Efficiency of unitized reversible fuel cell systems

Abstract

A pilot-scale unitized reversible fuel cell (URFC) system was installed in our laboratory (AIST) and its efficiency was comprehensively evaluated. This URFC system was composed of a proton exchange membrane (PEM)-based cell/stack and balance of plant (BOP) adaptable for both electrolysis (EL) and fuel cell (FC) operation modes. First, the efficiency of this URFC system was evaluated at both the stack level and system level. Next, the efficiency was also evaluated by considering the amount of recovered heat during EL and FC modes assuming combined heat and power (CHP) applications. Then, the calculated efficiency of the URFC system at each operation mode was compared with that of specialized devices of PEM electrolyzer and PEM fuel cell as reference standards. The comparison reveals that the stack performance of the operation modes of the URFC were comparable to that of the respective specialized device for EL and FC, whereas the system performance taking into account the BOP energy consumption requires improved efficiency for the FC mode of the URFC system. Finally, analysis of the system efficiency including heat recovery from the URFC system was used to evaluate quantitatively the potential of heat recovery, revealing that recovery and utilization of thermal energy are essential for improving the efficiency. It was identified that the round-trip efficiency of the system would be improved from 0.182 to 0.589 by utilizing thermal energy.