Method for removing V2O5 deposits in redox flow battery module

Abstract

The invention relates to a method for removing V2O5 deposits from at least one vanadium battery module (1) integrated in a battery system, comprising the following steps in sequence: identifying at least one battery module (1) having V2O5 deposits; closing the pump (4) of the at least one battery module (1) at a time t1; restarting the pump (4) of the at least one battery module (1) at a time t2; wherein the length of the time interval [Delta] t = t2-t1 is selected such that, at the point in time t2, the terminal voltage of the at least one battery module (1) is a negative value, but the overcharge of the electrolyte in the battery assembly (2) of the at least one battery module (1) is avoided, and the above steps (except the first step) are both carried out during the discharge of the battery system.

Description

Technical Field

[0001] The present invention relates to a method for removing vanadium oxide (V2O5) deposits in a vanadium redox flow battery module integrated in a battery system. The battery system comprises a plurality of vanadium battery modules connected in series. Background Art

[0002] When vanadium redox flow battery modules are operated at temperatures above 40°C and at high states of charge, V2O5 deposits may form in the positive half-cell, which can block the flow of electrolyte through the affected half-cell. In the worst case, this can lead to complete blockage of the battery module and thus failure. In addition, these deposits may also cause uneven electrolyte flow in the affected cells, which can trigger local cell overvoltages and cause damage to the battery. These deposits cannot be simply removed by reducing the temperature.

[0003] A variety of methods have been proposed in the prior art to avoid or solve the above problems. In addition to complex battery cooling technology, EP 3 024 080 A1 discloses a method for avoiding the formation of V2O5 by adding additives to the electrolyte. DE10 2019 106 588 A1 discloses a battery module comprising an additional pipe section and a valve for injecting an anolyte containing V2+ into a positive half-cell. V2+ is able to release V2O5 deposits. Obviously, in this case, the normal operation of the battery module must be interrupted. WO 2019 / 139566A1 discloses a method for releasing V2O5 deposits. In this method, the battery module is electrically isolated from the environment, that is, the charging or discharging of the battery module is interrupted. In addition, the flow of electrolyte in the half-cell affected by the deposit is interrupted, while the flow of electrolyte in the other half-cell continues. Subsequently, the battery assembly is short-circuited by wiring and resistance. The battery module is maintained in this state for a predetermined period of time. During this period, the deposits are released. Summary of the invention

[0004] The object of the present invention is to provide a method for cleaning V2O5 deposits in a vanadium redox flow battery module integrated in a battery system, which method can be implemented during normal operation of the battery system.

[0005] The object of the invention is achieved by the embodiments of the independent claim. Further advantageous embodiments of the invention can be found in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The present invention is described below in conjunction with the accompanying drawings. The accompanying drawings show the following in detail:

[0007] Figure 1 : a battery module;

[0008] Figure 2 : a battery system;

[0009] Figure 3 : A timing diagram of the terminal voltage in the method according to the first embodiment of the present invention; and

[0010] Figure 4 : A timing diagram of the terminal voltage in the method according to the second embodiment of the present invention. DETAILED DESCRIPTION

[0011] Figure 1 On the left is a schematic diagram of a vanadium-based battery module. The battery module is numbered 1. The battery module includes a battery assembly, numbered 2; a liquid storage device, numbered 3; two pumps, one of which is numbered 4; and optionally a measuring device for detecting the terminal voltage, numbered 5. The battery assembly 2 is an arrangement of multiple redox flow batteries, and the arrangement can be arbitrary. For example, it can be a single battery stack, a series connection of multiple stacks, a parallel connection of multiple stacks, or a combination of a series connection and a parallel connection of multiple stacks. The liquid storage device 3 is used to store electrolyte and provide electrolyte to the battery assembly 2. To this end, with a few exceptions, the liquid storage device 3 usually includes at least two liquid storage tanks and a pipeline system for connecting the liquid storage tanks to the battery assembly 2. Pump 4 is used to transport electrolyte. Figure 1 4. The electrolyte can also be delivered by a double-head pump, i.e. two pumps driven by a common motor. Furthermore, in principle, there can be more than one pump in each electrolyte circuit. This is advantageous in certain situations, for example when the battery modules are redundant. This means that if one of the pumps fails, the battery module can still function normally.

[0012] Figure 1 The right side shows a symbolic representation of the battery module 1. This symbolic representation will be used in the subsequent description.

[0013] Figure 2 A battery system consisting of a plurality of battery modules connected in series is schematically shown. The battery system includes at least two battery modules, one of which is numbered 1, a bidirectional power conversion system (PCS), numbered 6, and a control device, numbered 7. The battery modules 1 are connected in series and connected to the power conversion system 6. Figure 2 Four battery modules are shown in the figure, where the dashed lines connected in series represent any number of other modules. The power conversion system 6 is used to connect the battery system to a power grid or a higher-level power system. The control device 7 is configured to be able to detect the operating state of the power conversion system 6 and drive the pump 4 in the battery module 1. Optionally, the control device 7 can also be configured to additionally detect the measurement value of the measuring device 5 in the battery module 1.

[0014] In the following description, it is assumed that V2O5 deposits have been identified in one or more battery modules 1. A skilled person is aware of a variety of detection methods. For this, reference is made to paragraphs

[0037] to

[0041] of WO 2019 / 139566 A1 and to the Figure 1 Pressure sensor 52 (“pressure sensor 52”) is shown. Another method of identifying battery modules with V2O5 deposits without a pressure sensor is to monitor the terminal voltage of the battery modules during discharge. Although the same discharge current flows through all battery modules connected in series in the battery system, the terminal voltage of the battery modules with harmful V2O5 deposits decreases faster than that of unaffected battery modules.

[0015] The method of the present invention is described below. To this end, a sign convention is used, that is, during normal operation of the battery system, the terminal voltage of the battery module is a positive value.

[0016] According to the invention, a method for removing V2O5 deposits from at least one battery module 1 integrated in a battery system comprises the following steps in the specified order:

[0017] - identifying at least one battery module 1 having V2O5 deposits;

[0018] - switching off the pump 4 of at least one battery module 1 at time t1;

[0019] - At time t2 (slightly later), the pump 4 of at least one battery module 1 is switched on again;

[0020] The length of the time interval Δt=t2-t1 is selected so that the terminal voltage of the battery module 1 is negative at time point t2, but overcharging of the electrolyte in the battery components of the battery module is avoided, and these steps (except the first step) are performed when the battery system is discharged.

[0021] The first step, i.e. the identification of at least one battery module having V2O5 deposits, can be performed in any operating state of the battery system, i.e. regardless of whether the battery system is charging or discharging. However, the remaining steps can only be performed when the battery system is discharging. The identification step is a step for determining whether the subsequent steps of the inventive method need to be performed on the battery module. In other words, the method of the present invention in a narrow sense consists of the steps mentioned after the identification step. In some of the following explanations, the term "method of the present invention" is used in a narrow sense. This is the case when the context makes it clear that one or more battery modules have been identified.

[0022] In the implementation process of the method according to the present invention, the electrochemical process occurring in the degraded battery module will be Figure 3 For a more detailed explanation.

[0023] Figure 3The time variation of the terminal voltage of a battery module with V2O5 deposits when implementing the method of the present invention is shown. Before time point t1, the battery module with V2O5 deposits participates in the discharge process of the battery system like any other battery module of the battery system. During this period, the terminal voltage decreases over time because the electrolyte pumped into the battery assembly is partially discharged when flowing through the battery assembly.

[0024] When the pump is turned off at time t1, the electrolyte supply to the battery assembly stops, and the electrolyte retained in the battery assembly is discharged at a significantly faster rate because the discharge current flowing through the battery modules connected in series remains unchanged or changes only slightly during this process. Therefore, the terminal voltage of the battery module drops rapidly. When the terminal voltage drops to the zero line, the electrolyte in the battery assembly is reversely charged, causing the terminal voltage of the battery module to turn negative. This is an electrochemical characteristic of vanadium electrolyte.

[0025] Above the zero line of the terminal voltage, the electrolyte disposed in the positive half-cell is composed of a mixture of V5+ and V4+ ions. Below the zero line of the terminal voltage, the positive half-cell can be said to become a negative half-cell, the electrolyte contained in which is composed of a mixture of V3+ and V2+ ions. In this process, the battery undergoes a transition phase across the zero point, during which the mixture of vanadium ions changes. That is, after the transition phase, V2+ ions are generated in the positive half-cell containing the V2O5 deposits, and these ions release the V2O5 deposits.

[0026] However, the reverse charging process cannot last too long, otherwise the electrolyte in the battery pack may be overcharged, which may damage the battery module. Therefore, the pump is turned on again at the selected time point t2. At this time, fresh electrolyte flows back into the battery pack and flushes the released V2O5 deposits out of the positive half-cell. The state of charge of the supplied electrolyte is the same as before time point t1, so the terminal voltage rises again to the (positive) value corresponding to time point t1. Figure 3 In the curve shown, the pump is running at the same flow rate as in the period before time point t1. However, this is not a necessary condition. Instead, it only means that the battery module has returned to normal operation at time point t2. If for some reason, the normal operation of the battery module at time point t2 requires a pump flow rate different from that before t1, the pump is run at the required flow rate at time point t2.

[0027] The step "turning off the pump 4 of at least one battery module 1 at time t1" has the effect of causing the described polarity reversal. In principle, this can also be achieved by running the pump 4 at a very low flow rate. Therefore, in this document, "turning off the pump" is understood to mean an operation of the pump that leads to the described polarity reversal.

[0028] The time point t2 can be determined most simply by the control device detecting the terminal voltage: if the terminal voltage is sufficiently negative but has not yet fallen below a critical limit, the pump is switched on again. However, t2 can also be determined without detecting the terminal voltage. This can be done by calculation, which takes into account at least the following variables: the state of charge at the time point t1, the magnitude of the discharge current and the electrolyte volume of the individual battery components in the battery module. If t2 is to be determined via the terminal voltage, it must be possible to detect the negative terminal voltage.

[0029] During the shutdown of the pump, the discharge current through the battery system can also be reduced. This can delay the time point t2. That is, the time interval Δt=t2-t1 is thus increased.

[0030] If necessary, the method of the present invention can be repeated several times until the V2O5 deposits in the positive half-cell of the battery module are sufficiently reduced. In the repeated operation, the first step (identification step) does not necessarily need to be performed again. However, this step can be advantageously performed to check whether the battery module still has significant V2O5 deposits.

[0031] The inventors have found that the desired effect can be enhanced by extending the duration of the terminal voltage in the negative range. This can be achieved by running the pump at a reduced flow rate after turning on the pump at time point t2. Here, "reduced flow rate" refers to the pump flow rate compared to before t1. Figure 4 The terminal voltage diagram corresponding to this embodiment is shown. During the time interval t2 to t3, the pump operates at a pump flow rate when the terminal voltage remains in the negative range. Figure 4 In the case shown, the pump flow rate is selected so that the terminal voltage remains constant in the time interval between t2 and t3. Although this approach is advantageous, it is not absolutely necessary. If the pump flow rate is slightly different, the positive effect is almost equally significant. It is only necessary to avoid overcharging of the electrolyte present in the battery assembly during the entire time interval from t2 to t3. For this purpose, the pump flow rate does not necessarily have to be constant in the above time interval. For example, despite the reduction of the pump flow rate, the terminal voltage continues to drop and may exceed the critical limit. This can be avoided by further reducing the pump flow rate. That is, the control device can avoid overcharging by adjusting the pump flow rate during the time interval. At time point t3, the pump is then operated again at the flow rate at t1 so that the battery module returns to the corresponding (positive) terminal voltage. However, this is not a necessary condition. On the contrary, it is only to indicate that the battery module will resume normal operation at t3. If for some reason, the normal operation of the battery module at time point t3 requires a different pump flow rate than before time point t1, then at time point t3, the pump is operated at the flow rate required at that time.

[0032] It should be mentioned that when the pump is turned on at time t2, non-uniformity is formed in the battery assembly of the battery module. This is because fresh electrolyte with different composition from other electrolytes is supplied to the battery assembly at a specific point in the battery assembly. This leads to a local different potential state, resulting in the formation of a corresponding balancing current.

[0033] Between the time interval t2 and t3, the discharge current through the battery system can also be further reduced. Therefore, the pump flow rate in this time interval must be reduced to a lesser extent, or can be reduced to a lesser extent.

[0034] like Figure 4 As shown, the method for removing V2O5 deposits in at least one battery module 1 integrated in a battery system according to the present invention comprises the following steps performed in sequence:

[0035] - identifying at least one battery module 1 having V2O5 deposits;

[0036] - switching off the pump 4 of at least one battery module 1 at time t1;

[0037] - restarting the pump 4 of at least one battery module 1 at a time point t2 (slightly later) and operating the pump 4 in at least one battery module 1 at a first pump flow rate;

[0038] At time t3 (later than t2 ), the pump 4 in at least one battery module 1 is operated at a second pump flow rate.

[0039] The length of the time interval Δt=t2-t1 is selected so that at time point t2, the terminal voltage of at least one battery module 1 is negative; the first pump flow rate is selected so that between the time interval t2 and t3, the terminal voltage of at least one battery module 1 is still negative, but overcharging of the electrolyte in the battery assembly of at least one battery module 1 within the time interval t2 to t3 is avoided; the second pump flow rate is selected so that after time point t3, the terminal voltage of at least one degraded battery module 1 becomes positive; except for the first step, these steps are performed during the discharge of the battery system.

[0040] like Figure 4 The method of the invention shown in FIG. 1 can be performed several times in succession if necessary. In this case, it is the same as the method described above. Figure 3 The method described is the same.

[0041] Since the battery modules in the battery system are connected in series, the terminal voltages of the battery modules will be superimposed. This means that when implementing the method of the present invention, Figure 3 and Figure 4The voltage curve shown in Figure 1 is also reflected in the total voltage of the power conversion system. Therefore, the power conversion system must be able to cope with such voltage fluctuations. The same applies to the power of the battery system. If it is desired that the power output of the battery system remain constant, this can be achieved by increasing the discharge current accordingly between the time intervals t1 to t2 and t1 to t3.

[0042] To ensure that the battery system can continue to operate normally during the implementation of the method of the present invention, the total voltage on the power conversion system must not become negative. This limits the number of battery modules that can implement the method of the present invention simultaneously. In the extreme case, the number of battery modules that operate normally must be greater than the number of battery modules that are implementing the method of the present invention at any given time. In actual operation, it will be advantageous if the number of battery modules that operate normally is significantly greater than the number of battery modules that are implementing the method of the present invention at a given time.

[0043] In order to minimize the demands on the power conversion system, it may be advantageous if the method of the invention is performed on only one battery module at any given time. That is, if deposits are present on multiple battery modules, the method of the invention is performed on each affected battery module in turn.

[0044] The method according to the invention can also be advantageously implemented as a preventive measure. That is, the method is occasionally performed for each battery module in the battery system. Of course, in this case, it is not necessary to determine whether there are actually significant deposits on the battery modules. It is only necessary to assume that deposits will form on each battery module after a certain period of time. The step of "identifying at least one battery module with V2O5 deposits" merely includes determining at least one battery module on which the method according to the invention needs to be implemented. This can be achieved, for example, by first determining the operating time, after which the method according to the invention will be preventively performed on the battery module. Then, an identification step is performed to check whether there are battery modules in the battery system whose operating time exceeds the predetermined time since the start of operation or the last execution of the method according to the invention. If this is the case, the steps after the identification step of the method according to the invention are performed on the identified battery modules.

[0045] By implementing the method of the present invention, V2O5 deposits in the battery module can be at least partially released. The method can be carried out during normal operation of the battery system. The method can be performed without additional hardware support and can be completed using only the existing devices in ordinary battery modules. The only requirement is that the control device must be configured so that it can perform the steps of the method of the present invention. For this purpose, a corresponding computer program is run in the control device, which can also be stored on a computer-readable medium.

[0046] Reference Symbols List

[0047] 1-Battery module

[0048] 2-Battery Assembly

[0049] 3-Liquid storage device

[0050] 4-Pump

[0051] 5-Measuring device for detecting terminal voltage

[0052] 6- Power conversion system

[0053] 7-Control device

Claims

1. A method for removing V2O5 deposits in at least one vanadium battery module (1) integrated in a battery system, wherein the battery system comprises: At least three battery modules (1), a power conversion system (6) and a control device (7) connected in series, wherein the battery module (1) is connected to the power conversion system (6), each of the battery modules (1) comprises a battery assembly (2), a liquid storage device (3) for storing an electrolyte and two pumps (4) for conveying the electrolyte through the battery assembly (2), and the control device (7) is configured to detect the operating state of the power conversion system (6) and drive the pumps (4) in the battery module (1), characterized in that the method comprises the following steps performed in sequence: - identifying at least one battery module (1) having V2O5 deposits; - switching off the pump (4) of the at least one battery module (1) at time t1; - starting the pump (4) of the at least one battery module (1) at time t2; The length of the time interval Δt=t2-t1 is selected so that at the time point t2, the terminal voltage of the at least one battery module (1) is negative, but overcharging of the electrolyte in the battery assembly (2) of the at least one battery module (1) is avoided, and the above steps (except the first step) are performed during the discharge of the battery system.

2. The method according to claim 1, wherein the method comprises the following steps performed in sequence: - identifying at least one of said battery modules (1) having V2O5 deposits; - switching off the pump (4) of the at least one battery module (1) at time t1; - starting the pump (4) of the at least one battery module (1) at time t2 and operating the pump (4) of the at least one battery module (1) at a first pump flow rate; - operating the pump (4) of the at least one battery module (1) at a second pump flow rate at a time point t3 after the time point t2; in, The length of the time interval Δt=t2-t1 is selected so that at the time point t2 the terminal voltage of the at least one battery module (1) is a negative value; wherein the first pump flow rate is selected so that the terminal voltage of the at least one battery module (1) is negative during the time interval t2 to t3, but overcharging of the electrolyte in the battery assembly (2) of the at least one degraded battery module (1) is avoided during the time interval t2 to t3; The second pump flow rate is selected so that the terminal voltage of at least one degraded battery module (1) becomes positive after t3, and the above steps (except the first step) are performed during the discharge of the battery system.

3. A battery system, wherein the battery system comprises at least three vanadium redox flow battery modules (1) connected in series, a power conversion system (6) and a control device (7), The battery modules (1) are connected to the power conversion system (6), each battery module (1) comprising a battery assembly (2), a liquid storage device (3) for storing electrolyte and two pumps (4) for conveying electrolyte through the battery assembly (2), and The control device (7) is configured to detect the operating state of the power conversion system (6) to drive the pump (4) in the battery module (1), and to perform the steps of the method according to any of the preceding claims.

4. The battery system according to claim 3, wherein the control device (7) is capable of detecting the terminal voltage of the battery module (1).

5. A computer program comprising instructions for causing a battery system according to claim 3 or 4 to execute the method steps according to claim 1 or 2.

6. A computer readable medium having stored therein a computer program according to claim 5.

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