A research program is under way at the Idaho National Laboratory to assess the performance of solid-oxide cells operating in the steam electrolysis mode for hydrogen production over a temperature range of 800 to 900ºC. The research program includes both experimental and modeling activities. Selected results from both activities are presented in this paper. Experimental results were obtained from a ten-cell planar electrolysis stack, fabricated by Ceramatec 1 , Inc. The electrolysis cells are electrolyte-supported, with scandiastabilized zirconia electrolytes (~140 µm thick), nickel-cermet steam/hydrogen electrodes, and manganite air-side electrodes. The metallic interconnect plates are fabricated from ferritic stainless steel. The experiments were performed over a range of steam inlet mole fractions (0.1 -0.6), gas flow rates (1000 -4000 sccm), and current densities (0 to 0.38 A/cm 2 ). Hydrogen production rates up to 90 Normal liters per hour were demonstrated. Stack performance is shown to be dependent on inlet steam flow rate. A three-dimensional computational fluid dynamics (CFD) model was also created to model high-temperature steam electrolysis in a planar solid oxide electrolysis cell (SOEC). The model represents a single cell as it would exist in the experimental electrolysis stack. Mass, momentum, energy, and species conservation and transport are provided via the core features of the commercial CFD code FLUENT 1 . A solid-oxide fuel cell (SOFC) model adds the electrochemical reactions and loss mechanisms and computation of the electric field throughout the cell. The FLUENT SOFC user-defined subroutine was modified for this work to allow for operation in the SOEC mode. Model results provide detailed profiles of temperature, Nernst potential, operating potential, anode-side gas composition, cathode-side gas composition, current density and hydrogen production over a range of stack operating conditions. Mean model results are shown to compare favorably with the experimental results obtained from the ten-cell stack tested at INL.

A process model has been developed to evaluate the potential performance of a largescale high-temperature co-electrolysis plant for the production of syngas from steam and carbon dioxide. The co-electrolysis process allows for direct electrochemical reduction of the steam -carbon dioxide gas mixture, yielding hydrogen and carbon monoxide, or syngas. The process model has been developed using the Honeywell UniSim systems analysis code. Using this code, a detailed process flow sheet has been defined that includes all the components that would be present in an actual plant such as pumps, compressors, heat exchangers, turbines, and the electrolyzer. Since the electrolyzer is not a standard UniSim component, a custom one-dimensional co-electrolysis model was developed for incorporation into the overall UniSim process flow sheet. The one dimensional co-electrolysis model assumes local chemical equilibrium among the four process-gas species via the gas shift reaction. The electrolyzer model allows for the determination of co-electrolysis outlet temperature, composition (anode and cathode sides); mean Nernst potential, operating voltage and electrolyzer power based on specified inlet gas flow rates, heat loss or gain, current density, and cell area-specific resistance. The one-dimensional electrolyzer model was validated by comparison with results obtained from a fully three dimensional computational fluid dynamics model developed using FLUENT, and by comparison to experimental data. This paper provides representative results obtained from the UniSim flow sheet model for a 300 MW co-electrolysis plant, coupled to a high-temperature gas-cooled nuclear reactor. The coelectrolysis process, coupled to a nuclear reactor,

A one-dimensional model has been developed to predict the thermal and electrochemical behavior of a high-temperature steam electrolysis stack. This electrolyzer model allows for the determination of the average Nernst potential, cell operating voltage, gas outlet temperatures, and electrolyzer efficiency for any specified inlet gas flow rates, current density, cell active area, and external heat loss or gain. The model includes a temperature-dependent area-specific resistance (ASR) that accounts for the significant increase in electrolyte ionic conductivity that occurs with increasing temperature. Model predictions are shown to compare favorably with results obtained from a fully 3-D computational fluid dynamics model. The one-dimensional model was also employed to demonstrate the expected trends in electrolyzer performance over a range of operating conditions including isothermal, adiabatic, constant steam utilization, constant flow rate, and the effects of operating temperature. INTRODUCTIONA research program is under way at the Idaho National Laboratory to assess the performance of solid-oxide cells operating in the steam electrolysis mode for hydrogen production over a temperature range of 800 to 900ºC. The research program includes both experimental and modeling activities.Experimental activities, including both single button-cell testing and stack testing have been documented in several recent publications [e.g., 1-3]. The modeling activities include detailed computational fluid dynamics (CFD) simulations [4] and system-level modeling.In order to evaluate the potential hydrogen-production performance of large-scale high-temperature electrolysis (HTE) operations, we have developed an engineering process model at INL using the commercial system-analysis code HYSYS. Using this code, several detailed process flow sheets have been defined that include all of the components that would be present in an actual HTE plant such as pumps, compressors, heat exchangers, turbines, and the electrolyzer. Since the

The purpose of this report is to document the results of the thermal analyses performed to calculate the Advanced Gas-Cooled Reactor (AGR)-2 as-run daily temperatures of the fuel compacts. Time average volume average data provided by this report will be used to determine fuel performance in post-irradiation examination. 8. If revision, please state the reason and list sections and/or pages being affected: NA 9. Conclusions/Recommendations: This report documents the results of thermal analyses to predict the daily as-run temperatures for the AGR-2 experiment. Control gas gaps and compact-graphite holder gas gaps were modeled to change linearly with time (fast neutron fluence). Graphite shrinkage rates were taken from the AGC-1 Experiment. Daily heat rates for each compact and component in the models were input from daily as-run physics analyses. Daily gas compositions and component fast neutron fluences were also input. Six different finite element models were created for the six different AGR-2 capsules. Each capsule had a different gas gap that was implemented to control the temperature of the experiment. Gas mixture thermal conductivity was implemented using kinetic theory of gases. Fluence and temperature-dependent thermal conductivity was used for the graphite components and fuel compacts. Radiation heat transfer was implemented with emissivity of all graphite surfaces being 1.0 and stainless steel at 0.4. Ten of the fourteen predicted temperatures were within 20 to 50 °C of the measured thermocouples temperatures, while three of the other four were within about 100°C. All of the thermocouples failed during the experiment. Heat rates, and hence temperatures, were very sensitive to the outer shim control cylinders, which is typical for B experimental positions in the Advanced Test Reactor. Volume average time average temperature values were calculated and reported herein.

A three-dimensional computational fluid dynamics (CFD) electrochemical model has been created to model high-temperature electrolysis stack performance and steam electrolysis in the Idaho National Laboratory Integrated Lab Scale (ILS) experiment. The model is made of 60 planar cells stacked on top of each other operated as Solid Oxide Electrolysis Cells (SOEC). Details of the model geometry are specific to a stack that was fabricated by Ceramatec, Inc 1 . and tested at the Idaho National Laboratory. Inlet and outlet plenum flow and distribution are considered. Mass, momentum, energy, and species conservation and transport are provided via the core features of the commercial CFD code FLUENT 2 . A solid-oxide fuel cell (SOFC) model adds the electrochemical reactions and loss mechanisms and computation of the electric field throughout the cell. The FLUENT SOFC userdefined subroutine was modified for this work to allow for operation in the SOEC mode. Model results provide detailed profiles of temperature, Nernst potential, operating potential, activation overpotential, anode-side gas composition, cathode-side gas composition, current density and hydrogen production over a range of stack operating conditions. Variations in flow distribution, and species concentration are discussed. End effects of flow and per-cell voltage are also considered.

The purpose of this report is to document the results of the thermal analyses performed to calculate the Advanced Gas-Cooled Reactor (AGR)-2 as-run daily temperatures of the fuel compacts. Time average volume average data provided by this report will be used to determine fuel performance in post-irradiation examination. 8. If revision, please state the reason and list sections and/or pages being affected: NA 9. Conclusions/Recommendations: This report documents the results of thermal analyses to predict the daily as-run temperatures for the AGR-2 experiment. Control gas gaps and compact-graphite holder gas gaps were modeled to change linearly with time (fast neutron fluence). Graphite shrinkage rates were taken from the AGC-1 Experiment. Daily heat rates for each compact and component in the models were input from daily as-run physics analyses. Daily gas compositions and component fast neutron fluences were also input. Six different finite element models were created for the six different AGR-2 capsules. Each capsule had a different gas gap that was implemented to control the temperature of the experiment. Gas mixture thermal conductivity was implemented using kinetic theory of gases. Fluence and temperature-dependent thermal conductivity was used for the graphite components and fuel compacts. Radiation heat transfer was implemented with emissivity of all graphite surfaces being 1.0 and stainless steel at 0.4. Ten of the fourteen predicted temperatures were within 20 to 50 °C of the measured thermocouples temperatures, while three of the other four were within about 100°C. All of the thermocouples failed during the experiment. Heat rates, and hence temperatures, were very sensitive to the outer shim control cylinders, which is typical for B experimental positions in the Advanced Test Reactor. Volume average time average temperature values were calculated and reported herein.

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