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

•RSOFC coupled with waste steam with different fuel recycling methods was analyzed.•Effect of external heat source temperatures on fuel recirculation was analyzed.•Fuel recirculation by ejector can result in same system efficiency with by blower.•Fuel recycled at intermediate temperature showed the highest round-trip efficiency.•Using ejector, each waste steam temperature range suits specific system scenarios. 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 °C to slightly over 550 °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 °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.