid in the heat exchanger (see Figure 1-8d). These exergy losses are very significant, and even more so at higher temperatures.

Figure 1-8: Illustration of Heat Exchanger Temperature Profiles for (a) a Sensible Heat Transfer Fluid with an Isothermal Working Fluid, (b) a Sensible Heat Transfer Fluid Exchanging Heat with a Sensible Working Fluid, (c) an Isothermal Heat Transfer Fluid with an Isothermal Working Fluid, and (d) an Isothermal Heat Transfer Fluid with a Sensible Working Fluid

1.6 High Temperature Operation

It has been suggested that solar power is beneficial since the sun offers “free energy”; however, this is misleading, since while the energy itself comes from solar radiation, the collection of this energy into a useful form is expensive. While no sort of finite fuel is consumed in the generation of this energy, the collection and conversion of this energy into a usable form

takes a potentially large amount of effort and expense. As stated above, the collection and concentration of the solar resource (e.g., heliostats) generally compose over half of the capital cost of the entire solar plant. Because of this, any reduction in the number of heliostats needed for a given level of power production will have a significant impact on the overall capital cost of the plant. One way to do this is to run a higher efficiency power cycle. If the thermal-to-electric conversion efficiency is higher, then less thermal energy is required to produce the same amount of electric energy. One significant impact of this smaller thermal energy requirement is that fewer heliostats are needed.

The general power cycle efficiency equations are best described by the Carnot cycle. This idealized power cycle provides a useful illustration of the effect of process temperatures. The

Carnot cycle describes an ideal system which is the most efficient possible cycle for converting an amount of thermal energy into work [58]. The relationship between process temperatures is given in Equation 1-5, where η is the process efficiency, T is the process temperature, and T0 is the heat rejection temperature, typically taken to be ambient temperature.

Equation 1-5

However, there are a number of issues with operating at higher temperature. The first is higher thermal losses. While it seems trivial to indicate that higher temperature operation will lead to higher thermal losses, it is especially worth noting for the temperatures being considered.

The Department of Energy SunShot Initiative is funding research to attempt to run systems at temperatures ≥650°C [59]. Even higher temperatures are also of interest for even more efficient cycles. While conduction and convective heat losses scale linearly with temperature, thermal radiation losses become especially important at these higher temperatures. This is because

thermal radiation scales with T4, meaning that the thermal losses at high temperatures can very quickly become prohibitive for process operation.

Another thermal loss issue with operation at higher temperature is when the system has stopped and must restart. The transient nature of solar power means that the solar collection must restart at least once a day, and sometimes even more often. When the receiver system is not operating, it quickly returns to ambient temperature [17]; when the receiver begins starting up in order to begin operating at design temperatures, the equipment itself must first be warmed up from the ambient temperature. For large scale systems, this can be quite a significant thermal mass. This constitutes an energy penalty that must is incurred whenever the system starts up.

This is not necessarily accounted for when analyzing the system at the design point, this can constitute quite a significant loss for plant operation, since it must be incurred at least once a day.

Materials that are able to operate at high temperatures offer another obstacle to high temperature. Regardless of which heat transfer and storage media are used, some sort of containment materials must be used to hold them. This can lead to materials compatibility issues with the high temperatures. There are very few materials that are rated to operate at ultra-high temperatures. Generally, to operate at temperatures at or above 1000°C, ceramics must be used, but these types of materials are typically very brittle and not mechanically robust. Very few alloys can operate at temperatures approaching these, but the ones that can are Haynes® HR-

120® and HR-160® alloys, which can operate at temperatures up to 982°C [60, 61]. This has important implications for general containment materials, but especially for materials that will be used to run the power cycles. The Brayton cycle operates at high temperature and high pressure, and this can be very difficult to do from a materials standpoint. The Haynes® 230® alloy is the only alloy that is rated up to 982°C and 40 MPa pressure [62]. Some system components can use

ceramic containment materials, but some must necessarily have some strength in containment, and the options for these kinds of materials are severely limited.

One area of research for high temperature operation is the development of different heat transfer media. Materials that are able to withstand temperatures much greater than the current limits of molten nitrate salts are necessary to enable process operation along with a higher temperature power cycle. Solid particle receivers (SPRs) have been studied in the past for ultra- high temperature operation. This technology uses small and highly dense ceramic particles as a heat transfer “fluid”, using the solid particles themselves to absorb, transport, and store heat. A diagram of this type of process is shown in Figure 1-9, along with a diagram of an example receiver. This technology has been studied in a central receiver configuration, and generally with a 2-tank storage configuration. The particles are mechanically lifted to the top of the solar tower, where they fall through a cavity receiver while being heated with the concentrated solar radiation. The particles then fall into a hot particle storage tank, where they are held until electricity production is desired. When it is, the particles fall into a heat exchanger, where the heat is transferred to whatever power cycle is being utilized. The now cooled particles then fall into a cold storage tank, where they are held until being lifted to the on-sun receiver once again

[63].

a b

Figure 1-9: (a) Conceptual Design of a Solid Particle Receiver (b) Schematic Diagram of Solid Particle Receiver System (both from [63])

Solid particle receivers were initially studied in the 1980s at Sandia National

Laboratories, and this work was primarily focused on material selection for the solid particles and modeling of the receiver system. This initial work identified commercially available (at the time) zirconia and alumina particles as a material of interest for SPRs, and found that it is able to absorb solar radiation and flow without aggregation at temperatures up to 1200°C [63, 64]. The particles of interest were dark in color, and had very good (up to 95%) solar absorptivity, which is the optical absorptivity of the material weighted to solar wavelengths [63, 64]. Additionally, two-dimensional thermal modeling of the receiver suggested that open cavity receivers could achieve 60% thermal efficiencies, though this could be improved with windows or smaller apertures [63]. More recent efforts have made three-dimensional models to examine flow and thermal behavior in the receiver, and find that well-designed receivers can reach thermal efficiencies of 72% to 78%, and that convective and radiative heat losses are the most significant

at temperatures from 300 to 800°C [65]. Experimental validation of these models has also been performed, and it was found that the theoretical modeling generally agreed within 10% of experimental heat loss values [66]. Other efforts in solid particle receiver research are exploring the use of a gaseous screen (Aerowindow) instead of a physical window for the receiver [67] and a face-down receiver in which concentrated solar radiation enters the bottom of a cylindrical cavity receiver to irradiate the particles falling around the inside walls of the cylinder [68].

Finally, there has been an effort to determine the temperature ranges that SPRs can achieve through recirculation of the particles; when the particles are falling under gravity, it is difficult to achieve a significant temperature gain of particles in the receiver without the receiver length becoming unrealistic. However, it has been shown that particle recirculation should be able to achieve outlet temperatures of 800°C with an inlet temperature of 300°C [65, 68].

There are currently no power cycles that have demonstrated large scale power production at efficiencies of >50%, but there are a number of research efforts trying to make this a reality.

One power cycle that is well established is a Brayton Cycle. This is typically used in generators and jet engines with fossil fuels, but can be adapted for use in CSP as well. The general Brayton cycle has a gaseous working fluid which is compressed, heated, and then expanded through a turbine to produce electric power. The heat can either come from a fuel source or solar heat. The system can be open or closed; the working fluid can be drawn into the compressor from the surrounding atmosphere and subsequently released back into the atmosphere once expanded through the turbine in an open system, or recycled from the turbine back into the compressor in a closed system. Air is obviously used for open Brayton cycles, while inert gases such as helium are common for close systems [69]. This cycle is beneficial in that it can operate over a wide variety of operating conditions and power levels. Different heat transfer fluids and containment

materials can be used to operate over a wide variety of temperatures and pressures, and the cycle can maintain high efficiency over a variety of power output levels by changing the compression ratio [69]. Supercritical carbon dioxide (sCO2) has been studied as a potentially useful working fluid for Brayton cycles in the nuclear industry [70], and this cycle is currently being considered for its applicability to solar energy generation [71-73]. Specifically, a sCO2 cycle has the potential to operate at higher temperatures than steam, to provide thermal-to-electric efficiencies of >50%, and to operate with high energy density, leading to a more economical compact power cycle [71]. While promising, the sCO2 cycle has yet to be demonstrated at power generation levels of interest to solar power. The drawback to the Brayton cycle in general is the fact that it is operated at very high temperatures and potentially high pressures. There are few materials that can operate at these high temperature and high pressure conditions, especially at scale. This will be discussed further in Chapter II.

1.7 Scope of Thesis

The overall goal of this thesis is to explore the feasibility of storing both sensible and thermochemical energy at high temperatures, and explore the benefits and problems with doing so. Thermal energy storage is a major benefit to concentrated solar thermal power and provides a number of benefits to renewable energy production from the sun. This thesis focuses primarily on thermal energy storage, though entire systems are often considered and discussed. This is because any change made to a subsystem (such as thermal energy storage) will impact other subsystems and ultimately the overall system efficiency. As such, the thermal storage method must be considered in the context of system integration. There are many ways to store thermal energy, and the different ways of doing so have been studied extensively. However, there is not

as much consideration of thermochemical energy storage relative to other methods. Much of the previous work in this area has focused on either purely sensible or purely thermochemical energy storage, but there has not been an exploration of using both.

This thesis explores possible system configurations which could most benefit from this type of system, and presents conceptual system designs for consideration. Possible thermochemical reaction types are explored, and specific reactions are identified for consideration. Theoretical predictions are made of the possible energy storage that can be done with a particular thermochemical cycles that has been studied in the past for thermochemical water splitting. Additionally, an experimental exploration is begun for the chemical reaction of interest. Finally, future possibilities and directions using lessons learned in this initial exploration are identified and discussed.

Much greater energy density from the thermochemical aspect of the storage can be potentially realized while not discarding sensible energy to do so. This increased energy density is of great importance for higher temperature systems, which are of interest for the increased thermal to electric conversion efficiency that higher temperature systems could be able to achieve. Specifically, greater energy density can decrease the amount of material needed to be maintained at high temperature, which is difficult and expensive to do. This includes storing and insulating the material itself, but also includes the reduction in system size and throughput for other system components, which can also have large thermal losses.

In order to explore this possibility, theoretical predictions based on thermodynamic equilibrium are made using the FACTSageTM software. This allows for relatively quick exploration of various materials and reactions. From these predictions, the heat of reaction and conversion can be determined at various conditions, which can inform temperature ranges and

other conditions of interest. This ability to not only predict theoretical heats of reaction but also the reaction conversion is very important. It allows for a specific prediction of the amount of change in the system at various conditions, and to explore what sort of conditions would be necessary for better system performance. This is especially true for solid state solutions such as spinel materials, which can form complex ionic distributions in the solid structure. This makes the prediction of reaction very different than for distinct molecular interactions and reactions.

Theoretical predictions offer a useful indication of conditions and materials to explore, but experimental validation is imperative. This thesis begins such an experimental exploration of thermochemical energy storage, using thermo-gravimetric and calorimetric data obtained at

Sandia National Laboratories in Albuquerque, New Mexico. Additional data and material characterization experiments were done at the University of Colorado at Boulder. The results of this experimentation are compared to the sensible energy in the material calculated at the temperature of reaction. This allows for an observation of the potential benefit of thermochemical boosting to the sensible energy storage of the solid material at high temperature.

It should be noted that all of the above descriptions and discussions only make passing reference to economic considerations. Economics are one of the major decision points about which deployment and operational decisions are made, so they will have a major impact on which technologies will be used. Despite the importance, economics are not considered in depth in this work. This is due to the nature of actual plant deployments. Each case is different, and each potential plant will have a unique set of operating conditions, geographic location, environmental concerns, regulatory restrictions, materials pricing, and purchase power agreements. Because all of these (especially pricing and purchase agreements) can vary so

widely, they are not always explicitly considered, but rather general trends are noted and overall conclusions are drawn based on physical phenomena and energy efficiency studies.

As stated previously, the goal of this work is to explore the possibility of storing both thermochemical and sensible energy. This is done though a theoretical and experimental exploration of a single reaction. There are a number of other reactions of interest, which may or may not prove to be better suited to this type of application. This work presents a different way of storing high temperature thermal energy to solar power production and begins to explore some of the potential benefits and drawbacks to these types of systems. This work will help to inform future work and exploration in this area, and help to illustrate a different way of storing energy for renewable energy production.

References

1. World Energy Outlook 2012, International Energy Agency. 2012.

2. International Energy Outlook 2001, U.S. Energy Information Administration (EIA),

DOE/EIA-0484(2011). 2011.

3. Annual Energy Review 2011, u.s. energy Information Administration (EIA), DOE/EIA-

0384(2011). 2012: Washington, DC.

4. Siegel, N.P., Thermal energy storage for solar power production. Wiley Interdisciplinary

Reviews: Energy and Environment, 2012. 1(2): p. 119-131.

5. Technology Roadmap: Concentrating Solar Power, International Energy Agency. 2010.

6. Clean and Diversified Energy Initiative: Solar Task Force Report, Western Governors'

Association. 2006.

7. Electric Power Annual 2011, U.S. Energy Information Administration (EIA). 2013:

Washington, DC

8. Concentrating Solar Power Resource Maps. September 1, 2010 [cited 2013 March 25];

Available from: http://www.nrel.gov/csp/maps.html.

9. Denholm, P. and M. Hand, Grid flexibility and storage required to achieve very high

penetration of variable renewable electricity. Energy Policy, 2011. 39(3): p. 1817-1830.

10. Barlev, D., R. Vidu, and P. Stroeve, Innovation in concentrated solar power. Solar

Energy Materials and Solar Cells, 2011. 95(10): p. 2703-2725.

11. Price, H., et al., Advances in Parabolic Trough Solar Power Technology. Journal of Solar

Energy Engineering, 2002. 124(2): p. 109-125.

12. Falchetta, M., et al., Commissioning of the Archimede 5 MW molten salt parabolic trough

solar plant, in SolarPACES. 2010: Perpignan, France.

13. Kolb, G.J., Evaluation of Annual Performance of 2-Tank and Thermocline Thermal

Storage Systems for Trough Plants. Journal of Solar Energy Engineering, 2011. 133(3):

p. 031023-5.

14. Sargent and Lundy LLC Consulting Group, Assessment of Parabolic Trough and Power

Tower Solar Technology Cost and Performance Forecasts. 2003, National Renewable

Energy Laboratory (NREL): Report No. NREL/SR-550-34440.

15. Glatzmaier, G., Summary Report for Concentrating Solar Power Thermal Storage

Workshop: New Concept and Materials for Thermal Energy Storage and Heat-Transfer

Fluids May 20, 2011. 2011, National Renewable Energy Laboratory (NREL) Report No.

NREL/TP-5500-52134 Golden, CO

16. Kolb, G., et al., Freeze-Thaw Tests on Trough Receivers Employing a Molten Salt

Working Fluid. ASME Conference Proceedings, 2010. 2010(43956): p. 693-698.

17. Pacheco (Editor), J.E., et al., Final Test and Evaluation Results from the Solar Two

Project. 2002, Sandia National Laboratories: Report No. SAND2002-0120.

18. Dunn, R.I., P.J. Hearps, and M.N. Wright, Molten-Salt Power Towers: Newly

Commercial Concentrating Solar Storage. Proceedings of the IEEE, 2012. 100(2): p.

504-515.

19. BrightSource Energy. Ivanpah. 2012 [cited 2012 June 19]; Available from:

http://www.brightsourceenergy.com/ivanpah.

20. Tonopah Solar. Crescent Dunes Solar Energy Plant. 2012 [cited 2012 June 21];

Available from: http://www.tonopahsolar.com/project_overview.html.

21. Kolb, G.J., et al., Power Tower Technology Roadmap and Cost Reduction Plan. 2011,

Sandia National Laboratories: Report No. SAND2011-2419.

22. Bradshaw, R.W. and R.W. Carling, A Review of the Chemical and Physical Properties of

Molten Alkali Nitrate Salts and Their Effect on Materials Used For Solar Central

Receivers. 1987, Sandia National Laboratories: Report No. SAND87-8005 Livermore,

CA.

23. Kolb, G.J., An Evaluation of Possible Next-Generation High-Temperature Molten-Salt

Power Towers. 2011, Sandia National Laboratories: Report No. SAND2011-9320

Albuquerque, NM.

24. Andraka, C.E., K.S. Rawlinson, and N.P. Siegel, Technical feasibility of storage on large

dish stirling systems. 2012, Sandia National Laboratories: Report No. SAND2012-8352.

p. Medium: ED; Size: 73 p.

25. (2008) Sandia, Stirling Energy Systems set new world record for solar-to-grid conversion

efficiency. News Releases.

26. (2012) Ripasso Energy sets new solar-to-electricity world record!

27. Infinia Corporation. PowerDish(TM). Available from:

http://www.infiniacorp.com/solutions/powerdish/.

28. Cleanergy Inc. Utility Scale Solar Parks. Available from:

http://www.cleanergy.com/solar/utility-scale-solar-parks/.

29. Kalogirou, S.A., Solar thermal collectors and applications. Progress in Energy and

Combustion Science, 2004. 30(3): p. 231-295.

30. Turchi, C., et al., Current and Future Costs for Parabolic Trough and Power Tower

Systems in the US Market Preprint, in SolarPACES. 2010: Perpignan, France.

31. Herrmann, U. and D.W. Kearney, Survey of Thermal Energy Storage for Parabolic

Trough Power Plants. Journal of Solar Energy Engineering, 2002. 124(2): p. 145-152.

32. Electric Power Annual 2009, U.S. Energy Information Administration (EIA), DOE/EIA-

0348(2009). 2009.

33. Medrano, M., et al., State of the art on high-temperature thermal energy storage for

power generation. Part 2—Case studies. Renewable and Sustainable Energy Reviews,

2010. 14(1): p. 56-72.

34. Madaeni, S.H., R. Sioshansi, and P. Denholm, How Thermal Energy Storage Enhances

the Economic Viability of Concentrating Solar Power. Proceedings of the IEEE, 2012.

100(2): p. 335-347.

35. Gil, A., et al., State of the art on high temperature thermal energy storage for power

generation. Part 1—Concepts, materials and modellization. Renewable and Sustainable

Energy Reviews, 2010. 14(1): p. 31-55.

36. Flueckiger, S.M., Z. Yang, and S.V. Garimella, Review of Molten-Salt Thermocline Tank

Modeling for Solar Thermal Energy Storage. Heat Transfer Engineering, 2012. 34(10): p.

787-800.

37. Pacheco, J.E., S.K. Showalter, and W.J. Kolb, Development of a Molten-Salt

Thermocline Thermal Storage System for Parabolic Trough Plants. Journal of Solar

Energy Engineering, 2002. 124(2): p. 153-159.

38. Qin, F.G.F., et al., Thermocline stability criterions in single-tanks of molten salt thermal

energy storage. Applied Energy, 2012. 97(0): p. 816-821.

39. Yang, Z. and S.V. Garimella, Cyclic operation of molten-salt thermal energy storage in

thermoclines for solar power plants. Applied Energy, 2013. 103(0): p. 256-265.

40. Liu, M., W. Saman, and F. Bruno, Review on storage materials and thermal performance

enhancement techniques for high temperature phase change thermal storage systems.

Renewable and Sustainable Energy Reviews, 2012. 16(4): p. 2118-2132.

41. Dunn, R., K. Lovegrove, and G. Burgess, A Review of Ammonia-Based Thermochemical

Energy Storage for Concentrating Solar Power. Proceedings of the IEEE, 2012. 100(2):

p. 391-400.

42. Lovegrove, K., Developing ammonia based thermochemical energy storage for dish

power plants. Solar Energy, 2004. 76(1-3): p. 331-337.

43. Lovegrove, K., et al., Exergy Analysis of Ammonia-Based Solar Thermochemical Power

Systems. Solar Energy, 1999. 66(2): p. 103-115.

44. Brown, L.C., et al., High Efficiency Generation of Hydrogen Fuels Using Nuclear

Power: Final Technical Report for the Period August 1, 1999 Through September 30,

2002, in Other Information: PBD: 1 Jun 2003. 2003, General Atomics: Report No. GA-

A24285. p. Medium: ED; Size: 343 pages.

45. Corgnale, C. and W.a. Summers, Solar hydrogen production by the Hybrid Sulfur

process. International Journal of Hydrogen Energy, 2011. 36(18): p. 11604-11619.

46. Buckingham, R. Sulfur Based Thermochemical Heat Storage for Baseload Concentrating

Power. in CSP Subprogram Review. 2011. Golden, CO.

47. Wong, B., et al., Construction materials development in sulfur–iodine thermochemical

water-splitting process for hydrogen production. International Journal of Hydrogen

Energy, 2007. 32(4): p. 497-504.

48. Falcone, P.K., J.E. Noring, and J.M. Hruby, Assessment of a Solid Particle Receiver for a

High Temperature Solar Central Receiver System. 1985, Sandia National Laboratories:

Report No. SAND85-8208.

49. Wong, B. Thermochemical Heat Storage for Concentrated Solar Power Based on

Multivalent Metal Oxides. in CSP Subprogram Review. 2011. Golden, CO.

50. Neises, M., et al., Solar-heated rotary kiln for thermochemical energy storage. Solar

Energy, 2012. 86(10): p. 3040-3048.

51. Buckingham, R., et al., Metal Oxide Based Thermochemical Energy Storage for

Concentrated Solar Power - Thermodynamics and Parasitic Loads for Packed Bed

Reactors, in SolarPACES. 2010.

52. Edwards, S.E.B. and V. Materić, Calcium looping in solar power generation plants.

Solar Energy, 2012. 86(9): p. 2494-2503.

53. Schaube, ., A. W rner, and . Tamme, High Temperature Thermochemical Heat

Storage for Concentrated Solar Power Using Gas–Solid Reactions. Journal of Solar

Energy Engineering, 2011. 133(3): p. 031006-031006.

54. Schaube, F., et al., A thermodynamic and kinetic study of the de- and rehydration of

Ca(OH)(2) at high H2O partial pressures for thermo-chemical heat storage.

Thermochimica Acta, 2012. 538: p. 9-20.

55. Moore, R.C., et al., Design considerations for concentrating solar power tower systems

employing molten salt. 2010, Sandia National Laboratories: Report No. SAND2010-6978

Albuquerque, NM.

56. Mancini, T., et al., Dish-Stirling Systems: An Overview of Development and Status.

Journal of Solar Energy Engineering, 2003. 125(2): p. 135-151.

57. Stine, W.B. and R.B. Diver, A Compendium of Solar Dish/Stirling Technology. 1994,

Sandia National Laboratories: Report No. SAND93-7026 Albuquerque, NM.

58. Singh, N. and S.C. Kaushik, Optimum operating temperature and efficiency of solar

thermal power systems. Heat Recovery Systems and CHP, 1994. 14(6): p. 633-638.

59. SunShot Concentrating Solar Power (CSP) R&D Funding Opportunity Announcement,

Department of Energy, DE-FOA-0000595 CFDA Number 81.087. 2011: Golden, CO.

60. American Society of Mechanical Engineers (ASME), Case 2385-1 37Ni-30Co-28Cr-

2.75Si Alloy (UNS N12160) in Cases of Boiler and Pressure Vessel Code: Section VIII,

Division 1. 2004.

61. American Society of Mechanical Engineers (ASME), Case 2672 37Ni-33Fe-25Cr Alloy

(UNS N08120) Welded Construction Above 1650 F to 1800 F, in Cases of Boiler and

Pressure Vessel Code: Section VIII, Division 1. 2011.

62. American Society of Mechanical Engineers (ASME), Case 2671 57Ni-22Cr-14W-2Mo-

La Alloy (UNS N06230) Subpart 3, Fig. NFN-24 Up to 1,800 F (982 C), in Cases of

Boiler and Pressure Vessel Code: Section II, Part D. 2011.

63. Hruby, J.M., A Technical Feasibility Study of a Solid Particle Solar Central Receiver for

High Temperature Applications. 1986, Sandia National Laboratories: Report No.

SAND86-8211.

64. Hellmann, J.R. and V.S. McConnell, Characterization of Spherical Ceramic Particles for

Solar Thermal Transfer Media: A Market Survey. 1986, Sandia National Laboratories:

Report No. SAND86-1873 Albuquerque, NM.

65. Khalsa, S.S.S., et al., CFD Simulation and Performance Analysis of Alternative Designs

for High-Temperature Solid Particle Receivers, in ASME 2011 5th International

Conference on Energy Sustainability & 9th Fuel Cell Science, Engineering and

Technology Conference. 2011: Washington, DC.

66. Siegel, N.P., et al., Development and Evaluation of a Prototype Solid Particle Receiver:

On-Sun Testing and Model Validation. Journal of Solar Energy Engineering, 2010.

132(2): p. 021008.

67. Tan, T., et al., Wind effect on the performance of solid particle solar receivers with and

without the protection of an aerowindow. Solar Energy, 2009. 83(10): p. 1815-1827.

68. ger, M., et al., Face-Down Solid Particle Receiver Using Recirculation. Journal of

Solar Energy Engineering, 2011. 133(3): p. 031009-1 - 031009-8.

69. Forsberg, C.W., P.F. Peterson, and H. Zhao, High-Temperature Liquid-Fluoride-Salt

Closed-Brayton-Cycle Solar Power Towers. Journal of Solar Energy Engineering, 2007.

129(2): p. 141-146.

70. Dostal, V., P. Hejzlar, and M.J. Driscoll, High-performance supercritical carbon dioxide

cycle for next-generation nuclear reactors. Nuclear Technology, 2006. 154(3): p. 265-

282.

71. Turchi, C.S., et al., Thermodynamic Study of Advaned Supercritical Carbon Dioxide

Power cycles for High Performance Concentrating Solar Power Systems, in ASME 2012

6th International Conference on Energy Sustainability 2012: San Diego, CA, USA.

72. Chacartegui, R., et al., Alternative cycles based on carbon dioxide for central receiver

solar power plants. Applied Thermal Engineering, 2011. 31(5): p. 872-879.

73. Kim, Y.M., C.G. Kim, and D. Favrat, Transcritical or supercritical CO2 cycles using

both low- and high-temperature heat sources. Energy, 2012. 43(1): p. 402-415.

CHAPTER II

THERMOCHEMICAL AUGMENTATION ENERGY STORAGE CONCEPT

A thermochemical energy storage concept that is of interest would be a material that stores heat both sensibly and thermochemically. In this way, the reactants would be stored at the high temperature of sensible thermal energy storage, but augmented with a thermochemical cycle. This concept avoids the issue of cooling and re-heating reactants, but can still take advantage of the significant energy storage density benefits of thermochemical energy storage systems. There are many different ways in which to implement this cycle, and this chapter will provide a general overview of some of the most interesting proposed systems.

2.1 Use of Sensible and Thermochemical Energy Storage

The proposed technology does not rely on thermochemical energy storage alone, but rather enhances the effective heat capacity of the solid particles by storing addition thermochemical energy on top of the sensible heat already able to be stored at the high temperatures of interest. This hybrid approach of storing both sensible thermal and thermochemical energy would allow for significantly more energy storage than would be possible from either the sensible or thermochemical heat storage alone. Using both sensible and thermochemical energy storage is useful for a number of reasons and represents a novel approach to thermochemical energy storage. As was discussed in Chapter I, most of the past work in thermochemical energy storage focused on the benefits of cooling and storing reactants at ambient conditions, instead of utilizing sensible storage at the same time. The benefits and concerns with this new approach will be discussed below.

The main benefit of using both sensible and thermochemical energy storage is a large increase in thermal energy storage density. Whether considered from a mass or volume basis, higher energy density in thermal energy storage is a much more efficient way to store energy.

Higher energy density will lead to less thermal losses from the storage system. While the surface area exposed in a thermal mass will be significantly less than the total volume, a smaller volume of the same geometry will always have less surface area exposed, and so have less heat losses.

Even if the storage mass can be insulated well enough to have minimal heat losses, a higher temperature storage system will require much more insulation, which will lead to a higher capital cost. Heat losses are a much larger effect at higher temperatures, due to the temperature difference between the operating and ambient temperatures.

Another source of heat losses is not as immediately obvious. Solar power is an inherently transient process, and so the effects of process startup and shutdown must be considered. When the solar resource is strong enough, the concentrated solar radiation is focused on the receiver.

However, since the receiver was previously at (or near) ambient temperature, the thermal mass of the receiver itself must first heat up to the operating temperature. This typically involves cycling heat transfer fluid through the receiver as both it and the heat transfer fluid heat up, then finally beginning to cycle the hot heat transfer fluid through the process once it reaches operational temperatures. This is an effective energy penalty that occurs whenever the receiver is brought online. Since this happens at least once a day (sometimes more on cloudy days which can have highly variable irradiation), this can become a very large energy penalty for an operating plant.

However, if the heat transfer fluid could absorb and contain more thermal energy per unit mass/volume, the receiver and heat transfer flow rate would be much smaller. This would mean that the energy penalty associated with receiver startup would be smaller as well.

Another aspect of thermochemical energy storage that can be beneficial is the fact that energy can be stored via an endothermic thermochemical reaction, and then the products can be cooled and stored at ambient conditions. Since the products are stable, they can be stored for long periods of time, even for seasonal energy storage. However, there is an issue with cooling and the reaction products. Since the reactants will have to be re-heated later, there is an automatic energy penalty to recover the heat that was put into storage. This penalty becomes especially significant at higher temperatures, since the sensible heat from ambient to the temperature of the reaction can be a significant portion (if not more than) the thermochemical energy that was stored in the first place.

It should be noted that this heat can be recovered during cooling and either stored elsewhere or used to generate electricity. However, this would necessitate an additional heat storage system to recover this heat, which would add significant expense and system complexity.

There would also be an efficiency loss associated with recovering the heat from the reaction products and using this heat later. Depending on the power cycle design, heat at all temperatures down to ambient may not be usable, even if it can be recovered from the reaction products, meaning that there will always be some amount of energy loss for each reaction cycle. The other option would be to generate electricity directly from the reaction products cooling, but this would mean that the heat could only be recovered when the power cycle is operating; this means the system cannot simply charge the thermal energy storage system, which may be desirable for plant operation. Since the goal of thermal energy storage is to allow electricity generation on- demand rather than when the sun is shining, this amount of heat that would need to be recovered would go against this objective.

2.2 Gas-Solid Reactions

There are many different reactions that could potentially be used for this thermochemical augmentation. One particular class of reactions that are of interest is gas-solid reactions. These thermochemical cycles have both solid and gaseous reactants on one side of the reaction, such that a solid will evolve a gaseous product during the forward reaction, and recombine with this gaseous reactant on the reverse reaction. There are many different types of these reactions, as shown in Table 2-1.

Table 2-1: List of Potential Gas-Solid Reaction Cycles Reaction Type General Stoichiometry Example Reaction

Carbonate MCO3(s) ↔ MO(s) + CO2(g) CaCO3(s) ↔ CaO(s) + CO2(g)

Hydride MH2(s) ↔ M(s) + H2(g) MgH2(s) ↔ Mg + H2

Hydroxide M(OH)2(s) ↔ MO(s) + H2O(g) Ca(OH)2(s) ↔ CaO(s) + H2(g)

Oxide MO3(s) ↔ MO(s) + O2(g) Co3O4(s) ↔ 3 CoO(s) + ½ O2(g)

These types of gas-solid reactions are of interest for a number of reasons. From a thermodynamics standpoint, the entropy of the forward reaction (when the gas is evolved from the solid) increases when moving from reactants to products due to the fact that one of the products of the forward reaction is a gas. This helps to overcome the fact that the reaction is endothermic in order to proceed spontaneously under certain conditions. The Gibb’s ree Energy change is shown in Equation 2-1. As can be seen, an endothermic reaction (positive ΔH) will hinder spontaneous reaction (make the ΔG more positive); whereas a reaction with increasing entropy (positive ΔS) will promote spontaneous reaction (make the ΔG more negative).

Conversely, an exothermic reaction (negative ΔH) can help overcome a decrease in entropy

(negative ΔS) to promote spontaneous reaction (negative ΔG). Since thermochemical energy storage cycles will necessarily have an endothermic forward reaction and exothermic reverse reaction, gas-solid reactions are therefore very beneficial for thermochemical energy storage.

Equation 2-1

Another reason that gas-solid reactions are of interest is the relative ease of product separation. The products of the forward reaction must be separated in order to prevent re- combination into the reverse reaction, thereby releasing the energy that was meant to be stored.

For reactions that have two gaseous or two solid products, separating the products of the forward reactions can be quite difficult and energy intensive. However, separating a gaseous product from a solid product can be accomplished relatively easily by sweeping an inert gas stream.

One class of gas-solid reactions that is of particular interest is that of solid oxides. These materials tend thermally reduce in temperatures ranging from 800°C to over 2000°C [1, 2]. In the past, these types of materials have been studied extensively for the thermochemical splitting of water and carbon-dioxide to hydrogen and carbon-monoxide. As such, there is a rich literature from which to select materials for further study. Table 2-2 shows a variety of chemical reactions that have been studied for water and carbon-dioxide splitting, along with the associated thermal reduction temperatures under inert atmospheres. As can be seen, this class of chemical reactions matches well with temperatures of interest for high temperature concentrated solar powerusing the Air-Brayton power cycle, and can potentially be of use for thermal energy storage at these temperatures.

Table 2-2: List of Various Thermochemical Reactions Studied for Splitting H2O and CO2 Material Example Reduction Reaction Temperature Ref. Iron Oxide Fe3O4 → 3 eO + ½ O2 >1300°C [3] (on YSZ) Mixed Metal MnxZn1-xFe2O4 → MnxZn1-xFe2O3 + ½ O2 <1300°C [4] Oxides Cerium CeO2 → CeO2-d + d/2 O2 >1450°C [5] Oxide “Hercynite” (Co,Ni)Fe2O4 + 3 Al2O3 → (Co,Ni)Al2O4 + FeAl2O4 + ½ O2 1100-1400°C [1]

Additionally, these solid oxide thermochemical cycles also have relatively high

enthalpies of reaction compared to some other chemical reactions. This can also be seen in Table

2-2. The benefit of this is obvious, since a higher enthalpy of reaction means that more energy

can be stored in the active material.

Many of the solid oxide reaction identified above have been identified for beneficial

oxidation with H2O or CO2. Even with these select few materials, this oxidation reaction is still

difficult to achieve. This is due to the fact that during the oxidation reaction, the reduced material

needs to break a C-O or O-H bond, and this isn’t always favorable at temperatures of interest [6].

However, if the reduced material was merely re-oxidized with oxygen instead of H2O or CO2, it

would be much easier, since it would only be the reverse of the reduction reaction. Indeed, many

other solid oxide reactions were not studied for this application because they could not easily re-

oxidize with water or carbon-dioxide. However, the current application is interested only in the

heat absorption of the reduction reaction and the associated heat release upon direct re-oxidation

with oxygen. This oxidation is much easier to achieve, and so many more reactions are available

for study.

Finally, many thermochemical cycles for the production of hydrogen or carbon monoxide

must operate at different temperatures between the higher temperature reduction reaction and a

lower temperature oxidation reaction. This is not desirable for thermochemical energy storage

for reasons of exergetic efficiency. For thermal energy storage, it is important for the quality of the heat to be retained throughout the storage process. The expense and effort to obtain high quality/temperature heat can be extensive, and so heat of a similar quality should be regained from any reasonable storage process. As such, it is important for thermochemical energy storage system to release heat at a similar temperature to that at which it was put into storage. With direct re-oxidation with oxygen, the reverse of the reduction reaction is being performed. If this can be controlled by controlling the oxygen gas content near the active solid material, the two reactions should be able to be done at very similar (if not identical) temperatures.

2.3 Hercynite Thermochemical Cycle

It has been recently shown at the University of Colorado (CU) that cobalt or nickel ferrite can react with an alumina (Al2O3) substrate to undergo a redox cycle through a hercynite

(FeAl2O4) pathway: CoFe2O4 + 3 Al2O3 + ΔH ↔ CoAl2O4 + 2 FeAl2O4 + 1/2 O2 or NiFe2O4 + 3

Al2O3 + ΔH ↔ NiAl2O4 + 2 FeAl2O4 + 1/2 O2. This newly developed redox cycle has been used as a solar thermochemical water splitting cycle, where the oxidation is done with H2O or CO2 in order to produce H2 or CO, respectively. However, this cycle is also of interest for thermochemical energy storage for solar thermal power generation. If the endothermic reduction forward reaction can be used to absorb extra heat in the solar beam, the reduced material can then be oxidized later, releasing the captured heat. This allows for near isothermal and energy dense thermal energy storage at temperatures of interest to solar thermal power production.

Figure 2-1: Theoretical and Experimental Evolution of Oxygen via “Hercynite” Cycle at Various Temperatures (from [1])

The “hercynite cycle” can be reduced at 1000°C or higher (see Figure 2-1), which is a high-enough temperature to be of interest for high-temperature operation in a future CSP plant, and at the same time it is low enough to be possible for operation [1]. Other thermochemical cycles have issues with operating at temperatures that are too high for realistic operation

(source). While it is possible to operate the cycle at these extremely elevated temperatures, it is not necessarily feasible to operate these cycles for any appreciable length of time. This is especially important for CSP plants that are generally designed to have very long (~30 years) operational lives.

The ‘hercynite cycle’ has been shown thermodynamically and experimentally to predictably cycle in a repeatable manner (see Figure 2-2). Additionally, the reduction and oxidation cycling kinetics appear to be very fast and very repeatable (see Figure 2-2), meaning that the material should be effectively reduced even in the relatively short time the particles are exposed to the solar beam. Solid particles fall through the solar beam for only a couple of seconds, but can be recycled in the receiver for a number of minutes in order to reach higher temperatures, giving the material time to reduce.

Figure 2-2: Cycling of “Hercynite” Material Under Constant low of Oxygen (from [7])

Additionally, the “hercynite cycle” has been shown to reduce and oxidize at similar temperatures [1]. Further, it is expected that this effect would be larger for better control of oxygen content of the atmosphere surrounding the active material. This indicates that the temperature difference between reduction and oxidation could be very small, which means the loss of exergy would be minimal [8]. This would be a huge advantage for thermochemical energy storage, as described above.

This ability to reduce and oxidize at nearly the same temperatures means that the proposed technology could then meet and exceed the incredibly stringent technical targets of the

Department of Energy SunShot initiative, which aims for thermal storage systems to have exergetic efficiencies of >95% [9]. The proposed technology has a very good chance of far exceeding this aggressive goal by potentially achieving a maximum ~99% exergetic efficiency based on temperature differences. While this does not necessarily account for direct energy losses, it does provide a more forgiving upper limit compared to other thermochemical cycles

which could not achieve this exergetic efficiency even with zero thermal losses. Sensible energy storage has been demonstrated to achieve >98% energetic efficiency [10]. When an amount of energy enters and leaves storage at the same temperature, the overall exergetic efficiency can be nearly this high (see Equation 2-2 and Table 2-3). The proposed technology would also far exceed another SunShot technical target by operating at temperatures >1000°C, which is far about the SunShot target to operate at temperatures >650°C.

( ) ( ) Equation 2-2

( ) ( )

Table 2-3: Calculation of Exergetic Efficiency of “Hercynite” Cycle for Various Temperatures and Energetic Efficiency Levels, Showing the Effects of Different Operational Temperatures and Energy Efficiency Values Thigh Tlow Energy Efficiency Exergy Efficiency 700°C 500°C 100% 88.74% 1200°C 1000°C 100% 96.06% 1200°C 1150°C 100% 99.12% 1200°C 1150°C 98% 97.13% 1200°C 1200°C 100% 100% 1200°C 1200°C 98% 98%

2.4 Potential System Concepts

2.4.1 Chemically Augmented Solid Particle Receiver Concept

Two system concepts will be presented here. Both of these potential system designs utilize the overall general concept of augmenting sensible energy storage with a thermochemical cycle, but present two different ways in which this concept could be utilized. The first concept is augmentation of a solid particle receiver (SPR), where the solid particles serve as the heat

transfer and thermal storage media. These particles could be formulated to be able to thermochemically cycled in order to absorb and then release additional heat. The second concept would use a gaseous heat transfer fluid in order to absorb heat in a solar receiver, then use this heat to charge a stationary set of thermochemically active material.

The augmented SPR concept is similar to the SPR concept shown in Chapter I, except the receiver will also house the reduction reaction. The active material could then be re-oxidized in the heat exchanger section, in order to release the heat for electricity generation. This concept would likely be utilized in a central receiver configuration. The particles are elevated to the solar receiver at the top of the power tower and would fall through the receiver to be heated by the concentrated solar radiation. The receiver would also be the reduction reactor, where the particles would endothermically reduce to absorb additional energy. If the particles need additional time in the receiver, either to more fully reduce or simply to sensibly get up to temperature, the particles can be recycled through the receiver; this has been suggested for SPRs in the past [11]. The reduced particles would then fall into a hot storage tank, where they are kept at temperature until it is desirable to recover the thermal energy. At this point, they are sent through a heat exchanger to transfer the thermal energy to a power cycle for electricity generation. It is at this point that oxygen would be re-introduced, which would oxidize the particles and allow the stored thermochemical energy to be released in the exothermic reaction.

The oxidized and cool particles would then fall into a cold storage tank, where they would be stored until they are sent through the solar receiver again, completing the cycle.

This type of system configuration has a number of inherent benefits. The main benefit to this type of system is the fact that the reactive solid particles are both the heat transfer fluid and storage media. This means that there are no intermediate heat transfer steps in the process; the

concentrated solar energy is transferred to the active particles, stored, and then transferred to the power cycle on-demand. This eliminates the need for additional transfers of energy, each of which will necessarily impart an energetic or exergetic penalty, lowering the system efficiency.

Another potential benefit of a solid particle design is the fact that the dark colored particles could potentially be directly radiated by the concentrated sunlight to absorb energy.

This would have an efficiency advantage over indirect receivers, in which the heat transfer fluid flows through a receiver tube to absorb energy. Since the concentrated sunlight would be directly irradiating the particles themselves, it does not have to heat a receiver tube, which then transfers heat to the particles. Similar to the benefit gained from using the reactive particles as the heat transfer fluid and storage medium, direct irradiation of the particles would mean that one fewer heat transfers would need to occur. However, this can be difficult for a large scale system to achieve; because the particles need to be separate from oxygen during reduction, the receiver must be closed to the atmosphere. This can be done with a transparent optical window, but a large scale system would require a large window, which would not be very economical, and impossible to do with any sort of pressure differential. Additionally, at the temperatures of interest, vitrification of the window would be a concern over a long plant life, where solid dust and volatilized materials could deposit on the cooler window. That said, central receiver systems at the ultra-high temperatures of interest will likely not have a large single receiver, but rather be a collection of cavity receivers. Smaller windows would be far more feasible than a single large window, but vitrification and other temperatures issues are still a concern. This can be mitigated by reactor design that would protect the window and keeping solid particle dust to a minimum, but still remains an important concern for directly irradiated systems.

The major benefit to this system concept is the fact that more energy could be stored per particle, since both sensible and thermochemical heat would be stored. The benefits of this are twofold: smaller system flow rates and smaller storage volumes. If more energy is stored per reactive particle, then the mass of material needed to store a constant amount of energy would be much less than an associated inert particle. As discussed above, this leads to smaller flow rates in the receiver and other system components, meaning that the energy penalties associated with system startup would be much less. This means that the receiver, piping, and heat exchangers could all be much smaller, meaning they would be less expensive and have less thermal losses.

Additionally, the mass of material needed to store heat in the thermal energy storage subsystem would have a much smaller volume. This leads to less surface area that needs to be insulated and lower thermal losses. While thermal losses from storage are typically small (such as <2% for molten salt storage [10]), this is due to extensive insulation, which can add significant cost to the system for the temperatures being considered.

However, an augmented SPR system does have some potential problems as well. First, the solid particles are difficult to move and control; while liquid heat transfer fluids can be pumped to wherever they are desired, solid particles do not flow and so must be lifted by elevators of some kind. These can introduce potentially significant parasitic loads, and particle elevators can be difficult to operate at elevated temperatures. Another problem with this type of system is the fact that the particles are being moved and falling and so are impacted often. This type of physical stress can lead to particle attrition and breakage, especially over a long (~30 year) plant life. This issue is exacerbated by the fact that the particles undergo severe thermal shocks as well as physical ones. Particle breakage can lead to a loss of reactivity and ability to flow as small particles can sinter or clog.

One main concern of the proposed technology is the fact that the SPR configuration is best suited for use with highly-dense sintered particles [12, 13]. This high density could lead to reaction conversion limitations of the material, since potentially only the surface of each particle will be able to react. However, with small particles, the surface area to volume ratio increases, meaning that more material will be available for reaction.

It should be noted that most of the issues with this concept are inherent issues with the solid particle receiver concept in general. While there are some issues with reactive particles as opposed to inert particles, the main process issue is that the oxygen content near the particles can be controlled (this is discussed below).

2.4.2 Dish with Chemically Augmented Solid Storage System Concept

Another potential system configuration for this type of thermochemical energy storage is for dish systems. The basic storage concept is shown in Figure 2-3. In this type of system, a gaseous heat transfer fluid, such as helium, is used to transport the heat from the receiver to the storage and power generation systems. The heat transfer fluid passes through a receiver to get up to temperature, and then flows to the storage sub-system. The heat is then transferred to a collection of solid particles for storage. These particles can be reduced to absorb additional heat, similar to the previous system description. The solid storage system is split into many different units of solid material; this is so that each individual unit can be quickly brought up to temperature when the storage system is charging, instead of slowly bringing up the whole solid block of material at once. This is done so that the sensible heat transfer will not degrade as the temperatures change. Thus, when the system is charging, individual units are brought up to the high operating temperature in succession; when the storage system is to be discharged, individual

units will be used to heat the heat transfer fluid in succession. When it is desirable to produce electric power, the heat transfer fluid can take heat from the receiver or storage subsystems and deliver it to the power cycle. The flow of the hot heat transfer fluid can be directed either to the storage of power generation subsystems (or to a controllable mixture between the two) merely by opening and closing valves.

Figure 2-3: Schematic Diagram of Dish Concept (not to scale)

The placement of the storage and power cycle subsystems behind the dish has a number of advantages. The main advantage is the fact that the storage and power cycle subsystems do not block potential reflective surfaces [14]. When area is blocked in front of the focus of the reflective dish, this reflective area must be made up elsewhere to deliver the same amount of heat. This means that additional mirrors must be added to the outside edge of the reflective area to deliver the same amount of concentrated sunlight, and these edge mirrors are far less effective than mirrors near the middle. This is due to cosine losses; when the mirror directly faces the sunlight, solar flux equal to 100% of its reflective area is reflected back ( ( ) ). However,

when the mirror is faced away from the sun slightly, then the amount of solar flux reflected is less than the total area of the mirror (e.g., ( ) ). Finally, when the mirror surface is parallel to the sunlight, no solar flux is reflected ( ( ) ).

In addition to the potential area blocked from any subsystems that are set at the focus of the dish (such as the receiver and power cycle), there is an additional blockage from a large boom arm or supports for those subsystems. From a structural standpoint, the boom arm in front of the dish does not necessarily need to support a large amount of weight, since both the storage and power generation system would be behind the dish. This means that the boom arm could be much smaller, which would lead to much less blockage. Additionally, if the boom arm could be a straight support out from the center of the dish to the receiver instead of the commonly used

“elbow” design used in current dishes (see Dish Stirling Schematic in Chapter I), and if the center of gravity of the dish system could be behind the dish, then the “slot” at the bottom of most current dishes could be eliminated [14]. This would allow for more effective reflective area to be employed, and make the dish much more structurally sound, since it would be a continuous area [14].

Similar to the solid particle receiver concept, the main benefit of this proposed system is the fact that the high temperature heat can be stored in a significantly smaller volume. As discussed in Chapter I, putting storage on a dish system is difficult to do, primarily because the mass of the thermal storage would need to be small enough to be supported up on the dish structure itself, or the dish would need to connect to a storage system on the ground. Ground connections for dish systems would need to include expensive rotary joints, which are not always able to operate at high temperatures [14]. The proposed system here would allow for more energy to be stored in a smaller volume and mass than would otherwise be possible, making it

much more realistic to place the storage on the dish structure itself. This is a significant benefit, since this would take advantage of the extremely high volumetric and specific energy densities associated with thermochemical energy storage to be able to store significant thermal energy on a dish system. At the same time, the sensible heat would not be discarded in order to utilize the thermochemical heat storage, making the entire system much more efficient.

A major difference in this system is that the thermal storage media is stationary, and so does not suffer the same durability issues as a solid particle receiver. In this system, the heat is moved around the system by a gaseous heat transfer fluid, meaning that there are no solids that need to be transferred and moved around the system. This avoids many of the issues that occur in a solid particle receiver type system.

While there are a number of advantages to the dish system, there are some drawbacks as well. The stationary storage media does not need to be moved around the system, but the heat still needs to be transferred into and out of this media. When the particles are moving around a solid particle receiver, they can have good heat transfer with a gaseous heat transfer fluid; this is a reason why fluidized beds have such good heat transfer properties. However, when the solid material is stationary, the heat transfer will not be nearly as good. This is somewhat mitigated by the fact that some gaseous heat transfer fluids have very good heat transfer properties, such as helium [15]. Additionally, there are a few different ways in which the solid/gas heat transfer issue can be mitigated. One way is to have the active solid storage media can be placed in blocks of graphite; graphite has excellent heat transfer properties and can survive at high temperatures

[16], provided it is kept away from oxygen [17]. The inert helium heat transfer fluid would then be able to transfer heat to and from the graphite material very well, due to the fact that helium is compatible with graphite and both materials have beneficial heat transfer properties [17].

Graphite has been studied in the past for solid thermal energy storage as well, and has beneficial heat transfer and heat storage properties [18]. The graphite could be protected from the active material (and associated oxygen) by a coating such as Silicon-Carbide (SiC); these types of coatings have been successfully used in the past to protect high temperature graphite from oxidation [19]. The coated graphite could then have good contact with the active storage material, and transfer heat through solid/solid conduction. This concept is illustrated in Figure 2-

Figure 2-4: Schematic Diagram of Solid Redox Storage Unit Concept (not to scale)

However, despite these mitigations there remains the issue of heat exchange. No matter how efficient the heat exchange is, it will not be 100% efficient. Some loss will always be incurred in heat exchange, be it energetic or exergetic. The temperature difference necessary for heat exchange means that each time energy goes through a heat exchanger, it will lose exergy

due to the fact that it will be at a lower temperature. This is a general issue for any sort of indirect storage system (where the heat transfer fluid is not the storage media) and not unique to this system. It may be that indirect storage is a more reasonable storage method for ultra-high temperature operation; it is difficult to store such high temperature heat in anything other than some sort of solid material, and moving solid material as a heat transfer fluid is also difficult.

This issue will need to be considered in any possible system design and consideration.

2.5 Oxygen Separation

Aside from operating temperatures, the equilibrium of the thermochemical cycle depends on the partial pressure of oxygen. The oxygen content of the solar reactor can be controlled via an ion transport membrane solid electrolyte oxygen separator (ITM SEOS). An ITM SEOS device is composed of a stack of one or more electrochemical cells, as shown in Figure 2-5. A porous cathode layer accepts oxygen onto its surface, where it undergoes the electrochemical reduction shown in the bottom of Figure 2-5. The oxygen ions travel through the solid electrolyte to the anode layer, where they rejoin with the separated electrons to produce pure oxygen on the anode side of the cell [20].

Figure 2-5: Schematic operation of a single electrochemical cell in an ITM SEOS device (from [20])

ITM SEOS stacks are generally operated at 800-900°C [20]. The high temperature

(600°C minimum) is necessary to ensure oxygen ion conductivity in the electrolyte [20].

Additionally, the cell can de-oxygenate a gaseous feed stream even when the oxygen in the feed is very dilute. Cells have demonstrated de-oxygenation capabilities at concentrations as low as 2 ppm [21]. This is especially helpful for the reduction reaction, when temperatures are highest and oxygen concentrations must be kept low.

The applied electric current and the number of cells in a stack will determine the flow rate of oxygen for a ITM SEOS stack, as shown in Equation 2-3 and Figure 2-6 [20]. In Equation

2-3, QO2 is the oxygen flow rate (mol/sec) for ITM SEOS stack, I is the electric current, F is

araday’s constant, and Ncells is the number of cells in the stack [20]. This electrical requirement will need to be accounted for against the electrical production of the plant, but it should be a minimal parasitic load for a well-designed system.

Equation 2-3

Figure 2-6: O2 production of a Three-cell Stack Test (from [20])

One potential issue with running the process at scale will be the oxygen removed from the material upon reduction. The evolved oxygen will need to be separated from the reduced material to prevent re-oxidation, but since the oxygen will be at the high temperature of the reduction, releasing the oxygen to the surroundings would discard a potentially substantial amount of thermal energy. To avoid this, the oxygen would have to be stored at temperature, or the heat recovered from the oxygen by some means of recuperation. Recent advances in oxygen storage materials have demonstrated reversible oxygen storage at temperatures up to 900°C with no degradation of storage capacity [22], and others have found oxygen storage materials stable up to 1200°C [23]. These temperatures are very close to the temperatures of interest and may provide an advantageous solution to provide oxygen storage at temperature. However, this additional process may involve too many additional parasitic loads on the system and may or may not turn out to make the process more efficient.

As stated previously, the reduction-oxidation cycle can be controlled by temperature differences or presence of products/reactants. There is a great benefit to operating the reduction- oxidation cycle at near-isothermal temperatures, so the reactions must therefore be controlled by controlling the presence of oxygen. As a product of the reduction reaction and subsequently a reactant in the oxidation reaction, the presence or absence of oxygen near the active material can be used to control the reactions. As the only gaseous component in an otherwise solid-phase reaction cycle, it is also the most easily removed and reintroduced. This type of commercial electrochemical cell can then allow for operation at or near isothermal conditions, which is a great benefit to thermochemical energy storage.

2.6 Conclusions

The use of a thermochemical cycle to augment sensible energy storage has been suggested for concentrated solar power. Many previous systems focused either on purely sensible or purely thermochemical energy storage to store thermal energy. However, sensible energy storage can lead to large storage volumes, which can become prohibitive for high temperatures or large scale systems. Purely thermochemical storage systems suffer from exergy losses due to the fact that the two reactions of the cycles are typically operated at widely different temperatures. Additionally, thermochemical storage systems are typically suggested for long-term storage of products/reactants at ambient conditions, but this means that the active materials must be heated prior to reaction. This energy penalty can be mitigated somewhat during operation by utilizing the heat as reactant products are cooled, but this will necessarily results in a loss of system efficiency.

A system that utilizes both thermochemical and sensible energy storage would store an active material at the reaction temperatures of interest, thus avoiding the need to cool and re-heat the active material. Additionally, thermochemical energy storage has been shown to have very high energy storage density; a thermochemically-augmented storage material would therefore have potentially much higher energy densities than a sensible-only system. This could drastically reduce the storage mass and volume necessary to storage an amount of thermal energy, thus lessening issues with sensible energy storage at high temperatures and large scales.

A further benefit is gained if the thermochemical energy storage can be done isothermally. Since the collection of high temperature solar energy is so expensive, it is important to be able to utilize this high-grade heat at the temperatures at which it is collected.

Thermochemical energy storage cycles can have large differences between the temperatures at which the heat is stored and when it is released, thus losing a large amount of valuable exergy.

This particular exergy loss would be minimized if the two reactions in the thermochemical cycles could be done at (or very near) the same temperature.

There are many potential reactions to consider for such an energy storage system. One class of reactions that are particularly of interest is solid oxide materials; these solid materials evolve oxygen in the endothermic reduction reaction, and can then endothermically re-oxidize with oxygen. This is beneficial because these materials tend to have large reaction enthalpies, which makes for more effective thermochemical energy storage. These materials also reduce at high temperatures that are of interest for future concentrated solar power development. Finally, solid oxides make reactant separation a simple matter, since one of the products of the reduction reaction is a reduced solid and the other is a gas that is easily removed. A number of these types of materials have been studied for solar thermochemical water and carbon dioxide splitting,

where the reduced solid is oxidized with H2O or CO2 to produce H2 or CO. As such, there is an extensive literature from which to explore possible reactive materials. Furthermore, the reverse of the thermal reduction is thermodynamically easier to carry out than oxidation with water or carbon dioxide, which drastically extends the list possible reactions to consider.

One particular reaction cycle of interest is the so-called “hercynite cycle”, in which a mixed-metal ferrite is thermally reduced with aluminum oxide to form hercynite (iron aluminate), cobalt or nickel aluminate, and evolve oxygen. This cycle has been demonstrated to be effective at reacting with water and carbon dioxide, and reduces at temperatures above

1000°C. This high temperature is however somewhat lower than some other thermochemical cycles studied, making this cycle more realistic for operation. Additionally, preliminary results suggest that the hercynite cycle should be able to cycle isothermally, with the reduction and oxidation reactions controlled by the presence or absence of oxygen. As such, this reaction cycles was examined to explore the possibility of this type of thermochemically augmented energy storage.

Two different processes were suggested for use with a solid oxide reaction cycle such as the hercynite cycle. The first is an augmented solid particle receiver, in which solid particles serve as both the heat transfer media and thermal storage media for a central receiver solar system. The solid particles absorb the concentrated solar radiation in a receiver, where the materials could also be thermally reduced under an inert atmosphere. The solid particles are then stored until it is desirable to release the stored thermal energy to produce electric power. This cooling and oxidation reaction would release both the sensible and thermochemical heat stored in the material, and could transfer this thermal energy to a power cycle through a heat exchanger.

The cooled and oxidized particles would then be stored until being sent through the solar

receiver again. This is a modification to a solid particle receiver which has been studied in the past for high temperature solar thermal power production, where the sensible heat capacity of the solid particles would be augmented by a thermochemical reaction cycle.

Another potential process that is suggested for use with solid oxide thermochemical augmented energy storage is a dish system which an inert gaseous heat transfer fluid such as helium absorbs the solar radiation at the focus of the dish and transfers it to a series of solid blocks of material to store the thermal energy. These blocks could be composed of both the active solid material and a material to enhance heat transfer, such as graphite. The solid oxide material could then be reduced at the same time by removing oxygen from around the particles, thus absorbing additional heat. The same gaseous heat transfer can also transfer heat to a power cycle for electric power production. This removes the need for the solid material to be moved around the system, and can potentially enable large amounts of high-temperature heat to be stored on a dish system.

Isothermal thermochemical energy storage is important to be able to achieve for high- exergetic efficiency values for a system. The reduction and oxidation reactions must therefore be controlled in some other way. One way that is suggested to do this is to use commercially available electrochemical cells which can separate oxygen from an otherwise inert stream of gas.

These cells operate at elevated temperatures, making them more appropriate for operation at or near the thermal reduction temperatures of the solid oxide materials of interest. This would allow for highly effective control of the thermochemical cycle without a change in temperature. The electrochemical cell would add an electrical parasitic load to the system, which must be taken into consideration.

In-depth system models are necessary to fully examine the benefits and drawbacks to these potential systems. This would allow for a thorough analysis of potential trade-offs in the system, and an exploration of ideal operating conditions. However, there is limited amount of information about the reaction enthalpy and equilibrium compositions of the hercynite cycle (and other solid oxide cycles) under various temperature and oxygen concentrations. A determination of this design space is therefore necessary before a useful system model can be done.

References

1. Scheffe, J.R., J. Li, and A.W. Weimer, A spinel ferrite/hercynite water-splitting redox

cycle. International Journal of Hydrogen Energy, 2010. 35(8): p. 3333-3340.

2. Neises, M., et al., Solar-heated rotary kiln for thermochemical energy storage. Solar

Energy, 2012. 86(10): p. 3040-3048.

3. Coker, E.N., et al., Oxygen transport and isotopic exchange in iron oxide/YSZ

thermochemically-active materials via splitting of C(18O)2 at high temperature studied

by thermogravimetric analysis and secondary ion mass spectrometry. Journal of

Materials Chemistry, 2012. 22(14): p. 6726-6732.

4. Agrafiotis, C., et al., Solar water splitting for hydrogen production with monolithic

reactors. Solar Energy, 2005. 79(4): p. 409-421.

5. Chueh, W.C., et al., High-flux solar-driven thermochemical dissociation of CO2 and

H2O using nonstoichiometric ceria. Science (New York, N.Y.), 2010. 330(6012): p.

1797-801.

6. Miller, J., et al., Metal oxide composites and structures for ultra-high temperature solar

thermochemical cycles. Journal of Materials Science, 2008. 43(14): p. 4714-4728.

7. Arifin, D., et al., CoFe2O4 on a porous Al2O3 nanostructure for solar thermochemical

CO2 splitting. Energy & Environmental Science, 2012. 5: p. 9438-9443.

8. Glatzmaier, G., Summary Report for Concentrating Solar Power Thermal Storage

Workshop: New Concept and Materials for Thermal Energy Storage and Heat-Transfer

Fluids May 20, 2011. 2011, National Renewable Energy Laboratory (NREL) Report No.

NREL/TP-5500-52134 Golden, CO

9. High Energy Advanced Thermal Storage (HEATS) Funding Opportunity Announcement,

Advanced Research Projects Agency (ARPA-E) Department of Energy, DE-FOA-

0000471 CFDA Number 81.135. 2011.

10. Pacheco (Editor), J.E., et al., Final Test and Evaluation Results from the Solar Two

Project. 2002, Sandia National Laboratories: Report No. SAND2002-0120.

11. ger, M., et al., Face-Down Solid Particle Receiver Using Recirculation. Journal of

Solar Energy Engineering, 2011. 133(3): p. 031009-1 - 031009-8.

12. Hruby, J.M., A Technical Feasibility Study of a Solid Particle Solar Central Receiver for

High Temperature Applications. 1986, Sandia National Laboratories: Report No.

SAND86-8211.

13. Hellmann, J.R., et al., Evaluation of Spherical Ceramic Particles for Solar Thermal

Transfer Media. 1987, Sandia National Laboratories: Report No. SAND86-0981

Albuquerque, NM. p. 51.

14. Andraka, C.E., K.S. Rawlinson, and N.P. Siegel, Technical feasibility of storage on large

dish stirling systems. 2012, Sandia National Laboratories: Report No. SAND2012-8352.

p. Medium: ED; Size: 73 p.

15. Forsberg, C.W., P.F. Peterson, and H. Zhao, High-Temperature Liquid-Fluoride-Salt

Closed-Brayton-Cycle Solar Power Towers. Journal of Solar Energy Engineering, 2007.

129(2): p. 141-146.

16. Weimer, A.W., et al., Rapid carbothermal reduction of boron oxide in a graphite

transport reactor. AIChE Journal, 1991. 37(5): p. 759-768.

17. Overholser, L.G. and J.P. Blakely, Oxidation of graphite by low concentrations of water

vapor and carbon dioxide in helium. Carbon, 1965. 2(4): p. 385-394.

18. Yuan, H.-W., et al., Mechanical and thermal properties of cement composite graphite for

solar thermal storage materials. Solar Energy, 2012. 86(11): p. 3227-3233.

19. Jafari, H., et al., Nano-SiC/SiC anti-oxidant coating on the surface of graphite. Applied

Surface Science, 2013. 264(0): p. 128-132.

20. Meixner, D.L., et al., Electrochemical Oxygen Separation Using Solid Electrolyte Ion

Transport Membranes. Journal of the Electrochemical Society, 2002. 149(9): p. D132-

D132.

21. Nachlas, J.A. and D.M. Taylor, Inert gas purifying system, U.S.P.a.T. Office, 5,454,923

Editor^Editors. 1995, Ceramatec, Inc. : United States.

22. Motohashi, T., et al., Remarkable Oxygen Intake/Release Capability of BaYMn2O5+δ:

Applications to Oxygen Storage Technologies. Chemistry of Materials, 2010. 22(10): p.

3192-3196.

23. Machida, M., et al., Layered Pr-dodecyl sulfate mesophases as precursors of Pr2O2SO4

having a large oxygen-storage capacity. Journal of Materials Chemistry, 2006. 16(30): p.

3084-3090.

CHAPTER III

THEORETICAL PREDICTIONS

3.1 Introduction

In order to examine the effect of a thermochemical boost on a sensible thermal energy storage system, it is useful to examine various reaction chemistries at various conditions and make predictions about the amount of energy that can be stored. This is most easily done with theoretical equilibrium prediction software, so that various chemistries and conditions can be examined quickly. This will help to identify specific chemistries and materials which warrant future testing.

The main goal of this theoretical prediction effort is to determine the heat of reaction for various materials at various conditions. The heat of reaction is a critical parameter for thermochemical energy storage, as it can provide a direct value for the amount of energy that can be stored via a particular thermochemical reaction. Additionally, reaction products can be predicted at various conditions. This ability is useful for a number of reasons; the materials of interest can be screened for side reactions with possible contaminants or containment materials.

This ability to predict reaction products also predicts the reactant to product conversion at equilibrium for a particular set of conditions. This is useful for determining the energy storage limit that can reasonably be expected for a set of conditions. Theoretical equilibrium reaction conversion can also be compared to experimental values in order to check for kinetic or other limitations.

Here, the FACTSageTM 6.2 software is used to predict reaction chemistries and heats of reaction [1]. This calculation is done by calculating the Gibbs free energy, which includes

contributions from the pure phase components, ideal entropy, an excess Gibbs energy term, and terms to include contributions to Gibbs free energy from changes in molar volumes and magnetic ordering [2].The Gibbs free energy is minimized by assuming that the ideal solution equations are used, then replacing the chemical potentials by the associated non-ideal values; this effect cancels itself out when a minimum is reached [2]. The specifics of the software are proprietary, but have been validated for mixed metal oxide systems, among others [3].

3.2 Methods

The Equilib block was primarily used to calculate thermodynamic equilibrium through

Gibb’s ree Energy minimization. In this block, the reactants are identified and their amounts specified. In addition to the solid reactants of interest, an inert gas is included as a dilutant for the reduction reaction. This is done to reflect the situation in which the materials of interest will be surrounded by inert gas in order to sweep oxygen away and drive the reduction reaction further towards completion. In order to calculate a change in enthalpy, initial conditions are specified, including the phase, temperature, and pressure. If there are multiple entries for a reactant in multiple databases, the specific phase in the particular database would be selected at this point as well. Once the reactants have been specified, the final conditions are specified, including the temperature and pressure. Lastly, the possible product species are identified. This is done by selecting all possible products in all possible phases, including solid solution species. Duplicates between databases are suppressed, but otherwise all possible products are included. This leads to a lengthy calculation output file, but is done for completeness. The calculations are then performed, and the output examined. For many of the calculations that were done, the products were then reacted with other materials or at other conditions. This was done by using the Recycle

All Streams function in FACTSageTM, which allows for the products of one calculation to be used as reactant inputs into the next calculation. The temperature can then be changed, as well as the addition or removal of oxygen. Oxygen removal or addition was done by setting the product gas stream amount to zero, then adding a new stream with the desired amounts of oxygen and inert gas. For all of the following calculations, the pressure is left at a constant 1 atmosphere.

One of the first calculations to be done is a prediction of the solid state synthesis method for producing the mixed metal oxide. For this calculation, the three solid oxide ingredients will be combined and temperature and atmospheric profiles similar to what would be expected from a realistic synthesis will be applied. The resulting material will be compared to a second set of calculations using the desired materials (the “Synthesis” and “Comparison” cases below, respectively). The temperature profile will reflect calcination in air at various temperatures between 25°C and 1000°C after starting at ambient conditions (23°C), then a cooling of the material in air. Since the desired materials are CoFe2O4 and Al2O3 for the Hercynite Cycle

(CoFe2O4 + 3 Al2O3 ↔ CoAl2O4 + 2 FeAl2O4 + ½ O2), the starting ingredients will be CoO,

Fe2O3, and Al2O3. The “Synthesis” case will use 1 mole of CoO, 1 mole of e2O3, and 3 moles of

Al2O3 for the solid materials, and the air will be represented by 21 moles of O2 and 79 moles of

N2. The “Comparison” case will use similar amounts of all of these components, but will use 1 mole of CoFe2O4 in place of the 1 mole of CoO and the 1 mole of Fe2O3. A more general case is also examined for the hercynite materials (CoFe2O4 + 3 Al2O3) for temperatures from 23°C up to

1400°C under reducing conditions (100 moles argon) to examine the composition of the solid at these temperatures.

In order to obtain some idea of the design space for thermochemical and sensible energy storage, the amount of energy that could be stored at a variety of temperatures was examined. It

has been discussed previously that at or near isothermal thermochemical storage is useful for maintaining high exergetic efficiencies. Unlike lower temperature reactions, reactions at temperatures >900°C are able to have moderate temperature differences without large exergetic efficiency losses (see Chapter II). While isothermal energy storage may be more beneficial from a purely exergetic standpoint, small to moderate temperatures may allow for more energy to be stored thermochemically, which enhances the energy storage density of the material. As such, a study was done using FACTSageTM thermodynamic prediction software to estimate the amount of thermochemical energy that can be stored at various temperatures. Additionally, since the concentration of oxygen near the active material will directly affect the equilibrium constant for the forward or reverse reactions, the effect of oxygen concentration is also examined.

The heat of reaction for the reduction reaction was determined at temperatures from

900°C to 1400°C and for oxygen concentrations of 0% to 10%. This was done in the

FACTSageTM software by starting with stoichiometric amounts of solid material for the cobalt- ferrite hercynite cycle (1 mole CoFe2O4 and 3 moles Al2O3). Excess oxygen was added (100 moles), and the equilibrium was found at 900°C. This initial step was done to ensure a starting material that was fully oxidized and at thermodynamic equilibrium near the temperatures of interest. Calculations were then performed to find the thermodynamic equilibrium at all combinations of conditions for temperatures between 900°C to 1400°C in increments of 50°C and for oxygen concentrations of 0% to 10% in increments of 0.5% (balance argon up to 100 total gaseous moles). In finding the equilibrium composition of every phase at each of these conditions, the software also calculated the total change in enthalpy between the starting material

(fully oxidized at 900°C) and the final conditions. It should be noted that this total enthalpy change will include all possible enthalpy contributions, including sensible and chemical

enthalpic changes in the material. As such, equilibrium calculations were performed on the gas amounts alone at the same gas compositions and temperatures in order to find the enthalpic contributions from the gas phase (due to sensible energy changes and small amounts of gas-gas reactions). This enthalpy was subtracted from each of the total enthalpy changes for the calculations with the active material, leaving the total enthalpy changes for the solid material alone. Since this remaining enthalpy change includes sensible energy changes which can be significant for large temperature changes, the sensible energy was estimated for the active solid material. This was done in Excel, and is described below. The sensible energy due to the change in temperature from the starting material to the final equilibrium condition was subtracted from each of the enthalpy change values, leaving the enthalpy change of the thermochemical reaction.

This estimates a design space for various reduction/oxidation temperature combinations, and allows for an exploration of the possibilities of this material.

Isothermal reduction-oxidation cycles are of particular interest to this study. As was discussed previously, isothermal redox allows for potentially highly exergetic efficiency for thermochemical energy storage. In order to explore the possibly of isothermal thermochemical energy storage using the hercynite reaction, calculations were done in which the active materials were stabilized at various reaction temperatures from 1000°C to 1500°C under argon. The temperature was held constant and the gases present were alternated between air and oxygen 4 times each. The cycling was done to ensure that the equilibrium predictions were consistent and unchanging. The solid material was recycled each time while the gas streams are changed. The specific conditions are shown in Table 3-1 for a cycle done at 1200°C. For each case, the starting solid materials are 1 mole of CoFe2O4 and 3 moles of Al2O3, since these are the materials of interest for the hercynite cycle.

Table 3-1: Calculation Inputs and Methodology for Isothermal Redox at 1200°C Step Step Name Initial Final Sold Stream Gas Stream No. Temp. Temp. st 1 1 Reduction 1200°C 1200°C 1 mol Co Fe2O4 100 mol Ar 3 mol Al2O3 st 2 1 Oxidation 1200°C 1200°C (recycled) 100 mol O2 3 2nd Reduction 1200°C 1200°C (recycled) 100 mol Ar nd 4 2 Oxidation 1200°C 1200°C (recycled) 100 mol O2 5 3rd Reduction 1200°C 1200°C (recycled) 100 mol Ar rd 6 3 Oxidation 1200°C 1200°C (recycled) 100 mol O2 7 4th Reduction 1200°C 1200°C (recycled) 100 mol Ar th 8 4 Oxidation 1200°C 1200°C (recycled) 100 mol O2

One of the major goals of this study is to determine the benefit gained from thermochemical storage over sensible energy storage. As such, the sensible heat stored in the materials of interest is calculated and compared to the calculated heats of reaction. Equation 3-1 is used to find the sensible energy available at a particular temperature T, using an ambient temperature T0. Here,

T0 is taken to be 23°C, and are dummy variables for temperature inside the integration.

( ) ∫ ( ) Equation 3-1

This calculation is done by using equations for the specific heat capacity of each of the starting materials in order to find a weight-averaged equation for the active material specific heat capacity. Specifically, the heat capacities of CoO, Fe2O3, and Al2O3 were combined with their respective molar fractions in the stoichiometric material. The heat capacities of the components are found in [4]. The resulting molar heat capacity was then converted to a specific heat capacity using the combined molar mass of the material. For temperatures in increments of 25°C, the energy at each of these increments is calculated by finding the average heat capacity at each

temperature, then multiplying this by the difference between the incremental temperatures; this is summarized in Equation 3-2.

( ) ( ) Equation 3-2

A numerical estimation of the sensible energy was then found by adding all the energies for temperatures above the ambient. The exergy at a temperature of interest can then be found in the resulting table.

3.3 Results

The predicted equilibrium conditions for the calcination reaction in air are shown in

Appendix A. As can be seen, the cobalt oxide and iron oxide being to combine to form a single spinel phase at all temperatures examined. The composition of this spinel phase of the

CoO/Fe2O3 mixture is the same as for the fully formed cobalt-ferrite mixed metal oxide, as is shown by the CoFe2O4 line in. This is indicates that it is predicted that the combination of CoO and Fe2O3 in the correct amounts will form CoFe2O4 at all temperatures above 25°C at equilibrium in air. As such, a moderately high temperature to increase the kinetics of this formation should be sufficient to form the CoFe2O4 from a mixture of CoO and Fe2O3.

A general composition study based on equilibrium solid compositions under reducing conditions at temperatures from ambient up to 1400°C was also done, and the results are shown in Figure 3-1. The study finds the predicted solid composition at thermodynamic equilibrium at the indicated temperatures in an inert atmosphere. The four lines plotted in the figure indicate the mass of various solid phases; the “Spinel Mass”, “Cor Mass”, and “Cor2 Mass” lines indicate the

solid mass (in grams) of the spinel phase, corundum phase, and a second corundum phase, respectively. The “Total Mass” line indicates the total solid mass of the material. As can be seen, there is some phase separation in the corundum (M2O3, where M is a metal such as Fe or Al) phases at temperatures below 500°C, but these would not affect the reaction of interest, which occurs at temperatures >900°C. This reaction predication can be seen from the plot by noting where the total mass begins to drop from the loss of evolved oxygen. The spinel phase contains many of the reactants and products in the Hercynite Cycle reduction reaction, including the starting cobalt ferrite (CoFe2O4) and the products cobalt aluminate (CoAl2O4) and hercynite itself (FeAl2O4). However, the other reactant (Al2O3) is in the corundum phase. As such, the reaction can also be examined by looking at the mass increase in the spinel phase and associated mass drop in the corundum phase, and the Al2O3 reactions to form the aluminate products.

Hercynite Cycle Reduction Composition Study 600

500

400

Spinel Mass 300

Cor Mass Mass(g) 200 Cor2 Mass Total Solids 100

0 0 200 400 600 800 1000 1200 1400 1600 Temperature (deg C)

Figure 3-1: Predicted Composition of Solid Material at Thermodynamic Equilibrium at Various Temperatures with Inert Dilutant

Equilibrium predictions were calculated for isothermal reduction-oxidation cycles at various temperatures. The full results are given in Appendix A, and are summarized in Figure 3-

2 and Figure 3-3. These calculations estimate the extent to which thermochemical energy can be stored with no temperature change, using only the presence or absence of oxygen to control the reaction. This may not directly reflect the way this type of process will operate in practice, but is useful to illustrate the extent to which heat can be stored at a single temperature using only the heat of the thermochemical reaction. Since the quality (exergy) of the heat is so important for concentrated solar power, it is important to know that the thermal energy storage can deliver stored heat to the power cycle with minimal exergy losses. As can be seen in Figure 3-2, the maximum amount of heat that can be stored isothermally varies with temperature, as do the changes in mass.

Figure 3-2: Isothermal Thermochemical Energy Storage for Hercynite Cycle Using Changes in Equilibrium Compositions and Enthalpy

Figure 3-3: Changes in Amount of Solid Mass Present during Four Isothermal Reduction/Oxidation Cycles at Various Temperatures from 1000°C to 1400°C

Theoretical predictions of the amount of heat able to be stored in active hercynite materials using both sensible and thermochemical heat storage were calculated, and these predictions are summarized in Figure 3-4. The heat storage is estimated using the total enthalpy changes from the FACTSageTM predictions for the temperatures indicated, which includes both sensible and thermochemical enthalpy changes. These thermodynamic predictions are then compared to the sensible heat at the same temperatures using heat capacity data and calculated in

Excel. The full results to both of these calculations are given in Appendix A.

Heat of Reaction for Various Temperature and O2 Concentrations (kJ/kg) 10

9 100 150

5 150 100

O2 O2 concentration (%) 3

50 1 100 150

0 200 900 950 1000 1050 1100 1150 1200 1250 1300 1350 1400 Temperature (deg C) Figure 3-4: Thermochemical Reaction Enthalpy Change for Stoichiometric Hercynite Cycle Material Based on Thermodynamic Equilibrium Calculations at Temperatures between 900°C and 1400°C and Oxygen Concentrations between 0% and 10% with Balance Inert Argon

Calculations were done to estimate the sensible energy in a stoichiometric reactive mixture of CoO + Fe2O3 + 3 Al2O3. This was done to compare the thermochemical energy to the sensible energy stored at the temperatures of interest. The complete table of results of these calculations is shown in Appendix A. The specific heat capacity of this material and the cumulative exergy of the material are shown in Figure 3-5 and Figure 3-6, respectively.

Specific Heat of CoO:Fe2O3:3Al2O3 1400

1200

K) - 1000

800

600

400 Specific Heat Capacity (J/kg Heat Specific 200

0 0 200 400 600 800 1000 1200 1400 1600 Temperature (°C)

Figure 3-5: Calculated Specific Molar-Weighted Heat Capacity of Reactive Solid Material

Exergy of CoO:Fe2O3:3Al2O3 1800

1600

1400

1200

1000

800 Exergy (kJ/kg) Exergy 600

400

200

0 0 200 400 600 800 1000 1200 1400 1600 Temperature (°C)

Figure 3-6: Calculated Cumulative Exergy of Reactive Solid Material

3.4 Discussion

The thermodynamic predictions above indicate that the reduction reaction for the hercynite cycle can proceed in significant amounts at temperatures above 1000°C and at O2 concentration levels below 0.5%. This level of reaction does depend substantially on the concentration of oxygen present; Figure 3-4 shows that there is a large difference in the temperature necessary to reach a particular level of reduction (as seen by the reaction enthalpy) between O2 concentrations up to ~2%, whereas O2 concentrations above this level have less of an impact on the conversion. Though similar to Figure 3-4, this effect can be seen more directly in

Figure 3-7, which shows the remaining solid mass as a percentage of the original amount of solid

material present. Since the solid material losses mass as oxygen evolves in the reduction reaction, this is a direct measure of the reduction reaction conversion. This is useful to compare to Figure 3-4 in order to clearly illustrate the fact that the predicted thermochemical reaction enthalpy at particular conditions is a direct consequence of the reaction extent at those conditions. As was discussed in Chapter II, it is feasible to operate a solar process at temperatures up to 1500°C or higher and with oxygen concentration levels below 1% O2 concentration. As such, the hercynite cycle does appear to be able to realistically react in these conditions, and therefore can be potentially useful for high temperature solar thermal storage.

Remaining Solid Mass (% of Initial Material) 10

99.8 99.6 99.4 7 99.2

4 O2 O2 concentration (%)

99.6 3 99.8 99.4 99.2 99

99.6 0 99.8 99.4 99.2 99 98.8 98.6 900 950 1000 1050 1100 1150 1200 1250 1300 1350 1400 Temperature (deg C) Figure 3-7: Remaining Solid Mass of Reduction Reaction as a Percentage of Original Solid Material Present for Various Temperatures and O2 Concentrations

It was discussed previously that it is of interest to consider thermochemical energy storage in relation to the sensible energy that exists in the solid material at the same temperature.

As such, the sensible energy at the temperatures indicated for this temperature and O2 concentration study was compared to the thermochemical heat of reaction. This was done, and the results are shown in Figure 3-8. The thermodynamic predictions for isothermal heat storage above indicate that the reaction needs to occur at least 1200°C and 0.5% O2 in order to have a reasonably significant effect on thermal energy storage of at least 10% of the sensible energy. It should be noted that these thermochemical enthalpy changes are for a reduction reaction from a fully oxidized material at 900°C. At temperatures below 1000°C, the thermochemical energy storage potential is less than 5% of the sensible energy stored at that temperature, even for oxygen concentrations near 0%. The thermochemical fraction of energy storage increases at higher temperatures from the heat of reaction, despite the fact that the amount of sensible energy at higher temperatures will be higher as well. This reaches a maximum at 18.53%, which occurs at 1400°C at 0% O2.

Fractional ThermoChemical Boost to Sensible Exergy (%) 10

4 6 8

10 9 12

4 6 8 5 10 12

4 O2 O2 concentration (%) 3 14

2 4 6 8 1 10 12 16 14 0 900 950 1000 1050 1100 1150 1200 1250 1300 1350 1400 Temperature (deg C) Figure 3-8: Comparison of Thermochemical Heat of Reaction to Total Sensible Exergy at Indicated Temperature

This sensible energy comparison can be somewhat misleading, however. The sensible exergy that the thermochemical enthalpy is being compared to is the total amount of sensible energy estimated to be contained in the solid material from the indicated temperature down to an assumed ambient temperature of 23°C. This temperature difference from 23°C to >1000°C is very large, and so the sensible exergy of this temperature difference is subsequently very large. It is implicit in this comparison of exergy is that the sensible energy is useful from the indicated high temperature all the way down to ambient. While this is a common assumption in the comparison of exergy, this may not be necessarily true for a real system. Operational limits for materials in use in the system may restrict operation from such as extreme (>1000°) temperature change. This is not an absolute certainty, since the system can be designed such that any one

component does not have to endure such drastic temperature changes in a short amount of time, such as multiple heat exchangers to limit the temperature swing in any one unit and bottoming cycles to produce useful electricity from lower grade (temperature) heat. That said, it may be that the system would be more effective (or at least realistically operable, even if it was not at the optimal conditions) if the temperature swing was more limited. As such, it is useful to compare the predicted thermochemical heat of reaction to a more limited exergy. As a limiting case on the other end of the spectrum from the maximum exergy (with a minimum temperature of 23°C) would be a minimum exergy case, in which the exergy minimum temperature is 900°C. In this case, the exergy is only the sensible energy of the solid material from 900°C to the indicated temperature of interest. This comparison was done, and is displayed in Figure 3-9.

Fractional Thermochemical Boost to Sensible Exergy from 900 C (%) 10

25 9 20

7 20

6 30 20 25 5 35

4 O2 O2 concentration (%) 3 20 25 30 2 25 35 1 40 35 30 30 35 40 40 45 0 45 45 900 950 1000 1050 1100 1150 1200 1250 1300 1350 1400 Temperature (deg C) Figure 3-9: Comparison of Thermochemical Heat of Reaction to Sensible Exergy at Indicated Temperature from 900°C

Comparing the thermochemical heat of reaction to this limited exergy reveals some implications which are somewhat counterintuitive. The maximum fraction of the thermochemical energy to sensible exergy is 66.08% and occurs at 950°C and 0% O2. This is due to the fact that

950°C has the smallest exergy from 900°C except for 900°C, which has no exergy and so is not included in Figure 3-9. In one sense, it is obvious that this point would give the maximum thermochemical benefit, as it is being compared to the smallest amount of exergy. It is also obvious that low O2 concentrations would give a higher thermochemical benefit, since this means that the conversion of the reduction reaction is driven further to completion. However, what is somewhat surprising is the fact that at 950°C, the extent of reduction is very low (see

Figure 3-7), since this is comparatively low temperature compared to 1400°C. As such, there will not be a very large thermochemical enthalpy change to compare to the sensible exergy. However, the fact that the comparison takes into account a very small exergy for that low temperature means that the thermochemical benefit will be larger. This illustrates an important point that the temperature change over which a thermochemical cycle operates should match the overall operational temperature change of the system itself as closely as possible. This is not necessarily saying that a beneficial system using the hercynite cycle should only operate at temperatures down to 900°C; operating at such a high rejection temperature means that there is a very large amount of potential exergy not being utilized for electricity production or in some other useful manner. The point is that the thermochemical energy storage is more useful relative to the sensible energy at temperatures of interest if the temperature range of the thermochemical cycle is relatively close to the total operational temperature range.

The thermochemical heat storage is being evaluated as an augmentation to the sensible energy storage that can be stored in the solid material. Therefore, it is useful to compare the isothermal heat storage as shown in Figure 3-2 to the sensible energy results shown in Figure 3-

6. This is shown in Table 3-2. As can be seen, the effect of the isothermal energy storage is moderately sized when compared to the full sensible exergy of the material. The highest proportion is at around 1300°C, and does not quite reach 10% of the sensible heat. However, this comparison should be seen as a limiting case for a number of reasons. First, the sensible exergy is calculated with reference to the ambient temperature of 23°C, whereas in a realistic system it is unlikely that the heat will be able to be used at the temperatures indicated (>1000°C). While this would obviously be preferable from an exergetic standpoint, this would necessitate extremely wide temperature swings in materials throughout the system. For example, current state-of-the-art molten salt systems are only able to utilize heat down to 290°C due to the limitations in the salt. Even if the solid materials under consideration can handle these wide temperature swings, there would likely be a number of heat exchangers and associated power cycles to fully utilize the widely disparate temperature heat. This may or may not turn out to be useful in a realistic system. Additionally, the thermochemical heat being compared in Table 3-2 is obtained via thermodynamic prediction of multiple redox cycles. The material is therefore not fully reduced or oxidized at any point in the process, so the full potential heat of reaction is not utilized at any point. This is due to the fact that the hercynite reaction progresses to different equilibrium extents at various reactions, making it able to store more energy at higher temperatures. Similarly, the oxidation is not complete at the higher temperatures, meaning that not as much energy is released exothermically either. This partial reduction and oxidation can be seen in Figure 3-3 from the mass changes that differ with temperature and never fully return to

the initial mass level. While this type of isothermal reaction is interesting and useful from an exergetic standpoint, it is more likely that the thermochemical heat can be utilized over a wider range of temperatures, since the sensible heat utilization will require this temperature change anyway. As such, this can be considered somewhat of a lower bound on the thermochemical benefit to the sensible energy storage capacity of the material.

Table 3-2: Comparison of Isothermal Thermochemical Heat Storage to Sensible Heat Storage at Various Temperatures Sensible Thermochemical Temperature Heat Heat Qthermochem/Qsensible (kJ/kg) (kJ/kg) 1000°C 984.48 41.82 4.25% 1100°C 1094.05 70.04 6.40% 1200°C 1204.68 99.91 8.29% 1300°C 1316.34 123.47 9.38% 1400°C 1428.94 131.33 9.19%

As can be seen from these predictions, the hercynite cycle should be able to slightly augment sensible thermal energy storage in high temperature solid materials. However, there is one issue that is somewhat unique to the hercynite cycle. The predictions above are found in the

FACTSageTM software are given in a single enthalpy change value, based on the amount of material and conditions at thermodynamic equilibrium. This value is then corrected for the mass of the solid material, in order to find a specific heat in terms of energy per unit mass. The stoichiometry of the hercynite cycle has 4 moles of solid material (CoFe2O4 + 3 Al2O3) on each side the reaction; this fairly large amount of material makes the specific heat of the reaction much smaller. Many other solid oxide reactions have similar absolute heats of reaction, but since these other reactions can be for a single mole of material reacting, the specific heat of reaction can be two or three times higher. This can be significant when comparing the specific

thermochemical heat to the specific sensible heat of a material, and can make a large difference in the usefulness of these reactions for operating realistic systems.

It is therefore useful to examine other potential solid oxide reactions for thermochemical energy storage. By way of illustration, initial FACTSageTM predictions of various other solid oxide reactions are given in Table 3-3. These predictions were done for the reduction reaction only, at the temperatures indicated, with 1 mole of the solid oxide and 100 mole of inert argon as a dilutant. Three other material reactions were examined: iron oxide, cobalt oxide, and cerium oxide. All of these materials have been considered in the past for thermochemical cycles with

H2O and CO2 splitting. Each material was combined with 100 moles of inert argon as a dilutant and calculated at the temperatures indicated (listed as starting temperature – final temperature, or as an isothermal calculation). For the temperature range indicated for the iron oxide cycle, the material was corrected for sensible energy of both gas and solid, similar to methods described previously for the hercynite cycle. The cerium oxide cycle is a non-stoichiometric cycle, where δ is the extent of reaction. Other non-stoichiometric mixed metal oxides (especially mixed metal ferrites) are also of interest, due to the fact that they are structurally similar to the hercynite cycle, but without the need to react with large amounts of aluminum oxide, meaning these reactions can potentially have high heats of reaction per unit mass.

Table 3-3: Predictions of Reaction Enthalpies for Other Solid Oxide Thermochemical Reduction Reactions Material Name Reaction Temperature Reaction Enthalpy Iron Oxide Fe2O3 ↔ 2/3 e3O4 + 1/6 O2 1000°C -1200°C 471 kJ/kg Cobalt Oxide Co3O4 ↔ 3 CoO + 1/2 O2 Isothermal 800°C 901 kJ/kg Cerium Oxide CeO2 ↔ CeO(2-δ) + δ /2 O2 Isothermal 1600°C 4.9 kJ/kg*

As can be seen, other solid oxides can react with very high heats of reaction per unit mass. This can be potentially very useful for thermochemical energy storage. Some important considerations for each of the above cycles should be noted. First, it is important to note that the various materials undergo reduction reactions at very different temperatures. Cobalt oxide is completely done reducing at 800°C, whereas the cerium oxide undergoes only a small amount of reduction at temperatures as high as 1600°C. This is by no means any sort of complete list that encompasses all possible reaction temperatures, but rather meant to give an idea of the wide range of reaction cycle possibilities. Another aspect that should be noted is the low reaction enthalpy of the cerium oxide cycle, which is smaller than the other materials listed in Table 3-3 by multiple orders of magnitude. Aside from the fact that different materials inherently have different reaction enthalpies, this is primarily due to two reasons. First, the cerium oxide cycle does not reduce to the same extent as the other materials (d is very small, ≪1). Secondly, while data exists in the FACTSageTM database for cerium oxide, it does not directly exist for the temperatures of interest, and so the values are extrapolated (indicated by the asterisk). Therefore, the cerium oxide values are much more uncertain than the other theoretical predictions. This material was included because it has been studied in the past for thermochemical reaction cycles

[5-7], and is very different from the other reactions, and so can be useful for an illustrative comparison of different possible cycles.

Another important aspect to the solid oxide reduction reaction is the temperatures at which it begins and ends. As can be seen for the hercynite cycle reduction reaction in Figure 3-1, the reaction begins at about 800°C, but is not fully complete at temperatures as high as 1400°C.

By contrast, the composition study (similar to the one described for the hercynite cycle) was done for the cobalt oxide reaction cycle. The results of this study are shown in Figure 3-10,

which displays the equilibrium solid mass of the starting material and reduced material with 100 moles of inert argon gas. The total solid mass is not displayed, as the two materials shown are the only components present in the solid material at the temperatures indicated. As can be seen in

Figure 3-10, the full reduction reaction occurs over a much smaller temperature range than the hercynite cycle. While the material composition begins to change to the reduced material at temperatures as low as 650°C, the transition does not begin in earnest until temperatures above

740°C. Indeed, half the mass transition occurs between 780°C and 800°C. This means that nearly the entire reduction-oxidation cycle could feasibly be run between temperatures of 720°C and

800°C. This can be very promising for thermochemical energy storage, since such a small temperature change is needed to drive the reaction. As was discussed previously, it is not necessarily a requirement that the entire reaction conversion be used to store thermochemical energy, as energy can still be stored isothermally (see Figure 3-2 for isothermal energy storage with the hercynite cycle). However, the ability to utilize the full extent of reduction and oxidation with a small temperature change is very beneficial for thermochemical energy storage.

Figure 3-10: Predicted Composition of Solid Components at Thermodynamic Equilibrium for Cobalt Oxide Reaction at Transition Temperatures and with Inert Atmosphere

It must be stressed that these values do not mean that these other reactions will necessarily be more useful in a system, as there can be other problems associated with these reactions. For example, pure iron oxide will sinter very well, which could severely limit the amount of reactive material. Though it does contain iron oxide, the hercynite reaction has shown promise at being able to cycle repeatedly with minimal loss of activity over multiple cycles [8,

9]. Obviously further testing over hundreds or thousands of cycles should be done to ensure that the activity and structural integrity of this type of material continues, but these reported results are promising for this material of interest. These and other factors must be considered when designing a potential system for study, but these higher thermochemical energy storage numbers do provide reactions of interest for future study.

3.5 Conclusions

The possibility for thermochemical energy storage using the so-called “hercynite cycle” has been explored using theoretical thermodynamic equilibrium predictions. The FACTSageTM proprietary software was used, which predicts thermodynamic equilibrium for components using a large materials databases and Gibbs free energy minimization algorithms. Predictions of the material compositions at thermodynamic equilibrium were obtained for the hercynite cycle materials under various conditions of interest; namely, low concentrations of oxygen in an otherwise inert atmosphere and high temperatures to drive the reduction reaction to completion.

Shomate equation for the estimation of solid heat capacity. The enthalpy of the thermochemical reaction at various conditions was normalized per unit mass of solid material and compared to the sensible energy in the same amount of solid material at various temperatures and over various temperature ranges.

Specifically, the compositions of the hercynite cycle materials were predicted in an inert atmosphere at various temperatures from ambient up to 1400°C. This was done both to explore the thermochemical potential of the material, obtain an idea of what temperatures were necessary for significant reaction, and to predict the material that would form for solid state synthesis using various component metal oxides. It was found that the component oxides matched exactly the mixed-metal oxide composition for all temperatures and thermodynamic equilibrium. It was further found that for an inert atmosphere, the hercynite cycle will being reducing at

temperatures above 800°C, with significant reduction beginning to occur at temperatures above

1000°C.

The enthalpy change for the thermochemical reduction reaction was found at various temperatures and oxygen concentrations. When cycled isothermally between inert atmosphere for reduction and oxygen atmosphere for oxidation at various temperatures, it was found that the isothermal reaction enthalpy per unit mass ranged from approximately 40 kJ/kg at 1000°C to approximately 130 kJ/kg at 1400°C. These isothermal reaction enthalpy values were compared to the sensible energy at the indicated temperatures, and it was found that the thermochemical to sensible energy ratio ranged from 4.25% at 1000°C to 9.2% at 1400°C. While isothermal operation may be beneficial in general, these values show a relatively small increase to sensible energy storage. This may be overtaken by additional parasitic losses in the process that will be necessary for operating the thermochemical cycle.

Furthermore, the thermochemical enthalpy change for a temperature change from a fully oxidized material at 900°C to various temperatures from 900°C to 1400°C and various oxygen concentrations from 0% to 10%. The predicted reaction enthalpies for these temperature change calculations range from 2.84 kJ/kg at 900°C and 10% O2 to 264.8 kJ/kg at 1400°C and 0% O2.

These predicted reaction enthalpies were compared to the sensible energy from ambient (23°C) to the final reduction temperature. The thermochemical reaction enthalpies ranged from 0.32% to

18.5% of the sensible energy at 900°C and 1400°C, respectively. This reaction enthalpy was also compared to the sensible exergy starting at 900°C instead of 23°C, and this comparison found that the highest fraction of the thermochemical reaction enthalpy compared to this more limited exergy was 66.1% at 950°C. This was found to be true despite the fact that the reduction reaction had relatively minor conversion when going from 900°C to 950°C (meaning a smaller reaction

enthalpy) 950°C had the smallest amount of exergy above 900°C, making the proportion of the thermochemical reaction enthalpy larger.

Both of the above comparisons show that the hercynite cycle provides only a moderate benefit to sensible energy storage. While the 18.5% thermochemical boost to the full exergy is by no means insignificant, it is unlikely that this will ultimately prove to be beneficial to process operation. Even the 66% boost from the limited exergy is not necessarily enough to be realistically beneficial to a process. A ~10% reduction in process flow rates and storage volumes will certainly make a difference, but probably not enough to justify operating an entire thermochemical cycle. Thermochemical operation will involve additional parasitic losses, such as oxygen separation and pumping, which will cause significant efficiency losses for the rest of the process. A major thermochemical benefit would be needed to make it worth doing from a process operational standpoint. The exact boost that is needed is not known, because it depends so much on the particular system and associated parasitic losses.

Finally, additional metal oxide reduction reactions were explored. The reduction of iron oxide, cobalt oxide, and cerium oxide were explored at various temperatures with an inert atmosphere, and the associated predicted reaction enthalpies were found. Fe2O3 is predicted to reduce to Fe3O4 from temperatures of 1000°C to 1200°C, with a reaction enthalpy of 471 kJ/kg over this temperature range. Co3O4 is predicted to reduce to CoO at temperatures below 800°C with a reaction enthalpy of 901 kJ/kg. The FACTSageTM materials database did not contain data for cerium oxide at the temperatures considered for reduction, but the extrapolated values predicted a reduction reaction enthalpy of 4.9 kJ/kg at 1600°C, which is likely due to the small level of reduction this non-stoichiometric material undergoes. Furthermore, a composition study was done for the cobalt oxide reduction reaction between temperatures of 650°C and 800°C. It

was found that most of the reduction reaction equilibrium transition for cobalt oxide occurs between 720°C and 800°C, with half of the transition occurring between 780°C and 800°C.

Based on these theoretical predictions, it appears as though the hercynite cycle reduces at temperatures above 1000°C and O2 concentrations less than 2%. The reaction enthalpies are of moderate magnitude, and can contribute somewhat to sensible energy under particular conditions. For cycling between 1400°C and 900°C with <0.5% O2, it is predicted that the hercynite cycle can contribute a reaction enthalpy that is 18.5% of the total sensible energy at these high temperatures. It was also found that isothermal reduction-oxidation cycles are possible, and the thermochemical benefit gained isothermally can be up to 9.2% of the sensible energy at 1400°C. While not insignificant, these values of thermochemical boost are fairly low in terms of providing a reasonable boost to thermochemical energy storage.

However, the reaction enthalpies of other solid oxide materials were found to be much higher per unit mass than the hercynite cycle. This is due to the fact that the hercynite cycle has four moles of active material (CoFe2O4 + 3 Al2O3), whereas other solid oxides will only have a single mole of material (e.g., Co3O4). This can be a large difference in the mass of the active material for the reduction and oxidation reactions for reactions that can have similar enthalpy changes per “mole” of reaction. Iron and cobalt oxides were explored and appear to be very promising for further exploration of thermochemical energy storage, with predicted reaction enthalpies of 471 kJ/kg and 901 kJ/kg, respectively. Additionally, other mixed non- stoichiometric metal oxides are of interest, but were unable to be modeled using the

FACTSageTM software. That said, there can be other potential benefits to the hercynite cycle, including an ability to cycle repeatedly with no predicted (or observed) slag phase, and very little

reported loss of activity due to material sintering. Other solid oxides may not be able to cycle repeatedly due to structural or surface area issues.

In addition to the specific predictions summarized above, some general concepts were illustrated for thermochemical energy storage boosting of sensible energy storage. As discussed previously, it is important to be able to have a small temperature change between the reduction and oxidation reactions, so that there is not a large exergy loss for the stored energy. This can help narrow down a search for potential materials of interest, as it would be very beneficial for the reaction being considered to be able to fully reduce or oxidize over a small temperature range. This is not a hard requirement, but an important consideration for thermochemical energy storage in general, and for thermochemical boosting of sensible thermal energy storage.

Finally, it is important to match the temperature change associated the thermochemical energy storage cycle of interest to the process temperature change. Any process for the thermal production of electricity (or any other use of thermal energy) will likely have some energy change associated with the transfer of sensible thermal energy. If this temperature change can be closely matched to the reaction temperature change, the benefit to thermochemical energy storage will increase drastically. For the hercynite cycle in particular, it was found that the thermochemical energy gained from a temperature change from 900°C to 1400°C is 18.5% of the sensible energy at the high temperature of 1400°C. This is a significant boost, as this is direct increase in the energy storage capacity of the material without the need for increased storage material mass or a further temperature increase. The thermochemical benefit between the much smaller temperature swing of 900°C to 950°C is only 3.8% of the sensible energy at 950°C.

However, if the process in question had a lower temperature bound of 900°C, the thermochemical benefit of that same temperature swing of 900°C to 950°C becomes 66.1% of

the sensible energy, which is larger benefit. As discussed above, these specific values for the hercynite cycle are lower than desired, but the illustration of matching the thermochemical cycle to sensible temperature change is important. A process with a heat rejection lower bound of

900°C is unlikely, but this does illustrate the large difference between a thermochemical benefit only over the upper temperatures in a process, and if the process more directly matched the thermochemical cycle.

References

1. Bale, C.W., et al., FactSage thermochemical software and databases — recent

developments. Calphad, 2009. 33(2): p. 295-311.

2. Eriksson, G. and K. Hack, ChemSage—A computer program for the calculation of

complex chemical equilibria. Metallurgical Transactions B, 1990. 21(6): p. 1013-1023.

3. Jung, I.-H., et al., Thermodynamic evaluation and modeling of the Fe–Co–O system. Acta

Materialia, 2004. 52(2): p. 507-519.

4. Domalski, E.S. and E.D. Hearing, Condensed Phase Heat Capacity Data, in NIST

Chemistry WebBook, Standard Reference Database Number 69, P.J. Linstrom and W.G.

Mallard, Editors, National Institute of Standards and Technology: Gaithersburg, MD.

5. Chueh, W.C., et al., High-flux solar-driven thermochemical dissociation of CO2 and

H2O using nonstoichiometric ceria. Science (New York, N.Y.), 2010. 330(6012): p.

1797-801.

6. Siegel, N.P., et al., Cerium Oxide Materials for the Solar Thermochemical

Decomposition of Carbon Dioxide. ASME Conference Proceedings, 2010. 2010(43956):

p. 89-95.

100

7. Scheffe, J.R. and A. Steinfeld, Thermodynamic Analysis of Cerium-Based Oxides for

Solar Thermochemical Fuel Production. Energy & Fuels, 2012. 26(3): p. 1928-1936.

8. Scheffe, J.R., J. Li, and A.W. Weimer, A spinel ferrite/hercynite water-splitting redox

cycle. International Journal of Hydrogen Energy, 2010. 35(8): p. 3333-3340.

9. Arifin, D., et al., CoFe2O4 on a porous Al2O3 nanostructure for solar thermochemical

CO2 splitting. Energy & Environmental Science, 2012. 5: p. 9438-9443.

101

CHAPTER IV

EXPERIMENTAL EXAMINATION OF HERCYNITE CYCLE

4.1 Introduction

The previously discussed theoretical predictions provide useful information about the possibility of thermochemical energy storage, but experimentation is needed to actually demonstrate this. There are many different aspects of the proposed concept to explore, and only a few are presented here. The goal of this experimental exploration is to validate theoretical predictions and demonstrate the possibility of thermochemical augmentation to the sensible energy storage in a solid material. In the course of this exploration, the specific reaction of interest will be checked for any kinetic and thermodynamic limitations. This all can inform future work and decisions about thermochemical energy storage.

First, it is useful to visualize the material being studied. This can help draw conclusions about behavior of the samples under experimental conditions. To this end, the samples can be analyzed using a scanning electron microscope (SEM). This provides a real image at very high magnification, allowing features to be seen at very small length scales. Additionally, the SEM is able to analyze the molar composition of the sample via energy-dispersive X-ray spectroscopy

(EDX). This allows for the molar composition to be studied by identifying the presence of particular elements of interest. This is done on the SEM images, so that the molar composition of the features being examined can be analyzed for patterns.

It is important to verify that the chemical reactions of interest are occurring as expected.

Such verification can help to explain discrepancies in other experimental values and help to identify reaction completeness. X-ray diffraction (XRD) is a useful technique for the

102

examination of crystalline materials. X-ray diffraction works by bombarding a sample with X- rays and measuring the diffraction at various angles. If the X-rays impact a crystal plane at a particular angle, the detector can register a stronger signal. The resulting diffraction patter of signal counts at various angles is then compared to literature values of standard materials in order to find a match.

While X-ray diffraction is a very useful technique, there are other ways to analyze materials. Raman spectroscopy was also used for sample analysis. Raman spectroscopy works by shining a laser beam onto a sample, which then interacts with molecular vibrations and other excitations in the system, causing it to shift. The resulting shifted light is then measured in a detector, and the intensities of the shifts are plotted. The spectrum is then matched to standard spectra in the literature to match the sample to known material.

A thermo-gravimetric analyzer was primarily used for this experimentation. This instrument contains a very precise balance in a furnace with a controllable atmosphere. The instrument is then able to measure changes in weight at various temperatures. This is useful for a wide variety of analysis, including vaporization and absorption/desorption. Thermo-gravimetric analysis (TGA) is especially useful for solid oxide reactions, since the weight change associated with the evolution of oxygen during thermal reduction (and the associated weight gain upon re- oxidation) can be measured precisely. This weight change can be used to accurately measure reaction conversions.

The ability to measure the heat of reaction is very important for the current study. This can be done with differential scanning calorimetry (DSC). DSC works by measuring temperature differences between a reference crucible and a crucible with a sample in it. Two thermocouples are used, which are each attached to one of the crucibles. Thermocouples are comprised of two

103

different metallic wires attached at a point; the dissimilar metals or alloys generate a voltage at this junction which changes with temperature. In this way, the temperature of each of the crucibles is measured. Additionally, the potential difference between the similar wires from each of the two thermocouples can be measured. This potential difference is proportional to a temperature difference and therefore a heat flow between the two samples. This proportionality can be found by running standards in the DSC of pure materials which melt at known temperature and with a known latent heat of fusion. Different standard materials with different melting points are run in order to cover the entire temperature range of interest. In this way, the heat flow into or out of the material can be found, such as when the material undergoes reduction or oxidation.

4.2 Experimental Setup and Methods

The sample powder analyzed in this study was prepared via a solid state synthesis method. The component materials were weighed out in molar ratios of interest and placed into a plastic bottle. Zirconia milling media of nominal diameter of 1.5 mm were added and ethanol was added to cover the powder and milling media. The bottle was sealed and placed on a ball mill for 8 hours. The ethanol and powder mixture was then poured onto a drying plate and the ethanol was allowed to evaporate in a fume hood. The resulting powder was then collected and placed into a furnace to calcine at 850°C for 2 hours. The “Base” material was formulated using

Sigma-Aldrich® Cobalt(II) oxide, -325 mesh,; Sigma-Aldrich® Iron(II,III) oxide, powder,

<5μm, 95%; and Sigma-Aldrich® Aluminum oxide, powder, <10 micron, 99.7%. The material was weighed out in the stoichiometric ratio of the hercynite reaction, with 1:1:3 for

104

CoO:Fe3O4:Al2O3. Though not strictly in the nominal stoichiometric ratio, the Fe3O4 powder was used due to material availability.

In order to examine the potential differences between a powder made from Fe2O3 and one made from Fe3O4, a composition study was performed using the FACTSageTM thermodynamic prediction software. Molar amounts of the starting materials were entered into the software, such that the “ e2O3” case was composed of 1 mole CoO, 1 mole of e2O3, and 3 moles of Al2O3. For comparison, the “ e3O4” case was composed of 1 mole CoO, 1 mole of e3O4, and 3 moles of

Al2O3. For each case, the equilibrium composition of each of these mixtures was found at temperatures between 400°C and 1400°C, and the resulting molar compositions were compared.

Both cases had 21 moles of O2 and 79 moles of N2 present to represent equilibrium in air. The results of this study are fully listed in Appendix A, and are summarized in Figure 4-1. As can be seen, the general trends in the molar composition of the two cases are nearly identical. The primary difference is the continued presence of a second corundum phase above 550°C. At temperatures between ~500°C and ~900°C, the molar amounts of the two cases are identical in the spinel and corundum phases; above these temperatures, the molar amounts begin to differ somewhat, with the Fe3O4 case having more moles of material in both the spinel and corundum phases as the second corundum phase in the Fe3O4 case begins to decrease and transfer its material to the other solid phases. The second corundum phase in both cases is composed primarily of Fe2O3. It therefore makes sense that the oxidized Fe3O4 in the Fe3O4 case would form additional Fe2O3. Based on this analysis, while the differing powder composition is expected to have some effect, it is expected to be minor (though not negligible). This is especially true at temperatures above 1100°C, where the second corundum phase in the Fe3O4 case is no longer expected to exist.

105

Figure 4-1: Comparison of Predicted Equilibrium Compositions of Solid Mixtures of CoO+Fe2O3+3Al2O3 and CoO+Fe3O4+3Al2O3

Additionally, sample powders were prepared with different formulations using different powders. These samples, hereafter referred to as Alumina-6 and Alumina- 9, were made using molar ratios for CoO:Fe2O3:Al2O3 of 1:1:6 and 1:1:9, respectively. These samples used the same powders as above, but used Alfa Aesar® Iron(III) oxide, 325 mesh, 99.5% (metals basis), instead of the Fe3O4 powder used above. They were mixed and calcined in the same manner as the base powder. Once mixed and dried, the base powder was analyzed via SEM/EDX with no additional processing.

Samples were prepared for SEM/EDX analysis by mounting double-sided carbon tape on a 12.2mm x 10mm JEOL aluminum specimen mount. The powders were then placed on the carbon tape, tapping off any un-adhered particles. The samples were then analyzed using a JEOL

JSM-6480LV SEM.

106

Sample powder was spread evenly on a glass slide, and the slide was then placed into a

PANalytical X’Pert P O for analysis. The typical scan done would scan at angles between 10° and 80°. Samples from the Base, Alumina6, and Alumina9 powders were analyzed pre- and post- calcination, and after being cycled in the TGA. Peak identification for the resulting scans was performed with help from the Jade software from MDI Products.

Additionally, a Scintag PAD X diffractometer from Thermo Electron, Inc., (Waltham,

MA) was used to analyze samples in-situ at high temperatures. Sample preparation for this high temperature XRD (HT-XRD) was done by mixing the sample powder with acetone using a small mortar and pestle. This slurry was then spread on a glass slide and allowed to dry. The slide was then placed onto the instrument stage for analysis. The program would do scans as the sample was heated under inert conditions for thermal reduction, then cooled somewhat, then oxidized in air. The typical scans in-situ were done from 2-θ angles of 10.0° to 80.0° in increments of 0.05°, with 1.5 second scans done at each position. After an initial scan at room temperature, the sample was heated. Scans were done from 200°C to 800° in increments of 200°C, and from

800°C to 1400°C in increments of 50°C. For each measurement temperature, the instrument furnace would hold the temperature constant during the scan. After the sample reached 1400°C and was scanned, the sample was cooled to 1000°C and scanned twice. Air was then introduced, and three sample scans were done in quick succession. Finally, the sample was cooled under air and a final scan was performed at 50°C.

Sample preparation was done for Raman spectroscopy by mixing the sample powder with acetone in a small mortar and pestle. This slurry was then spread onto a glass slide and allowed to dry. Analysis was done with a Laser Quantum® torus® single frequency 532nm CW laser.

Measurements were done at 10% laser power to reduce fluorescence from the sample.

107

Sample powders of approximately 115 mg were placed in alumina crucibles for

TGA/DSC analysis. A NETZSCH® STA 409 C/CD was used, with a TG-DSC sample carrier with radiation shield and type S thermocouples. After the sample and reference crucibles were placed on the stage, the furnace was closed and sealed, the chamber evacuated and back-filled with argon gas three times. 20 sccm of argon gas was then continuously flowed through the chamber as a protective gas for the SiC furnace. The temperature and gas flow profile was programmed into the control computer. For reduction, 40 sccm of argon gas was used, and 40 sccm of air was used for oxidation steps (both in addition to the protective gas flow). In each experimental case when the reduction and oxidation temperatures were different, the sample allowed to settle at the oxidation temperature for 15 minutes. This stabilization was to allow the

DSC signal to steady, so that the exothermic peak could be clearly observed. For each new temperature profile, an empty crucible was first run in order to provide a baseline and correction for buoyancy effects. Calibration was done for the DSC using NETZSCH® In, Sn, Bi, Zn, Al,

Ag, Au, and Ni melting standards. Temperature profiles were set up for the calibration runs such that the temperature would rise at 5 K/min from room temperature to 30 K above the melting point of the standard. The temperature would then ramp down at 10 K/min to 100 K below the melting point of the standard to ensure complete solidification. The temperature then repeats these steps and ramp rates twice more. Of these three runs, the two most consistent are selected for inclusion in the analysis. The TGA and DSC data were analyzed using NETZSCH®

Proteus® – Thermal Analysis – Version 5.2.0 software.

Finally, simulations of three experimental runs in the TGA/DSC were done using the

FACTSageTM Gibbs free energy minimization software. The three experimental runs were chosen such that there was one for each material formulation evaluated. The Base powder

108

formulation was evaluated using the molar amounts described above to match the sample preparation, which was 1 mole CoO, 1 mole of Fe3O4, and 3 moles of Al2O3. These solid materials were simulated using the Recycle functionality to match a temperature profile which cycles three times between reduction in argon (100 moles Ar) at 1400°C and oxidation in air (21 moles O2 and 79 moles N2) at 1000°C. The Alumina-6 material formulation was modeled by 1 mole CoO, 1 mole of Fe2O3, and 6 moles of Al2O3. The Alumina-9 material formulation was modeled by 1 mole CoO, 1 mole of Fe2O3, and 9 moles of Al2O3. Each of the Alumina formulations was simulated to reflect an experimental run in which the material was reduced in argon (100 moles Ar) at 1100°C, 1200°C, 1300°C, and 1400°C with oxidation in air (21 moles

O2 and 79 moles N2) at 1000°C after each reduction step. The last step is an isothermal reduction-oxidation step at 1200°C using the same amounts of gas as above. The solid mass predicted for each step was noted, and the total enthalpy change for each step was corrected for the enthalpic contributions of the gas present, normalized per unit mass, and corrected for the sensible energy changes of the solid material. This solid sensible energy estimation was done with a similar sensible energy calculation as shown in Chapter III, except using the appropriate molar amounts for each material formulation.

4.3 Results

The sample powders were prepared and mixed as described above. The resulting powder was dark in color, and was imaged via a Scanning Electron Microscope (SEM). The resulting micrograph is shown in Figure 4-2. As can be seen, the powder particle sizes in the sample vary widely.

109

Figure 4-2: SEM Image of Base Powder

The complete results for the XRD analysis are shown in Appendix B, including scans done for the Base Powder, Alumina-6, and Alumina-9 material formulations for samples of uncalcined material, calcined material, and material that had been cycled in the TGA.

Results from the in-situ High Temperature XRD analysis are shown in Figure 4-3. This figure shows a contour plot of peak height for all scans done. Obvious peaks in the XRD patterns are identified. The scan temperatures and the point at which oxygen was introduced to the sample are noted. The complete results for the High-Temperature in-situ XRD analysis are shown in Appendix B.

110

High-Temperature in-situ XRD 50C 1000C 1000C Al2O3  1000C Air Introduced 1000C FeAl2O4  1000C  FeAl2O4 1400C FeAl2O4  1350C 1300C 1250C 1200C 1150C 1100C

Temperature 1050C FeO  1000C 950C 900C 850C  CoFe2O4 (right side) 800C 600C CoFe2O4  400C 200C  Al2O3 Fe2O3   Co3O4 25C 25 30 35 40 45 2- (degrees) Figure 4-3: X-Ray Diffraction Patterns for “Base” Sample Powder for X D Scans Done in-situ at the Temperatures Indicated

The results in Figure 4-3 show the composition transition as the material reduces and the associated change upon oxidation. A selection of scans from the HT-XRD data is shown in

Figure 4-4 to de-convolute some of the important peaks and shifts that occur for 2-θ values around 35°. These scans are for the calcined powder at room temperature, the reduced powder at

1400°C and under flowing argon gas, and the re-oxidized powder at 1000°C in air.

111

Figure 4-4: High Temperature in-situ X- ay Diffraction Patterns for “Base” Sample Powder for Scans Done on the Initial Sample, Under Reducing Conditions, and After Re-Oxidation

The aman spectra for “base” powder samples are shown in Figure 4-5 for scans done on samples before and after calcination and after TGA cycling. The complete results for all Raman spectra are shown in Appendix B. The literature spectra for comparison are found in Refs. [1, 2].

112

Base Powder 15000

10000 un-calcined calcined post-TGA CoFe2O4 FeAl2O4

intensity (counts) CoAl2O4 5000

0 200 300 400 500 600 700 800 wavenumbers (cm-1) Figure 4-5: aman Spectra Patterns for “Base” Sample Powder for Scans Done with Unaltered Mixed Powder, Calcined Powder, and Sample Powder that had been cycled in the TGA/DSC

The results of the TGA/DSC data analysis are shown in Figure 4-6. This figure was constructed from the output all included experimental runs. Each horizontal line in Figure 4-6 shows a single heat of oxidation value obtained at the reduction and oxidation temperatures indicated at the endpoints of the horizontal line. Lines that appear as single points are for isothermal redox. As with previous experimental results, the complete results of the TGA/DSC data are given in Appendix B.

113

Heat of Oxidation 110

100

60 Base Powder Alumina 6 50 Alumina 9

30 Specific of Heat (-J/g) Reaction 20

0 800 900 1000 1100 1200 1300 1400 1500 Temperature (deg C) Figure 4-6: Heat of Oxidation for Base, Alumina-6, and Alumina-9 Material Formulations at Indicated Reduction and Oxidation Temperatures

Finally, selected experimental runs were compared to theoretical predictions, as described above. The results from this theoretical predictions are shown along with the experimental values are shown in Table 4-1 for the Base Powder 1400-1000 Cycles experiment, Table 4-2 for the

Alumina-6 Variable Thermal Reduction and Isothermal Redox experiment, and Table 4-3 for the

Alumina-9 Variable Thermal Reduction and Isothermal Redox experiment.

114

Table 4-1: Theoretical Prediction of Base Powder 1400-1000 Cycles Experimental Run and Comparison to Data Step Red1 Ox1 Red2 Ox2 Red3 Ox3 Temperature 1400°C 1000°C 1400°C 1000°C 1400°C 1000°C Fractional Theoretical 97.003 99.040 97.199 99.060 97.201 99.060 Mass (%) Experimental 97.31 98.77 97.28 98.76 97.26 98.77 1 Experimental 96.51 97.97 96.5 97.96 96.51 97.96 2 Reaction Theoretical -267.924 249.215 -243.629 251.819 -243.311 Enthalpy Experimental -80.6 -79.1 -79.2 (J/g) 1 Experimental -72.6 -70.8 -70.7 2

Table 4-2: Theoretical Prediction of Alumina-6 Variable Thermal Reduction and Isothermal Redox Experimental Run and Comparison to Data Fractional Mass (%) Reaction Enthalpy (J/g) Step Temperature Theoretical Experimental Theoretical Experimental Red1_1100 1100°C 99.670 98.62 -55.691 Ox1_1000 1000°C 99.964 98.89 -43.863 -14.95 Red1_1200 1200°C 99.481 98.49 73.192 Ox1_1000 1000°C 99.964 98.79 -72.104 -15.14 Red1_1300 1300°C 99.275 98.39 105.639 Ox1_1000 1000°C 99.964 98.74 -103.290 -16.5 Red1_1400 1400°C 99.069 98.27 140.632 Ox1_1000 1000°C 99.964 98.67 -136.525 -17.94 IsoRed_1200 1200°C 99.481 98.35 73.184 IsoOx_1200 1000°C 99.832 98.64 -46.784 -13.12

115

Table 4-3: Theoretical Prediction of Alumina-9 Variable Thermal Reduction and Isothermal Redox Experimental Run and Comparison to Data Fractional Mass (%) Reaction Enthalpy (J/g) Step Temperature Theoretical Experimental Theoretical Experimental Red1_1100 1100°C 99.787 99.59 -92.036 Ox1_1000 1000°C 99.979 99.83 -26.551 -11.82 Red1_1200 1200°C 99.664 99.47 45.028 Ox1_1000 1000°C 99.979 99.82 -44.319 -16.25 Red1_1300 1300°C 99.524 99.34 66.727 Ox1_1000 1000°C 99.979 99.82 -65.174 -22.11 Red1_1400 1400°C 99.376 99.21 91.375 Ox1_1000 1000°C 99.979 99.8 -88.604 -28.63 IsoRed_1200 1200°C 99.664 99.46 45.023 IsoOx_1200 1200°C 99.900 99.71 -30.663 -12.25

4.4 Discussion

Two important pieces of information are found from the experimental TG and DSC data: the mass change on reduction and the exothermic energy flow on oxidation. These are both found using the Proteus® analysis software. The mass change is found by a simple difference in mass from the TG data from when reduction visually begins to the lowest point it reaches. This mass change can be expressed as an absolute mass or a percentage change. The start of reduction mass loss is verified by matching it to the mass gain upon oxidation of the material. The energy flows are found by finding the area under the DSC curve using a built-in function in the

Proteus® software. The DSC signal calibration using the aforementioned standards can be incorporated into the analysis automatically if specified before the experimental run, or the area under the DSC curve can be manually transformed to an energy flow later. The energy flow under the DSC curve is normalized per unit sample mass, which was specified at the beginning of the experiment.

The summary figure of the DSC data in Figure 4-6 shows a lot of important information about the reduction and oxidation cycles that were examined. Each horizontal line represents a

116

single data point reading of the enthalpy change associated with oxidation. The endpoints of the line indicate the temperatures at which the sample for that particular reading had been reduced and oxidized. There are a number of things to note from this figure. First, it can be easily seen that the longer lines (indicating larger temperature changes between reduction and oxidation) have higher heats of reaction per unit mass. This is due to the fact that with larger temperature changes, the conversion of the forward (reduction) and reverse (oxidation) reactions are larger, meaning that the observed enthalpy change for that reaction will be larger.

Another important feature to note about Figure 4-6 is the differences between the Base,

Alumina-6, and Alumina-9 formulations. While each material formulation follows the general trend noted above (that larger temperature differences lead to larger observed thermochemical enthalpy changes), there are clear differences between the Base and Alumina formulations. This is partially due to the material formulation differences in the iron oxide used to make the samples, but also due to the fact that the Alumina samples have additional aluminum oxide, as their name suggests. This was originally done to explore the effect of the extra alumina beyond the stoichiometric amount, and see whether this would lower the reaction enthalpy. It was also thought that these materials could potentially have better reactivity, since they could have better contact between the cobalt ferrite and the alumina. As can be seen from Figure 4-6, the Alumina-

6 and Alumina-9 formulations do appear to have lower observed oxidation enthalpy changes than corresponding temperature differences in the Base powder formulation.

An important consideration in the enthalpic data being considered is the variability of the data. This is difficult to do with a limited data set, but important to consider as much as possible all the same. The variability in the data displayed in Figure 4-6 is visually very wide-ranging.

Some samples have observed enthalpy measurements that are very close to each other, even for

117

different temperature ranges; this is especially true for the Alumina-6 samples. Conversely, the

Base Powder TR and Isothermal sample has enthalpy measurements that span the entire range of the plot. It should be noted that the Alumina-6 sample run data did have potentially problematic drifting issues in the data set, which may explain some of the suspiciously similar enthalpy change values; this can be seen in Appendix B.

Additionally, there were two separate experimental runs in which Base powder formulation samples were subjected to the same temperature profiles, and the temperature profiles each cycled between the same temperatures (1400°C reduction and 1000°C oxidation) three times. This provides an opportunity for a direct comparison, albeit in a fairly limited sense.

Basic statistics for these data points were calculated, and are shown in Table 4-4. It can be seen that the variability of the two samples is fairly similar, but the means of the two samples are very different. This was confirmed by a 2-sample Student’s t-test, which gives a very small p-value of

0.001 (when not assuming equal variances). This means that the null hypothesis of the t-test should be rejected based on the experimental data and the two samples give significantly different mean values of the oxidation enthalpy changes. However, this also illustrates the very small amount of replicate data currently available, and strongly suggests the need for additional experimentation.

Table 4-4: Basic Statistics for Base Powder 1400°C-1000°C Cycles Sample Mean Number of Standard 95% Confidence Points Deviation Interval on Mean All (#1 and #2) 75.5 J/g 6 4.60 J/g ± 4.84 J/g Sample #1 79.63 J/g 3 0.84 J/g ± 2.08 J/g Sample #2 71.37 J/g 3 1.14 J/g ± 2.66 J/g

118

The experimental values obtained do not match the theoretical predictions from

FACTSageTM. This is clearly shown in Table 4-1, Table 4-2, and Table 4-3. As can be seen in the tables, the experimental values obtained for the Base Powder comparison tend to be approximately 30% of the theoretical reaction enthalpy values. The difference between the theoretical and experimental values can be explained by a number of potential reasons. The first and most obvious is kinetic limitations. The thermodynamic predictions done by the

FACTSageTM software are for thermodynamic equilibrium, and the samples in the experimental runs might not have reached equilibrium. In order to explore this possibility, the mass changes from the experimental data were compared to the theoretical mass changes from the

FACTSageTM predictions. Both mass changes were normalized to the original starting mass of solid material. The results of this analysis are shown for the Base Powder in Table 4-1, but are plotted in Figure 4-7 to examine this effect visually.

Base Powder 1400-1000C Cycles Mass Changes 99.5

98.5

97.5 Theoretical 97 Experimental 1

% of Initial %of Initial Mass 96.5 Experimental 2

95.5

95 Red1 Ox1 Red2 Ox2 Red3 Ox3 Experimental Step Figure 4-7: Comparison of Fractional Mass Changes from Experimental Results for Base Powder 1400-1000 Cycles to Theoretical Predictions

119

As can be seen from this figure, the experimental results vary somewhat. The first sample appears to match the theoretical results fairly closely, but the second sample does not. However, it must be noted that while the results appear to be close, the entire range of these fraction mass results only cover 3% of the mass. The small mass changes can therefore have a big impact on the fraction mass results shown above. This can occur in a systematic fashion if there was an error in either calibrating the thermo-gravimetric or in weighing the initial sample mass. If there were issues with the calibration or weighing of a sample, this would affect the entire sample run, if not multiple sample runs. It is unclear which, if any, of these experimental errors are significant, and further testing is needed to explore this. One reason by which the “Experimental

1” fractional mass results in Figure 4-7 are questionable is the large differences between the experimental and theoretical reaction enthalpies. This is also shown in Table 4-1, but is plotted in

Figure 4-8 for a visual analysis.

120

Base Powder 1400-1000C Cycles Reaction Enthalpy 300

200

100

Theoretical 0 Experimental 1 Red1 Ox1 Red2 Ox2 Red3 Ox3 Experimental 2

-100 ReactionEnthalpy (J/g)

-200

-300 Experimental Step

Figure 4-8: Comparison of Reaction Enthalpies from Experimental Results for Base Powder 1400-1000 Cycles to Theoretical Predictions

Both the values shown in Table 4-1, and plotted in Figure 4-8 show a large discrepancy between the predicted and experimental reaction enthalpy values. Furthermore, both of the experimental values appear similarly very different from the theoretical predictions, unlike the fractional mass change differences in Figure 4-7. Both of these figures show a systematic difference between the theoretical and experimental results. This is clearly not random error, and can be due to a number of reasons. Experimental error was mentioned previously. There are a number of ways in which this could affect the results indicated. An error in the initial sample mass reading would affect the subsequent fractional mass results. An error in the calibration of the DSC signal would also systematically affect the reaction enthalpy readings. Additional experimentation is needed with careful calibration in order to ascertain the extent of these potential experimental errors.

121

However, this type of error may have something to do with the Base powder formulation.

It is therefore useful to compare the experimental results and theoretical predictions in a visual manner. Figure 4-9 and Figure 4-10 display the fractional mass changes for the Alumina-6 and

Alumina-9 material formulations, respectively.

Alumina-6 TR and IsoRedox Mass Changes 100.5

100

99.5

98.5

Theoretical % of Initial %of Initial Mass 98 Experimental

97.5

Experimental Step

Figure 4-9: Comparison of Reaction Enthalpies from Experimental Results for Alumina-6 Variable Thermal Reduction and Isothermal Redox to Theoretical Predictions

122

Alumina-9 TR and IsoRedox Mass Changes 100.2

100

99.8

99.6

99.4

Theoretical % of Initial %of Initial Mass 99.2 Experimental

98.8

Experimental Step

Figure 4-10: Comparison of Reaction Enthalpies from Experimental Results for Alumina-9 Variable Thermal Reduction and Isothermal Redox to Theoretical Predictions

As can be seen from these figures, the Alumina material formulations do not match the theoretical results either. The differences in reaction enthalpy values are also substantially lower

(see Table 4-2 and Table 4-3), which is a similar trend as the Base powder. Also similar to the experimental data seen in the Base powder material formulation experiments (and discussed above), there does appear to be significantly variability in the experimental data obtained.

Alumina-6 has a particularly wide gap between the fractional mass changes predicted from thermodynamic equilibrium and the experimental values.

Aside from systematic experimental error, another possible explanation of the disparity between experimental and theoretical results is kinetic limitations. The theoretical predictions are calculated at thermodynamic equilibrium, and if the experimental samples are not reaching equilibrium, they will not match these theoretical predictions. This is possible, but many of the

123

experimental runs were done such that the mass had stopped changing significantly. That is not to say that stopped moving during reduction or oxidation completely, but the rate of change had certainly slowed. While it may not necessarily be expected that kinetic limitations would be an issue at such high temperatures, there is also the diffusion of oxygen in the solid to consider.

Additionally, it can be seen that the relative difference between the fractional masses and reaction enthalpies is a different magnitude (see Table 4-1). As such, it is not enough to simply scale the partial conversion achieved in the experimental results by the theoretical fractional mass change. This is difficult to do in a realistic manner, since the differences from the theoretical and experimental extents of reaction for the reduction and oxidation reactions are unknown. The experimental data is only able to directly observe the oxidation exotherm, making it difficult to know how much to change this value. Even though the relative mass change differences between experimental and theoretical values are known for both the reduction and oxidation, it is not clear how much these different values should scale the oxidation enthalpy change value. This is due to the fact that the enthalpy of oxidation depends on the previous extent of reduction, but this relationship is difficult to determine. Despite all this, it can be seen from rough estimations of scaling the reaction enthalpy values in Table 4-1 by the extent of reaction from the mass change that this will not make the experimental values match the theoretical values. This suggests that kinetic limitations alone are not enough to explain the differences. Regardless of the specific source of the limitations, it is clear that the experimental samples are not at thermodynamic equilibrium, and it does not appear that kinetic limitations alone.

Since the experimental value difference cannot be explained by purely kinetic limitations, there must be other considerations to consider. It is of interest to consider if the expected

124

chemistry is actually occurring. This can be done by analyzing the various XRD patterns and comparing them to published results. The spectra of interest are shown in Figure 4-3. This figure shows the evolution of the reaction as observed by XRD in-situ. As can be seen, the initial scan shows the presence of Al2O3 and CoFe2O4, as would be expected. The scan also shows hematite

(Fe2O3), which is present from the oxidized residual Fe3O4 that was used to make the material.

Additionally, there appears to be some cobalt oxide which did not calcine fully, either from limited time at the calcination temperature, or from not being in contact with the iron oxide. As the temperature is raised in a reducing argon atmosphere, the cobalt oxide and hematite disappear, presumably to form additional cobalt ferrite, as this peak does increase in intensity.

Wüstite (FeO) appears between 800°C and 1250°C. At around 1200°C, the expected reduction reaction appears to occur in earnest. Above this temperature, the aluminum oxide peaks reduce in intensity significantly, and peaks signifying hercynite (CoFe2O4) and cobalt aluminate (CoAl2O4) appear. Both of these peaks are indicated by the hercynite label, since the peaks for both of these materials are very similar. There is little change when the material is cooled under argon, as is expected; since there is no oxygen present, the material will not re-oxidize.

There is a noticeable shift upon the introduction of air, however. The aluminum oxide peaks regain some of their intensity, though not enough for the peak at 2-θ=25.5° to reappear on the contour map (it can be seen in the individual scans). However, despite the fact that it is near the aluminum oxide peak, the cobalt ferrite peak does not clearly reappear. Instead, the peaks associated with the spinel phase (hercynite and cobalt aluminate) shift to the right, while other peaks do not. This shift can be seen in the contour plot, but is more easily seen (along with the stationary aluminum oxide peaks) in Figure 4-4. This shift is also seen in the XRD scans of the cooled material, shown in Appendix B. This shift is also seen in the Raman spectroscopy data

125

(see Figure 4-5); the uncalcined and calcined materials show peaks that match literature peaks for the expected materials (CoFe2O4) relatively well, but the material that had been cycled in the

TGA showed much less clear characteristics. The broad amorphous peaks do not clearly match any of the literature spectra, though it can be seen that some of the expected spinel peaks

(CoAl2O4 and FeAl2O4) come close.

There are likely two main reasons for this discrepancy between the expected and observed experimental results. The first is the fact that the sample powders were made by solid state synthesis, i.e. physical mixing. Small powder sizes were used, and the powders were well- mixed, but there was no other mechanism to ensure that the reactants were in good contact throughout the reaction. This can greatly limit the reaction extent, since the materials cannot react if they are not in contact. However, it was seen previously that the smaller reaction extent is not enough to fully explain this difference. The other related way in which poor reactant contact affects the results is the possibility of side reactions. If the reactants are not in good contact in the proper amounts, then other, unexpected reactions can occur, rather than the preferred and expected reaction. These reactions can contribute to or detract from the observed enthalpy changes in the samples. Since it is unknown what materials were in contact and in what proportions, these reactions are difficult to predict and account for. The possibility for this effect can be seen in Figure 4-2; the sample powders are loosely packed at best, and there are obvious voids between particles. Additionally, it does appear from the EDX data in Appendix B that some of the particles are richer in one material over the others, as can be expected from mixing powders.

Unfortunately, this reaction contact issue is difficult to quantify and study directly, since it is difficult to determine which reactants are in contact and in which proportions, and then

126

estimate what reactions this will produce. That is not to say that the hercynite cycle material does not occur; it has been previously shown that the hercynite reaction does occur as expected and can be cycled repeatedly [2, 3]. As can be seen in Figure 4-11, the Raman spectra for the reduced and subsequently oxidized material matches expected literature spectra for the expected materials. This was done with reactants made via atomic layer deposition (ALD), and so the reactants would be ensured very good contact [2]. This plot indicates that the reaction should proceed as expected when reactant contact is better ensured.

Figure 4-11: Raman Spectra of Active Material Made via Atomic Layer Deposition with Comparison to Literature Spectra (from [2])

4.5 Conclusions

In order to pursue an experimental exploration of the hercynite cycle for thermochemical energy storage, various material formulations were made via solid state synthesis. Small powders in stoichiometric amounts were mixed in a mill and were subsequently calcined to form the

127

mixed metal oxide (cobalt ferrite CoFe2O4) of interest for the hercynite cycle. X-Ray Diffraction and Raman Spectroscopy were performed on these sample powders before and after calcination to ensure the desired materials were formed. Material formulations were made for stoichiometric amounts of the components of the hercynite cycle, and with different amounts of excess aluminum oxide.

A prediction of material composition was performed using the FACTSageTM Gibbs free energy minimization software. The results of these calculations indicate that mixtures of cobalt oxide (CoO) and iron oxide (Fe2O3) at thermodynamic equilibrium should form the same distribution of products as the mixed metal oxide (CoFe2O4) for all temperatures above ambient that were considered. This indicates that the temperatures should be raised enough in the calcination step to overcome kinetic barriers to equilibrium. Additionally, thermodynamic predictions were done for a material made with Fe3O4 instead of Fe2O3. It was found that there was a significant mass and molar difference that arose from the different oxidation state of the original iron oxide, but that the general trends of reduction and oxidation occurred at the same temperatures, since the predicted compositions of different iron oxide formulations were very similar at high temperatures.

Experimental runs were performed in a combination TGA/DSC in order to monitor both the mass changes and heat flows associated with reduction and oxidation of the materials.

Experimental runs were performed at various temperatures between 900°C and 1500°C using inert argon gas during reduction steps and flowing air during oxidation steps. Additionally, experimental runs were done to demonstrate isothermal energy storage, where reduction and oxidation reactions were performed at a single temperature. The resulting oxidation enthalpy changes spanned an order of magnitude, ranging from 10 – 100 kJ/kg. This was shown to be

128

strongly due to the fact that the temperature change between reduction and oxidation affects the equilibrium conditions of the material, meaning that the conversion of the reduction and oxidation reactions will be different for different temperatures. Experimental results indicate that a large temperature difference between reduction and oxidation resulted in a higher enthalpy change of reaction upon oxidation. Furthermore, it was expected that material formulations with excess aluminum oxide would tend to have lower heats of reaction per unit mass due to the additional inert material, and this was found to be the case.

While the experimental results are useful and informative, there are some surprising anomalies in the experimental results. Experimental results show a relatively wide variability in the values obtained, and two samples run under the same conditions produced significantly different results. As such, additional experimentation is needed to isolate issues with experimental results and lend more weight statistical weight to results. Additionally, the heats of reaction obtained for the oxidation exotherms were significantly lower than equilibrium predictions of a material that undergoes an identical temperature profile. These theoretical results were obtained using the FACTSageTM Gibbs free energy minimization software, and indicate from the reaction enthalpy and mass change values that the samples were not reaching thermodynamic equilibrium at the temperatures under consideration. While kinetic and diffusional limitations potentially have an effect, it was also pointed out that physically mixed powders produced via solid state synthesis do not have guaranteed reaction contact. Since the thermochemical reactions cannot occur if the solid reactants are not in physical contact.

However, all of these effects do not seem to be able to fully account for the differences between the experimental and theoretical values for reaction enthalpy, indicating that there is either an

129

error with the observation of the reaction enthalpy or side reactions not predicted by well-mixed thermodynamic equilibrium are occurring and contributing to changes the total reaction enthalpy.

Analysis of the materials was done via X-Ray Diffraction (XRD) prior to, during, and after thermal reaction cycling. XRD scans indicate that the initial material contains expected components, suggesting that the synthesis method was successful in creating appreciable amounts of the mixed metal oxide. High-temperature XRD was performed in-situ on a single reduction and oxidation cycle. Scans suggest that the reduction reaction performs more or less as expected, with expected reduction products being formed at the high temperature. However, the oxidation reaction does not appear to directly re-form the original reactants. Scans done in-situ indicate a shift in the peaks corresponding to the spinel phase of the material, and this is corroborated by XRD scans done after cycling multiple samples in the TGA. The specific materials formed are not able to be clearly identified, and additional analysis done by Raman

Spectroscopy also indicates an unclear mixture of components. Since previous work has shown that the hercynite cycle can repeatedly reform the original reactant products, this suggests that side reactions are occurring.

Despite potential issues in the magnitudes of the values obtained for the reaction enthalpy of oxidation at various temperatures, this experimental exploration does contribute some informative conclusions. Reduction and oxidation reactions using the hercynite cycle were demonstrated to have the potential to store significant amounts of heat per unit mass. Reduction and oxidation cycles done at the same temperature demonstrated that this thermochemical energy storage can also occur completely isothermally, using only the presence or absence of oxygen to control the reactions. While not as high as other reaction cycles done at different temperatures, this isothermal thermochemical energy storage was demonstrated to occur in appreciable

130

amounts. Finally, it was shown that additional aluminum oxide does not improve the thermochemical energy storage potential of the active material. Instead of providing better contact for reactions, the aluminum oxide appears to serve an inert material which tends to lower the thermochemical energy storage potential per unit mass.

4.6 Future Work

This experimental exploration of the hercynite cycle for thermochemical energy storage helps to inform future work in this area. One of the main areas suggested for future work is to examine the thermochemical energy storage potential of the hercynite cycle using active materials that are ensured to have better reactant contact. Samples with the mixed metal ferrite deposited directly on aluminum oxide supports are suggested as a potentially useful way to explore this effect. A study using both physically mixed powders and deposited samples will aid in the exploration of the loss of reactivity that mixed powders can have. In general, a more thorough exploration effort of the hercynite cycle is needed to explore potential sources of experimental error and lending statistical weight to experimentally obtained values. Additionally, heat capacity data of the active materials both while undergoing reaction and while not reacting

(such as in an atmosphere of excess oxygen) would directly illustrate a thermochemical boost to sensible energy storage.

Reduction-oxidation reaction cycles beyond the hercynite cycle are also of interest.

Theoretical predictions have indicated that other metal oxides such as iron and cobalt oxide have potentially very high heats of reaction. Since it was found both theoretically and experimentally that the added mass of the aluminum oxide does contribute to a lower value of reaction enthalpy per unit mass for the hercynite cycle, other solid oxides that do not have this added weight could

131

conceivably have much higher reaction enthalpy values. While there are a number of reasons why various materials have operational limitations (such as loss of active surface area from material sintering), it is beneficial to explore other reaction cycles to explore the potential benefits and drawbacks to different materials. The lessons learned from the current study will be helpful for this further exploration, both in terms of experimental operation and in selecting additional materials for further consideration.

References

1. Downs, R.T., The RRUFF Project: an integrated study of the chemistry, crystallography,

Raman and infrared spectroscopy of minerals, in Programs and Abstracts of the 19th

General Meeting of the International Mineralogical Association in Kobe, Japan O03-13.

2006.

2. Arifin, D., et al., CoFe2O4 on a porous Al2O3 nanostructure for solar thermochemical

CO2 splitting. Energy & Environmental Science, 2012. 5: p. 9438-9443.

3. Scheffe, J.R., J. Li, and A.W. Weimer, A spinel ferrite/hercynite water-splitting redox

cycle. International Journal of Hydrogen Energy, 2010. 35(8): p. 3333-3340.

132

Chapter V

Conclusions

An initial exploration was done on the feasibility of storing both sensible and thermochemical energy at high temperatures. Thermal energy storage is a major benefit to concentrated solar thermal power and provides a number of benefits to renewable energy production from the sun. This thesis focuses primarily on thermal energy storage, though entire systems are often considered and discussed. This is because any change made to a subsystem

(such as thermal energy storage) will impact other subsystems and ultimately the overall system efficiency. As such, the thermal storage method must be considered in the context of system integration. There are many ways to store thermal energy, and some of the different ways of doing so have been studied extensively in the past.

5.1 Thermochemical Augmentation of Sensible Energy Storage Concept

133

during operation by utilizing the heat as reactant products are cooled, but this will necessarily results in a loss of system efficiency.

A thermochemically active material that is stored at the temperatures of interest would utilize both thermochemical and sensible energy storage, avoiding the need to cool and re-heat the active material. Additionally, thermochemical energy storage has been shown to have very high energy storage density. A thermochemically-augmented storage material would therefore have potentially much higher energy densities than a sensible-only system. This could drastically reduce the storage mass and volume necessary to store an amount of thermal energy, thus lessening issues with sensible energy storage at high temperatures and large scales.

Isothermal thermochemical energy storage is an important way to increase the exergetic efficiency of the system. Since the collection of high temperature solar energy is so expensive, it is important to be able to utilize this high-grade heat at the temperatures at which it is collected.

Thermochemical energy storage cycles can have large differences between the temperatures at which the heat is stored and when it is released, thus losing a large amount of valuable exergy due to the temperature difference. This particular exergy loss would be minimized if the two reactions in the thermochemical cycles could be done at (or very near) the same temperature.

While there are many different thermochemical reactions to consider, solid oxides are of particular interest. These solid materials evolve oxygen in the endothermic reduction reaction, and can then endothermally re-oxidize. This is beneficial because these materials tend to have large reaction enthalpies, which makes for more effective thermochemical energy storage. These materials also reduce at high temperatures that are of interest for future concentrated solar power development. Finally, solid oxides make reactant separation a simple matter, since one of the products of the reduction reaction is a reduced solid and the other is a gas that is easily removed.

134

A number of these types of materials have been studied for solar thermochemical water and carbon dioxide splitting, where the reduced solid is oxidized with H2O or CO2 to produce H2 or

CO. As such, there is an extensive literature from which to explore possible reactive materials.

Furthermore, the reverse of the thermal reduction is thermodynamically easier to carry out than oxidation with water or carbon dioxide, which drastically extends the list possible reactions to consider.

Two potential processes were suggested for use with a solid oxide reaction. The first is an augmented solid particle receiver, in which solid particles serve as both the heat transfer media and thermal storage media for a central receiver solar system. The solid particles absorb the concentrated solar radiation in a receiver, where the materials could also be thermally reduced under an inert atmosphere. The solid particles are then stored until it is desirable to release the stored thermal energy to produce electric power. This cooling and oxidation reaction would release both the sensible and thermochemical heat stored in the material, and could transfer this thermal energy to a power cycle through a heat exchanger. The cooled and oxidized particles would then be stored until being sent through the solar receiver again. This is a modification to a solid particle receiver which has been studied in the past for high temperature solar thermal power production, where the sensible heat capacity of the solid particles would be augmented by a thermochemical reaction cycle.

Another potential process that is suggested for use with solid oxide thermochemical augmented energy storage is a dish system, in which an inert gaseous heat transfer fluid such as helium absorbs the solar radiation at the focus of the dish and transfers it to a series of solid blocks of material to store the thermal energy. These blocks could be composed of both the active solid material and a material to enhance heat transfer, such as graphite. The solid oxide

135

material could then be reduced at the same time by removing oxygen from around the particles, thus absorbing additional heat. The same gaseous heat transfer can also transfer heat to a power cycle for electric power production. This removes the need for the solid material to be moved around the system, and can potentially enable large amounts of high-temperature heat to be stored on a dish system.

5.2 Theoretical Exploration of Hercynite Cycle for Thermochemical Augmentation of Thermal

Energy Storage

One reaction cycle of interest is the so-called “hercynite cycle”, in which a mixed-metal ferrite is thermally reduced with aluminum oxide to form hercynite (iron aluminate), cobalt or nickel aluminate, and evolve oxygen. This cycle has been demonstrated to be effective at reacting with water and carbon dioxide and reduces at temperatures above 1000°C. This high temperature is however somewhat lower than some other thermochemical cycles studied, making this cycle more realistic for operation. Additionally, preliminary results suggest that the hercynite cycle should be able to cycle isothermally, with the reduction and oxidation reactions controlled by the presence or absence of oxygen. As such, this reaction cycle was examined to explore the possibility of this type of thermochemically augmented energy storage.

136

otherwise inert atmosphere and high temperatures to drive the reduction reaction to completion.

Stoichiometric amounts of the solid material were used in order to obtain a prediction of the enthalpy changes over different temperature and O2 concentration conditions. These enthalpy changes were corrected for sensible energy changes over the same temperature changes using an estimation of solid heat capacity. The enthalpy of the thermochemical reaction at various conditions was normalized per unit mass of solid material and compared to the sensible energy in the same amount of solid material at various temperatures and over various temperature ranges.

The compositions of the hercynite cycle materials were predicted in an inert atmosphere at various temperatures from ambient up to 1400°C. This was done to explore the thermochemical potential of the material, obtain an idea of what temperatures were necessary for significant reaction, and to predict the material that would form for solid state synthesis using various component metal oxides. It was found that the component oxides matched exactly the mixed-metal oxide composition for all temperatures and thermodynamic equilibrium. It was further found that for an inert atmosphere, the hercynite cycle will being reducing at temperatures above 800°C, with significant reduction beginning to occur at temperatures above

1000°C.

137

The thermochemical enthalpy change for a temperature change from a fully oxidized material at 900°C to various temperatures from 900°C to 1400°C and various oxygen concentrations from 0% to 10% was also found. The predicted reaction enthalpies for these temperature change calculations range from 2.84 kJ/kg at 900°C and 10% O2 to 264.8 kJ/kg at

1400°C and 0% O2. These predicted reaction enthalpies were compared to the sensible energy from ambient (23°C) to the final reduction temperature. The thermochemical reaction enthalpies ranged from 0.32% to 18.5% of the sensible energy at 900°C and 1400°C, respectively. The reaction enthalpy was also compared to a more limited sensible exergy starting at 900°C instead of 23°C. This comparison found that the highest fraction of the thermochemical reaction enthalpy compared to this more limited exergy was 66.1% at 950°C. This was found to be true despite the fact that the reduction reaction had relatively minor conversion when going from

900°C to 950°C (meaning a smaller reaction enthalpy) 950°C had the smallest amount of exergy above 900°C, making the proportion of the thermochemical reaction enthalpy larger.

Both of these sensible-to-thermochemical comparisons show that the hercynite cycle provides only a moderate benefit to sensible energy storage. While the 18.5% thermochemical boost to the full exergy is by no means insignificant, it is unlikely that this will ultimately prove to be beneficial to process operation. Even the 66% boost from the limited exergy is not necessarily enough to be realistically beneficial to a process. A ~10% reduction in process flow rates and storage volumes will certainly make a difference, but probably not enough to justify operating an entire thermochemical cycle. Thermochemical operation will involve additional parasitic losses, such as oxygen separation and pumping, which will cause significant efficiency losses for the rest of the process. A major thermochemical benefit would be needed to make it

138

worth doing from a process operational standpoint. The exact boost that is needed is not known, because it depends so much on the particular system and associated parasitic losses.

Additional metal oxide reduction reactions were explored using similar techniques for comparison purposes. The reduction reactions of iron oxide, cobalt oxide, and cerium oxide were explored at various temperatures with an inert atmosphere, and the associated predicted reaction enthalpies were found. Fe2O3 is predicted to reduce to Fe3O4 from temperatures of 1000°C to

1200°C, with a reaction enthalpy of 471 kJ/kg over this temperature range. Co3O4 is predicted to reduce to CoO at temperatures below 800°C with a reaction enthalpy of 901 kJ/kg. The

FACTSageTM materials database did not contain data for cerium oxide at the temperatures considered for reduction, but the extrapolated values predicted a reduction reaction enthalpy of

4.9 kJ/kg at 1600°C, which is likely due to the small level of reduction this non-stoichiometric material undergoes. Furthermore, a composition study was done for the cobalt oxide reduction reaction between temperatures of 650°C and 800°C; it was found that most of the reduction reaction equilibrium transition occurs between 720°C and 800°C, with half of the transition occurring between 780°C and 800°C.

The theoretical calculations predict that the hercynite cycle reduces at temperatures above

1000°C and O2 concentrations less than 2%. The reaction enthalpies are of moderate magnitude and can contribute significantly to sensible energy under particular conditions. For cycling between 1400°C and 900°C with <0.5% O2, it is predicted that the hercynite cycle can contribute a reaction enthalpy that is 18.5% of the total sensible energy at these high temperatures. It was also found that isothermal reduction-oxidation cycles are possible, and the thermochemical benefit gained isothermally can be up to 9.2% of the sensible energy at 1400°C. These values of

139

thermochemical boost are not insignificant, but are fairly low in terms of providing a reasonable boost to thermochemical energy storage.

Reaction enthalpies of other solid oxide materials that were examined were found to be higher per unit mass than the hercynite cycle. This is due to the fact that the hercynite cycle has four moles of active material (CoFe2O4 + 3 Al2O3), whereas other solid oxides will only have a single mole of material (e.g., Co3O4). This can be a large difference in the mass of the active material for the reduction and oxidation reactions for reactions that can have similar enthalpy changes per “mole” of reaction. Iron and cobalt oxides were explored and appear to be very promising for further exploration of thermochemical energy storage, with predicted reaction enthalpies of 471 kJ/kg and 901 kJ/kg, respectively. Additionally, other mixed non- stoichiometric metal oxides are of interest but were unable to be modeled using the FACTSageTM software. That said, there can be other potential benefits to the hercynite cycle, including an ability to cycle repeatedly with no predicted (or observed) slag phase, and very little reported loss of activity due to material sintering.

It is important to be able to have a small temperature change between the reduction and oxidation reactions, so that there is not a large exergy loss for the stored energy. This can help narrow down a search for potential materials of interest, as it would be very beneficial for the reaction being considered to be able to fully reduce or oxidize over a small temperature range.

This is not an inflexible requirement, but an important consideration for thermochemical energy storage in general, and for thermochemical boosting of sensible thermal energy storage.

Any process for the thermal production of electricity (or any other use of thermal energy) will likely have some temperature change associated with the transfer of sensible thermal energy.

If this temperature change can be closely matched to the reaction temperature change, the benefit

140

to thermochemical energy storage will increase drastically. For the hercynite cycle in particular, it was found that the thermochemical energy gained from a temperature change from 900°C to

1400°C is 18.5% of the sensible energy at the high temperature of 1400°C. This is a significant boost, as this is direct increase in the energy storage capacity of the material without the need for increased storage material mass or a further temperature increase. The thermochemical benefit between the much smaller temperature change of 900°C to 950°C is only 3.8% of the sensible energy at 950°C. However, if the process in question had a lower temperature bound of 900°C, the thermochemical benefit of that same temperature swing of 900°C to 950°C becomes 66.1% of the sensible energy, which is larger benefit. A process with a heat rejection lower bound of

5.3 Experimental Investigation of Hercynite Cycle for Thermochemical Energy Storage Potential

An initial experimental investigation was done to explore the potential for thermochemical energy storage using the hercynite cycle. Various material formulations were made via solid state synthesis, in which small powders in stoichiometric amounts were mixed in a mill and were subsequently calcined to form the mixed metal oxide (cobalt ferrite CoFe2O4) of interest for the hercynite cycle. X-Ray Diffraction and Raman Spectroscopy were performed on these sample powders before and after calcination to ensure the desired materials were formed, and a visual examination of the powder was done via SEM/EDX. Material formulations were made for stoichiometric amounts of the components of the hercynite cycle and with different amounts of excess aluminum oxide. A theoretical prediction was done to predict the resulting

141

material composition of materials made in this manner, and it was found that mixtures of cobalt oxide (CoO) and iron oxide (Fe2O4) at thermodynamic equilibrium should form the same distribution of products as the mixed metal oxide (CoFe2O4) for all temperatures above ambient that were considered. As such, the calcination for the sample powders was done to reach equilibrium.

Experimental runs were performed in a TGA/DSC at various temperatures between

900°C and 1500°C using inert argon gas during reduction steps and flowing air during oxidation steps. Additionally, experimental runs were done to demonstrate isothermal energy storage, where reduction and oxidation reactions were performed at a single temperature. The resulting oxidation enthalpy changes spanned an order of magnitude, ranging from 10 – 100 kJ/kg. This was shown to be strongly due to the fact that the temperature change between reduction and oxidation affects the equilibrium conditions of the material, meaning that the conversion of the reduction and oxidation reactions will be different for different temperatures. This was predicted by thermodynamics, and experimental results generally indicate that a large temperature difference between reduction and oxidation resulted in a higher enthalpy change of reaction upon oxidation. Furthermore, it was expected that material formulations with excess aluminum oxide would tend to have lower heats of reaction per unit mass due to the additional inert material, and this was found to be the case.

Experimental results show a relatively wide variability in the values obtained, and two samples run under the same conditions produced significantly different results. Additional experimentation is needed to isolate issues with experimental results and lend more weight statistical weight to results. Additionally, the heats of reaction obtained for the oxidation exotherms were significantly lower than equilibrium predictions of a material that undergoes an

142

identical temperature profile. These theoretical results were obtained using the FACTSageTM

Gibbs free energy minimization software and indicate from the reaction enthalpy and mass change values that the samples were not reaching thermodynamic equilibrium at the temperatures under consideration. While kinetic and diffusional limitations potentially have an effect, it was also pointed out that physically mixed powders produced via solid state synthesis do not have guaranteed reaction contact. The thermochemical reactions cannot occur if the solid reactants are not in physical contact. However, all of these effects do not seem to be able to fully account for the differences between the experimental and theoretical values for reaction enthalpy, indicating that there is either an error with the observation of the reaction enthalpy or side reactions not predicted by well-mixed thermodynamic equilibrium are occurring and contributing to changes the total reaction enthalpy.

X-Ray Diffraction (XRD) was done before, during, and after thermal reaction cycling.

XRD scans indicate that the starting material contains expected components, suggesting that the synthesis method was successful in creating appreciable amounts of the mixed metal oxide.

High-temperature XRD was performed in-situ on a single reduction and oxidation cycle. Scans suggest that the reduction reaction performs more or less as expected, with expected reduction products being formed at the high temperature. However, the oxidation reaction does not appear to re-form the original reactants. Scans done in-situ indicate a shift in the peaks corresponding to the spinel phase of the material, and this is corroborated by XRD scans done after cycling multiple samples in the TGA. The specific materials formed are not able to be clearly identified, and additional analysis done by Raman Spectroscopy also indicates an unclear mixture of components. Since previous work has shown that the hercynite cycle can repeatedly reform the

143

original reactant products, this suggests that side reactions are occurring due to the material synthesis method in this study.

Reduction and oxidation reactions using the hercynite cycle was demonstrated to have the potential to store significant amounts of heat per unit mass. Reduction and oxidation cycles done at the same temperature demonstrated that this thermochemical energy storage can also occur completely isothermally, using only the presence or absence of oxygen to control the reactions.

While not as high as other reaction cycles done at different temperatures, this isothermal thermochemical energy storage was demonstrated to occur in appreciable amounts. Finally, it was shown that additional aluminum oxide does not improve the thermochemical energy storage potential of the active material; instead of providing better contact for reactions, the aluminum oxide appears to serve an inert material which tends to lower the thermochemical energy storage potential per unit mass.

5.4 Future Work

In-depth system modeling would allow for a thorough analysis of potential trade-offs in the system, and an exploration of ideal operating conditions. As such, information gleaned from the theoretical exploration done in this study could be used to inform system models. This would aid in performing a useful system study, using the reaction enthalpy and equilibrium compositions of the hercynite cycle (and other solid oxide cycles) under various temperature and oxygen concentrations to demonstrate potential benefits and issues with the system. System models could identify beneficial operating conditions and further inform future explorations

(both theoretical and experimental) of other materials and reaction cycles for thermochemical storage.

144

A further examination the thermochemical energy storage potential of the hercynite cycle using active materials that are ensured to have better reactant contact would be very useful.

Samples with the mixed metal ferrite deposited directly on aluminum oxide supports could be used to explore this effect. A study using both physically mixed powders and deposited samples will explore the loss of reactivity that mixed powders with poor reactant contact can have. While the current study does have a number of useful insights, a more thorough exploration effort of the hercynite cycle is needed to explore potential sources of experimental error and lending statistical weight to experimentally obtained values. Additionally, it would be useful to obtain heat capacity data of the active materials both while undergoing reaction and while not reacting in order to directly illustrate the thermochemical boost to sensible energy storage.

The hercynite cycle is potentially useful for thermochemical energy storage, but other reaction cycles could be even more effective. Theoretical predictions have indicated that other metal oxides such as iron and cobalt oxide have very high heats of reaction. Since it was found both theoretically and experimentally that the added mass of the aluminum oxide does contribute to a lower value of reaction enthalpy per unit mass for the hercynite cycle, other solid oxides that do not have this added weight could feasibly have much higher values for the specific heat of reaction. While there are a number of reasons why various materials have operational limitations

(such as loss of active surface area from material sintering), it is beneficial to explore other reaction cycles to explore the potential benefits and drawbacks to different materials. The lessons learned from the current study will be helpful for this further exploration, both in terms of material synthesis for experimental operation and in using compositional studies to select additional materials for further consideration.

145

BIBLIOGRAPHY

Agrafiotis, C., M. Roeb, et al. (2005). "Solar water splitting for hydrogen production with monolithic reactors." Solar Energy 79(4): 409-421.

Andraka, C. E., K. S. Rawlinson, et al. (2012). Technical feasibility of storage on large dish stirling systems, Sandia National Laboratories: Medium: ED; Size: 73 p.

Arifin, D., V. J. Aston, et al. (2012). "CoFe2O4 on a porous Al2O3 nanostructure for solar thermochemical CO2 splitting." Energy & Environmental Science 5: 9438-9443.

Bale, C. W., E. Bélisle, et al. (2009). "FactSage thermochemical software and databases — recent developments." Calphad 33(2): 295-311.

Barlev, D., R. Vidu, et al. (2011). "Innovation in concentrated solar power." Solar Energy Materials and Solar Cells 95(10): 2703-2725.

Bradshaw, R. W. and R. W. Carling (1987). A Review of the Chemical and Physical Properties of Molten Alkali Nitrate Salts and Their Effect on Materials Used For Solar Central Receivers. Livermore, CA, Sandia National Laboratories.

BrightSource Energy. (2012). "Ivanpah." Retrieved June 19, 2012, from http://www.brightsourceenergy.com/ivanpah.

Brown, L. C., G. E. Besenbruch, et al. (2003). High Efficiency Generation of Hydrogen Fuels Using Nuclear Power: Final Technical Report for the Period August 1, 1999 Through September 30, 2002. Other Information: PBD: 1 Jun 2003, General Atomics: Medium: ED; Size: 343 pages.

Buckingham, R. (2011). Sulfur Based Thermochemical Heat Storage for Baseload Concentrating Power. CSP Subprogram Review, Golden, CO.

Buckingham, R., B. Wong, et al. (2010). Metal Oxide Based Thermochemical Energy Storage for Concentrated Solar Power - Thermodynamics and Parasitic Loads for Packed Bed Reactors. SolarPACES.

Chacartegui, R., J. M. Muñoz de Escalona, et al. (2011). "Alternative cycles based on carbon dioxide for central receiver solar power plants." Applied Thermal Engineering 31(5): 872-879.

Chueh, W. C., C. Falter, et al. (2010). "High-flux solar-driven thermochemical dissociation of CO2 and H2O using nonstoichiometric ceria." Science (New York, N.Y.) 330(6012): 1797-1801.

Cleanergy Inc. "Utility Scale Solar Parks." from http://www.cleanergy.com/solar/utility-scale- solar-parks/.

Coker, E. N., J. A. Ohlhausen, et al. (2012). "Oxygen transport and isotopic exchange in iron oxide/YSZ thermochemically-active materials via splitting of C(18O)2 at high temperature

146

studied by thermogravimetric analysis and secondary ion mass spectrometry." Journal of Materials Chemistry 22(14): 6726-6732.

Corgnale, C. and W. a. Summers (2011). "Solar hydrogen production by the Hybrid Sulfur process." International Journal of Hydrogen Energy 36(18): 11604-11619.

Denholm, P. and M. Hand (2011). "Grid flexibility and storage required to achieve very high penetration of variable renewable electricity." Energy Policy 39(3): 1817-1830.

Domalski, E. S. and E. D. Hearing Condensed Phase Heat Capacity Data. NIST Chemistry WebBook, Standard Reference Database Number 69. P. J. Linstrom and W. G. Mallard. Gaithersburg, MD, National Institute of Standards and Technology.

Dostal, V., P. Hejzlar, et al. (2006). "High-performance supercritical carbon dioxide cycle for next-generation nuclear reactors." Nuclear Technology 154(3): 265-282.

Downs, R. T. (2006). The RRUFF Project: an integrated study of the chemistry, crystallography, Raman and infrared spectroscopy of minerals. Programs and Abstracts of the 19th General Meeting of the International Mineralogical Association in Kobe, Japan O03-13.

Dunn, R., K. Lovegrove, et al. (2012). "A Review of Ammonia-Based Thermochemical Energy Storage for Concentrating Solar Power." Proceedings of the IEEE 100(2): 391-400.

Dunn, R. I., P. J. Hearps, et al. (2012). "Molten-Salt Power Towers: Newly Commercial Concentrating Solar Storage." Proceedings of the IEEE 100(2): 504-515.

Edwards, S. E. B. and V. Materić (2012). "Calcium looping in solar power generation plants." Solar Energy 86(9): 2494-2503.

Eriksson, G. and K. Hack (1990). "ChemSage—A computer program for the calculation of complex chemical equilibria." Metallurgical Transactions B 21(6): 1013-1023.

Falchetta, M., G. Liberati, et al. (2010). Commissioning of the Archimede 5 MW molten salt parabolic trough solar plant. SolarPACES. Perpignan, France.

Falcone, P. K., J. E. Noring, et al. (1985). Assessment of a Solid Particle Receiver for a High Temperature Solar Central Receiver System, Sandia National Laboratories.

Flueckiger, S. M., Z. Yang, et al. (2012). "Review of Molten-Salt Thermocline Tank Modeling for Solar Thermal Energy Storage." Heat Transfer Engineering 34(10): 787-800.

Forsberg, C. W., P. F. Peterson, et al. (2007). "High-Temperature Liquid-Fluoride-Salt Closed- Brayton-Cycle Solar Power Towers." Journal of Solar Energy Engineering 129(2): 141-146.

Gil, A., M. Medrano, et al. (2010). "State of the art on high temperature thermal energy storage for power generation. Part 1—Concepts, materials and modellization." Renewable and Sustainable Energy Reviews 14(1): 31-55.

147

Glatzmaier, G. (2011). Summary Report for Concentrating Solar Power Thermal Storage Workshop: New Concept and Materials for Thermal Energy Storage and Heat-Transfer Fluids May 20, 2011. Golden, CO National Renewable Energy Laboratory (NREL)

Hellmann, J. R., M. O. Eatough, et al. (1987). Evaluation of Spherical Ceramic Particles for Solar Thermal Transfer Media. Albuquerque, NM, Sandia National Laboratories: 51.

Hellmann, J. R. and V. S. McConnell (1986). Characterization of Spherical Ceramic Particles for Solar Thermal Transfer Media: A Market Survey. Albuquerque, NM, Sandia National Laboratories.

Herrmann, U. and D. W. Kearney (2002). "Survey of Thermal Energy Storage for Parabolic Trough Power Plants." Journal of Solar Energy Engineering 124(2): 145-152.

Hruby, J. M. (1986). A Technical Feasibility Study of a Solid Particle Solar Central Receiver for High Temperature Applications, Sandia National Laboratories.

Infinia Corporation. "PowerDish(TM)." from http://www.infiniacorp.com/solutions/powerdish/.

Jafari, H., N. Ehsani, et al. (2013). "Nano-SiC/SiC anti-oxidant coating on the surface of graphite." Applied Surface Science 264(0): 128-132.

Jung, I.-H., S. A. Decterov, et al. (2004). "Thermodynamic evaluation and modeling of the Fe– Co–O system." Acta Materialia 52(2): 507-519.

Kalogirou, S. A. (2004). "Solar thermal collectors and applications." Progress in Energy and Combustion Science 30(3): 231-295.

Khalsa, S. S. S., J. M. Christian, et al. (2011). CFD Simulation and Performance Analysis of Alternative Designs for High-Temperature Solid Particle Receivers. ASME 2011 5th International Conference on Energy Sustainability & 9th Fuel Cell Science, Engineering and Technology Conference. Washington, DC.

Kim, Y. M., C. G. Kim, et al. (2012). "Transcritical or supercritical CO2 cycles using both low- and high-temperature heat sources." Energy 43(1): 402-415.

Kolb, G., C. Ho, et al. (2010). "Freeze-Thaw Tests on Trough Receivers Employing a Molten Salt Working Fluid." ASME Conference Proceedings 2010(43956): 693-698.

Kolb, G. J. (2011). "Evaluation of Annual Performance of 2-Tank and Thermocline Thermal Storage Systems for Trough Plants." Journal of Solar Energy Engineering 133(3): 031023- 031025.

Kolb, G. J. (2011). An Evaluation of Possible Next-Generation High-Temperature Molten-Salt Power Towers. Albuquerque, NM, Sandia National Laboratories.

Kolb, G. J., C. K. Ho, et al. (2011). Power Tower Technology Roadmap and Cost Reduction Plan, Sandia National Laboratories.

148

Liu, M., W. Saman, et al. (2012). "Review on storage materials and thermal performance enhancement techniques for high temperature phase change thermal storage systems." Renewable and Sustainable Energy Reviews 16(4): 2118-2132.

Machida, M., K. Kawamura, et al. (2006). "Layered Pr-dodecyl sulfate mesophases as precursors of Pr2O2SO4 having a large oxygen-storage capacity." Journal of Materials Chemistry 16(30): 3084-3090.

Meixner, D. L., D. D. Brengel, et al. (2002). "Electrochemical Oxygen Separation Using Solid Electrolyte Ion Transport Membranes." Journal of the Electrochemical Society 149(9): D132- D132.

Miller, J., M. Allendorf, et al. (2008). "Metal oxide composites and structures for ultra-high temperature solar thermochemical cycles." Journal of Materials Science 43(14): 4714-4728.

Motohashi, T., T. Ueda, et al. (2010). "Remarkable Oxygen Intake/Release Capability of BaYMn2O5+δ: Applications to Oxygen Storage Technologies." Chemistry of Materials 22(10): 3192-3196.

Nachlas, J. A. and D. M. Taylor (1995). Inert gas purifying system. U. S. P. a. T. Office. United States, Ceramatec, Inc. .

Neises, M., S. Tescari, et al. (2012). "Solar-heated rotary kiln for thermochemical energy storage." Solar Energy 86(10): 3040-3048.

Overholser, L. G. and J. P. Blakely (1965). "Oxidation of graphite by low concentrations of water vapor and carbon dioxide in helium." Carbon 2(4): 385-394.

Pacheco (Editor), J. E., R. W. Bradshaw, et al. (2002). Final Test and Evaluation Results from the Solar Two Project, Sandia National Laboratories.

ger, M., . Amsbeck, et al. (2011). " ace-Down Solid Particle Receiver Using Recirculation." Journal of Solar Energy Engineering 133(3): 031009-031001 - 031009-031008.

Scheffe, J. R., J. Li, et al. (2010). "A spinel ferrite/hercynite water-splitting redox cycle." International Journal of Hydrogen Energy 35(8): 3333-3340.

Scheffe, J. R. and A. Steinfeld (2012). "Thermodynamic Analysis of Cerium-Based Oxides for Solar Thermochemical Fuel Production." Energy & Fuels 26(3): 1928-1936.

Siegel, N. P., S. Livers, et al. (2010). "Cerium Oxide Materials for the Solar Thermochemical Decomposition of Carbon Dioxide." ASME Conference Proceedings 2010(43956): 89-95.

Tonopah Solar. (2012). "Crescent Dunes Solar Energy Plant." Retrieved June 21, 2012, from http://www.tonopahsolar.com/project_overview.html.

149

Turchi, C. S., Z. Ma, et al. (2012). Thermodynamic Study of Advaned Supercritical Carbon Dioxide Power cycles for High Performance Concentrating Solar Power Systems. ASME 2012 6th International Conference on Energy Sustainability San Diego, CA, USA.

Weimer, A. W., R. P. Roach, et al. (1991). "Rapid carbothermal reduction of boron oxide in a graphite transport reactor." AIChE Journal 37(5): 759-768.

Wong, B. (2011). Thermochemical Heat Storage for Concentrated Solar Power Based on Multivalent Metal Oxides. CSP Subprogram Review, Golden, CO.

Yuan, H.-W., C.-H. Lu, et al. (2012). "Mechanical and thermal properties of cement composite graphite for solar thermal storage materials." Solar Energy 86(11): 3227-3233.

(September 1, 2010). "Concentrating Solar Power Resource Maps." Retrieved March 25, 2013, from http://www.nrel.gov/csp/maps.html.

(2009). Electric Power Annual 2009. U.S. Energy Information Administration (EIA).

(2010). Technology Roadmap: Concentrating Solar Power. International Energy Agency.

(2011). High Energy Advanced Thermal Storage (HEATS) Funding Opportunity Announcement. Advanced Research Projects Agency (ARPA-E) Department of Energy.

(2011). International Energy Outlook 2011. U.S. Energy Information Administration (EIA).

(2012). Annual Energy Review 2011. u.s. energy Information Administration (EIA). Washington, DC.

(2012). World Energy Outlook 2012. International Energy Agency.

(2013). Electric Power Annual 2011. U.S. Energy Information Administration (EIA). Washington, DC

American Society of Mechanical Engineers (ASME) (2004). Case 2385-1 37Ni-30Co-28Cr- 2.75Si Alloy (UNS N12160) Cases of Boiler and Pressure Vessel Code: Section VIII, Division 1.

American Society of Mechanical Engineers (ASME) (2011). Case 2671 57Ni-22Cr-14W-2Mo- La Alloy (UNS N06230) Subpart 3, Fig. NFN-24 Up to 1,800 F (982 C). Cases of Boiler and Pressure Vessel Code: Section II, Part D.

American Society of Mechanical Engineers (ASME) (2011). Case 2672 37Ni-33Fe-25Cr Alloy (UNS N08120) Welded Construction Above 1650 F to 1800 F. Cases of Boiler and Pressure Vessel Code: Section VIII, Division 1.

150

APPENDIX A: OUTPUT FROM THERMODYNAMIC EQUILIBRIUM PREDICTIONS

A.1: Calcination Composition Comparison Prediction Results

T(C) Component (Phase) Synthesis Case (mol) Comparison Case (mol) 25 Mol-Fe3O4(SPINA#1) 2.291E-20 2.291E-20 25 Mol-Fe3O4[1-](SPINA#1) 7.577E-27 7.577E-27 25 Mol-Fe3O4[1+](SPINA#1) 1.422E-11 1.422E-11 25 Mol-Fe3O4[2-](SPINA#1) 1.221E-35 1.221E-35 25 Mol-Fe1O4[5-](SPINA#1) 1.344E-21 1.344E-21 25 Mol-Fe1O4[6-](SPINA#1) 2.165E-30 2.165E-30 25 Mol-Fe1Al2O4(SPINA#1) 9.701E-19 9.701E-19 25 Mol-Al3O4[1+](SPINA#1) 6.092E-08 6.092E-08 25 Mol-Al1Fe2O4[1-](SPINA#1) 7.665E-25 7.665E-25 25 Mol-Al1O4[5-](SPINA#1) 1.359E-19 1.359E-19 25 Mol-Fe1Al2O4[1+](SPINA#1) 6.021E-10 6.021E-10 25 Mol-Al1Fe2O4[1+](SPINA#1) 1.439E-09 1.439E-09 25 Mol-Co1Al2O4(SPINA#1) 3.789E-03 3.789E-03 25 Mol-Al1Co2O4[1-](SPINA#1) 4.385E-11 4.385E-11 25 Mol-Co3O4[2-](SPINA#1) 2.727E-06 2.727E-06 25 Mol-Co1O4[6-](SPINA#1) 8.454E-15 8.454E-15 25 Mol-Co1Fe2O4(SPINA#1) 8.947E-05 8.947E-05 25 Mol-Fe1Co2O4[1-](SPINA#1) 4.334E-13 4.334E-13 25 Mol-Fe1Co2O4[2-](SPINA#1) 6.983E-22 6.983E-22 25 Mol-Co1Fe2O4[2-](SPINA#1) 4.767E-20 4.767E-20 25 Mol-Co1Co2O4(SPINA#1) 3.320E-01 3.320E-01 25 Mol-Fe1Co2O4(SPINA#1) 8.502E-17 8.502E-17 25 Mol-Co1Co2O4[1+](SPINA#1) 6.111E-60 6.111E-60 25 Mol-Co1Co2O4[1-](SPINA#1) 5.019E-65 5.019E-65 25 Mol-Co1Fe2O4[1-](SPINA#1) 0.000E+00 0.000E+00 25 Mol-Fe1Co2O4[1+](SPINA#1) 5.277E-08 5.277E-08 25 Mol-Co1Fe2O4[1+](SPINA#1) 1.647E-63 1.647E-63 25 Mol-Al1Co2O4[1+](SPINA#1) 5.339E-06 5.339E-06 25 Mol-Co1Al2O4[1+](SPINA#1) 6.973E-62 6.973E-62 25 Mol-Co1O4[5-](SPINA#1) 0.000E+00 0.000E+00 25 Mol-Al2O3(CORU#1) 2.996E+00 2.996E+00 25 Mol-Fe2O3(CORU#1) 3.703E-04 3.703E-04 25 Mol-Al2O3(CORU#2) 4.511E-04 4.511E-04 25 Mol-Fe2O3(CORU#2) 9.995E-01 9.995E-01 500 Mol-Fe3O4(SPINA#1) 6.451E-06 6.451E-06 500 Mol-Fe3O4[1-](SPINA#1) 1.564E-05 1.564E-05

151

T(C) Component (Phase) Synthesis Case (mol) Comparison Case (mol) 500 Mol-Fe3O4[1+](SPINA#1) 3.918E-01 3.918E-01 500 Mol-Fe3O4[2-](SPINA#1) 2.575E-10 2.575E-10 500 Mol-Fe1O4[5-](SPINA#1) 5.799E-06 5.799E-06 500 Mol-Fe1O4[6-](SPINA#1) 9.547E-11 9.547E-11 500 Mol-Fe1Al2O4(SPINA#1) 1.222E-06 1.222E-06 500 Mol-Al3O4[1+](SPINA#1) 4.848E-04 4.848E-04 500 Mol-Al1Fe2O4[1-](SPINA#1) 1.022E-07 1.022E-07 500 Mol-Al1O4[5-](SPINA#1) 3.789E-08 3.789E-08 500 Mol-Fe1Al2O4[1+](SPINA#1) 7.421E-02 7.421E-02 500 Mol-Al1Fe2O4[1+](SPINA#1) 2.560E-03 2.560E-03 500 Mol-Co1Al2O4(SPINA#1) 4.742E-03 4.742E-03 500 Mol-Al1Co2O4[1-](SPINA#1) 2.783E-03 2.783E-03 500 Mol-Co3O4[2-](SPINA#1) 2.722E-02 2.722E-02 500 Mol-Co1O4[6-](SPINA#1) 3.706E-07 3.706E-07 500 Mol-Co1Fe2O4(SPINA#1) 2.504E-02 2.504E-02 500 Mol-Fe1Co2O4[1-](SPINA#1) 4.260E-01 4.260E-01 500 Mol-Fe1Co2O4[2-](SPINA#1) 7.012E-06 7.012E-06 500 Mol-Co1Fe2O4[2-](SPINA#1) 9.997E-07 9.997E-07 500 Mol-Co1Co2O4(SPINA#1) 8.997E-04 8.997E-04 500 Mol-Fe1Co2O4(SPINA#1) 2.318E-07 2.318E-07 500 Mol-Co1Co2O4[1+](SPINA#1) 2.024E-27 2.024E-27 500 Mol-Co1Co2O4[1-](SPINA#1) 6.124E-26 6.124E-26 500 Mol-Co1Fe2O4[1-](SPINA#1) 2.249E-30 2.249E-30 500 Mol-Fe1Co2O4[1+](SPINA#1) 1.408E-02 1.408E-02 500 Mol-Co1Fe2O4[1+](SPINA#1) 5.633E-26 5.633E-26 500 Mol-Al1Co2O4[1+](SPINA#1) 9.198E-05 9.198E-05 500 Mol-Co1Al2O4[1+](SPINA#1) 1.067E-26 1.067E-26 500 Mol-Co1O4[5-](SPINA#1) 8.338E-31 8.338E-31 500 Mol-Al2O3(CORU#1) 2.916E+00 2.916E+00 500 Mol-Fe2O3(CORU#1) 7.519E-02 7.519E-02 500 Mol-Al2O3(CORU#2) 1.293E-03 1.293E-03 500 Mol-Fe2O3(CORU#2) 5.230E-02 5.230E-02 850 Mol-Fe3O4(SPINA#1) 3.299E-03 3.299E-03 850 Mol-Fe3O4[1-](SPINA#1) 2.657E-03 2.657E-03 850 Mol-Fe3O4[1+](SPINA#1) 3.202E-01 3.202E-01 850 Mol-Fe3O4[2-](SPINA#1) 2.737E-05 2.737E-05 850 Mol-Fe1O4[5-](SPINA#1) 8.989E-04 8.989E-04 850 Mol-Fe1O4[6-](SPINA#1) 9.261E-06 9.261E-06 850 Mol-Fe1Al2O4(SPINA#1) 1.291E-03 1.291E-03 850 Mol-Al3O4[1+](SPINA#1) 1.015E-02 1.015E-02

152

T(C) Component (Phase) Synthesis Case (mol) Comparison Case (mol) 850 Mol-Al1Fe2O4[1-](SPINA#1) 2.150E-04 2.150E-04 850 Mol-Al1O4[5-](SPINA#1) 7.276E-05 7.276E-05 850 Mol-Fe1Al2O4[1+](SPINA#1) 1.253E-01 1.253E-01 850 Mol-Al1Fe2O4[1+](SPINA#1) 2.592E-02 2.592E-02 850 Mol-Co1Al2O4(SPINA#1) 3.652E-02 3.652E-02 850 Mol-Al1Co2O4[1-](SPINA#1) 2.277E-02 2.277E-02 850 Mol-Co3O4[2-](SPINA#1) 8.197E-02 8.197E-02 850 Mol-Co1O4[6-](SPINA#1) 2.619E-04 2.619E-04 850 Mol-Co1Fe2O4(SPINA#1) 9.328E-02 9.328E-02 850 Mol-Fe1Co2O4[1-](SPINA#1) 2.814E-01 2.814E-01 850 Mol-Fe1Co2O4[2-](SPINA#1) 2.899E-03 2.899E-03 850 Mol-Co1Fe2O4[2-](SPINA#1) 7.740E-04 7.740E-04 850 Mol-Co1Co2O4(SPINA#1) 8.755E-04 8.755E-04 850 Mol-Fe1Co2O4(SPINA#1) 3.096E-05 3.096E-05 850 Mol-Co1Co2O4[1+](SPINA#1) 2.467E-22 2.467E-22 850 Mol-Co1Co2O4[1-](SPINA#1) 2.309E-20 2.309E-20 850 Mol-Co1Fe2O4[1-](SPINA#1) 2.181E-22 2.181E-22 850 Mol-Fe1Co2O4[1+](SPINA#1) 3.005E-03 3.005E-03 850 Mol-Co1Fe2O4[1+](SPINA#1) 2.628E-20 2.628E-20 850 Mol-Al1Co2O4[1+](SPINA#1) 2.432E-04 2.432E-04 850 Mol-Co1Al2O4[1+](SPINA#1) 1.029E-20 1.029E-20 850 Mol-Co1O4[5-](SPINA#1) 7.378E-23 7.378E-23 850 Mol-Al2O3(CORU#1) 2.797E+00 2.797E+00 850 Mol-Fe2O3(CORU#1) 1.831E-01 1.831E-01 850 Mol-Al2O3(CORU#2) 0.000E+00 0.000E+00 850 Mol-Fe2O3(CORU#2) 0.000E+00 0.000E+00 1000 Mol-Fe3O4(SPINA#1) 1.345E-02 1.345E-02 1000 Mol-Fe3O4[1-](SPINA#1) 8.119E-03 8.119E-03 1000 Mol-Fe3O4[1+](SPINA#1) 2.613E-01 2.613E-01 1000 Mol-Fe3O4[2-](SPINA#1) 4.179E-04 4.179E-04 1000 Mol-Fe1O4[5-](SPINA#1) 2.503E-03 2.503E-03 1000 Mol-Fe1O4[6-](SPINA#1) 1.288E-04 1.288E-04 1000 Mol-Fe1Al2O4(SPINA#1) 8.205E-03 8.205E-03 1000 Mol-Al3O4[1+](SPINA#1) 2.908E-02 2.908E-02 1000 Mol-Al1Fe2O4[1-](SPINA#1) 1.481E-03 1.481E-03 1000 Mol-Al1O4[5-](SPINA#1) 4.567E-04 4.567E-04 1000 Mol-Fe1Al2O4[1+](SPINA#1) 1.594E-01 1.594E-01 1000 Mol-Al1Fe2O4[1+](SPINA#1) 4.768E-02 4.768E-02 1000 Mol-Co1Al2O4(SPINA#1) 6.784E-02 6.784E-02 1000 Mol-Al1Co2O4[1-](SPINA#1) 3.936E-02 3.936E-02

153

T(C) Component (Phase) Synthesis Case (mol) Comparison Case (mol) 1000 Mol-Co3O4[2-](SPINA#1) 9.181E-02 9.181E-02 1000 Mol-Co1O4[6-](SPINA#1) 1.065E-03 1.065E-03 1000 Mol-Co1Fe2O4(SPINA#1) 1.112E-01 1.112E-01 1000 Mol-Fe1Co2O4[1-](SPINA#1) 2.157E-01 2.157E-01 1000 Mol-Fe1Co2O4[2-](SPINA#1) 1.110E-02 1.110E-02 1000 Mol-Co1Fe2O4[2-](SPINA#1) 3.456E-03 3.456E-03 1000 Mol-Co1Co2O4(SPINA#1) 9.815E-04 9.815E-04 1000 Mol-Fe1Co2O4(SPINA#1) 1.187E-04 1.187E-04 1000 Mol-Co1Co2O4[1+](SPINA#1) 4.383E-22 4.383E-22 1000 Mol-Co1Co2O4[1-](SPINA#1) 4.100E-20 4.100E-20 1000 Mol-Co1Fe2O4[1-](SPINA#1) 1.543E-21 1.543E-21 1000 Mol-Fe1Co2O4[1+](SPINA#1) 2.306E-03 2.306E-03 1000 Mol-Co1Fe2O4[1+](SPINA#1) 4.967E-20 4.967E-20 1000 Mol-Al1Co2O4[1+](SPINA#1) 4.207E-04 4.207E-04 1000 Mol-Co1Al2O4[1+](SPINA#1) 3.030E-20 3.030E-20 1000 Mol-Co1O4[5-](SPINA#1) 4.758E-22 4.758E-22 1000 Mol-Al2O3(CORU#1) 2.676E+00 2.676E+00 1000 Mol-Fe2O3(CORU#1) 2.115E-01 2.115E-01 1000 Mol-Al2O3(CORU#2) 0.000E+00 0.000E+00 1000 Mol-Fe2O3(CORU#2) 0.000E+00 0.000E+00

154

A.2: Composition Predictions for Hercynite Cycle Material with Inert Atmosphere

Temperature Spinel Spinel2 Cor Mass Cor2 Total Solids (deg C) Mass (g) Mass (g) (g) Mass (g) Mass (g) 25 0.057 0.000 305.902 0.000 540.505 50 206.038 0.000 255.459 79.007 540.505 75 206.239 0.000 255.869 78.396 540.505 100 206.534 0.000 256.478 77.492 540.505 125 206.959 0.000 257.353 76.193 540.505 150 207.565 0.000 258.594 74.346 540.505 175 208.453 0.000 260.391 71.660 540.505 200 231.058 0.000 300.936 8.511 540.505 225 231.134 0.000 301.499 7.872 540.505 250 231.174 0.000 302.078 7.253 540.505 275 231.186 0.000 302.688 6.631 540.505 300 231.173 0.000 303.338 5.994 540.505 325 231.135 0.000 304.032 5.338 540.505 350 231.072 0.000 304.767 4.666 540.505 375 230.979 0.000 305.540 3.985 540.505 400 230.854 0.000 306.340 3.311 540.505 425 230.687 0.000 307.151 2.667 540.505 450 230.468 0.000 307.946 2.090 540.505 475 230.180 0.000 308.681 1.643 540.504 500 229.999 0.000 309.649 0.856 540.503 525 229.868 0.000 310.633 0.000 540.501 550 229.598 0.000 310.898 0.000 540.496 575 229.375 0.000 311.115 0.000 540.490 600 229.211 0.000 311.268 0.000 540.479 625 229.124 0.000 311.342 0.000 540.465 650 229.134 0.000 311.313 0.000 540.446 675 229.264 0.000 311.158 0.000 540.422 700 229.541 0.000 310.848 0.000 540.389 725 229.995 0.000 310.353 0.000 540.349 750 230.658 0.000 309.640 0.000 540.298 775 231.563 0.000 308.672 0.000 540.235 800 232.749 0.000 307.409 0.000 540.159 825 234.258 0.000 305.809 0.000 540.066 850 236.135 0.000 303.822 0.000 539.956 875 238.431 0.000 301.395 0.000 539.826 900 241.202 0.000 298.470 0.000 539.672 925 244.509 0.000 294.983 0.000 539.492

155

Temperature Spinel Spinel2 Cor Mass Cor2 Total Solids (deg C) Mass (g) Mass (g) (g) Mass (g) Mass (g) 950 248.411 0.000 290.872 0.000 539.283 975 252.966 0.000 286.078 0.000 539.044 1000 258.218 0.000 280.554 0.000 538.772 1025 264.192 0.000 274.275 0.000 538.467 1050 270.890 0.000 267.239 0.000 538.130 1075 278.291 0.000 259.472 0.000 537.762 1100 286.351 0.000 251.016 0.000 537.367 1125 295.015 0.000 241.933 0.000 536.948 1150 304.218 0.000 232.291 0.000 536.508 1175 313.889 0.000 222.163 0.000 536.052 1200 323.960 0.000 211.624 0.000 535.584 1225 334.363 0.000 200.745 0.000 535.108 1250 345.040 0.000 189.588 0.000 534.628 1275 355.945 0.000 178.205 0.000 534.149 1300 367.046 0.000 166.629 0.000 533.675 1325 378.335 0.000 154.876 0.000 533.211 1350 389.828 0.000 142.930 0.000 532.758 1375 401.574 0.000 130.747 0.000 532.321 1400 413.662 0.000 118.240 0.000 531.902

156

A.3: Isothermal Reduction-Oxidation Equilibrium Prediction Results for Hercynite Cycle

Delta H (J) Temp (deg C) Step 1000 1100 1200 1300 1400 1st Red -23842.16 -985.58 26760.71 56877.85 87423.62 1st Ox -21833.40 -36129.76 -51079.41 -62507.93 -65456.92 2nd Red 22601.81 37854.70 54002.38 66734.53 70981.52 2nd Ox -22598.57 -37838.21 -53928.88 -66459.36 -70098.70 3rd Red 22601.81 37854.70 54002.38 66734.53 70981.51 3rd Ox -22598.57 -37838.21 -53928.88 -66459.36 -70098.69 4th Red 22601.81 37854.70 54002.38 66734.53 70981.50 4th Ox -22598.57 -37838.21 -53928.88 -66459.36 -70098.68

Solid Mass (g) Step Temp (deg C) 1st Red 1st Ox 2nd 2nd Ox 3rd Red 3rd Ox 4th Red 4th Ox Red 1000 538.772 540.248 538.722 540.248 538.722 540.248 538.722 540.248 1100 537.367 539.886 537.249 539.886 537.249 539.886 537.249 539.886 1200 535.584 539.315 535.377 539.315 535.377 539.315 535.377 539.315 1300 533.675 538.525 533.378 538.525 533.378 538.525 533.378 538.525 1400 531.902 537.597 531.534 537.597 531.534 537.597 531.534 537.597

157

A.4: Estimation of Thermochemical Heat of Reaction for Stoichiometric Hercynite Cycle at Various Temperatures and O2 Concentrations

Thermo- Thermo- chemical Solid Specific Exergy Thermo- chemical Boost to Solid Fraction Sensible O2 Total ΔH Solid ΔH Solid Above chemical Boost to Sensible T(C) Gas ΔH(J) Mass of Initial Energy (%) (J) (J) ΔH 900°C ΔH Sensible Exergy (g) Material (kJ/kg) (kJ/kg) (kJ/kg) (kJ/kg) Energy Above (%) (%) 900°C (%) 0 900 11528.86 0.00 11528.86 539.66 99.84 21.33 876.02 0.00 21.33 2.43 N/A 0.5 900 4971.48 0.02 4971.45 540.08 99.92 9.20 876.02 0.00 9.20 1.05 N/A 1 900 3988.45 0.04 3988.41 540.15 99.93 7.38 876.02 0.00 7.38 0.84 N/A 1.5 900 3469.63 0.04 3469.59 540.18 99.94 6.42 876.02 0.00 6.42 0.73 N/A 2 900 3125.79 0.05 3125.73 540.21 99.95 5.78 876.02 0.00 5.78 0.66 N/A 2.5 900 2872.40 0.06 2872.34 540.22 99.95 5.31 876.02 0.00 5.31 0.61 N/A 3 900 2673.70 0.06 2673.63 540.24 99.95 4.95 876.02 0.00 4.95 0.56 N/A 3.5 900 2511.36 0.07 2511.29 540.25 99.95 4.65 876.02 0.00 4.65 0.53 N/A 4 900 2374.83 0.07 2374.75 540.26 99.96 4.39 876.02 0.00 4.39 0.50 N/A 4.5 900 2257.46 0.08 2257.38 540.27 99.96 4.18 876.02 0.00 4.18 0.48 N/A 5 900 2154.85 0.08 2154.77 540.28 99.96 3.99 876.02 0.00 3.99 0.46 N/A 5.5 900 2063.94 0.09 2063.85 540.28 99.96 3.82 876.02 0.00 3.82 0.44 N/A 6 900 1982.48 0.09 1982.39 540.29 99.96 3.67 876.02 0.00 3.67 0.42 N/A 6.5 900 1908.83 0.10 1908.74 540.29 99.96 3.53 876.02 0.00 3.53 0.40 N/A 7 900 1841.72 0.10 1841.62 540.30 99.96 3.41 876.02 0.00 3.41 0.39 N/A 7.5 900 1780.16 0.10 1780.06 540.30 99.96 3.29 876.02 0.00 3.29 0.38 N/A 8 900 1723.37 0.11 1723.26 540.31 99.96 3.19 876.02 0.00 3.19 0.36 N/A 8.5 900 1670.70 0.11 1670.59 540.31 99.97 3.09 876.02 0.00 3.09 0.35 N/A 9 900 1621.65 0.11 1621.53 540.32 99.97 3.00 876.02 0.00 3.00 0.34 N/A 9.5 900 1575.78 0.12 1575.66 540.32 99.97 2.92 876.02 0.00 2.92 0.33 N/A 10 900 1532.73 0.12 1532.61 540.32 99.97 2.84 876.02 0.00 2.84 0.32 N/A 0 950 152489.88 103940.51 48549.37 539.27 99.77 89.82 930.11 54.08 35.74 3.84 66.08 0.5 950 144292.15 104312.87 39979.28 539.82 99.87 73.97 930.11 54.08 19.88 2.14 36.76 1 950 143163.50 104685.19 38478.31 539.92 99.89 71.19 930.11 54.08 17.11 1.84 31.63 1.5 950 142739.43 105057.50 37681.93 539.98 99.90 69.72 930.11 54.08 15.63 1.68 28.90 2 950 142583.16 105429.80 37153.36 540.01 99.91 68.74 930.11 54.08 14.65 1.58 27.10 2.5 950 142565.77 105802.11 36763.66 540.04 99.91 68.02 930.11 54.08 13.93 1.50 25.76 3 950 142632.45 106174.41 36458.04 540.06 99.92 67.45 930.11 54.08 13.37 1.44 24.72 3.5 950 142755.08 106546.71 36208.37 540.08 99.92 66.99 930.11 54.08 12.91 1.39 23.86 4 950 142917.43 106919.01 35998.42 540.09 99.92 66.60 930.11 54.08 12.52 1.35 23.15 4.5 950 143109.30 107291.31 35817.99 540.10 99.93 66.27 930.11 54.08 12.18 1.31 22.53 5 950 143323.90 107663.61 35660.29 540.11 99.93 65.98 930.11 54.08 11.89 1.28 21.99 5.5 950 143556.50 108035.90 35520.60 540.12 99.93 65.72 930.11 54.08 11.63 1.25 21.51

158

Thermo- Thermo- chemical Solid Specific Exergy Thermo- chemical Boost to Solid Fraction Sensible O2 Total ΔH Solid ΔH Solid Above chemical Boost to Sensible T(C) Gas ΔH(J) Mass of Initial Energy (%) (J) (J) ΔH 900°C ΔH Sensible Exergy (g) Material (kJ/kg) (kJ/kg) (kJ/kg) (kJ/kg) Energy Above (%) (%) 900°C (%) 6 950 143803.67 108408.20 35395.47 540.13 99.93 65.49 930.11 54.08 11.40 1.23 21.08 6.5 950 144062.87 108780.50 35282.37 540.14 99.93 65.28 930.11 54.08 11.19 1.20 20.70 7 950 144332.14 109152.80 35179.34 540.15 99.93 65.09 930.11 54.08 11.00 1.18 20.34 7.5 950 144609.96 109525.09 35084.87 540.16 99.94 64.91 930.11 54.08 10.83 1.16 20.02 8 950 144895.11 109897.39 34997.72 540.16 99.94 64.75 930.11 54.08 10.67 1.15 19.72 8.5 950 145186.62 110269.68 34916.94 540.17 99.94 64.60 930.11 54.08 10.52 1.13 19.45 9 950 145483.70 110641.98 34841.72 540.17 99.94 64.46 930.11 54.08 10.38 1.12 19.19 9.5 950 145785.68 111014.27 34771.41 540.18 99.94 64.33 930.11 54.08 10.25 1.10 18.95 10 950 146092.01 111386.57 34705.44 540.18 99.94 64.21 930.11 54.08 10.13 1.09 18.72 0 1000 295322.38 207880.14 87442.24 538.76 99.68 161.78 984.48 108.46 53.32 5.42 49.17 0.5 1000 285305.24 208628.87 76676.37 539.46 99.81 141.86 984.48 108.46 33.41 3.39 30.80 1 1000 283856.82 209377.49 74479.33 539.61 99.83 137.80 984.48 108.46 29.34 2.98 27.05 1.5 1000 283430.28 210126.09 73304.19 539.69 99.85 135.62 984.48 108.46 27.17 2.76 25.05 2 1000 283396.91 210874.69 72522.22 539.74 99.86 134.18 984.48 108.46 25.72 2.61 23.71 2.5 1000 283568.30 211623.27 71945.03 539.78 99.87 133.11 984.48 108.46 24.65 2.50 22.73 3 1000 283863.94 212371.85 71492.09 539.81 99.87 132.27 984.48 108.46 23.81 2.42 21.96 3.5 1000 284242.38 213120.43 71121.95 539.83 99.88 131.59 984.48 108.46 23.13 2.35 21.33 4 1000 284679.63 213869.01 70810.62 539.85 99.88 131.01 984.48 108.46 22.55 2.29 20.79 4.5 1000 285160.61 214617.58 70543.03 539.87 99.88 130.51 984.48 108.46 22.06 2.24 20.34 5 1000 285675.30 215366.15 70309.15 539.89 99.89 130.08 984.48 108.46 21.63 2.20 19.94 5.5 1000 286216.68 216114.72 70101.96 539.90 99.89 129.70 984.48 108.46 21.24 2.16 19.59 6 1000 286779.68 216863.29 69916.39 539.92 99.89 129.36 984.48 108.46 20.90 2.12 19.27 6.5 1000 287360.51 217611.86 69748.65 539.93 99.89 129.04 984.48 108.46 20.59 2.09 18.98 7 1000 287956.27 218360.42 69595.85 539.94 99.90 128.76 984.48 108.46 20.31 2.06 18.72 7.5 1000 288564.72 219108.99 69455.73 539.95 99.90 128.50 984.48 108.46 20.05 2.04 18.48 8 1000 289184.05 219857.55 69326.50 539.96 99.90 128.26 984.48 108.46 19.81 2.01 18.26 8.5 1000 289812.83 220606.12 69206.71 539.97 99.90 128.04 984.48 108.46 19.59 1.99 18.06 9 1000 290449.85 221354.68 69095.17 539.97 99.90 127.84 984.48 108.46 19.38 1.97 17.87 9.5 1000 291094.15 222103.24 68990.91 539.98 99.90 127.64 984.48 108.46 19.19 1.95 17.69 10 1000 291744.91 222851.80 68893.11 539.99 99.91 127.46 984.48 108.46 19.01 1.93 17.52 0 1050 439997.04 311819.00 128178.04 538.12 99.56 237.15 1039.13 163.11 74.04 7.13 45.39 0.5 1050 428247.70 312947.99 115299.71 538.98 99.72 213.32 1039.13 163.11 50.21 4.83 30.78 1 1050 426308.80 314076.70 112232.10 539.18 99.76 207.64 1039.13 163.11 44.54 4.29 27.31 1.5 1050 425775.43 315205.36 110570.07 539.30 99.78 204.57 1039.13 163.11 41.46 3.99 25.42 2 1050 425793.10 316334.01 109459.09 539.37 99.79 202.51 1039.13 163.11 39.41 3.79 24.16 2.5 1050 426099.86 317462.63 108637.23 539.43 99.80 200.99 1039.13 163.11 37.89 3.65 23.23

159

Thermo- Thermo- chemical Solid Specific Exergy Thermo- chemical Boost to Solid Fraction Sensible O2 Total ΔH Solid ΔH Solid Above chemical Boost to Sensible T(C) Gas ΔH(J) Mass of Initial Energy (%) (J) (J) ΔH 900°C ΔH Sensible Exergy (g) Material (kJ/kg) (kJ/kg) (kJ/kg) (kJ/kg) Energy Above (%) (%) 900°C (%) 3 1050 426582.72 318591.25 107991.47 539.47 99.81 199.80 1039.13 163.11 36.69 3.53 22.50 3.5 1050 427183.19 319719.86 107463.33 539.51 99.82 198.82 1039.13 163.11 35.71 3.44 21.90 4 1050 427867.33 320848.46 107018.87 539.54 99.82 198.00 1039.13 163.11 34.89 3.36 21.39 4.5 1050 428613.76 321977.05 106636.71 539.56 99.83 197.29 1039.13 163.11 34.18 3.29 20.96 5 1050 429408.23 323105.65 106302.58 539.59 99.83 196.67 1039.13 163.11 33.57 3.23 20.58 5.5 1050 430240.76 324234.24 106006.52 539.61 99.83 196.13 1039.13 163.11 33.02 3.18 20.24 6 1050 431104.13 325362.82 105741.31 539.63 99.84 195.64 1039.13 163.11 32.53 3.13 19.94 6.5 1050 431992.96 326491.40 105501.56 539.64 99.84 195.19 1039.13 163.11 32.08 3.09 19.67 7 1050 432903.11 327619.98 105283.13 539.66 99.84 194.79 1039.13 163.11 31.68 3.05 19.42 7.5 1050 433831.38 328748.56 105082.82 539.67 99.85 194.42 1039.13 163.11 31.31 3.01 19.20 8 1050 434775.19 329877.13 104898.06 539.68 99.85 194.08 1039.13 163.11 30.97 2.98 18.99 8.5 1050 435732.49 331005.71 104726.78 539.70 99.85 193.76 1039.13 163.11 30.65 2.95 18.79 9 1050 436701.58 332134.28 104567.30 539.71 99.85 193.46 1039.13 163.11 30.36 2.92 18.61 9.5 1050 437681.07 333262.85 104418.22 539.72 99.86 193.19 1039.13 163.11 30.08 2.89 18.44 10 1050 438669.78 334391.42 104278.36 539.73 99.86 192.93 1039.13 163.11 29.82 2.87 18.28 0 1100 586297.10 415757.17 170539.93 537.36 99.42 315.52 1094.05 218.03 97.49 8.91 44.72 0.5 1100 573209.57 417270.30 155939.27 538.36 99.60 288.51 1094.05 218.03 70.48 6.44 32.33 1 1100 570670.73 418782.78 151887.95 538.64 99.66 281.01 1094.05 218.03 62.98 5.76 28.89 1.5 1100 569943.76 420295.16 149648.60 538.79 99.68 276.87 1094.05 218.03 58.84 5.38 26.99 2 1100 569947.35 421807.49 148139.86 538.89 99.70 274.08 1094.05 218.03 56.05 5.12 25.71 2.5 1100 570338.83 423319.78 147019.05 538.97 99.72 272.01 1094.05 218.03 53.98 4.93 24.76 3 1100 570968.18 424832.04 146136.14 539.03 99.73 270.37 1094.05 218.03 52.34 4.78 24.01 3.5 1100 571757.08 426344.29 145412.79 539.08 99.74 269.03 1094.05 218.03 51.00 4.66 23.39 4 1100 572659.81 427856.52 144803.29 539.12 99.75 267.91 1094.05 218.03 49.88 4.56 22.88 4.5 1100 573647.47 429368.74 144278.73 539.16 99.75 266.94 1094.05 218.03 48.91 4.47 22.43 5 1100 574700.73 430880.95 143819.78 539.19 99.76 266.09 1094.05 218.03 48.06 4.39 22.04 5.5 1100 575806.05 432393.15 143412.90 539.22 99.76 265.33 1094.05 218.03 47.30 4.32 21.70 6 1100 576953.58 433905.34 143048.24 539.25 99.77 264.66 1094.05 218.03 46.63 4.26 21.39 6.5 1100 578135.97 435417.53 142718.44 539.27 99.77 264.05 1094.05 218.03 46.02 4.21 21.11 7 1100 579347.61 436929.71 142417.90 539.29 99.78 263.49 1094.05 218.03 45.46 4.16 20.85 7.5 1100 580584.09 438441.88 142142.21 539.31 99.78 262.98 1094.05 218.03 44.95 4.11 20.62 8 1100 581841.91 439954.05 141887.86 539.33 99.78 262.51 1094.05 218.03 44.48 4.07 20.40 8.5 1100 583118.25 441466.22 141652.03 539.34 99.79 262.08 1094.05 218.03 44.05 4.03 20.20 9 1100 584410.78 442978.38 141432.40 539.36 99.79 261.67 1094.05 218.03 43.64 3.99 20.02 9.5 1100 585717.60 444490.53 141227.07 539.37 99.79 261.29 1094.05 218.03 43.26 3.95 19.84 10 1100 587037.09 446002.69 141034.40 539.39 99.79 260.93 1094.05 218.03 42.90 3.92 19.68

160

Thermo- Thermo- chemical Solid Specific Exergy Thermo- chemical Boost to Solid Fraction Sensible O2 Total ΔH Solid ΔH Solid Above chemical Boost to Sensible T(C) Gas ΔH(J) Mass of Initial Energy (%) (J) (J) ΔH 900°C ΔH Sensible Exergy (g) Material (kJ/kg) (kJ/kg) (kJ/kg) (kJ/kg) Energy Above (%) (%) 900°C (%) 0 1150 733927.45 519694.73 214232.72 536.50 99.26 396.36 1149.24 273.22 123.14 10.72 45.07 0.5 1150 720081.70 521596.09 198485.61 537.61 99.47 367.23 1149.24 273.22 94.01 8.18 34.41 1 1150 716948.48 523496.04 193452.44 537.96 99.53 357.91 1149.24 273.22 84.70 7.37 31.00 1.5 1150 715990.34 525395.75 190594.59 538.16 99.57 352.63 1149.24 273.22 79.41 6.91 29.06 2 1150 715941.55 527295.35 188646.20 538.30 99.59 349.02 1149.24 273.22 75.80 6.60 27.75 2.5 1150 716383.86 529194.87 187188.99 538.40 99.61 346.33 1149.24 273.22 73.11 6.36 26.76 3 1150 717130.36 531094.34 186036.02 538.48 99.63 344.19 1149.24 273.22 70.98 6.18 25.98 3.5 1150 718082.29 532993.76 185088.53 538.55 99.64 342.44 1149.24 273.22 69.22 6.02 25.34 4 1150 719181.46 534893.15 184288.31 538.61 99.65 340.96 1149.24 273.22 67.74 5.89 24.79 4.5 1150 720390.92 536792.52 183598.40 538.66 99.66 339.68 1149.24 273.22 66.47 5.78 24.33 5 1150 721685.79 538691.86 182993.93 538.70 99.67 338.56 1149.24 273.22 65.35 5.69 23.92 5.5 1150 723048.59 540591.18 182457.41 538.74 99.67 337.57 1149.24 273.22 64.35 5.60 23.55 6 1150 724466.60 542490.49 181976.11 538.77 99.68 336.68 1149.24 273.22 63.46 5.52 23.23 6.5 1150 725930.27 544389.78 181540.49 538.80 99.69 335.88 1149.24 273.22 62.66 5.45 22.93 7 1150 727432.29 546289.06 181143.23 538.83 99.69 335.14 1149.24 273.22 61.92 5.39 22.66 7.5 1150 728966.93 548188.33 180778.60 538.86 99.70 334.47 1149.24 273.22 61.25 5.33 22.42 8 1150 730529.61 550087.59 180442.02 538.88 99.70 333.84 1149.24 273.22 60.63 5.28 22.19 8.5 1150 732116.64 551986.84 180129.80 538.90 99.70 333.27 1149.24 273.22 60.05 5.23 21.98 9 1150 733725.00 553886.07 179838.93 538.92 99.71 332.73 1149.24 273.22 59.51 5.18 21.78 9.5 1150 735352.17 555785.31 179566.86 538.94 99.71 332.22 1149.24 273.22 59.01 5.13 21.60 10 1150 736996.05 557684.53 179311.52 538.96 99.72 331.75 1149.24 273.22 58.53 5.09 21.42 0 1200 882569.83 623631.74 258938.09 535.58 99.09 479.07 1204.68 328.66 150.41 12.49 45.76 0.5 1200 868585.73 625925.92 242659.81 536.75 99.31 448.95 1204.68 328.66 120.29 9.99 36.60 1 1200 864987.69 628217.17 236770.52 537.17 99.38 438.06 1204.68 328.66 109.40 9.08 33.29 1.5 1200 863829.09 630507.94 233321.15 537.42 99.43 431.68 1204.68 328.66 103.01 8.55 31.34 2 1200 863732.37 632798.46 230933.91 537.59 99.46 427.26 1204.68 328.66 98.60 8.18 30.00 2.5 1200 864221.03 635088.82 229132.21 537.72 99.49 423.93 1204.68 328.66 95.26 7.91 28.99 3 1200 865076.95 637379.06 227697.89 537.83 99.51 421.27 1204.68 328.66 92.61 7.69 28.18 3.5 1200 866183.08 639669.22 226513.86 537.91 99.52 419.08 1204.68 328.66 90.42 7.51 27.51 4 1200 867469.77 641959.32 225510.45 537.98 99.53 417.23 1204.68 328.66 88.56 7.35 26.95 4.5 1200 868892.36 644249.36 224643.00 538.05 99.55 415.62 1204.68 328.66 86.96 7.22 26.46 5 1200 870420.66 646539.35 223881.31 538.10 99.56 414.21 1204.68 328.66 85.55 7.10 26.03 5.5 1200 872033.29 648829.30 223203.99 538.15 99.57 412.96 1204.68 328.66 84.30 7.00 25.65 6 1200 873714.66 651119.22 222595.44 538.20 99.57 411.83 1204.68 328.66 83.17 6.90 25.31 6.5 1200 875453.02 653409.10 222043.92 538.24 99.58 410.81 1204.68 328.66 82.15 6.82 25.00 7 1200 877239.34 655698.96 221540.38 538.27 99.59 409.88 1204.68 328.66 81.22 6.74 24.71

161

Thermo- Thermo- chemical Solid Specific Exergy Thermo- chemical Boost to Solid Fraction Sensible O2 Total ΔH Solid ΔH Solid Above chemical Boost to Sensible T(C) Gas ΔH(J) Mass of Initial Energy (%) (J) (J) ΔH 900°C ΔH Sensible Exergy (g) Material (kJ/kg) (kJ/kg) (kJ/kg) (kJ/kg) Energy Above (%) (%) 900°C (%) 7.5 1200 879066.52 657988.80 221077.72 538.31 99.59 409.02 1204.68 328.66 80.36 6.67 24.45 8 1200 880928.90 660278.62 220650.28 538.34 99.60 408.23 1204.68 328.66 79.57 6.61 24.21 8.5 1200 882821.88 662568.41 220253.47 538.37 99.61 407.50 1204.68 328.66 78.84 6.54 23.99 9 1200 884741.70 664858.19 219883.51 538.39 99.61 406.82 1204.68 328.66 78.15 6.49 23.78 9.5 1200 886685.21 667147.95 219537.26 538.42 99.61 406.17 1204.68 328.66 77.51 6.43 23.58 10 1200 888649.80 669437.70 219212.10 538.44 99.62 405.57 1204.68 328.66 76.91 6.38 23.40 0 1250 1031891.70 727568.28 304323.42 534.62 98.91 563.04 1260.38 384.36 178.68 14.18 46.49 0.5 1250 1018348.90 730260.79 288088.11 535.82 99.13 533.00 1260.38 384.36 148.64 11.79 38.67 1 1250 1014512.70 732947.53 281565.17 536.30 99.22 520.93 1260.38 384.36 136.57 10.84 35.53 1.5 1250 1013253.10 735633.31 277619.79 536.59 99.28 513.64 1260.38 384.36 129.27 10.26 33.63 2 1250 1013161.70 738318.60 274843.10 536.79 99.31 508.50 1260.38 384.36 124.14 9.85 32.30 2.5 1250 1013728.70 741003.57 272725.13 536.95 99.34 504.58 1260.38 384.36 120.22 9.54 31.28 3 1250 1014714.70 743688.32 271026.38 537.07 99.37 501.44 1260.38 384.36 117.07 9.29 30.46 3.5 1250 1015989.10 746372.90 269616.20 537.18 99.39 498.83 1260.38 384.36 114.47 9.08 29.78 4 1250 1017473.30 749057.35 268415.95 537.27 99.40 496.61 1260.38 384.36 112.24 8.91 29.20 4.5 1250 1019116.40 751741.69 267374.71 537.34 99.42 494.68 1260.38 384.36 110.32 8.75 28.70 5 1250 1020883.60 754425.93 266457.67 537.41 99.43 492.98 1260.38 384.36 108.62 8.62 28.26 5.5 1250 1022750.40 757110.10 265640.30 537.47 99.44 491.47 1260.38 384.36 107.11 8.50 27.87 6 1250 1024698.50 759794.20 264904.30 537.53 99.45 490.11 1260.38 384.36 105.75 8.39 27.51 6.5 1250 1026714.30 762478.24 264236.06 537.58 99.46 488.87 1260.38 384.36 104.51 8.29 27.19 7 1250 1028787.20 765162.23 263624.97 537.62 99.47 487.74 1260.38 384.36 103.38 8.20 26.90 7.5 1250 1030908.90 767846.17 263062.73 537.67 99.48 486.70 1260.38 384.36 102.34 8.12 26.63 8 1250 1033072.60 770530.07 262542.53 537.71 99.48 485.74 1260.38 384.36 101.38 8.04 26.38 8.5 1250 1035273.10 773213.93 262059.17 537.74 99.49 484.85 1260.38 384.36 100.48 7.97 26.14 9 1250 1037505.80 775897.75 261608.05 537.78 99.50 484.01 1260.38 384.36 99.65 7.91 25.93 9.5 1250 1039767.00 778581.54 261185.46 537.81 99.50 483.23 1260.38 384.36 98.87 7.84 25.72 10 1250 1042053.50 781265.31 260788.19 537.84 99.51 482.49 1260.38 384.36 98.13 7.79 25.53 0 1300 1181580.30 831504.37 350075.93 533.67 98.74 647.69 1316.34 440.31 207.38 15.75 47.10 0.5 1300 1168976.80 834602.35 334374.45 534.85 98.95 618.64 1316.34 440.31 178.33 13.55 40.50 1 1300 1165185.30 837689.38 327495.92 535.37 99.05 605.91 1316.34 440.31 165.60 12.58 37.61 1.5 1300 1163979.60 840774.61 323204.99 535.69 99.11 597.97 1316.34 440.31 157.66 11.98 35.81 2 1300 1163992.10 843858.90 320133.20 535.93 99.15 592.29 1316.34 440.31 151.98 11.55 34.52 2.5 1300 1164706.10 846942.60 317763.50 536.11 99.19 587.91 1316.34 440.31 147.59 11.21 33.52 3 1300 1165873.40 850025.87 315847.53 536.25 99.21 584.36 1316.34 440.31 144.05 10.94 32.72 3.5 1300 1167356.00 853108.82 314247.18 536.37 99.24 581.40 1316.34 440.31 141.09 10.72 32.04 4 1300 1169069.80 856191.52 312878.28 536.48 99.26 578.87 1316.34 440.31 138.56 10.53 31.47

162

Thermo- Thermo- chemical Solid Specific Exergy Thermo- chemical Boost to Solid Fraction Sensible O2 Total ΔH Solid ΔH Solid Above chemical Boost to Sensible T(C) Gas ΔH(J) Mass of Initial Energy (%) (J) (J) ΔH 900°C ΔH Sensible Exergy (g) Material (kJ/kg) (kJ/kg) (kJ/kg) (kJ/kg) Energy Above (%) (%) 900°C (%) 4.5 1300 1170959.90 859274.01 311685.89 536.57 99.27 576.66 1316.34 440.31 136.35 10.36 30.97 5 1300 1172988.50 862356.33 310632.17 536.65 99.29 574.71 1316.34 440.31 134.40 10.21 30.52 5.5 1300 1175128.80 865438.50 309690.30 536.72 99.30 572.97 1316.34 440.31 132.66 10.08 30.13 6 1300 1177360.60 868520.54 308840.06 536.79 99.31 571.40 1316.34 440.31 131.08 9.96 29.77 6.5 1300 1179668.90 871602.47 308066.43 536.85 99.32 569.97 1316.34 440.31 129.65 9.85 29.45 7 1300 1182041.90 874684.30 307357.60 536.90 99.33 568.65 1316.34 440.31 128.34 9.75 29.15 7.5 1300 1184470.40 877766.04 306704.36 536.95 99.34 567.45 1316.34 440.31 127.13 9.66 28.87 8 1300 1186946.80 880847.70 306099.10 537.00 99.35 566.33 1316.34 440.31 126.01 9.57 28.62 8.5 1300 1189465.10 883929.28 305535.82 537.04 99.36 565.28 1316.34 440.31 124.97 9.49 28.38 9 1300 1192020.20 887010.80 305009.40 537.08 99.37 564.31 1316.34 440.31 124.00 9.42 28.16 9.5 1300 1194608.00 890092.26 304515.74 537.12 99.38 563.40 1316.34 440.31 123.08 9.35 27.95 10 1300 1197224.90 893173.66 304051.24 537.16 99.38 562.54 1316.34 440.31 122.22 9.29 27.76 0 1350 1331412.90 935440.07 395972.83 532.75 98.57 732.60 1372.53 496.51 236.10 17.20 47.55 0.5 1350 1320142.70 938953.18 381189.52 533.89 98.78 705.25 1372.53 496.51 208.75 15.21 42.04 1 1350 1316692.50 942446.34 374246.16 534.43 98.88 692.41 1372.53 496.51 195.90 14.27 39.46 1.5 1350 1315729.20 945936.22 369792.98 534.78 98.94 684.17 1372.53 496.51 187.66 13.67 37.80 2 1350 1315977.60 949424.41 366553.19 535.03 98.99 678.17 1372.53 496.51 181.67 13.24 36.59 2.5 1350 1316938.30 952911.50 364026.80 535.23 99.02 673.50 1372.53 496.51 176.99 12.90 35.65 3 1350 1318365.30 956397.83 361967.47 535.39 99.05 669.69 1372.53 496.51 173.18 12.62 34.88 3.5 1350 1320120.30 959883.57 360236.73 535.53 99.08 666.49 1372.53 496.51 169.98 12.38 34.24 4 1350 1322117.80 963368.84 358748.96 535.65 99.10 663.74 1372.53 496.51 167.23 12.18 33.68 4.5 1350 1324301.40 966853.74 357447.66 535.75 99.12 661.33 1372.53 496.51 164.82 12.01 33.20 5 1350 1326632.10 970338.33 356293.77 535.84 99.14 659.19 1372.53 496.51 162.69 11.85 32.77 5.5 1350 1329081.80 973822.64 355259.16 535.92 99.15 657.28 1372.53 496.51 160.77 11.71 32.38 6 1350 1331629.60 977306.72 354322.88 536.00 99.17 655.55 1372.53 496.51 159.04 11.59 32.03 6.5 1350 1334259.60 980790.59 353469.01 536.07 99.18 653.97 1372.53 496.51 157.46 11.47 31.71 7 1350 1336959.40 984274.28 352685.12 536.13 99.19 652.52 1372.53 496.51 156.01 11.37 31.42 7.5 1350 1339719.10 987757.81 351961.29 536.19 99.20 651.18 1372.53 496.51 154.67 11.27 31.15 8 1350 1342530.90 991241.19 351289.71 536.24 99.21 649.93 1372.53 496.51 153.43 11.18 30.90 8.5 1350 1345388.10 994724.44 350663.66 536.30 99.22 648.78 1372.53 496.51 152.27 11.09 30.67 9 1350 1348285.40 998207.57 350077.83 536.34 99.23 647.69 1372.53 496.51 151.19 11.02 30.45 9.5 1350 1351218.40 1001690.60 349527.80 536.39 99.24 646.67 1372.53 496.51 150.17 10.94 30.24 10 1350 1354183.30 1005173.50 349009.80 536.43 99.25 645.72 1372.53 496.51 149.21 10.87 30.05 0 1400 1481343.10 1039375.40 441967.70 531.90 98.41 817.70 1428.94 552.92 264.78 18.53 47.89 0.5 1400 1471701.40 1043317.20 428384.20 532.98 98.61 792.57 1428.94 552.92 239.65 16.77 43.34 1 1400 1468873.50 1047223.90 421649.60 533.52 98.71 780.11 1428.94 552.92 227.19 15.90 41.09

163

164

A.5: Sensible Energy Calculations for Stoichiometric Hercynite Material

The following calculations are for molar amounts of 1 mole CoO, 1 mole Fe2O3, and 3 moles of Al2O3.

Combined Mean Energy Molar Heat Capacity (J/mol-K) Molar Cumulative Temp Temp Specific Stored at Heat Exergy (degC) (K) Heat Temp Capacity (kJ/kg) Al2O3 Fe2O3 CoO (J/kgK) (kJ/kg) (J/mol-K) 25 298.15 78.8043 103.7753 54.9834 79.0343 731.1160 1.4622 1.4622 50 323.15 84.5774 108.5756 54.4374 83.3490 771.0298 18.7768 20.2391 75 348.15 89.2891 112.7837 54.0632 86.9428 804.2745 19.6913 39.9304 100 373.15 93.2100 116.5468 53.8134 89.9980 832.5371 20.4601 60.3905 125 398.15 96.5286 119.9681 53.6554 92.6419 856.9941 21.1191 81.5096 150 423.15 99.3795 123.1219 53.5661 94.9653 878.4872 21.6935 103.2032 175 448.15 101.8608 126.0632 53.5286 97.0348 897.6319 22.2015 125.4046 200 473.15 104.0456 128.8334 53.5308 98.9002 914.8874 22.6565 148.0611 225 498.15 105.9890 131.4642 53.5635 100.5990 930.6024 23.0686 171.1298 250 523.15 107.7336 133.9808 53.6199 102.1603 945.0456 23.4456 194.5754 275 548.15 109.3123 136.4029 53.6948 103.6069 958.4278 23.7934 218.3688 300 573.15 110.7512 138.7469 53.7843 104.9569 970.9161 24.1168 242.4856 325 598.15 112.0709 141.0262 53.8852 106.2248 982.6450 24.4195 266.9051 350 623.15 113.2883 143.2519 53.9954 107.4225 993.7238 24.7046 291.6097 375 648.15 114.4170 145.4337 54.1131 108.5595 1004.2425 24.9746 316.5843 400 673.15 115.4681 147.5797 54.2369 109.6442 1014.2761 25.2315 341.8158 425 698.15 116.4509 149.6972 54.3659 110.6832 1023.8873 25.4770 367.2928 450 723.15 117.3731 151.7927 54.4994 111.6823 1033.1297 25.7127 393.0055 475 748.15 118.2412 153.8719 54.6368 112.6465 1042.0491 25.9397 418.9453 500 773.15 119.0608 155.9399 54.7779 113.5800 1050.6850 26.1592 445.1044 525 798.15 119.8364 158.0016 54.9225 114.4867 1059.0720 26.3720 471.4764 550 823.15 120.5722 160.0613 55.0705 115.3697 1067.2406 26.5789 498.0553 575 848.15 121.2716 162.1233 55.2220 116.2320 1075.2179 26.7807 524.8360 600 873.15 121.9377 164.1914 55.3771 117.0763 1083.0279 26.9781 551.8141 625 898.15 122.5730 166.2693 55.5360 117.9049 1090.6926 27.1715 578.9856 650 923.15 123.1800 168.3605 55.6989 118.7199 1098.2318 27.3616 606.3472 675 948.15 123.7606 170.4683 55.8661 119.5233 1105.6637 27.5487 633.8959 700 973.15 124.3167 150.6240 56.0380 115.9224 1072.3538 27.2252 661.1211 725 998.15 124.8499 150.6240 56.2150 116.2778 1075.6408 26.8499 687.9710 750 1023.15 125.3617 150.6240 56.3974 116.6213 1078.8188 26.9307 714.9018 775 1048.15 125.8534 150.6240 56.5856 116.9540 1081.8961 27.0089 741.9107 800 1073.15 126.3262 140.5736 56.7802 115.2665 1066.2856 26.8523 768.7630 825 1098.15 126.7811 140.7431 56.9815 115.6136 1069.4968 26.6973 795.4602

165

Combined Mean Energy Molar Heat Capacity (J/mol-K) Molar Cumulative Temp Temp Specific Stored at Heat Exergy (degC) (K) Heat Temp Capacity (kJ/kg) Al2O3 Fe2O3 CoO (J/kgK) (kJ/kg) (J/mol-K) 850 1123.15 127.2193 140.9188 57.1901 115.9533 1072.6396 26.7767 822.2369 875 1148.15 127.6415 141.0997 57.4064 116.2861 1075.7179 26.8545 849.0914 900 1173.15 128.0486 141.2847 57.6310 116.6123 1078.7355 26.9307 876.0221 925 1198.15 128.4415 141.4730 57.8644 116.9323 1081.6960 27.0054 903.0275 950 1223.15 128.8208 141.6637 58.1070 117.2466 1084.6031 27.0787 930.1062 975 1248.15 129.1872 141.8561 58.3596 117.5554 1087.4601 27.1508 957.2570 1000 1273.15 129.5414 142.0497 58.6225 117.8593 1090.2706 27.2216 984.4786 1025 1298.15 129.8839 142.2439 58.8963 118.1584 1093.0377 27.2914 1011.7700 1050 1323.15 130.2154 142.4381 59.1817 118.4532 1095.7649 27.3600 1039.1300 1075 1348.15 130.5363 142.6320 59.4791 118.7440 1098.4553 27.4278 1066.5578 1100 1373.15 130.8473 142.8251 59.7892 119.0312 1101.1120 27.4946 1094.0524 1125 1398.15 131.1487 143.0171 60.1125 119.3151 1103.7382 27.5606 1121.6130 1150 1423.15 131.4410 143.2077 60.4495 119.5960 1106.3369 27.6259 1149.2389 1175 1448.15 131.7248 143.3965 60.8009 119.8743 1108.9113 27.6906 1176.9295 1200 1473.15 132.0003 143.5834 61.1673 120.1503 1111.4642 27.7547 1204.6842 1225 1498.15 132.2681 143.7681 61.5491 120.4243 1113.9988 27.8183 1232.5025 1250 1523.15 132.5285 143.9505 61.9471 120.6966 1116.5179 27.8815 1260.3840 1275 1548.15 132.7819 144.1303 62.3618 120.9676 1119.0244 27.9443 1288.3283 1300 1573.15 133.0288 144.3073 62.7938 121.2375 1121.5213 28.0068 1316.3351 1325 1598.15 133.2694 144.4816 63.2436 121.5067 1124.0115 28.0692 1344.4042 1350 1623.15 133.5042 144.6528 63.4223 121.7175 1125.9620 28.1247 1372.5289 1375 1648.15 133.7334 144.8211 63.7884 121.9619 1128.2228 28.1773 1400.7062 1400 1673.15 133.9575 144.9861 64.1578 122.2033 1130.4553 28.2335 1428.9397 1425 1698.15 134.1767 145.1479 64.5304 122.4417 1132.6608 28.2890 1457.2286 1450 1723.15 134.3914 145.3064 64.9060 122.6773 1134.8408 28.3438 1485.5724 1475 1748.15 134.6019 145.4616 65.2846 122.9104 1136.9968 28.3980 1513.9704 1500 1773.15 134.8086 145.6134 65.6659 123.1410 1139.1301 28.4516 1542.4220 1525 1798.15 135.0117 145.7617 66.0498 123.3693 1141.2422 28.5047 1570.9266 1550 1823.15 135.2116 145.9066 66.4363 123.5955 1143.3346 28.5572 1599.4838

166

A.6: Equilibrium Composition Predictions of Cobalt Oxide

Temperature (deg C) CoO Mass (g) Co3O4 Mass (g) 650 0.150 240.636 660 0.267 240.511 670 0.469 240.295 680 0.813 239.926 690 1.393 239.305 700 2.360 238.270 710 3.952 236.564 720 6.547 233.784 730 10.731 229.302 740 17.410 222.148 750 27.966 210.841 760 44.490 193.141 770 70.120 165.687 780 109.529 123.473 790 169.629 59.095 800 224.798 0.000

167

A.7: Comparison of Fe3O4 and Fe2O3 Synthesis Using Equilibrium Compositions of Solid Material

Case Temperature (deg C) Corundum (mol) Corundum2 (mol) Spinel (mol) Fe2O3 400 2.6641 0.94831 0.59169 Fe2O3 450 2.558 0.92334 0.67913 Fe2O3 500 2.9915 0.053589 0.96995 Fe2O3 550 3.0032 0.02903 0.9785 Fe2O3 600 3.0138 0.0098162 0.98434 Fe2O3 650 3.0175 0.98845 Fe2O3 700 3.0121 0.9921 Fe2O3 750 3.005 0.997 Fe2O3 800 2.9948 1.004 Fe2O3 850 2.9802 1.014 Fe2O3 900 2.959 1.0286 Fe2O3 950 2.929 1.0492 Fe2O3 1000 2.8877 1.0776 Fe2O3 1050 2.8321 1.1159 Fe2O3 1100 2.7592 1.166 Fe2O3 1150 2.666 1.2302 Fe2O3 1200 2.5495 1.3106 Fe2O3 1250 2.4065 1.4097 Fe2O3 1300 2.2331 1.5306 Fe2O3 1350 2.0229 1.6782 Fe2O3 1400 1.7643 1.8618 Fe3O4 400 2.6564 1.4557 0.59193 Fe3O4 450 2.5479 1.433 0.67942 Fe3O4 500 2.9788 0.56617 0.97002 Fe3O4 550 2.9864 0.54584 0.97855 Fe3O4 600 2.9901 0.53342 0.98437 Fe3O4 650 2.9869 0.52997 0.98888 Fe3O4 700 2.9841 0.52596 0.9935 Fe3O4 750 2.9796 0.52184 0.99942 Fe3O4 800 2.9733 0.51584 1.0079 Fe3O4 850 2.9653 0.50536 1.0207 Fe3O4 900 2.9559 0.48661 1.0402 Fe3O4 950 2.9458 0.45401 1.0699 Fe3O4 1000 2.9358 0.3992 1.115 Fe3O4 1050 2.9266 0.30977 1.1836 Fe3O4 1100 2.9184 0.16646 1.2894 Fe3O4 1150 2.8841 1.4293

168

Case Temperature (deg C) Corundum (mol) Corundum2 (mol) Spinel (mol) Fe3O4 1200 2.747 1.5233 Fe3O4 1250 2.5751 1.6418 Fe3O4 1300 2.3567 1.7934 Fe3O4 1350 2.0779 1.9884 Fe3O4 1400 1.7198 2.2418

169

APPENDIX B: EXPERIMENTAL DATA

B.1: X-Ray Diffraction Data

Base Powder

uncalcined 5000 calcined post-TGA

4000

3000

intensity (counts) 2000

1000

0 30 32 34 36 38 40 42 44 46 2-theta (degrees)

170

Alumina-6 Powder

uncalcined 12000 calcined post-TGA 10000

8000

6000 intensity (counts) 4000

2000

0 30 32 34 36 38 40 42 44 46 2-theta (degrees)

Alumina-9 Powder

uncalcined 12000 calcined post-TGA 10000

8000

6000 intensity (counts) 4000

2000

0 30 32 34 36 38 40 42 44 46 2-theta (degrees)

171

B.2: High-Temperature in-situ X-Ray Diffraction Data

High-Temperature in-situ XRD 50C 1000C 1000C 1000C Air Introduced 1000C 1000C 1400C 1350C 1300C 1250C 1200C 1150C 1100C

Temperature 1050C 1000C 950C 900C 850C 800C 600C 400C 200C 25C 10 20 30 40 50 60 70 80 2- (degrees)

172

B.3: Raman Spectra

173

Alumina-6

14000 un-calcined calcined 12000 post-TGA

10000

8000

6000 intensity (counts)

4000

2000

0 200 300 400 500 600 700 800 wavenumbers (cm-1)

Alumina-9 10000 un-calcined 9000 calcined post-TGA 8000

7000

6000

5000

4000 intensity (counts) 3000

2000

1000

0 200 300 400 500 600 700 800 wavenumbers (cm-1)

174

B.4: SEM/EDX Data of Base Powder

20 µm

SEM Image of Base Powder

SEM/EDX Showing Cobalt

175

SEM/EDX Showing Iron

SEM/EDX Showing Aluminum

176

B.5: TGA/DSC Results B.5.1: Summarized Results Experimental Run Reduction Oxidation Reduction Oxidation Temperature Temperature Mass Loss Heat Flow 1100°C 1000°C -0.16% 7.3 J/g Base Powder – 1200°C 1000°C -0.87% 42.1 J/g Thermal 1300°C 1000°C -1.34% 67.9 J/g Reduction and 1400°C 1000°C -1.78% 97.0 J/g Isothermal Redox 1200°C 1200°C -1.11% 32.6 J/g 1500°C 1500°C -1.97% 27.6 J/g 1500°C 15.4 J/g 1500°C 1450°C -1.02% 31.5 J/g 1500°C 1400°C -1.08% 34.2 J/g 1500°C 1350°C -1.25% 39.6 J/g 1500°C 1300°C -1.36% 45.9 J/g 1500°C 1250°C -1.49% 51.3 J/g Oxidation 1500°C 1200°C -1.58% 57.0 J/g Temperature Vary 1500°C 1150°C -1.65% 63.2 J/g 1500°C 1100°C -1.72% 71.1 J/g 1500°C 1050°C -1.78% 75.8 J/g 1500°C 1000°C -1.86% 83.0 J/g 1500°C 950°C -1.93% 90.1 J/g 1500°C 900°C -1.97% 96.3 J/g 1500°C 850°C -1.97% 99.7 J/g 1500°C 800°C -1.99% 95.4 J/g Base Powder – 1400°C 1000°C -1.52% 80.6 J/g 1400-1000 1400°C 1000°C -1.49% 79.1 J/g Cycling 1400°C 1000°C -1.47% 79.2 J/g Base Powder – 1400°C 1000°C -1.50% 72.6 J/g 1400-1000 1400°C 1000°C -1.47% 70.8 J/g Cycling #2 1400°C 1000°C -1.45% 70.7 J/g Base Powder – 1500°C 1000°C -1.50% 74.4 J/g Thermal 1000°C -0.33% 4.2 J/g Reduction 1500 1500°C 1000°C -1.53% 74.1 J/g Variable Ramp 1000°C -0.34% (4.0) J/g Rate 1500°C 1000°C -1.66% 79.6 J/g 1100°C 1000°C -0.31% 15.0 J/g Alumina6 – 1200°C 1000°C -0.41% 15.1 J/g Thermal Reduction 1300°C 1000°C -0.41% 16.5 J/g and Isothermal 1400°C 1000°C -0.46% 17.9 J/g Redox 1200°C 1200°C -0.33% 13.1 J/g 1100°C 1000°C -0.22% 11.8 J/g Alumina9 – 1200°C 1000°C -0.36% 16.3 J/g Thermal Reduction 1300°C 1000°C -0.47% 22.1 J/g and Isothermal 1400°C 1000°C -0.61% 28.6 J/g Redox 1200°C 1200°C -0.35% 12.3 J/g

177

B.5.2: Base Powder – Thermal Reduction and Isothermal Redox

Sample mass: 92.9 mg Step: Reduction Oxidation Redox Oxidation Sensitivity Energy Storage Temp. Temp. Mass Exotherm Loss Area 1 1100°C 1000°C (0.16%) -2.64 0.36 2.64/0.36 = 7.3333 uVs/mg uV/mW J/g 2 1200°C 1000°C (0.87%) -14.3 0.34 14.3/0.34 = 42.0588 uVs/mg uV/mW J/g 3 1300°C 1000°C (1.34%) -22.42 0.33 22.42/0.33 = uVs/mg uV/mW 67.9394 J/g 4 1400°C 1000°C (1.78%) -30.07 0.31 30.07/0.31 = 97.0 uVs/mg uV/mW J/g Isothermal 1200°C 1200°C (1.11%) -11.07 0.34 11.07/0.34 = Redox uVs/mg uV/mW 32.5588 J/g

178

B.5.3: T-Oxidation Variance

179

Step: Reduction Oxidation Reduction Oxidation Sensitivity Energy Flow Temp. Temp. Mass Loss Exotherm Area 1 1500°C 1500°C 2.06 mg -8.293 0.3 8.293/0.3 = (1.97%) uVs/mg uV/mW 27.6433 J/g (2) 1500°C (4.629 0.3 ( 4.629/0.3 = uVs/mg) uV/mW 15.43 J/g 2 1500°C 1450°C 1.06 mg -9.675 0.3071 9.675/0.3071 = (1.02%) uVs/mg uV/mW 31.5044 J/g 3 1500°C 1400°C 1.13 mg -10.74 0.3143 10.74/0.3143 = (1.08%) uVs/mg uV/mW 34.1712 J/g 4 1500°C 1350°C 1.30 mg -12.72 0.3214 12.72/0.3214 = (1.25%) uVs/mg uV/mW 39.5769 J/g 5 1500°C 1300°C 1.41 mg -15.07 0.3286 15.07/0.3286 = (1.36%) uVs/mg uV/mW 45.8612 J/g

180

6 1500°C 1250°C 1.56 mg -17.21 0.3357 17.21/0.3357 = (1.49%) uVs/mg uV/mW 51.266 J/g 7 1500°C 1200°C 1.64 mg -19.55 0.3429 19.55/0.3429 = (1.58%) uVs/mg uV/mW 57.0137 J/g 8 1500°C 1150°C 1.72 mg -22.12 0.35 22.12/0.35 = 63.2 (1.65%) uVs/mg uV/mW J/g 9 1500°C 1100°C 1.79 mg -24.9 0.3571 24.9/0.35 = (1.72%) uVs/mg uV/mW 71.1429 J/g 10 1500°C 1050°C 1.86 mg -27.62 0.3643 27.62/0.3643 = (1.78%) uVs/mg uV/mW 75.8166 J/g 11 1500°C 1000°C 1.93 mg -30.82 0.3714 30.82/0.3714 = (1.86%) uVs/mg uV/mW 82.9833 J/g 12 1500°C 950°C 2.01 mg -34.1 0.3786 34.1/0.3786 = (1.93%) uVs/mg uV/mW 90.0687 J/g 13 1500°C 900°C 2.05 mg -37.16 0.3857 37.16/0.3857 = (1.97%) uVs/mg uV/mW 96.3443 J/g 14 1500°C 850°C 2.05 mg -39.17 0.3929 39.17/0.3929 = (1.97%) uVs/mg uV/mW 99.6946 J/g 15 1500°C 800°C 2.07 mg -38.17 0.4 38.17/0.4 = (1.99%) uVs/mg uV/mW 95.425 J/g

181

B.5.4: Base Powder 1400-1000 Cycling

Sample size: 115.1 mg

Step: Reduction Oxidation Redox Oxidation Sensivity Energy Temp. Temp. Mass Exotherm Area Loss 1 1400°C 1000°C (1.52%) -29.83 uVs/mg 0.37 -29.83/0.37 = - uV/mW 80.6216 J/g 2 1400°C 1000°C (1.49%) -29.28 uVS/mg 0.37 -29.28/0.37 = - uV/mW 79.1351 J/g 3 1400°C 1000°C (1.47%) -29.32 uVs/mg 0.37 -29.32/0.37 = - uV/mW 79.2432 J/g

182

B.5.5: 1400-1000 Cycle Test #2

Step: Reduction Oxidation Redox Energy Temp. Temp. Mass Loss 1 1400°C 1000°C -1.50% -26.85/0.37 = - 72.5676 J/g 2 1400°C 1000°C -1.47% -26.19/0.37 = - 70.7838 J/g 3 1400°C 1000°C -1.45% -26.16/0.37 = - 70.7027 J/g

183

B.5.6: Thermal Reduction 1500 Variable Ramp Rate

Step: Reduction Oxidation Redox Energy Temp. Temp. Mass Loss 1 1500°C 1000°C -1.50% -27.52/0.37 = - 74.3784 J/g (1.5) 1000°C -0.33% 1.545/0.37 = 4.1757 J/g 2 1500°C 1000°C -1.53% -27.4/0.37 = - 74.0541 J/g (2.5) 1000°C -0.34% 1.468/0.37 = 3.9676 J/g 3 1500°C 1000°C -1.66% -29.44/0.37 = - 79.5676 J/g

184

B.5.7: Alumina-6 – Thermal Reduction and Isothermal Redox

Step: Reduction Oxidation Redox Oxidation Exotherm Temp. Temp. Mass Loss Area 1 1100°C 1000°C (0.31%) -14.95 J/g 2 1200°C 1000°C (0.41%) -15.14 J/g 3 1300°C 1000°C (0.41%) -16.5 J/g 4 1400°C 1000°C (0.46%) -17.94 J/g Isothermal 1200°C 1200°C (0.33%) -13.12 J/g Redox

185

B.5.8: Alumina-9 – Thermal Reduction and Isothermal Redox

Step: Reduction Oxidation Redox Oxidation Exotherm Temp. Temp. Mass Loss Area 1 1100°C 1000°C (0.22%) -11.82 J/g 2 1200°C 1000°C (0.36%) -16.25 J/g 3 1300°C 1000°C (0.47%) -22.11 J/g 4 1400°C 1000°C (0.61%) -28.63 J/g Isothermal 1200°C 1200°C (0.35%) -12.25 J/g Redox

186