GCEP award #51922: A Novel Solid Oxide Flow Battery Utilizing H-C-O Chemistry

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1 1 GCEP award #51922: A Novel Solid Oxide Flow Battery Utilizing H-C-O Chemistry Investigators Gareth Hughes, Graduate Researcher; David Kennouche, Graduate Researcher; Zhan Gao, Post-doctoral Researcher; Scott A Barnett, Professor, Materials Science and Engineering, Northwestern University Chris Wendel, Graduate Researcher, Pejman Kazempoor, Post-doctoral Researcher, Robert Braun, Assistant Professor, Robert Kee, Professor, Mechanical Engineering, CSM Abstract This project centers on a novel device that stores energy electrochemically in tanks like a flow battery, while the materials and chemistry are more akin to those of a solidoxide fuel cell hence the name Solid Oxide Flow Battery (SOFB). The SOFB is well suited for grid-scale energy storage because the energy storage capacity can be increased by increasing tank storage capacity. The objective of the project is to carry out the fundamental studies of the materials, cells, stacks, and system designs needed to validate the device concept and provide the information needed for further development. One set of challenges for SOFB technology includes the development of cells that yield the desired performance at the desired operating temperature and pressure, and that can work without significant degradation over thousands of electrolysis/fuelcell cycles. Other challenges are to develop viable stack and system designs that can yield the predicted ideal round-trip efficiencies of ~ 80%. In the final year of this project, good progress has been made on a number of fronts. Our novel LSGM-electrolyte cells have been improved to yield outstanding intermediatetemperature performance. A number of different electrodes have been tested under pressurized conditions, providing important input for system modeling as well as showing which materials are best suited for pressurization. Extensive life testing has been carried out to determine conditions where solid oxide cells can operate with long lifetimes. Results on cyclic testing of air electrodes show that degradation is eliminated for current density below ~ 0.6 A/cm 2. Initial results on the stability of Ni nanoparticle anodes have also been obtained. System-level modeling and techno-economic analyses have continued to chart out how to design and operate entire SOFB-based energy storage systems along with their economic implications. In the past year a range of system configurations intended for smaller scale distributed energy storage have been explored; these include simple systems with minimal balance-of-plant components to more complex systems including turbine expansion for increased electrical efficiency, and separating water for higher energy density storage. Results indicate that a roundtrip efficiency of nearly 74% is achieved with relatively low tanked energy density (~20 kwh/m 3 ) for systems configured to store water-vapor containing gases. Separately storing condensed water increases energy density of storage, but limits efficiency to 68% based on the energetic cost of evaporating reactant water during electrolysis operation. Further increases in energy density require either higher storage pressures (e.g., >50-bar nominal) which lower roundtrip efficiency to about 65% or the addition of a separate methanation reactor. Cost of energy storage is found to be strongly influenced by stack power and system energy densities because the storage tanks and stack comprise a

2 majority of the system capital cost. As a capstone of the project, a comprehensive analysis of a promising large-scale, bulk energy storage system configuration, where a pressurized, intermediate-temperature solid oxide cell is coupled with pressurized underground storage caverns, providing high round-trip efficiency (> 72%) along with relatively low storage cost (<3 /kwh) and a storage capacity exceeding other available technologies. Introduction The need for grid-scale electrical energy storage is becoming more acute as increasing amounts of intermittent renewable electrical sources come on line, making it increasingly difficult to match fluctuations in both supply and demand [1, 2]. Daily supply/demand cycles mean storage times of ~ 12 h, requiring the ability to store very large amounts of energy. Currently available methods generally fail to meet at least one of the main storage-technology criteria cost, efficiency, storage capacity, durability, and widespread availability [1-4]. Hydroelectric water pumping and underground compressed air storage are well established but limited to specific naturally-occurring geographic sites. Secondary batteries and ultracapacitors currently have limitations for storing large amounts of energy cost effectively. Flow batteries may provide more scalable storage, although electrolyte volume and cost scale with energy stored [5, 6]. Reversible fuel cells have received only limited consideration for energy storage although they have potential for large-scale storage, round-trip efficiency is usually relatively low [7, 8]. This project aims to demonstrate and develop a novel Solid Oxide Flow Battery (SOFB) for large-scale energy storage. This unique device occupies a new space that has similarities to, but is quite distinct from, both solid oxide fuel cells and flow batteries. The devices are different than flow batteries since the storage fluids are very-low-cost gases rather than expensive liquid electrolytes, and the operating temperature is higher. They are also different than solid oxide fuel cells, as they operate reversibly rather than in one current direction, and at lower temperature and higher pressure. Figure 1 shows a simplified schematic of the proposed SOFB energy storage system. The membrane-electrode assembly (MEA) is based upon ceramic components that are designed to operate at C. In solid-oxide fuel cell (SOFC) mode, electricity is produced (discharge) as the fuel flows from the SOFC Fuel tank through the MEA, with the exhaust collected in SOEC Fuel tank. In solid-oxide electrolysis cell (SOEC) mode, the flow reverses direction, with electricity stored chemically (charging) in the SOFC Fuel tank. Oxygen flows through the lower channel. Oxygen ions are transferred through a dense ceramic membrane within the MEA. 2

3 Figure 1. Simplified process flow diagram for the SOFB concept. Discharging is shown with solid lines and charging with dashed lines. In conventional high-temperature H 2 O or CO 2 electrolysis, relatively high electrolysis voltages are used to provide the input energy needed to compensate for the highly endothermic reactions. This ultimately leads to relatively low round-trip storage efficiency. Our SOFB concept is designed to overcome efficiency limitations by altering the products of electrolysis of H 2 O-CO 2 mixtures. That is, by producing CH 4 in addition to H 2 and CO, the overall reaction becomes less endothermic, allowing lower electrolysis voltages and higher efficiency. In our proposed method, CH 4 production is increased by operating the SOFB with a C-H-O fuel-gas mixture at either (1) a relatively low temperature of 600 o C and 1 atm, or (2) an increased pressure of 3-15 atm and a more usual operating temperature of 750 o C. In a more recent embodiment, the cells operate at 600 o C and under pressurization of ~ 20 atm, which has the same advantages as the other modes but, in addition, it results in a very CH 4 -rich product that can be stored in existing pressurized natural gas underground caverns. With the above unique approach used to improve efficiency, the SOFB has the potential to meet the desired large-scale energy storage criteria and also has some unique advantages: High round-trip efficiency; Large amounts of energy can be stored over time scales of hours or days, limited only by the size of the storage tanks, and yet the SOFB system can also respond quickly; Reasonable cost: using current cost targets for solid oxide fuel cells and an energy storage time of 12 h yields a very desirable stack cost for energy storage of approximately $70/kWh; Solid electrolyte and tank leakage are minimal, yielding negligible energy loss during storage; The technology should have minimal environmental impact and uses mostly abundant and non-toxic materials of construction. SOFBs will integrate into the electrical network similar to other electrochemical storage devices; since energy is stored primarily as methane, they have the additional unique feature of allowing integration with the natural gas network; SOFB technology development will benefit substantially by leveraging prior/current development of solid oxide fuel cells, that has already advanced towards establishing 3

4 good long-term stability, a route to low-cost large-scale manufacturing, and scale-up with > 100 kw demonstrations; Given that no other widely available technology meets all the requirements, this innovative concept has the potential to be a game changer and play an important role in enabling extensive use of renewable energy. However, the technology will require substantial development from the state of the art in the closely related technology solid oxide fuel cells. As noted above, SOFB requirements are significantly different than fuel cells, due to the unique operating conditions (low temperature and/or high pressure) and the need to work reliably upon many cycles between charge and discharge mode. This requires fundamental new cell materials development work, along with development of novel stack and system designs. The research project aims to validate that the SOFB concept can yield high efficiency, and greatly extend current knowledge on solid oxide cell operation at high pressures and low temperatures. New cell materials, microstructures, and designs are being developed. State-of-the-art methods for observing materials evolution during accelerated testing are being employed to assess and improve long-term stability. The work benefits from synergies between innovative cell development, experimental evaluation, fundamental physically-based modeling at the cell and stack level, and system-level modeling including balance-of-plant performance. Achieving high efficiency while producing enough excess heat to offset thermal losses and maintain the stack operating temperature ultimately depends on combined cell/system optimization. Thus, cell/system modeling is employed to guide cell performance targets and materials development in a concurrent R&D engineering approach that is expected to accelerate technology development. The research is intended to provide sufficient proof-of-concept data to justify further investment in larger-scale development and implementation; the next stage would be development of a moderate-scale (~ kw) device. If expected targets are reached, the technology is anticipated to compete quite effectively against other storage technologies, providing strong motivation for commercial development. Background The field of energy storage using solid oxide cells continues to grow. This area, along with that in the broader field of solid oxide fuel cells, is important because it improves the science and technology background supporting the present work. Much of the recent work is on unidirectional cell operation, converting input renewable energy into chemical fuels. The group at Danish Technical University (Energy Conversion Department) continues to lead this field. The research group at Julich, Germany has also been active, in particular publishing two reports recently on the long-term stability of solid oxide cells under electrolysis and fuel cell modes.[9, 10] The main lab in the U.S. developing solid oxide electrolysis applications, Idaho National Labs, has been active in the past but currently has limited funding in this area. Outside of our group, there was previously little work published related to the use of solid oxide cells for reversible electricity storage, but we are seeing increased activity and interest, for example a recent article by the Danish group.[11] Results The results are divided into four main parts: 4

5 1) Development of solid oxide cells specifically designed for the energy storage application; 2) Testing of solid oxide cells under the pressurized and intermediate temperature conditions needed for efficient energy storage; 3) Life testing to determine which cells and materials are most stable, and what conditions are conducive to long cell lifetime; and 4) Detailed and physically realistic system-level modeling based on measured cell characteristics to determine optimal operating conditions and to predict system characteristics including round-trip efficiency, storage capacity, etc. 1. Solid Oxide Cell Development Solid oxide cells with (La0.9Sr0.1)0.98(Ga0.8Mg0.2)O3-δ (LSGM) electrolyte have been further developed, improved, and tested. The cells made using tape casting, a method widely used for large-scale manufacturing, have been substantially improved. They consist of a (Sr,La)TiO 3 support, a Ni-impregnated LSGM anode functional layer, and thin dense LSGM electrolyte, and a (La,Sr)(Fe,Co)O 3 - GDC composite cathode. The peak power density of 1.6 Wcm -2 is reached when the operating temperature is 650 o C, and 1.2 Wcm -2 at 600 o C. Figure 2 shows the cell performance when measured in both electrolysis and fuel cell modes. The cell area specific resistance is remarkably low 0.18 Ωcm 2 at 650C. 5 Voltage(V) ELECTROLYSIS 1.8 FUEL CELL 650 o C o C o C %H 2 /50%H 2 O Current density(macm -2 ) Fig. 2. ReSOC test results. Voltage versus current density at varying operating temperatures, in electrolysis and fuel cell modes, for a button cell with an LSGM electrolyte. 2. Solid Oxide Cell Testing Cell Testing In Realistic Fuel Compositions The above results are for hydrogen-steam mixtures, usually used for ease of testing. However, the actual devices will operate in complex H-O-C gas mixtures, with the exact constitution determined by the gas composition and the temperature. Thus, we have also Voltage(V)

6 operated the cells in such mixtures, predicted assuming that the gases are equilibrated and using the conditions found in our system calculations to yield good storage efficiency. the cell performance is not strongly dependent on the fuel gas constitution used. Indeed, the results in the complex mixtures were similar to that for H 2 -H 2 O. High-Pressure Electrochemical Testing We have carried out measurements of electrode performance under pressurized conditions; the results are important input for modeling of solid oxide cells operated under pressurized conditions. The work has focused on oxygen electrodes, which typically provide the main limitation on intermediate-temperature cell performance. Furthermore, the electrolyte resistance does not typically vary with pressure. EIS data measured at 800 o C from a typical LSM-YSZ electrode cell indicate a reduction in total polarization resistance R P with increasing po 2 (Figure 3). Both arcs decrease in resistance and increase in peak frequency as po 2 increases. Figure 3 shows that the total resistance data are fit well using R P po Figure 3. Polarization resistances at 800 o C of an LSM-YSZ composite electrode symmetric cell with a YSZ electrolyte as a function of po 2. Resistances of the lower frequency (LF) and higher frequency (HF) responses are given along with the total resistances R P. EIS data measured at 800 o C from a typical LSCF-GDC electrode cell, given in Figure 4, indicate a reduction in R P with increasing po 2. Both arcs decrease in resistance and increase in peak frequency as po 2 increases. Figure 4 also shows that the total resistance data are fit well using R P po

7 Figure 4. Polarization resistances at 600 o C of an LSCF-GDC composite electrode symmetric cell with a GDC electrolyte as a function of po 2. Resistances of the lower frequency (LF) and higher frequency (HF) responses are given along with the total resistances R P. In summary, both the electrode materials showed significant improvement with increasing po 2, with R P reduced by a factor of ~2 upon increasing po 2 from a nominal value of 0.2 atm (air) to 10 atm. 3. SOFB Durability Testing Effect of Current Density & Overpotential In the present report, LSM-YSZ composite oxygen electrodes were investigated in order to more accurately determine the effect of J and electrode η on degradation rate. (Note that the relative importance of J as opposed to η in causing degradation has not yet been elucidated, so both quantities are included in the present report.) In order to focus on the oxygen electrode, which appears to be the cell component most prone to degradation, symmetrical LSM-YSZ / YSZ / LSM-YSZ cells were tested. This has the advantage of minimizing extraneous degradation effects in full cells that may obfuscate the LSM-YSZ electrode degradation of interest here. Furthermore, symmetric applied current cycles, with equal current densities and times in each direction, were utilized in order to maintain symmetrical electrodes throughout the test despite the tendency for more rapid degradation in electrolysis mode. This enables more straightforward electrochemical impedance analysis. Long-term tests were carried out at current densities J ranging from 0.5 A/cm 2 to 1.5 A/cm 2, in order to more accurately assess how the degradation rate varies with J. Figure 5 shows the total, ohmic, and polarization resistance values obtained from fits to the impedance spectroscopy data taken throughout the life test. The life test at 0.5 A/cm 2 shows no detectable change in total resistance. In some cases, the cell resistance varied strongly initially, but then stabilized after ~ 70 h. The cell tested at 0.5 A/cm 2 showed a 7

8 slight steady decrease in R P that was almost exactly compensated by an increase in R Ω, such that there was no net change in total resistance. The overpotential η across a single electrode, calculated from J and polarization resistance, was 0.17 V. For J increased from 0.6 to 0.9A/cm 2 (initial η value from 0.33 to 0.38 V), the rate of decrease of R P lessened, while the rate of increase of R Ω increases, resulting in a faster total resistance increase. At J = 1.5 A/cm 2 (initial η = 0.93 V), both R P and RΩ increased rapidly with time. Figure 6 plots the measured total resistance degradation rates, obtained from best fits to the cell resistance versus time data in Fig. 5 neglecting any initial break-in effects, versus η (a) and J (b). Figure 6 gives a reasonable idea of how the degradation rate increases with current. Furthermore, the constant-current 0.5 A/cm 2 and 0.6 A/cm 2 tests provide a fairly clear indication that measurable degradation starts at a current between these values. SEM-EDS analysis was completed on a cell that was annealed with zero current along with the 0.8 and 0.9 A/cm 2 cells. Example images from these are shown in Figure 7. The cell annealed without current shows uniform electrode structure right up to the interface, whereas in the 0.8 A/cm 2 and 0.9 A/cm 2 cases there are microstructural changes near the electrode/electrolyte interface and throughout the active layer. The electrode structure near the electrode-electrolyte interface appears to be depleted of material, containing fewer electrode particles and larger pores. There is significant coarsening of the LSM particles in the active layer for the cells run at 0.8 A/cm 2 and 0.9 A/cm 2, compared to the no-current cell, but the YSZ particles remain un-coarsened. Also, the electrolyte shows pits and pores near the electrode/electrolyte interface, especially in the 0.9 A/cm 2 cell. The line scans in Figure 8 quantitatively show that La and Mn (i.e., LSM) are depleted near the electrode-electrolyte interface in the 0.8 and 0.9 A/cm 2 samples, in agreement with the images in Figure 7. The La and Mn signals increase with increasing distance from the electrolyte. On the other hand, the main effect of current on the Zr composition profile is a stronger local variation in intensity, presumably related to the presence of enlarged LSM particles. 8

9 Figure 5. (a) Ohmic (b) polarization and (c) total resistance versus time during reversingcurrent life tests of LSM-YSZ electrodes tested at 800C at current density values from 0.5 to 1.5 A/cm 2. The values are derived from fits to EIS data. 9

10 Figure 6. Degradation rates, derived from the slopes of the total resistance versus time data in Figure 5, as a function of a) overpotential and b) current density. Figure 7. A summary of: low magnification SEM images (left column), higher magnification SEM images (middle column), and EDS compositional maps (right column) from the electrode/electrolyte regions of cells with no current (top row), 0.8 A/cm 2 (middle row), and 0.9 A/cm 2 (bottom row). The color scheme of the composition maps is shown in the upper right frame. Figure 8. (right side). EDS linescans for a) annealed b) 0.8 A/cm 2 and c) 0.9 A/cm 2 cells. The SEM-EDS results for 0.8 and 0.9 A/cm 2 suggest that the amount of LSM in the interface region also decreases after cell operation under current. There appears to be a gradual increase in the amount of LSM with increasing distance from the interface, perhaps indicating that material has been transported away from the interface during cell 10

11 operation. Finally, the LSM particle size within the functional layers has increased after degradation. There does not appear to be a prior report of such changes in the LSM phase in LSM-YSZ functional layers. While there is no obvious explanation, cation transport has been observed in similar perovskite oxygen electrode materials under an oxygen gradient, as was present in the present experiments during cell operation.[12] Given the extensive structural/chemical changes within the electrode functional layer for 0.8 and 0.9 A/cm 2, it is perhaps surprising that the increases in polarization resistance are not larger. Indeed, SEM-EDS analysis of the electrode/electrolyte interface appears to be a very sensitive means for detecting low levels of degradation, perhaps more so than the EIS measurements. The apparent removal of material from near the electrode/electrolyte interface presumably represents an early stage of the oxygen-electrode delamination that was reported for LSM-YSZ electrodes after ~ 1000 h at a higher J of 1.5 A/cm 2, both in current switching and electrolysis modes.[13] That is, continuing removal of material ultimately results in a complete interfacial opening or delamination. Such delamination is likely even at the lower J of 0.8 A/cm 2 for sufficiently long-term cell operation. In summary, the present results show that the overall resistance degradation rate increases with increasing current density J (overpotential η), increasing above the present detection limit of 1%/kh for J somewhere between 0.5 and 0.6 A/cm 2 (η = 0.17 to 0.33 V). This result is consistent with SEM observations: substantial changes in the nearinterface structure were observed for J = 0.8 and 0.9 A/cm 2 (η = 0.39 and 0.37 V), whereas there was little observable change in structure reported previously (for the same cells and test conditions) at 0.5 A/cm 2.[13] These results are consistent with a recent report[14] where degradation was observed for J = 1.0 A/cm 2 (η = ~0.25 V), unless the fraction of the time in electrolysis mode during the reversing-current cycle was reduced. For J increased above 0.6 A/cm 2 (0.33 V), the rate of degradation increases rapidly to > 3 %/kh, fast enough to severely limit device lifetime and hence the total amount of energy that can be stored/converted. Stability of Nano-Scale Ni Fuel Electrodes Although the thin-lsgm-electrolyte cells with infiltrated-ni LSGM anode functional layers (AFLs) are shown above to provide very good IT-SOFC performance (Fig. 2),[15-18] their long-term performance has not been explored. One of the key questions is the stability of the nano-scale Ni in the AFL. Initial life tests were carried out at 650 C, which was higher than the nominal C operating temperature in order to accelerate any degradation. A similar accelerated testing strategy was employed previously to study infiltrated perovskite SOFC cathodes.[19] Impedance spectroscopy measurements and equivalent circuit fitting was used to isolate the infiltrate Ni anode polarization resistance. A model based on coarsening theory was used to correlate changes in polarization resistance with the observed Ni particle coarsening. In addition, identical IT-SOFCs were life tested for varying times and the AFLs were subsequently examined by fracture cross-sectional SEM to observe their microstructure. Figure 9a and 9b show examples of the AFL morphology for a cell tested for 60 h, while Figures 9c and 9d show a cell tested for 550 h. In Figures 9a and b, the nickel has formed a connected network of ~ 100 nm size particles. After 550 h, Figures 9c and 9d show 11

12 evidence of significant Ni coarsening, although there is still a percolated Ni network. Ni particles have a much larger range of sizes from ~ 50 nm particles to agglomerated networks that can extend up to ~ 1 µm in length. There is no visible change in the LSGM scaffold structure. Figure 9. SEM images of AFL structures from a cell tested for 60 h (a), and a cell tested for 550 h (c). The images in (b) and (d) show magnified views of the AFLs in (a) and (c), respectively. Impedance spectra were measured at 550 o C periodically during a typical life test, with the cell ageing at 650 o C with no cell current during most of the test. The impedance response at ~10 4 Hz, associated with the anode,[15] continued to increase throughout the test, suggesting that it will become an increasingly contribute to cell degradation. The anode response was well separated from the cathode response, making it feasible to separate by fitting the impedance spectra, and thereby track its time evolution. Figure 10 shows a plot of the anode resistance values, obtained from the impedance data, versus time. Although there is some scatter in the data, overall the resistance increases continuously during the test, approximately doubling over 550 h. This increase is presumably explained by the Ni coarsening shown in Figure 9, which should result in a substantial decrease in anode three-phase boundary (TPB) density. The fitted curve in Figure 10 results from an electrochemical model that takes the resistance as proportional to the inverse of the TPB density, and calculates the time variation of the TPB density based on power-law coarsening Ni particles. 12

13 R p (Ohm cm 2 ) Measured Fit Time (h) Figure 10. Raw EIS data taken at 550 C (triangle markers) and the corresponding n = 4 fit (dotted line) for cell operated at 650 C. 4. Results from Physically Based Cell and Systems Modeling Large-Scale Storage by Coupling a SOFB With Underground Gas Caverns As a capstone of the GCEP project, we have carried out a comprehensive study of a very large scale SOFB storage system in which reversible solid oxide electrochemical cells (ReSOCs) were coupled with sub-surface storage of CO 2 and CH 4, thereby enabling large-scale electricity storage with a round-trip efficiency exceeding 70% and an estimated storage cost around 3 /kwh. The ReSOCs use electricity to convert H 2 O and CO 2 into H 2, CO and O 2 ( electrolysis mode ) and produce power by the reverse process ( fuel cell mode ). By operating the ReSOCs at relatively low temperature and high pressure, the produced CO and H 2 can be catalytically converted to a CH 4 -rich gas inside the cell [20]. The heat generated by the exothermic CH 4 forming reaction can be used by the endothermic CO and H 2 forming reaction, thereby minimizing heat losses and optimizing round-trip efficiency [21, 22]. Cost-effective storage of H 2 O in reservoirs [23], and underground storage of pressurized CO 2 and CH 4 [24] enable unprecedented large energy capacity and long storage times. Figure 11 shows the proposed storage system schematically. The main components are the electrochemical conversion module consisting of a stack of ReSOCs, underground caverns for storing a CH 4 -rich gas and a CO 2 -rich gas, and a water reservoir. The ReSOC is represented in a simplified way as an electrode/electrolyte/electrode tri-layer, and the relevant simplified electrochemical reactions are given at each electrode. Additional balance-of-plant (BOP) needed for gas/water processing, not shown in Fig. 1, includes heat exchangers for efficiently heating/cooling gases as they enter/exit the ReSOC; compressors/expanders for pressurizing/expanding gases between ambient air, and a water condenser for removing steam from, and an evaporator for adding steam to, the gases. 13

14 Fig. 11. Schematic diagram of the proposed large-scale electricity storage system. When storing renewable electricity (Electrolysis mode) the reversible solid oxide cell (ReSOC) converts CO 2 + H 2 O into CH 4, by extracting oxygen (green lines). The process is reversed (red lines) when producing electricity on demand (fuel cell mode). Storage caverns and reservoirs are also shown. The complete energy storage cycle can be described as follows. During electricity storage (electrolysis mode), compressed CO 2 -rich gas is expanded, mixed with H 2 O, heated, and then introduced into the ReSOC, which uses electrical power to extract oxygen and produce a CH 4 -rich fuel gas; water is condensed and removed from this gas upon cooling, and then CH 4 is finally compressed for storage. During electricity production (fuel cell mode), compressed CH 4 -rich fuel gas is expanded, mixed with H 2 O, heated, and then introduced into the ReSOC, which oxidizes the fuel to generate power and produce a CO 2 /H 2 O-rich mixture; the H 2 O is condensed out upon cooling, and the CO 2 -rich gas is compressed for storage. This method is similar to that proposed earlier by Bierschenk et al. [22], but introduces new concepts that are critical to making this a viable large-scale electricity storage technology. Paramount among these are 1) pressurized and intermediate temperature operation of the ReSOC in order to produce a methane-rich product, 2) coupling with low-cost, underground, pressurized storage and 3) condensation of H 2 O. As noted above, during electrolysis mode (charging), oxygen is extracted from the gas entering the ReSOC moving to the left on the blue dashed line in the C-O-H composition diagram in Fig. 12a. During fuel cell mode (discharging), oxygen is added moving to the right. A C/H composition ratio of 1/5.67 was chosen to avoid formation of solid carbon which can destroy the fuel electrode (expected in the region to the upper left in Fig. 12a). The oxygen content used in the storage cycle ranges from about 4 to 40%, as indicated by the points on the blue line. Deleterious processes associated with complete reactant conversion (e.g., nickel catalyst oxidation, reactant starvation) are avoided by 14

15 setting the oxygen contents so that the gas mixture is neither fully oxidized nor fully reduced within the ReSOC. These compositions also allow a relatively wide range of oxygen contents in order to maximize energy storage capacity. (a) (b) Fig. 12. ReSOC operating conditions (A) C-O-H composition triangle showing the range of gas compositions for the proposed storage cycle, with a C/H ratio of 1/5.67, and oxygen content ranging from 4% to 40%. Also shown are the regions where the gas is fully oxidized (blue), and where carbon deposition is expected (grey) for a temperature of 650 C and pressures of 20 and 150 bars. (B) Predicted equilibrium gas constitution versus oxygen content, showing how the gas constitution is expected to change as it moves through the ReSOC for an operating temperature of 650 C and a pressure of 20 bar. Fig. 12b shows the predicted equilibrium gas constitutions, for an operating temperature of 650 C and pressure of 20 bar, versus oxygen content. Prior studies suggest that typical Ni-based ReSOC electrodes are sufficiently catalytic that the gas mixtures approach reasonably close to these predicted equilibrium constitutions [22, 25]. The gas that exits the ReSOC during fuel cell mode is mainly H 2 O with some CO 2 and H 2 (shown as the blue dot on the right of Fig 12a at about 40% oxygen) whereas the gas that exits the ReSOC during electrolysis mode is mainly CH 4 with smaller amounts of H 2 and H 2 O (shown as the red dot on the left of Fig 12a at about 4% oxygen). After the H 2 O is 15

16 condensed out upon cooling, this latter gas is rich in CH 4 (58%) with substantial H 2 (40%). This mixture has an energy density greater than 70% (based on higher heating value) of that of pure methane. The proposed storage technology is only viable employing ReSOCs that are able to operate with low internal resistance at temperatures 650 o C, i.e., with high current density at relatively low overpotentials. This is difficult to achieve by simply adapting conventional solid oxide fuel cells, which typically provide low resistance only at a higher temperature of ~ 800 o C, for use as ReSOCs. However, intermediate-temperature ReSOCs recently developed in this project can yield suitable performance; a value of 0.2 Ω cm 2 was assumed in the system modeling outlined in the next paragraph. As shown in Fig. 2 above, our recent test data for a ReSOC button cell with a thin (La 0.9 Sr 0.1 ) 0.98 Ga 0.8 Mg 0.2 O 3-δ (LSGM) electrolyte shows that this is feasible, as the resistance is only 0.18 Ω cm 2 at 650 C. System-level simulation was carried out to identify attractive system designs and to enable prediction of realistic round-trip storage efficiencies, here defined as the ratio of the electrical energy (DC power) generated in fuel cell mode to the electrical energy used in electrolysis mode. Fig. 13 depicts total system efficiency as a function of stack pressure for a temperature of 650 C, a storage pressure of 160 bar, and three different operating current densities. The curves in Fig. 13 are only shown above a specific pressure value, different for each current density, where it is possible to operate the system without an external heat source. These are the thermo-neutral points above which the ReSOC stack produces enough heat, in both operating modes, to maintain its operating temperature and satisfy system gas processing needs that include gas preheat and steam generation. Increasing the pressure shifts the product gas constitution, which reduces the thermo-neutral voltage of the electrolysis reaction to values close to the cell equilibrium (Nernst) voltage and thereby decreases the heat requirements [22]. The thermo-neutral point in Fig. 13 shifts to lower pressure with increasing current density because the ReSOC produces more heat at higher currents. Fig 13. System modeling results. Stack and system round-trip efficiency vs. stack pressure at three different current densities. 16

17 Fig. 13 also shows the ReSOC stack efficiency. The difference between stack and system efficiency is caused by the parasitic auxiliary power utilized by the BOP hardware. The stack efficiency varies little with pressure, because the cell resistance (0.2 Ωcm 2 ) has been assumed independent of pressure. However, the stack efficiency does decrease with increasing cell current density due to increasing cell over-potentials. System efficiency decreases with increasing pressure for each current density because of increasing parasitic power requirements for gas compression turbomachinery in the air processing BOP, while the decrease in power requirements for fuel processing BOP is relatively small. At 0.8 A/cm 2 and stack pressures near 20 bar, the auxiliary power required by the BOP is either net neutral or can even yield some net power as a result of efficient expansion of the high temperature, high pressure stack exhaust gases. Round-trip system efficiency peaks above 72% at a stack pressure near 20 bar and a current density of 0.7 A cm -2. Importantly, this value is high enough to be competitive with other storage technologies and recent advances in ReSOC pressure testing support that 20 bar operation is feasible [26, 27]. Although an optimal efficiency is achieved at 20 bar, higher pressure, lower temperature and higher O-content allows more methane and less hydrogen in the generated fuel. This increases the energy density and makes the gas more suitable for distribution in natural gas networks. Higher pressure operation is further expected to reduce the possibility for carbon deposition such that more carbonaceous reactants can be used at higher pressure. The downside of lowering the cell temperature is that the cell internal resistance increases with decreasing temperature, which decreases current density and thereby increases investment costs. However, future improvements in ReSOC technology may help enable such approaches. In addition to storing large quantities of energy with high efficiency, the ReSOC system must also have a reasonable capital cost a key criterion for any energy storage technology [28]. The storage cost estimate presented here was made assuming a 250 MW ReSOC system and gas caverns large enough to store 500 GWh, or almost 3 months of electricity supply. The expense distribution per annum, assuming 5 year stack and 20 year BOP lifetimes, is presented in Fig. 14a. This estimation is fairly complete, including buildings, labor, maintenance, etc. Note that the cavern expense is only 14% of the total, meaning that a 50% increase of the maximum storage capacity would only result in an increase of the total plant cost (TPC) of 7%. Assuming a capacity factor of 100%, i.e. that the system has no idling time, the storage cost using the calculation method proposed by Yang et al. [29] is estimated at about 2.8 /kwh. This cost is lower than CAES, batteries and H 2 storage and, in some cases, is comparable with hydropower as shown in Fig. 14b [29, 30]. The figure summarizes the key advantages of the proposed storage system the combination of relatively low cost, high round-trip efficiency, and ability to store large quantities of energy for long durations. 17

18 (a) (b) Fig. 14. Cost distribution for the proposed storage system and comparison with other storage technologies. (A) Annual ReSOC storage system expenses in %. (B) Energy storage technologies amended from literature [29, 30] including the ReSOC technology as function of investment cost per kwh per cycle and maximum discharge hours. Efficiency is denoted by the color from purple to blue. Electricity arbitrage is possible for technologies with high efficiency and placed in the upper left part of the graph. As shown in Fig. 14b, the only other technologies that can match the combination of high efficiency and low-cost large-scale energy storage, where electricity arbitrage becomes possible are pumped hydro and, to some extent, CAES. These technologies have in common the use of very low cost storage media (e.g. water or air) stored in geologicscale natural formations. However, compared with pumped hydro and CAES, the ReSOC technology has the advantage that the energy storage is chemical, rather than by potential energy, delivering much higher energy density and hence the longer cost-effective storage times. Furthermore, natural gas underground storage and infrastructure needed for the ReSOC technology are widely available compared to pumped hydro, which is limited in capacity by confined geographic availability. Hydrogen storage has been widely considered, but has disadvantages relative to the proposed storage technology due to the lower energy density of hydrogen compared to CH 4 and the low round-trip efficiency [22, 31, 32]. Finally, secondary and flow batteries utilize relatively more expensive storage media (solid or liquid electrode/electrolyte materials) and hence are more suitable for short-time electricity storage [29, 33]. 18

19 Distributed-scale Energy Storage Using SOFBs In addition to large-scale, bulk energy storage system studies, we have performed system design and techno-economic analyses for smaller-scale (100 kw / 1 MWh) electrical energy storage applications. These studies have included SOC stack-level simulations which examine the influence of reactant flow configuration (co- vs. counter) on desirable reversible SOC performance. At the system-level, a series of system configurations have been investigated starting with a stored-vapor case, in which tanked reactants are stored at elevated temperature to maintain the vapor phase of the humid gas. This system was observed to incur relatively low energy density, but achieves high roundtrip efficiency due not only to avoiding the expenditure of thermal energy to reboil feedstock steam, but also when an expansion turbine to recuperate losses from compressing gas to tanks pressure is included in the system. An alternate approach has also been considered in which water is condensed out of the gas streams and stored in a separate container prior to compression. This approach enables lower temperature and higher energy density storage for more economical tank sizing. The efficiency of this water separation configuration is limited by the energetic requirement to re-boil reactant water. Within each configuration, we have performed parametric studies which reveal the impact of key operating conditions such as fuel utilization, current density, and tank pressure. A brief synopsis of these activities is provided here and detailed results can be found in the forthcoming publications cited in the Publications and Patents section below. ReSOC Stack-level Simulation The flow configuration meaning whether the oxidant and reactant streams are flowing in the same direction, opposite direction, or perpendicular to each other also impacts the cell and system performance. Typically cross-flow performance falls in between that of co- and counter-flow. The one-dimensional model employed in this study is formulated to simulate either co- or counter-flow operation. Table 1 shows performance results for all combinations of flow-configuration in SOFC and SOEC mode. The stack roundtrip efficiency is maximized when both modes are operated in counter-flow. This is in agreement with previous studies showing highest electrical performance for counter-flow in SOFCs and SOECs. Perhaps more interesting is the effect of flow configuration on stack heat generation. Operating the SOFC mode in coflow and/or operating the SOEC mode in counter-flow leads to higher SOEC mode heat generation (see air temperature change, ΔT air ). These trends are explained by the higher change in methane mole fraction from fuel channel inlet to outlet in SOEC mode. Counter-flow operation results in lower outlet gas temperatures because the fuel channel outlet corresponds with the air cooling inlet. Alternatively, co-flow operation leads to higher outlet temperatures. When the SOFC is operated in a co-flow configuration, the higher outlet temperature results in lower outlet methane content when compared to counter-flow (see Table 1). Further, when SOEC mode is operated in counter-flow, the lower outlet temperature results in higher methane content. Thus, when the SOFC is operated in co-flow and SOEC in counter-flow (i.e., co/counter-flow), the amount of exothermic methane formation is largest compared to the other configurations, leading to 19

20 maximum SOEC air temperature rise of 29 C. This configuration also leads to more methane reforming in the SOFC mode, and an associated minimum SOFC air temperature rise. For practical systems, the 29 C temperature rise is not sufficient to satisfy pinch temperatures in a preheating heat exchanger, so oxidant airflow may be reduced to increase the temperature rise with a minor electrical performance penalty. The high air temperature increase for the co/counter-flow case is primarily due to increased methanation, but also partially explained by the lower roundtrip efficiency. To quantify this impact, the efficiency of the co/counter-flow case is increased to the value of the counter/counter-flow case (i.e., 72.8%) by reducing the current density to A/cm 2. For equivalent efficiency, the heat generation is higher when the stack is operated in co/counter-flow (see Table 1). However, the discharge power density at the lower current density is reduced by 3% compared with the counter/counter-flow case. Table 1. Performance summary of co- and counter-flow configurations in both operating modes. Flow SOFC mode SOEC mode Roundtrip Configuration V cell dt/dx ΔT air x CH4 V cell dt/dx ΔT air x CH4 efficiency SOFC SOEC mv C/mm C % mv C/mm C % (%) co co counter counter co counter counter co co/counter, A/cm The cell durability is also an important consideration in choosing the flow configuration, as represented here through the cell temperature gradient. Two trends are observed: 1) higher temperature gradient is seen for counter-flow operation, which is consistent with previous SOFC modeling studies and 2) higher temperature gradient is seen in SOFC mode, compared with SOEC mode. The second trend is explained by the cell net heat generation where higher heat generation also leads to higher temperature gradients. The high efficiency and near-equilibrium reactant compositions used for this ReSOC application lead to lower axial temperature gradients compared to typical SOFC applications. Thus, the temperature gradients are not critical in selecting the most suitable stack flow configurations. 20

21 Fig. 15. Axial profiles for (a) species, (b) temperature and current density magnitude in both SOFC and SOEC mode. Co-flow in SOFC mode and counter-flow in SOEC mode with reversed reactant flow direction and constant airflow direction. Other practical design considerations when selecting flow configuration include reducing localized variations in the cell environment between operating modes including local temperature, composition, and current density swings. For the co/counter-flow configuration suggested by the results in Table 1, it may be envisioned that the airflow direction is unaltered between operating modes, while the fuel-channel flow switches direction along with mode switching. Axial species, temperature, and current density profiles for the co/counter-flow configuration are shown in Fig. 15. For this simulation, the air temperature increase is set to 100 C in both operating modes, replacing the excess air ratio parameter assignment in the base case. This approach is more suitable to system simulation to ensure the pinch temperature in the air preheater can be met by the air exhaust stream. However, the excess air ratios are 6.1 and 1.4 for SOFC and SOEC mode, respectively, which reduces the practicality of using the same air blower in both operating modes. Additional parameter adjustment may be employed in system simulations to reduce the difference in airflow rate between operating modes. In Fig. 15(a), the composition at a given axial location in the stack varies less than if the reactants entered the fuel channel from the same side in each operating mode. However, system implementation of changing the flow direction in the fuel channel must be addressed. The temperature and current density magnitude profiles are shown in Fig. 15(b). The cell temperature increases in the fuel-flow direction in both modes, meaning that the cell will undergo localized temperature cycling of up to 50 C. Similarly, the 21

22 current density profiles show opposite trends throughout the cell, indicating galvanic cycling between 0.49 and A/cm 2 at the air inlet, and between 0.43 and A/cm 2 at the air outlet. The flow configuration may be selected to either maintain relatively constant gas constitution in the fuel channel, or more constant temperature profiles (if the fuel-channel flow direction is the same in each operating mode). ReSOC System Studies Distributed energy storage system concepts have been generated in which it should be noted that while the systems are intended for dual mode operation, process flowsheet details such as heat exchanger bypass flows are not depicted here for sake of simplicity. The operating conditions for these studies assume a nominal stack temperature of 600 C, stored fuel composition with hydrogen-to-carbon ratio of 9.9, and fuel oxygen content of 5%. The reactant gas flow configuration within the ReSOC stack is preferentially switched between co- and counter-flow arrangements based on prior analysis (above). The stack is operated in co-flow in SOFC mode and in counter-flow for SOEC operation, which benefits system thermal performance. The stored vapor system schematic and statepoint data for SOFC and SOEC modes are shown in Fig. 16. This system includes heat exchangers to preheat reactants and cool stack products, and compressors to pump stack products back to the storage tanks. The reactant species are preheated from the storage tank temperature (fixed to 50 C above saturation) to the stack inlet temperature, which requires a thermal load of 7-9 kw depending on the operating mode. The gas streams are heated and electrochemically converted in the stack which operates at sufficiently high overpotentials to be net exothermic. Thus, the hot fuel and air channel tail-gases can be used to recuperatively preheat the incoming reactant streams. Additional preheating is provided from hot gases downstream of the fuel compressors. The stack inlet reactant gas temperatures are 50 C and 72 C below nominal stack temperature in SOFC and SOEC modes, respectively. The lower stack inlet temperature in SOEC mode ensures that the pinch temperature is met in the fuel preheater due to the higher heat capacitance rate of the cold-side gas. The fuel heat exchanger serves two functions: to preheat reactant gases and to cool product gases for compression. Because of the imbalance in flow rates and heat of compression, the fuel preheat does not lower the stack tail-gas enough to achieve a sufficiently low compressor inlet temperature. Thus, pre-compressor cooling and inter-stage cooling using system airflow as a heat sink lowers compression temperature to a minimum compressor inlet temperature of 50 C above saturation. The air preheater and intercooling processes increase the ambient air temperature from 25 C to an inlet ReSOC oxygen channel temperature of 460 C (SOFC) or 544 C (SOEC) as shown in Fig. 16. The air flowrate and stack-inlet temperature are selected such that a nominal stack temperature (e.g., 600 C) is achieved with a specified air temperature rise of 150 C (SOFC) and 50 C (SOEC) across the air electrode side of the stack. Despite the internal chemical reactions balancing the thermal load between operating modes, the stack is more exothermic in SOFC mode. Thus, the air temperature rise in SOFC mode is higher compared to SOEC mode resulting in more similar air flowrates and exhaust temperatures (~ C). The stack and system efficiencies for the stored vapor system are 83.6% and 65.4%, respectively, indicating that a significant efficiency penalty is incurred from 22

23 compression parasitics. In each operating mode, the fuel compressor load is 7-12 times larger than the air blower load due to the much higher pressure ratio. However, the preheat load is higher on the air-side due to the higher flowrate of air compared to fuel and exhaust. The thermal or electric loads on the BOP are similar enough between SOFC and SOEC modes that the system may be dual-mode operated using the same components, favourably reducing system capital cost; although system simulation with specified hardware is required to validate this design approach. The volumetric energy density is relatively low at 18.9 kwh/m 3 for the stored vapor system. For an 8 hour operating duration at 100 kw stack discharge power, the fuel and exhaust tank volumes are 14.3 and 21.6 m 3, respectively. The temperature of the compressed fuel and exhaust streams enter their respective storage tanks above the tank temperature. The elevated gas inlet temperature allows for heat loss to the environment through the tank insulation, but additional heat may need to be removed from the stream prior to tanking to account for compression heat within the rigid tank. In any case, higher fidelity analysis to explore the transient effects of storage with specified tank geometry and materials is necessary to complete this element of the system design. Fig. 16. Stored vapor system schematic and statepoint data. Improving system efficiency requires reducing the auxiliary power loads. The performance impacts of operating conditions, including fuel utilization and current density, and the addition of a fuel expander are considered for the stored vapor. 23

24 The system efficiency is improved by operating the stack more efficiently (i.e., at lower current density), but the ultimate merit of this design condition is best informed through an economic analysis that judiciously weights low power density stack operation, capital cost associated with system thermal management, and operating costs (electrical energy, O&M). At lower current density, the stack generates less waste heat and therefore preheating process streams with reasonable pinch temperatures is problematic. Fig. 17a shows the roundtrip efficiency for varied current density for the stored vapor system. Improvement to 66.8% system efficiency is achieved at 0.15 A/cm 2. However, at lower current density the system efficiency degrades because much less cooling airflow is available for compressor intercooling in SOEC mode as the SOEC approaches the thermoneutral point at about 0.10 A/cm 2. Fig. 17 (a.) Efficiency vs. current density for the stored vapor system at nominal stack temperature of 600 C, fuel utilization of 65%, and tank pressure of 20 bar, (b) Efficiency and tank volume vs. tank pressure for the stored vapor system with and without a recuperative expander. An alternate operating strategy might consider operating at voltages below the thermoneutral voltage and supplying the deficit thermal energy in the SOEC stack with, for example, an electric heater integrated with the stack to preheat reactant gases or otherwise maintain stack temperature. The electric heater allows the SOEC stack to operate under endothermic conditions, but the power supplied to the heater must be included in the efficiency definition as auxiliary power. The SOEC stack efficiency is constant with current density if the stack is at or below the thermoneutral voltage. However, a roundtrip efficiency benefit may be realized due to a higher operating voltage in SOFC mode, which is required to satisfy the charge balance constraint with equal charge/discharge durations. Ultimately, this strategy is not considered in the present work because the thermoneutral voltage is reached in SOEC mode at low overpotential (<50 mv), so operating at even lower power density values is not considered economically feasible. The electric heater approach may be more realistically considered in steamhydrogen systems where reaching the thermoneutral voltage requires high overpotential. Another method for system performance improvement is integrating a fuel expansion turbine to recover energy as reactant streams are discharged from pressurized tanks. For the conditions in Fig. 15, including an 80% isentropic efficiency turbine in place of the tank discharge valve in both operating modes increases system efficiency to 73.7% an 8.5-percentage point increase. Small-scale (1-10 kw) scroll expanders developed for 24

25 PEM fuel cell systems and experimental organic Rankine cycles suggest that high efficiency expansion is viable at the proposed scale. The system efficiency and tank energy density are shown as a function of tank pressure in Fig. 17b (recall that this nominal tank pressure is approximately half of the maximum tank pressure). Using a turbine expander increases the system efficiency, particularly at high tank pressure. Furthermore, the energy density increases significantly with increased storage pressure and the turbine configuration enables higher storage pressure while maintaining >70% roundtrip efficiency. The energy density trend shown in Fig. 17b is explained because less volume is required to store the required quantity of gas at high pressure. In fact, the stored compositions do not deviate from those shown in Fig. 15 as storage pressure increases. The compositions in the tanks are not chemically altered between the stack fuel-channel outlet and storage tank, meaning that they are not in chemical equilibrium at the storage conditions. The composition is expected to remain stable over the time-scales of roundtrip storage (e.g., days to weeks) when uncatalyzed. The energy density improves slightly when a turbine is included in the system because the system roundtrip efficiency is higher and therefore less gas must be stored for equivalent energy discharge capacity. That is, because the net power generation in SOFC mode is higher due to the added turbine, more energy can be extracted from a fixed amount of stored fuel. Additional system studies including economic analyses have been performed including methods to achieve energy storage densities greater than 100 kwh/m 3. The details of these results are given elsewhere and can be found in the publication listing at the end of this report. However, here we show the annualized component cost breakdown for the highest and lowest cost systems and is given in Fig. 18. The annualized cost reflects the shorter lifetime of the ReSOC stack compared to other system components. The majority costs in the baseline stored vapor case are the ReSOC stack (46.1%) and gaseous storage tanks (34.9%) with smaller contributions from the relatively simple BOP. The lowest cost system (water separation + fuel expander, 50 bar nominal storage) has substantially lower tank cost due to the reduced volume. The system BOP cost is a larger share of total system cost because of increased heat exchanger area, particularly from the air-cooled condenser. The results show of our analyses report system performance at high roundtrip efficiency but at lower energy storage density. However, the economic results indicate that the higher stack power density and smaller tanks required in the water separation systems lead to lower cost of energy storage. This suggests that the operating conditions to achieve optimal efficiency may be distinct from the lowest cost system. Our work above has reported that large-scale, bulk storage systems achieve roundtrip system efficiency >70% for the water separation system with pressurized ReSOC stacks. The lower efficiency reported here is attributed to both differences in system configuration and operating conditions. The pressurized stack allows more carbonaceous fuel mixtures to be used without depositing carbon, such that a higher rate of in-situ methanation occurs in the SOEC stack. Furthermore, additional turbomachinery including an air turbine at the pressurized stack exhaust recuperates compression energy and results in net energy generation from the BOP, allowing system efficiency to exceed stack efficiency by efficiently expanding air exhaust. 25

26 Fig. 18. Annualized component cost contributions for the highest (basecase stored vapor) and lowest (water separation + fuel expander systems) cost systems. Summary of ReSOCs for Distributed-scale Storage This past year s work has focused on stand-alone energy storage systems utilizing ReSOCs and several systems are conceptualized and analysed for 100 kw-scale distributed applications. These systems use carbonaceous reactants and promote heterogeneous reforming and fuel synthesis reactions to achieve higher efficiency compared to low temperature reversible fuel cells, or those operated only on steamhydrogen reactants. System modeling is used to calculate efficiency and energy density metrics using a calibrated ReSOC model based on intermediate temperature LSGMelectrolyte data at 600 C. The system results compare stored vapor and water separation strategies, which are distinguished by the phase of the H 2 O constituent during tanked storage. The stored vapor system achieves higher roundtrip system efficiency of 73.7% compared to 68.3% in the water separation case. The water separation case is limited by the thermal load required to re-boil water in SOEC mode. A fuel expander is necessary to achieve high roundtrip efficiency, particularly at elevated storage pressure in the stored vapor system. The energy density is higher in the water separation system as a result of the lower quantity of stored gas and lower storage temperature. Energy density of 89.2 kwh/m 3 is achieved in the water separation case for a storage pressure of 50 bar compared to 44.2 kwh/m 3 at the same storage pressure for the stored vapor case. The preliminary system capital costs are estimated to be $/kwh. The cost of energy storage is strongly influenced by stack power density and energy density because storage tanks and ReSOC stack comprise a majority of the system cost. The water separation systems show lower cost of energy storage compared to the stored vapor systems, and increased storage pressure lowers system cost in both cases. The effects of improvement in ReSOC cell-stack performance and transient tanked storage on system efficiency are also discussed. Improved cell performance does not allow increased roundtrip system efficiency because roundtrip efficiency is limited by energetic preheating loads. However, reducing present day cell resistance from about 0.40 to 0.20 Ωcm 2 at 600 C will increase power density from 0.19 to 0.38 W/cm 2 and reduce annualized system cost by about 20% for fixed system efficiency. System 26