Graphical and mathematical analysis of fuel cell/battery passive hybridization with K factors

Hybridization is a promising method for enhancing the quality of the power supplying system including fuel cells which is not capable of meeting load demand statically or dynamically. Though there have been much research advances on hybridization, systematic studies are insufficient to reveal fundamental characteristics. In this study, we systematically categorize passive hybrid topologies, which are battery state of charge (SoC) controlled, fuel cell relative humidity (RH) controlled, and battery-fuel cell controlled, respectively. Each hybrid topology can be analyzed based on the graphical and mathematical method for fundamentally understanding and designing the hybrid system. First, in the graphical method, I-V curves, which represent the characteristics of the intrinsic properties of each device, are used for the understanding of the current sharing and power sharing of the hybrid system. Second, the mathematical method based on the relations deduced from each characterization curve is used for a more detailed understanding on topology to find key factors of hybridization. The results show that the power sharing of hybridization is strongly connected to the fundamental properties of each device, and it can be expressed by a combination of two factors K_V and K_R, which represent the electrical potential and internal resistance ratio of each device, respectively.