Method for modeling all-vanadium redox flow battery and improving coulombic efficiency

Abstract

The invention discloses a method for modeling an all-vanadium redox flow battery and improving the coulombic efficiency of the battery, aiming at the composition and principle of VRB, a circuit model comprising electrochemistry, fluid mechanics, temperature, bypass current and vanadium battery multi-stack equivalent loss is established, and the influence of pumping loss and bypass current in an all-vanadium redox flow battery pipeline system on the vanadium battery is explored. By establishing the relation between the pumping loss and the pumping loss current, a bypass current model and a multi-stack series vanadium battery equivalent loss circuit model, all the models are converted into a whole, the kernel mechanism of the vanadium battery considering the dynamic response is elaborated, and the dynamic response of the vanadium battery is analyzed according to key factors involved in modeling research of the VRB, including the pumping loss and the bypass current. And how the factors comprehensively influence the performance and the efficiency of the battery is elaborated, and an idea is provided for pipeline design and improvement of the energy efficiency of the battery for manufacturers.

Description

Technical Field

[0001] The invention relates to the field of all-vanadium liquid flow battery modeling and battery coulombic efficiency, and in particular to a method for all-vanadium liquid flow battery modeling and improving coulombic efficiency. Background Art

[0002] In the development of energy technology in recent years, vanadium liquid flow battery (VRFB) has become an important solution in large-scale energy storage system due to its excellent scalability, long life and excellent safety. In order to further optimize the performance and economic efficiency of this battery system, academia and industry are constantly exploring ways to improve its operating efficiency.

[0003] The multi-field coupling model of all-vanadium liquid flow batteries has been studied at home and abroad, combining the fluid mechanics model, electrochemical model, temperature model, and circuit model, and combining the "three transmissions and one reverse" in VRB with the electrical relationship. However, it is limited to the small tank system of a single-stack battery. However, the bypass current of the battery in a large-scale system of multiple stacks is an issue that cannot be ignored. The study did not consider the impact of the bypass current model on the battery; Ye Qiang's team at Shanghai Jiaotong University regarded the bypass current channel as an ohmic resistor, established a circuit equivalent model of the bypass current, and studied the impact of the bypass current on the coulomb efficiency and energy efficiency of the system, but did not model the battery's electrochemical model, fluid mechanics model, temperature model, and equivalent circuit loss model. Only a DC power supply was used to represent the battery, ignoring the complex changes and coupling relationships of the battery; Li Xin's team at Hefei University of Technology revealed the impact of the relevant parameters of the vanadium battery pipeline on the pumping loss of a single-stack all-vanadium liquid flow battery.

[0004] In order to understand the bypass current loss of VRB under different design conditions, the present invention studies its influence on battery performance by establishing a refined vanadium battery model including electrochemical model, fluid mechanics model, temperature model, and equivalent circuit loss model. The influence of battery pipeline electrolyte (equivalent to resistance) on bypass current in battery string modeling is analyzed and discussed through matlab-simulink joint simulation; finally, the influence of bypass current and pumping loss on battery performance is comprehensively analyzed. The results show that in the vanadium liquid flow battery system, the use of longer and thicker pipelines can effectively reduce the energy loss caused by the flow resistance of the electrolyte, and at the same time can reduce the local bypass current caused by the excessively high electrolyte flow rate; at the same time, the flow characteristics of the electrolyte, the friction resistance of the pipeline and the energy efficiency requirements of the system should be considered in the system design, and the length and diameter of the pipeline should be reasonably selected to ensure a higher coulomb efficiency. Summary of the invention

[0005] The purpose of the present invention is to provide a method for modeling an all-vanadium liquid flow battery and improving the coulombic efficiency. By establishing the relationship between pumping loss and pumping loss current, a bypass current model, and an equivalent loss circuit model of multiple stacks of vanadium batteries in series, each model is "integrated", the core mechanism of the vanadium battery taking into account the dynamic response and the key factors involved in the modeling research of VRB, including pumping loss and bypass current, and how these factors comprehensively affect the performance and efficiency of the battery, thereby improving the efficiency.

[0006] The technical solution of the present invention is as follows:

[0007] Compared with the prior art, the present invention has the following beneficial effects:

[0008] Comprehensiveness and accuracy: By building a comprehensive model that includes bypass current (S1), electrochemistry (S2), fluid mechanics (S3), and temperature (S4), this method can fully reflect the actual working state of the all-vanadium liquid flow battery. This multi-dimensional and multi-level modeling approach greatly improves the accuracy and reliability of the model and helps to more accurately predict battery performance.

[0009] Application of equivalent loss circuit model: Establishing the equivalent loss circuit model (S5) and combining it with the previously established model can more accurately simulate the energy loss of the battery during actual operation. This helps to evaluate the battery's energy conversion efficiency and system stability, and provides an important basis for the optimal design of the battery.

[0010] Scientific basis for coulombic efficiency optimization: By establishing a coulombic efficiency model (S6) and changing the pipeline parameters to observe the changes in the battery model, this method can scientifically evaluate the impact of different pipeline sizes on the battery coulombic efficiency. This helps to find the best pipeline size configuration, thereby optimizing battery design, improving coulombic efficiency, and reducing energy consumption costs.

[0011] Reduce R&D costs and time: Compared with traditional trial and error or empirical methods, this method can significantly reduce R&D costs and time by guiding battery design optimization through scientific modeling and simulation analysis. This will help accelerate the commercialization of all-vanadium liquid flow batteries and promote their widespread application in the field of energy storage.

[0012] High-efficiency energy storage: All-vanadium liquid flow batteries have the characteristics of high efficiency and long life, and have broad application prospects in the field of energy storage. Through modeling research, battery design and performance can be optimized to improve energy conversion efficiency. The problem of signal demodulation and wave head identification of the initial fault traveling wave after the nonlinear multi-component signal is decomposed into a single-component signal is solved.

[0013] Renewable energy integration: All-vanadium flow batteries can be combined with renewable energy sources such as solar and wind to achieve energy balance and stable output. Modeling can help optimize the integration and management of energy systems.

[0014] Environmental protection and sustainable development: All-vanadium flow battery is an environmentally friendly energy storage technology that does not produce pollutants and helps reduce the impact on the environment. Modeling can help evaluate the environmental friendliness and sustainability of the battery.

[0015] Driven by innovative technologies: As an emerging technology, the modeling research of all-vanadium liquid flow battery can promote technological innovation and development in related fields and promote progress in the entire energy field. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 All-vanadium redox flow battery series model.

[0017] Figure 2 It is the equivalent circuit diagram of liquid flow battery and pipeline.

[0018] Figure 3 This is a block diagram of a hybrid model of an all-vanadium liquid flow battery string taking into account bypass current. DETAILED DESCRIPTION

[0019] The present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0020] The object of the present invention is to provide a method for modeling an all-vanadium liquid flow battery and improving coulombic efficiency, so as to solve the modeling and efficiency optimization problems of multiple-stack all-vanadium liquid flow batteries proposed in the above background technology.

[0021] A method for modeling an all-vanadium liquid flow battery and improving coulombic efficiency comprises the following steps:

[0022] S1: Establish the bypass current model part of the overall model of all-vanadium liquid flow battery;

[0023] S2: Establish the electrochemical model part of the overall model of all-vanadium liquid flow battery;

[0024] S3: Establish the fluid mechanics model part of the overall model of all-vanadium liquid flow battery;

[0025] S4: Establish the temperature model part of the overall model of all-vanadium liquid flow battery;

[0026] S41 stack temperature change

[0027] S42 Liquid storage tank temperature change part

[0028] S43 Effect of temperature on electrolyte concentration, viscosity and ion diffusion coefficient

[0029] (2) S5: establish the equivalent loss circuit model part of the overall model of the all-vanadium liquid flow battery; S1-S4 are coupled to S5;

[0030] (3) S6: Establish the coulombic efficiency model of the all-vanadium liquid flow battery, change the pipeline parameters to observe the change results of the vanadium battery model, judge the change results through the coulombic efficiency model, find the optimal pipeline size, so as to optimize and improve the battery coulombic efficiency.

[0031] See also Figure 1-Figure 3 The details are as follows:

[0032] 1 All-vanadium redox flow battery All-vanadium redox flow battery series model

[0033] The core part of the all-vanadium flow battery is the battery stack, which is usually composed of two electrode plates (anode and cathode) and an ion exchange membrane. The electrode plates are generally made of graphite felt or carbon paper, which have good conductivity and chemical stability. The ion exchange membrane usually uses a proton exchange membrane (such as Nafion), which allows hydrogen ions to pass through but prevents other ions from penetrating. The electrolyte storage system includes two independent storage tanks, which store the positive electrode electrolyte (containing VO2 + / VO 2+ ion solution) and the negative electrode electrolyte (containing V 2+ / V 3+ ions). The two storage tanks are connected to the battery stack through pipes and pumps to form an electrolyte circulation system. All-vanadium liquid flow batteries use the redox reaction of vanadium to store and release energy. The battery system includes two electrolytic cells, one of which contains an anode solution containing oxygenated vanadium ions and the other contains a cathode solution containing trivalent vanadium ions. The two solutions are connected by an electrolyte fluid called an anode liquid flow circulation body and a cathode liquid flow circulation body. During the charging process, an external power supply applies a voltage so that the oxygenated vanadium is oxidized to vanadium ions at the anode, while the trivalent vanadium is reduced to metallic vanadium at the cathode. These ions flow in the solution to complete the charging process. During the discharge process, these reactions are reversed, and the vanadium ions recrystallize into oxygenated vanadium or metallic vanadium in the solution, releasing the stored energy. The current collector (the liquid flow circulation body between the electrodes) plays a key role in the operation of the battery. It is responsible for transferring the electrons generated by the electrode reaction, thereby realizing energy storage and release. Figure 1 Shown is a series model of all-vanadium liquid flow batteries.

[0034] The specific reaction formula is as follows:

[0035] positive electrode:

[0036] negative electrode:

[0037] In the battery series configuration, it is necessary to use large tanks to store electrolytes. Large tanks of electrolyte can provide more active materials for the entire system, which means that more energy can be stored and released in a single charge and discharge cycle. Compared with configuring multiple small tanks, using a single or several large tanks can reduce manufacturing and maintenance costs and facilitate unified management of the electrolyte status, such as concentration, temperature, and chemical purity. However, in the battery series configuration, bypass current (i.e., current that bypasses the normal electrochemical reaction path) is a key issue that must be considered because it can lead to energy loss and reduced system efficiency. In order to effectively manage and optimize the battery series system, it is crucial to establish an accurate series circuit model.

[0038] 2 All-vanadium liquid flow battery string bypass current model

[0039] An equivalent circuit model of the flow battery stack is established, and a set of linear equations is established based on Kirchhoff's current and voltage laws to solve the bypass current. Figure 2 for Figure 1 The equivalent circuit of the CLP stack system, each battery is equivalent to a power supply and a resistor, each section of the pipeline filled with electrolyte is equivalent to a resistor. Due to the different lengths and diameters of the main pipe and the branch pipe, the main pipe and the branch pipe are simplified to the equivalent resistance R. The ohmic losses of the main pipe and the branch pipe are different, where the subscript m represents the main pipe, c represents the branch pipe, and Ri represents the internal resistance of the battery. Figure 2 It is the circuit model of the bypass current pipeline.

[0040] RNm, RNc, RPm, and RPc represent the equivalent resistances of the negative electrode main pipe, the negative electrode branch pipe, the positive electrode main pipe, and the positive electrode branch pipe, respectively.

[0041] The current at each node in the above figure satisfies Kirchhoff's current law:

[0042]

[0043] Where m is the number of branches at the node.

[0044] The equivalent resistance of the main pipe and branch pipe meets the following requirements:

[0045]

[0046] Where ρ is the resistivity of the electrolyte; L k and A k is the length and cross-sectional area of ​​the pipe.

[0047] 3 Hybrid model of all-vanadium liquid flow battery taking into account bypass current

[0048] 3.1 Electrochemical model

[0049] In the electrochemical process of the all-vanadium liquid flow battery (VRFB), mathematical models can help understand and predict the performance of the battery. The following will introduce the main electrochemical reaction models and some important equations, including the Nernst equation and the dynamic differential equations of vanadium ion concentration in the storage tank and the stack.

[0050]

[0051] In the above formula, Us represents the single stack voltage of the battery; Ueq represents the standard electrode potential; Tstack represents the temperature of the electrolyte flowing into the stack; R represents the universal gas constant; z is the number of electron transfers in the electrochemical reaction; Cs2, Cs5, Cs3, and Cs4 represent the concentration of vanadium ions in the stack, and the numbers represent the valence of the reaction ions. The input is the valence of the ions, which changes with battery charging and discharging, electrolyte circulation, and ion transmembrane reactions.

[0052] The concentration of vanadium ions in the storage tank and the battery stack is determined by their dynamic differential equations, as follows:

[0053]

[0054] In the formula, the subscripts s and t represent the battery stack and the liquid storage tank respectively, the numbers represent the ion valence, and V in the text represents the volume, so Vs and Vt represent the volumes of the battery stack and the liquid storage tank, and the following text follows this logic; C represents the vanadium ion concentration; Q is the flow rate; Ncell represents the number of single-chip batteries; k is the vanadium ion diffusion coefficient; S represents the ion exchange membrane area; d represents the ion exchange membrane thickness.

[0055] 3.2 Fluid mechanics model

[0056] When the electrolyte flows through the pipeline, the stack electrode and the stack flow channel, it will produce a flow pressure drop, thereby generating pump power loss. The present invention takes into account the pipeline friction loss pressure drop, which is related to the length Lk of the pipeline, the diameter Dh of the pipeline, the friction coefficient fd, the electrolyte flow rate v, the conductivity ρ, the pipeline design, etc.; the electrode flow pressure drop is related to the electrode length Le, the cross-sectional area Ae, the permeability Ke, the electrolyte viscosity μ, the fiber diameter df of the electrode carbon felt material, the Kozeny-Carman constant K, and the porosity ε of the electrode; the stack flow channel is relatively complex, and existing literature has simplified the calculation method of flow resistance by using the finite element method, and the data is quoted here. A is the cross-sectional area of ​​the pipe, L is the length of the pipe, and Rcell is the flow resistance. The expression is as follows:

[0057]

[0058] 3.3 Temperature model

[0059] Temperature is one of the important parameters of the performance of all-vanadium redox flow batteries. It mainly affects the viscosity, density, concentration, and ion diffusion coefficient of the electrolyte; changes in electrolyte density and viscosity mainly affect the calculation of losses in the fluid mechanics model, while concentration and ion diffusion coefficient mainly affect the electrochemical model.

[0060] 3.3.1 Stack temperature change

[0061] The temperature of the electrolyte inside the battery stack is mainly affected by the following four factors: the heat of the electrolyte transport in the stack tank, the outside air temperature, the self-discharge reaction, and the heat generated by the battery stack. In order to better understand and predict the impact of these factors on the electrolyte temperature, we can establish a model to describe the temperature change inside the battery stack by studying each factor. The relationship is as follows:

[0062]

[0063] In the equation, Cp represents the specific heat capacity of the electrolyte, Hs represents the total heat transfer capacity of the stack; ΔH2, ΔH3, ΔH4 and ΔH5 represent the enthalpy changes of different vanadium ions in the self-discharge reaction; QC p ρ(T pipe -T stack ) represents the heat generated during the transfer of electrolyte from the pipeline to the battery stack; H s (T air -T stack ) represents the effect of the outside air temperature on the electrolyte temperature in the battery stack, I 2 R represents the rate at which heat is generated by the internal resistance of the fuel cell stack.

[0064] 3.3.2 Temperature change of liquid storage tank

[0065] The temperature of the liquid storage tank is mainly affected by two factors: the heat of electrolyte transportation and the change of external temperature. The temperature change formula is:

[0066]

[0067] In the formula: QC p ρ(T pipe -T stack ) represents the heat generated by the electrolyte during transportation between the storage tank and the pipeline; Hn represents the overall heat transfer capacity of the storage tank; H n (T air -T stack ) represents the effect of external temperature on the electrolyte temperature in the tank.

[0068] 3.3.3 Effect of temperature on electrolyte concentration, viscosity and ion diffusion coefficient

[0069] The electrolyte density is mainly affected by the vanadium ion concentration, sulfuric acid concentration and temperature; the electrolyte concentration is affected by the vanadium ion concentration and temperature.

[0070] Due to the complex quantitative relationship, A. Mousa proposed an empirical formula for electrolyte concentration and density based on experimental data, as shown in equations (14) and (15):

[0071]

[0072] A1, B1, C1, D1, E1, and F1 in the formula are empirical coefficients, which are 1.003 20, 0.163 22, -0.006 56, 0.073 63, -0.004 91, and -0.000 61 respectively; is the concentration of divalent vanadium ions; is the concentration of sulfuric acid.

[0073]

[0074] In the formula, A2, B2, D2, E2, and F2 are empirical coefficients, which are 0.838 8, -1.324 6, 1.130 0, -0.096 9, and 0.001 3 respectively; c is the vanadium ion concentration.

[0075] The ion exchange membrane cannot completely isolate the positive and negative active substances, so each valence vanadium ion has a corresponding ion diffusion coefficient, and its relationship with temperature is as follows:

[0076]

[0077] In the formula, A0 is the pre-factor; Ea is the activation energy of the diffusion process.

[0078] 3.4 Equivalent loss circuit model

[0079] The equivalent loss circuit model mainly reflects the electrical relationship between various components during the operation of the battery. It mainly includes the charge and discharge current Id; the pump loss current Ipump is caused by the power consumption of the pump when the electrolyte circulates inside the battery, which is closely related to the flow rate, viscosity and pipeline design of the electrolyte; the bypass current Ishunt is the loss of part of the current bypassing the electrode and flowing through the electrolyte or other conductive paths due to the voltage difference between the batteries, which is related to the battery stack design and pipeline structure; the fixed loss Rf is the energy consumption of the battery management system (BMS), the power consumption of the controller and the power consumption of the cooling system; the total resistance generated by the battery material (ion exchange membrane, electrode, bipolar plate, electrolyte, etc.) is the ohmic internal resistance R0 and the reaction internal resistance R1~Rn caused by the chemical reaction; C1~Cn is the stack capacitance, which reflects the dynamic response of the battery during the charge and discharge process; Us1~Usn is the battery stack voltage (the value is determined by the electrochemical model).

[0080] In the all-vanadium liquid flow battery system, the various models are coupled with each other and jointly affect the performance and efficiency of the battery. The electrochemical reaction rate is affected by temperature. The higher the temperature, the faster the reaction rate. However, too high a temperature may lead to an increase in side reactions, affecting the efficiency and life of the battery. The heat generated by the electrochemical reaction will change the system temperature, thereby affecting the viscosity and conductivity of the electrolyte. The electrolyte flow rate affects the vanadium ion concentration gradient on the electrode surface, thereby affecting the electrochemical reaction efficiency, while the flow and pressure information provided by the fluid mechanics model is used to calculate the electrochemical reaction rate and pumping loss. The heat generated by the ohmic loss and pumping loss in the equivalent loss circuit model will affect the system temperature, and temperature changes will change the conductivity and flow resistance of the electrolyte, thereby affecting these losses. The pumping loss generated by the flow of the electrolyte affects the energy consumption of the system, and the pipeline design and flow rate affect the size of the bypass current, thereby affecting the efficiency of the system.

[0081] The electrochemical model obtains the concentration of vanadium ions of various valences through equations (4) and (5), and obtains the single stack voltage Us1~Usn of the battery stack through the Nernst equation of equation (3) and feeds it back to the equivalent loss circuit, thereby coupling the electrochemical model with the equivalent loss circuit model; the electrolyte concentration and density affected by the temperature model, as the input of the fluid mechanics model, affect the total pressure of the battery, and then the pump loss current is calculated from the pump loss power and the battery stack voltage and fed back to the equivalent loss circuit model, realizing the coupling of the temperature model, the fluid mechanics model and the equivalent loss circuit model; the bypass current output calculated by the bypass current model is fed back to the equivalent loss circuit model to realize the coupling effect between the bypass current model and the equivalent loss circuit model; at the same time, the single stack battery model is connected in series to realize the modeling of multiple vanadium battery stacks.

[0082] according to Figure 3 As shown in Equation (17), during charging, part of the current bypasses the battery and flows into the electrolyte loop, part of the current is due to the pressure loss of the electrolyte flowing in the pipeline and the battery stack, which will affect the size of the pump loss current, and part of the parasitic loss current of the energy storage system, resulting in a smaller current actually flowing into the battery stack. As shown in Equation (17), the actual charging current of the battery is less than the nominal charging current. During discharge, the bypass current, pump loss current and parasitic loss current will be provided by the battery, as shown in Equation (18), and the actual discharge current cannot reach the nominal discharge current.

[0083] I s =I d -I f -I pump -I shunt (17)

[0084] I s =I d +I f +I pump +Ishunt (18)

[0085] P total =P pump +P shunt +P f +P 1～n +P0 (19)

[0086] In the above formula, Ppump, Pshunt, Pf, P1~n, and P0 represent pumping loss, bypass loss, system fixed loss, reaction loss of series-connected batteries 1~n, and ohmic loss, respectively.

[0087] 4 Coulombic efficiency model of all-vanadium liquid flow battery

[0088] When charging, the nominal current (Inom) refers to the normal charging current designed for the system. The presence of bypass current will cause the current (Ic) that actually enters the electrode to participate in the chemical reaction to become smaller. This means that each battery can actually only convert a current less than Inom into chemical energy for storage. Therefore, the energy storage efficiency of each battery is reduced, and the charged amount cannot be fully utilized. When discharging, theoretically the same amount of current (Id) as when charging should be converted from chemical energy back to electrical energy. However, due to the presence of bypass current, the current (Id) that actually participates in the chemical reaction becomes larger and exceeds the nominal current Inom of the system. This means that although the battery tries to release more electrical energy, the current that ultimately reaches the external load is still limited by Inom. In the formula, ε η It is the coulomb loss, which reduces the energy efficiency and economy of the battery system. td and tc represent the charge and discharge time.

[0089]

[0090] 5 Model parameters

[0091] The present invention adopts a VRB-15kw system, which is composed of three VRB-5kw connected in series, and the corresponding battery fixed parameters are GEC-VRB-5kW / 20kWhVRB.

[0092] Table 1 VRB-5kw full flow battery parameters

[0093] Tab.1 Parameters of VRB-5 kw full-flow battery

[0094]

[0095]

Claims

1. A method for modeling all-vanadium liquid flow batteries and improving coulombic efficiency, It is characterized in that The following steps are involved: (1) Perform one of S1-S4 or any combination thereof: S1: Establish the bypass current model part of the overall model of all-vanadium liquid flow battery; S2: Establish the electrochemical model part of the overall model of all-vanadium liquid flow battery; S3: Establish the fluid mechanics model part of the overall model of all-vanadium liquid flow battery; S4: Establish the temperature model part of the overall model of all-vanadium liquid flow battery; S41 stack temperature change S42 Liquid storage tank temperature change part S43 Effect of temperature on electrolyte concentration, viscosity and ion diffusion coefficient (2) S5: establish the equivalent loss circuit model part of the overall model of the all-vanadium liquid flow battery; one of S1-S4 or any combination thereof is coupled to S5; (3) S6: Establish the coulombic efficiency model of the all-vanadium liquid flow battery, change the pipeline parameters to observe the change results of the vanadium battery model, judge the change results through the coulombic efficiency model, find the optimal pipeline size, so as to optimize and improve the battery coulombic efficiency.

2. A method for modeling and improving coulombic efficiency of an all-vanadium liquid flow battery according to claim 1, It is characterized in that S5: Establishing the equivalent loss circuit model part of the overall model of the all-vanadium liquid flow battery: The equivalent loss circuit model mainly reflects the electrical relationship between various components during the operation of the battery. It mainly includes the charge and discharge current Id; the pump loss current Ipump is caused by the power consumption of the pump when the electrolyte circulates inside the battery; the bypass current Ishunt is the loss caused by the voltage difference between batteries causing part of the current to bypass the electrode and flow through the electrolyte or other conductive paths; Fixed loss Rf is the energy consumption of the battery management system (BMS), the power consumption of the controller and the power consumption of the cooling system, etc.; the total resistance generated by the battery material is the ohmic internal resistance R0 and the reaction internal resistance R1~Rn caused by the chemical reaction; C1~Cn is the stack capacitance, which reflects the dynamic response of the battery during charge and discharge; Us1~Usn is the battery stack voltage; The electrochemical model obtains the concentration of vanadium ions of various valences through equations (4) and (5), and obtains the single stack voltage Us1~Usn of the battery stack through the Nernst equation of equation (3) and feeds it back to the equivalent loss circuit. As shown in equation (17), the actual charging current of the battery is less than the nominal charging current. During discharge, the bypass current, pump loss current and parasitic loss current will be provided by the battery, as shown in equation (18). The actual discharge current cannot reach the nominal discharge current. I s =I d -I f -I pump -I shunt (17) I s =I d +I f +I pump +I shunt (18) P total =P pump +P shunt +P f +P 1～n +P 0 (19) In the above formula, Ppump, Pshunt, Pf, P1~n, and P0 represent pumping loss, bypass loss, system fixed loss, reaction loss of series-connected cells 1~n, and ohmic loss, respectively; One of the models of S1, S2, S3, S4 or any combination thereof is coupled into an all-vanadium liquid flow battery model to characterize the electrical characteristics of the all-vanadium liquid flow battery and lay the foundation for the research of S6.

3. A method for modeling and improving coulombic efficiency of an all-vanadium liquid flow battery according to claim 1 or 2, It is characterized in that S6: Establishing a coulombic efficiency model of an all-vanadium liquid flow battery for judging the change results to optimize and improve the coulombic efficiency of the battery: When charging, the nominal current (Inom) refers to the normal charging current designed for the system; where ε η is the Coulomb loss, which reduces the energy efficiency and economy of the battery system; td and tc represent the charge and discharge time; When the temperature is set, the length and cross-sectional area of ​​the pipeline in the all-vanadium liquid flow battery model are changed, which affects the bypass current and pump loss current of the battery, causing the bypass current and pump loss current input into the S5 model to change. As a result, the charge and discharge current of the battery changes during the charge and discharge process, affecting the coulombic efficiency of the battery. With the help of the S6 model, the appropriate pipeline size is designed to improve the coulombic efficiency of the battery.

4. The method for modeling and improving coulombic efficiency of an all-vanadium liquid flow battery according to claim 1, It is characterized in that S1: Establish the bypass current model part of the overall model of all-vanadium liquid flow battery: Establish an equivalent circuit model of the flow battery stack, and establish a linear equation system based on Kirchhoff's current and voltage laws to solve the bypass current; The equivalent circuit of the liquid flow battery stack system, each battery is equivalent to a power supply and a resistor, each section of the pipeline filled with electrolyte is equivalent to a resistor. Due to the different lengths and diameters of the main pipe and the branch pipe, the main pipe and the branch pipe are simplified to the equivalent resistance R, and the ohmic loss of the main pipe and the branch pipe is different, where the subscript m represents the main pipe, c represents the branch pipe, and Ri represents the internal resistance of the battery; RNm, RNc, RPm, and RPc represent the equivalent resistances of the negative electrode main pipe, the negative electrode branch pipe, the positive electrode main pipe, and the positive electrode branch pipe, respectively; The current at each node satisfies Kirchhoff's current law: Where m is the number of branches at the node; The equivalent resistance of the main pipe and branch pipe meets the following requirements: Where ρ is the resistivity of the electrolyte; L k and A k is the length and cross-sectional area of ​​the pipeline; When the charging current is input, the bypass current result will be obtained from this model, and the result will be coupled to the entire battery model through S5.

5. The method for modeling and improving coulombic efficiency of an all-vanadium liquid flow battery according to claim 1, characterized in that S2: establishing the electrochemical model part of the overall model of the all-vanadium liquid flow battery: In the above formula, Us represents the single stack voltage of the battery; Ueq represents the standard electrode potential; Tstack represents the temperature of the electrolyte flowing into the battery stack; R represents the universal gas constant; z is the number of electron transfers in the electrochemical reaction; Cs2, Cs5, Cs3, and Cs4 represent the vanadium ion concentration in the battery stack, and the numbers represent the valence of the reaction ions; the input is the ion valence, which changes with battery charging and discharging, electrolyte circulation, and ion transmembrane reaction; The concentration of vanadium ions in the storage tank and the battery stack is determined by their dynamic differential equations, as follows: In the formula, subscripts s and t represent the battery stack and the liquid storage tank, respectively, the numbers represent the ion valence, V represents the volume, so Vs and Vt represent the volume of the battery stack and the liquid storage tank, and the following text is the same logic; C represents the vanadium ion concentration; Q is the flow rate; Ncell represents the number of single-chip cells; k is the vanadium ion diffusion coefficient; S represents the ion exchange membrane area; d represents the ion exchange membrane thickness; The above mathematical expressions are used to obtain the concentrations of vanadium ions of various valences in the battery stack and the liquid storage tank, as well as the single stack voltage of the battery stack. The results are then coupled to the overall battery model through S5.

6. The method for modeling and improving coulombic efficiency of an all-vanadium liquid flow battery according to claim 1, It is characterized in that S3: Establish the fluid mechanics model of the overall model of the all-vanadium liquid flow battery: When the electrolyte flows through the pipeline, the stack electrode and the stack flow channel, it will produce a flow pressure drop, which will lead to pump power loss. Considering the pipeline friction loss pressure drop, it is related to the pipeline length Lk, pipeline diameter Dh, friction coefficient fd, electrolyte flow rate v, conductivity ρ, pipeline design, etc.; the electrode flow pressure drop is related to the electrode length Le, cross-sectional area Ae, permeability Ke, electrolyte viscosity μ, fiber diameter df of the electrode carbon felt material, Kozeny-Carman constant K, and electrode porosity ε; cited data A is the cross-sectional area of ​​the pipe, L is the length of the pipe, and Rcell is the flow resistance; the expression is as follows: By establishing a fluid mechanics sub-model, the length and diameter of the pipeline are discussed, and the pressure drop is compared to obtain the pump loss current under different sizes, which is then input into S5 for coupling with the overall battery model.

7. The method for modeling and improving coulombic efficiency of an all-vanadium liquid flow battery according to claim 1, It is characterized in that S4: Establishing the temperature model part of the overall model of the all-vanadium liquid flow battery: S41 stack temperature change The temperature of the electrolyte inside the battery stack is mainly affected by the following four aspects: the heat of the electrolyte transport in the stack tank, the outside air temperature, the self-discharge reaction and the heat generated by itself; a model is established to describe the temperature change inside the battery stack; the relationship is as follows: In the equation, Cp represents the specific heat capacity of the electrolyte, Hs represents the total heat transfer capacity of the stack; ΔH2, ΔH3, ΔH4 and ΔH5 represent the enthalpy changes of different vanadium ions in the self-discharge reaction; QC p ρ(T pipe -T stack ) represents the heat generated during the transmission of the electrolyte from the pipeline to the stack; Hs (Tair-Tstack) represents the effect of the outside air temperature on the temperature of the electrolyte in the stack, I 2 R represents the rate of heat generated by the internal resistance of the stack; S42 Liquid storage tank temperature change part The temperature of the liquid storage tank is mainly affected by two factors: the heat of electrolyte transportation and the change of external temperature. The temperature change formula is: In the formula: QC p ρ(T pipe -T stack ) represents the heat generated by the electrolyte during transportation between the storage tank and the pipeline; Hn represents the overall heat transfer capacity of the storage tank; H n (T air -T stack ) represents the effect of the external temperature on the electrolyte temperature in the tank; S43 Effect of temperature on electrolyte concentration, viscosity and ion diffusion coefficient The electrolyte density is mainly affected by the vanadium ion concentration, sulfuric acid concentration and temperature; the electrolyte concentration is affected by the vanadium ion concentration and temperature; The empirical formulas for electrolyte concentration and density are shown in equations (14) and (15): A1, B1, C1, D1, E1, and F1 in the formula are empirical coefficients, which are 1.003 20, 0.163 22, -0.006 56, 0.07363, -0.004 91, and -0.000 61 respectively; is the concentration of divalent vanadium ions; is the sulfuric acid concentration; Wherein, A2, B2, D2, E2, F2 are empirical coefficients, which are 0.838 8, -1.324 6, 1.130 0, -0.096 9, 0.001 3 respectively; c is the concentration of vanadium ions; The ion exchange membrane cannot completely isolate the positive and negative active substances, so each valence vanadium ion has a corresponding ion diffusion coefficient, and its relationship with temperature is as follows: In the formula, A0 is the pre-factor; Ea is the activation energy of the diffusion process; The temperature model established through steps S41, S42, and S43 affects the viscosity, density, and ion diffusion coefficient of the electrochemical model electrolyte, and then affects the overall battery model through the impact of the electrochemical model output on the S5 model.

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