To improve the power generation efficiency of a solid oxide fuel cell (SOFC) system, we tried to operate the system at extremely high fuel utilization around 90% while keeping lower fuel utilization for individual cell stacks. We designed the system with two-stage SOFC stacks and a fuel regeneration process. The system was operated at a high total fuel utilization rate of 88.4%, and the gross power generation efficiency reached DC 71.2% at its lower heating value and gross power output of 2 kW. In addition, our system was operated for more than 500 hours without degradation. The technologies of the two-stage SOFC stacks and fuel regeneration process were proven to make stable power generation possible even with high fuel utilization in the system.

A solid oxide fuel cell (SOFC) module using two-stage stacks and a fuel regeneration process between them was developed in this study for the first time, to the best of the authors' knowledge. Upon configuring the first-stage and secondstage stacks and a steam reformer between them in the SOFC module, a gross output power of DC 2.27 kW was generated with gross power generation efficiency of DC 69.2% (lower heating value (LHV)), at a total fuel utilization rate of 86.3%. This technology enables operation at a very high total fuel utilization rate even while operating the stacks at a moderate fuel utilization rate (below 70%). Considering an auxiliary device loss (6%) and inverter loss (5%), the net power generation efficiency is estimated to be AC 61.8% (LHV); hence, the module is considered to exhibit a high power generation efficiency. Further increases in the power generation efficiency could be realized in the future by removing the CO 2 from the anode off-gas during the fuel regeneration process and/or operating the stacks at higher temperatures by decreasing heat leakage from the module.