Hybrid Energy Storage System for Electric Vehicle

Author Name(s): Prachi S. Hatwar, Alok Ranjan, Akshay D. Kadu, Pankaj R. Sawarkar, Sanjay B. Bodkhe, Vinay T. Barhate
Author Email: alokranjan152294@gmail.com

Abstract

Majority of the electric vehicles (EV) available commercially are powered by electrochemical batteries which have their own limitations related with cost, size, lifecycle and power density. A vehicle is supposed to perform satisfactorily under various dynamic conditions and on different road terrains requiring high torque and high motor current many times. The battery which is not a high power device is forced to deliver these high currents. Consequently, it results in reduction of battery life as well as low vehicle mileage per charging. This paper presents a hybrid energy storage system (HESS) for EV which involves coordinated usage of battery and ultracapacitor (UC) based on the shared bus topology. One boost converter and a bidirectional buck boost converter are employed to create the active parallel configuration. The proposed scheme is simulated in Matlab/Simulink environment and tested at different operating conditions. Simulation results prove the effectiveness of the proposed scheme in overcoming the limitations of battery alone EV.

Introduction

EV are believed to be an important means for reducing both, the greenhouse gas emission and energy consumption of global transport. They are potentially more fuel efficient than comparable internal combustion engines. The energy required by an EV is supplied by the battery which is the high energy density storage device. Various dynamic operating conditions like starting, acceleration, running on a positive ramp etc. requires high torque and therefore high motor current. Drawing of high current from battery reduces its life. Therefore, in order to fullfill the tractive requirements of the vehicle, the on-board batteries strength may be increased but this demands higher cost, more space and increased maintenance. Secondly, the regenerative braking causes repeated cycles of charging and discharging of the battery and leads to a depletion in the battery life. The shortcomings of battery have attracted the researchers to develop a HESS for electric vehicle. UC is an energy storage device with high power density and therefore, it can be a good companion with the battery. Proper coordination between them will satisfy both high energy density as well as high power density requirement of an EV. However, due to different performance characteristics of battery and UC, direct connection between them is not possible [1]. Many HESS topologies are proposed in literature for the hybridization of more than one energy storage devices [2-5]. Parallel active topology enables independent control over the power flow of the battery and UC. It is also called as shared bus topology. During regenerative braking, the kinetic energy of the electric vehicle can be transferred to the supercapacitor or the battery by using appropriate switching algorithm for interfacing converters. This harvested energy can be used during acceleration and also for avoiding deep discharge of the battery [4]. The system efficiency can be improved by maintaining optimum dc bus voltage if HESS is designed to have the boosting features by connecting the dc/dc converter output in series with UC [5].

Conclusion

A HESS consisting of battery and UC are proposed for the EV in this paper. The control system is designed to ensure that the battery supplies base load power while peak power demanded during dynamic conditions is provided by UC. The regenerated power from the PMDC machine is used for charging of UC as and when the vehicle runs under the influence of its kinetic energy. These measures minimize the excessive power drain from battery and reduce its repeated charging/discharging. Consequently, it will lead to enhanced battery life and improved mileage of the vehicle per charging. Though, this work is not focused on speed control, the speed of PMDC motor can be controlled by varying the dc bus voltage. The simulation results presented for different operating conditions reflect the feasibility and effectiveness of proposed scheme.

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