Development of a new configuration and optimization of the 5-kW class energy conversion system combining a solid oxide fuel cell and internal combustion engine using a spark-assisted ignition

Abstract

For the optimal operation of an energy conversion system combining a solid oxide fuel cell (SOFC) and an internal combustion engine using spark-assisted ignition (SAI), a novel configuration based on natural gas was analyzed. This is the first study to determine the optimal configuration of an SOFC–SAI engine hybrid system through a case study that includes all possible configurations, considering the heat transfer quality for balance of plant (BOP) operation. A system-level analysis was conducted by integrating the single-cylinder engine experimental results with a simulation model for the SOFC and BOPs. The optimal operating conditions affecting power generation, such as load distribution and air supply related to the electrochemical and combustion reactions in the SOFC and SAI engine, were investigated using the response surface method (RSM) based on a system-level analysis. Through a comparative analysis of the configurations, the most efficient configuration was developed, which included three external reformers using the thermal energy of the anode/cathode off-gas and a heat exchanger to recover the residual heat after steam generation, equivalent to 3–4 % of the lower heating value of the fuel in the system. Based on the response surface, which is a multiple linear regression model using the least squares method (LSM), the optimal design point was investigated by considering the system operability, such as combustion stability and thermal self-sustainability. The combustion stability was confirmed through an engine experiment, and the SAI engine achieved over 90 % combustion efficiency under all operable ranges, such as fuel utilization and equivalence ratio. Consequently, the 5-kW class SOFC–SAI engine hybrid system can achieve an efficiency of ∼63.2 % at optimal operation with the newly developed design, that is, fuel utilization of ∼75 %, air utilization of ∼25 %, and equivalence ratio of ∼0.55. The SAI engine had responsibility for approximately 9 % of the hybrid system power.