Dynamic behavior of high-temperature CO2/H2O co-electrolysis coupled with real fluctuating renewable power

•A 2D SOEC model for dynamic behavior analysis is developed.•The model employs real-time photovoltaic power supply for co-electrolysis.•A high CO2 mole fraction can help inhibit temperature fluctuation.•The inlet gas temperature has little effect on H2/CO ratio at high voltages.•Output H2/CO ratio of 60% gas utilization is higher than that of 80%. Direct utilization of fluctuational renewable powers leads to rapid changes of working conditions and brings difficulties in the operation of solid oxide electrolysis cells (SOECs). Herein, a multi-physics SOEC model is established to investigate its dynamic characteristics using a real photovoltaic power supply for co-electrolysis of H2O and CO2. Dynamic responses of key performances including the current density, the average SOEC temperature, the H2O/CO2 conversion rate and the output H2/CO ratio are analyzed over a whole day. It is found that a high CO2 mole fraction can help inhibit average temperature fluctuation, where the maximum temperature difference decreases from 110 to 57 K with the inlet CO2 mole fraction increasing from 0.2 to 0.8. Besides, the largest temperature gradient occurs in the middle of the cell in the morning and gradually migrates to the inlet. Generally, a high inlet gas temperature can increase the outlet H2/CO ratio especially at low voltages. The outlet H2/CO ratio is also found to be closely related with the gas utilization rate, where a utilization rate of 0.6 shows 10% higher H2/CO ratio than that of 0.8. This study can provide a guideline for the performance optimization of SOECs with fluctuating power supply.