Redox flow battery system

Abstract

This redox flow battery system comprises: a plurality of banks; power conversion devices respectively provided to the plurality of banks; and a controller that controls the power conversion devices to control the charge states of the plurality of banks. The plurality of banks each comprise a battery cell that performs charging and discharging through the supply of an electrolytic solution. The plurality of banks comprise a plurality of first banks controlled to be a first charging state, and at least one second bank other than the first banks. The controller controls the charging state of the second bank so as to be a second charging state different from the first charging state.

Description

[Technical Field]

[0001] The present disclosure relates to redox flow battery systems. This application claims priority based on Japanese Patent Application No. 2021-102299 filed on June 21, 2021, and incorporates by reference all of the contents of said Japanese application. [Background technology]

[0002] Patent Document 1 discloses a method for operating a redox flow battery system in which a positive electrode electrolyte and a negative electrode electrolyte are supplied to a battery cell to charge and discharge the battery. Patent Document 1 also describes controlling the voltage variation between banks in a redox flow battery system having multiple banks. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-40648 Summary of the Invention

[0004] The redox flow battery system of the present disclosure includes a plurality of banks, a power conversion device provided in each of the plurality of banks, and a controller that controls the power conversion device to control the state of charge of each of the plurality of banks, each of the plurality of banks including battery cells that are charged and discharged by supplying an electrolyte, the plurality of banks including a plurality of first banks controlled to a first state of charge and one or more second banks excluding the first banks, and the controller controls the state of charge of the second bank to a second state of charge different from the first state of charge. [Brief explanation of the drawings]

[0005] [Figure 1] FIG. 1 is a schematic diagram showing the configuration of a redox flow battery system according to an embodiment. [Figure 2]FIG. 2 is a schematic diagram showing the basic configuration of a bank. DETAILED DESCRIPTION OF THE INVENTION

[0006] [Problem to be solved by this disclosure] There is a demand for higher energy density in redox flow battery systems. Increasing energy density would enable an increase in battery capacity. However, increasing energy density increases the likelihood of side reactions occurring during charging or discharging. These side reactions can lead to electrolyte degradation and other degradation of battery performance.

[0007] Generally, in redox flow batteries, the usable range of the state of charge (SOC) is limited to prevent degradation of battery performance due to side reactions. The technology described in Patent Document 1 makes the SOC of each bank as equal as possible by performing voltage control to reduce the voltage difference between the banks. The technology described in Patent Document 1 operates so that the SOC of all banks is equal within the SOC range in which no side reactions occur, and therefore cannot expand the usable range of the SOC.

[0008] An object of the present disclosure is to provide a redox flow battery system that can achieve high energy density while improving degradation of battery performance due to side reactions.

[0009] [Effects of this disclosure] The redox flow battery system of the present disclosure can achieve high energy density while improving the degradation of battery performance due to side reactions.

[0010] [Description of the embodiments of the present disclosure] The present inventors have conducted extensive research into increasing the energy density of redox flow batteries and the effect of increasing the energy density on battery performance, and as a result have reached the following findings.

[0011] One way to increase the energy density of redox flow batteries is to expand the range of SOC usage, but expanding the range of SOC usage can lead to side reactions that adversely affect battery performance.

[0012] Typically, in redox flow batteries, the SOC is controlled to be maintained within a certain range to prevent degradation of battery performance due to side reactions. In high or low SOC regions, active material ions contained in the electrolyte may precipitate due to side reactions. Precipitation of active material ions may cause the electrolyte to cease functioning as an active material. Precipitation of active material ions deteriorates the electrolyte and reduces battery capacity. Furthermore, active material ion precipitates may adhere to the electrodes that make up the battery cell, causing battery cell degradation. Thus, charging or discharging in high or low SOC regions promotes side reactions such as active material ion precipitation. Increased production of side reaction products, such as active material ion precipitates, leads to a decrease in battery performance. Therefore, conventional redox flow batteries are generally controlled to charge and discharge within an SOC range where side reactions are unlikely to occur. In other words, the usable SOC range is limited.

[0013] The inventors have discovered that products of side reactions can sometimes be re-dissolved as ions in the electrolyte through a reversible reaction. For example, if a side reaction produces a precipitate of active material ions in a high SOC range, it may be possible to re-convert the precipitate into ions in a low SOC range. Alternatively, if a side reaction produces a precipitate of active material ions in a low SOC range, it may be possible to re-convert the precipitate into ions in a high SOC range. In such cases, by periodically controlling the SOC to maintain the SOC in a low or high SOC range for a certain period of time, it is possible to re-dissolve the precipitate of active material ions as ions. Therefore, even if a side reaction occurs, it is possible to re-dissolve side reaction products, such as precipitates of active material ions, by controlling the SOC and recover the electrolyte. This may potentially improve the degradation of battery performance due to side reactions. However, controlling the SOC to re-dissolve the side reaction products requires forcibly controlling the SOC to a specific range, and therefore cannot be performed simultaneously with charging or discharging. Therefore, the SOC control must be performed when the battery cell is not being charged or discharged. When the SOC control is performed, the operation of the redox flow battery is stopped, which imposes restrictions on operation and reduces the user's operational freedom.

[0014] The present inventors propose a redox flow battery system with multiple banks in which the SOC is varied to achieve high energy density. Specifically, the SOC of some banks is controlled to be different from that of all banks, rather than being controlled to be equal.

[0015] First, embodiments of the present disclosure will be listed and described.

[0016] (1) A redox flow battery system according to an embodiment of the present disclosure includes a plurality of banks, a power conversion device provided in each of the plurality of banks, and a controller that controls the power conversion device to control the state of charge of each of the plurality of banks, each of the plurality of banks including battery cells that are charged and discharged by supplying an electrolyte, the plurality of banks including a plurality of first banks controlled to a first state of charge and one or more second banks excluding the first banks, and the controller controls the state of charge of the second bank to a second state of charge different from the first state of charge.

[0017] The redox flow battery system of the present disclosure can achieve high energy density while improving the degradation of battery characteristics due to side reactions. The state of charge of a bank refers to the state of charge of the battery cells that make up the bank. When there are multiple battery cells, the state of charge of a battery cell is the average value of the states of charge of the multiple battery cells. Hereinafter, the state of charge will be simply referred to as "SOC."

[0018] In the redox flow battery system of the present disclosure, the first bank is a bank that charges from a power source such as a power generation facility and discharges to a load. The SOC of the first bank fluctuates during charging and discharging. The SOC of the first bank is controlled to a first SOC. The first SOC is a specific value within an expanded range of the normal SOC usage range. The normal SOC usage range is an SOC range in which side reactions and side reaction products are unlikely to occur even with repeated charging and discharging. The second bank is a bank that performs electrolyte recovery operations such as redissolving side reaction products. The second bank is controlled to a second SOC. The second SOC is a specific value within an SOC range in which side reaction products can be redissolved by a reversible reaction.

[0019] In the redox flow battery system of the present disclosure, the energy density can be increased by expanding the range of SOC use in the first bank. This ensures battery capacity for the entire system. By controlling the SOC of the second bank to an SOC at which the by-reaction products re-dissolve, the electrolyte in the second bank can be restored. This reduces the degradation of battery performance due to side reactions. The redox flow battery system of the present disclosure allows the electrolyte in the second bank to be restored while charging or discharging the first bank. Since the electrolyte can be restored without shutting down the system, there are fewer operational constraints. The above statement that side reactions redissolve is broadly defined as returning an electrolyte solution whose state has changed due to charging or discharging within a specific SOC range to its original state. This is not limited to returning solid precipitates to ions in the electrolyte solution, but may or may not involve a phase change between gas, solid, and liquid. It also includes controlling the amount of electrodeposition in electrolytes containing solid active materials such as zinc. Side reactions can cause active materials to deposit on the electrode surface. When side reaction deposits adhere to the electrode, the electrode's reactivity deteriorates, resulting in a decrease in battery performance. Catalysts and other materials can be attached to the electrode surface to improve electrode reactivity. Deposits from side reactions that cover the catalyst or other materials can also result in a decrease in battery performance. Redissolving the deposits on the electrode surface can maintain the state of the electrode surface, thereby preventing a decrease in battery performance.

[0020] (2) In the redox flow battery system of the present disclosure, the second state of charge may differ from the first state of charge by ±2% or more.

[0021] The redox flow battery system controls the SOC of the second bank so that it deviates from the SOC of the first bank by ±2% or more. The state in which the difference between the SOC of the first bank and the SOC of the second bank is ±2% or more can be achieved by intentionally shifting the SOC of the second bank from the SOC of the first bank.

[0022] (3) In the redox flow battery system of the present disclosure, the first state of charge may have a first state of charge range defined by an upper limit and a lower limit, the second state of charge may have a second state of charge range defined by an upper limit and a lower limit, and the second state of charge range may be 30% or less of the first state of charge range.

[0023] In the above-described redox flow battery system, the electrolyte in the second bank can be easily restored.

[0024] (4) In the redox flow battery system of the present disclosure, the ratio of the number of the second banks to the total number of the plurality of banks may be 30% or less.

[0025] In the redox flow battery system described above, the number of second banks is smaller than the number of first banks among the multiple banks. In other words, the number of first banks is larger. Since the number of first banks that perform charging and discharging is greater than the number of second banks that perform electrolyte recovery work, it is easy to ensure battery capacity in the entire system.

[0026] (5) In the redox flow battery system of the present disclosure, the controller may control each of the plurality of banks to include a time period during which the banks are in the first state of charge and a time period during which the banks are in the second state of charge.

[0027] The above-described redox flow battery system can perform electrolyte recovery work for all banks. During a time period in which the bank is in a first state of charge, the bank functions as a first bank. During a time period in which the bank is in a second state of charge, the bank functions as a second bank. In other words, each bank has a time period in which it is the first bank, where charging and discharging are performed, and a time period in which it is the second bank, where electrolyte recovery work is performed. Electrolyte recovery work can be performed for each bank at predetermined intervals.

[0028] (6) In the redox flow battery system described in (5) above, a ratio of a time period during which the battery is in the second state of charge to a total time period during which the battery is in the first state of charge and a time period during which the battery is in the second state of charge may be 30% or less.

[0029] In the above-described redox flow battery system, the time for performing the electrolyte recovery operation for each bank is set to be relatively short.

[0030] (7) In the redox flow battery system of the present disclosure, when transitioning a specific bank among the plurality of first banks from the first state of charge to the second state of charge, the controller may charge or discharge the second bank from the specific bank.

[0031] The above-described redox flow battery system can transition a specific bank from a first state of charge to a second state of charge by charging or discharging between the banks, even if charging or discharging is not performed between the banks and an external power source or load. In the above-described embodiment, the second bank can also be transitioned from the second state of charge to the first state of charge at the same time.

[0032] (8) In the redox flow battery system disclosed herein, a ratio of the number of the second banks to the total number of the plurality of banks may be x, and when the controller transitions a specific bank among the plurality of first banks from the first state of charge to the second state of charge, the controller may discharge or charge the specific bank by a value that is 1 / (1-x) times or more greater than an average value obtained by dividing the value of the discharge amount or charge amount of the multiple first banks by the total number of the multiple banks.

[0033] In the above-described redox flow battery system, when discharging or charging a plurality of first banks, a specific bank is preferentially discharged or charged over the other banks, thereby transitioning to the second bank, which shortens the transition time compared to when all banks are charged or discharged at the average value.

[0034] (9) In the redox flow battery system disclosed herein, a ratio of the time spent in the second state of charge to the total time spent in the first state of charge and the second state of charge may be y, and when the controller transitions a specific bank among the plurality of first banks from the first state of charge to the second state of charge, the controller may discharge or charge the specific bank by a value that is 1 / (1-y) times or more greater than an average value obtained by dividing the value of the discharge amount or charge amount for the plurality of first banks by the total number of the plurality of banks.

[0035] In the above-described redox flow battery system, when discharging or charging a plurality of first banks, a specific bank is preferentially discharged or charged over the other banks, thereby transitioning to the second bank, which shortens the transition time compared to when all banks are charged or discharged at the average value.

[0036] (10) In the redox flow battery system of the present disclosure, each of the plurality of banks may include a counter that quantifies the amount of a by-product produced in the electrolyte.

[0037] The above-described redox flow battery system can appropriately determine the timing for carrying out the electrolyte recovery work.

[0038] (11) In the redox flow battery system described in (10) above, the counter may digitize the amount of a by-product produced in the electrolyte as a function of a state-of-charge value in each of the plurality of banks, a temperature of the electrolyte, and time.

[0039] The above-described redox flow battery system can appropriately determine the amount of by-products produced.

[0040] [Details of the embodiments of the present disclosure] A specific example of the redox flow battery system of the present disclosure will be described with reference to the drawings. Hereinafter, the redox flow battery may be referred to as an "RF battery." The same reference numerals in the drawings indicate the same or corresponding parts. Note that the present invention is not limited to these examples, but is defined by the claims and is intended to include all modifications within the meaning and scope equivalent to the claims.

[0041] <Overview of the RF battery system> Referring to FIGS. 1 and 2, the RF battery system 1 according to an embodiment will be described. As shown in FIG. 1, the RF battery system 1 includes a plurality of banks 2, a power conversion device (PCS: Power Conditioning System) 7 provided in each of the plurality of banks 2, and a battery control device (BMS: Battery Management System) 6. Each bank 2 includes battery cells 10.

[0042] Typically, the RF battery system 1 is connected to an external power source 91 and a load 92 via a substation facility 90. The RF battery system 1 can charge the power supplied from the power source 91 or discharge the charged power to the load 92. The power source 91 is a power generation facility using natural energy such as solar power generation or wind power generation, or other general power plants. The load 92 is a power grid, a consumer, or the like. The RF battery system 1 is used, for example, for load leveling, momentary low compensation, emergency power supply, and other applications, and for output smoothing of natural energy power generation.

[0043] One of the features of the RF battery system 1 of the embodiment is that in the plurality of banks 2, the SOCs of all the banks are not the same and the SOCs of some banks are deviated. Specifically, the plurality of banks 2 include a first bank 2a and a second bank 2b. And it includes a controller 60 that controls the SOC of each bank 2. The controller 60 controls so that the SOC of the second bank 2b is different from the SOC of the first bank 2a. Hereinafter, first, the basic configuration of the bank 2 constituting the RF battery system 1 will be described. Then, the features of the RF battery system 1 will be described in detail.

[0044] <Bank> Each bank 2 can independently control the charging and discharging of the battery cells 10. The bank 2 includes the battery cells 10, an electrolyte tank 20, and a power conversion device 7. The battery cells 10 are supplied with electrolyte from the electrolyte tank 20. The electrolyte tank 20 stores the electrolyte. The electrolyte tank 20 has a positive electrode electrolyte tank 22 and a negative electrode electrolyte tank 23. The positive electrode electrolyte tank 22 stores the positive electrode electrolyte. The negative electrode electrolyte tank 23 stores the negative electrode electrolyte. The bank 2 charges and discharges by supplying the electrolyte from the electrolyte tank 20 to the battery cells 10. The bank 2 can function as a secondary battery by itself.

[0045] (battery cell) The battery cell 10 is a cell of a secondary battery. That is, the battery cell 10 is charged and discharged by supplying an electrolyte. As shown in FIG. 2, the battery cell 10 has a positive electrode 14, a negative electrode 15, and a membrane 11 disposed between the positive electrode 14 and the negative electrode 15. The battery cell 10 is separated by the membrane 11 into a positive electrode cell 12 and a negative electrode cell 13. The positive electrode 14 is built in the positive electrode cell 12. The negative electrode 15 is built in the negative electrode cell 13.

[0046] A positive electrode electrolyte is supplied to the positive electrode cell 12. A negative electrode electrolyte is supplied to the negative electrode cell 13. In this example, the bank 2 includes an outward pipe 42 and a return pipe 44 that connect the battery cell 10 and the positive electrode electrolyte tank 22. The bank 2 also includes an outward pipe 43 and a return pipe 45 that connect the battery cell 10 and the negative electrode electrolyte tank 23. Pumps 46, 47 are provided on the outward pipes 42, 43, respectively. The positive electrode electrolyte is supplied from the positive electrode electrolyte tank 22 through the outward pipe 42 by the pump 46 to the positive electrode cell 12. The positive electrode electrolyte that has passed through the positive electrode cell 12 and is discharged from the positive electrode cell 12 is returned to the positive electrode electrolyte tank 22 through the return pipe 44. The negative electrode electrolyte is supplied from the negative electrode electrolyte tank 23 through the outward pipe 43 by the pump 47 to the negative electrode cell 13. The negative electrode electrolyte that passes through the negative electrode cell 13 and is discharged from the negative electrode cell 13 is returned to the negative electrode electrolyte tank 23 through the return pipe 45. In other words, the electrolyte is circulated between the electrolyte tank 20 and the battery cell 10.

[0047] The bank 2 may be configured to include a cell stack 100 in which a plurality of battery cells 10 are stacked. The cell stack 100 includes a laminate in which a cell frame 5, a positive electrode 14, a diaphragm 11, and a negative electrode 15 are repeatedly stacked in this order, and two end plates 101 that sandwich the laminate from both ends. The cell stack 100 is configured by sandwiching the laminate between the two end plates 101 and fastening the end plates 101 with fastening members 102. The cell stack 100 generally has a structure in which a predetermined number of battery cells 10 form sub-stacks (not shown), and multiple sub-stacks are stacked on top of each other.

[0048] The cell frame 5 has a bipolar plate 51 arranged between the positive electrode 14 and the negative electrode 15, and a frame 50 provided around the bipolar plate 51. The positive electrode 14 and the negative electrode 15 are housed inside the frame 50, with the bipolar plate 51 sandwiched between them. A single battery cell 10 is formed by placing the positive electrode 14 and the negative electrode 15 between the bipolar plates 51 of adjacent cell frames 5, with a diaphragm 11 sandwiched between them. A seal member 55 is placed between the frame 50 to prevent electrolyte leakage from the battery cell 10.

[0049] The bank 2 may be configured to include a plurality of cell stacks 100. The plurality of cell stacks 100 may be connected in series or in parallel. When the bank 2 includes a plurality of cell stacks 100, the bank 2 may be configured to include one electrolyte tank 20 for the plurality of cell stacks 100, or may be configured to include a plurality of electrolyte tanks 20. When the bank 2 includes a single electrolyte tank 20, the bank 2 may include one positive electrode electrolyte tank 22 and one negative electrode electrolyte tank 23. When the bank 2 includes a plurality of electrolyte tanks 20, the bank 2 may include a plurality of positive electrode electrolyte tanks 22 and a plurality of negative electrode electrolyte tanks 23. When the bank 2 includes a plurality of battery cells 10, each battery cell 10 is supplied with an electrolyte having a uniform SOC.

[0050] (electrolyte) The electrolyte is typically a solution containing active material ions. The active material ions function as an active material. Metal ions whose valence changes through oxidation-reduction can be used as the active material ions. The active material ions are, for example, ions of elements selected from the group consisting of manganese, vanadium, iron, chromium, titanium, and zinc.

[0051] <Characteristics of the electrolyte> At least one of the positive electrode electrolyte and the negative electrode electrolyte has the following properties. (1) Depending on the range of SOC during charging or discharging, side reactions occur and by-products are generated. (2) By-products can be redissolved by a reversible reaction. The electrolyte is, for example, an electrolyte that generates side reactants through side reactions in a high SOC region where the SOC is high, and that can redissolve the side reactants through a reversible reaction in a relatively low SOC region. Alternatively, the electrolyte is an electrolyte that generates side reactants through side reactions in a low SOC region where the SOC is low, and that can redissolve the side reactants through a reversible reaction in a relatively high SOC region. Even if a side reaction occurs in such an electrolyte, it is possible to restore the electrolyte to its original state by redissolving the side reactants through SOC control. The side reactants generated by the side reaction are, for example, precipitates of active material ions.

[0052] The electrolyte is, for example, an electrolyte containing manganese ions. When a positive electrode electrolyte containing manganese ions is used, manganese oxide may precipitate in the positive electrode electrolyte in a high SOC region during charging. The manganese oxide may decompose into manganese ions in a low SOC region and redissolve in the positive electrode electrolyte. When the positive electrode electrolyte is an electrolyte containing manganese ions, the negative electrode electrolyte may be, for example, an electrolyte containing titanium ions.

[0053] (power conversion device) The power conversion device 7 controls the charging and discharging of the battery cell 10. The battery cell 10 is charged and discharged via the power conversion device 7. Each bank 2 can be independently controlled by the power conversion device 7 for the charging and discharging of the battery cell 10. The power conversion device 7 is controlled by the battery control device 6. For the power conversion device 7, for example, an AC / DC conversion device can be used.

[0054] (Monitor cell) As shown in FIG. 1, each bank 2 includes a monitor cell 30 for measuring the SOC. The monitor cell 30 is supplied with the same electrolyte as that supplied to the battery cell 10. That is, the electrolyte supplied to the battery cell 10 and the electrolyte supplied to the monitor cell 30 are supplied from the same electrolyte tank 20. For the monitor cell 30, a single cell having basically the same configuration as the above-described battery cell 10 can be used. The monitor cell 30 does not charge or discharge. The SOC of the bank 2 can be obtained from the open circuit voltage (OCV) of the monitor cell 30.

[0055] The monitor cell 30 may be provided separately from the battery cell 10, or a part of the cells of the above-described cell stack 100 may be configured as a monitor cell. When the monitor cell 30 is provided separately, it is preferable to provide the monitor cell 30 upstream of the battery cell 10. The monitor cell 30 provided upstream of the battery cell 10 is supplied with the electrolyte before being charged or discharged by the battery cell 10. This is because if the monitor cell 30 is provided downstream of the battery cell 10, the SOC detected by the monitor cell 30 will deviate from the SOC of the electrolyte in the electrolyte tank 20. When the bank 2 includes a plurality of battery cells 10, the monitor cell 30 measures the average value of the SOCs of the plurality of battery cells 10.

[0056] <Features of the RF battery system> As shown in FIG. 1, the RF battery system 1 includes a plurality of banks 2 and a battery control device 6. The number of banks 2 is, for example, four or more. The greater the number of banks 2, the greater the battery capacity of the entire system. The number of banks 2 may be 11 or more, or 16 or more. If the number of banks 2 is large, the entire system may become large and management of the banks 2 may become complicated. The upper limit of the number of banks 2 is, for example, 100 or less, or even 50 or less. The number of banks 2 may be, for example, 11 to 100 or 16 to 50. In FIG. 1, each bank 2 is represented as No. 1, No. 2, ... No. n, where n is a natural number.

[0057] (multiple banks) The multiple banks 2 include multiple first banks 2a and one or more second banks 2b. The first bank 2a is a bank among the multiple banks 2 that is controlled to a first SOC. The second bank 2b is a bank among the multiple banks 2 other than the first bank 2a. The second bank 2b is a bank among the multiple banks 2 that is controlled to a second SOC different from the first SOC. Here, the first bank 2a and the second bank 2b are not physically fixed. Each bank 2 can function as both the first bank 2a and the second bank 2b. Therefore, during operation of the RF battery system 1, there may be a state where all banks 2 operate as first banks 2a, i.e., a state where only multiple first banks 2a exist. In other words, there may be a state where no bank 2 functions as a second bank 2b, and the number of second banks 2b is zero. The RF battery system 1 of this embodiment not only always includes a second bank 2b, but also includes a state where one or more second banks 2b exist only at a certain point during operation.

[0058] Daiichi Bank The first bank 2a is a bank that charges and discharges between an external power source 91 and a load 92. The SOC of the first bank 2a fluctuates depending on the charging and discharging. During charging and discharging, the SOC of the first bank 2a is controlled to a first SOC. The first SOC has a first SOC range. The first SOC is a specific value within the first SOC range. The first SOC range is a numerical range defined by the upper and lower limits of the first SOC. In other words, the first SOC range is the difference between the upper and lower limits of the range that the first SOC can take. The first SOC range is an expanded range of the normal SOC range. The normal SOC range is an SOC range in which side reactions are unlikely to occur and side reaction products are unlikely to be produced. In other words, it is an SOC range in which the electrolyte is unlikely to deteriorate due to side reactions and battery performance can be maintained. The SOC range in which the above-mentioned by-products are unlikely to be generated is, for example, a range in which, when charging and discharging within a certain SOC range for one week, the proportion of active material consumed in generating by-products is 10 mol % or less, where the total amount of elements that constitute the active material in the electrolyte is 100 mol %. The period of charging and discharging within the above SOC range is preferably two weeks, and more preferably one month. The proportion of active material consumed in generating by-products is preferably 5 mol % or less, and even more preferably 1 mol % or less. For example, when manganese oxide is generated as a by-product in a positive electrode electrolyte containing manganese ions as the active material, manganese is consumed in generating by-products. The range in which the normal SOC range is expanded includes, in addition to the normal SOC range, an SOC range in which side reactions occur upon charging or discharging and by-products are generated. The first SOC includes at least one of a high SOC region and a low SOC region. The high SOC region is a region in which the SOC is higher than the upper limit of the normal SOC range. The low SOC region is a region in which the SOC is lower than the lower limit of the normal SOC range. In the high SOC region and the low SOC region, side reaction products may be generated. The first SOC is a range that combines the normal SOC utilization range with at least one of the high SOC region and the low SOC region.

[0059] The normal SOC range varies depending on the electrolyte used. The normal SOC range is determined in advance by testing using the electrolyte used. The ranges that are wider than the normal SOC range, i.e., the high SOC region and the low SOC region, can be set appropriately. Expanding the SOC range in the first bank 2a increases the battery capacity of the first bank 2a. As a result, the energy density of the entire system can be increased. The high SOC region or the low SOC region is, for example, 5% or more, 10% or more, or 15% or more of the normal SOC range. The above-mentioned range is the difference between the upper and lower limits of the range. For example, if the normal SOC range is 20% to 80%, the difference between the upper and lower limits is 80% - 20% = 60%. If the high SOC region is 10% of the range, the high SOC region is 60% x 10% = 6%. The combined SOC utilization range of the normal SOC utilization range and the high SOC region is 20% to 86%. In this case, the first SOC range is 86% - 20% = 66%. In this case, the SOC utilization range is 10% larger than the normal SOC utilization range, so the battery capacity of the first bank 2a increases by 10%. If the low SOC region range is set to 10% of the utilization range, the SOC utilization range is 14% to 80%. In this case, the first SOC range is 80% - 14% = 66%. In this case, the battery capacity of the first bank 2a also increases by 10%. The reason for the improvement in the energy density of the entire system will be explained in detail in the section below titled "Example of Energy Density Calculation."

[0060] The number of first banks 2a is, for example, 10 or more, and furthermore, 15 or more. The greater the number of first banks 2a, the easier it is to ensure battery capacity in the entire system.

[0061] The multiple first banks 2a are controlled so that the SOC of each first bank 2a does not vary during charging and discharging. In other words, the SOC of all first banks 2a is controlled to be the same. Specifically, the difference in SOC between the first banks 2a is less than ±2%, and even less than ±1%. The difference in SOC between the first banks 2a is the difference between the average SOC of all first banks 2a and the SOC value of each first bank 2a.

[0062] <Second Bank> The second bank 2b is a bank that performs an electrolyte recovery operation. The SOC of the second bank 2b is controlled to a second SOC. The second SOC has a second SOC range. The second SOC is a specific value within the second SOC range. The second SOC range is , th The second SOC range is a numerical range defined by the upper and lower limits of the second SOC. That is, the second SOC range is the difference between the upper and lower limits of the range that the second SOC can take. The second SOC range is the SOC range in which the by-products can be redissolved by a reversible reaction.

[0063] The SOC range in which the above-mentioned side reaction products redissolve varies depending on the electrolyte used. The SOC range in which the side reaction products redissolve is determined in advance by testing using the electrolyte used. For example, if the side reaction products redissolve in an SOC range of 30% or less, the second SOC may be 30% or less, or may be 20% or less. In this case, the lower the SOC, the faster the reversible reaction rate, and therefore the more likely the side reaction products redissolve. If the side reaction products redissolve in an SOC range of 70% or more, the second SOC may be 70% or more, or may be 80% or more. In this case, the higher the SOC, the faster the reversible reaction rate, and therefore the more likely the side reaction products redissolve. The second SOC may be a specific value within the SOC range in which the above-mentioned side reaction products redissolve, and may partially overlap with the first SOC range. For example, when the first SOC range is 20% to 86%, the second SOC range may be 20% to 30%, or 0 to 30%. If the second SOC range overlaps at least partially with the first SOC range, the second bank 2b may be charged and discharged in that overlapping range. That is, in the second bank 2b, the battery cells 10 may be charged and discharged during the electrolyte recovery operation. Of course, the battery cells 10 do not have to be charged and discharged in the second bank 2b.

[0064] The second SOC range is, for example, smaller than the first SOC range. The second SOC range may be, for example, 30% or less of the first SOC range. If the first SOC range is, for example, 66% as described above, then 30% is approximately 20%. If the second SOC range is, for example, 20% to 30% and the difference between its upper and lower limits is 10%, then the second SOC range is approximately 15% of the first SOC range. In other words, the second SOC range is 30% or less of the first SOC range.

[0065] The number of second banks 2b is smaller than the number of first banks 2a. The ratio x of the number of second banks 2b to the total number of the multiple banks 2 is, for example, 30% or less. Of the multiple banks 2, the number of second banks 2b is in the minority. In other words, the number of first banks 2a is in the majority. Because there are many first banks 2a, it is easy to ensure battery capacity for the entire system. The ratio x may be, for example, 20% or less, or even 10% or less.

[0066] (Examples of the first and second SOC) Specific examples of the first and second SOC ranges are shown below. This example uses a titanium / manganese-based electrolyte. In titanium / manganese-based electrolytes, the positive electrode electrolyte contains manganese ions, and the negative electrode electrolyte contains titanium ions. For titanium / manganese-based electrolytes, the typical SOC range is approximately 20% to 80%. In the high SOC range (over 80%), manganese oxide may precipitate in the positive electrode electrolyte as a side reaction product. The high SOC range is, for example, 3% or more, further 6% or more, or 12% or more. The larger the range of the high SOC range, the wider the SOC range compared to the normal SOC range, thereby increasing the usable battery capacity. The SOC range during charging and discharging, i.e., the first SOC range, is, for example, 20% to 83%, further 20% to 86%, or 20% to 92%. The higher the SOC, the faster the side reaction rate, resulting in increased manganese oxide precipitation. The upper limit of the first SOC range is, for example, 100%, or even 95%. If the upper limit of the first SOC range is 95%, it is easy to suppress a decrease in battery efficiency due to an increase in internal resistance.

[0067] The SOC range in which manganese oxide in the positive electrode electrolyte can be redissolved by a reversible reaction is generally 30% or less. In other words, the SOC range in which by-products can be redissolved by a reversible reaction, i.e., the second SOC range, is, for example, 0% to 30%, or even 0% to 20%. The lower the SOC, the faster the reversible reaction rate, making it easier for manganese oxide to be redissolved. The second SOC may be lower than the above-mentioned normal SOC range. The second SOC may be less than 20%. If the upper limit of the second SOC range is less than 20%, manganese oxide is more easily redissolved. The second SOC may partially overlap with the above-mentioned normal SOC range. The second SOC may be, for example, 20% to 30%.

[0068] Other electrolytes include, for example, vanadium-based electrolytes. In vanadium-based electrolytes, both the positive and negative electrode electrolytes contain vanadium ions. The vanadium ions in the positive and negative electrode electrolytes have different valences. For vanadium-based electrolytes, the typical SOC range is approximately 5% to 95%.

[0069] (Battery control device) The battery control device 6 not only controls the operation of the RF battery system 1, but also monitors the status of each bank 2 and controls operations necessary to improve it. The battery control device 6 independently manages the charging and discharging of each bank 2. Therefore, it is possible to perform electrolyte recovery work for the second bank 2b while charging or discharging the first bank 2a. The battery control device 6 issues operation commands to the power conversion device 7 of each bank 2. The power conversion device 7 of each bank 2 controls the charging and discharging of the battery cells 10 based on the operation commands from the battery control device 6. The battery control device 6 issues charge / discharge commands to the power conversion device 7 of each first bank 2a when charging or discharging between the power source 91 and the load 92, for example. In the first bank 2a, the battery cells 10 are charged or discharged according to the requested charge or discharge amount. The battery control device 6 issues a recovery work command to the power conversion device 7 of the second bank 2b. In the second bank 2b, electrolyte recovery work is performed.

[0070] The battery control device 6 is typically configured by a computer. The computer includes a processor, memory, timer, etc. The memory stores a control program to be executed by the processor and various data. The processor reads and executes the control program stored in the memory. The program includes a set of instructions related to processing by the controller 60.

[0071] Controller (SOC control) The battery control device 6 includes a controller 60. The controller 60 controls the power conversion device 7 to control the SOC of each bank 2. The controller 60 performs SOC control to set the SOC of the second bank 2b to a second SOC different from the first SOC. The second SOC preferably differs from the first SOC by ±2% or more. Therefore, in the SOC control, the SOC of the second bank 2b is controlled to deviate from the SOC of the first bank 2a by ±2% or more. A state in which the difference between the SOC of the first bank 2a and the SOC of the second bank 2b is ±2% or more can be achieved by intentionally shifting the SOC of the second bank 2b. The second SOC may differ from the first SOC by ±3% or more, or even by ±4% or more.

[0072] Through the SOC control, the SOC of the second bank 2b is controlled to a second SOC different from the first SOC. By maintaining the SOC of the second bank 2b at the second SOC for a certain period of time, the by-products in the electrolyte of the second bank 2b are redissolved. This allows the electrolyte of the second bank 2b to be restored. This improves the deterioration of battery performance in the second bank 2b. The second bank 2b may be charged and discharged within the second SOC range, or may be in standby or stopped state without being charged or discharged. After the SOC of the second bank 2b reaches the second SOC, the time for maintaining the second SOC is equal to or longer than the time required for the by-products in the electrolyte of the second bank 2b to be redissolved and the electrolyte to return to its original state. The longer the time for maintaining the second SOC, the more the by-products in the electrolyte are redissolved and reduced. After the electrolyte has returned to its original state, the second SOC may be maintained, but the by-products cannot be further redissolved. The time for maintaining the second SOC may be, for example, 1 minute to 24 hours, 2 hours to 16 hours, or 4 hours to 12 hours. After the SOC of the second bank 2b reaches the second SOC, the pump circulating the electrolyte in the second bank 2b that is not being charged or discharged may or may not be stopped. Stopping the pump reduces power consumption by the pump.

[0073] (Rotation control) The controller 60 may perform rotation control to control each of the multiple banks 2 so that it includes a time period during which the SOC is a first SOC and a time period during which the SOC is a second SOC. During a time period during which the controller 60 sets the SOC of a bank 2 to the first SOC, that bank 2 is a bank that performs charging and discharging. During a time period during which the controller 60 sets the SOC of a bank 2 to the second SOC, that bank 2 is a bank that performs electrolyte recovery work. That is, each bank 2 has a time period during which it is the first bank 2a that performs charging and discharging, and a time period during which it is the second bank 2b that performs electrolyte recovery work. For example, suppose that during a certain time period, one bank 2 functions as the first bank 2a and another bank 2 functions as the second bank 2b. During another time period, one bank 2 transitions from the state of the first bank 2a to the state of the second bank 2b, and another bank 2 transitions from the state of the second bank 2b to the state of the first bank 2a. By switching each bank 2 to the second bank 2b in turn at predetermined time intervals through the rotation control, the electrolyte recovery work can be performed on all of the plurality of banks 2 in rotation.

[0074] The time period for determining the first SOC is set, for example, to the time period required for the amount of side reaction products produced in the electrolyte in the first bank 2a to reach a predetermined amount. If the amount of side reaction products produced in the electrolyte is less than the predetermined amount, the battery performance of the first bank 2a can be maintained even if a certain amount of side reaction products is produced. The time period for determining the second SOC is set to be at least the time required for the side reaction products in the electrolyte in the second bank 2b to redissolve and for the electrolyte in the second bank 2b to return to its original state. The return of the electrolyte to its original state refers to a state in which the amount of side reaction products produced in the electrolyte is below a certain level. The time required for the electrolyte to return to its original state is determined in advance by testing using the electrolyte to be used.

[0075] The ratio y of the time during which the battery is at the second SOC to the total time during which the battery is at the first SOC and the time during which the battery is at the second SOC is, for example, 30% or less. If the ratio y of the time during which the battery is at the second SOC is 30% or less, the time during which the battery functions as the second bank 2b is short. In other words, the time required for the electrolyte recovery process in each bank 2 is relatively short. The ratio y may be, for example, 20% or less, or even 10% or less.

[0076] <Example of rotation control> For example, suppose the total number of banks 2 is 11. The number of first banks 2a is 10. The number of second banks 2b is 1. In this case, the proportion x of the number of second banks 2b is 1 / 11 ≒ 9.1%. Let the individual banks 2 be No. 1, No. 2, ... No. 11. At a certain time, of the multiple banks 2, No. 1 to No. 10 are first banks 2a controlled at a first SOC, and the remaining No. 11 is second bank 2b controlled at a second SOC. Suppose that each bank 2 is periodically switched to second bank 2b in turn. Here, the time period during which the second SOC is used is assumed to be one day.

[0077] When one day has passed from a certain time, No. 11 is switched from the second bank 2b to the first bank 2a. Instead of No. 11, one of No. 1, No. 2, No. 10 is switched from the first bank 2a to the second bank 2b. In this case, No. 10 is switched to the second bank 2b. At this time, No. 1 to No. 9 and No. 11, excluding No. 10, become the first bank 2a. The number of first banks 2a and the number of second banks 2b do not change. Furthermore, Nikkei When this time has elapsed, No. 10 is switched to the first bank 2a and No. 9 is switched to the second bank 2b. In this way, by switching from the first bank 2a to the second bank 2b in the order of No. 11 → No. 10 → No. 9 → ··· No. 1 → No. 11 → ··· as time passes, the electrolyte recovery work can be performed for all banks 2. Under the above conditions, the time period during which a specific bank 2 is continuously at the first SOC is 10 days. In other words, the time during which charging and discharging is possible using the first bank 2a is 10 days. The cycle from when an individual bank 2 goes from the state of second bank 2b to the next state of second bank 2b is 11 days. The time period during which the bank 2 is at the first SOC and the second SOC are two The ratio y of the time spent with the second SOC to the total time spent with the first SOC is 1 (days) / 11 (days) ≒ 0.091.

[0078] (Transition Control) The controller 60 may perform transition control for a specific first bank 2a among the multiple first banks 2a, causing the bank 2a to transition from a first SOC to a second SOC. Transitioning from a first SOC to a second SOC means switching the specific first bank 2a to a second bank 2b. If the SOC of the specific first bank 2a is higher than the second SOC, the controller 60 discharges the specific first bank 2a to lower the SOC of the specific first bank 2a to the second SOC. Conversely, if the SOC of the specific first bank 2a is lower than the second SOC, the controller 60 charges the specific first bank 2a to raise the SOC of the specific first bank 2a to the second SOC. The transition control may be, for example, one of the following first, second, and third transition controls.

[0079] First Transition Control The first transition control charges or discharges from a specific first bank 2a to a second bank 2b.

[0080] First example of transition control An example of a first transition control will be described below when the SOC of a specific first bank 2a is higher than the second SOC. As described in the example of rotation control above, consider the time when one day has passed since a certain time. Specifically, consider switching No. 10 from the first SOC to the second SOC and switching from the first bank 2a to the second bank 2b. In this case, when switching No. 11 from the second bank 2b to the first bank 2a, a command is issued to each power conversion device 7 to discharge No. 10 and charge No. 11. By discharging No. 10, No. 10 changes from the first bank 2a to the second bank 2b. By charging No. 11, No. 11 changes from the second bank 2b to the first bank 2a. Because charging or discharging is performed between the specific first bank 2a and the second bank 2b, transition control can be performed even without charging or discharging between the external power source 91 and the load 92.

[0081] <Second Transition Control> The second transition control preferentially discharges or charges a specific first bank 2a over other banks 2 when discharging or charging between the external power source 91 and the load 92. Specifically, the specific first bank 2a is discharged or charged at a value that is 1 / (1-x) times or more the average value obtained by dividing the values ​​of the discharge amounts or charge amounts for the multiple first banks 2a by the total number of the multiple banks 2. Here, x is the ratio of the number of second banks 2b to the total number of the multiple banks 2.

[0082] <Second example of transition control> We will explain an example in which all banks 2 are operating as first banks 2a, and only a specific bank 2 is migrated to second bank 2b. The total number of banks 2 is 11. The number of first banks 2a after the migration is 10. The number of second banks 2b after the migration is 1. The ratio x of the number of second banks 2b is 1 / 11 ≒ 0.091. 1 / (1-x) is 1.1. The rated output of one bank 2 is 100kW. The rated output of the entire system, minus second bank 2b, is 100(kW) x 10 = 1000kW.

[0083] In this example, we consider a situation where banks 1 to 11 are the first bank 2a, as described in the rotation control example. Then, bank 11 is shifted from the first SOC to the second SOC, switching from the first bank 2a to the second bank 2b. In this case, when discharging to the external load 92, the allocation of the discharge amount to bank 11 is increased so that bank 11 is given priority for discharge. Specifically, bank 11 is discharged at a value exceeding the average value obtained by dividing the discharge amounts for the multiple first banks 2a by the number of all banks 2. For example, when a discharge command of 1000 kW is received, bank 11 is discharged at its rated output of 100 kW. Banks 1 to 10, which are the first bank 2a, are discharged at 90 kW, the average of the remaining 900 kW discharge amount. When the discharge amount to each bank 2 is evenly allocated, the average value obtained by dividing the discharge command value by the total number of banks 2 is 1000 / 11 ≒ 90.9 kW. Dividing the discharge rate of 100 kW for No. 11 by this average value gives 100 / (1000 / 11) = 1.1. The discharge rate of 100 kW for No. 11 is 1 / (1-x) = 1.1 times or more the average value. Since No. 11 is preferentially discharged at a value 1.1 times or more the average value, the transition time can be shortened.

[0084] As another example of the second transition control, consider the case where one day has passed since a certain time, as described in the example of rotation control above. Specifically, consider transitioning No. 10 from the first SOC to the second SOC and switching from the first bank 2a to the second bank 2b. In this case, when discharging to the external load 92, the allocation of the discharge amount to No. 10 is increased so that No. 10 is given priority for discharge. Specifically, No. 10 is discharged at a value equal to or greater than the average value obtained by dividing the discharge amounts for the multiple first banks 2a by the number of first banks 2a. For example, when a discharge command of 550 kW is received, No. 10 is discharged at its rated output of 100 kW. At this time, No. 11 is not discharged because its transition from the second SOC to the first SOC has not yet been completed. Nos. 1 to 9 of the first bank 2a, excluding No. 10, are discharged at the average of the remaining discharge amounts of 450 kW, i.e., 50 kW. When the discharge amount to each bank 2 is evenly distributed, the average value obtained by dividing the discharge command value by the total number of banks 2 is 550 / 11 = 50 kW. The value obtained by dividing the above-mentioned discharge amount of 100 kW of No. 10 by this average value is 100 / (550 / 11) = 2. The discharge amount of 100 kW of No. 10 is 1 / (1-x) = 1.1 times or more the above average value. Since No. 10 is preferentially discharged at a value 1.1 times or more the above average value, the transition time can be shortened.

[0085] For No. 11, the SOC is shifted from the second to the first bank 2b, and the bank 2a is switched from the second bank 2b to the first bank 2a. In this case, when charging from the external power source 91, the charge allocation to No. 11 is increased so that No. 11 is given priority for charging. Specifically, No. 11 is charged to a value equal to or greater than the average value obtained by dividing the charge values ​​for the multiple first banks 2a by the number of first banks 2a. For example, when a 550 kW charge command is received, No. 11 is charged at the rated output of 100 kW. At this time, No. 10 is not charged. Nos. 1 to 9 are charged at the average value of the remaining charge amount of 450 kW, i.e., 50 kW. To evenly allocate the charge amount to each bank 2, the average value obtained by dividing the charge command value by the total number of banks 2 is 550 / 11 = 50 kW. As with the transition control for No. 10 described above, No. 11 is given priority for charging at a value equal to or greater than 1.1 times the average value, thereby shortening the transition time.

[0086] <Third Transition Control> Similar to the second transition control described above, the third transition control preferentially discharges or charges a specific first bank 2a over other banks 2 when discharging or charging between the external power source 91 and the load 92. Unlike the second transition control, the third transition control discharges or charges a specific first bank 2a at a value that is at least 1 / (1-y) times the average value obtained by dividing the values ​​of the discharge amounts or charge amounts for the multiple first banks 2a by the total number of the multiple banks 2. Here, y is the ratio of the time during which the battery is at the second SOC to the total time during which the battery is at the first SOC and the second SOC.

[0087] <Third example of transition control> As with the second example of transition control, an example will be described in which all banks 2 are operating as first banks 2a, but only a specific bank 2 is transitioned to second bank 2b. The total number of banks 2 is 11. The number of first banks 2a after the transition is 10. The number of second banks 2b after the transition is 1. The ratio y of the time at the second SOC to the total time at the first SOC and the time at the second SOC is 1 (days) / 11 (days) ≒ 0.091, as explained in the example of rotation control above. 1 / (1 - y) is 1.1. The rated output of one bank 2 is 100 kW. The rated output of the entire system, excluding second bank 2b, is 100 (kW) x 10 = 1000 kW.

[0088] As in the second transition control example, consider switching No. 11 from the first SOC to the second SOC and switching from the first bank 2a to the second bank 2b. In the third transition control, the basic concept of discharging No. 11 at a value greater than the average value obtained by dividing the discharge amounts for the multiple first banks 2a by the total number of banks 2 is the same as in the second transition control. For example, when a discharge command of 1000 kW is received, No. 11 is discharged at the rated output of 100 kW. The first banks 2a, Nos. 1 to 10, are discharged at the average value of the remaining 900 kW discharge amount, i.e., 90 kW. When the discharge amount to each bank 2 is evenly distributed, the average value obtained by dividing the discharge command value by the total number of banks 2 is 1000 / 11 ≒ 90.9 kW. The value obtained by dividing the discharge amount of No. 11 (mentioned above, 100 kW) by this average value is 100 / (1000 / 11) = 1.1. The discharge amount of 100 kW for No. 11 is 1 / (1-y) = 1.1 times or more the above average value. Since No. 11 is preferentially discharged at a value 1.1 times or more the above average value, the transition time can be shortened.

[0089] (counter) In this embodiment, each bank 2 is provided with a counter 80 that quantifies the amount of by-products produced in the electrolyte, as shown in Fig. 1. By providing the counter 80, the timing for the controller 60 to shift from the first bank 2a to the second bank 2b can be appropriately determined based on the amount of by-products produced.

[0090] The amount of by-products produced can be determined, for example, by measuring the optical properties and physical properties of the electrolyte, or as a function of the SOC value, the temperature of the electrolyte, and time. Optical properties of the electrolyte include, for example, the color and transparency of the electrolyte. Depending on the electrolyte, the color and transparency may change due to the generation of by-products. The greater the amount of by-products produced, the greater the change in color and transparency tends to be. Therefore, the amount of by-products produced can be quantitatively determined by measuring the optical properties of the electrolyte. The optical properties of the electrolyte can be measured, for example, by image processing of images taken with a camera or using a spectrophotometer. Physical properties of the electrolyte include, for example, viscosity, conductivity, and density. Depending on the electrolyte, the generation of by-products may change the viscosity, conductivity, and density. The greater the amount of by-products produced, the greater the change in viscosity, conductivity, and density tends to be. Therefore, the amount of by-products produced can also be quantitatively determined by measuring the physical properties of the electrolyte.

[0091] In addition, side effects thing The amount of side reaction products produced depends on the parameters of the SOC value, the temperature of the electrolyte, and time. The higher or lower the SOC, the more likely side reactions occur, and the greater the amount of side reaction products produced. The higher the temperature of the electrolyte, the faster the side reaction occurs, and the greater the amount of side reaction products produced. The longer the battery is held at an SOC at which side reactions occur, the greater the amount of side reaction products produced. Therefore, the amount of side reaction products produced can be quantitatively determined as a function of the above parameters. In this embodiment, the SOC is managed from electrical information such as the amount of electricity during charging or discharging in each bank 2 and the charge / discharge time.

[0092] <Energy density calculation example> The energy density of the RF battery system 1 according to the embodiment was calculated. Here, the energy densities of a working model using the embodiment and a comparative model with a conventional design are calculated and compared. The energy density is calculated as the battery capacity per unit amount of electrolyte.

[0093] (Comparative model) The comparative model is an RF battery system having multiple banks, all of which are configured to charge and discharge. That is, the comparative model differs from the embodiment in that all of the banks are first banks and the comparative model does not have a second bank. The comparison model sets the SOC utilization range from 20% to 80%. The basic specifications of the comparison model are shown in Table 1. The effective number of banks is the number of banks used for charging and discharging. In the comparison model, the effective number of banks is the same as the total number of banks.

[0094] (Implementation model) The working model has the same number of banks as the comparison model. The number of first banks is 15. The number of second banks is 1. The implementation model sets the SOC utilization range from 20% to 86%. The SOC utilization range of the implementation model is 10% larger than that of the comparison model. Therefore, the time capacity of the implementation model increases by 10% compared to the comparison model. The basic specifications of the implementation model are shown in Table 2. In the implementation model, the number of effective banks is the same as the number of first banks. In other words, the number of effective banks is the total number of banks minus the number of second banks. In Table 2, "t+δ" represents the time capacity, and "δ" indicates the expansion of the time capacity due to the expansion of the SOC usage range.

[0095] [Table 1]

[0096] [Table 2]

[0097] Tables 1 and 2 show that the working model has a smaller number of effective banks than the comparative model, but its energy density is higher than that of the comparative model. This is because the working model has a wider SOC range than the comparative model, which increases the hourly capacity. The energy density calculation formula suggests that a larger number of first banks makes it easier to achieve an improved energy density. Conditions under which the working model can be more effective include a large total number of banks n and a large expansion range δ of hourly capacity. Specifically, it is more effective when the number of banks n is, for example, 30 or more, and the expansion range δ is, for example, 20% or more.

[0098] <Effects of the embodiment> The RF battery system 1 of the above-described embodiment can improve the degradation of battery characteristics due to side reactions while realizing a high energy density for the following reasons.

[0099] Expanding the range of SOC available in the first bank 2a allows for increased energy density. Controlling the SOC of the second bank 2b to a level at which by-products redissolve allows for recovery of the electrolyte in the second bank 2b. This reduces the risk of battery performance degradation due to side reactions. Furthermore, recovery of the electrolyte in the second bank 2b can be performed while charging or discharging the first bank 2a. Since the electrolyte recovery can be performed without shutting down the system, fewer operational constraints are imposed. By using rotation control by the controller 60 to switch each bank 2 to the second bank 2b in turn at predetermined intervals, the electrolyte recovery can be performed on all of the multiple banks 2 in rotation. [Explanation of symbols]

[0100] 1 RF battery system 2 banks 2a First bank, 2b Second bank 10 battery cells 11 Diaphragm 12 positive electrode cell, 13 negative electrode cell 14 positive electrode, 15 negative electrode 20 Electrolyte Tank 22 positive electrode electrolyte tank, 23 negative electrode electrolyte tank 30 Monitor Cell 42,43 Outward piping, 44,45 Return piping 46,47 Pump 5 cell frame, 50 frame body, 51 bipolar plate, 55 sealing member 100 cell stack, 101 end plate, 102 fastening member 6 Battery control device 60 Controller 7 Power conversion equipment 80 counters 90 Substation equipment, 91 Power source, 92 Load

Claims

1. Multiple banks and a power conversion device provided in each of the plurality of banks; a controller that controls the power conversion device to control the state of charge of each of the plurality of banks; each of the plurality of banks includes battery cells that are charged and discharged by supplying an electrolyte; The plurality of banks include: a plurality of first banks controlled to a first state of charge; one or more second banks excluding the first bank, the controller controls the state of charge of the second bank to a second state of charge different from the first state of charge; Redox flow battery system.

2. 2. The redox flow battery system of claim 1, wherein the second state of charge differs from the first state of charge by ±2% or more.

3. the first state of charge has a first state of charge range defined by an upper limit and a lower limit; the second state of charge has a second state of charge range defined by an upper limit and a lower limit; 3. The redox flow battery system according to claim 1, wherein the second state-of-charge range is 30% or less of the first state-of-charge range.

4. 4. The redox flow battery system according to claim 1, wherein a ratio of the number of the second banks to a total number of the plurality of banks is 30% or less.

5. 5. The redox flow battery system according to claim 1, wherein the controller controls each of the plurality of banks to include a time period during which the banks are in the first state of charge and a time period during which the banks are in the second state of charge.

6. 6. The redox flow battery system according to claim 5, wherein a ratio of a time period during which the battery is in the second state of charge to a total time period during which the battery is in the first state of charge and a time period during which the battery is in the second state of charge is 30% or less.

7. 7. The redox flow battery system according to claim 1, wherein when a specific bank among the plurality of first banks is caused to transition from the first state of charge to the second state of charge, the controller charges or discharges the second bank from the specific bank.

8. a ratio of the number of the second banks to the total number of the plurality of banks is x; 7. The redox flow battery system according to claim 1, wherein when the controller causes a specific bank among the plurality of first banks to transition from the first state of charge to the second state of charge, the controller discharges or charges the specific bank by a value that is 1 / (1-x) times or more higher than an average value obtained by dividing a value of a discharge amount or a charge amount for the plurality of first banks by a total number of the plurality of banks.

9. a ratio of the time spent in the second state of charge to the total time spent in the first state of charge and the second state of charge is y; 7. The redox flow battery system according to claim 1, wherein when the controller causes a specific bank among the plurality of first banks to transition from the first state of charge to the second state of charge, the controller discharges or charges the specific bank by a value that is 1 / (1-y) times or more greater than an average value obtained by dividing a value of a discharge amount or a charge amount for the plurality of first banks by a total number of the plurality of banks.

10. 10. The redox flow battery system according to claim 1, wherein each of the plurality of banks includes a counter that digitizes an amount of a by-product produced in the electrolyte.

11. 11. The redox flow battery system according to claim 10, wherein the counter quantifies the amount of a by-product produced in the electrolyte as a function of a state of charge value in each of the plurality of banks, a temperature of the electrolyte, and time.

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