Numerical investigation of the transient performance of a reversible solid oxide cell during the mode switching process

Abstract

Reversible solid oxide cells (RSOC) have attracted much attention due to their unrivaled efficiency. As one of the most promising energy storage and conversion technologies, the RSOC will have to switch frequently between fuel cell and electrolysis mode to follow the nature of intermittent renewable energy sources and meet time-varying electricity demand. In this paper, a 3D dynamic model of a single repeating unit was developed and validated. Based on this model, the interaction between the transient external electrical performance with the temporal-spatial evolution of the reactant and product concentrations was investigated during the RSOC switching from SOFC to SOEC. Once the mode switching occurs, the power density decreases to a lower value and then asymptotically recovers to a new quasi-steady state. The recovery time of the concentration field within porous electrodes determines the total transient time. In addition, detailed parametric studies for the key design and operational parameters are conducted to find out their impact on the power density overshoot (PDO) and relaxation time (τ0). The results suggest that shortening the cell length can effectively reduce PDO and τ0. With the hydrogen molar fraction of the incoming fuel gas (xH2) equal to 0.2 and 0.8, respectively, the maximum values of PDO and τ0 are attained. Compared with the maximum value, when adopting 0.4 ≤ xH2 ≤ 0.6, the PDO and τ0 can be reduced by 40 % and 25 %, respectively. Extending the mode switching time can effectively reduce PDO and τ0. The power density drops directly into the quasi-steady state without oscillations when the switching time is more than 0.3 s, and the τ0 is slightly shorter than the mode switching time.