Study of the oversized capacity and the increased energy loss of hybrid energy storage systems and design of an improved controller based on the low-pass filter

Abstract

A hybrid energy storage system (HESS) consisting of batteries and supercapacitors (SCs) is an effective approach to stability problems brought by renewable energy sources (RESs) in microgrids. This paper investigates the energy exchange between the two energy storage devices (ESDs) caused by the low-pass filter (LPF), which leads to the oversized capacity of HESSs. In addition, the energy exchange between the ESDs leads to more energy loss of HESSs. Based on the analysis of the power flows, this paper proposes an improved controller based on the LPF controller. A power direction control strategy eliminates the non-beneficial power flow to reduce the capacity of HESSs and improve the round-trip energy efficiency. In addition, a SOC control strategy regime balances the desired state of charge (SOC) of the ESDs instead of depending on the LPF. In this paper, the case study shows that the improved LPF controller reduces the capacity of the HESS to the minimized capacity and improves the round-trip energy efficiency. Furthermore, it has no adverse effect on battery aging and achieves the battery lifetime extension with a smaller capacity. A scaled-down HESS experimental setup validates the effectiveness of the improved LPF controller and the simulation results. Finally, the proposed improved controller is compared with various existing controllers to verify the performance.