전류 및 유량 조건에 따른 VRFB 시스템 특성 분석 및 BMS 알고리즘 설계

Abstract

Analysis of VRFB system characteristics and BMS algorithm design according to current and flow conditions Jung, HyeonHong Advisor : Prof. Lee, Seongjun, Ph.D. Department of Mechanical Engineering, Graduate School of Chosun University In recent years, the renewable energy industry has been attracting attention in the energy sector due to resource depletion problems and environmental pollution. Due to a chronic problem with renewable energy, renewable energy generation systems have intermittent generation characteristics depending on environmental conditions. Energy storage systems (ESSs), which are applied for stable and efficient power use, such as frequency regulation and peak load reduction, are also growing steadily to solve the problems of renewable energy. ESS systems are currently used as energy storage devices for renewable energy generation systems such as solar, wind, and hydropower. They are made of various types of batteries, including lead-acid batteries, lithium-ion batteries, RFBs, Nas, super capacitors, and CAES. In most applications, lithium-ion batteries with high energy density and efficiency are applied. However, as is known from the large-capacity lithium batteries applied to electric vehicles, the limitations of lithium-ion battery life and fire vulnerability are also problems that occur in ESSs. Accordingly, VRFBs, which have the strengths of longer life and fire safety due to aqueous electrolytes, are being actively studied recently. VRFBs have lower energy density and efficiency than lithium-ion batteries, and there are high initial installation costs. In addition, it has a disadvantage that the capacity decreases due to the electrolyte crossover phenomenon, in which the electrolyte volume becomes unbalanced due to the difference in ion concentration during charging and discharging. Therefore, research is needed to increase the efficiency of VRFB systems, including research on optimal efficiency operation for efficient energy management and use of VRFBs and research that can recover capacity through the rebalancing process of mixing the electrolyte of the anode and cathode to solve the capacity reduction caused by the electrolyte crossover phenomenon. As a result, research on VRFB optimal efficiency operation and rebalancing, as mentioned earlier, is being conducted on VRFBs recently. Previous studies have presented results on battery efficiency under different current and flow conditions, but internal parameter analysis and available power for optimal efficiency operation were not analyzed. In addition, research is being conducted on capacity and state estimation for efficient energy management, safety and life management of VRFBs. However, the results are presented for short-term experimental conditions with little capacity change, without considering the electrolyte rebalancing process. Therefore, this paper presents the results of voltage/current analysis, charge/discharge efficiency, energy efficiency, system efficiency, internal parameter analysis, and available power for optimal efficiency operation through 40W-class VRFB stack charging/discharging cycling experiments considering currents and electrolyte flow conditions, OCV and internal parameter measurement cycling, and DCIR experiments. In addition, we propose a VRFB capacity estimation method using the cumulative charge amount and electrolyte volume used in long-term cycling conditions with rebalancing at the 10W-class single cell of VRFBs. We also propose the state of charge estimation method based on current integration method using the capacity estimation model presented in this study.