Redox flow battery system and operation method thereof

Abstract

The invention provides a method of operating a redox flow battery system in which an intervention is performed on a battery module (1), said intervention comprising the steps of:-shutting down at least one pump of an associated battery module (1) in order to stop the supply of electrolyte to said cell device (2); -short-circuiting the associated battery module (1) when the terminal voltage of the associated battery module (1) falls below a predefined value; -performing the action; opening at least one pump of the associated battery module (1); and-disconnecting the short circuit of the associated battery module (1).

Description

Technical Field

[0001] The present invention relates to a redox flow battery system and a method of operating a redox flow battery system, wherein the redox flow battery system includes a plurality of battery modules connected in series, and wherein the method involves reducing or eliminating imbalances occurring between the battery modules connected in series during charging and discharging of the battery system, maintaining the battery modules or disconnecting one or more battery modules to optimize a partial load mode. Background Art

[0002] Redox flow battery systems and methods for reducing or eliminating imbalances that occur between battery modules connected in series during charging and discharging of the battery system are known in the prior art. For example, DE 10 2020 108053 A1 discloses such a system and such a method. Measures for reducing or eliminating such imbalances are generally referred to as "balancing". Summary of the invention

[0003] The object of the present invention is to provide a redox flow battery system which is simpler in structure than the systems known in the prior art, and to provide a method of operating such a system. BRIEF DESCRIPTION OF THE DRAWINGS

[0004] According to the invention, this object is achieved by a redox flow battery system and a method according to the independent claims. Further advantageous embodiments of the invention can be found in the dependent claims.

[0005] In the following, the present invention is explained with reference to the accompanying drawings. The accompanying drawings show in detail:

[0006] Figure 1 Battery modules;

[0007] Figure 2 A redox flow battery system according to the prior art;

[0008] Figure 3 Unbalanced charge / discharge cycles;

[0009] Figure 4 Perform balanced charge / discharge cycles according to prior art;

[0010] Figure 5 A redox flow battery system according to the present invention;

[0011] Figure 6 Performing balanced charge / discharge cycles according to the present invention;

[0012] Figure 7 Performing balanced charge / discharge cycles according to the present invention;

[0013] Figure 8 Performing balanced charge / discharge cycles according to the present invention;

[0014] Fig. 9 Terminal voltage curve during the equalization intervention according to the invention. DETAILED DESCRIPTION

[0015] Figure 1 A schematic representation of a battery module is shown on the left. The battery module is designated 1. The battery module comprises a cell device designated 2, a tank device designated 3, and a measuring device for detecting controlled variables. The cell device 2 is a device having a large number of redox liquid flow cells, which can be arranged as required. For example, it can 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 electrolyte and supply electrolyte to the cell device 2. With a few exceptions, the tank device 3 comprises at least two tanks, a piping system for connecting the tanks to the cell device 2, and a pump for supplying electrolyte. Here, Figure 1 Two separate pumps are shown. The electrolyte can also be pumped with a double-head pump, ie two pumps driven by a common motor. The tank device 3 is designed such that it can supply all cells of the cell arrangement 2 with electrolyte.

[0016] Figure 1 The battery module 1 shown in comprises two measuring devices for providing a measured variable, which represents a measured value of the state of charge (SoC—state of charge) of the associated battery module. The measuring device designated 4 is a measuring device for providing the so-called open circuit voltage (OCV). The OCV value is a measured value of the state of charge (SoC) of the battery module. The measuring device designated 5 is a measuring device for providing the terminal voltage of the cell device 2 and therefore also of the battery module 1. When the battery module 1 is charged or discharged, the difference between the terminal voltage and the open circuit voltage lies in the voltage drop across the internal resistance of the cell device 2. If the charging or discharging current and the internal resistance of the cell device 2 are known, the terminal voltage is a measured value of the state of charge of the battery module.

[0017] exist Figure 1 The right side of is shown a symbolic representation of the battery module 1. This symbolic representation will be used hereinafter.

[0018] Figure 2 A schematic representation of a battery system according to the prior art is shown. The battery system comprises at least two battery modules (one of which is designated as 1), a bidirectional power conversion system (PCS) (designated as 7), and a control device (designated as 8). The battery modules 1 are connected in series and connected to the conversion system 7. Figure 2Four battery modules are shown, wherein the dashed lines of the series connection are intended to indicate an arbitrary number of additional modules. A conversion system 7 is used to connect the battery system to the mains or a higher-level electrical system. The battery system further comprises a first switch (one of which is designated 8) and a second switch (one of which is designated 9) for each battery module 1. The first switch 8 is arranged in series with the battery module 1, wherein a switch is arranged in front of or behind each battery module 1, respectively. This means that the series connection of the battery modules can be interrupted by each first switch 8. The second switches 9 are respectively arranged in a bypass around the corresponding battery module 1 and the associated first switch 8. In Figure 2 In the embodiment, all switches 8 and 9 are in the open state. In fact, the control device 7 drives these switches in such a way that exactly one switch is closed and one switch is open in each pair of switches of the first switch and the second switch (alternatingly open and closed). This means that a pair of switches has exactly two switch positions, wherein, in the first switch position (the first switch 8 is closed and the second switch 9 is open), the relevant battery module 1 is in the series connection of the battery system, and in the second switch position (the first switch 8 is open and the second switch 9 is closed), the relevant battery module 1 is separated from the series connection of the battery system through the bypass. Therefore, disconnecting the first switch 8 while the second switch 9 is closed prevents the module from discharging via the bypass. Since the bypass represents a short circuit, the discharge of the battery module via the bypass can also lead to very large currents, thereby damaging or even destroying the battery module.

[0019] In such Figure 2 In the battery system shown with identical battery modules 1, no harmful imbalances can occur. However, actual battery modules 1 differ from one another due to manufacturing differences and aging processes. In addition, different operating conditions, such as temperature differences between the individual modules, can cause them to behave differently. For these reasons, actual battery modules have different efficiency values ​​and different internal resistances. For a given charging or discharging current, the higher the efficiency, the faster the associated battery module reaches the final state. Because the same current will flow through Figure 2All battery modules 1 in the series connection shown, so the modules with higher efficiency will reach the final state faster than the modules with lower efficiency. In order to avoid damage, the charging or discharging process must be interrupted as soon as a module reaches the corresponding final state. In this way, without compensating for this effect, the available storage capacity of such a battery system will decrease with each cycle ("capacity fade"). The different internal resistances of the modules will also have similar effects. The terminal voltage has an upper limit that must not be exceeded and a lower limit that must not be lowered. Even with the same efficiency, the module with higher internal resistance will reach the corresponding limit of the terminal voltage faster during charging or discharging than the module with lower internal resistance. If the first module reaches this limit, the corresponding process must be terminated, which also leads to a reduction in the available capacity of the battery system. Alternatively, the power of the system may also be reduced. In either case, these effects will lead to damage to the system. Balancing aims to reduce or completely eliminate the described effects in order to permanently keep the available capacity of the battery system at a higher level or eliminate the described damage. On the other hand, successful balancing makes it possible to use cells with relatively large differences in efficiency and / or internal resistance, which of course will also be reflected in reduced production costs.

[0020] Figure 3 Two charge / discharge cycles of two battery modules connected in series with different efficiency values ​​are shown. Here, the efficiency difference chosen is very high for the purpose of illustrating this. In a real battery system, the efficiency difference is much smaller. Figure 3 In FIG. 1 , the SoC curve of the battery module with higher efficiency is shown as a solid line, and the SoC curve of the battery module with lower efficiency is shown as a dotted line. Figure 3 In the diagram, the lowest state of charge is marked with 0% and the highest state of charge is marked with 100%.

[0021] The more efficient battery module reaches a SoC value of 100% when the less efficient battery module is not yet fully charged. Since the current flowing through both battery modules is the same, the charging process must now be terminated. If the battery system is now to be discharged, the process begins when the less efficient battery module is not yet fully charged. Due to this unequal starting point of discharge and the lower efficiency, the less efficient battery module reaches a SoC value of 0% when the more efficient battery module is not yet fully discharged. Because the described effects are cumulative, the SoC curves of the two battery modules will deviate further and further as the number of cycles increases and the usable capacity of the battery system continues to decrease.

[0022] The negative effects described can be avoided by taking balancing measures.

[0023] Figure 4Two diagrams are shown, each of which is a charging cycle of two battery modules connected in series with different efficiencies. In this case, a balancing intervention is performed in each case within a half cycle. The SoC curve shown can be generated by disconnecting one or more battery modules from the series connection for a certain period of time, so that the battery module no longer participates in the charging or discharging of other battery modules during this period. Ideally, the SoC curve of the module is horizontal when the coupling is disconnected (self-discharge can be ignored). For this purpose, as shown in FIG. Figure 2 As shown, a first switch 8 and a second switch 9 are used in the battery system.

[0024] The more efficient battery modules are disconnected during the charging half cycle, while the less efficient battery modules are disconnected during the discharging half cycle. The difference between the two diagrams is the timing of the disconnection.

[0025] exist Figure 4 In the left diagram of , the relevant battery modules are only disconnected when the two curves are equal. For example, during charging, the high-efficiency battery module is disconnected until the low-efficiency battery module reaches the same state of charge as the high-efficiency battery module currently exhibits.

[0026] exist Figure 4 In the right diagram of , the relevant battery module is disconnected for a longer time, so that the two curves of the battery modules intersect. For example, during the charging half-cycle, the battery module with higher efficiency is disconnected until the battery module with lower efficiency obtains a sufficiently high "advance" during charging, so that the battery module with higher efficiency only catches up with the battery module with lower efficiency at the SoC value of 100%. This equalization intervention can be described as a temporary overcompensation, because the equalization only occurs for a period of time after the intervention.

[0027] The inventors have recognized that by omitting the switch 8 for interrupting the series connection, a battery system of this type can be constructed more simply, wherein the balancing intervention is performed differently from the prior art. This will be explained in more detail below. The inventors have also recognized that a battery system constructed according to the invention is capable of disconnecting single or multiple battery modules in order to perform maintenance or optimize a partial load mode. This will be explained in more detail after the description of the balancing intervention.

[0028] Figure 5 The battery system according to the present invention is shown. Figure 2 The battery system shown differs in that the switch 8 for interrupting the series switching is missing and the control device 7 is designed so that it can perform the balancing intervention or maintenance intervention or optimization intervention described below. Therefore, a battery system suitable for performing the method according to the invention comprises a device for short-circuiting each battery module. The simplest embodiment of such a device is, for example, Figure 5 As shown, the short-circuit circuit includes a short-circuit line and a switch 9 arranged on the short-circuit line. The switch 9 can be a relay or a semiconductor device for switching. More complex short-circuit devices can also be envisioned. For example, a short-circuit rail can be connected in parallel with the battery string, and each battery module can be connected to the short-circuit rail by means of two switches.

[0029] The balancing interventions according to the invention are characterized in that they are triggered only during the discharge of the battery system. Thus, in a cyclically operated battery system, either all balancing interventions occur only in the discharge half cycle, or, if the application of the battery system permits, the battery system briefly switches to discharge during the charging half cycle if a balancing intervention is required during the charging half cycle. In the second case, the battery system switches back to the charging state after or during the balancing intervention.

[0030] Figure 6 Figure 2 shows a charge / discharge cycle with balanced intervention according to the present invention. In the discharge half cycle, the SoC curve is Figure 4 There is no difference in the SoC curves shown. In the left diagram, the intervention lasts until both SoC curves are equal to each other. In the charging half-cycle, there is a switch to discharge during the intervention, where only the more efficient battery modules actually experience a significant discharge. In the right diagram, the balancing intervention only occurs in the discharge half-cycle. This intervention is of the "overcompensation" type. Figure 7 Another charge-discharge cycle with equalizing interventions according to the invention is shown. There is one intervention per half cycle. Both interventions are of the "overcompensation" type. Due to the equalizing interventions of the "overcompensation" type in the charging half cycle, the charge / discharge cycle is significantly extended.

[0031] Figure 8 Another charge-discharge cycle with equalization intervention according to the invention is shown. Figure 7 On the contrary, it is not the less efficient battery module that is short-circuited during the charging half-cycle, but the more efficient battery module. Figure 7 As shown, the battery system switches to discharge during the intervention, but only for a short time. It then switches back to charging. The time period of switching to discharge can be very short. It only needs to be enough to perform the first two steps of the equalization intervention (see the next section). Both interventions are of the "overcompensation" type. Figure 8 The embodiment shown is characterized in that Figure 7 The cycle time is shorter than that of the embodiment shown.

[0032] The sequence of the balancing intervention according to the present invention will be described in more detail below. Here, it is assumed that the battery system is in a discharge mode and a balancing intervention is to be performed on at least one battery module. The balancing intervention comprises the following steps:

[0033] - Turn off the pump of the relevant battery module

[0034] -When the terminal voltage of the relevant battery module drops below a predefined value, short-circuit the relevant battery module

[0035] - Wait until the final equilibrium state is reached

[0036] - Turn on the pump of the relevant battery module

[0037] -Disconnect the short circuit of the relevant battery module

[0038] In step "waiting until the final equilibrium state is reached", the battery system either Figure 6 and Figure 7 The discharge is performed as shown, wherein the discharge current flows through all battery modules except the battery module that performs the balancing intervention, or as shown Figure 8 In the charging half cycle, the battery system switches to charging after a short discharge phase. In this case, during the step "waiting until the final balancing state is reached", the discharge or charging current also flows through all battery modules except the battery module that performs the balancing intervention.

[0039] The final equilibrium state can be either equilibrium or overcompensation.

[0040] Since the internal resistance of a battery module is much greater than the resistance of the associated short-circuit line, the discharge current flows (almost entirely) through the short-circuited battery module via the closed short-circuit line, thereby reaching the desired end state of the balancing intervention over time. The balancing intervention according to the invention can also be performed simultaneously on multiple battery modules.

[0041] refer to Fig. 9 The process in a battery module with a balancing intervention according to the invention is explained in more detail. Fig. 9 shows the terminal voltage V of the relevant battery module K Changes over time t. Before the equalization intervention, the terminal voltage of the relevant battery module is V0. V0 depends on factors such as the state of charge of the relevant battery module. At time t0, the pump of the relevant battery module is turned off so that electrolyte is no longer supplied to the single unit. As a result, the electrolyte in the single unit will be discharged quickly because there is no "fresh" electrolyte supplied. This will cause the terminal voltage to drop rapidly. If the terminal voltage drops to a predefined threshold voltage V s Below, the relevant battery module is short-circuited. Fig. 9In the illustration of , this occurs at time t1. This leads to a complete collapse of the terminal voltage. This state is maintained until the desired final state for the equalization intervention is reached. During this entire time, no large amounts of "fresh" electrolyte are supplied to the individual units of the relevant battery module. This can be ensured, for example, by locking the pump or closing a valve arranged in the supply line. The above-mentioned measures can be understood as being included in the step "turning off the pump of the relevant battery module".

[0042] In the short time interval immediately after the short circuit, all the remaining energy in the cells of the relevant battery module is converted into heat in the cells. The energy content must be small enough to prevent damage to the cells. This is achieved by choosing a sufficiently low threshold voltage V s This is ensured by the fact that the energy content of a single device is proportional to the terminal voltage. Therefore, the threshold voltage V can be determined based on energy considerations. s , where at least the following variables must be considered: the volume of electrolyte in the monomer device, the structure of the monomer device (including the number of monomers, electrode shape, electrode material, thermal coupling between the electrode and the environment), discharge current and state of charge. Due to the complexity of possible influencing factors, it is recommended to specify the threshold voltage V s The effectiveness is verified experimentally.

[0043] The criterion "when the terminal voltage of the relevant battery module drops below a predefined value" can also be implemented by leaving a sufficiently long time between shutting down the pump and the short circuit, i.e., by selecting a sufficiently long time interval Δt=t1-t0. Here, the Δt to be selected depends in particular on the state of charge of the relevant battery module at time t0 and the discharge current flowing through the battery module in the time interval between t0 and t1. In the battery system studied by the inventors, Δt is in the range of several seconds. This means that the method according to the invention can be implemented without measuring and detecting the terminal voltage of the relevant battery module.

[0044] The balancing intervention described according to the invention can advantageously be combined with other known types of balancing intervention, for example, with a balancing intervention in which a load is connected in parallel with the relevant battery module. In this case, the balancing intervention during discharge can be achieved by the intervention according to the invention, and the balancing intervention during charging can be achieved by the aforementioned parallel connection of the load. In this way, switching to discharge mode during the charging half-cycle can be avoided. Auxiliary systems of the battery modules, such as pumps, can be regarded as loads. However, the load can also be a resistor in order to convert the energy dissipated by the balancing into heat.

[0045] The battery system according to the invention is also suitable for carrying out maintenance work on at least one battery module. In this case, only the step "waiting until the final equilibrium state is reached" is replaced by the step "performing maintenance measures on the relevant battery module". The maintenance measures may be all possible measures to repair the relevant battery module after a fault or to prevent future faults. This may also involve completely replacing the relevant battery module. The new battery module will then replace the relevant battery module (which will also be handled in the subsequent process steps).

[0046] In addition, the battery system according to the present invention is suitable for disconnecting one or more battery modules from the series connection to make the operation in partial load mode more efficient. The disconnection reduces the internal resistance of the battery system, thereby reducing losses. This is just to replace the step "waiting until the final equilibrium state is reached" with the step "operating the battery system in partial load mode".

[0047] In order to cover all these possibilities, the step "waiting until a final equilibrium state is reached" is referred to as "performing measures" in the independent claim. In the dependent claims, these measures are specified as "waiting until a final equilibrium state is reached" or "performing maintenance measures on the relevant battery module" or "operating the battery system in partial load mode".

[0048] It should be noted that the two subsequent steps of "starting the pump of the relevant battery module" and "breaking the short circuit of the relevant battery module" can be started at the same time or performed one after another. The order of the above steps is not important as long as the time between the two steps is not too long.

[0049] These two steps recouple the battery modules that were previously disconnected by the first two steps into the series connection. If the intervention is an equalization intervention, this of course ensures that the recoupled battery modules have a suitable SoC. For the other two types of intervention described, care must be taken to ensure that the coupling is only carried out if the SoC of the battery module to be coupled corresponds approximately to the SoC of the other battery modules. This means that when coupling, the deviation between the SoC of the battery module to be coupled and the SoC of the other battery modules should not exceed 10%, preferably not more than 5%. The SoC can be further adjusted by subsequent equalization interventions.

[0050] A further embodiment of the method according to the invention comprises the following steps in the order shown:

[0051] - Turn off the pump of the relevant battery module

[0052] -When the terminal voltage of the relevant battery module drops below a predefined value, short-circuit the relevant battery module

[0053] - Enforcement measures

[0054] - Turn on the pump of the relevant battery module

[0055] -Measure the current flowing through the single cell device of the relevant battery module

[0056] - Once the measured current exceeds a predefined threshold, the short circuit of the relevant battery module is disconnected

[0057] The advantage of this embodiment is that when the short circuit is disconnected, the voltage jump of the battery system can be reduced to a minimum.

[0058] It is worth mentioning that the method according to the present invention can also be used in a battery system in which the battery module includes a single-cell device, which includes a plurality of sub-cell devices connected in parallel. For example, US 10 263 270 B2 discloses such a battery system. Here, each sub-cell device can also be short-circuited individually, wherein the short-circuiting can occur simultaneously or successively. A suitable short-circuiting device must be provided.

[0059] In order to carry out the method according to the invention, the control device 7 must be designed accordingly. Here, the control device controls at least the pump and the device 9 for short-circuiting the individual battery modules and, if necessary, also controls other devices provided to stop the supply of electrolyte to the cell device 2. If necessary, the control device 7 also detects the terminal voltage of the battery module, i.e. it is connected to the measuring device 5 to determine the terminal voltage. In addition, a computer program installed in the control device is provided for carrying out the steps of the method according to the invention. The computer program according to the invention can be stored on a data carrier.

[0060] List of Reference Numerals

[0061] 1 Battery Module

[0062] 2 Monomer device

[0063] 3 Storage tank equipment

[0064] 4 Measurement equipment for determining OCV

[0065] 5 Measuring equipment for determining terminal voltage

[0066] 6Bidirectional Conversion System (PCS)

[0067] 7. Control Equipment

[0068] 8Switch for interrupting series connection

[0069] 9. Device for short-circuiting a battery module

Claims

1. A method of operating a redox flow battery system, the system comprising at least two battery modules (1), a bidirectional conversion system (6) and a control device (7), wherein: The battery modules (1) are connected in series and to the conversion system (6), and wherein, for each battery module (1), a device (9) is provided for short-circuiting the associated battery module (1), and wherein each battery module (1) comprises a cell device (2) having a plurality of redox flow cells and a tank device (3) for storing an electrolyte and supplying the electrolyte to the cell device (2) and at least one pump for conveying the electrolyte, in, An intervention is performed on at least one battery module (1), the intervention comprising the following steps: - turning off at least one pump of the corresponding battery module (1) to stop supplying electrolyte to the single cell device (2); - when the terminal voltage of the corresponding battery module (1) drops below a predefined value, short-circuiting the corresponding battery module (1); - Enforcement measures; - turning on at least one pump of the corresponding battery module (1); and - Disconnect the short circuit of the corresponding battery module (1).

2. The method according to claim 1, wherein: The intervention includes the following steps in the order shown: - turning off at least one pump of the corresponding battery module (1) to stop supplying electrolyte to the cell assembly (2); - when the terminal voltage of the corresponding battery module (1) drops below a predefined value, short-circuiting the corresponding battery module (1); - Enforcement measures; - switching on at least one pump of the corresponding battery module (1); - measuring the current flowing through the single cell device (2) of the corresponding battery module (1); and - After the measured current exceeds a predefined threshold, the short circuit of the corresponding battery module (1) is opened.

3. The method according to claim 1 or 2, wherein: The measures include waiting until a final equilibrium state is reached.

4. The method according to claim 1 or 2, wherein: The measures involve carrying out maintenance measures on the respective battery module (1).

5. The method according to claim 1 or 2, wherein: The measures involve operating the redox flow battery system in a partial load mode.

6. The method according to any one of claims 1 to 5, wherein: The battery system is in a discharge mode prior to the intervention.

7. The method according to any one of claims 1 to 5, wherein: The battery system is in charging mode before the intervention, and the intervention comprises the following steps as a first step: - Switching the battery system to discharge mode.

8. A redox flow battery system configured to perform the method according to any one of claims 1 to 7.

9. A computer program for executing the steps of the method according to any one of claims 1 to 7.

10. A data carrier having stored thereon a computer program according to claim 9.

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