## 1 Introduction  
As the demand for electric energy is rising worldwide, the amounts of harmful gases, such as carbon dioxide, generated from fossil fuels are increasing. Accordingly, the interest in various new and renewable energy sources is growing [1]. The output characteristics of new and renewable energy fluctuate according to location conditions, such as the natural environment and geographic characteristics, which mainly affect the output quality [2]. To solve this problem, energy storage systems (ESSs), which are one of the technologies for storing and supplying new and renewable energy, have attracted considerable attention. ESSs are a stand-alone system that produces, stores, and supplies electricity at the kWh scale. An ESS is divided into mechanical and electrochemical energy storage devices according to the configuration type. An example of a mechanical energy storage device is a non-battery type device that uses pumping-up power generation and compressed air energy storage. In addition, lead-acid batteries, NaS batteries, lithium batteries, and redox flow batteries (RFBs) are a few examples of electrochemical energy storage devices [3; 4]. A long cycle life, energy efficiency, and low construction cost are required to build a large capacity ESS using a chemical energy storage device. RFBs are attracting considerable attention as the most suitable devices for this purpose [5]. One of the main characteristics of the RFBs is that they can be used in large-capacity storage devices. In terms of power and energy, the system has a wide operating range, which is suitable for stationary applications where a large amount of stored energy is required [6], for example, energy can be stored during periods of low energy demand and used for periods of high energy demand, regardless of weather conditions [7]. A RFB is a representative ESS that generates or stores energy using the oxidation/reduction reaction between the active materials that constitute the anode and cathode electrolyte, and its output characteristics are determined accordingly. RFBs can be designed for the specific applications because the size of the stack that determines the output and the size of the electrolyte tank that determines the capacity are controlled independently of each other [8]. As the RFB supplies the stored electrolyte through a pump, the shape of the flow channel can affect the system efficiency [9]. Flow channel design is a critical part of the flow battery system development process, and there are two aspects that can be considered to improve the power and efficiency of the RFB. First, the overall efficiency of the RFB decreases when the electrolyte enters the electrode at an extremely low velocity or when there is reverse flow [10]. This can be solved by maximizing the reaction area by making the flow of the electrolyte uniform through an appropriate flow channel design, thereby improving the output. Second, the power consumption of the pump can be reduced by minimizing the pressure drop using a simplified flow channel [11]. Currently, many chemical species, such as zinc bromine, iron-chromium, bromine polysulfide, and vanadium are employed as RFB redox couples in ESSs [12]. Among them, vanadium with an electromotive force of 1.4 V, provides the advantages of a relatively high energy efficiency and relatively low ion crossover. Studies for improving the system efficiency of the RFB are as follows: [PERSON] et al. [13] confirmed the flow field of the parallel structure of the flow battery through computational fluid dynamics (CFD) and experiments, and the flow battery performance was calculated according to the flow of the electrolyte. [PERSON] et al. [14; 15] defined a formula for evaluating the non-uniformity of electrolyte and proposed a cell design that minimized the non-uniformity by using a commercial code verified by the experimental pressure drop data. [PERSON] et al. [16] analyzed the battery capacity and system efficiency characteristics according to the electrolyte flow rate of a vanadium redox flow battery.