Dynamic behavior and control strategy study of CO₂/H₂O co-electrolysis in solid oxide electrolysis cells

The co-electrolysis of CO₂ and H₂O in high temperature solid oxide electrolysis cells (SOECs) is a promising energy storage method for intermittent renewable energy sources. In this paper, a three-dimensional (3D) continuum model of a 3-kW 40-cell planar SOEC stack is employed to study the dynamic behavior and control strategy under variable working conditions. The dynamic responses of stack power, current density, output H₂/CO ratio and stack temperature are evaluated for a scaled real-time wind power input over a whole day. The fluctuation of the wind power input leads to SOEC stack temperature fluctuation, which illustrates the need for temperature control. Two representative cases with voltage step changes in both endothermic and exothermic operation modes are studied to predict the temperature control by the variation of excess air ratio. The effects of excess air ratio on both the steady-state temperature gradient and the transient temperature variation rate are analyzed in both cases. The temperature fluctuation is successfully controlled by applying an excess air ratio profile that changes with the wind power input.