Comparison of a One-Dimensional Model of a High-Temperature Solid-Oxide Electrolysis Stack With CFD and Experimental Results

Abstract

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

Optimized Flow Sheet for a Reference Commercial-Scale Nuclear-Driven High-Temperature Electrolysis Hydrogen Production Plant

“…The electrolyzer model has been validated by comparison with results obtained from a fully 3-D computational fluid dynamics model and by comparison with experimental results. These comparisons may be found in Reference [3].…”

“…However, higher system pressures also require heavier and more expensive components, and can have a negative impact on system performance and reliability. Based on these considerations, and the results of analyses performed at system operating pressures of 3.5 MPa and 7.0 MPa which indicated slightly lower overall hydrogen production efficiencies at the higher HTE operating pressure [3], an HTE operating pressure of 5.0 MPa was selected for the reference design. The decision to operate at 5.0 MPa was also influenced by the need to deliver the hydrogen gas at elevated pressure for either storage or pipeline transport.…”

“…An additional process heater is used to directly add heat during the electrolysis process to maintain isothermal electrolyzer operating conditions. The custom electrolyzer module developed at INL for direct incorporation into the UniSim system analysis code has been described in detail previously [3]. Downstream of the electrolyzer, the hydrogen -rich product stream flows through the electrolysis recuperator where the product stream is cooled and the inlet process stream is preheated.…”

Characterization of the Transient Response of the ILS with One Module Installed to Heatup Changes in Power Level and Cooldown

“…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. Details concerning this one-dimensional model and its implementation in UniSim have been reported in [2,3].…”

“…As explained in references [2,3], when the electrolysis process is operated below the thermal neutral voltage (V tn = 1.287 V/cell for 800°C operating temperature), heat must be added to overcome the endothermic reaction heat requirement. At thermal neutral conditions, the module operation is adiabatic and isothermal.…”