Optimum Battery Size for Fuel Cell Hybrid Electric Vehicle— Part I

This study explores different hybridization levels of a midsized vehicle powered by a polymer electrolyte membrane fuel cell stack. The energy buffer considered is a lead-acid-type battery. The effects of the battery size on the overall energy losses for different drive cycles are determined when dynamic programming determines the optimal current drawn from the fuel cell system. The different hybridization levels are explored for two cases: (i) when the battery is only used to decouple the fuel cell system from the voltage and current demands from the traction motor to allow the fuel cell system to operate as close to optimally as possible and (ii) when regenerative braking is included in the vehicle with different efficiencies. The optimal power-split policies are analyzed to quantify all the energy losses and their paths in an effort to clarify the hybridization needs for a fuel cell vehicle. Results show that without any regenerative braking, hybridization will not decrease fuel consumption unless the vehicle is driving in a mild drive cycle (city drive with low speeds). However, when the efficiency of the regenerative braking increases, the fuel consumption (total energy losses) can be significantly lowered by choosing an optimal battery size.