REDOX FLOW BATTERY SYSTEM

A redox flow battery system includes: a plurality of banks; a power conversion device provided in each of the plurality of banks; and a controller that controls a state of charge of each of the plurality of banks by controlling the power conversion device. In the redox flow battery system, each of the plurality of banks includes a battery cell that performs charging and discharging by a supply of an electrolytic solution, the plurality of banks include: a plurality of first banks controlled to be in a first state of charge; and one or more second banks excluding the first banks, and the controller controls a state of charge of the second bank to be a second state of charge different from the first state of charge.

TECHNICAL FIELD

The present disclosure relates to a redox flow battery system.

This application claims priority from Japanese Patent Application No. 2021-102299, filed on Jun. 21, 2021, the contents of which are incorporated herein by reference in their entirety.

BACKGROUND ART

PTL 1 discloses an operation method of a redox flow battery system that performs charging and discharging by supplying a positive electrolyte and a negative electrolyte to a battery cell. In PTL 1, it is described that a variation in voltage between a plurality of banks is controlled in the redox flow battery system including the banks.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

A 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 a state of charge of each of the plurality of banks by controlling the power conversion device. In the redox flow battery system, each of the plurality of banks includes a battery cell that performs charging and discharging by the supply of an electrolytic solution, the plurality of banks include: a plurality of first banks controlled to be in a first state of charge; and one or more second banks excluding the first banks, and the controller controls a state of charge of the second bank to be a second state of charge different from the first state of charge.

DETAILED DESCRIPTION

Problem to be Solved by the Present Disclosure

It is required to enhance the energy density in a redox flow battery system. When the energy density can be increased, the battery capacity can be enhanced. However, when the energy density is to be increased, a side reaction easily occurs at the time of charging or discharging. There is a possibility that a decrease in the battery performance such as the deterioration of an electrolytic solution is caused by the side reaction.

In general, in a redox flow battery, the usage range of the state of charge (SOC) is limited to suppress the decrease in the battery performance due to a side reaction. The technique described in PTL 1 causes the SOCs of the banks to be as equal as possible by performing voltage control that reduces the difference in voltage between the banks. In the technique described in PTL 1, operation is performed so as to cause the SOCs of all of the banks to be equal within an SOC range in which a side reaction does not occur. Therefore, the usage range of the SOC cannot be expanded.

An object of the present disclosure is to provide a redox flow battery system capable of improving the decrease in battery performance due to a side reaction while being able to realize high energy density.

Advantageous Effect of the Present Disclosure

The redox flow battery system of the present disclosure is capable of improving the decrease in the battery performance due to the side reaction while being able to realize high energy density.

Description of Embodiments

The inventors of the present invention have gained the following insight as a result of intensive research regarding acquisition of high energy density in a redox flow battery and the influence of the acquisition of the high energy density on the battery performance.

One measure to enhance the energy density of the redox flow battery is to expand the usage range of the SOC. However, when the usage range of the SOC is expanded, a side reaction may occur, and the battery performance may be adversely affected.

In general, in the redox flow battery, the SOC is managed to be maintained within a certain range in order to suppress the decrease in the battery performance due to a side reaction. In a region in which the SOC is high or a region in which the SOC is low, an active material ion included in the electrolytic solution may be deposited by the side reaction. When the active material ion is deposited, there is a possibility that the active material ion may not function as an active material. The deposition of the active material ion causes the deterioration of the electrolytic solution and the decrease in the battery capacity. A deposit of the active material ion causes the deterioration of a battery cell by adhering to an electrode constituting the battery cell, for example. As above, when charging or discharging is performed in the region in which the SOC is high or the region in which the SOC is low, a side reaction such as the deposition of an active material ion is facilitated. When the generation amount of a side reaction product such as the deposit of the active material ion increases, the battery performance decreases. Thus, in the related-art redox flow battery, control is commonly performed so as to perform charging and discharging within an SOC range in which a side reaction hardly occurs. In other words, the usage range of the SOC is limited.

The inventors of the present invention have found that it may be possible to dissolute a product generated by a side reaction into the electrolytic solution as an ion again by a reversible reaction. For example, when a deposit of an active material ion is generated by a side reaction in the region in which the SOC is high, it may be possible to cause the deposit to return into an ion again in the region in which the SOC is low. Alternatively, when a deposit of an active material ion is generated by a side reaction in the region in which the SOC is low, it may be possible to cause the deposit to return into an ion again in the region in which the SOC is high. In such case, it becomes possible to redissolve the deposit of the active material ion as an ion by controlling the SOC to be periodically maintained in the region in which the SOC is low or the region in which the SOC is high for a certain amount of time. Therefore, even when a side reaction occurs, it becomes possible to recover the electrolytic solution by redissolving the side reaction product such as the deposit of the active material ion by controlling the SOC. It can be possible to improve the decrease in the battery performance due to a side reaction. However, the control of the SOC for redissolving the side reaction product forcibly controls the SOC into a particular range, and hence cannot perform charging or discharging at the same time. Therefore, the control of the SOC described above needs to be performed when the battery cell is not performing charging and discharging. When the control of the SOC described above is executed, the operation of the redox flow battery is stopped, and hence a restriction occurs in the operation. There is less freedom of operation for a user.

The inventors of the present invention propose shifting the SOCs between the plurality of banks in the redox flow battery system including the banks in order to realize high energy density. In detail, control is not performed so as to cause the SOCs of all of the banks to be equal, and control is performed such that the SOCs of some banks are different.

An embodiment of the present disclosure is listed and described first.

(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 a state of charge of each of the plurality of banks by controlling the power conversion device. In the redox flow battery system, each of the plurality of banks includes a battery cell that performs charging and discharging by a supply of an electrolytic solution, the plurality of banks include: a plurality of first banks controlled to be in a first state of charge; and one or more second banks excluding the first banks, and the controller controls a state of charge of the second bank to be a second state of charge different from the first state of charge.

The redox flow battery system of the present disclosure is capable of improving the decrease in the battery characteristics due to the side reaction while being able to realize high energy density. The state of charge of the bank means the state of charge of a battery cell constituting the bank. When there are a plurality of battery cells, the state of charge of the battery cell is an average value of the states of charge of the plurality of battery cells. The state of charge is simply referred to as an “SOC” below.

In the redox flow battery system of the present disclosure, the first bank is a bank that performs charging from a power source such as a power generation facility and performs discharging to a load. The SOC of the first bank fluctuates in accordance with charging and discharging. The SOC of the first bank is controlled to be the first SOC. The first SOC is a particular value within a range obtained by expanding the usage range of a normal SOC. The usage range of the normal SOC is an SOC range in which a side reaction hardly occurs and a side reaction product is hardly generated even when charging and discharging are repeated. The second bank is a bank that performs recovery work of the electrolytic solution such as the redissolution of the side reaction product. The SOC of the second bank is controlled to be the second SOC. The second SOC is a particular value within an SOC range in which a product generated by a side reaction can be redissolved by a reversible reaction.

In the redox flow battery system of the present disclosure, it becomes possible to enhance the energy density by expanding the usage range of the SOC in the first bank. The battery capacity can be secured as the entire system. It becomes possible to recover the electrolytic solution of the second bank by controlling the SOC of the second bank to be the SOC in which the side reaction product is redissolved. Therefore, it becomes possible to improve the decrease in the battery performance due to the side reaction. In the redox flow battery system of the present disclosure, it becomes possible to perform the recovery work of the electrolytic solution of the second bank while performing charging or discharging in the first bank. It becomes possible to perform the recovery work of the electrolytic solution without stopping the system, and hence there are less operation restrictions.

The expression of “the side reaction product is redissolved” described above is defined as causing the state of the electrolytic solution that has changed by charging or discharging in a particular SOC range to return to the original state in a broad sense. This is not limited to causing a solid deposit to return to an ion in the electrolytic solution and may or may not be associated with changes in phase of gas, solid, and liquid. In the case of an electrolytic solution including a solid active material such as zinc, controlling the amount of electrodeposition thereof is also included. An active material may be deposited on a surface of the electrode by a side reaction. When a deposit due to a side reaction adheres to the electrode, the reactivity of the electrode deteriorates. In other words, the battery performance decreases. A catalyst and the like may be caused to adhere to a surface of the electrode for the purpose of improving the reactivity of the electrode. The decrease in the battery performance is also caused when the deposit due to the side reaction is deposited so as to cover the catalyst and the like. It becomes possible to maintain the state of the surface of the electrode by redissolving the deposit that has adhered to the surface of the electrode. As a result, it becomes possible to suppress the decrease in the battery performance.

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

The redox flow battery system described above controls the SOC of the second bank to be shifted 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 realized by intentionally shifting the SOC of the second bank from the SOC of the first bank.

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

The redox flow battery system described above easily performs the recovery work of the electrolytic solution of the second bank.

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

In the redox flow battery system described above, out of the plurality of banks, the second banks are smaller in number than the first banks. In other words, the first banks are greater in number. The number of the first banks that perform charging and discharging is greater than the number of the second banks that perform the recovery work of the electrolytic solution, and hence the battery capacity is easily secured as the entire system.

(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 set to the first state of charge and a time period set to the second state of charge.

The redox flow battery system described above is capable of performing the recovery work of the electrolytic solution for all of the banks. The bank functions as the first bank in the time period set to the first state of charge. The bank functions as the second bank in the time period set to the second state of charge. In other words, there are a time period in which each bank is the first bank that performs charging and discharging and a time period in which each bank is the second bank that performs the recovery work of the electrolytic solution. The recovery work of the electrolytic solution can be performed by each predetermined amount of time for each of the banks.

(6) In the redox flow battery system according to (5) described above, a rate of an amount of time set to the second state of charge to a total amount of time of an amount of time set to the first state of charge and the amount of time set to the second state of charge may be 30% or less.

In the redox flow battery system described above, the amount of time the recovery work of the electrolytic solution is performed for each of the banks is set to be relatively short.

(7) In the redox flow battery system of the present disclosure, when a particular bank out of the plurality of first banks is caused to transition from the first state of charge to the second state of charge, the controller may perform charging or discharging from the particular bank to the second bank.

In the redox flow battery system described above, it becomes possible to cause a particular bank to transition from the first state of charge to the second state of charge by performing charging or discharging between the banks even when charging or discharging is not performed with respect to an external power source or a load. In the example described above, it becomes possible to perform the transition of the second bank from the second state of charge to the first state of charge at the same time.

(8) In the redox flow battery system of the present disclosure, a rate of a number of the second banks to a total number of the plurality of banks may be represented by x, and the controller may discharge or charge a particular bank out of the plurality of first banks by a value that is 1/(1−x) times or more of an average value obtained by dividing a value of a discharging amount or a charging amount to the plurality of first banks by the total number of the plurality of banks when the particular bank is caused to transition from the first state of charge to the second state of charge.

The redox flow battery system described above causes a particular bank to transition to the second bank by preferentially discharging or charging the particular bank as compared to other banks when the plurality of first banks are discharged or charged. Therefore, a shorter amount of transition time is required as compared to a case where all of the banks are charged or discharged by an average value.

(9) In the redox flow battery system of the present disclosure, a rate of an amount of time set to the second state of charge to a total amount of time of an amount of time set to the first state of charge and the amount of time set to the second state of charge may be represented by y, and the controller may discharge or charge a particular bank out of the plurality of first banks by a value that is 1/(1−y) times or more of an average value obtained by dividing a value of a discharging amount or a charging amount to the plurality of first banks by the total number of the plurality of banks when the particular bank is caused to transition from the first state of charge to the second state of charge.

The redox flow battery system described above causes a particular bank to transition to the second bank by preferentially discharging or charging the particular bank as compared to other banks when the plurality of first banks are discharged or charged. Therefore, a shorter amount of transition time is required as compared to a case where all of the banks are charged or discharged by an average value.

(10) In the redox flow battery system of the present disclosure, each of the plurality of banks may have a counter that quantifies a generation amount of a side reaction product in the electrolytic solution.

The redox flow battery system described above is capable of suitably determining a timing for performing the recovery work of the electrolytic solution.

(11) In the redox flow battery system according (10) described above, the counter may quantify the generation amount of the side reaction product in the electrolytic solution as a function of each of a value of a state of charge, a temperature of the electrolytic solution, and an amount of time in each of the plurality of banks.

The redox flow battery system described above is capable of suitably obtaining the generation amount of the side reaction product.

DETAILS EMBODIMENT OF PRESENT DISCLOSURE

A specific example of the redox flow battery system of the present disclosure is described with reference to the drawings. The redox flow battery may be referred to as an “RF battery” below. The same reference characters in the drawings indicate the same or equivalent parts.

The invention is not limited to the exemplifications above and is defined by the scope of claims. All modifications made within the scope and spirit equivalent to those of the claims are intended to be included in the invention.

<Overview of RF Battery System>

With reference toFIG.1andFIG.2, an RF battery system1according to the embodiment is described. As shown inFIG.1, RF battery system1includes a plurality of banks2, power conversion devices (PCS: Power Conditioning System)7respectively provided in plurality of banks2, and a battery control device (BMS: Battery Management System)6. Each bank2includes a battery cell10.

RF battery system1is representatively connected to an external power source91and an external load92via a transformer facility90. RF battery system1can be charged with electric power supplied from power source91and discharge the charged electric power to load92. Power source91is a power generation facility using natural energy such as solar power and wind power or other general power plants. Load92is a power system, a consumer, or the like. RF battery system1is used for the purpose of load leveling, the purpose of instantaneous voltage drop compensation, an emergency power source, and the like, and the purpose of output smoothing of natural energy power generation, for example.

One feature of RF battery system1of the embodiment is as follows. In plurality of banks2, not all of the SOCs of the banks are aligned with each other, and the SOCs of some banks are shifted from each other. Specifically, plurality of banks2include a first bank2aand a second bank2b. A controller60that controls the SOC of each bank2is included. Controller60performs control such that the SOC of second bank2bis different from the SOC of first bank2a.

A basic configuration of banks2constituting RF battery system1is described first below. Then, the features of RF battery system1are described in detail.

Individual banks2can independently control the charging and discharging of battery cells10. Bank2includes battery cell10, an electrolytic solution tank20, and a power conversion device7. An electrolytic solution is supplied to battery cell10from electrolytic solution tank20. Electrolytic solution tank20stores therein the electrolytic solution. Electrolytic solution tank20has a positive electrolyte tank22and a negative electrolyte tank23. Positive electrolyte tank22stores therein a positive electrolyte. Negative electrolyte tank23stores therein a negative electrolyte. Bank2performs charging and discharging by supplying the electrolytic solution to battery cell10from electrolytic solution tank20. Bank2can function as a secondary battery by itself.

Battery cell10is a cell of a secondary battery. In other words, battery cell10performs charging and discharging by the supply of the electrolytic solution. As shown inFIG.2, battery cell10has a positive electrode14, a negative electrode15, and a membrane11disposed between positive electrode14and negative electrode15. Battery cell10is separated into a positive cell12and a negative cell13by membrane11. Positive electrode14is built in positive cell12. Negative electrode15is built in negative cell13.

The positive electrolyte is supplied to positive cell12. The negative electrolyte is supplied to negative cell13. Bank2of this example includes a supply pipe42and a return pipe44that connect battery cell10and positive electrolyte tank22to each other. Bank2includes a supply pipe43and a return pipe45that connect battery cell10and negative electrolyte tank23to each other. Pumps46,47are provided in each of supply pipes42,43. The positive electrolyte is supplied to positive cell12from positive electrolyte tank22through supply pipe42by pump46. The positive electrolyte that has passed through positive cell12and been discharged from positive cell12is returned to positive electrolyte tank22through return pipe44. The negative electrolyte is supplied to negative cell13from negative electrolyte tank23through supply pipe43by pump47. The negative electrolyte that has passed through negative cell13and been discharged from negative cell13is returned to negative electrolyte tank23through return pipe45. In other words, the electrolytic solution circulates between electrolytic solution tank20and battery cell10.

Bank2may include a cell stack100in which plurality of battery cells10are stacked. Cell stack100includes a stacked body in which a cell frame5, positive electrode14, membrane11, and negative electrode15are repeatedly stacked in the stated order, and two end plates101that sandwich the stacked body from both sides. Cell stack100is configured by sandwiching the stacked body by two end plates101and fastening the place between end plates101by fastening members102. In general, cell stack100has a structure in which a predetermined number of battery cells10constitute a sub stack (not shown) and a plurality of sub stacks are stacked.

Cell frame5has a bipolar plate51disposed between positive electrode14and negative electrode15, and a frame body50provided around bipolar plate51. On the inner side of frame body50, positive electrode14and negative electrode15are housed across bipolar plate51. One battery cell10is configured by disposing positive electrode14and negative electrode15across membrane11between bipolar plates51of adjacent cell frames5. A seal member55is disposed between frame bodies50in order to suppress the leakage of the electrolytic solution from battery cells10.

Bank2may include plurality of cell stacks100. Plurality of cell stacks100may be connected in series or parallelly connected. When bank2includes plurality of cell stacks100, one electrolytic solution tank20or plurality of electrolytic solution tanks20may be included for plurality of cell stacks100. When one electrolytic solution tank20is included, one positive electrolyte tank22and one negative electrolyte tank23are included. When plurality of electrolytic solution tanks20are included, plurality of positive electrolyte tanks22and plurality of negative electrolyte tanks23are included. When bank2includes plurality of battery cells10, electrolytic solution with uniform SOC is supplied to each battery cell10.

The electrolytic solution is representatively a solution including an active material ion. The active material ion is an ion that functions as an active material. A metal ion of which valence changes by oxidoreduction can be used as the active material ion. The active material ion is an ion of an element selected from a group consisting of manganese, vanadium, iron, chromium, titanium, and zinc, for example.

The electrolytic solution of at least one of the positive electrolyte and the negative electrolyte has the following characteristics.(1) A side reaction occurs and a side reaction product is generated in accordance with the SOC range at the time of charging or discharging.(2) The side reaction product can be redissolved by a reversible reaction.

The electrolytic solution is an electrolytic solution in which a side reaction product can be generated by a side reaction in a high SOC region in which the SOC is high and the side reaction product can be redissolved by a reversible reaction in a region in which the SOC is relatively low, for example. Alternatively, the electrolytic solution is an electrolytic solution in which a side reaction product can be generated by a side reaction in a low SOC region in which the SOC is low and the side reaction product can be redissolved by a reversible reaction in a region in which the SOC is relatively high. Even when a side reaction occurs, the electrolytic solution as above can be recovered to the original state by redissolving the side reaction product by the control of the SOC. The side reaction product generated by the side reaction is a deposit of the active material ion, for example.

The electrolytic solution is an electrolytic solution containing a manganese ion, for example. When a positive electrolyte containing manganese ion is used, the manganese oxide may be deposited in the positive electrolyte in the high SOC region at the time of charging. The manganese oxide may be decomposed into a manganese ion and redissolved into the positive electrolyte in the low SOC region. When the positive electrolyte is an electrolytic solution containing a manganese ion, the negative electrolyte is an electrolytic solution containing a titanium ion, for example.

Power conversion device7controls the charging and discharging of battery cell10. Battery cell10performs charging and discharging via power conversion device7. Each bank2can independently control the charging and discharging of battery cell10by power conversion device7. Power conversion device7is controlled by a battery control device6. An AC/DC conversion device can be used for power conversion device7, for example.

As shown inFIG.1, each bank2includes a monitor cell30for measuring the SOC. An electrolytic solution in common with the electrolytic solution supplied to battery cell10is supplied to monitor cell30. In other words, the electrolytic solution supplied to battery cell10and the electrolytic solution supplied to monitor cell30are supplied from same electrolytic solution tank20. A single cell having basically the same configuration as battery cell10described above can be used as monitor cell30. Monitor cell30does not perform charging and discharging. The SOC of bank2can be obtained from open circuit voltage (OCV) of monitor cell30.

Monitor cell30may be provided separately from battery cell10or some cells of cell stack100described above may be configured as the monitor cell. When monitor cell30is separately provided, monitor cell30may be provided on the upstream side of battery cell10. The electrolytic solution before being charged or discharged by battery cell10is supplied to monitor cell30provided upstream from battery cell10. This is because the SOC detected by monitor cell30differs from the SOC of the electrolytic solution in electrolytic solution tank20when monitor cell30is provided downstream from battery cell10. When bank2includes plurality of battery cells10, monitor cell30measures an average value of the SOCs of plurality of battery cells10.

<Features of RF Battery System>

As shown inFIG.1, RF battery system1includes plurality of banks2and battery control device6. The number of banks2is four or more, for example. The battery capacity of the entire system can be increased as the number of banks2becomes larger. The number of banks2may be further 11 or more or 16 or more. When the number of banks2becomes greater, the entire system becomes enormous and the management of bank2becomes complicated. The upper limit of the number of banks2is 100 or less or further 50 or less, for example. The number of banks2may be 11 or more and 100 or less, or 16 or more and 50 or less, for example. InFIG.1, each bank2is indicated as No. 1, No. 2 . . . . No. n. Here, n is a natural number.

Plurality of banks2include plurality of first banks2aand one or more second banks2b. First banks2aare banks controlled to be in the first SOC out of plurality of banks2. Second bank2bis a bank other than first banks2aout of plurality of banks2. Second bank2bis a bank controlled to be in the second SOC different from the first SOC. Here, first banks2aand second bank2bare not physically fixed. Individual banks2may function as first banks2aor second banks2b. Therefore, there may be a state in which all banks2are operating as first banks2a, in other words, a state in which only plurality of first banks2aexist during the operation of RF battery system1or at a certain time point. In other words, there may be a state in which there are no banks2that function as second bank2band the number of second banks2bbecomes zero. RF battery system1of this embodiment is not only placed in a state in which second bank2bis constantly included but also includes being in a state in which one or more second banks2bare included only at a certain time point during the operation.

First bank2ais a bank that performs charging and discharging with respect to external power source91or load92. The SOC of first bank2afluctuates in accordance with charging and discharging. The SOC of first bank2ais controlled to be in the first SOC at the time of charging and discharging. The first SOC has a first SOC range. The first SOC is a particular value within the first SOC range. The first SOC range is a numerical value width defined by an upper limit and a lower limit of the first SOC. In other words, the first SOC range is a difference between the upper limit and the lower limit of a range possible for the first SOC. The first SOC range is a range obtained by expanding a usage range of a normal SOC. The usage range of the normal SOC is an SOC range in which a side reaction hardly occurs and a side reaction product is hardly generated. In other words, the usage range of the normal SOC is an SOC range in which the deterioration of the electrolytic solution due to a side reaction hardly progresses and the battery performance can be maintained. The SOC range in which the side reaction product is hardly generated described above is a range in which the rate of the active material consumed by the generation of the side reaction product is 10 mol % or less where the total amount of elements serving as the active material included in the electrolytic solution is 100 mol % when charging and discharging are performed for one week within a certain SOC range, for example. It is preferred that the period of time for charging and discharging within the SOC range be two weeks or further one month. It is preferred that the rate of the active material consumed by the generation of the side reaction product be 5 mol % or less, or further 1 mol % or less. For example, when manganese oxide is generated as a side reaction product in a positive electrolyte containing a manganese ion as the active material, manganese is consumed in the generation of the side reaction product. The range obtained by expanding the usage range of the normal SOC includes an SOC range in which a side reaction occurs and a side reaction product is generated by charging or discharging in addition to the usage range of the normal SOC. 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 an upper limit value of the usage range of the normal SOC. The low SOC region is a region in which the SOC is lower than a lower limit value of the usage range of the normal SOC. In the high SOC region and the low SOC region, a side reaction product may be generated. The first SOC is a range obtained by combining the usage range of the normal SOC and at least one of the high SOC region and the low SOC region.

The usage range of the normal SOC described above differs depending on the electrolytic solution to be used. The usage range of the normal SOC is obtained in advance by a test using the electrolytic solution to be used. The range expanded from the usage range of the normal SOC, in other words, the range of each of the high SOC region and the low SOC region only needs to be set as appropriate. The battery capacity of first bank2aincreases by expanding the usage range of the SOC in first bank2a. As a result, the energy density of the entire system can be enhanced. The range of the high SOC region or the range of the low SOC region is 5% or more, further 10% or more, or 15% or more of the usage range of the normal SOC, for example. The usage range described above is a difference between the upper limit and the lower limit of the usage range. For example, when the usage range of the normal SOC is from 20% to 80%, the difference between the upper limit and the lower limit is 80%−20%=60%. When the range of the high SOC region is 10% of the usage range, the range of the high SOC region is 60%×10%=6%. The usage range of the SOC obtained by combining the usage range of the normal SOC and the high SOC region is from 20% to 86%. The first SOC range in this case is 86%−20%=66%. In this case, the usage range of the SOC is expanded by 10% as compared to the usage range of the normal SOC, and hence the battery capacity of first bank2ais increased by 10%. When the range of the low SOC region is 10% of the usage range, the usage range of the SOC is from 14% to 80%. The first SOC range in this case is 80%−14%=66%. The battery capacity of first bank2aincreases by 10% in this case as well. The reason the energy density of the entire system improves is described in detail in the section of <Trial Calculation Example of Energy Density> described below.

The number of first banks2ais 10 or more, or further 15 or more, for example. The battery capacity is secured more easily as the entire system as the number of first banks2abecomes greater.

The plurality of first banks2aare controlled such that the SOCs of first banks2aare not shifted from each other at the time of charging and discharging. In other words, all of the SOCs of first banks2aare controlled to be aligned with each other. Specifically, the difference in the SOC between first banks2ais less than ±2%, or further ±1% or less. The difference in the SOC between first banks2ais a difference between an average value of the SOCs of all first banks2aand the value of the SOC of each first bank2a.

Second bank2bis a bank that performs recovery work of the electrolytic solution. The SOC of second bank2bis controlled to be the second SOC. The second SOC has a second SOC range. The second SOC is a particular value within the second SOC range. The second SOC range is a numerical value width defined by an upper limit and a lower limit of the second SOC. In other words, the second SOC range is a difference between the upper limit and the lower limit of a range possible for the second SOC. The second SOC range is an SOC range in which the side reaction product described above can be redissolved by a reversible reaction.

The SOC range in which the side reaction product described above is redissolved differs depending on the electrolytic solution to be used. The SOC range in which the side reaction product is redissolved is obtained in advance by a test using the electrolytic solution to be used. For example, when the side reaction product is redissolved in a region in which the SOC is 30% or less, the second SOC only needs to be 30% or less and may be 20% or less. In this case, as the SOC becomes lower, the speed of the reversible reaction becomes higher, and hence the side reaction product tends to be easily redissolved. When the side reaction product is redissolved in a region in which the SOC is 70% or more, the second SOC only needs to be 70% or more and may be 80% or more. In this case, as the SOC becomes higher, the speed of the reversible reaction becomes higher, and hence the side reaction product tends to be easily redissolved. The second SOC only needs to be a particular value within the SOC range in which the side reaction product described above is redissolved and may partially overlap with the first SOC range described above. For example, when the first SOC range is from 20% to 86%, the second SOC range may be from 20% to 30% or may be from 0 to 30%. When the second SOC range at least partially overlaps with the first SOC range, second bank2bmay perform charging and discharging in the overlapping range. In other words, in second bank2b, battery cell10can also perform charging and discharging during the recovery work of the electrolytic solution. Needless to say, battery cell10does not necessarily need to perform charging and discharging in second bank2b.

The second SOC range is smaller than the first SOC range, for example. The second SOC range may be 30% or less of the first SOC range, for example. For example, when the first SOC range is 66% as described above, 30% of the first SOC range is about 20%. For example, when the second SOC range is from 20% to 30% and the difference between the upper limit and the lower limit thereof is 10%, the second SOC range is about 15% of the first SOC range. In other words, the second SOC range is 30% or less of the first SOC range.

The number of second banks2bis smaller than the number of first banks2a. A rate x of the number of second banks2bto the total number of plurality of banks2is 30% or less, for example. Out of plurality of banks2, second banks2bare smaller in number. In other words, first banks2aare greater in number. The number of first banks2ais great, and hence the battery capacity is easily secured as the entire system. Rate x may be 20% or less, or further 10% or less, for example.

(Specific Examples of First SOC and Second SOC)

An example of specific ranges of the first SOC and the second SOC is shown. Here, an example of a case where a titanium/manganese-based electrolytic solution is used as the electrolytic solution is shown. In the titanium/manganese-based electrolytic solution, a positive electrolyte contains a manganese ion, and a negative electrolyte contains a titanium ion. In the case of the titanium/manganese-based electrolytic solution, the usage range of the normal SOC is roughly from 20% to 80%. In a high SOC region in which the SOC exceeds 80%, manganese oxide may be deposited in the positive electrolyte as a side reaction product. The range of the high SOC region is 3% or more, further 6% or more, or 12% or more, for example. As the range of the high SOC region becomes greater, a usable battery capacity increases because the usage range of the SOC becomes expanded than the usage range of the normal SOC. The usage range of the SOC at the time of charging and discharging, in other words, the first SOC range is from 20% to 83%, further from 20% to 86%, or from 20% to 92%, for example. As the SOC becomes higher, the speed of the side reaction becomes higher, and hence the deposition amount of manganese oxide increases. The upper limit of the first SOC range is 100%, or further 95%, for example. When the upper limit of the first SOC range is 95%, the decrease in battery efficiency due to the increase of internal resistance is easily suppressed.

The SOC range in which manganese oxide in the positive electrolyte can be redissolved by a reversible reaction is roughly 30% or less. In other words, the SOC range in which the side reaction product can be redissolved by the reversible reaction, that is, the second SOC range is from 0% to 30% or further from 0% to 20%, for example. As the SOC becomes lower, the speed of the reversible reaction becomes higher, and hence the manganese oxide is redissolved more easily. The second SOC may be lower than the usage range of the normal SOC described above. The second SOC may be less than 20%. When the upper limit of the second SOC range is less than 20%, the manganese oxide is redissolved more easily. The second SOC may partially overlap with the usage range of the normal SOC described above. The second SOC may be from 20% to 30%, for example.

Another electrolytic solution is a vanadium-based electrolytic solution, for example. In the vanadium-based electrolytic solution, both of a positive electrolyte and a negative electrolyte contain a vanadium ion. The valences of the vanadium ion of the positive electrolyte and the vanadium ion of the negative electrolyte are different from each other. In the case of the vanadium-based electrolytic solution, the usage range of the normal SOC is roughly from 5% to 95%.

Battery control device6not only controls the operation of RF battery system1but also includes controlling actions necessary for monitoring and improving the state of each bank2. Battery control device6independently manages the charging and discharging of each bank2. Therefore, it becomes possible to perform the recovery work of the electrolytic solution of second bank2bwhile performing charging or discharging in first banks2a. Battery control device6issues an action command to power conversion device7of each bank2. Power conversion device7of each bank2controls the charging and discharging of battery cell10based on the action command from battery control device6. Battery control device6issues a charging/discharging command to power conversion device7of each first bank2awhen charging and discharging are performed with respect to power source91or load92, for example. In first bank2a, battery cell10performs charging or discharging in accordance with the requested charging amount or discharging amount. Battery control device6issues a recovery work command to power conversion device7of second bank2b. In second bank2b, the recovery work of the electrolytic solution is executed.

Battery control device6is representatively configured by a computer. The computer includes a processor, a memory, a timer, and the like. The memory has a control program to be executed by the processor and various data stored therein. The processor reads out and executes the control program stored in the memory. The program includes an instruction group relating to processing by controller60.

Battery control device6has controller60. Controller60controls power conversion device7and controls the SOC of each bank2. Controller60performs SOC control that controls the SOC of second bank2bto be the second SOC different from the first SOC. It is preferred that the second SOC be different from the first SOC by ±2% or more. Therefore, in the SOC control, the SOC of second bank2bis controlled to be shifted from the SOC of first bank2aby ±2% or more. A state in which the difference between the SOC of first bank2aand the SOC of second bank2bis ±2% or more can be realized by intentionally shifting the SOC of second bank2b. The second SOC may be different from the first SOC by ±3% or more or may be further different from the first SOC by ±4% or more.

The SOC of second bank2bis controlled to be the second SOC different from the first SOC by the SOC control described above. By maintaining the SOC of second bank2bto be the second SOC for a certain amount of time, the side reaction product in the electrolytic solution of second bank2bis redissolved. As a result, the electrolytic solution of second bank2bcan be recovered. The decrease in the battery performance in second bank2bcan be improved. Second bank2bmay perform charging and discharging within the second SOC range described above or may stand by or stop without performing charging and discharging. The amount of time by which the SOC of second bank2bis maintained at the second SOC after reaching the second SOC is an amount of time it takes for the side reaction product in the electrolytic solution of second bank2bto be redissolved and for the electrolytic solution to recover to the original state or more. As the amount of time maintained in the second SOC becomes longer, the side reaction product in the electrolytic solution decreases more by redissolving. After the electrolytic solution recovers to the original state, the second SOC may be maintained, but the side reaction product cannot be dissolved any further. The amount of time maintained in the second SOC may be one minute or more and 24 hours or less, further two hours or more and 16 hours or less, or four hours or more and 12 hours or less, for example. The pump that circulates the electrolytic solution may be stopped or may not be stopped in second bank2bthat does not perform charging and discharging after the SOC of second bank2breaches the second SOC. By stopping the pump, the electric power consumption by the pump can be reduced.

Controller60may perform rotation control so as to include the time period set to the first SOC and the time period set to the second SOC for each of plurality of banks2. In the time period in which controller60sets the SOC of bank2to be the first SOC, bank2is the bank that performs charging and discharging. In the time period in which controller60sets the SOC of bank2to be the second SOC, bank2is the bank that performs the recovery work of the electrolytic solution. In other words, in each bank2, there are a time period in which each bank2is first bank2athat performs charging and discharging and a time period in which each bank2is second bank2bthat performs the recovery work of the electrolytic solution. For example, in a certain time period, certain bank2functions as first bank2a, and another bank2functions as second bank2b. In another time period, certain bank2transitions from a state of first bank2ato a state of second bank2b, and another bank2transitions from a state of second bank2bto a state of first bank2a. It becomes possible to perform the recovery work of the electrolytic solution in rotation for all of plurality of banks2by switching each bank2to second bank2bin turn by every predetermined amount of time by the rotation control described above.

The time period set to the first SOC is set to a time period within the amount of time it takes for the generation amount of the side reaction product in the electrolytic solution in first bank2ato reach a predetermined amount, for example. When the generation amount of the side reaction product in the electrolytic solution is less than the predetermined amount, the battery performance of first bank2acan be maintained even when the side reaction product is generated by a certain amount. The time period set to the second SOC is set to an amount of time it takes for the side reaction product in the electrolytic solution of second bank2bto be redissolved and for the electrolytic solution of second bank2bto recover to the original state or more. The expression of “the electrolytic solution recovers to the original state” means a state in which the generation amount of the side reaction product in the electrolytic solution is a certain amount or less. The amount of time it takes for the electrolytic solution to recover to the original state is obtained in advance by a test with use of the electrolytic solution to be used.

A rate y of the amount of time set to the second SOC to the total amount of time of the amount of time set to the first SOC and the amount of time set to the second SOC is 30% or less, for example. When rate y of the amount of time set to the second SOC is 30% or less, the amount of time functioning as second bank2bis short. In other words, the amount of the recovery work time of the electrolytic solution in each bank2is relatively short. Rate y may be 20% or less, or further 10% or less, for example.

For example, the total number of plurality of banks2is 11. The number of first banks2ais 10. The number of second banks2bis 1. In this case, rate x of the number of second banks2bis 1/11≈9.1%. Each of banks2is set to No. 1, No. 2 . . . . No. 11. At a certain time, out of plurality of banks2, No. 1 to No. 10 are first banks2acontrolled to be in the first SOC, and remaining No. 11 is second bank2bcontrolled to be in the second SOC. Each of banks2is periodically switched to second bank2bin turn. Here, the time period set to the second SOC is one day.

No. 11 is switched from second bank2bto first bank2aafter one day passes from a certain time. Any of No. 1, No. 2 . . . . No. 10 is switched from first bank2ato second bank2bin place of No. 11. Here, No. 10 is switched to second bank2b. At this time, No. 1 to No. 9 and No. 11 excluding No. 10 are first banks2a. The number of first banks2aand the number of second banks2bdo not change. When another day passes, No. 10 is switched to first bank2a, and No. 9 is switched to second bank2b. The recovery work of the electrolytic solution can be performed for all banks2by performing switching from first bank2ato second bank2bin turn in the order of No. 11, No. 10, No. 9, . . . . No. 1, No. 11 . . . , for example, as time elapses as above. In the condition described above, a time period in which particular bank2is continuously set to be the first SOC is ten days. In other words, the amount of time charging and discharging is possible by first bank2ais ten days. A cycle from when individual banks2changes from the state of second bank2bto the state of second bank2bagain is 11 days. Rate y of the amount of time set to the second SOC to the total amount of time of the amount of time set to the first SOC and the amount of time set to the second SOC is 1(day)/11(day)≈0.091.

Controller60may perform transition control that causes particular first bank2aout of plurality of first banks2ato transition from the first SOC to the second SOC. The expression of “causing transition from the first SOC to the second SOC” means to switch particular first bank2ato second bank2b. When the SOC of particular first bank2ais higher than the second SOC, particular first bank2ais discharged, and the SOC of particular first bank2ais lowered to the second SOC. When the SOC of particular first bank2ais lower than the second SOC, particular first bank2ais charged, and the SOC of particular first bank2ais raised to the second SOC. The transition control performs one of first transition control, second transition control, and third transition control shown below, for example.

The first transition control performs charging or discharging from particular first bank2ato second bank2b.

<Example of First Transition Control>

An example of the first transition control when the SOC of particular first bank2ais higher than the second SOC is described. Here, as described in the example of the rotation control, a case where one day has passed from a certain time is conceived. In other words, a case where transition from the first SOC to the second SOC is performed and switching from first bank2ato second bank2bis performed for No. 10 is conceived. At this time, when No. 11 is switched from second bank2bto first bank2a, a command is issued to each power conversion device7so as to discharge No. 10 and charge No. 11. By discharging No. 10, No. 10 is changed from first bank2ato second bank2b. By charging No. 11, No. 11 is changed from second bank2bto first bank2a. The transition control can be performed even when charging or discharging is not performed with respect to external power source91or load92because charging or discharging is performed between particular first bank2aand second bank2b.

The second transition control preferentially discharges or charges particular first bank2aas compared to other banks2when discharging or charging is performed with respect to external power source91or load92. Specifically, particular first bank2ais discharged or charged by a value that is 1/(1−x) times or more of an average value obtained by dividing the discharging amount or the charging amount to the plurality of first banks2aby the total number of plurality of banks2. Here, x represents a rate of the number of second banks2bto the total number of plurality of banks2.

<Example of Second Transition Control>

An example in which only particular bank2transitions to second bank2bfrom a state in which all banks2are operating as first banks2ais described. The total number of plurality of banks2is 11. The number of first banks2aafter the transition is 10. The number of second banks2bafter the transition is 1. Rate x of the number of second banks2bis 1/11≈0.091. Here, 1/(1−x) is 1.1. The rated output of one bank2is 100 KW. The rated output of the entire system obtained by subtracting second bank2bis 100 (KW)×10=1000 kW.

Here, a case where No. 11 is switched from first bank2ato second bank2band transitions from the first SOC to the second SOC from a state in which No. 1 to No. 11 are first banks2adescribed in the example of the rotation control is conceived. In this case, when discharging to external load92is performed, the allocation of the discharging amount to No. 11 is increased so as to preferentially discharge No. 11. Specifically, No. 11 is discharged by a value exceeding an average value obtained by dividing the value of the discharging amount to plurality of first banks2aby the number of all banks2. For example, when a discharging command for 1000 kW is received, No. 11 is discharged by a rated output of 100 kW. No. 1 to No. 10 that are first banks2aare discharged by a value obtained by averaging the remaining discharging amount of 900 kW, in other words, 90 kW. When the discharging amount to each bank2is evenly allocated, an average value obtained by dividing the discharging command value by the total number of banks2is 1000/11≈90.9 kW. A value obtained by dividing the discharging amount of 100 KW of No. 11 described above by this average value is 100/(1000/11)=1.1. The discharging amount of 100 kW of No. 11 is 1/(1−x)=1.1 times or more of the average value described above. No. 11 is preferentially discharged by a value that is 1.1 times or more of the average value described above, and hence the amount of transition time can be caused to be shorter.

As an example of another second transition control, a case where one day has passed from a certain time is conceived as described in the example of the rotation control described above. In other words, a case where transition from the first SOC to the second SOC is performed and switching from first bank2ato second bank2bis performed for No. 10 is conceived. In this case, when discharging to external load92is performed, the allocation of the discharging amount to No. 10 is increased so as to preferentially discharge No. 10. Specifically, No. 10 is discharged by a value equal to or more than an average value obtained by dividing the value of the discharging amount to plurality of first banks2aby the number of first banks2a. For example, when a discharging command for 550 kW is received, No. 10 is discharged by a rated output of 100 kW. At this time, it is considered that No. 11 has not finished transitioning from the second SOC to the first SOC, and No. 11 is not discharged. Out of first banks2a, No. 1 to No. 9 excluding No. 10 are discharged by a value obtained by averaging the remaining discharging amount of 450 kW, in other words, 50 kW. When the discharging amount to each bank2is evenly allocated, an average value obtained by dividing the discharging command value by the total number of banks2is 550/11=50 kW. A value obtained by dividing the discharging amount of 100 KW of No. 10 described above by this average value is 100/(550/11)−2. The discharging amount of 100 KW of No. 10 is 1/(1−x)=1.1 times or more of the average value described above. No. 10 is preferentially discharged by a value that is 1.1 times or more of the average value described above, and hence the amount of transition time can be caused to be shorter.

No. 11 is caused to transition from the second SOC to the first SOC and switched from second bank2bto first bank2a. In this case, when charging to external power source91is performed, the allocation of the charging amount to No. 11 is increased so as to preferentially charge No. 11. Specifically, No. 11 is charged by a value equal to or more than an average value obtained by dividing the value of the charging amount to plurality of first banks2aby the number of first banks2a. For example, when a charging command for 550 kW is received, No. 11 is charged by a rated output of 100 kW. At this time, No. 10 is not charged. No. 1 to No. 9 are charged by a value obtained by averaging the remaining charging amount of 450 kW, in other words, 50 kW. When the charging amount to each bank2is evenly allocated, an average value obtained by dividing the charging command value by the total number of banks2is 550/11=50 kW. As with the transition control of No. 10 described above, No. 11 is preferentially charged by a value that is 1.1 times or more of the average value described above, and hence the amount of transition time can be caused to be shorter.

As with the second transition control described above, the third transition control preferentially discharges or charges particular first bank2aas compared to other banks2when discharging or charging is performed with respect to external power source91or load92. Unlike the second transition control, in the third transition control, particular first bank2ais discharged or charged by a value that is 1/(1−y) times or more of an average value obtained by dividing the discharging amount or the charging amount to the plurality of first banks2aby the total number of plurality of banks2. Here, y represents the rate of the amount of time set to the second SOC to the total amount of time of the amount of time set to the first SOC and the amount of time set to the second SOC.

<Example of Third Transition Control>

As with the example of the second transition control, an example in which only particular bank2transitions to second bank2bfrom a state in which all banks2are operating as first banks2ais described. The total number of plurality of banks2is 11. The number of first banks2aafter the transition is 10. The number of second banks2bafter the transition is 1. As described in the example of the rotation control described above, rate y of the amount of time set to the second SOC to the total amount of time of the amount of time set to the first SOC and the amount of time set to the second SOC is 1 (day)/11 (day)≈0.091. Here, 1/(1−y) is 1.1. The rated output of one bank2is 100 KW. The rated output of the entire system obtained by subtracting second bank2bis 100 (KW)×10=1000 kW.

As with the example of the second transition control, a case where transition from the first SOC to the second SOC is performed and switching from first bank2ato second bank2bis performed for No. 11 is conceived. In the third transition control, the basic concept in which No. 11 is discharged by a value exceeding an average value obtained by dividing the value of the discharging amount to plurality of first banks2aby the number of all banks2is similar to that of the second transition control. For example, when a discharging command for 1000 kW is received, No. 11 is discharged by a rated output of 100 KW. No. 1 to No. 10 that are first banks2aare discharged by a value obtained by averaging the remaining discharging amount of 900 KW, in other words, 90 kW. When the discharging amount to each bank2is evenly allocated, an average value obtained by dividing the discharging command value by the total number of banks2is 1000/11≈90.9 kW. A value obtained by dividing the discharging amount of 100 KW of No. 11 described above by this average value is 100/(1000/11)−1.1. The discharging amount of 100 kW of No. 11 is 1/(1−y)=1.1 times or more of the average value described above. No. 11 is preferentially discharged by a value that is 1.1 times or more of the average value described above, and hence the amount of transition time can be caused to be shorter.

In the present embodiment, as shown inFIG.1, each bank2includes a counter80that quantifies the generation amount of the side reaction product in the electrolytic solution. By including counter80, a timing for causing transition from first bank2ato second bank2bby controller60can be suitably determined based on the generation amount of the side reaction product.

The generation amount of the side reaction product can be obtained by optical characteristics, physical properties, and the like of the electrolytic solution or obtained as functions of a value of the SOC, the temperature of the electrolytic solution, and the amount of time, for example. The optical characteristics of the electrolytic solution include color, transparency, and the like of the electrolytic solution, for example. In some electrolytic solutions, the color and the transparency may change when a side reaction product is generated. As the generation amount of the side reaction product becomes greater, the change in the color and the transparency tends to become greater. Therefore, the generation amount of the side reaction product can be quantitatively obtained by measuring the optical characteristics of the electrolytic solution. The optical characteristics of the electrolytic solution can be measured by performing image processing of an image photographed by a camera, or by using a spectrophotometer, for example. The physical properties of the electrolytic solution include viscosity, conductivity, density, and the like, for example. In some electrolytic solutions, the viscosity, the conductivity, and the density may change when a side reaction product is generated. As the generation amount of the side reaction product becomes greater, the change in the viscosity, the conductivity, and the density tends to become greater. Therefore, the generation amount of the side reaction product can also be quantitatively obtained by measuring the physical properties of the electrolytic solution.

The generation amount of a side reaction product depends on each of parameters of the value of the SOC, the temperature of the electrolytic solution, and the amount of time. As the SOC becomes higher or the SOC becomes lower, a side reaction occurs more easily, and the generation amount of a side reaction product increases. As the temperature of the electrolytic solution becomes higher, the speed of the side reaction becomes greater, and the generation amount of the side reaction product increases. As the amount of time maintained in the SOC in which the side reaction occurs becomes longer, the generation amount of the side reaction product increases. Therefore, the generation amount of the side reaction product can be quantitatively obtained as the function of each parameter described above. In the present embodiment, the SOC is managed by electrical information such as the quantity of electricity and the amount of charging/discharging time at the time of charging or discharging in each bank2.

<Trial Calculation Example of Energy Density>

A trial calculation of the energy density in RF battery system1of the embodiment has been performed. Here, the trial calculation of the energy density of each of an embodiment model using the embodiment and a comparative model of a related-art design is performed and compared, and the energy densities are compared with each other. The energy density is obtained as the battery capacity per unit of the electrolytic solution amount.

The comparative model is an RF battery system including a plurality of banks. In the RF battery system, all the banks perform charging and discharging. In other words, the comparative model is different from the embodiment in that all the banks are the first banks and the second bank is not included.

The comparative model sets the usage range of the SOC to 20% to 80%.

A basic specification of the comparative model is shown in Table 1. The effective number of banks is the number of banks used in charging and discharging. In the comparative model, the effective number of banks is the same as the total number of banks.

The embodiment model includes the same number of banks as the comparative model. The number of the first banks is 15. The number of the second banks is 1.

The embodiment model sets the usage range of the SOC to 20% to 86%. The usage range of the SOC of the embodiment model is expanded from the comparative model by 10%. Therefore, the time capacity of the embodiment model increases by 10% as compared to the comparative model.

A basic specification of the embodiment model is shown in Table 2. In the embodiment model, the effective number of banks is the same as the number of the first banks. In other words, the effective number of banks is a value obtained by subtracting the number of the second banks from the total number of banks. In Table 2, out of “t+δ” expressing the time capacity, “δ” represents an expanded width of the time capacity obtained by expanding the usage range of the SOC.

From Table 1 and Table 2, the following is understood. In the embodiment model, the effective number of banks is smaller than that of the comparative model, but the energy density improves as compared to the comparative model. The reason thereof is because the time capacity is increased in the embodiment model because the usage range of the SOC is expanded as compared to the comparative model. From a calculation expression of the energy density, it can be said that an effect of improving the energy density is easily obtained when the number of the first banks is greater. In the embodiment model, the condition in which the effect is obtained more is when a total number of banks n is great or when expanded width δ of the time capacity is great, for example. Specifically, it is more effective when number of banks n is 30 or more, for example, or when the expanded width δ is 20% or more, for example.

Effects of Embodiment

RF battery system1of the embodiment described above is capable of improving the decrease in battery characteristics due to the side reaction while being able to realize the high energy density from the following reasons.

It becomes possible to enhance the energy density by expanding the usage range of the SOC in first bank2a. It becomes possible to recover the electrolytic solution of second bank2bby controlling the SOC of second bank2bto be the SOC in which a side reaction product is redissolved. Therefore, it becomes possible to improve the decrease in the battery performance due to a side reaction. It becomes possible to perform the recovery work of the electrolytic solution of second bank2bwhile performing charging or discharging in first banks2a. It becomes possible to perform the recovery work of the electrolytic solution without stopping the system, and hence there are less operation restrictions. It becomes possible to perform the recovery work of the electrolytic solution in rotation for all of plurality of banks2by switching each bank2to second bank2bin turn by every predetermined amount of time by the rotation control by controller60.

REFERENCE SIGNS LIST