Drive Cycle Energy Efficiency of Fuel Cell/Supercapacitor Passive Hybrid Vehicle System

The electric vehicle with passive hybridization of fuel cells and supercapacitors leads to lower cost and compactness due to the absence of DC/DC converters. This paper models such a vehicle and evaluates the energy efficiency of its powertrain system. The powertrain component losses, as functions of electric machine torque, speed and DC-link voltage, are modelled with a high level of detail which are verified against available test data. Compared to a pure fuel cell system, the fuel cell efficiency is higher when supercapacitors are introduced under pulse current load, and it is higher at lower current amplitude. As the pulse current frequency increases, the fuel cell efficiency also increases due to higher proportional current from the high-efficiency supercapacitors. A multiplicity of drive cycles is selected, divided into a low, middle, and high speed category to analyze the powertrain efficiency. The total powertrain energy efficiency varies between 53%-71% during propulsion for the studied drive cycles, whereas it is higher during braking ranging from 84% to 94%. The differences are closely related to the speed, acceleration, and DC-link voltage levels. The lower powertrain efficiency causes higher hydrogen consumption, leading to a reduced fuel cell efficiency at high speed, high acceleration and low DC-link voltage