Method for operating a redox flow battery

Abstract

In order to efficiently operate a redox flow battery, the positive electrolyte pump (9a) and / or the negative electrolyte pump (9b) is operated in a pulsed manner during a pulsed operating time (tP), as a result of which the flow rate (M+) of the positive electrolyte (15a) and / or the flow rate (M-) of the negative electrolyte (15b) is varied between a specified maximum flow rate (Mmax) and a specified minimum flow rate (Mmin) during a pulse cycle (PZ), and a plurality of pulse cycles (PZ) follow one another in the pulsed operating time (tP) during the operation of the redox flow battery (1).

Description

[0001] Method for operating a redox flow battery

[0002] The present invention relates to a method for operating a redox flow battery, and to a redox flow battery, having at least one cell stack consisting of a plurality of individual cells, wherein each individual cell comprises a positive half-cell with a positive electrode and a negative half-cell with a negative electrode, wherein a positive electrolyte flows through the positive half-cell during operation of the redox flow battery and a negative electrolyte flows through the negative half-cell during operation of the redox flow battery, wherein the positive electrolyte is circulated through the positive half-cells by a positive electrolyte pump with a positive flow rate and the negative electrolyte is circulated through the negative half-cells by a negative electrolyte pump with a negative flow rate.

[0003] A redox flow battery is an electrochemical energy storage device for electrochemically based energy storage and typically consists of storage tanks for storing positive and negative electrolytes, as well as pumps and lines for circulating the electrolytes through one or more cell stacks containing a number of individual cells. The individual cells of the cell stack are each formed by a positive half-cell and a negative half-cell arranged side by side, with the positive and negative half-cells of an individual cell separated from each other by a semipermeable membrane, typically an ion exchange membrane. The semipermeable membrane is, for example, a cation and / or anion exchange membrane, e.g., based on sulfonated tetrafluoroethylene polymer (PTFE). The positive half-cell contains a positive electrode located in a frame, through which the positive electrolyte flows.The negative half-cell contains a negative electrode located in a frame, through which the negative electrolyte flows. The positive and negative electrolytes are circulated separately through the half-cells. The positive and negative electrodes are usually made of porous graphite felts through which the electrolyte can flow. Electrode plates, such as bipolar plates, are arranged between adjacent individual cells of the cell stack as current collectors. These plates are usually made of a composite material of carbon and plastic.On the axial outer sides of the axially outer individual cells of the cell stack, there are current collectors on the electrode plates, via which an electrical contact is made to the outside in order to tap an electrical voltage across the entire cell stack (discharging the redox flow battery) or to apply an electrical voltage to the cell stack (charging the redox flow battery). The cell stack is closed off on the axial outer sides by an end plate, which holds the cell stack together. The electrolyte of a redox flow battery is a liquid and essentially comprises an electrochemical redox couple consisting of a first redox element and a second redox element, each in the form of a redox-active element or ion or combinations of redox-active elements and / or ions with different electrical charges (oxidation numbers). There are many different redox-active elements or ions.Combinations of redox-active elements and / or ions that can be used as redox couples in a redox flow battery. The redox elements of a redox couple are usually dissolved in the electrolyte. The electrolyte is usually an aqueous acid, such as aqueous sulfuric acid. Redox flow batteries with a wide variety of combinations of redox couples are known. Some non-exhaustive examples of known combinations of redox couples are V. 2+ / V 3+ vs. V 4+ / V 5+ (in a vanadium redox flow battery), V 2+ / V 3+ vs. Br / CIBr2, Br2 / Br vs. S / S 2 ; Br / Br2vs. Zn 2+ / Zn, Ce 4+ / Ce 3+ vs. V 2+ / V 3+ , Fe 3+ / Fe 2+ vs. Br2 / Br, Fe 3+ / Fe 2+ vs. Cr 3+ / Cr 2+ , Mn 2+ / Mn 3+ vs. Br2 / Br, Fe 3+ / Fe 2+ vs. Ti2+ / Ti 4+ and others. The redox elements can be contained in the electrolyte in various chemical compounds, for example in the form of sulfates, such as vanadium sulfate, or chlorides, such as vanadium chloride. The vanadium-based redox flow battery is the most widely used, which is why the following discussion will primarily focus on the vanadium redox flow battery, without restricting its generality.

[0004] In a vanadium-based redox flow battery, the positive electrolyte in the charged state consists of a redox pair in the form of vanadium with the oxidation number +4 (also known as V lv or V 4+ and vanadium with the oxidation number +5 (also known as V v or V 5+ The negative electrolyte, in the charged state, consists of a redox pair in the form of vanadium with the oxidation number +2 (also known as V" or V 2+referred to) and vanadium with the oxidation number +3 (also known as V 111 or V 3+ ) - whereby the negative electrolyte has a more negative electrochemical potential than the positive electrolyte.

[0005] During operation of a redox flow battery, electrical energy is delivered to a consumer or absorbed from an energy source. In the example of a vanadium-based redox flow battery, the following well-known chemical reaction (redox reaction) occurs during charging / discharging in a single cell of the redox flow battery: y5+ _|_ y2+ y4+ _|_ y3 + During the charging process, the reaction is initiated with the help of an external current or voltage source, which applies an electrical voltage to a single cell (or the entire cell stack).

[0006] This structure and function of a redox flow battery is well known, for example from WO 2018 / 087220 A1 or WO 2014 / 131702 A1 .

[0007] A cell stack is therefore understood to be a cell stack consisting of several individual cells connected in series. The serial electrical connection of individual cell stacks is called a cell string and results in higher electrical voltages. Several cell strings can be electrically connected in parallel to increase the electrical output. For example, a battery system can comprise three individual batteries, each consisting of 6 cell strings connected in parallel, with each cell string consisting of 7 cell stacks. Each individual battery therefore comprises 42 cell stacks. The individual batteries in the battery system can be electrically connected in series or parallel, as required. Each individual battery can have its own electrolyte tanks, piping, pumps, etc. to supply the individual cells of the respective individual battery with electrolyte independently of the other individual batteries.

[0008] Each cell stack has an electrolyte capacity limited by its geometry. At the same time, during operation of the redox flow battery, the available material turnover of the redox reaction during charging / discharging per unit of time is limited by the internal components of the cell stack (electrodes, membrane, flow field, etc.) and also depends on the degree of charge (SoC). Therefore, oversupplying a cell stack does not increase the efficiency of the redox flow battery, but rather leads to losses due to excess power required to operate the electrolyte pumps. However, undersupplying a cell stack must always be avoided, as this adversely affects the efficiency of the redox flow battery.

[0009] To operate a redox flow battery, the method of circulating the electrolytes through a cell stack is essential. Constant, continuous operation of the electrolyte pumps results in higher consumption due to unnecessary pumping power, which is particularly inefficient when there is no or low electrical power input / output and significantly reduces the efficiency of the redox flow battery. Reducing the pumping power using an appropriate method can therefore significantly increase the efficiency of the redox flow battery, but at the same time, it must not be allowed to become too low.

[0010] It is therefore an object of the present invention to provide a method for operating the electrolyte pumps for circulating the electrolytes through a cell stack of a redox flow battery, which enables efficient operation of the redox flow battery.

[0011] This object is achieved according to the invention in that the positive electrolyte pump and / or the negative electrolyte pump are operated in a pulsed manner during a pulse operating time, whereby the flow rate of the positive electrolyte pump and / or the flow rate of the negative electrolyte pump is varied in a pulse cycle between a predetermined maximum flow rate and a predetermined minimum flow rate, and several pulse cycles follow one another during the pulse operating time during operation of the redox flow battery. The pulse cycles are preferably repeated cyclically during the pulse operating time, whereby the pulse cycles themselves, in particular with regard to the pulse cycle durations and the maximum and minimum flow rates, do not necessarily have to be identical.The pulsed operation of the electrolyte pump results in less energy being required to operate the electrolyte pump, which improves the efficiency of the redox flow battery compared to an operation with permanent operation of the electrolyte pumps, but without adversely affecting the charge / discharge operation of the redox flow battery.

[0012] A pulse cycle is preferably implemented such that the flow rate in a pulse cycle is increased from zero to the maximum flow rate of the pulse cycle, the flow rate is maintained at greater than zero during an on-time, and the flow rate is reduced to zero again after the on-time has elapsed and is maintained at zero during an off-time. This enables a very simple implementation of the invention. The flow rate can be maintained at the predetermined maximum flow rate of the pulse cycle during the on-time, which is simple to implement, but can also vary during the on-time, thus providing greater flexibility in controlling the electrolyte pumps.

[0013] In order to take into account current operating parameters of the redox flow battery, it is advantageously provided that the switch-on time or the switch-off time or the maximum flow rate differ in at least two pulse cycles of the pulse operating time.

[0014] In an alternative embodiment of the invention, the flow rate is reduced during a pulse duration of a pulse cycle from a predetermined first maximum flow rate according to a predetermined curve to a predetermined minimum flow rate and then increased again according to a predetermined curve to a predetermined second maximum flow rate. This enables a more continuous flow rate profile, which can be advantageous during operation of the redox flow battery. In order to be able to take current operating parameters of the redox flow battery into account, it is advantageously provided that the pulse duration or the first maximum flow rate or the second maximum flow rate differ in at least two pulse cycles of the pulse operating time.

[0015] The present invention will be explained in more detail below with reference to Figures 1 to 6, which show exemplary, schematic and non-limiting advantageous embodiments of the invention.

[0016] Fig.1 the basic operating principle of a redox flow battery,

[0017] Fig.2 a redox flow battery with a cell stack,

[0018] Fig.3 shows the structure of a cell stack of a redox flow battery,

[0019] Fig.4 a design of a redox flow battery with several cell strings,

[0020] Fig.5 a pulsed operation of an electrolyte pump and

[0021] Fig.6 an alternative pulsed operation of an electrolyte pump.

[0022] In order to better understand the invention, the structure and operation of a redox flow battery 1 is briefly explained below with reference to Fig.1 to Fig.4.

[0023] Fig. 1 shows a schematic structure of a redox flow battery 1 using a single cell 2 of a cell stack 10 to explain the well-known functional principle of a redox flow battery 1. For better explanation and illustration, Fig. 1 shows only a single cell 2 of a cell stack 10 of a redox flow battery 1, whereby a cell stack 10 will generally have a plurality of single cells 2 arranged side by side in the cell stack 10.

[0024] A single cell 2 consists of two half-cells 2a, 2b, which form a positive reaction chamber 3a and a negative reaction chamber 3b. The two half-cells 2a, 2b, or the positive reaction chamber 3a and the negative reaction chamber 3b, are separated by a semipermeable, in particular ion-selective, membrane 4. The reaction chambers 3a, 3b are formed, for example, in recesses 6a, 6b of frames 5a, 5b. A cell electrode 7a, 7b is arranged in each of the frames 5a, 5b, in the reaction chambers 3a, 3b, or in the recesses 6a, 6b, which is in contact with the respective electrolyte 15a, 15b located in the half-cell 2a, 2b. Electrolytes 15a, 15b with different electrical charges (positive and negative electrolyte) flow through the recesses 6a, 6b and, if applicable, the cell electrodes 7a, 7b arranged therein of a single cell 2.Each of the electrolytes 15a, 15b contains a redox pair with specific, time-varying concentrations (depending on the state of charge) of redox elements. The semipermeable, particularly ion-selective, membrane 4 can be made, for example, of sultanate-modified polytetrafluoroethylene (PTFE), with the trade name Nation™, and enables ions to achieve charge equalization between the positive reaction chamber 3a and the negative reaction chamber 3b (or between the electrolytes 15a, 15b contained therein). Power connections 11, 12 are also provided on a cell stack 10 of a redox flow battery 1 in order to tap an electrical cell stack voltage Vz applied to the cell stack 10 via a consumer 14 (discharging the redox flow battery 1) or to apply an electrical cell stack voltage Vz to the cell stack 10 (charging the redox flow battery 1).

[0025] An electrical load 14 can take any form. Based on the electrical voltage, current, or power requirements of the electrical load 14, a cell stack 10 can be configured in a redox flow battery 1 to provide the necessary electrical voltage and / or the necessary electrical current. Redox flow batteries 1 are often used as stationary energy storage devices, for example, to serve as emergency power systems for industrial plants, storage systems for renewable energy (photovoltaics, wind power), and the like. Consequently, depending on the application, a person skilled in the art can design or select a cell stack 10, or a parallel and / or serial connection of multiple cell stacks or cell strings consisting of multiple cell stacks, and redox pairs in a redox flow battery 1.

[0026] The electrolytes 15a, 15b are stored in storage tanks 13a, 13b and are circulated from there through the cell stack 10 by electrolyte pumps 9a, 9b, specifically through a half-cell 2a, 2b of a single cell 2 of the cell stack 10. For this purpose, a supply line 16a, 16b and a discharge line 17a, 17b are provided for each electrolyte 15a, 15b, which are connected via electrolyte connections 22a, 22b, 23a, 23b to the corresponding electrolyte channels 18a, 18b, 19a, 19b (see Fig. 3) in the cell stack 10.

[0027] In a cell stack 10 with several adjacent individual cells 2, an electrode plate 8, such as a bipolar plate, is arranged between each two adjacent individual cells 2 (Fig. 3). At the outer ends of the cell stack 10, a power connection 11, 12 can be located on the outer electrode plates 8 or on the outer half-cells 2a, 2b (or cell electrodes 7a, 7b) of the cell stack 10, which can be electrically contacted from the outside.

[0028] The typical structure of a cell stack 10 of a redox flow battery 1 is explained in more detail with reference to Figs. 2 and 3. A cell stack 10 of a redox flow battery 1 comprises at least one individual cell 2, generally a plurality of individual cells 2, which in turn are each formed from two frames 5a, 5b of half-cells 2a, 2b. A frame 5a, 5b is preferably made of a plastic, such as an elastomer, such as a polyolefinic thermoplastic elastomer (TPE or TPO), such as Santoprene®, or a thermoplastic vulcanate (TPV), in particular using an injection molding process. In the stack direction R (in the direction in which the individual cells 2a, 2b are arranged next to one another) between two frames 5a, 5b of an individual cell 2, a semipermeable membrane 4, typically an ion exchange membrane (either cation or anion exchange membrane, e.g. Nation®) is arranged in the cell stack 10.The membrane 4 separates the reaction spaces 3a, 3b, recesses 6a, 6b of the half-cells 2a, 2b of a single cell 2, the cell electrodes 7a, 7b arranged therein, and the electrolyte liquids 15a, 15b contained therein. Between each two individual cells 2 adjacent in the stack direction R, an electrode plate 8, e.g., a bipolar plate, is arranged in the cell stack 10. The electrode plate 8 is, as shown in Fig. 3, inserted into mutually facing recesses 32 in the frames 5a, 5b. The frames 5a, 5b have central recesses 6a, 6b that run through in the stack direction R, each forming a reaction space 3a, 3b and in which cell electrodes 7a, 7b, e.g., mats made of carbon fiber, are arranged, as shown in Fig. 3.

[0029] The differently charged electrolytes 15a, 15b are pumped through the recesses 6a, 6b in the frames 5a, 5b through the individual cells 2, with an electrolyte 15a, 15b with a different electrical charge flowing through the cell electrode 7a, 7b of each half-cell 2a, 2b of an individual cell 2. The electrolytes 15a, 15b are supplied and removed from the outside via electrolyte connections 22a, 22b, 23a, 23b and are then distributed internally via an electrolyte channel system provided in the frames 5a, 5b with electrolyte channels 18a, 18b, 19a, 19b. The electrolyte connections 22a, 22b, 23a, 23b are provided, for example, on an end plate 24 of the cell stack 10, as shown in Fig.3, although other arrangements of the electrolyte connections 22a, 22b, 23a, 23b, for example on an end frame 20, are also possible.

[0030] The cell stack 10 can be closed off in the stack direction R at each of its two axial ends by an end frame 20. An electrically conductive current collector 21 is arranged in the end frame 20, e.g. in a recess on one end face of the end frame 20, which is connected to an electrical current connection 11, 12 leading to the outside. In the embodiment shown, the current collector 21 rests against the last electrode plate 8 of the last individual cell 2 in order to establish electrical contact. The current collector 21 or a current connection 11, 12 could, however, also be designed differently. Likewise, the end frame 20 could be omitted from the cell stack 10. In the embodiment shown, the cell stack 10 is arranged between two rigid end plates 24 and pressed together by clamping means 25. The clamping means 25 are designed, for example, with through-reaching bolts 26, nuts 27, washers 28 and springs 29, as shown in Fig.2.However, the cell stack 10 can also be held together in other ways; in particular, the clamping device 25 can be designed differently. The two end plates 24 can also be arranged between two pressure plates 30, which are pressed together by the clamping device 25, as shown in Fig. 2. To prevent the frames 5a, 5b from settling due to the contact pressure of the clamping device 25, a spacer 31 can also be provided between the end plates 24.

[0031] However, the present invention is not limited to a specific embodiment of a half-cell 2a, 2b, a single cell 2, a cell stack 10, or the redox flow battery 1. Nor is the invention limited to a specific electrolyte 15a, 15b or to specific redox elements in the electrolytes 15a, 15b. The above statements regarding a redox flow battery 1 serve merely to facilitate understanding of the invention.

[0032] From the above description and the basic operating principle, it is also obvious that there is a positive electrolyte 15a and a negative electrolyte 15b, as well as related thereto a positive half cell 2a and a negative half cell 2b of a single cell 2 and a positive electrolyte pump 9a and a negative electrolyte pump 9b.

[0033] A redox flow battery 1 can also comprise a plurality of cell stacks 10, as explained with reference to Fig. 4. In the embodiment according to Fig. 4, two cell strings 40 are provided, wherein each cell string 40 comprises a plurality of cell stacks 10, for example as described above. However, in a redox flow battery 1, only one cell string 40 can of course also be provided. The cell stacks 10 in a cell string 40 can be electrically connected in series via the power connections 11, 12 of the cell stacks 10 (not shown in Fig. 4 for reasons of clarity). However, a different electrical connection of the cell stacks 10 in a cell string 40 is also possible, such as a parallel connection or a mixture of series and parallel connections. In the embodiment of Fig. 4, the cell strings 40 are electrically connected in parallel (not shown in Fig. 4 for reasons of clarity).However, a different electrical connection of the cell strings 40 is also possible, such as a parallel connection or a mixture of series and parallel connections. Such an electrical connection of cell strings 40 can also be referred to as a single battery 41. A redox flow battery 1 can comprise several such single batteries 41, wherein the single batteries 41 can in turn be electrically connected to one another by a series or parallel connection or a mixture of a series and parallel connection. In this way, a redox flow battery 1 with the desired electrical output voltage and the desired electrical power can be realized.

[0034] Each cell stack 10 is supplied with a positive electrolyte 15a and a negative electrolyte 15b. A storage tank 13a for the positive electrolyte 15a and a storage tank 13b for the negative electrolyte 15b are provided for this purpose. The positive electrolyte 15a is circulated through the cell stack 10 at a positive flow rate M+ by a positive electrolyte pump 9a, and the negative electrolyte 15b is circulated at a negative flow rate M- by a negative electrolyte pump 9b, wherein the positive flow rate M+ and the negative flow rate M- are preferably approximately equal.

[0035] In the case of multiple individual batteries 41 of the redox flow battery 1, a storage tank 13a for the positive electrolyte 15a and a storage tank 13b for the negative electrolyte 15b, as well as a positive electrolyte pump 9a and a negative electrolyte pump 9b, can be provided for each individual battery 41. Thus, the individual batteries 41 would be electrically connected to one another, but would be designed with separate electrolyte circuits.

[0036] In the minimal configuration, the redox flow battery 1 comprises at least one cell stack 10 consisting of a plurality of individual cells 2, wherein each individual cell 2 comprises a positive half-cell 2a with a positive cell electrode 7a and a negative half-cell 2b with a negative cell electrode 7b, wherein the positive half-cell 2a is flowed through by the positive electrolyte 15a during operation of the redox flow battery 1 and the negative half-cell 2b is flowed through by the negative electrolyte 15b during operation of the redox flow battery 1.

[0037] For a specific electrical (charging or discharging) current in the cell stack 10, a specific flow rate M+, M- of electrolytes 15a, 15b through the cell stack 10 is required. The flow rate M+, M- is adjusted by the positive electrolyte pump 9a and the negative electrolyte pump 9b.

[0038] According to the invention, the positive electrolyte pump 9a and / or the negative electrolyte pump 9b are operated in a pulsed manner. Preferably, both electrolyte pumps 9a, 9b are operated in the same way. However, it is also possible for only one of the two electrolyte pumps 9a, 9b to be operated in a pulsed manner. It is also conceivable for both electrolyte pumps 9a, 9b to be operated in a pulsed manner, but for the type and manner of pulsed operation of the electrolyte pumps 9a, 9b to differ. In the following, therefore, no distinction is made between the positive flow rate M+ and the negative flow rate M-, but only a general reference is made to a flow rate M, which can refer to both the positive flow rate M+ and the negative flow rate M-.

[0039] By means of a pulsed operation of an electrolyte pump 9a, 9b, the flow rate M of the electrolyte 15a, 15b circulated thereby is varied in a pulse cycle PZ between a predetermined maximum flow rate M ma x and a given minimum flow rate Mmin.

[0040] The minimum flow rate is M m ax > M m in^0. The maximum flow rate M ma x results from the design of the redox flow battery 1, in particular the parts of the redox flow battery 1 that influence the circulation of the electrolytes 15a, 15b. The maximum flow rate M ma x does not necessarily have to be the maximum possible flow rate, but can also be limited by other factors, such as avoiding turbulent flow, etc.

[0041] Pulse cycles PZ are repeated cyclically and consecutively during the operation of the cell stack 10 of the redox flow battery 1 within a pulse operating time tp, wherein the pulse operating time tp comprises at least two consecutive pulse cycles PZ. Outside of a pulse operating time tp, the electrolyte pump 9a, 9b can be operated continuously at a specific flow rate M.

[0042] Fig. 5 shows an embodiment of pulsed operation of an electrolyte pump 9a, 9b. A plurality of pulse cycles PZ are provided in a pulse operating time tp. During a pulse cycle PZ, the electrolyte pump 9a, 9b is switched on for a switch-on time t on with a given maximum flow rate M ma x, operated. The rest of the pulse cycle PZ is the electrolyte pump 9a, 9b during a switch-off time t Ofr is switched off. This results in a pulse cycle duration of tpz=ton+toff. The pulses of the pulse cycles PZ shown in Fig. 5 are only idealized; in real operation, pulses with a finitely rising leading edge and falling edge will result.

[0043] The specified maximum flow rate M ma x and / or the minimum flow rate Mmin need not be the same for each pulse cycle PZ, but can vary (as indicated by the dashed line in Fig. 5). Likewise, the pulse cycle duration tpz does not have to be the same for all pulse cycles PZ, nor do the switch-on times t on and the switch-off times t O fr.

[0044] The permissible switch-off time t Ofr depends primarily on the electrical charge Q absorbed or released by the cell stack 10. The critical downtime T is the time required to fully charge or discharge the available electrochemically convertible redox element species in the electrolyte 15a, 15b (e.g., vanadium) (e.g., according to the well-known Nernst equation). The critical downtime T can also depend on the charge level SoC, but also on the design of the cell stack 10, such as the cell volume and fluid circuit (Cell II structure, fluid distribution, etc.). In general, the maximum permissible off-time t O fr corresponds to the downtime T, which means t O for T.

[0045] The electric charge Q (in C(oulomb)) can be given, for example, as Q = Q0± i \lt. Where t0 is the time at which the electrolyte pump 9a, 9b is switched off, Q0 is the charge of the cell stack 10 or the electrolyte 15a, 15b when the electrolyte pump 9a, 9b is switched off, and T is the critical downtime. I is the electrical current flowing through the cell stack 10, which can be measured. Q can be obtained, for example, from the Nernst equation. This allows the critical downtime T to be determined.

[0046] The electrolyte pump 9a, 9b can thus, for example, be operated in such a way that when the critical downtime T is reached, the electrolyte pump 9a, 9b is started for the specified switch-on time ton. In this way, the switch-off time t O fr. This can also result in different pulse cycle durations tpz of the pulse cycles PZ.

[0047] Other operating parameters of the cell stack 10 can also influence the pulsing with switch-on time ton and switch-off time t O fr These operating parameters can, for example, influence the change between switch-on time t on and switch-off time t O fr have, or on the switch-on time t on , the pulse height (maximum flow rate M ma x) a pulse cycle PZ or the minimum flow rate M m in a pulse cycle PZ.

[0048] An important operating parameter of the cell stack 10 is the current charge level (SoC). The charge level (SoC) is determined by default in a redox flow battery 1 and can therefore be assumed to be known.

[0049] A high SoC means that, due to the high charge, there are many convertible redox element species and a low risk of depletion of the electrolyte 15a, 15b. However, the viscosity of the electrolyte 15a, 15b can change due to the SoC. In a vanadium redox flow battery 1, for example, the viscosity increases due to the higher V v -portion. Therefore, a longer pulse duration (switch-on time ton), a shorter pulse pause (switch-off time t O fr) and / or a higher pulse height (flow rate Mmax) may be provided.

[0050] At a medium SoC, a moderate proportion of convertible redox element species is present, but there is still a low risk of depletion. This allows for a comparatively shorter pulse duration and / or lower pulse height (pump frequency) with longer pulse pauses. At a low SoC, only a small proportion of convertible redox element species are present, resulting in a high risk of depletion. Therefore, more electrolyte 15a, 15b needs to be circulated, which can be achieved by longer pulse durations, shorter pulse pauses, and / or higher pulse heights.

[0051] The operating mode of the redox flow battery, i.e., whether charging or discharging, can also be taken into account. During charging, there may be a risk of overcharging individual cells 2 within the cell stack 10 due to inhomogeneous voltage behavior of individual cells 2. Therefore, shorter pulse pauses and / or higher pulse heights can be provided. During discharging, pulse duration, pulse height, and pulse pauses can have a direct influence on the metabolism / efficiency of the electrochemical conversion. The pulse cycles can be selected so that a drop in the cell voltage does not occur.

[0052] At lower electrolyte temperatures, efficiency may decrease, which may require more mass transport of redox elements. Therefore, pulse cycles PZ with longer pulse durations, shorter pulse pauses, and / or higher pulse heights can be provided. At higher electrolyte temperatures, an increase in efficiency may occur, but there is a risk of precipitation of a redox element (e.g., V v ) and to the increase in signs of aging. Therefore, pulse cycles PZ with shorter pulse duration, longer pulse pauses, and / or lower pulse height can be provided.

[0053] These influences can be determined empirically or by simulation for a specific design of the redox flow battery 1 and can then be stored in the control of the redox flow battery 1, for example in the form of characteristic curves, characteristic maps or formulaic relationships, in order to control the electrolyte pump 9a, 9b during operation of the redox flow battery 1, also depending on the above or other operating parameters.

[0054] Outside the pulse operating time tp, the electrolyte pump 9a, 9b can be operated continuously with a certain flow rate, as indicated in Fig.5.

[0055] During the switch-on time t on The flow rate M does not necessarily have to remain constant, but could also be within the switch-on time t on However, the flow rate M decreases during the switch-on time t on not to the minimum flow rate Mmin during the switch-off time tO fr from.

[0056] Fig. 6 shows a further embodiment of pulsed operation of an electrolyte pump 9a, 9b. A plurality of pulse cycles PZ are again provided in a pulse operating time tp. In this embodiment, the electrolyte pump 9a, 9b is controlled such that the flow rate M during a pulse cycle PZ in a pulse cycle duration tpz varies from a predetermined first maximum flow rate M ma xi, to a given minimum flow rate M m in (>0) and then again to a predetermined second maximum flow rate M ma x2. The decrease and increase of the flow rate M occurs according to a predetermined curve, for example, an exponential curve. The pulse cycle duration tpz and the predetermined curve, as well as the maximum flow rates M ma xi, M ma x2 and the minimum flow rate M min, can in turn depend on the operating parameters of the redox flow battery 1 described above. This relationship can again be determined empirically or by simulation. The pulse cycle durations tpz of different pulse cycles PZ do not have to be the same.

[0057] In Fig.6, an alternative curve for decreasing and increasing the flow rate M is shown in dashed lines. According to this curve, the flow rate M is kept at the maximum flow rate M for a short period of time at the beginning and end of the pulse cycle PZ. ma x held.

[0058] Also in the embodiment according to Fig.6, neither the pulse cycle durations tpz nor the maximum flow rate M ma x or minimum flow rates M m in be the same for each pulse cycle PZ.

[0059] In addition to the pulse cycles described, other forms of pulse cycles PZ are of course conceivable. For example, in a pulse cycle PZ in Fig. 5, only the falling edge could be falling according to a curve as in Fig. 6, or only the rising edge could be rising according to a curve.

[0060] However, the pulsed operation of the electrolyte pump 9a, 9b always results in less energy being used to operate the electrolyte pump 9a, 9b, which increases the efficiency of the redox flow battery 1 compared to the permanent operation of the electrolyte pump 9a, 9b, but without adversely affecting the operation of the redox flow battery 1.

[0061] To control the electrolyte pumps 9a, 9b, a control unit 50 is provided (Fig. 4), which controls the electrolyte pumps 9a, 9b via corresponding control signals Sa, Sb in order to set a desired flow rate M+, M- of the respective electrolyte 15a, 15b through the at least one cell stack 10.

[0062] The control unit 50 is preferably a processor-based computer hardware on which control software for adjusting the flow rate M+, M- is installed and runs. However, the control unit 50 can also be implemented as an integrated circuit, such as a field-programmable gate array (FPGA) or an application-specific integrated circuit (ASIC).

Claims

Patent claims 1 . Method for operating a redox flow battery (1) with at least one cell stack (10) consisting of a plurality of individual cells (2), wherein each individual cell (2) comprises a positive half-cell (2a) with a positive cell electrode (7a) and a negative half-cell (2b) with a negative cell electrode (7b), wherein a positive electrolyte (15a) flows through the positive half-cell (2a) during operation of the redox flow battery (1) and a negative electrolyte (15b) flows through the negative half-cell (2b) during operation of the redox flow battery (1), wherein the positive electrolyte (15a) is circulated through the positive half-cells (2a) by a positive electrolyte pump (9a) with a positive flow rate (M+) and the negative electrolyte (15b) is circulated through the negative half-cells (2b) by a negative electrolyte pump (9b) with a negative flow rate (M-) is characterized bythat the positive electrolyte pump (9a) and / or the negative electrolyte pump (9b) is operated in a pulsed operating time (tp), whereby the flow rate (M+) of the positive electrolyte (15a) and / or the flow rate (M-) of the negative electrolyte (15b) in a pulse cycle (PZ) between a predetermined maximum flow rate (M, ma x) and a specified minimum flow rate (M m in) is varied and several pulse cycles (PZ) follow one another in the pulse operating time (tp) during operation of the redox flow battery (1).

2. Method according to claim 1, characterized in that the flow rate (M+) of the positive electrolyte (15a) and / or the flow rate (M-) of the negative electrolyte (15b) is increased in a pulse cycle (PZ) from zero to the maximum flow rate (Mmax) of the pulse cycle (PZ) and the flow rate (M+) of the positive electrolyte (15a) and / or the flow rate (M-) of the negative electrolyte (15b) is kept greater than zero during a switch-on time (ton) and that the flow rate (M+) of the positive electrolyte (15a) and / or the flow rate (M-) of the negative electrolyte (15b) is reduced to zero again after the switch-on time (ton) has elapsed and during a switch-off time (t O ff) is kept at zero.

3. Method according to claim 2, characterized in that the flow rate (M+) of the positive electrolyte (15a) and / or the flow rate (M-) of the negative electrolyte (15b) is maintained at the predetermined maximum flow rate (Mmax) of the pulse cycle (PZ) during the switch-on time (ton).

4. Method according to one of claims 2 or 3, characterized in that the switch-on time (ton) or the switch-off time (t O ff) or the maximum flow rate (M ma x) in at least two pulse cycles (PZ) of the pulse operating time (tp).

5. Method according to claim 1, characterized in that the flow rate (M+) of the positive electrolyte (15a) and / or the flow rate (M-) of the negative electrolyte (15b) during a pulse duration (tpz) of a pulse cycle (PZ) is reduced from a predetermined first maximum flow rate (M ma xi) according to a given curve to a given minimum flow rate (Mm in) and then increased again according to a predetermined curve to a predetermined second maximum flow rate (Mmaxz).

6. Method according to claim 5, characterized in that the first maximum flow rate (M ma xi) and the second maximum flow rate (M ma x2) of the pulse cycle (PZ) are equal.

7. Method according to claim 5, characterized in that the pulse duration (tpz) or the first maximum flow rate (M ma xi) or the second maximum flow rate (Mmaxz) in at least two pulse cycles (PZ) of the pulse operating time (tp).

8. Redox flow battery with at least one cell stack (10) consisting of a plurality of individual cells (2), wherein each individual cell (2) comprises a positive half-cell (2a) with a positive cell electrode (7a) and a negative half-cell (2b) with a negative electrode, wherein the positive half-cell (2a) is flowed through by a positive electrolyte (15a) during operation of the redox flow battery (1) and the negative half-cell (2b) is flowed through by a negative electrolyte (15b) during operation of the redox flow battery (1), wherein a positive electrolyte pump (9a) is provided which circulates the positive electrolyte (15a) with a positive flow rate (M+) through the positive half-cells (2a) and a negative electrolyte pump (9b) is provided which circulates the negative electrolyte (15b) with a negative flow rate (M-) through the negative half-cells (2b), characterized in that the redox flow battery (1) is provided with a control unit (50),which is designed to operate the positive electrolyte pump (9a) and / or the negative electrolyte pump (9b) in a pulsed operating time (tp) by the control unit (50) adjusting the flow rate (M+) of the positive electrolyte (15a) and / or the flow rate (M-) of the negative electrolyte (15b) in a pulse cycle (PZ) between a predetermined maximum flow rate (M, ma x) and a predetermined minimum flow rate (Mmin) and several pulse cycles (PZ) follow one another in the pulse operating time (tp) during operation of the redox flow battery (1).

9. Redox flow battery according to claim 8, characterized in that the control unit (50) is designed to adjust the flow rate (M+) of the positive electrolyte (15a) and / or the flow rate (M-) of the negative electrolyte (15b) in one pulse cycle (PZ) from zero to the maximum flow rate (M ma x) to increase the pulse cycle (PZ) and to keep the flow rate (M+) of the positive electrolyte (15a) and / or the flow rate (M-) of the negative electrolyte (15b) greater than zero during a switch-on time (ton), and that the control unit (50) is designed to reduce the flow rate (M+) of the positive electrolyte (15a) and / or the flow rate (M-) of the negative electrolyte (15b) to zero again after the switch-on time (ton) has elapsed and to keep it at zero during a switch-off time (ton).

10. Redox flow battery according to claim 9, characterized in that the control unit (50) is designed to maintain the flow rate (M+) of the positive electrolyte (15a) and / or the flow rate (M-) of the negative electrolyte (15b) during the switch-on time (ton) at the predetermined maximum flow rate (M ma x) of the pulse cycle (PZ).

11. Redox flow battery according to claim 8, characterized in that the control unit (50) is designed to reduce the flow rate (M+) of the positive electrolyte (15a) and / or the flow rate (M-) of the negative electrolyte (15b) during a pulse duration (tpz) of a pulse cycle (PZ) from a predetermined first maximum flow rate (Mmaxi) according to a predetermined curve to a predetermined minimum flow rate (M m in) and then again according to a given curve to a given second maximum flow rate (M ma x2).

12. Redox flow battery according to claim 11, characterized in that the first maximum flow rate (M ma xi) and the second maximum flow rate (M ma x2) are equal.

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