Solid oxide fuel cell operation in a solid oxide fuel cell–internal combustion engine hybrid system and the design point performance of the hybrid system

A solid oxide fuel cell (SOFC)–internal combustion engine (ICE) hybrid system is a recently-proposed distributed electric power generation system to achieave extremely high efficiency beyond that of current technologies. The objectives of this study are to find the operation characteristics of the SOFC in the hybrid system and determine the operational design point of the hybrid system. To accomplish these objectives, operation of a 5 kW-class SOFC–ICE hybrid system was analysed by integrating the experimental results of the internal combustion engine with the simulation models of other system components. Two unique characteristics of SOFC operation were found and analysed. First, the SOFC in the hybrid system should utilize anode inlet gas with low temperature (e.g., 750–800 K) and low external reforming rate (e.g., 30–40%), which decreases the SOFC temperature, especially at the entrance, where the current density becomes very low, decreasing SOFC performance. Second, the overall effects of pressure pulsation caused by the engine on SOFC operation are insignificant since the flow path between the SOFC and the engine acts as a damper, reducing the pressure pulsation amplitude. The estimated variations were 0.2 mV in SOFC cell voltage. To determine the operational design point of the hybrid system, parametric analyses of system operation were conducted while varying several control parameters, e.g., SOFC fuel utilization factor. The design point of system operation was determined by considering system performance and operational stability. Near-zero pollutant emissions and 59.0% system efficiency were achieved at the determined design point, a 7.9% absolute and 15.5% relative improvement compared to that of an SOFC stand-alone system.