Intelligent controller for a hybrid energy storage system

Abstract

The performance and range of electric vehicles are largely determined by the characteristics of the electrical energy storage (EES) device used. The EES should be sufficiently sized to be able to provide the necessary power and energy requirements of the vehicle. Batteries are typically energy dense, although batteries that are both energy and power dense exist, they are much more expensive. The life and usable capacity of batteries are negatively impacted by power impulses. Battery packs in electric vehicles (EV) are typically oversized to be able to provide enough power during these impulses experienced when the vehicle accelerates. Hybrid energy storage systems (HESS) have been proposed in the literature to solve these problems. HESS beneficially combines two or more EES devices with complementary characteristics. An additional EES device with a high-power density, such as an ultracapacitor, can be used as a buffer to provide power during power surges to reduce the power impulses experienced by the battery. Isolating the battery from the power impulses would allow the EV to utilize more energy dense batteries, increasing the range of the EV as well as increasing the lifetime of the utilized batteries. The research presented in this paper documents the implementation of an active HESS that combined a battery pack and an ultracapacitor bank. The implemented HESS was used to reduce the peak-power that the battery needs to provide to the load. An active topology utilising two DC/DC converters and a switch was used to implement the hybrid energy storage system. Fuzzy logic was used as a close-loop control structure to control the DC/DC converters in the topology, whilst a rule-based control strategy was used to control the operating states of the HESS. Experimental implementation of the system showed that the system was able to actively control the flow of power throughout the HESS in order to limit the power drawn from the battery to a user-defined limit. The performance of the fuzzy logic controllers was also experimentally found to be sufficient when used in conjunction with the rule-based control strategy. The system allows one to utilize batteries that are optimized for energy density seeing that the system was able to actively limit the power drawn from the battery, whilst providing the required power to the load by utilising the ultracapacitor bank. The controller and HESS were simulated in MATLAB®/Simulink® and practically implemented through the Simulink® Real-Time environment with a STM32 Nucleo microcontroller. The active topology reduced the peak-power drawn from the battery by 46.05% for a pulse train load profile whilst the system reduced the peak-power drawn from the battery by 90.1% for a real-world drive cycle. The developed active HESS is not only suitable for EVs, but can be used to hybridize different energy sources, such as fuel cells, photovoltaic cells and any other EES devices that have complementary characteristics.