Flow channel structure for improving mass transfer performance of positive electrode of zinc-bromine flow battery and design method of flow channel structure

Abstract

The invention discloses a runner structure for improving the mass transfer performance of a zinc-bromine flow battery positive electrode and a design method of the runner structure, and belongs to the field of flow batteries. Aiming at the problem of non-uniform two-phase flow mass transfer of positive electrode electrolyte (a zinc bromide solution and an organic bromine complex) of a zinc-bromine flow battery, a scheme of arranging rib plate structures which are arranged in a staggered manner on the inner wall of a snake-shaped flow channel of the bipolar plate is provided. The rib plate structure increases flow velocity change, enhances turbulence disturbance and promotes two-phase uniform mixing; meanwhile, the pressure drop of the runner is increased, and the infiltration amount of electrolyte to the electrode is improved. Simulation results show that under the conditions that the electrolyte inlet flow velocity is 0.1 m / s and the electrode permeability is 1 * 10 <-11 > m < 2 >, the average volume fractions of bromine complexes in the electrode with the size of 2 cm * 2 cm are increased by 140% (the included angle between the bromine complexes and the flow direction is 45 degrees) and 123% (the included angle between the bromine complexes and the flow direction is 135 degrees) respectively compared with a traditional S-shaped flow channel, and the relative standard deviation of reactant phase holdup is reduced by 4.3% and 7.9% respectively. The mass transfer condition of the positive electrode electrolyte of the zinc-bromine flow battery is remarkably improved, and the positive electrode electrolyte can be applied to a large-scale energy storage zinc-bromine flow battery system.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of flow battery, and particularly relates to a flow channel structure for improving the mass transfer performance of a zinc-bromine flow battery positive electrode and a design method thereof. BACKGROUND

[0002] At present, renewable energy power generation (such as solar energy, wind energy, etc.) has significant intermittency, instability and unpredictability characteristics, and large-scale energy storage technology is one of the key technologies to solve this problem. The redox flow battery technology has obvious advantages in large-scale energy storage technology due to its long energy storage time, large scale, high safety, high cost performance in the whole life cycle, flexible power and capacity configuration, etc.

[0003] The redox flow battery can be divided into various systems according to different active substances. The zinc-bromine flow battery is a more successful commercialized flow battery technology except for the all-vanadium flow battery. Compared with the all-vanadium flow battery, the zinc-bromine flow battery has lower raw material cost, is easier to obtain, has higher theoretical energy density, and has more outstanding economic advantages, and is a flow battery system with great potential for large-scale commercial development.

[0004] There is energy loss in the charging and discharging process of the flow battery, and the concentration difference overpotential is one of the reasons for the energy loss. When the electrolyte flows through the electrode reaction area, there will be uneven distribution of reactants in some areas, and the supply of reaction ions is insufficient. This uneven mass transfer will cause a concentration difference overpotential, affecting the overall efficiency of the battery. During the discharging process of the zinc-bromine flow battery, the positive electrolyte is a mixture of zinc bromide solution and organic bromine complex, and there is a liquid-liquid two-phase flow problem during operation. Compared with single-phase electrolyte, the uneven mass transfer phenomenon is more serious. Therefore, designing a reasonable flow channel structure and improving the mass transfer effect of the reactant (organic bromine complex) during the discharging process is of great significance to improve the overall operation efficiency of the zinc-bromine flow battery. SUMMARY

[0005] The purpose of the present application is to overcome the defects in the prior art and provide a flow channel structure for improving the mass transfer performance of a zinc-bromine flow battery positive electrode and a design method thereof. The flow channel structure designed in the present application is based on the traditional snake-shaped flow channel and is processed on the bipolar plate near the membrane side. A rib plate structure is added inside the snake-shaped flow channel to form a structure similar to a "fishbone" shape. The rib plate produces a disturbance effect on the flowing electrolyte, enhancing the mixing of the complex and the zinc bromide solution two-phase, so that the reactants are more dispersedly distributed in the electrolyte. At the same time, the rib plate optimizes the pressure distribution in the electrolyte flow process to some extent, thereby increasing the amount of reactants entering the porous electrode and improving the uniformity of reactant mass transfer.

[0006] The specific technical solutions adopted by the present application are as follows:

[0007] In a first aspect, the present invention provides a design method for a flow channel structure to improve the mass transfer performance of the positive electrode in a zinc-bromine flow battery, as detailed below:

[0008] A serpentine flow channel is formed on the near-film side surface of the bipolar plate; a first flow channel opening is formed at one end of the serpentine flow channel, and a second flow channel opening is formed at the other end; ribs are staggered on the inner walls of both sides of the serpentine flow channel, and the arrangement of the ribs increases the average volume fraction of bromine complex in the positive electrode of the battery. The ribs are greater than the average volume fraction of bromine complexes in the original serpentine channel without ribs; the extension direction of the ribs forms an angle θ with the flow direction of the electrolyte in the serpentine channel, and satisfies 30°≤θ≤135°; if the relative standard deviation χ of the bromine complex phase content in the serpentine channel with ribs is greater than the relative standard deviation of the bromine complex phase content in the original serpentine channel without ribs, then the arrangement of the ribs is re-optimized.

[0009] Preferably, the first flow channel and the second flow channel serve as the inlet and outlet of the electrolyte, respectively.

[0010] Preferably, the average volume fraction of the bromine complex Calculated using COMSOL Multiphysics finite element software.

[0011] Preferably, the formula for calculating the relative standard deviation χ of the bromine complex phase content is as follows:

[0012]

[0013] In the formula, c d Let A be the bromine complex phase content at each measurement point in the positive electrode of the battery, and let A be the reaction area.

[0014] Preferably, the included angle θ is 45° or 135°.

[0015] Preferably, the width w of the rib is 10% to 30% of the width of the serpentine flow channel.

[0016] Preferably, the length L of the rib plate satisfies:

[0017] When the width of the serpentine flow channel is 3mm, L = 1.5mm;

[0018] When the width of the serpentine flow channel is 1.8 mm, L = 1.0 mm.

[0019] Preferably, the serpentine flow channel satisfies the following conditions: flow channel depth H = 1 mm; width W1 = 3 mm for the portion of the flow channel where the first and second flow channel openings are located; and width W2 = 1.8 mm for the main flow channel.

[0020] In a second aspect, the present invention provides a flow channel structure for improving the mass transfer performance of the positive electrode of a zinc-bromine flow battery by utilizing the design method described in any one of the first aspects.

[0021] Thirdly, the present invention provides a zinc-bromine flow battery containing a flow channel structure as described in the second aspect to improve the mass transfer performance of the positive electrode of the zinc-bromine flow battery.

[0022] Compared with the prior art, the present invention has the following advantages:

[0023] The positive electrode flow channel structure of this invention has significant advantages when applied to zinc-bromine flow batteries. Under the same electrolyte flow rate conditions, the reactants penetrate more deeply into the electrode, are more evenly distributed, and exhibit better mass transfer performance.

[0024] In flow simulations, the zinc-bromine flow battery employing the positive electrode channel structure of this invention exhibits a lower average volume fraction of reactants in its electrodes compared to the traditional cross-channel design. The efficiency was increased by approximately 135.6%, and the relative standard deviation (χ) of the reactant phase content in the electrode decreased by approximately 7.88% (electrolyte inlet flow rate was 0.1 m / s, electrode permeability was 1 × 10⁻⁶). -11 m 2 ). Attached Figure Description

[0025] Figure 1 (a) the flow channel structure of the present invention and (b) the conventional serpentine flow channel structure;

[0026] Figure 2 These are schematic diagrams of the simulated flow channel structure in Embodiments 1 and 2 of the present invention;

[0027] Figure 3 This is a schematic diagram illustrating the parameter identification of the flow channel structure of the present invention;

[0028] The attached diagram is labeled as follows: bipolar plate 1, rib plate 2, first flow channel opening 3, and second flow channel opening 4. Detailed Implementation

[0029] The present invention will be further described and illustrated below with reference to the accompanying drawings and specific embodiments. The technical features of each embodiment of the present invention can be combined accordingly, provided that there is no mutual conflict.

[0030] This invention provides a design method for a flow channel structure to improve the mass transfer performance of the positive electrode in a zinc-bromine flow battery. The specific design method is as follows:

[0031] In existing technologies, bipolar plates 1 typically have serpentine flow channels formed on their near-membrane side surface, such as... Figure 1As shown in (b), a first flow channel opening 3 is opened at one end of the serpentine flow channel, and a second flow channel opening 4 is opened at the other end of the serpentine flow channel. The first flow channel opening 3 and the second flow channel opening 4 serve as the inlet and outlet of the electrolyte, respectively.

[0032] The design method of this invention is based on the traditional serpentine flow channel, with ribs 2 staggered on the inner walls of both sides of the serpentine flow channel, such as... Figure 1 As shown in (a), the ribs 2 are used to enhance the mixing of the two phases of the electrolyte and increase the electrode penetration pressure drop. Their arrangement should be such that the average volume fraction of bromine complexes in the positive electrode of the battery is... Average volume fraction of bromine complex This can be calculated using COMSOL Multiphysics finite element software. The extension direction of rib 2 forms an angle θ with the flow direction of the electrolyte in the serpentine channel, satisfying 30°≤θ≤135°. If the relative standard deviation χ of the bromine complex phase content in the serpentine channel after setting rib 2 is greater than the relative standard deviation of the bromine complex phase content in the original serpentine channel without rib 2, it may be that the design dimensional parameters are unreasonable or the effect is not good under this condition, and the arrangement (i.e., parameters) of rib 2 needs to be re-optimized.

[0033] In a preferred embodiment of the present invention, to reflect the amount and uniformity of reactant penetration and distribution in the electrode, the average volume fraction of the bromine complex phase in the electrode is used. To describe the infiltration rate, the relative standard deviation χ of the bromine complex phase content is calculated using data from various points obtained through gridding (or by selecting calculation points as needed). The calculation formula is as follows:

[0034]

[0035] In the formula, c d Let A be the bromine complex phase content at each measurement point in the positive electrode of the battery, and let A be the reaction area.

[0036] The coefficient χ reflects the uniformity of the reactant phase content distribution at each point in the electrode. The smaller the χ, the more uniform the phase distribution.

[0037] In a preferred embodiment of the present invention, the included angle θ is 45° or 135°, such as... Figure 2 As shown.

[0038] As a preferred embodiment of the present invention, such as Figure 3As shown, the width w of the rib 2 is 10% to 30% of the width of the serpentine flow channel (preferably 0.4 mm); the length L of the rib 2 is adapted to the width of the serpentine flow channel: when the width of the serpentine flow channel (i.e., W1) is 3 mm, L = 1.5 mm; when the width of the serpentine flow channel (i.e., W2) is 1.8 mm, L = 1.0 mm; the geometric parameters of the serpentine flow channel satisfy: the channel depth H = 1 mm; the width W1 of the part of the flow channel where the first flow channel opening 3 and the second flow channel opening 4 are located is 3 mm; the width W2 of the main flow channel is 1.8 mm.

[0039] This invention introduces a rib structure into the traditional serpentine flow channel, which enables:

[0040] 1) Effect on pressure distribution in the flow channel: The addition of the rib structure will increase the flow resistance of the electrolyte in the flow channel, resulting in an increase in the flow pressure drop in the flow channel. The increase in pressure drop will promote the penetration of electrolyte into the electrode.

[0041] 2) Effect on velocity distribution in the flow channel: The addition of small rib structure inside the flow channel significantly increases the velocity variation inside the flow channel compared to the traditional serpentine flow channel. The higher velocity variation gives the two-phase electrolyte flow a greater disturbance effect, promoting the mixing of the two phases of the electrolyte.

[0042] 3) Effect on the distribution of reactants in the electrode: Due to the increased flow pressure drop of the electrolyte in the flow channel, the flow rate of the electrolyte in the electrode is significantly increased, and the amount of reactants penetrating into the electrode is greatly increased; at the same time, due to the rib structure promoting the two-phase mixing of the electrolyte, the uniformity of the two-phase distribution of the electrolyte in the flow channel is improved, and the distribution of reactants penetrating into the electrode is also more uniform.

[0043] The design method of the present invention and the effect of the resulting flow channel structure will be specifically illustrated through the following examples.

[0044] Example 1

[0045] This embodiment applies the design method to a zinc-bromine flow battery, providing a flow channel structure design method to improve the performance of the zinc-bromine flow battery, resulting in a novel positive electrode flow channel structure, such as... Figure 1 (a) and Figure 2 As shown.

[0046] In this embodiment, the novel flow channel structure is a serpentine flow channel structure with added ribs, machined on the near-film side surface of the bipolar plate. The ribs in the flow channel are staggered along both sides of the flow path, with the rib extension direction forming an angle θ of 135° with the electrolyte flow direction to enhance the disturbance effect on the electrolyte. In this embodiment, as... Figure 2 As shown, the first flow channel 3 is used as the electrolyte inlet, and the second flow channel 4 is used as the electrolyte outlet.

[0047] Example 2

[0048] This embodiment is largely the same as embodiment 1 in its setup, with the following differences: Figure 2 As shown, the first flow channel 3 is used as the electrolyte outlet, the second flow channel 4 is used as the electrolyte inlet, and the angle θ between the extension direction of the rib and the electrolyte flow direction is 45°.

[0049] Simulations were performed on the serpentine flow channel structures after adding ribs in Examples 1 and 2, respectively. The COMSOL Multiphysics software was used for the solutions, and the specific implementation details are as follows:

[0050] (1) Model building

[0051] Geometric Structure: The positive electrode flow channel depth is set to 1mm, the flow channel width at the inlet and outlet is 3mm, and the remaining section is set to 1.8mm. The flow channel rib width is set to 0.83mm, and the flow channel length is set to 20mm. The ribs distributed on both sides of the 3mm wide flow channel have a center length of 1.5mm, and the ribs distributed on both sides of the 1.8mm wide flow channel have a center length of 1mm. All ribs are 0.4mm wide, with a smaller angle of 45° to the side edges, and are arranged in a staggered pattern. Figure 2 The electrode area is 4 cm². 2 (2cm×2cm), electrode thickness is 1mm.

[0052] Mesh generation: The physics-controlled mesh in COMSOL was used, employing a conventional mesh. Mesh independence verification was performed.

[0053] Physical fields: Brinkman physical field and horizontal concentrated physical field are used for free and porous media flow; Electrolyte properties: Zinc bromide electrolyte (aqueous phase) ρ = 1000 kg / m³ 3 μ = 1 mPa·s, bromine complex (oil phase) ρ = 1680 kg / m 3 μ = 2.5 mPa·s.

[0054] (2) Boundary conditions

[0055] Inlet: velocity inlet (0.1 m / s), volume fraction of reactant bromine complex phase 0.222;

[0056] Outlet: Pressure outlet (0 Pa gauge pressure);

[0057] Electrode interface: porous medium (permeability 1×10⁻¹¹) m 2 (Porosity 0.8).

[0058] (3) Simulation results

[0059] The average volume fraction of reactants in the electrode was obtained through simulation calculations. It can describe the amount of reactant penetrating into the electrode; the relative standard deviation (χ) of the reactant phase content reflects the uniformity of reactant distribution in the electrode. The calculation time is 5 seconds. The simulation results of Examples 1 and 2 are compared with the simulation results of a conventional serpentine flow channel.

[0060] The amount of reactant penetrating into the electrode: After adopting the optimized structure, the average volume fraction of the reactant phase in the electrode in Examples 1 and 2 reached 0.1079 and 0.1166, respectively, which is 123% and 140% higher than that of the conventional serpentine flow channel (0.0485).

[0061] Uniformity of reactant distribution in the electrode: After adopting the optimized structure, the relative standard deviations of reactant phase content in the electrodes of Examples 1 and 2 were 1.262 and 1.211, respectively, which were 7.9% and 4.3% lower than the control group (1.370).

[0062] The results show that the flow channel structure of the present invention, by adding a rib structure, improves the pressure distribution in the zinc-bromine flow battery electrode, promotes the mixing of the two phases of the electrolyte, and effectively improves the penetration amount and distribution uniformity of the reactants in the electrode.

[0063] Furthermore, based on the above embodiments, different basic flow channel structures can be used to adjust the size and angle of the ribs to further optimize the mass transfer effect, thereby further improving battery performance under the advantages of the present invention.

[0064] This invention addresses the problem of uneven mass transfer in the two-phase flow of the positive electrode electrolyte (zinc bromide solution and organic bromine complex) in zinc-bromine flow batteries by proposing a staggered rib structure on the inner wall of the serpentine flow channel of the bipolar plate. The rib structure increases the velocity variation, enhances turbulence, and promotes uniform mixing of the two phases; simultaneously, it increases the pressure drop in the flow channel, thereby increasing the amount of electrolyte penetrating into the electrode. Simulation results show that at an electrolyte inlet velocity of 0.1 m / s, the electrode permeability is 1 × 10⁻⁶. -11 m 2 Under these conditions, the average volume fraction of bromine complexes in the electrode is increased by 140% (at an angle of 45° with the flow direction) and 123% (at an angle of 135° with the flow direction) compared to the traditional serpentine flow channel, respectively, and the relative standard deviation of reactant phase content is reduced by 4.3% and 7.9%, respectively. This invention significantly improves the mass transfer of the positive electrode electrolyte in zinc-bromine flow batteries and can be applied to large-scale energy storage zinc-bromine flow battery systems.

[0065] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the invention. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the invention. Therefore, all technical solutions obtained through equivalent substitution or transformation fall within the protection scope of the present invention.

Claims

1. A method for designing a flow channel structure to improve the mass transfer performance of the positive electrode in a zinc-bromine flow battery, characterized in that, Specifically as follows: A serpentine flow channel is formed on the near-film side surface of the bipolar plate (1); a first flow channel opening (3) is formed at one end of the serpentine flow channel, and a second flow channel opening (4) is formed at the other end; ribs (2) are staggered on the inner walls of both sides of the serpentine flow channel, and the arrangement of the ribs (2) increases the average volume fraction of bromine complex in the positive electrode of the battery. The average volume fraction of bromine complex in the original serpentine channel without ribs (2) is greater than that of the original serpentine channel without ribs (2); the extension direction of the ribs (2) forms an angle θ with the flow direction of the electrolyte in the serpentine channel, and satisfies 30°≤θ≤135°; if the relative standard deviation χ of the bromine complex phase content in the serpentine channel after setting ribs (2) is greater than that of the bromine complex phase content in the original serpentine channel without ribs (2), then the arrangement of ribs (2) is re-optimized.

2. The design method for the flow channel structure to improve the mass transfer performance of the positive electrode in a zinc-bromine flow battery according to claim 1, characterized in that, The first flow channel (3) and the second flow channel (4) serve as the inlet and outlet of the electrolyte, respectively.

3. The design method for the flow channel structure to improve the mass transfer performance of the positive electrode in a zinc-bromine flow battery according to claim 1, characterized in that, The average volume fraction of the bromide complex Calculated using COMSOL Multiphysics finite element software.

4. The design method of the flow channel structure for improving the mass transfer performance of the positive electrode in a zinc-bromine flow battery according to claim 1, characterized in that, The formula for calculating the relative standard deviation χ of the bromine complex phase content is as follows: In the formula, c d Let A be the bromine complex phase content at each measurement point in the positive electrode of the battery, and let A be the reaction area.

5. The design method of the flow channel structure for improving the mass transfer performance of the positive electrode in a zinc-bromine flow battery according to claim 1, characterized in that, The included angle θ is 45° or 135°.

6. The design method of the flow channel structure for improving the mass transfer performance of the positive electrode in a zinc-bromine flow battery according to claim 1, characterized in that, The width w of the rib (2) is 10% to 30% of the width of the serpentine flow channel.

7. The design method of the flow channel structure for improving the mass transfer performance of the positive electrode in a zinc-bromine flow battery according to claim 1, characterized in that, The length L of the rib (2) satisfies: When the width of the serpentine flow channel is 3mm, L = 1.5mm; When the width of the serpentine flow channel is 1.8 mm, L = 1.0 mm.

8. The design method of the flow channel structure for improving the mass transfer performance of the positive electrode in a zinc-bromine flow battery according to claim 1, characterized in that, The serpentine flow channel satisfies the following conditions: flow channel depth H = 1 mm; the width W1 of the flow channel where the first flow channel opening (3) and the second flow channel opening (4) are located is 3 mm; and the width W2 of the main flow channel is 1.8 mm.

9. A flow channel structure for improving the mass transfer performance of the positive electrode of a zinc-bromine flow battery, obtained by the design method according to any one of claims 1 to 8.

10. A zinc-bromine flow battery containing a flow channel structure as described in claim 9 to improve the mass transfer performance of the positive electrode of a zinc-bromine flow battery.

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