Sensitivity analysis of a solid oxide co-electrolysis cell system with respect to its key operating parameters and optimization with its performance map

This study performs thermodynamic optimization of solid oxide co-electrolysis cell system's key operating parameters (current density, current-to-reactant ratio, and air ratio) by using high-fidelity and empirical-based system component models. For each operating parameter, a sensitivity analysis is conducted to elucidate optimal operating points for high system performance evaluated by indices such as system efficiency, reactant conversion, and H₂:CO ratio. At a high current density of 1.2 A/cm², the highest system efficiency of 58.21%, the highest reactant conversion of 63.84%, and the lowest H₂:CO ratio of 1.258 are obtained. Likewise, a high current-to-reactant ratio of 0.9 allows obtaining the high system efficiency of 58.26%, the high reactant conversion of 83.79%, and the lowest H₂:CO ratio of 1.230. Contrary to the two parameters, at the lowest air ratio of 3, the maximum system efficiency of 63.38% and the maximum H₂:CO ratio of 1.516 are obtained, whereas the highest reactant conversion of 62.78% is obtained at the highest air ratio. Based on the single parameter analysis, a performance map for each system performance index is derived as a function of current density and current-to-reactant ratio under the fixed air ratio given that the air ratio is more related to the thermal inertia of the stack. To gain high system efficiency and high reactant conversion, a high current-to-reactant ratio of 0.75 ∼ 0.90 is necessary. The high current density of 1.2A/cm² is also recommended for obtaining high performance, a small temperature gradient inside the SOEC stack, and small temperature variation with the change of the current-to-reactant ratio. However, if the system aims to obtain a low H₂:CO ratio, low current-to-reactant ratio and low current density are suggested at the expense of system efficiency and reactant conversion. The maximum system efficiency is obtained at 1.2A/cm² and the current-to-reactant ratio of 0.75, and the maximum reactant conversion of 88.91% is obtained at 1.2A/cm² and the current-to-reactant ratio of 0.90. On the other hand, the highest H₂:CO ratio is obtained at 0.2 A/cm² and the current-to-reactant ratio of 0.15. The extensive results for the solid oxide co-electrolysis cell system obtained in this study enable figuring out the coupling effects of key operating parameters and capturing optimal operating conditions of a solid oxide co-electrolysis cell system without performing costly experiments under various operating conditions.