Effect of Supercapacitor Modelling and Unit Cell Capacitance Selection Towards Economic Sizing of Energy Storage System in Electric Vehicle

Supercapacitors (SCs) are well established as Energy Storage System (ESS) in pure Electric Vehicles (EVs) as well as in hybrid EVs due to their numerous advantages like high power density, longer lifetime, high efficiency, etc. In this paper, the effect of SC modelling and elementary cell capacitance selection on the sizing of ESS for a SC-based city bus is investigated. The bus operates on an urban route with few stops in between, covering a total distance of 240 km in its whole. We present two methods for unit cell capacitance selection considering five different Maxwell SC cells: a) simple analytical method based on powertrain and driving cycle specifications b) complex optimization-based sizing with high fidelity model of the powertrain. The optimization approach determines the ideal size of the Supercapacitor Module (SCM) by minimizing the cost and mass of the ESS investment using the particle swarm optimization algorithm. Further, considering three well-established equivalent circuit models of SC and identifying the model parameters of SC cells using experimental data, we analyze their performance on sizing and compare based on accuracy and robustness. The results indicate that both SC modelling and unit cell capacitance choices substantially affect the construction of the SCM. Finally, recommendations are made regarding the SC model and unit cell capacitance that should be used to address the various design goals of ESS as well as the concerns of manufacturers, consumers, and Original Equipment Manufacturers (OEMs) of EVs.