Redox flow battery array and method for state of charge balancing

Abstract

A redox flow battery array design and method for balancing the state of charge within the array are disclosed. Flow battery unit strings in an array comprising strings of flow battery units (units sharing a common electrolyte pair) are balanced by measuring the state of charge of the common electrolyte pair and appropriately adjusting flow in one or more of the associated anolyte and catholyte circuits to balance the state of charge in the flow battery unit strings. The required equipment, i.e., state of charge measurement device, flow regulator, and controller, represents a significant simplification over current art approaches.

Description

[Technical field]

[0001] The present invention relates to an array of redox flow batteries and to a method for balancing the state of charge of flow battery unit strings in the array and flow battery units within the strings. [Background technology]

[0002] Redox flow batteries, such as vanadium redox flow batteries, continue to show promise for storing large amounts of electrical energy, such as for load leveling applications. They convert electrical energy into chemical energy that can be stored and subsequently converted into electrical energy that can be released when there is demand. In operation, a liquid electrolyte is delivered to the flow battery to convert electrical energy into chemical energy or convert chemical energy into electrical energy that can be discharged to a power load via an electrical connection.

[0003] Each redox flow battery cell comprises an anode and a cathode separated by a separator. A negative liquid electrolyte, or anolyte, is pumped (flowed) from an anolyte storage tank to the anode, and a positive liquid electrolyte, or catholyte, is pumped (flowed) from a catholyte storage tank to the cathode, resulting in an electrochemically reversible redox reaction. The separator may be a microporous separator or an ion exchange membrane that separates the electrodes to prevent the electrolytes from mixing, but allows selected ions to pass through to complete the redox reaction. After allowing the redox reaction to occur, the anolyte and catholyte are returned to their respective storage tanks. Operation of such a redox flow battery therefore requires both an anolyte fluid circuit and a catholyte fluid circuit, each with supply and return lines connecting the anolyte and catholyte tanks to the anode and cathode, respectively, in the flow battery cell. The state of charge of the cell is determined by the chemical composition or condition of the two parts of the electrolyte: the anolyte and the catholyte.

[0004] Many applications, particularly load leveling applications, require a large number of flow battery cells to store and / or deliver large amounts of energy and power, respectively. Typically, large complex arrays of cells are employed in which the cells are interconnected in various series and parallel combinations. Desirably, many individual flow battery cells in such arrays will share common anolyte and catholyte supplies. Such a combination of cells is referred to herein as a flow battery unit. The cells in a flow battery unit may be interconnected in series and / or parallel, but all share a common anolyte and catholyte supply. As a result, all cells in the flow battery unit are at the same state of charge.

[0005] In larger arrays, the flow battery units may desirably be further interconnected into series and / or parallel strings of flow battery units, with such composite flow battery unit strings transmitting and receiving power for use in a desired application via one or more controllable bi-directional power converters (e.g., DC / DC and / or AC / DC converters that step up the operating DC, typically to over 800V, for connection to a utility AC / DC converter).

[0006] To date, flow battery units with a shared common electrolyte supply have not been used effectively in electrical series and / or parallel configurations due to the inability to efficiently and cost-effectively manage capacity balancing between all of them. In one past approach, the maximum voltage of the series connected cells in a flow battery unit may be limited to typically 800V for connection to a commercial AC / DC converter. Furthermore, the foregoing may be employed in conjunction with the use of electrical contacts to switch on and off the series connected stack of cells in a particular flow battery unit, varying the charge / discharge current in separate steps for capacity balancing. A DC / DC converter may still be used as before to manage and boost the current.

[0007] Boosting series stacks of cells within individual flow battery units increases system costs as shunt mitigation measures (such as longer and thinner flow channels between cells and stacks, and increased pumping power required for the fluid anolyte / catholyte) increase costs and reduce overall efficiency.

[0008] Unfortunately, all of the above approaches limit the maximum DC voltage generated from each flow battery unit, making it difficult to integrate flow batteries into the 800V+ DC connection requirements of current technology AC / DC converters. The use of multiple DC / DC converters in the main current circuit adds cost and reduces the overall round trip efficiency of the system, reducing the maximum performance and cost benefits that could be obtained from the flow battery. Also, the use of multiple contacts to switch the stack on and off is expensive. Also, the contacts have a short life when switched under load, and the cell stack continues to discharge electrolyte within the stack, reducing system efficiency.

[0009] In yet another approach, capacity balancing within a series string of flow battery units may be achieved using a subsystem that detects imbalances between flow battery units in the string and then adjusts the state of charge in one or more units accordingly by fluidly interconnecting and mixing portions of the anolyte and catholyte tanks to bring the state of charge down to a common target value (similar to discharging the cells in that unit with a bleeder resistor). Such a subsystem would obviously add unnecessary cost and complexity.

[0010] Although much progress has been made in developing viable redox flow battery arrays for large-scale commercial applications, there remains a need for further simplification, efficiency, and cost reduction. Improvements in state-of-charge balancing and energy storage capacity balancing in series and parallel strings of flow battery units would be particularly useful. The present invention addresses these needs and provides other benefits, as disclosed below. Summary of the Invention

[0011] The present invention discloses an improved redox flow battery array and a method for balancing the state of charge within the array. Balancing of flow battery unit strings in an array comprising strings of flow battery units in which the units share a common electrolyte pair is achieved by measuring the state of charge of the participating common electrolyte pair and then appropriately adjusting the flow in one or more of the associated anolyte and catholyte circuits. Although embodiments require state of charge measurement devices, flow regulators and controllers, the array of the present invention as a whole is significantly simplified compared to prior art approaches. For example, the present redox flow battery array requires only one controllable bidirectional power converter for each set of electrically parallel connected flow battery unit strings (i.e., there need not be additional controllable bidirectional power converters electrically connected to any of the participating flow battery unit strings). The array also does not require a subsystem, as in some prior art approaches, to fluidly interconnect and mix the anolyte and catholyte tanks to provide state of charge balancing.

[0012] Specifically, the present invention relates to a redox flow battery array comprising at least two flow battery unit strings electrically connected in parallel, each flow battery unit string comprising at least one flow battery unit. Such a flow battery unit comprises a flow battery string comprising at least one flow battery cell. However, a flow battery string may comprise multiple cells interconnected, typically in a series and / or parallel combination. Each cell comprises a flow battery unit electrolyte comprising an anode, a cathode, and an anolyte and a catholyte. The flow battery unit also comprises an anolyte tank and a catholyte tank for storage of the anolyte and catholyte of the flow battery unit electrolyte, respectively. In some embodiments, more than one tank for each of the anolyte or catholyte may be employed. However, the anolyte and catholyte in a given flow battery unit are common to all cells in the given unit. The flow battery unit also comprises an anolyte circuit with supply and return lines connecting the anolyte tank to the anodes in the flow battery cells in the flow battery string, and a catholyte circuit with supply and return lines connecting the catholyte tank to the cathodes in the flow battery cells in the flow battery string as well. The flow battery array further comprises a controllable bi-directional power converter electrically connected to each of the participating flow battery unit strings, although, as mentioned above, only one such controllable bi-directional power converter is needed for a participating flow battery unit string.

[0013] In the present invention, each flow battery unit comprises a state of charge measuring device capable of measuring the state of charge of the flow battery unit electrolyte in that unit. The array further comprises a flow regulator in at least one of the anolyte and catholyte circuits in at least one of the flow battery units in the at least two flow battery unit strings, the flow regulator adjusting the flow of at least one of the anolyte and catholyte therethrough. While this may be possible in some embodiments to achieve acceptable balancing, in a preferred embodiment, a flow regulator is provided in each of the anolyte and catholyte circuits in each of the flow battery units in each involved flow battery unit string. And, the array further comprises a controller having an input connected to each of the state of charge measuring devices in the flow battery units and an output connected to the involved flow regulator or regulators. The output of the controller controls the flow regulator or regulators to balance the state of charge in the at least two flow battery unit strings by adjusting the flow of at least one of the anolyte and catholyte to the anode and cathode in the flow battery cells in the flow battery string according to the state of charge of the flow battery unit electrolyte in the flow battery unit.

[0014] An array of the present invention may comprise a plurality of flow battery unit strings (i.e., at least two or more such flow battery unit strings) electrically connected in parallel, or the flow battery unit strings involved may comprise two or more flow battery units electrically connected in series.

[0015] A voltage sensor may be incorporated across the flow battery unit and a current sensor in series therewith to measure the resistance and / or rate of change of resistance of the flow battery unit involved. Thus, the resistance and rate of change of resistance of the flow battery unit string may provide additional input for predictive control of the array.

[0016] In embodiments comprising two or more flow battery units electrically connected in series in a string of flow battery units, a suitable current bypass device may be employed to balance the units within the string, for example, each of the flow battery unit strings may comprise a current bypass device electrically connected in parallel to one or more of the flow battery units in those strings for this purpose.

[0017] Various types of devices can be considered for measuring the state of charge. In one desirable approach, an unloaded (open circuit) reference flow battery cell can be taken for each flow battery unit that is fed with the same anolyte and catholyte that is fed to that unit. In this approach, a voltage sensor is connected across the reference flow battery cell to measure the open circuit voltage of the electrolyte entering the stack, and therefore to measure the state of charge of both the reference flow battery cell and therefore all the cells in the flow battery unit.

[0018] In another approach, the state of charge measurement device employed may be a colorimetric or spectroscopic device having optical access to the flow battery unit electrolyte (e.g., via suitable windows in the anolyte and catholyte circuits or tanks) that can be used to determine the electrolyte's state of charge, as is known in the art. Alternatively, potentiometric devices other than the reference cells described above may be employed to separately measure the anolyte and catholyte states of charge against a common reference electrode (e.g., saturated calomel, silver chloride, etc.), each being summed to determine the total state of charge.

[0019] The present invention is suitable for use in redox flow battery arrays in general, but particularly in arrays comprising vanadium redox flow batteries, and is suitable for use in applications including a single array or in larger, more complex applications comprising two or more arrays of the invention.

[0020] Balancing the states of charge of the flow battery unit strings in the aforementioned redox flow battery array can be achieved by a method comprising the steps of measuring a state of charge of a flow battery unit electrolyte in a flow battery unit in each of the at least two flow battery unit strings using a state of charge measuring device; inputting the measured state of charge to a controller; determining using the controller a control signal for regulating a flow of at least one of anolyte and catholyte to the anode and cathode in a flow battery cell in at least one of the flow battery units according to the state of charge of the flow battery unit electrolyte in the flow battery unit so as to balance the states of charge in the at least two flow battery unit strings; and outputting the control signal to a flow regulator.

[0021] In embodiments in which each of the flow battery units comprises an unloaded reference cell and a voltage sensor as described above, measuring the state of charge may comprise measuring the open circuit voltage of the unloaded reference flow battery cell.

[0022] In embodiments where each flow battery unit string comprises at least two flow battery units electrically connected in series, and each flow battery unit string comprises a current bypass device electrically connected in parallel to at least one of the flow battery units in the flow battery unit string, balancing of the state of charge between the flow battery units in the flow battery unit string can be achieved by shunting a portion of the total current to one or more of the current bypass devices.

[0023] Unlike some approaches in the prior art, an advantageous effect of the present invention is that the step of state of charge balancing can be performed when the array is charging or discharging via a controllable bi-directional power converter, and the method does not need to be performed on every charge or discharge, but perhaps only when the electrolyte of the flow battery unit is in a low or high state of charge. [Brief description of the drawings]

[0024] [Figure 1] FIG. 1 shows a schematic diagram of a simple embodiment of the present invention comprising two strings of flow battery units electrically connected in parallel, with only one flow battery unit in each string and only one flow battery cell in each unit. [Diagram 2] 2 shows a schematic diagram of an embodiment of the invention comprising n strings of flow battery units electrically connected in parallel, with at least one flow battery unit in each of the N strings. In FIG. 2, optional voltage and current sensors are provided. [Diagram 3] 1 shows a schematic diagram of an embodiment of the invention comprising two strings of flow battery units electrically connected in parallel, each string having n flow battery units, in each of the strings a current bypass device is provided across the first flow battery unit. [Figure 4] 1 shows a schematic diagram of an embodiment of the invention comprising two strings of flow battery units electrically connected in parallel, each string having n flow battery units, in each of the strings a current bypass device and a voltage sensor are provided across each flow battery unit, and a current sensor is provided in series with the string. [Diagram 5] 1 shows a schematic diagram of a composite embodiment of the present invention suitable for use in commercial AC power applications. The embodiment shown comprises an array of N' flow batteries of the present invention electrically connected in parallel. The array comprises N strings of flow battery units electrically connected in parallel and n flow battery units in each of the flow battery unit strings. As shown, a current bypass device is provided across each flow battery unit. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] Unless the context requires otherwise, throughout this specification and the claims, the words "comprises," "comprising," and the like, are to be interpreted in an open and inclusive sense. The words "a," "an," and the like, are to be interpreted as meaning at least one, and not limited to only one.

[0026] The term "string" is used herein to refer to any combination of electrically interconnected components in both series and / or parallel configurations.

[0027] For redox flow batteries, the state of charge is a function of the state of the flow battery electrolyte. Since the electrolyte comprises both an anolyte and a catholyte, the total state of charge is a function of the combined state of the anolyte and catholyte.

[0028] In the following description, redox flow battery arrays are designated with the symbol AR and numbered 1 through N' (e.g., AR1, AR2, ... ARN'). Flow battery unit strings are designated with the symbol S and numbered A through N (e.g., SA, SB, ... SN). Flow battery units are designated according to the unit string in which they appear and numbered 1 through n (e.g., A1, B3, N1, Nn, etc.).

[0029] FIG. 1 shows a schematic diagram of a simple embodiment of a redox flow battery array AR1 of the present invention comprising only two flow battery unit strings SA, SB electrically connected in parallel, where each string has only one flow battery unit A1, B1 and each unit has only one flow battery cell 2a, 2b. Each cell comprises an anode 3a, 3b, a cathode 4a, 4n, and a flow battery unit electrolyte comprising an anolyte 5a, 5b and a catholyte 6a, 6b. An anolyte tank 7a, 7b and a catholyte tank 8a, 8b are provided for storage of the anolyte 5a, 5b and catholyte 6a, 6b, respectively, for each flow battery unit electrolyte in the array. (Note that in some embodiments, more than one tank may be employed to store a common anolyte or a common catholyte supply. However, each cell in a given flow battery unit shares a common supply of anolyte and a common supply of catholyte. Consequently, each cell in a given flow battery unit also shares the same state of charge.) An anolyte circuit 9a, 9b, comprising supply lines 10a, 10b and return lines 11a, 11b, is also provided to connect each anolyte tank 7a, 7b to cell anodes 3a, 3b of one or more cells. Similarly, a catholyte circuit 12a, 12b, comprising supply lines 13a, 13b and return lines 14a, 14b, is also provided to connect each catholyte tank 8a, 8b to cell cathodes 4a, 4b of one or more cells. A controllable bidirectional power converter 16 is electrically connected to each flow battery unit string SA, SB to provide appropriate conversion of power from or to the array during discharging or charging of the array, respectively. Converter 16 may be operated in voltage mode, current mode or power mode at any given time and may be either a DC / DC converter or a DC / AC converter.

[0030] In the present invention, a state of charge (SOC) measuring device 17a, 17b is provided to measure the state of charge for each flow battery unit electrolyte. A flow regulator is also provided in at least one of the anolyte and catholyte circuits to regulate the flow of anolyte and / or catholyte therethrough. In FIG. 1, a flow regulator 18 is shown in each anolyte supply line 10a, 10b and each catholyte supply line 13a, 13b of each of the flow battery units A1, B1. A controller 19 is also provided to monitor the state of charge and control the flow regulator 18. The controller 19 thus has inputs 20a, 20b (shown in dashed lines in this figure) connected to each of the state of charge measuring devices 17a, 17b and outputs 21a, 21b (shown in dashed lines in this figure) connected to the flow regulator 18. The outputs 21a, 21b of the controller 19 control the flow regulators 18 to adjust the flow of anolyte 5a, 5b and catholyte 6a, 6b to the anodes 3a, 3b and cathodes 4a, 4b in the flow battery cells 2a, 2b in response to the state of charge of the flow battery unit electrolytes in the flow battery units A1, B1. By adjusting these flows, the resistance of the cells in the flow battery units, and therefore the resistance of the flow battery units themselves, can be actively increased or decreased as needed to properly balance the state of charge in the flow battery unit strings. In particular, a state of charge measuring device is used to measure the state of charge of the flow battery unit electrolytes. The measured state of charge is then input to the controller, which determines appropriate control signals to adjust the flow of anolyte and catholyte to bring about state of charge balancing in the flow battery unit strings, and then outputs those control signals to the flow regulators.

[0031] Additional information may be obtained about the array to refine operation of the state of charge controller. For example, the rate of change of state of charge (d(SOC) / dt, where SOC is the state of charge) may be determined. As described below, the resistances of the individual flow battery units may be determined during operation (e.g., by including voltage and current sensors in the array as appropriate), along with the rate of change of resistance of those units. That information may then be used by the controller for predictable and more robust and accurate control of the state of charge balancing method.

[0032] In a desirable embodiment, the state of charge measurement device used in such an array may simply comprise an unloaded reference flow battery cell supplied with the same anolyte and catholyte of the flow battery unit electrolyte. The reference cell will therefore be at the same state of charge as the cells in that unit. A voltage sensor is then connected across the reference flow battery cell to measure its open circuit voltage and hence the state of charge of that flow battery unit.

[0033] However, in alternative embodiments, other devices known to those skilled in the art may be employed to measure the state of charge. For example, a suitable colorimetric or spectroscopic device configured in the electrolyte supply or circuit hardware to provide optical access to the flow battery unit electrolyte may be used to do so. Alternatively, potentiometric devices other than the reference cells described above may be employed to separately measure the state of charge of the anolyte and catholyte against a common reference electrode (e.g., saturated calomel, silver chloride, etc.), with each summed to determine the total state of charge.

[0034] Advantageously, the balancing technique illustrated in FIG. 1 enables state-of-charge balancing between strings of flow battery units without requiring a separate controllable bi-directional power converter electrically connected to each string. Instead, a single controllable bi-directional power converter may serve the needs of each array. Also, no subsystem is required to fluidly interconnect and mix the anolyte and catholyte tanks to achieve state-of-charge balancing.

[0035] A further advantage of this approach is that the required measurement and control steps can be performed when the array is fully operational, i.e., charging or discharging via the controllable bidirectional power converter, and the method steps may be performed periodically, such as when the flow battery unit electrolyte is at a low or high state of charge.

[0036] 2 shows a schematic diagram of a flow battery array comprising N flow battery unit strings SA, SB,...SN electrically connected in parallel, where there is at least one flow battery unit A1, B1,...N1 in each of the N strings. The locations of optional voltage sensors 24A, 24B,...24N and current sensors 25A, 25B,...25N are shown here for purposes of measuring the resistance and rate of change of resistance of individual flow battery units in the strings.

[0037] 3 shows a schematic diagram of a flow battery array comprising two electrically parallel connected strings of flow battery units SA, SB, again with n flow battery units A1, A2, ... An and B1, B2, ... Bn in each string, respectively. Here, current bypass devices (CBDs) 26A1 and 26B1 are shown as being provided across the first flow battery units A1, B1 in each of the strings. The current bypass devices can be used to balance the state of charge between the flow battery units in the flow battery unit strings by appropriately shunting the current to the current bypass devices.

[0038] 4 also shows a schematic diagram of a flow battery array with two electrically parallel connected strings of flow battery units SA, SB, each string having n flow battery units A1, A2,...An and B1, B2,...Bn, respectively. For suitable measurement and balancing purposes, current bypass devices 26A1, 26A2,...26An, 26B1, 26B2,...26Bn and voltage sensors 24A1, 24A2,...24An, 24B1, 24B2,...24Bn are provided across each flow battery unit in each of the strings, and current sensors 25A and 25B are provided in series with the strings. As one skilled in the art will appreciate, each flow battery unit in string SA provides its SOC status input (not shown) to controller 19, which can identify via output signals (not shown) which current bypass devices 26A1, 26A2, ... 26An etc. should direct bypass current to maintain all flow battery units in string SA at the same electrolyte state of charge. This activation control method works within a single flow battery unit string, but not for multiple flow battery unit strings electrically connected in parallel.

[0039] FIG. 5 shows a schematic diagram of a composite embodiment of the present invention suitable for commercial connection to AC utilities. The illustrated embodiment comprises an array of N' flow batteries of the present invention (i.e., AR1, AR2, ... ARN') electrically connected in parallel. The array comprises N flow battery unit strings (i.e., SA, SB, ... SN) electrically connected in parallel and n flow battery units in each of the flow battery unit strings (i.e., A1, A2, ... An, B1, B2, ... Bn, N1, N2, ... Nn). [FIG. 5 also includes an expanded view of battery unit A1, itself comprising a stack of parallel cells, designated herein as CS1-CSx. Components in common with FIG. 1 are similarly labeled. The expanded view also shows other hardware associated with the battery unit strings, such as pumps, fans, auxiliary equipment, and communication lines.] As shown in FIG. 5, a current bypass device is provided across each flow battery unit. Here, each array associated with a controllable bi-directional power converter 16 includes a DC connection panel 16i and associated AC / DC converter 16ii. Figure 5 also shows the ESS connection panel and other hardware typically found in connecting to an AC installation, including switchgear, main breakers, metering devices, field loads, and AC lines.

[0040] All of the above U.S. patents, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to herein are hereby incorporated by reference in their entirety.

[0041] While particular elements, embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited thereto, since modifications may be made by those skilled in the art without departing from the spirit and scope of the present disclosure, particularly in light of the teachings above. Such modifications are deemed to be within the purview and scope of the claims appended hereto.

Claims

1. 1. A redox flow battery array, comprising: at least two strings of flow battery units electrically connected in parallel, each string including at least one flow battery unit; a flow battery string comprising at least one flow battery cell, each cell comprising an anode, a cathode, and a flow battery unit electrolyte comprising an anolyte and a catholyte; an anolyte tank and a catholyte tank for storing the anolyte and the catholyte of the flow battery unit electrolyte, respectively; an anolyte circuit including a supply line and a return line connecting the anolyte tank to the anode in the flow battery cell of the flow battery string; a catholyte circuit including the supply line and the return line connecting the catholyte tank to the cathode in the flow battery cell of the flow battery string; a state of charge measuring device capable of measuring a state of charge of the flow battery unit electrolyte in the flow battery unit; The flow battery unit string comprising: a flow regulator in at least one of the anolyte circuit and the catholyte circuit, which regulates a flow of at least one of the anolyte and the catholyte through at least one of the flow battery units in the at least two strings of flow battery units; a controllable bidirectional power converter electrically connected to each of the at least two strings of flow battery units; a controller having an input connected to each of the state of charge measuring devices in the flow battery units and an output connected to the flow regulator, the output of the controller controlling the flow regulator to balance the state of charge in at least two of the flow battery unit strings by adjusting the flow of at least one of the anolyte and the catholyte to the anode and the cathode in the flow battery cells in the flow battery string in response to a state of charge of the flow battery unit electrolyte in the flow battery unit; A redox flow battery array comprising:

2. 10. The redox flow battery array of claim 1, wherein there is no additional controllable bi-directional power converter electrically connected to any of the flow battery unit strings.

3. 2. The redox flow battery array of claim 1, wherein there is no subsystem for fluidly interconnecting and mixing the anolyte tank and the catholyte tank depending on the state of charge of the flow battery unit electrolyte in the flow battery unit.

4. The redox flow battery array according to claim 1 , comprising a plurality of the flow battery unit strings electrically connected in parallel.

5. 5. The redox flow battery array of claim 4, comprising a voltage sensor across at least one of the flow battery units in each of the flow battery unit strings and a current sensor in series with at least one of the flow battery units in each of the flow battery unit strings.

6. 2. The redox flow battery array of claim 1, wherein each of the flow battery unit strings comprises at least two of the flow battery units electrically connected in series.

7. 7. The redox flow battery array of claim 6, wherein each of the flow battery unit strings comprises a current bypass device electrically connected in parallel to at least one of the flow battery units in the flow battery unit string.

8. 8. The redox flow battery array of claim 7, wherein the current bypass device is electrically connected in parallel to each of the flow battery units in each of the flow battery unit strings.

9. 2. The redox flow battery array of claim 1, further comprising a flow regulator in each of the anolyte and catholyte circuits in each of the flow battery units in each of the flow battery unit strings that regulates the flow of each of the anolyte and catholyte circuits therethrough, and the controller has an output connected to each of the flow regulators in each of the anolyte and catholyte circuits.

10. The state of charge measuring device in each of the at least one flow battery unit an unloaded reference flow battery cell supplied with the anolyte and the catholyte of the flow battery unit electrolyte; a voltage sensor connected across the reference flow battery cell to measure the open circuit voltage of the reference flow battery; The redox flow battery array of claim 1 .

11. 2. The redox flow battery array of claim 1, wherein the state of charge measuring device in each of the at least one flow battery unit is selected from the group consisting of a colorimetric device having optical access to the flow battery unit electrolyte, a spectroscopic device having optical access to the flow battery unit electrolyte, and a potentiometric device.

12. 2. The redox flow battery array of claim 1, wherein the flow battery cells in the flow battery string are vanadium redox flow batteries.

13. A redox flow battery system comprising at least two redox flow battery arrays according to claim 1 electrically connected in parallel.

14. 2. A method for balancing the state of charge of the flow battery units in a redox flow battery array according to claim 1, comprising: balancing the state of charge of the flow battery unit strings, the step of balancing the state of charge of the flow battery unit strings comprising: measuring a state of charge of the flow battery unit electrolyte of the flow battery units in each of the at least two flow battery unit strings using the state of charge measuring device; inputting the measured state of charge into the controller; determining, with the controller, a control signal for adjusting a flow of at least one of the anolyte and the catholyte to the anode and the cathode in the flow battery cells in at least one of the flow battery units in response to a state of charge of the flow battery unit electrolyte in the flow battery units to balance states of charge in at least two of the flow battery unit strings; outputting the control signal to the flow regulator; A method for providing the above.

15. The state of charge measuring device in each of the at least one flow battery unit an unloaded reference flow battery cell supplied with the anolyte and the catholyte of the flow battery unit electrolyte; a voltage sensor connected across the reference flow battery cell to measure the open circuit voltage of the reference flow battery; Equipped with 15. The method of claim 14, wherein measuring the state of charge comprises measuring the open circuit voltage of the unloaded reference flow battery cell.

16. Each of the flow battery unit strings comprises at least two of the flow battery units electrically connected in series, each of the flow battery unit strings comprises a current bypass device electrically connected in parallel to at least one of the flow battery units in the flow battery unit string, and the method further comprises:

15. The method of claim 14, further comprising balancing the state of charge between flow battery units within the string of flow battery units by shunting current to one or more of the current bypass devices.

17. The method of claim 14 , wherein each of the steps occurs when an array is charging or discharging via the controllable bi-directional power converter.

18. 15. The method of claim 14, wherein each step of the method is performed when the flow battery unit electrolyte is in a low or high state of charge.

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