Optimal Sizing of a Battery-Ultracapacitor Hybrid Energy Storage System for Urban Transit Buses

This study investigates - for city buses operating in typical low speed start-stop driving patterns - the potential improvement in battery life obtained using a battery ultracapacitor hybrid energy storage system (HESS). A simulation tool for estimating the energy consumption of battery powered as well as HESS-powered electric buses is developed. A rule-based control strategy to determine the power-split between the battery and ultracapacitor is designed. The optimal number of ultracapacitor cells in series and parallel, and the threshold power, a key control strategy parameter, are determined using a genetic algorithm. The objective is to minimize a cost function, that is a combination of the battery capacity loss over 100 cycles of constant power charging and discharging over the Manhattan Bus cycle (MHBC), the fraction of the ultracapacitor cost relative to that of the battery powered bus, and the fraction of net usable energy contained in the ultracapacitor that remains unutilized. The simulation of the battery capacity loss due to cycling upto the end of life (20% loss in capacity) shows that with the HESS, the battery is able to withstand about 35% greater number of cycles of discharging over the MHBC followed by constant power charging at 100 kW.