Vanadium Redox Flow Battery System Research Articles

Machine‐Learning‐Based Accurate Prediction of Vanadium Redox Flow Battery Temperature Rise Under Different Charge–Discharge Conditions

ABSTRACTAccurate prediction of battery temperature rise is very essential for designing efficient thermal management scheme. In this paper, machine learning (ML)‐based prediction of vanadium redox flow battery (VRFB) thermal behavior during charge–discharge operation has been demonstrated for the first time. Considering different currents with a specified electrolyte flow rate, the temperature of a kW scale VRFB system is studied through experiments. Three different ML algorithms; linear regression (LR), support vector regression (SVR), and extreme gradient boost (XGBoost) have been used for prediction. The training and validation of ML algorithms have been done by the practical dataset of a 1 kW 6 kWh VRFB storage under 40 , 45, 50, and 60 A charge–discharge currents and 10 L min−1 of flow rate. A comparative analysis among ML algorithms is done by performance metrics such as correlation coefficient (R2), mean absolute error (MAE), and root mean square error (RMSE). XGBoost shows the highest R2 value of around 0.99, which indicates its higher prediction accuracy compared to other ML algorithms used. The ML‐based prediction results obtained in this work can be very useful for controlling the VRFB temperature rise during operation and act as an indicator toward further development of an optimized thermal management system.

Low-cost and durable polyvinyl alcohol modified polyethylene separator for vanadium redox flow battery

Porous separators are considered a viable alternative to the high cost Nafion membrane for vanadium redox flow battery (VRFB) commercialization. However, the porous separators suffer from high vanadium ion crossover due to the presence of large micron size pores, which leads to decay in capacity of the VRFB system. To tackle the same, a composite separator is synthesized wherein a cross-linked polyvinyl alcohol (PVA) layer is coated on to the porous polyethylene silica separator (PE separator) to mitigate the vanadium ion crossover rate and employed in the VRFB application. Scanning electron microscopy (SEM) analysis confirmed the successful coating of the PVA film on to the PE separator. Synthesized PE-PVA separator showed a significant decrease in the vanadium ion crossover by 31.44 times compared to the pristine PE separator. Consequently, the self-discharge time increases by 8 and 1.66 times compared to the pristine PE separator and the Nafion117 membrane, respectively. VRFB cell equipped with the synthesized separator showed better capacity retention with a capacity fade rate of 0.2 % Ah·cycle−1 than the Nafion117 membrane (0.5 % Ah·cycle−1). In-situ and ex-situ oxidative stability of the separator is explored in detail and a degradation mechanism of the cross-linked PVA membrane is proposed accordingly. The synthesized single side coated PE-PVA´ separator exhibits higher coulombic efficiency of 99 % and comparable energy efficiency of 71 % at a current density of 120 mA·cm−2 and demonstrated excellent lifetime durability tested up to 1600 charge-discharge cycles at 80 mA·cm−2.

A comprehensive study in experiments combined with simulations for vanadium redox flow batteries at different temperatures

Ensuring the appropriate operation of Vanadium Redox Flow Batteries (VRFB) within a specific temperature range can enhance their efficiency, fully exploiting the advantages of renewable energy. This study employs a comprehensive approach combining experimentation and simulation to systematically investigate the impact of temperature on VRFB performance. Experimental evaluations of VRFB performance across various temperatures were conducted through charge and discharge experiments, electrochemical impedance spectroscopy, hydraulic pressure drop testing, and cycling charge and discharge trials. Additionally, a three-dimensional multi-field coupled model was developed to provide a theoretical analysis and elucidate VRFB performance under varying temperature conditions, incorporating temperature-dependent variations in the physical parameters of different components. The results suggest that in VRFB systems, an increase in temperature leads to a reduction in overpotential and pressure drop, resulting in improved system efficiency, albeit accompanied by a slight decrease in Coulombic efficiency. Furthermore, compared to ohmic overpotential and electrochemical overpotential, the impact of temperature on concentration overpotential appears to be relatively insignificant. Increasing the flow rate or temperature could contribute to a more stable degradation rate of capacity and Coulombic efficiency during the battery cycling process. Higher flow rates enhance the voltage efficiency of the battery, with the degree of improvement decreasing as temperature increases.

(Invited) Redox Flow Batteries – Exploring Electrolyte Additives and Hybrid Organic/Inorganic Redox Pairs

Renewable energy sources are intermittent and thus require provision to store the energy when available using energy storage devices. These energy storage devices should be safe, efficient, and have a long life cycle for use in grid-scale energy storage systems. Lead-acid, Li-ion, and Redox Flow Batteries (RFBs) are examples of such energy storage devices. Among these energy storage systems, RFBs have unique advantages that make them attractive for large-scale energy storage applications, like a high cycle life of about 10,000 cycles.[1] The current generation of commercial RFBs use vanadium solution in different oxidation states as electrolytes (anolyte and catholyte), carbon-based electrodes and Nafion as the ion-exchange membrane. High cost, risk of electrolyte crossover, low volumetric energy density, and pumping losses limit the vanadium redox flow batteries (VRFBs).[2] Also, the Nafion membrane accounts for about 40% of the system's total cost.[3] To ensure high cycle life, the electrolytes must be stable (active material must remain dissolved in the supporting electrolyte), and vanadium ions should not crossover the Nafion membrane. The VRFBs tend to lose vanadium from electrolyte solutions due to the precipitation of V2O5 during charging, resulting in a significant loss of energy density. Additives help in maintaining the long-term stability of vanadium electrolytes. In this work, we have monitored the solubility and electrochemical characteristics of vanadium electrolyte solutions with V2O5 in the presence of different additives, namely HCl and MSA (methanesulfonic acid), for over three months.[4] The findings of this study provide insight into using these additives for VRFBs. Further research in RFBs has recently shifted towards redox-active aqueous-organic-based electrolytes consisting of Earth-abundant elements (C, H, O, N, S), accommodating the need for green, safe, and low-cost energy storage. We have studied the feasibility of using quinone-based organic electrolytes for RFBs. A proof-of-concept membrane-free two-compartment cell with an auxiliary electrode in each compartment was also demonstrated. In the conventional design, the hydrogen ions move through the Nafion membrane to balance charges but in the auxiliary electrode-based membrane-free setup, the auxiliary electrode in each compartment undergoes redox reactions opposite to the primary electrodes to balance charges.[5]References[1] Y.E. Durmus, H. Zhang, F. Baakes, G. Desmaizieres, H. Hayun, L. Yang, M. Kolek, V. Küpers, J. Janek, D. Mandler, S. Passerini, Y. Ein‐Eli, Side by Side Battery Technologies with Lithium‐Ion Based Batteries, Adv. Energy Mater. 10 (2020). https://doi.org/10.1002/aenm.202000089.[2] B. Turker, S. Arroyo Klein, E.M. Hammer, B. Lenz, L. Komsiyska, Modeling a vanadium redox flow battery system for large scale applications, Energy Convers. Manag. 66 (2013) 26–32. https://doi.org/10.1016/j.enconman.2012.09.009.[3] J. Ye, D. Yuan, M. Ding, Y. Long, T. Long, L. Sun, C. Jia, A cost-effective nafion/lignin composite membrane with low vanadium ion permeation for high performance vanadium redox flow battery, J. Power Sources. 482 (2021) 229023. https://doi.org/10.1016/j.jpowsour.2020.229023.[4] O.H. Nguyen, P. Iyapazham Vaigunda Suba, M. Shoaib, V. Thangadurai, Investigating the Electro-Kinetics and Long-Term Solubility of Vanadium Electrolyte in the Presence of Inorganic Additives, J. Electrochem. Soc. 170 (2023) 110523. https://doi.org/10.1149/1945-7111/ad0a75.[5] S.V. Venkatesan, K. Karan, S.R. Larter, V. Thangadurai, An auxiliary electrode mediated membrane-free redox electrochemical cell for energy storage, Sustain. Energy Fuels. 4 (2020) 2149–2152. https://doi.org/10.1039/c9se00734b.

Electrochemical performance of MXene/Fe3O4 composite derived from recycled carbon for vanadium redox flow battery

MXenes are emerging 2D materials that have demonstrated excellent combined metallic electrical conductivity and hydrophilicity. They have been extensively studied in applications like energy storage, catalysis, optoelectronics, and so on. In this work, the carbon sourced from carbon soot extracted from fossil fuel combustion wastes was used to produce Ti3C2Tx MXene for the first time. The recyclable carbon-derived MXene/iron oxide (Fe3O4) composites with different concentrations of Fe3O4 were synthesized via a hydrothermal route. The successful synthesis of the MXene/Fe3O4 composites was confirmed by the SEM, HRTEM, EDX, UV–Vis, XRD, FTIR, and XPS analyses. Electron images morphologically revealed the existence of Fe3O4 within the as-synthesized composites, while UV–Vis spectra detected the surface plasmon resonance peaks of MXene and Fe3O4, respectively. The XRD spectra identified the crystalline structure of MXene at 5.7° (002) and 25.3° (006), as well as the magnetite Fe3O4 at 18.2° (111) and 38.2° (311), which aligned well with the presence of Ti-O and Fe-O bands observed in the FTIR data. The XPS study also confirmed the presence of the Fe element within the composites. The electrochemical performance of composite as electrode material towards the cathode VO2+/VO2+ redox reaction was investigated. The synergistic effect of both MXene and metal oxide improved the electrode kinetic activity towards the redox reaction, as evidenced by the voltage peak separation (ΔE) and the ratio of peak currents (IO/IR) values determined in cyclic voltammetry (CV), and the charge transfer process probed by electrochemical impedance spectroscopy (EIS). The specific capacity of the battery with the composites was increased along with the improvement of retention strength. The MXene/Fe3O4 5 wt% composite displayed the highest electrochemical catalytic activity with the lowest ΔE (124 mV), IO/IR of about 1, producing specific capacities of 10.2 Ah L-1 and energy efficiency of 70.1 %. The nature of MXene and Fe3O4, morphological features, and higher surface area of composites increased the kinetic activity of the electrode, enabling the composites to be potentially applied in the vanadium redox flow battery (VRFB) system.

Crossover analysis in a commercial 6 kW/43kAh vanadium redox flow battery utilizing anion exchange membrane

The volumetric and species transport is a significant source of capacity decay in vanadium redox flow batteries (VRFBs). However, even with the prevalent use of anion exchange membranes (AEMs) in commercial systems, the understanding of the crossover mechanisms in this context remains limited. The primary objective of this study was to gain a deeper comprehension of these mechanisms through experimental investigations conducted on a 6 kW/43kAh VRFB system using AEMs. It is concluded that protons serve as the primary charge carriers, owing to the high ionic concentrations of the electrolyte. During normal operation, there was a consistent pattern of volume transfer from the positive to the negative half-cell (∼0.5 % of the total electrolyte per cycle/day). Experimental assessments performed at different states-of-charge (SoCs) revealed that the volumetric transport towards the anolyte increases with SoC. The osmotic effects are concluded to be the main contributors to the volumetric transport. The osmotic pressure difference is hypothesized to arise from the asymmetric diffusion coefficients of the vanadium ions, changes in pH affecting sulfate and bisulfate equilibrium alongside their different diffusion coefficients, and a likely predominance of the bisulfate concentration gradient in the water transport through the membrane.

A flexible integrated microsensor embedded in hydrogen/vanadium redox flow battery for real-time microscopic measurement

The vanadium redox flow battery system is an emerging energy storage technology, which has many advantages in application, such as high efficiency, long life, high power, and high safety. The latest literature indicates that hydrogen/vanadium redox flow batteries (HVRFBs) have better energy density and efficiency than the vanadium redox flow battery. In addition, the hydrogen concentration, electrical conductivity, voltage, current, temperature, electrolyte flow, and runner pressure inside the HVRFBs will influence their performance and life. Therefore, this study used micro-electro-mechanical systems (MEMS) technology to develop a flexible 7-in-1 microsensor, which is embedded in HVRFBs for real-time microscopic measurements to optimize the operating conditions and enhance performance and safety.

Extending the lifetime of vanadium redox flow batteries by reactivation of carbon electrode materials.

The degradation and aging of carbon felt electrodes is a main reason for the performance loss of Vanadium Redox Flow Batteries over extended operation time. In this study, the chemical mechanisms for carbon electrode degradation are investigated and distinct differences in the degradation mechanisms on positive and negative electrodes have been revealed. A combination of surface analysis techniques such as X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and Electrochemical Impedance Spectroscopy (EIS) was applied for this purpose. In addition to understanding the chemical and physical alterations of the aged electrodes, a thermal method for reactivating aged electrodes was developed. The reactivation process was successfully applied on artificially aged electrodes as well as on electrodes from a real-world industrial vanadium redox flow battery system. The aforementioned analysis methods provided insight and understanding into the chemical mechanisms of the reactivation procedure. By applying the reactivation method, the lifetime of vanadium redox flow batteries can be significantly extended.

Shunt Current Analysis of Vanadium Redox Flow Battery System with Multi-Stack Connections

Vanadium redox flow battery (VRFB) systems with multiple stacks due to long lifetime, low self-discharge, and flexible design, are commonly used in large-scale electrical energy storage applications In a VRFB system, pumps deliver positive and negative electrolytes to each stack through a piping system including channels and manifolds. However, the electrolyte flowing between cells through channels and manifolds and the electrolyte flowing between stacks through pipes are electrically conductive. Shunt currents are generated due to the voltage difference between the cells and between the stacks, which reduce the energy efficiency of the battery system. In particular, under different load connections, the shunt currents of a multi-stack VRFB system have different distributions and cause different impairments to the system efficiency. Therefore, it is important to predict the shunt currents of the multi-stack system under different load connections before the actual construction of the system. In this paper, a multi-stack VRFB system with 120 cells was explored by circuit-based modeling to evaluate the shunt currents according to the stack configuration such as serial, parallel, and mixed connections.Then, the Coulomb efficiencies (CEs) were investigated with operating currents at 36 and 54A. The results show that shunt currents are generally more significant at the center cell of a stack than at other cells under these stack configurations and exhibit a positive correlation with the battery state of charge (SOC), i.e., larger shunt currents increase with SOC. In addition, the CEs of the system are higher at 54A compared to those at 36A due to smaller shunt current losses regardless of the stack configurations. Furthermore, regardless of the operating current levels, the multi-stack system connected by parallel loads achieves the highest CEs compared to the series and mixed connected systems due to the absence of shunt current losses in the piping system. Figure 1

Experimental and Numerical Study of Electrolyte Mixing in the Tanks of Vanadium Redox Flow Batteries: Insights from Conventional and 3D-Printed Tanks

Vanadium redox flow batteries (VRFBs) are a promising technology for large-scale grid energy storage. Despite extensive research on the electrochemical and fluid dynamic phenomena in the cells or stacks [1, 2], the impact of electrolyte flow and mixing in the tanks has received much less attention. Previous research has shown that the cell potential and battery capacity may substantially differ from those predicted under the perfect mixing assumption [3] (i.e., continuous stirred-tank reactor). Recent numerical simulations have detected the formation of segregated zones within the tanks that cause inhomogeneous electrolyte utilization and significant capacity decay [4]. The authors have confirmed these results by means of transient laminar numerical simulations [5] and experimental testing of an industrial VRFB system [6]. However, these first attempts should be carefully extended, using a well-controlled lab-scale system, so that the effects of the different components can be independently studied.In this study, we investigate the effect of electrolyte mixing on VRFB response using a medium-size lab-scale system. Two experimental campaigns were conducted to explore i) the impact of flow rate and applied current on the predicted fluid behavior, and ii) the performance of 3D-printed tanks designed for improving electrolyte mixing with reduced dead recirculation zones. In both campaigns, the transient response of the system was investigated to assess the effect of electrolyte mixing on battery performance. The experimental results were also compared with numerical simulations, which showed good agreement with the experimental data.This work makes a novel contribution to the field by using 3D-printed tanks to investigate the impact of electrolyte mixing on RFB performance. The results extend previous research on this topic and provide insights into the design of RFBs for improved energy storage. Specifically, the use of 3D-printed tanks could be a promising approach to address the issue of inhomogeneous electrolyte exploitation in RFBs. The findings of this study could serve in the design of RFBs for large-scale grid energy storage and facilitate the adoption of this technology as a sustainable energy storage solution.

A Comparative Study of Electrode Parameters of Vanadium Redox Flow Batteries for Improved Design and Performance

Large-scale energy storage has become an urgent priority to integrate variable renewable energy sources into the electricity grid. Redox flow batteries offer attractive energy storage platforms due to the ease of integration of renewable energy, higher round-trip efficiency, location flexibility, and scalability [1]. Vanadium redox flow batteries (VRFBs) have already received widespread attention due to their recent commercialization for large-scale storage applications [2]. The electrode plays a significant role in determining the performance of the VRFB, which is composed of a porous microstructure characterized by fiber diameter, porosity, specific surface area, and permeability. These properties control various parameters that affect the performance of the VRFB system, such as pressure differences between the inlet and outlet of the electrode, overpotentials, cell potential, and crossover from the membrane. Various studies have reported varying electrode porosity's influence on cell charge-discharge cycles of VRFB systems [3–5]. However, these studies lack a consistent link between porous electrode microstructures and cell performance. Hence, the interrelated nature of the individual parameters and accurately predicting their effect on cell performance pose a challenge in predicting cell performance.In this work, we present a comparative study of porous electrode parameters using a 2-dimensional physics-based model of a VRFB incorporating mass, momentum, charge transport, and kinetics. We have established a consistent relationship between electrode parameters and cell performance which is incorporated into the model to verify the dependency of overpotential, pressure difference, and crossover of ions on porosity and fiber diameter by utilizing a correlation for the surface area and permeability as a function of these parameters in the porous electrode. Pressure difference at the inlet and out of the electrode is one of the driving forces for the crossover of ions across one electrode to another, decreasing with the higher value of fibre diameters during the charging of the cell (as shown in Figure 1(a)). The effect of the operating parameter, flow rate, is negligible on the ion crossover. However, the crossover is impacted by the current, which further increases by increasing the current value, as shown in Figure 1(b). We also found that the system efficiency increases with an increase in the fibre diameter and reaches a maximum at the range of 40-70 µm, but voltage efficiency decreases with the higher value of fibre diameter, as shown in Figure 1(c). This study/analysis/work will include the impact on the crossover of vanadium ions by varying the electrode and operating parameters. The system's efficiency with and without considering the pumping energy will also be presented for various parameters. Our key findings provide important insight for optimizing VRFB design. Developing a 2D model that considers multiple key parameters enables a better understanding of the complex interactions between electrode microstructure and system performance.

Conductive nanoparticle-embedded carbon composite bipolar plates for vanadium redox flow batteries

Electrochemical reduction and oxidation reactions of vanadium ions can enable efficient power management by a secondary battery, such as a vanadium redox flow battery (VRFB). To increase the energy efficiency of a VRFB system, the electrical resistance of the cell component, such as a bipolar plate, should be reduced. In this study, particles of polyaniline (PANI), a conductive polymer, were embedded in a carbon/epoxy composite by spraying on the prepreg surface to increase the electrical conductivity of the bipolar plate. The effect of the PANI particles on the electrical and mechanical properties was investigated. Furthermore, the optimum PANI content of 3 wt% achieved an area-specific resistance of 16.7 mΩ·cm2, 55.2 % lower than that of the neat specimen. The tensile strength was improved to 586 MPa, a 13.8 % increase compared with the neat specimen. Finally, the cell performance of the fabricated bipolar plate was verified via permeability and single-cell charge/discharge tests.

A 2D Thermal Model of Stacked Vanadium Redox Flow Batteries for Efficient Thermal Management

In an industrial‐scale vanadium redox flow battery system, cells are stacked to form a compact module that serves as the power system's most fundamental unit in its scale‐up. Considering the narrow temperature range restricted by the precipitation of vanadium ions in the electrolyte and the relatively strong current employed, heat generation and dissipation determine the stack's temperature, significantly affecting the performance. Using the mass, momentum, energy conservation, electrochemical kinetics in each cell, and circuit analysis in the manifolds and channels through which electrolyte flows and is distributed to each cell, a 2D thermal model is developed in this study to investigate the temperature distribution and variation within the cell stack. Benefit from the integrated simulation of a 2D model and analytical circuit in flow frames in the stack, the temperature distribution and hotspots within the stack during the dynamic charge–discharge cycles can be precisely determined. In the results, it is indicated that the global consideration of transport and electrochemical processes demonstrate a more systematic understanding of heat generation and transfer throughout the stack.

A flow-rate-aware data-driven model of vanadium redox flow battery based on gated recurrent unit neural network

The vanadium redox flow battery (VRB) system involves complex multi-physical and multi-timescale interactions, where the electrolyte flow rate plays a pivotal role in both static and dynamic performance. Traditionally, fixed flow rates have been employed for operational convenience. However, in today’s highly dynamic energy market environment, adjusting flow rates based on operating conditions can provide significant advantages for improving VRB energy conversion efficiency and cost-effectiveness. Unfortunately, incorporating the electrolyte flow rate into conventional multi-physical models is overly complex for VRB management and control systems, as real-time operations demand low-computational and low-complexity models for onboard functionalities. This paper introduces a novel data-driven approach that integrates flow rates into VRB modeling, enhancing data processing capabilities and prediction accuracy of VRB behaviors. The proposed model adopts a gated recurrent unit (GRU) neural network as its fundamental framework, exhibiting exceptional proficiency in capturing VRB’s nonlinear voltage segments. The GRU network structure is carefully designed to optimize the predictive ability of the model, with flow rate considered as a crucial input parameter to account for its influence on VRB behavior. Model refinement involves analyzing well-designed simulation results obtained during VRB operations under various flow rates. Laboratory experiments were also designed and conducted, covering different conditions of currents and flow rates to validate the proposed data-driven modeling method. Comparative analyses were performed against several state-of-the-art algorithms, including equivalent circuit models and other data-driven models, demonstrating the superiority of the proposed GRU-based VRB model considering flow rates. Thanks to the GRU’s outstanding capability in processing time series data, the proposed model delivers impressively accurate terminal voltage predictions with a low error margin of no more than 0.023 V (1.3%) under wide operating ranges. These results indicate the efficacy and robustness of the proposed approach, highlighting the novelty and significance of accounting for flow rates in accurate VRB modeling for management and control system design.

Shunt current analysis of vanadium redox flow battery system with multi-stack connections

In vanadium redox flow batteries (VRFBs), the electrolyte flowing between cells through channels and manifolds and the electrolyte flowing between stacks through pipes are electrically conductive. Shunt currents are generated due to the voltage difference between the cells and between the stacks, and reduce the energy efficiency of the battery system. In particular, under different load connections, the shunt currents of a multi-stack VRFB system have different distributions and cause different impairments to the system efficiency. Therefore, it is important to predict the shunt currents of a multi-stack system under different load connections before the actual construction of the system. In this paper, a multi-stack VRFB system with 120 cells was explored by circuit-based modeling to evaluate the shunt currents according to the stack configuration such as having serial, parallel, and mixed connections. Thereafter, the Coulomb efficiencies (CEs), voltage efficiencies (VEs), and energy efficiencies (EEs) were evaluated at the operating current of 36 A and 54 A. The results show that shunt currents are generally more significant at the center cell of a stack than at the other cells under these stack configurations and exhibit a positive correlation with the battery state of charge (SOC), i.e., larger shunt currents increase with the SOC. In addition, the CEs of the system are higher at 54 A compared with those at 36 A due to smaller shunt current losses regardless of the stack configurations while the VEs and EEs show the opposite trend to the CEs. Furthermore, regardless of the operating current levels, the multi-stack system connected by parallel loads achieves the highest CEs and EEs compared with the series and mixed connected systems due to the absence of shunt current losses in the piping system.

A novel online learning-based linear quadratic regulator for vanadium redox flow battery in DC microgrids

This paper proposes a novel learning-based linear quadratic regulator (LQR) to overcome the long-lasting problem of model dependency in the existing vanadium redox flow battery (VRB) control approaches. Compared to the conventional model-dependent control methods, such as PI control and model predictive control (MPC), the proposed method automatically updates the optimal control policy through the online learning mechanism without any knowledge of the VRB system dynamics. The ability of the proposed method to handle uncertainties is verified by simulations under various scenarios.

Evaluation of Asymmetric Flow Rates for Better Performance Vanadium Redox Flow Battery

AbstractElectrolyte imbalance caused by water and ion crossover is one of the main factors affecting the capacity of vanadium redox flow battery system over cycling. Ion crossover and the associated water transfer towards the positive half‐cell is mainly caused by the transfer of the vanadium ions, its bounded water and the water driven by osmosis. The different viscosity of the vanadium (III) ions in the negative half‐cell builds inconsistent pressure profiles at both half‐cells during charge‐discharge cycling and therefore magnifies ions and water transfer tendency. To mitigate the effect of electrolyte imbalance, herein we report an experimental study on the effect of using asymmetric flow rates in the negative and positive half‐cells. Over different current densities of 50, 100, and 150 mA cm−2, the use of one magnitude flow factor lower in the negative half‐cell and one magnitude flow factor higher in the positive half‐cell is superior to the symmetric flow rates on reducing electrolyte imbalance and subsequently, improving the capacity retention. At 100 mA cm−2, a gain of 5 % in discharge capacity coupled with energy efficiency improvement of around 3 % is achieved using a cell with asymmetric flow rates compared to a cell with the symmetric flow rates after 50 cycles. Low magnitude of flow rate asymmetry can sufficiently compensate for the half‐cells pressures imbalance, higher increment in flow rate asymmetry showed that positive‐to‐negative crossover become more favourable.

Model-based nonlinear dynamic optimisation for the optimal flow rate of vanadium redox flow batteries

The control of the electrolyte flow rate is crucial to ensure the efficient operation of a vanadium redox flow battery (VRFB) system. In this paper, a model-based nonlinear dynamic optimisation (MNDO) method is proposed and implemented in MATLAB/Simulink to study the optimal flow rate under constant current (CC) and constant current–constant voltage (CC–CV) charging methods for a VRFB. A 5 kW/3 kWh VRFB system is considered to investigate the feasibility and accuracy of the established model. System efficiency is the optimisation objective in this study, where all case studies are carried out within 15% to 85% state of charge (SOC). The simulation results show that the proposed method enhanced battery performance by improving the overall VRFB system efficiency under different charging methods. Furthermore, the simulation results show that the CC–CV method is more energy-efficient than the CC method but requires more charging time. An in-depth analysis is carried out to discuss the underlying merits of the proposed method in balancing the losses caused by the concentration overpotential and pump energy consumption in a varying power environment. Moreover, further analyses are carried out to highlight the merits of the CC–CV charging method in saving energy while charging and reducing system imbalance. Finally, a 2D lookup table is designed based on the results from the proposed MNDO method that offers a practical controller of the electrolyte flow rate without requiring excessive computational resources. The performance of the 2D lookup table has been evaluated within 100 charging/discharging cycles. It achieves a system efficiency gain of up to 1.48% under CC–CV charging operation compared with the CC charging method and optimal conventional flow rate control method.

(Digital Presentation) Mathematical Approximations for Monitoring the State of Charge and Computing Electrolyte Volume in Vanadium Redox Flow Battery Systems

During the last years, several attempts have been carried out to provide clean energy storage as an alternative to overcome the energy demand. Vanadium redox flow batteries (VRFBs) appear as a suitable device that converts the chemical energy present in the electrolyte into electrical energy in a noiseless and clean manner. In the mentioned device, the electrolyte is polarized by using renewable energy, and later the stored energy is used in several applications from mobile to stationary scenarios. There are two fundamental design parameters to be considered in the sizing of a VRFB: the state of charge (SoC), and the electrolyte volume. The mentioned parameters are related to two independent parameters of great importance, the energy consumption per unit of time (power delivered by the device), and the amount of energy that can be stored in the system (energy capacity).The purpose of the current study is, based on collected data and theoretical approximations, analyze the SoC and the required electrolyte volume for several applications. To estimate the SoC for the different VRFB, a Nernst-modified equation is used. In this case, the number of cells was a key part of achieving the desired results. A correction factor (λ) was introduced to visualize the trend of the generated curve family in a better manner. On the other hand, to calculate the volume of electrolyte, an equation derived from the definition of electric charge is implemented. Similarly, due to the power applications range covering the study, a correction factor (γ) was also introduced. Considering the obtained results, as the correction factor increases the load state in the system will tend to present higher values of open circuit voltage and nevertheless constant values of load state. Based on the energy capacity calculations, and considering common vanadium concentrations among the electrolytes, it was found that the higher the vanadium concentration the required electrolyte volume will tend to be minor. In addition, on a large scale, there is a greater convergence of the required electrolyte volume to values greater than 10 000 m3. Figure 1. Trend of the open circuit voltage (OCV) when the state of charge (SoC) is increasing. Due to the wide range of output power analyzed in the current study, all the OCV presented values have been determined by using a correction factor (λ). Figure 1

Densification, Precipitation, and Dissolution of Vanadium Electrolyte for Vanadium Redox Flow Battery Systems

The all-vanadium redox flow battery system (VRFB) is the most mature RFB technology since it uses a single active species, which does not degrade. Rebalancing electrolytes to maintain maximum energy capacity in an RFB system that uses a single active species, or symmetrical electrolytes, is simple and enables the use of relatively low-selectivity separators, which have higher ionic conductivities.1 Conversely, finite-lifetime RFB chemistries, especially asymmetric chemistries, result in significant negative impacts on the levelized cost of storage (LCOS).2 Currently, the vanadium electrolyte used in a MWh-scale VRFB system is typically shipped to the installation site in liquid form in multiple totes and the VRFB-system tanks are filled onsite. A new approach explored in this study is to densify and precipitate the electrolyte in a solid form, which can then be quickly dissolved to its intended concentration onsite by the addition of de-ionized water.3 This approach can enable multiple benefits. First, shipping the electrolyte in solid form provides more stability and higher density during shipping, which significantly reduces shipping costs. Second, the solid electrolyte can be shipped in the actual containers that will be used onsite, which thus simplifies the required onsite work. This presentation will discuss a method used to densify the vanadium electrolyte, as well as nucleation and precipitation processes that generate the solid electrolyte form, which will quickly dissolve when deionized water is added onsite. The nucleation and precipitation process leverages recent work on the solid-liquid high-energy storage density method for RFB systems.4 References M. L. Perry, J. D. Saraidaridis, and R. Darling, “Crossover Mitigation Strategies for Redox-Flow Batteries,” Current Opinion in Electrochemistry, 21 (2020). 10.1016/j.coelec.2020.03.024 K. E. Rodby, M. L. Perry, and F. R. Brushett, “Assessing Capacity Loss Remediation Methods for Asymmetric Redox Flow Battery Chemistries Using Levelized Cost of Storage,” J. Power Sources, 506 (2021). 10.1016/j.jpowsour.2021.230085 M. L. Perry, Provisional U.S. Patent Application 63/312,999 filed on Feb. 2, 2022.Yuanchao Li and Trung Van Nguyen, “A Solid/Liquid High-Energy-Density Storage Concept for Redox Flow Batteries and Its Demonstration in an H2-V System,” J. Electrochem. Soc., 169 (2022). 10.1149/1945-7111/ac97c6