Method for removing v2o5 deposits in a redox flow battery module

Abstract

The invention relates to a method for removing V2O5 deposits in at least one vanadium battery module (1) which is incorporated in a battery system, the method comprising the following steps in the order indicated: - identifying at least one battery module (1) having V2O5 deposits; - switching off the pumps (4) of the at least one battery module (1) at a time t1; - switching on the pumps (4) of the at least one battery module (1) at a time t2; wherein the length of the time interval ∆t= t2-t1 is selected such that, at time t2, a terminal voltage of the at least one battery module (1) is negative, but overcharging of the electrolyte located in the cell assembly (2) of the at least one battery module (1) is avoided, and wherein these steps, with the exception of the first step, take place while the battery system is being discharged.

Description

Method for removing V2O5 deposits in a redox flow battery module The invention relates to a method for removing V2O5 deposits in a vanadium redox flow battery module integrated into a battery system. The battery system comprises several vanadium battery modules connected in series. When vanadium redox flow battery modules are operated at temperatures above 40°C and at high states of charge, V2O5 deposits can form in the positive half-cell, impeding the flow of electrolyte through the affected half-cell. In the worst case, this can lead to a complete blockage and thus to the failure of the battery module. Furthermore, the deposits can cause uneven flow through the affected cell, resulting in local cell overvoltages that can damage the cell. The deposits cannot be removed simply by lowering the temperature. Several methods have been disclosed in the prior art to avoid or remedy the described problem. In addition to the complex cooling of the cells, EP 3 024 080 A1 discloses the addition of additives to the electrolyte to prevent the formation of V2O5. DE 10 2019 106 588 A1 discloses a battery module that includes additional pipe sections and valves so that the anolyte, which contains V 2+ contains, can be directed into the positive half-cell. By V 2+The V2O5 deposit is dissolved. It is clear that this must interrupt the normal operation of the battery module. WO 2019 / 139566 A1 discloses a method for dissolving V2O5 deposits. The battery module is electrically isolated from the environment, i.e., the charging or discharging of the battery module is interrupted. In addition, the electrolyte flow through the half-cell with deposits is interrupted, while the electrolyte flow through the other half-cell continues. The cell arrangement is then heated via a line and an electrical resistor. short-circuited. The battery module is left in this state for a predetermined period of time. During this time, the deposit is dissolved. The object of the invention is to provide a method for removing V2O5 deposits in a vanadium redox flow battery module which is integrated in a battery system, wherein the method can be carried out during normal operation of the battery system. The object is achieved according to the invention by an embodiment according to the independent claim. Further advantageous embodiments of the present invention can be found in the subclaims. The invention is explained below with reference to the figures. The figures show in detail: Fig.1 battery module; Fig.2 Battery system; Fig. 3 Time profile of the terminal voltage during the method according to the invention in a first embodiment; Fig. 4 Time profile of the terminal voltage during the method according to the invention in a second embodiment; Figure 1 shows a schematic representation of a vanadium-based battery module on the left. The battery module is designated 1. The battery module comprises a cell arrangement, designated 2, a tank device, designated 3, two pumps, one of which is designated 4, and optionally a measuring device for detecting the terminal voltage, designated 5. The cell arrangement 2 is an arrangement of a plurality of redox flow cells, which can be arranged in any desired manner. For example, it could be a single cell stack, a series connection of several stacks, a parallel connection of several stacks, or a combination of series and parallel connection of several stacks. The tank device 3 serves to Storing the electrolyte and supplying the cell arrangement 2 with electrolyte. For this purpose, the tank device 3 comprises, with a few exceptions, at least two tanks and a pipe system for connecting the tanks to the cell arrangement 2. The pumps 4 are used to pump the electrolyte. Figure 1 shows two separate pumps 4. The electrolyte could just as well be pumped with a double-head pump, i.e. with two pumps driven by a common motor. In principle, there can be more than one pump per electrolyte circuit. This can be advantageous, for example, if the battery module is to be designed redundantly. This means that if one pump fails, the battery module remains functional. On the right side of Figure 1, a symbolic representation of the battery module 1 is shown. This symbolic representation will be used below. Figure 2 shows a schematic representation of a battery system with a plurality of battery modules connected in series. The battery system comprises at least two battery modules, one of which is designated 1, a bidirectional power conversion system (PCS), designated 6, and a control device, designated 7. The battery modules 1 are connected in series and connected to the converter 6. Figure 2 shows four battery modules, with the dashed lines in the diagram indicating any number of additional modules. The converter 6 connects the battery system to the grid or to a higher-level electrical system. The control device 7 is designed such that it can detect the operating state of the converter 6 and control the pumps 4 in the battery modules 1.Optionally, the control device 7 can be designed so that it can additionally record the measured values ​​of the measuring devices 5 of the battery modules 1. For the following explanations, it is assumed that one or more battery modules 1 have been identified that suffer from V2O5 deposits. Several possibilities are known to the person skilled in the art. Reference is made to the sections

[0037] to

[0041] and the pressure sensors (“pressure transducers 52”) shown in Fig.1 52 of WO 2019 / 139566 A1. An alternative method for identifying battery modules with V2O5 deposits, which does not require pressure sensors, is to monitor the terminal voltage of the battery modules during discharging. Although the same discharge current flows through all series-connected battery modules of a battery system, the terminal voltage drops more rapidly in battery modules with harmful V2O5 deposits than in unaffected battery modules. The method according to the invention is described below. A sign convention is used, according to which the terminal voltage of the battery modules has a positive sign during normal operation of the battery system. The method according to the invention for removing V2O5 deposits in at least one battery module 1 which is integrated in a battery system comprises the following steps in the specified order: - Identification of at least one battery module 1 with V2O5 deposits; - switching off the pumps 4 of the at least one battery module 1 at a time ti ; - Switching on the pumps 4 of the at least one battery module 1 at a (later) time t2j, wherein the length of the time interval Δt= t2-t1 is selected such that at time t2 the terminal voltage of the battery module 1 is negative, but overcharging of the electrolyte in the cell arrangement of the battery module 1 is avoided, and wherein these steps, with the exception of the first step, take place while the battery system is being discharged. The first step, ie the identification of at least one battery module with V2O5 deposits, can be carried out during any operating state of the battery system, i.e. both while the battery system is being charged and while the battery system is being discharged. The other steps, however, can only be carried out when the battery system is being discharged. The identification step is a step which checks whether the subsequent steps of the method according to the invention should be carried out on a battery module. This means that the method according to the invention in the narrower sense consists of the steps mentioned after the identification step. In some cases, the term "method according to the invention" is used in this narrower sense in the following explanations. This is the case when it is clear from the context that one or more battery modules have already been identified. The electrochemical processes that occur in the degraded battery module in question during the implementation of the method according to the invention are explained in more detail with reference to Figure 3. Figure 3 shows the temporal progression of the terminal voltage of a battery module with V2O5 deposits during the implementation of the method according to the invention. Before time t1, the battery module with V2O5 deposits participates in the discharge process of the battery system like any other battery module in the battery system. The terminal voltage drops over time because the electrolyte pumped through the cell arrangement by the pumps is partially discharged while it remains in the cell arrangement. When the pumps are switched off at time t1, the supply of electrolyte to the cell arrangement ceases, and the electrolyte permanently present in the cell arrangement during this state is therefore discharged much more quickly because the discharge current flowing through the series-connected battery modules does not change or changes only negligibly as a result of the process. The terminal voltage of the battery module in question drops accordingly quickly.When the terminal voltage reaches zero, the electrolyte in the cell array is charged in reverse, causing the terminal voltage of the battery module in question to become negative. This is an electrochemical characteristic of the vanadium electrolyte. Above the zero terminal voltage, the electrolyte in the positive half-cell has a composition that is a mixture of V. 5+ and V 4+ ions. Below the zero line of the terminal voltage, the positive half-cell becomes the negative half-cell and the electrolyte it contains has a composition that is a mixture of V 3+ and V 2+ ions. During the zero crossing, the cell passes through a transition phase with different mixtures of vanadium ions. This means that after passing through the transition phase, the positive half-cell, in which the V2O5 deposits are located, contains V 2+Ions in which the V2O5 deposits dissolve. However, the charging process with the opposite sign cannot be continued indefinitely, as otherwise the electrolyte in the cell arrangement would be overcharged, which would lead to damage to the battery module. Therefore, the pumps are switched on again at a correspondingly selected time t2. Fresh electrolyte now flows into the cell arrangement and washes the dissolved V2O5 deposits out of the positive half-cell. This supplied electrolyte has a state of charge similar to that which existed shortly before time t1, so that the terminal voltage rises again to the (positive) value corresponding to t1. In the curve shown in Figure 3, the pumps are operated at the same flow rate as before time t1. However, this is not a necessary condition. Rather, it is only intended to express that the battery module in question returns to normal operation at time t2.If, for any reason, the normal operation of the battery module at time t2 requires a different pumping rate than that which existed shortly before time ti, the pumps would be operated at time t2 at the pumping rate required at that time. The effect of the step "switching off the pumps 4 of the at least one battery module 1 at a time t1" is to initiate the described polarity reversal. In principle, this can also be achieved by operating the pumps 4 at a very low delivery rate. Therefore, in this document, "switching off the pumps" is understood to mean operation of the pumps that leads to the described polarity reversal. The easiest way to determine time t2 is to have the control device detect the terminal voltage: If the terminal voltage is sufficiently negative but has not yet fallen below the critical limit, the pumps are switched on again. However, time t2 can also be determined without detecting the terminal voltage. This can be done by a calculation that includes at least the following variables: state of charge at time ti, magnitude of the discharge current, and volume of electrolyte in a cell of the cell arrangement of the battery module in question. If the terminal voltage is to be used to determine t2, it must be capable of detecting voltages with a negative sign. While the pumps are off, the discharge current flowing through the battery system can also be reduced. This allows time t2 to be pushed back in time. That is, the time interval Δt=t2-t1 becomes longer. If necessary, the method according to the invention can be carried out several times in succession until the V2O5 deposits in the positive half-cell of the battery module in question have been sufficiently removed. When repeating the process, the first step does not necessarily have to be performed. However, it can be advantageously performed to check whether the battery module in question still contains significant V2O5 deposits. The inventors have recognized that the desired effect can be increased by extending the period during which the terminal voltage is in the negative range. This can be advantageously achieved by operating the pumps that are switched on at time t2 at a reduced pumping rate thereafter. The term "reduced" refers to the pumping rate that existed before time t1. Figure 4 shows the terminal voltage curve corresponding to this embodiment. In the time interval between t2 and ts, the pumps are operated at a pumping rate such that the terminal voltage remains negative. In the case shown in Figure 4, the pumping rate was selected such that the terminal voltage remains constant in the time interval between t2 and ts. remains. This is advantageous but not absolutely necessary. The positive effect would be almost as strong if the pump rate were slightly different. It is only necessary that overcharging of the electrolyte in the cell arrangement is avoided during the entire time interval between t2 and ts. For this to happen, the pump rate does not necessarily have to be constant in the said time interval. For example, it could happen that the terminal voltage continues to decrease despite a reduced pump rate and would exceed the critical limit. This can then be avoided by a further reduction in the pump rate. This means that the pump rate can be regulated by the control device in the said time interval so that overcharging is avoided. At time ts, the pumps are then operated again at the pump rate of ti so that the battery module returns to the corresponding (positive) terminal voltage. However, this is not a necessary condition.Rather, it simply indicates that the battery module in question returns to normal operation at time ts. If, for whatever reason, the normal operation of the battery module at time ts requires a different pumping rate than that which existed shortly before time ti, the pumps would be operated at the required pumping rate at time ts. It should be noted that when the pumps are switched on at time t2, inhomogeneities form in the cell array of the respective battery module. These arise from the fact that fresh electrolyte is supplied to the cell array at a specific location, which has a different composition than the rest of the electrolyte in the cell array. This leads to locally different potential states, which in turn generate corresponding compensating currents. In the time interval between t2 and ts, the discharge current flowing through the battery system can also be reduced. This means that the pump rate can be reduced less during this time interval. The method according to the invention according to Figure 4 for removing V2O5 deposits in at least one battery module 1 which is integrated in a battery system comprises the following steps in the given order: - Identification of at least one battery module 1 with V2O5 deposits; - switching off the pumps 4 of the at least one battery module 1 at a time ti ; - switching on the pumps 4 of the at least one battery module 1 at a (later) time t2 and operating the pumps 4 of the at least one battery module 1 at a first pumping rate; - Operating the pumps 4 of the at least one battery module 1 at a time ts after t2 at a second pumping rate; wherein the length of the time interval Δt=t2-t1 is selected such that at time t2 the terminal voltage of the at least one battery module 1 is negative, and wherein the first pumping rate is selected such that the terminal voltage of the at least one battery module 1 is negative during the time interval between t2 and ts, but overcharging of the electrolyte in the cell arrangement of the at least one battery module 1 is avoided in the time interval between t2 and ts, and wherein the second pumping rate is selected such that the terminal voltage of the at least one degraded battery module 1 becomes positive after ts, and wherein these steps, with the exception of the first step, take place while the battery system is discharging. The inventive method according to Figure 4 can also be carried out several times in succession if necessary. The above statement regarding the method according to Figure 3 applies analogously. Since the battery modules in the battery system are connected in series, the terminal voltages of the battery modules add up. This means that the voltage curve shown in Figures 3 and 4 is also reflected in the total voltage applied to the converter when the method according to the invention is carried out. Therefore, the converter must be designed to cope with this voltage variability. can cope. The same applies to the performance of the battery system. If the power delivered by the battery system is to remain constant, this can be achieved by increasing the discharge current accordingly in the time intervals between ti and t2 or ti and ts. To ensure that the battery system continues to function smoothly while the method according to the invention is being carried out, the total voltage applied to the converter must not become negative. This limits the number of battery modules on which the method according to the invention can be carried out simultaneously. In the limiting case, the number of normally operated battery modules must be greater than the number of battery modules on which the method according to the invention is being carried out at a given time. In practice, it will be advantageous if the number of normally operated battery modules is significantly greater than the number of battery modules on which the method according to the invention is being carried out at a given time. To keep the demands on the converter as low as possible, it is advantageous if the method according to the invention is only carried out on one battery module at a given time. This means that if more than one battery module has deposits, the method according to the invention is carried out successively on each affected battery module. The method according to the invention can advantageously also be carried out prophylactically. This means that the method is carried out from time to time on each battery module of the battery system. Of course, it is not necessary to first determine whether the battery modules actually contain deposits to a significant extent. It is simply assumed that deposits are present in each battery module after a certain time. The step "Identification of at least one battery module 1 with V2O5 deposits" then only consists in determining at least one battery module on which the method according to the invention is to be carried out. This can be done, for example, by specifying an operating period in advance, after which a prophylactic The method according to the invention is to be carried out on a battery module. The said identification step then consists in checking the battery modules of the battery system to determine whether there are any battery modules that have been operated for longer than the previously defined operating time since commissioning or the last time the method according to the invention was carried out on them. If this is the case, the steps of the method according to the invention that follow the identification step are carried out on the battery modules thus identified. Using the method according to the invention, V2O5 deposits in battery modules can be at least partially dissolved. The method can be carried out during normal operation of the battery system. No additional hardware is required for implementation; the resources available with conventional battery modules are sufficient.Only the control device must be designed to carry out the method steps according to the invention. For this purpose, a corresponding computer program runs in the control device, which can also be stored on a computer-readable medium. List of reference symbols 1 battery module 2 cell arrangement 3 tank device 4 Pump 5 Measuring device for detecting the terminal voltage 6 inverters 7 Control device

Claims

Patent claims 1. A method for removing V2O5 deposits in at least one vanadium battery module (1) integrated into a battery system, wherein the battery system comprises at least three battery modules (1) connected in series, an inverter (6), and a control unit (7), and wherein the battery modules (1) are connected to the inverter, and wherein each battery module (1) comprises a cell assembly (2), a tank assembly (3) for storing electrolyte, and two pumps (4) for conveying electrolyte through the cell assembly (2), and wherein the control unit (7) is configured to detect an operating state of the inverter (6) and to control the pumps (4) in the battery modules (1), characterized in that the method comprises the following steps in the specified order: - Identification of at least one battery module (1 ) with V2O5 deposits; - Switching off the pumps (4) of the at least one battery module (1) at a time ti ; - Switching on the pumps (4) of the at least one battery module (1) at time t2j, wherein the length of the time interval At= t2-ti is chosen such that at time t2 a terminal voltage of the at least one battery module (1) is negative, but an overcharging of the electrolyte located in the cell arrangement (2) of the at least one battery module (1) is avoided, and wherein these steps, with the exception of the first step, are carried out while the battery system is being discharged.

2. The method of claim 1, wherein the method comprises the following steps in the specified order: - Identification of at least one battery module (1 ) with V2O5 deposits; - Switching off the pumps (4) of the at least one battery module (1) at a time ti ; - Switching on the pumps (4) of the at least one battery module (1) at time t2 and operating the pumps (4) of the at least one battery module (1) with a first pump rate; - Operating the pumps (4) of the at least one battery module (1) at a time ts after t2 with a second pump rate; wherein the length of the time interval At= t2-ti is chosen such that at time t2 a terminal voltage of the at least one battery module (1) is negative, and wherein the first pump rate is chosen such that the terminal voltage of the at least one battery module (1) is negative during the time interval between t2 and ts, but an overcharging of the electrolyte in the cell arrangement (2) of the at least one degraded battery module (1) is avoided in the time interval between t2 and ts, and wherein the second pump rate is chosen such that the terminal voltage of the at least one degraded battery module (1) becomes positive after ts, and wherein these steps, with the exception of the first step, are carried out while the battery system is being discharged.

3. Battery system, wherein the battery system comprises at least three vanadium redox flow battery modules (1) connected in series, an inverter (6) and a control device (7), and wherein the battery modules (1) are connected to the inverter, and wherein each battery module (1) comprises a cell assembly (2), a tank assembly (3) for storing electrolyte and two pumps (4) for conveying electrolyte through the cell assembly (2), and wherein the control device (7) is configured to detect an operating state of the inverter (6), control the pumps (4) in the battery modules (1) and perform the steps of the method according to any of the preceding claims. Battery system according to claim 3, wherein the control device (7) can detect a terminal voltage of the battery modules (1). Computer program comprising instructions that cause the battery system according to claim 3 or 4 to perform the method steps according to claim 1 or 2. Computer-readable medium on which the computer program according to claim 5 is stored.

Images