Waste heat utilization in reversible solid oxide fuel cell systems for electrical energy storage: Fuel recirculation design and feasibility analysis

This study investigates a reversible solid oxide fuel cell system coupled with an external waste heat source for use of electrical energy storage. Steam from waste-fueled boilers is used as an external heat source over temperatures ranging from 150 degrees C to slightly over 550 degrees C. We considered three fuel recirculation configurations that represent the different temperatures of recirculated fuel. We also investigated the effect of different devices, including a recycle blower and ejector. Our results showed that fuel recirculation increases the overall round-trip efficiency up to 6.0% points when waste steam is at 500 degrees C. However, recirculation at the highest temperature did not lead to the highest efficiency improvement in the ejector systems. Instead, the warm-ejector configuration, our intermediate temperature case, resulted in the highest efficiency. Furthermore, the warm-ejector configuration showed efficiencies close to those of the blower system at a wide range of operating current densities. We also determined a range of external heat source temperatures that is favorable or workable for specific ejector system configurations so that the fuel recirculation configuration could be designed accordingly to achieve best performance.