Redox flow battery system and method for operating same

Abstract

The invention relates to a method for operating a redox flow battery system. An intervention is carried out for a battery module (1), said intervention having the following steps: - deactivating the at least one pump of the battery module (1) in question in order to terminate the supply of electrolyte into the cell assembly; - short-circuiting the battery module (1) in question if the terminal voltage of the battery module (1) in question has fallen below a defined value; - carrying out measures; - activating the at least one pump of the battery module (1) in question; and - opening the short-circuit of the battery module (1) in question.

Description

[0001] Redox flow battery system and method of operation

[0002] The invention relates to a redox flow battery system and a method for operating a redox flow battery system, wherein the redox flow battery system comprises a plurality of battery modules connected in series, and wherein the method relates to the reduction or elimination of imbalances occurring between series-connected battery modules during charging and discharging of the battery system, the maintenance of a battery module or the decoupling of one or more battery modules to optimize partial load operation.

[0003] Redox flow battery systems and methods for reducing or eliminating imbalances that occur between series-connected battery modules during charging and discharging of the battery system are known from the prior art. For example, DE 10 2020 108 053 ​​A1 discloses such a system and method. Measures to reduce or eliminate these imbalances are commonly referred to as "balancing."

[0004] The object of the invention is to provide a redox flow battery system which is simpler in construction than the systems known from the prior art, and to provide a method for operating such a system.

[0005] This object is achieved according to the invention by a redox flow battery system and by methods according to the independent claims. Further advantageous embodiments of the present invention can be found in the subclaims.

[0006] The invention is explained below with reference to the figures. The figures show in detail:

[0007] Fig.1 Battery module

[0008] Fig.2 Redox flow battery system according to the state of the art;

[0009] Fig.3 Charge / discharge cycles without balancing Fig.4 Charge / discharge cycles with balancing according to the state of the art

[0010] Fig.5 Redox flow battery system according to the invention

[0011] Fig.6 Charge / discharge cycles with balancing according to the invention Fig.7 Charge / discharge cycles with balancing according to the invention Fig.8 Charge / discharge cycles with balancing according to the invention Fig.9 Course of the terminal voltage during a balancing intervention according to the invention

[0012] Figure 1 shows a schematic representation of a 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, and a measuring device for recording a controlled variable. 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 connections of several stacks. The tank device 3 serves to store the electrolyte and to supply the cell arrangement 2 with electrolyte. For this purpose, the tank device 3 comprises, with a few exceptions, at least two tanks, a pipe system for connecting the tanks to the cell arrangement 2, and pumps for conveying the electrolyte.Figure 1 shows two separate pumps. The electrolyte could equally well be pumped with a double-head pump, i.e., with two pumps driven by a common motor. The tank device 3 is designed to supply all cells of the cell array 2 with electrolyte.

[0013] The battery module 1 shown in Figure 1 comprises two measuring devices for providing a measured value that represents a measure of the state of charge of the associated battery module (SoC). The measuring device, designated by 4, is a measuring device for providing the so-called open circuit voltage (OCV). The OCV value is a measure of the state of charge of the battery module (SoC). The measuring device, designated by 5, is a measuring device for providing the terminal voltage of the cell arrangement 2 and thus also of the battery module 1. When the battery module 1 is charging or discharging, the terminal voltage differs from the open circuit voltage by the voltage that drops across the internal resistance of the cell arrangement 2. With the charging or discharging current and the internal resistance of the cell arrangement 2 known, the terminal voltage represents a measure of the state of charge of the battery module.

[0014] On the right side of Figure 1, a symbolic representation of the battery module 1 is shown. This symbolic representation will be used below.

[0015] Figure 2 shows a schematic representation of a battery system according to the prior art. The battery system comprises at least two battery modules, one of which is designated 1, a bidirectional power conversion system (PCS), designated 7, and a control device, designated 8. The battery modules 1 are connected in series and to the converter 7. Figure 2 shows four battery modules, with the dashed lines in the series connection indicating any number of additional modules. The converter 7 connects the battery system to the grid or to a higher-level electrical system. The battery system further comprises, for each battery module 1, a first switch, one of which is designated 8, and a second switch, one of which is designated 9.The first switch 8 is arranged in series with the battery modules 1, with one switch being arranged in front of or behind each battery module 1. This means that with each of the first switches 8, the series connection of the battery modules can be interrupted. The second switches 9 are each arranged in a bypass line around a battery module 1 and the associated first switch s. In Figure 2, all switches 8 and 9 are shown in the open state. In reality, the switches are controlled by the control device 7 such that from each switch pair of a first and second switch, exactly one switch is closed and one switch is open (alternately open and closed). This meansA switch pair has exactly two switching positions, whereby in the first switching position (first switch 8 closed and second switch 9 open) the associated battery module 1 is in the series connection of the battery system, and in the second switch position (first switch 8 open and second switch 9 closed) the associated battery module 1 is separated from the series connection of the battery system by the bypass line. Opening the first switch 8 with the second switch 9 closed prevents the module from discharging via the bypass line. Since the bypass line represents a short circuit, such a discharge of the battery module via the bypass line would also lead to very high currents that could damage or even destroy the battery module.

[0016] In a battery system according to Figure 2 with completely identical battery modules 1, no harmful imbalance could arise. However, real battery modules 1 differ due to manufacturing variations and aging processes. In addition, different operating conditions, e.g. temperature differences between the individual modules, can cause them to behave differently. For these reasons, real battery modules have different efficiency values ​​and different internal resistances. For a given charging or discharging current, higher efficiency leads to the battery module in question reaching its final state more quickly. Since the same current flows through all of the battery modules 1 in the series circuit according to Figure 2, the high-efficiency modules reach the final state more quickly than the low-efficiency modules. To avoid damage, the charging or discharging current must beThe discharge process must be aborted as soon as a module reaches its respective final state. If this effect is not compensated for, the usable storage capacity of such a battery system decreases with each cycle (“capacity fading”). The different internal resistance of the modules has a similar effect. There are upper and lower limits for the terminal voltage, which must not be exceeded or undercut. Even with identical efficiency, a module with a higher internal resistance will reach the respective limit of the terminal voltage more quickly during charging or discharging than a module with a lower internal resistance. If the first module reaches this limit, the respective process must be aborted, which also leads to a reduction in the usable capacity of the battery system. Alternatively, the system's output could also be reduced.In any case, these effects lead to a degradation of the system. Balancing is intended to reduce or completely eliminate these effects in order to maintain the usable capacity of the battery system at a consistently high level or to eliminate the degradation described. On the other hand, successful balancing enables the use of cells with a comparatively high variation in efficiency and / or internal resistance, which naturally translates into reduced production costs.

[0017] Figure 3 shows two charge / discharge cycles of two battery modules connected in series with different efficiency values. For clarity, the efficiency difference is chosen to be very high. In real battery systems, the efficiency differences are much smaller. In Figure 3, the SoC curve of the battery module with the higher efficiency is shown as a solid line, and the SoC curve of the battery module with the lower efficiency is shown as a dashed line. In Figure 3, the minimum state of charge is marked as 0% and the maximum state of charge as 100%.

[0018] The more efficient battery module reaches the 100% SoC value when the less efficient battery module is not yet fully charged. Since both battery modules are subjected to the same current, the charging process must now be stopped. If you then want to discharge the battery system, the process begins when the less efficient battery module is not yet fully charged. Due to this unequal starting point for discharging and the lower efficiency, the less efficient battery module reaches the 0% SoC value when the more efficient battery module is not yet fully discharged. Since the described effects act cumulatively, the SoC curves of the two battery modules diverge further and further with the increasing number of cycles, and the usable capacity of the battery system continues to decrease.

[0019] The described negative effect can be avoided by taking balancing measures. Figure 4 shows two diagrams, each with a charging cycle of two battery modules connected in series with different efficiencies. In each case, a balancing intervention is carried out in one half-cycle. The SoC curves shown can be generated by decoupling one or more battery modules from the series connection for a certain time, so that they no longer participate in the charging or discharging of the other battery modules during this time. Ideally, the SoC curve for the decoupled modules is horizontal (self-discharge can be neglected). For this purpose, the first and second switches 8 and 9 are used in a battery system according to Figure 2.

[0020] During the charging half-cycle, the more efficient battery module is decoupled, and during the discharging half-cycle, the less efficient battery module is decoupled. The difference between the two diagrams lies in the different decouple times.

[0021] In the left diagram of Figure 4, the battery module in question is only decoupled until the two curves are equal. For example, during charging, the efficient battery module is decoupled until the less efficient battery module reaches the same state of charge as the more efficient battery module.

[0022] In the right-hand diagram of Figure 4, the battery module in question is decoupled for a longer period, so that the two curves of the battery modules cross. For example, in the charging half-cycle, the more efficient battery module is decoupled until the less efficient battery module has gained a sufficiently high "head start" in charging, so that the more efficient battery module catches up with the less efficient battery module just at 100% SoC. Such a balancing intervention could be described as a type of temporary overcompensation, since equalization only occurs some time after the intervention. The inventors have recognized that a generic battery system can be constructed more simply by omitting the switches 8 for interrupting the series connection, whereby the balancing interventions are implemented differently than known from the prior art. This is explained in more detail below.The inventors have further recognized that a battery system constructed according to the invention enables the decoupling of individual or multiple battery modules for maintenance purposes or to optimize partial load operation. This will be explained in more detail following the description of the balancing interventions.

[0023] Figure 5 shows a battery system according to the invention, which differs from the battery system shown in Figure 2 in that the switches 8 for interrupting the series switches are missing, and in that the control device 7 is designed such that it can carry out the balancing interventions, maintenance interventions, or optimization interventions described below. A battery system suitable for carrying out the method according to the invention therefore comprises means for short-circuiting each battery module. The simplest embodiment of such means is shown in Figure 5 and comprises a short-circuit line and a switch 9 arranged in the short-circuit line. The switch 9 can be a relay or a semiconductor arrangement for switching. More complex short-circuiting means are also conceivable.For example, a short-circuit bar could be provided parallel to the battery string, to which individual battery modules can be connected using two switches.

[0024] The balancing interventions according to the invention are characterized by the fact that they are only triggered during the discharging of the battery system. This means that in a cyclically operated battery system, either all balancing interventions take place only during the discharging half-cycles or, if the application of the battery system permits, the battery system is briefly switched to discharging during the charging half-cycle if a balancing intervention becomes necessary during the charging half-cycle. In the second case, the battery system is switched back to charging after or during the balancing intervention.

[0025] Figure 6 shows charge / discharge cycles with balancing interventions according to the invention. In the discharge half-cycle, the SoC curves do not differ from the SoC curves shown in Figure 4. In the left-hand diagram, the interventions continue until the two SoC curves have equalized. In the charge half-cycle, the system switches to discharge during the intervention, whereby only the more effective battery module actually experiences a significant discharge. In the right-hand diagram, a balancing intervention only occurs in the discharge half-cycle. The intervention is of the "overcompensation" type. Figure 7 shows another charge / discharge cycle with balancing interventions according to the invention. There is one intervention in each half-cycle. Both interventions are of the "overcompensation" type. The "overcompensation" type balancing intervention in the charge half-cycle significantly extends the charge / discharge cycle.

[0026] Figure 8 shows another charge-discharge cycle with balancing interventions according to the invention. In contrast to Figure 7, in the charging half-cycle, it is not the less efficient but the more efficient battery module that is short-circuited. As in Figure 7, the battery system is switched to discharging during the intervention, but only for a short time. Afterward, it is switched back to charging. The period for which the system switches to discharging can be very short. It only needs to be long enough to perform the first two steps of the balancing intervention (see next section). Both interventions are of the "overcompensation" type. The embodiment according to Figure 8 is characterized by a shorter cycle time compared to the embodiment according to Figure 7.

[0027] The following describes the process of a balancing intervention according to the invention in more detail. It is assumed that the battery system is in discharge mode and that a balancing intervention is to be performed on at least one battery module. The balancing intervention comprises the following steps: - Switching off the pumps of the respective battery module

[0028] - Short-circuiting the battery module in question if the terminal voltage of the battery module in question has fallen below a predefined value

[0029] - Wait until the final balancing state has been reached

[0030] - Switching on the pumps of the relevant battery module

[0031] - Opening the short circuit of the battery module in question

[0032] During the "Wait until the final balancing state is reached" step, the battery system is either discharged as shown in Figures 6 and 7, with the discharge current flowing through all battery modules except for the battery module undergoing the balancing intervention, or, as shown in Figure 8 in the charging half-cycle, the battery system is switched to charging after a short discharge phase. In this case, too, during the "Wait until the final balancing state is reached" step, the discharge or charge current flows through all battery modules except for the battery module undergoing the balancing intervention.

[0033] The final balancing state can be either an adjustment or an overcompensation.

[0034] Since the internal resistance of the battery modules is much greater than the resistance of the corresponding short-circuit lines, the discharge current flows (almost entirely) past the short-circuited battery module through the closed short-circuit line, which, over time, leads to the desired final state of the balancing intervention. Balancing intervention according to the invention can also be performed on several battery modules simultaneously.

[0035] The processes in the battery module on which a balancing intervention according to the invention is carried out are explained in more detail with reference to Figure 9. Figure 9 shows the curve of the terminal voltage VK of the battery module in question as a function of time t. Before the balancing intervention, the battery module in question has the terminal voltage Vo. Vo depends, among other things, on the state of charge of the battery module in question. At time t0, the pumps of the battery module in question are switched off so that no more electrolyte is supplied to the cell arrangement. As a result, the electrolyte in the cell arrangement discharges very quickly because no “fresh” electrolyte is supplied. This leads to a very rapid reduction in the terminal voltage. If the terminal voltage has fallen below the predefined threshold voltage Vs, the battery module in question is short-circuited. In the illustration in Figure 9, this happens at time t1.This causes the terminal voltage to collapse completely. This condition is maintained until the desired final state of the balancing intervention has been reached. During this entire time, no significant amount of "fresh" electrolyte may be supplied to the cell arrangement of the battery module in question. This can be ensured, for example, by locking the pumps or closing a valve located in the supply line. These measures are then included in the step "Switching off the pumps of the battery module in question."

[0036] During a very short time interval immediately after short-circuiting, the entire remaining energy content of the cell arrangement of the battery module in question is converted into heat. To prevent damage to the cell arrangement, the energy content must be sufficiently small. This is ensured by choosing a sufficiently small threshold voltage Vs, since the energy content of the cell arrangement scales with the terminal voltage. The threshold voltage Vs can therefore be determined based on energy considerations, whereby at least the following variables must be taken into account: electrolyte volume in the cell arrangement, structure of the cell arrangement (including number of cells, electrode shape, electrode material, thermal coupling of the electrodes to the environment), discharge current and state of charge.Due to the complexity of the possible influencing factors, an experimental test of the effectiveness of the specified threshold voltage Vs is recommended. The criterion “when the terminal voltage of the battery module in question has fallen below a predefined value” can also be implemented by allowing a sufficiently long time to elapse between switching off the pumps and short-circuiting, i.e. by selecting a sufficiently long time interval Δt = ti-Δt. The value Δt to be selected depends, among other things, on the state of charge of the battery module in question at time Δt and the discharge current flowing through the battery modules in the time interval between Δt and Δt. In the battery systems investigated by the inventors, Δt was in the range of a few seconds. This means that the method according to the invention can also be carried out without measuring and recording the terminal voltage of the battery module in question.

[0037] The described balancing interventions according to the invention can advantageously be combined with other known types of balancing interventions, e.g., with balancing interventions in which a load is connected in parallel with the battery module in question. In this case, the balancing interventions during discharging could be implemented using the interventions according to the invention, and the balancing interventions during charging could be implemented by the aforementioned parallel connection of a load. This can prevent switching to discharge mode during the charging half-cycle. Auxiliary systems of the battery modules, such as pumps, can be considered as loads. However, this can also be an electrical resistor, so that the energy dissipated by the balancing is converted into heat.

[0038] A battery system according to the invention is also suitable for carrying out maintenance interventions on at least one battery module. In this case, the step of "waiting until a final balancing state has been reached" is simply replaced by the step of "carrying out maintenance measures on the battery module in question." The maintenance measures can be any measures that repair the battery module in question after a fault or prevent future faults. This can also involve the complete replacement of the battery module in question. The new battery module then takes the place of the battery module in question (and is referred to as such in the subsequent method steps). Furthermore, the battery system according to the invention is suitable for decoupling one or more battery modules from the series connection in order to make operation more efficient during partial load operation.Decoupling reduces the internal resistance of the battery system, thus reducing losses. The process simply replaces the step of "waiting until a final balancing state is reached" with the step of "operating the battery system at partial load."

[0039] To encompass all these possibilities, the step "waiting until a balancing end state has been reached" is referred to in the independent claim as "performing measures." In the dependent claims, the measures are then specified as "waiting until a balancing end state has been reached," or as "performing maintenance measures on the battery module in question," or as "operating the battery system in partial load mode."

[0040] It should be noted that the two subsequent steps, "switching on the pumps of the respective battery module" and "opening the short circuit of the respective battery module," can be initiated at the same time or performed consecutively. The order of these steps is irrelevant as long as the time between the two steps is not too long.

[0041] Through these two steps, the battery module previously decoupled in the first two steps is recoupled into the series circuit. If the intervention was a balancing intervention, this naturally ensures that the recoupled battery module has a suitable SoC. With the other two described intervention types, care must be taken to ensure that the coupling only occurs when the SoC of the battery module to be coupled approximately corresponds to the SoC of the other battery modules. This means that the SoC of the battery module to be coupled should not deviate by more than 10% – preferably no more than 5% – from the SoC of the other battery modules during coupling. Further adjustment of the SoC can be achieved through a subsequent balancing intervention. Another embodiment of the method according to the invention comprises the following steps in the specified order:

[0042] - Switching off the pumps of the relevant battery module

[0043] - Short-circuiting the battery module in question if the terminal voltage of the battery module in question has fallen below a predefined value

[0044] - Implementation of measures

[0045] - Switching on the pumps of the relevant battery module

[0046] - Measuring the current flowing through the cell arrangement of the battery module in question

[0047] - Opening the short circuit of the battery module in question as soon as the measured current has exceeded a predefined threshold

[0048] The advantage of this design is that a jump in the voltage of the battery system when opening the short circuit can be minimized.

[0049] It should be noted that the method according to the invention can also be applied to battery systems in which the battery modules comprise cell arrangements that include several parallel sub-cell arrangements. US Pat. No. 10,263,270 B2, for example, discloses such a battery system. The individual sub-cell arrangements can also be short-circuited individually, whereby the short-circuiting can occur simultaneously or sequentially. Appropriate short-circuiting means must then be provided.

[0050] In order for the method according to the invention to be carried out, the control device 7 must be designed accordingly. The control device controls at least the pumps and the means 9 for short-circuiting the individual battery modules and, if necessary, further provided means for terminating the supply of electrolyte to the cell arrangement 2. If necessary, the control device 7 also records the terminal voltages of the battery modules, i.e., it is connected to the measuring devices 5 for determining the terminal voltage. Furthermore, a computer program installed in the control device is provided for executing the steps of the method according to the invention. The computer program according to the invention can be stored on a data carrier.

[0051] List of reference symbols

[0052] 1 battery module

[0053] 2 cell arrangement 3 tank device

[0054] 4 Measuring device for determining the OCV

[0055] 5 Measuring device for determining the terminal voltage

[0056] 6 Bidirectional converter (PCS)

[0057] 7 Control device 8 Switch for interrupting the series circuit

[0058] 9 Means for short-circuiting a battery module

Claims

Patent claims 1. A method for operating a redox flow battery system comprising at least two battery modules (1), a bidirectional inverter (6), and a control device (7), wherein the battery modules (1) are connected in series and to the inverter (6), and wherein means (9) for short-circuiting the respective battery module (1) are provided for each battery module (1), and wherein each battery module (1) comprises a cell arrangement (2) with a plurality of redox flow cells and a tank device (3) for storing electrolyte and supplying the cell arrangement (2) with electrolyte and at least one pump for conveying the electrolyte, characterized in that an intervention is carried out on at least one battery module (1) which comprises the following steps: - Switching off at least one pump of the relevant battery module (1) to stop the supply of electrolyte to the cell arrangement (2); - Short-circuiting the relevant battery module (1) if the terminal voltage of the relevant battery module (1) has fallen below a predefined value; - Implementing measures; - Switching on at least one pump of the relevant battery module (1 ); - Opening the short circuit of the relevant battery module (1 ).

2. The method of claim 1, wherein the intervention comprises the following steps in the specified order: - Switching off at least one pump of the relevant battery module (1) to stop the supply of electrolyte to the cell arrangement (2); - Short-circuiting the relevant battery module (1) if the terminal voltage of the relevant battery module (1) has fallen below a predefined value; - Implementing measures; - Switching on at least one pump of the relevant battery module (1); - Measuring the current flowing through the cell arrangement (2) of the battery module (1) in question; - Opening the short circuit of the relevant battery module (1) as soon as the measured current exceeds a predefined threshold. Method according to claim 1 or 2, wherein the measures consist of waiting until a balancing end state has been reached. Method according to claim 1 or 2, wherein the measures consist of performing maintenance on the relevant battery module (1). Method according to claim 1 or 2, wherein the measures consist of operating the redox flow battery system in partial load mode. Method according to any one of claims 1 to 5, wherein the battery system is in a discharge mode before the intervention. Method according to any one of claims 1 to 5, wherein the battery system is in a charge mode before the intervention and the intervention is performed as a first step: - Switching the battery system to a discharge mode; includes: a redox flow battery system designed to perform the method according to any one of claims 1 to 7; and a computer program for executing the steps of the method according to any one of claims 1 to 7.

10. Data carrier on which a computer program according to claim 9 is stored.

Images