Capacitors vs. Batteries in a Serial Hybrid Electric Bus*

Serial hybrid electric vehicles are propelled by electric traction motors, where electric power is delivered by an engine-generator set and an energy storage system. This paper compares the use of electric double layer capacitors (EDLCs) to that of NiMH-batteries in a 24 m long bi-articulated serial hybrid electric bus for public transportation in terms of fuel economy of the two systems. In order to analyze the influence of buffer size, the buffer-power ratio (ratio of maximum power of buffer and generator) is varied, while the overall power-to-weight ratio of the vehicle is held constant. As a reference, a diesel engine powered bus is simulated on the same driving mission. The vehicle designs are compared based on their optimal fuel economy, which is evaluated using dynamic programming. The results show that EDLCs outperform batteries for buffer-power ratios above 40%. This indicates that for the serial configuration, high buffer efficiency and specific power are more important than high energy density. In order to achieve better fuel economy than conventional, the hybrid is required to have a minimum buffer-power ratio, which is about 20% for urban driving, and 30% for mixed driving. The maximum fuel saving potential compared to conventional is about 16% for urban, and 8% for mixed driving.