A flow battery system with isolation function and direct current boosting method thereof

Abstract

The application belongs to the field of energy consumption optimization of energy storage systems, and discloses a liquid flow battery system with an isolation function and a direct current voltage boosting method thereof, the direct current voltage boosting method comprising the following steps: calculating the stack loss in the liquid flow battery energy storage system to obtain a first mathematical model between the stack loss and the stack voltage; obtaining the conduction loss of the bidirectional isolation DC / DC converter to obtain a second mathematical model between the stack voltage and the conduction loss; taking the maximum overall efficiency of the energy storage module as the target, obtaining a third mathematical model based on the first mathematical model and the second mathematical model, and optimizing and solving the third mathematical model by using a genetic algorithm to obtain the stack voltage and the number of stacks of the energy storage module, and then obtaining the target voltage of the energy storage module; when any stack fails, the bidirectional isolation DC / DC converter can perform direct current voltage boosting based on the target voltage of the energy storage module. The application can effectively guarantee the stability of the direct current side voltage of the energy storage system, and improve the fault redundancy capability and operation reliability thereof.

Description

Technical Field

[0001] This invention belongs to the field of energy consumption optimization technology for energy storage systems, and more specifically, relates to a flow battery system with isolation function and its DC boost method. Background Technology

[0002] Due to the unique structure of flow batteries, the electrolytes at the positive and negative electrodes connect different potential points, creating an ionic short circuit. This generates leakage current within the battery system, leading to a decrease in system efficiency. Since individual flow battery cells have relatively low voltages, in practical engineering applications, dozens of cells are first assembled into a stack on the DC side. These stacks are then connected in series to form an energy storage module with a common conduit to achieve voltage boosting. Because of the leakage current, the voltage of a single stack cannot be too high. In engineering applications, multiple stacks need to be connected in series to achieve the required voltage. However, connecting multiple stacks in series increases the leakage current in the common conduit, and an excessive number of stacks in series reduces the overall reliability of the module. Therefore, DC boosting of flow battery energy storage systems is a critical issue in the construction of flow battery energy storage power stations.

[0003] Currently, the main methods to reduce leakage current in DC boost converters are to decrease the number of cells connected in series within a single fuel cell stack and increase the coil length. However, reducing the number of individual cells and the number of cells connected in series within the stack will lower the DC-side voltage, while increasing the coil length will increase system cost and pipeline flow resistance, leading to increased pump losses. Therefore, how to achieve a DC voltage for flow batteries that meets the requirements of AC-DC conversion while maintaining efficiency is a pressing issue that needs to be addressed. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the present invention aims to provide a flow battery energy storage system with isolation function and its DC boost method, so as to enhance the safety of the flow battery energy storage system, reduce leakage current, and improve the efficiency of the energy storage system.

[0005] To achieve the above objectives, the present invention provides a DC boost method for a flow battery system with isolation function, the DC boost method comprising the following steps:

[0006] S1 obtains the internal resistance loss and leakage current loss of the fuel cell stack to obtain a first mathematical model between the fuel cell stack loss and the fuel cell stack voltage.

[0007] S2 obtains the conduction loss of the bidirectional isolation DC / DC converter to obtain a second mathematical model between the stack voltage and the conduction loss;

[0008] S3 aims to maximize the overall efficiency of the energy storage module in the flow battery energy storage system. Based on the first and second mathematical models, a third mathematical model is obtained, and a genetic algorithm is used to optimize and solve the third mathematical model to obtain the stack voltage and the required number of series stacks n, thereby obtaining the target voltage of the energy storage module.

[0009] S4 utilizes n fuel cells connected in series to form the energy storage module, connecting both the cathode and anode of each fuel cell to the input terminal of the corresponding bidirectional isolated DC / DC converter, grounding the cathode of each fuel cell; multiple bidirectional isolated DC / DC converters are connected in series, and electrical isolation is achieved between the input and output terminals of each bidirectional isolated DC / DC converter; when any fuel cell fails, the bidirectional isolated DC / DC converter can perform DC boost based on the target voltage of the energy storage module.

[0010] Furthermore, in step S1, the internal resistance loss of the fuel cell stack is calculated using the following formula:

[0011] U b =k×U0

[0012] P e =k×P e,0

[0013] P e,0 =I e,0 2 R e

[0014] Among them, U b Where is the stack voltage, k is the number of cells connected in series within the stack, U0 is the voltage of a single flow cell, and P is the voltage of the individual flow cell. e，0 For the battery internal resistance loss, P e For the internal resistance loss of the fuel cell stack, I e，0 R is the battery's operating current. e This represents the internal resistance of a single cell.

[0015] Furthermore, the leakage current loss of the fuel cell stack is calculated using the following formula:

[0016]

[0017] Among them, P sc For the leakage current loss of the fuel cell stack, I d For the leakage current of the distribution pipe of the fuel cell stack, I c R is the leakage current in the common conduit of the fuel cell stack. d To distribute the internal resistance of the electrolyte in the pipeline, R c The internal resistance of the electrolyte in the public pipeline.

[0018] Furthermore, the leakage current of the distribution pipe of the fuel cell stack is calculated using the following formula:

[0019] I d =α d U b +β d

[0020] Among them, I d For the leakage current of the distribution pipe of the fuel cell stack, α d β d To distribute the leakage current coefficient of the pipeline, the longer the distribution pipeline, the greater α becomes. d The larger the value of β, the better. d The value is determined by the resistivity of the electrolyte itself;

[0021] Preferably, the leakage current of the common conduit of the fuel cell stack is calculated using the following formula:

[0022] I c =α c U b +β c

[0023] Among them, I c For the common conduit leakage current of the fuel cell stack, α c β c α is the leakage current coefficient of the public pipeline. The longer the public pipeline, the greater the leakage current coefficient. c The larger the value of β, the better. c The value is determined by the resistivity characteristics of the electrolyte itself.

[0024] Furthermore, the conduction loss of the bidirectional isolated DC / DC converter is calculated using the following formula:

[0025] P r =4·I s 2 ·R ds(on)

[0026] Among them, I s The effective value of the switching current in a bidirectional isolated DC / DC converter is determined by the switching transistor's duty cycle; R ds(on) P is the on-resistance of the switching transistor. r This refers to conduction loss.

[0027] Furthermore, the first mathematical model is:

[0028] P b =f1(U b )

[0029] Among them, P b For the stack loss, U b This is the stack voltage.

[0030] Furthermore, the second mathematical model is:

[0031] P DC / DC =f2(U b )

[0032] Among them, P DC To bidirectionally isolate DC / DC converter losses, U b This is the stack voltage.

[0033] Furthermore, in step S3, the third mathematical model is expressed as:

[0034] min f(U b )=nP b +nP DC / DC

[0035] st nU b =U DC

[0036] P b =f1(U b )

[0037] P DC / DC =f2(U b )

[0038] Where n is the number of series-connected fuel cells, P b For the stack loss, U b U is the stack voltage. DC Let P be the target voltage of the energy storage module consisting of n fuel cells connected in series. DC / DC This is to isolate the losses of the bidirectional DC / DC converter.

[0039] Furthermore, when the flow battery energy storage system is operating normally, the bidirectional isolated DC / DC converter does not perform a boost function; when any stack fails and is bypassed, the output voltage of the remaining stacks is U. DC The number of remaining non-faulty battery stacks, at which point the bidirectional isolated DC / DC converter will perform DC boost based on the output voltage of the remaining battery stacks.

[0040] According to another aspect of the present invention, a flow battery system with isolation function is also disclosed, the flow battery energy storage system being capable of implementing a DC boost method as described above, the flow battery energy storage system comprising:

[0041] Transformer, bidirectional AC / DC converter, energy storage module, positive pump and negative pump, wherein:

[0042] The output terminal of the transformer is connected to the grid connection point, and its input terminal is connected to the output terminal of the bidirectional AC / DC converter.

[0043] The energy storage module includes multiple series-connected fuel cells. The cathode and anode of each fuel cell are connected to the input terminals of a corresponding bidirectional isolated DC / DC converter, and the cathode of each fuel cell is grounded. The output terminals of the multiple bidirectional isolated DC / DC converters are connected in series and then connected to the input terminal of the bidirectional AC / DC converter. Each fuel cell includes a first input terminal and a second input terminal. The first input terminal is connected to the positive electrode pump through a positive common pipe, and the second input terminal is connected to the negative electrode pump through a negative common pipe. When the flow battery is working, the input and output terminals of the bidirectional isolated DC / DC converters are electrically isolated, so that the cathode and anode of the fuel cell are electrically disconnected from the series connection points of the multiple bidirectional isolated DC / DC converters.

[0044] Compared with the prior art, the above-described technical solutions conceived in this invention have the following main advantages:

[0045] 1. The DC boost method for the flow battery system of the present invention considers the stack loss in the flow battery system and the loss of the bidirectional isolated DC / DC converter connected to the stack. With the goal of optimizing overall efficiency, a genetic algorithm is used to solve for the target stack voltage, thereby obtaining the target voltage of the energy storage module. Since the stack voltage is determined by the number of batteries connected in series internally, it is easy to change. However, if the stack voltage is too high, it will cause excessive leakage current inside the stack, reducing the overall efficiency of the module. If the stack voltage is too low, it will increase the number of stacks required in the energy storage module. Correspondingly, more bidirectional isolated DC / DC converters are needed, which will increase the loss of the bidirectional isolated DC / DC converters and increase the cost. Therefore, the present invention uses the maximization of the overall efficiency of the energy storage module as the optimization goal to determine the target stack voltage and the required number of stacks, and constructs the corresponding energy storage module. In the event of a stack failure, the bidirectional isolated DC / DC converter realizes DC boost based on the calculated target voltage of the energy storage module.

[0046] 2. This invention addresses the low-voltage, high-current characteristics of flow batteries by installing a bidirectional isolation DC / DC converter before each stack. The input and output terminals of this bidirectional isolation DC / DC converter are electrically isolated, and the series connection between the stack electrodes and each bidirectional isolation DC / DC converter is disconnected. Since the cathodes of the stacks are all grounded, there is no potential difference between the common negative terminals of the stacks in the series circuit. Furthermore, since the voltage of each stack is equal, there is no potential difference between the common positive terminals of the stacks. This eliminates leakage current in the common positive and negative terminals of the stacks in the energy storage module, thereby improving the overall energy efficiency of the flow battery energy storage system.

[0047] 3. In the flow battery energy storage system of the present invention, the bidirectional isolated DC / DC converter does not perform the boost function under normal operating conditions in order to improve the efficiency of the bidirectional isolated DC / DC converter. When a single cell failure or other stack failure occurs during operation, resulting in a decrease in the utilization rate of the energy storage system, the flow battery energy storage system of the present invention has the function of real-time bypass of the faulty stack, which can effectively ensure the stability of the DC side voltage of the energy storage system and improve the fault redundancy capability and operational reliability of the energy storage system. Attached Figure Description

[0048] Figure 1 This is a schematic flowchart of a DC boost method for a flow battery system with isolation function provided in an embodiment of the present invention;

[0049] Figure 2 This is a schematic diagram of the topology of a flow battery system with isolation function provided in an embodiment of the present invention;

[0050] Figure 3 This is a schematic diagram of the topology of a conventional flow battery system provided in an embodiment of the present invention;

[0051] Figure 4 This is a schematic diagram of the bidirectional push-free converter topology provided in an embodiment of the present invention. Detailed Implementation

[0052] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0053] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0054] One embodiment of the present invention provides a DC boost method for a flow battery system with isolation function, which can be applied to applications such as... Figure 2 In the flow battery system shown, such as Figure 1 As shown, the DC boost method for a flow battery system with isolation function includes the following steps:

[0055] S1 obtains the battery internal resistance loss and the stack leakage current loss. Since the stack loss is the sum of the battery internal resistance loss and the stack leakage current loss, the first mathematical model representing the relationship between the stack loss and the stack voltage can be directly obtained; specifically, the stack loss P b The main consideration is the internal resistance loss P of the fuel cell stack. e and leakage current loss P sc Right now:

[0056] P b =P e +P sc (1)

[0057] Among them, the internal resistance loss is mainly determined by the internal resistance of the flow battery and the operating current; the leakage current loss is mainly affected by the resistance of the electrolyte in the pipeline and the number of batteries connected in series in the stack. The larger the resistance of the electrolyte in the pipeline, the smaller the leakage current; the more batteries connected in series in the stack and the higher the stack voltage, the larger the leakage current.

[0058] S2 obtains the conduction loss of the bidirectional isolated DC / DC converter, and based on the conduction loss, obtains a second mathematical model between the stack voltage and the conduction loss;

[0059] Specifically, the losses of a bidirectional isolated DC / DC converter mainly consider the conduction loss of the switching transistor, the turn-on loss, and the high-frequency transformer loss. However, the conduction loss of the switching transistor is the only loss of the bidirectional isolated DC / DC converter that is related to its input voltage (i.e., the stack voltage). Therefore, in this embodiment, only the conduction loss is obtained, and a second mathematical model representing the relationship between the stack voltage and the conduction loss can be obtained.

[0060] S3 aims to maximize the overall efficiency of the energy storage module in the flow battery energy storage system. Based on the first and second mathematical models, a third mathematical model is obtained, and a genetic algorithm is used to optimize and solve the third mathematical model to obtain the stack voltage and the required number of series stacks n, where n is a positive integer, thereby obtaining the target voltage of the energy storage module.

[0061] S4 uses n fuel cells connected in series to form an energy storage module. The cathodes and anodes of the fuel cells are connected to the input terminals of the corresponding bidirectional isolated DC / DC converters, and the cathodes of each fuel cell are grounded. Multiple bidirectional isolated DC / DC converters are connected in series, and electrical isolation can be generated between the input and output terminals of each bidirectional isolated DC / DC converter. When any fuel cell fails, the bidirectional isolated DC / DC converter can perform DC boost based on the target voltage of the energy storage module.

[0062] In a preferred embodiment, in step S1, the internal resistance loss of the fuel cell stack is calculated using the following formula:

[0063] U b =k×U0 (2)

[0064] P e =k×P e,0 (3)

[0065] P e,0 =I e,0 2 R e (4)

[0066] Among them, U b P is the stack voltage, k is the number of cells connected in series in the stack, and k is a positive integer; U0 is the voltage of a single flow cell, P e，0 For the battery internal resistance loss, P e For the internal resistance loss of the fuel cell stack, I e，0 R is the battery's operating current. e This represents the internal resistance of a single cell.

[0067] In a preferred embodiment, the leakage current loss of the fuel cell stack is calculated using the following formula:

[0068]

[0069] Among them, P sc For the leakage current loss of the fuel cell stack, I d For the leakage current of the distribution pipe of the fuel cell stack, I c R is the leakage current in the common conduit of the fuel cell stack. d To distribute the internal resistance of the electrolyte in the pipeline, R c The internal resistance of the electrolyte in the public pipeline.

[0070] In a more preferred embodiment, the leakage current of the fuel cell stack's distribution pipe is calculated using the following formula:

[0071] I d =α d U b +β d (6)

[0072] Among them, I d For the leakage current of the distribution pipe of the fuel cell stack, α d β d To distribute the leakage current coefficient of the pipeline, the longer the distribution pipeline, the greater α becomes. d The larger the value of β, the better. d The value is determined by the resistivity of the electrolyte itself;

[0073] Preferably, the leakage current of the common conduit of the fuel cell stack is calculated using the following formula:

[0074] I c =α c U b +β c (7)

[0075] Among them, I c For the common conduit leakage current of the fuel cell stack, α c β c α is the leakage current coefficient of the public pipeline. The longer the public pipeline, the greater the leakage current coefficient. c The larger the value of β, the better. c The value is determined by the resistivity characteristics of the electrolyte itself.

[0076] In a preferred embodiment, the conduction loss of the bidirectional isolated DC / DC converter is calculated using the following formula:

[0077] P r =4·I s 2 ·R ds(on) (8)

[0078] Among them, I s The effective value of the switching current in a bidirectional isolated DC / DC converter is determined by the switching transistor's duty cycle; R ds(on) P is the on-resistance of the switching transistor. r This refers to conduction loss.

[0079] In a preferred embodiment, the first mathematical model is further defined as follows:

[0080] P b =f1(U b (9)

[0081] Among them, P b For the stack loss, U b This is the stack voltage.

[0082] Furthermore, the second mathematical model is:

[0083] P DC / DC =f2(U b (10)

[0084] Among them, P DC To bidirectionally isolate DC / DC converter losses, U b This is the stack voltage.

[0085] In a preferred embodiment, in step S3, the third mathematical model is expressed as:

[0086]

[0087] Where n is the number of series-connected fuel cells, P b For the stack loss, U b U is the stack voltage. DC Let P be the target voltage of the energy storage module consisting of n fuel cells connected in series. DC / DCThis is to isolate the losses of the bidirectional DC / DC converter.

[0088] In a preferred embodiment, when the flow battery energy storage system is operating normally, the bidirectional isolated DC / DC converter does not perform the boost function; when any stack fails and is bypassed, the output voltage of the remaining stacks is U. DC The number of remaining non-faulty battery stacks determines the DC boost converter's output voltage based on the remaining battery stacks.

[0089] According to another aspect of the invention, such as Figure 2 As shown, a flow battery system with isolation function is also disclosed. This flow battery energy storage system can realize any of the aforementioned DC boost methods. The flow battery energy storage system includes:

[0090] Transformer, bidirectional AC / DC converter, energy storage module, positive pump and negative pump, wherein:

[0091] The output terminal of the transformer is connected to the grid connection point, and the input terminal of the transformer is connected to the output terminal of the bidirectional AC / DC converter.

[0092] The energy storage module includes multiple series-connected fuel cells. The cathode and anode of each fuel cell are connected to the input terminals of the corresponding bidirectional isolated DC / DC converters. The output terminals of the multiple bidirectional isolated DC / DC converters are connected in series, and the series connection points at both ends are connected to the input terminals of the bidirectional AC / DC converters. In this embodiment, the input and output terminals of the bidirectional isolated DC / DC converters are electrically isolated, so that the cathode and anode of the fuel cell are disconnected from the series connection points of the multiple bidirectional isolated DC / DC converters. Furthermore, the cathodes of the fuel cells are all grounded, so that there is no potential difference in the common negative electrode channel of each fuel cell.

[0093] The aforementioned bidirectional isolated DC / DC converter's internal power electronic components have also been replaced with MOSFETs, which are more suitable for this topology. Both are voltage-controlled electronic components. IGBTs measure losses based on a fixed voltage, so the higher the voltage, the smaller the proportion of losses, making them suitable for operation under medium to high voltage conditions. MOSFETs, on the other hand, measure losses based on internal resistance, making them suitable for operation under low voltage conditions. For the flow battery energy storage power station structure of this patent, the DC / DC side is at a low voltage. If IGBT power electronic components are still used, it will result in a large proportion of losses in the DC / DC module.

[0094] Specifically, the aforementioned bidirectional isolated DC / DC converter adopts a bidirectional push-pull converter, and its topology is as follows: Figure 3As shown, the input terminal of the bidirectional isolated DC / DC converter is connected to the positive and negative terminals of the fuel cell stack, and its output terminal is connected in series with the corresponding bidirectional isolated DC / DC converter of other fuel cell stacks to perform DC boost; the specific device selection is determined by the fuel cell stack voltage calculated in Example 1.

[0095] Because this converter has a higher bilateral excitation utilization rate compared to forward and flyback converters, and the two switches in the push-pull converter work alternately, it can output a large power. However, its switch voltage stress is large, so it is mainly suitable for low-voltage, high-power applications. Moreover, compared to bidirectional full-bridge converters, it requires fewer switches, thereby reducing costs.

[0096] To better illustrate the implementation details of the present invention, the following embodiments are provided to further illustrate the present invention. It should be understood that the following embodiments are only preferred implementation methods and are not intended to limit the scope of protection of the present invention in any way.

[0097] Example 1

[0098] The algorithm proposed in this embodiment for determining the stack voltage with the goal of maximizing the overall efficiency of the energy storage module is as follows:

[0099] Based on the topology of the flow battery energy storage system, the main losses of the flow battery system include stack losses and DC / DC converter losses.

[0100] I. Calculation of fuel cell stack losses

[0101] P stack loss b The main consideration is the internal resistance loss P of the fuel cell stack. e and leakage current loss P sc Right now:

[0102] P b =P e +P sc (12)

[0103] The internal resistance loss is mainly determined by the internal resistance of the flow battery and the operating current; the leakage current loss is mainly affected by the resistance of the electrolyte in the pipeline and the number of batteries connected in series in the stack. The higher the resistance of the electrolyte in the pipeline, the lower the leakage current; the more batteries connected in series in the stack and the higher the stack voltage, the greater the leakage current.

[0104] 1.1 Calculate the internal resistance loss P of the fuel cell stack e :

[0105] U b =k×U0 (13)

[0106] P e =k×P e,0 (14)

[0107] P e,0 =I e,0 2 R e (15)

[0108] Among them, U b Where is the stack voltage, k is the number of cells connected in series within the stack, U0 is the voltage of a single flow cell, and P is the voltage of the individual flow cell. e,0 For battery internal resistance loss, I e,0 R is the battery's operating current. e This represents the internal resistance of a single cell.

[0109] 1.2 Calculation of leakage current loss P of fuel cell stack sc for:

[0110]

[0111] I d =α d U b +β d (17)

[0112] I c =α c U b +β c (18)

[0113] Among them, I d For the leakage current of the distribution pipe of the fuel cell stack, R d To distribute the internal resistance of the electrolyte in the pipeline, I c R is the common conduit leakage current of the fuel cell stack. c For the internal resistance of the electrolyte in the public pipeline, α d β d To allocate the leakage current coefficient of the pipeline, α c β c α is the leakage current coefficient for public conduits; the longer the distribution conduit, the greater the leakage current coefficient. d The larger the value of α, the longer the public pipeline length. c The larger the value of β, the better. d With β c The value is determined by the resistivity of the electrolyte itself.

[0114] 1.3 In summary, the stack loss P can be obtained. b With the stack voltage U b The relationship is as follows:

[0115] P b =f1(U b (19)

[0116] II. Calculating the Losses of a Bidirectional Isolated DC / DC Converter

[0117] Calculate the loss P of a bidirectional isolated DC / DC converter DC / DC时 The main consideration is the conduction loss P of the switching transistor. r Turn-on loss P sc and high-frequency transformer losses P T ,Right now:

[0118] P DC / DC =P r +P sc +P T (20)

[0119] 2.1 Calculate the switching loss P of the switching transistor in the bidirectional isolation DC / DC converter. sc :

[0120] P sc =4·U b ·I d ·(t on +t off )·f s (twenty one)

[0121] Among them, U b I is the stack voltage. d t is the operating current of the fuel cell stack. on and t off and are the on-time and off-time of the switching transistor, respectively, f s The frequency of the switching transistor.

[0122] 2.2 Calculate the conduction loss P of the switching transistor r :

[0123] P r =4·I s 2 ·R ds(on) (twenty two)

[0124] Among them, I s R is the effective value of the switching current, determined by the switching transistor's duty cycle. ds(on) This is the on-resistance of the switching transistor.

[0125] 2.3 Calculate the loss P of the high-frequency transformer in the bidirectional isolation DC / DC converter. T :

[0126] P T =P fe +P cu (twenty three)

[0127] P cu =I s 2 (R1+R′2) (24)

[0128] Among them, P fe The iron loss of the transformer is a fixed value; P cu R1 and R2' are the copper losses of the transformer, which are specifically related to the current; R1 and R2' are the primary winding and the converted secondary winding of the transformer, respectively.

[0129] Combining the calculations in 2.1-2.3, it can be seen that the loss of the bidirectional isolated DC / DC converter that is related to the input voltage is the conduction loss of the switching transistor in step 2.2. Based on this conduction loss, the input voltage of the bidirectional isolated DC / DC converter, which is the stack voltage U, can be obtained. b And the stack voltage U b Its bidirectional isolated DC / DC converter loss P DC / DC The relationship is:

[0130] P DC / DC =f2(U b (25)

[0131] III. Stack voltage U b Optimize the solution:

[0132] Based on the aforementioned formula, the following third mathematical model is obtained:

[0133]

[0134] Among them, U DC Let be the voltage value of the energy storage module after n fuel cells are connected in series.

[0135] The specific form of the expression is as described above. By optimizing the solution using a genetic algorithm, the stack voltage U that maximizes the overall efficiency of the energy storage module can be obtained. b The specific principle for calculating the number of fuel cells in series, n, is as follows:

[0136] (1) Initialize the algorithm parameters;

[0137] (2) Initialize the population: Randomly generate x pile voltages U within the constraints. b The number of fuel cells connected in series, n, forms the initial population;

[0138] (3) Define the fitness function: use the inverse function of the objective function -f(U) b ) is the fitness function;

[0139] (4) Selection operation: In the current population, select the individual that maximizes the fitness function as the parent;

[0140] (5) Crossover operation: The parent generation is combined with the best combination selected in the previous round, and a new individual is generated by binary single-point crossover. Then, the new individual is merged with the original population, and x individuals that maximize the fitness function are selected to form a new population.

[0141] (6) Mutation operation: to cause individuals in the population to mutate with a certain probability, and binary selection randomly selects a bit of the binary code and flips it;

[0142] (7) Retain the individual with the largest fitness function in the previous optimization process and replace the worst individual in the current population to form a new population;

[0143] (8) When the set number of cycles is reached, the optimization ends and the final optimization result is obtained, which is the stack voltage U. b The number of fuel cells connected in series, n.

[0144] The DC boost method in this embodiment is as follows: The energy storage module of the flow battery system is composed of n battery stacks connected in series. The cathode and anode of each battery stack are connected to the input terminal of a bidirectional isolated DC / DC converter, and the output and input terminals of the bidirectional isolated DC / DC converter are electrically isolated. The cathode of each battery stack is grounded. During operation, when a single battery stack fails, the flow battery system can automatically bypass the faulty battery stack. That is, if one or more battery stacks fail during operation, the faulty battery stack will be automatically short-circuited. Since the voltage of each battery stack is equal, when an individual battery stack fails and is short-circuited, the distributed voltage of the other normal battery stacks needs to be increased. At this time, the adjustable output voltage characteristic of the bidirectional isolated DC / DC converter is used to increase the voltage of the corresponding battery stack to stabilize the overall output voltage of the series-connected battery stacks.

[0145] For example: when the series output voltage is U DC When the number of fuel cells connected in series is n, the output voltage of all fuel cell DC / DC modules is U. DC / n, when one of the fuel cells fails and is bypassed, the output voltage of the remaining n-1 fuel cells is U. DC / (n-1), so this method not only balances and stabilizes the output voltage of the fuel cell stack, but also promptly removes faults and maintains the stable output voltage of the series fuel cell stack.

[0146] Example 2

[0147] Traditional flow battery energy storage power station structure such as Figure 3 As shown, since the voltage of a single fuel cell stack is low, multiple fuel cell stacks need to be connected in series to boost the voltage, and then converted into alternating current by an inverter. Therefore, the voltage between the positive and negative terminals of fuel cell stacks 1 and 4 is:

[0148]

[0149] Among them: V1 + V1 - V4 represents the positive and negative electrode potentials of fuel cell stack 1. + V4 -Let be the positive and negative potentials of fuel cell 4, and U be the voltage between the positive and negative terminals of each fuel cell.

[0150] Therefore, the more fuel cells connected in series, the greater the voltage difference between the two ends, and the greater the leakage current in the common conduit.

[0151] During battery charging and discharging, the electrolyte is driven by the circulation pump from the inlet into the stack through the main pipeline. In the stack, it flows into the branch pipelines through the coil, flows in the individual cells, and flows out from the outlet. When the electrolyte in the pipeline circulates and discharges, it forms ion channels. When the individual cells are connected in series, electron channels are formed between the cells. Therefore, when the electron channels and ion channels form a closed loop, there is a large amount of leakage current in the pipeline and pump loops of the flow battery. The leakage current not only increases the power station loss, but also increases the burden on the temperature control system. Therefore, reducing or even eliminating the leakage current while ensuring the DC side voltage rise is an important means to improve the system efficiency.

[0152] The improved topology of the flow battery energy storage system in this embodiment is as follows: Figure 2 As shown, each fuel cell stack is equipped with an isolated DC / DC converter, and the cathode of each stack is grounded, while other structures remain unchanged. Because the input and output terminals of the isolated bidirectional DC / DC converter are electrically isolated through its internal high-frequency transformer, the stack electrodes are disconnected from the series connection point, and the negative terminal of the stack is grounded. Therefore, there is no potential difference in the common negative terminal pipe of each stack in the series circuit. Furthermore, since the distribution voltage of each stack is the same, there is also no potential difference in the common positive terminal pipe. This completely eliminates the leakage current in the common positive and negative terminal pipes of the fuel cell stack in the energy storage module, thereby improving the overall energy efficiency of the power station.

[0153] In this implementation, the bidirectional isolated DC / DC converter does not perform boost function under normal operating conditions, which improves the efficiency of the DC / DC converter.

[0154] Furthermore, the power electronic devices inside the aforementioned bidirectional isolated DC / DC converter have been replaced by MOSFETs, which are more suitable for this topology, instead of the currently more commonly used IGBTs. Both are voltage-controlled electronic components. IGBTs measure losses based on a fixed voltage, so the higher the voltage, the smaller the loss ratio, making them suitable for operation under medium to high voltage conditions. MOSFETs, on the other hand, measure losses based on internal resistance, making them suitable for operation under low voltage conditions. For the flow battery energy storage power station structure of this patent, the DC / DC side is under low voltage conditions. If IGBT power electronic devices are still used, it will cause problems such as a large proportion of losses in the DC / DC module.

[0155] Specifically, in this embodiment, the bidirectional isolated DC / DC converter adopts a bidirectional push-pull converter, and its topology is as follows: Figure 3As shown, its input terminal is connected to the positive and negative terminals of the fuel cell stack, and its output terminal is connected in series with the DC / DC module of other fuel cell stacks for DC boosting; the selection of specific electrical components such as capacitors C1 and C2, inductor L, and switching transistors S1-S4 is based on the calculated fuel cell stack voltage U. b Decide.

[0156] Compared to forward and flyback converters, the aforementioned bidirectional isolated DC / DC converter has a higher transformer bilateral excitation utilization rate, and the two switching transistors in the push-pull converter work alternately, which can output a larger power; however, its switching transistor voltage stress is relatively large, so it is mainly suitable for low-voltage, high-power applications. Moreover, compared to bidirectional full-bridge converters, it requires fewer switching transistors and has a lower cost.

[0157] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A DC boost method for a flow battery system with isolation function, characterized in that, The DC boost method includes the following steps: S1 obtains the internal resistance loss and leakage current loss of the fuel cell stack to obtain a first mathematical model between the fuel cell stack loss and the fuel cell stack voltage, wherein the fuel cell stack loss includes the internal resistance loss and the leakage current loss of the fuel cell stack. S2 obtains the conduction loss of the bidirectional isolation DC / DC converter to obtain a second mathematical model between the stack voltage and the conduction loss; S3 aims to maximize the overall efficiency of the energy storage module of the flow battery system. Based on the first and second mathematical models, a third mathematical model is obtained, and a genetic algorithm is used to optimize and solve the third mathematical model to obtain the stack voltage and the required number of series stacks n, thereby obtaining the target voltage of the energy storage module. S4 utilizes n fuel cells connected in series to form the energy storage module, connecting both the cathode and anode of each fuel cell to the input terminal of the corresponding bidirectional isolated DC / DC converter, grounding the cathode of each fuel cell; the output terminals of the multiple bidirectional isolated DC / DC converters are connected in series, and electrical isolation is achieved between the input and output terminals of each bidirectional isolated DC / DC converter; when any fuel cell fails, the bidirectional isolated DC / DC converter can perform DC boost based on the target voltage of the energy storage module.

2. The DC boost method for a flow battery system with isolation function as described in claim 1, characterized in that, In step S1, the internal resistance loss of the fuel cell stack is calculated using the following formula: in, U b This is the fuel cell stack voltage. k This refers to the number of batteries connected in series within the fuel cell stack. U 0 represents the voltage of a single flow cell. P e，0 This is due to the internal resistance loss of the battery. P e This refers to the internal resistance loss of the fuel cell stack. I e，0 This is the battery's operating current. R e This represents the internal resistance of a single cell.

3. The DC boost method for a flow battery system with isolation function as described in claim 1, characterized in that, The leakage current loss of the fuel cell stack is calculated using the following formula: in, P sc For the leakage current loss of the fuel cell stack, I d For the leakage current of the distribution pipes of the fuel cell stack, I c This refers to the leakage current in the common conduit of the fuel cell stack. R d To distribute the internal resistance of the electrolyte in the pipeline, R c The internal resistance of the electrolyte in the public pipeline.

4. The DC boost method for a flow battery system with isolation function as described in claim 3, characterized in that, The leakage current of the fuel cell stack's distribution pipe is calculated using the following formula: in, α d , β d To distribute the leakage current coefficient of the pipeline, the longer the distribution pipeline, the more... α d The smaller the value, β d The value is determined by the resistivity of the electrolyte itself; The leakage current of the common conduit of the fuel cell stack is calculated using the following formula: in, α c , β c This is the leakage current coefficient for public pipelines. The longer the public pipeline, the more... α c The smaller the value, β c The value is determined by the resistivity characteristics of the electrolyte itself.

5. The DC boost method for a flow battery system with isolation function as described in claim 1, characterized in that, The conduction loss of the bidirectional isolated DC / DC converter is calculated using the following formula: in, I s The effective value of the switching transistor current in a bidirectional isolated DC / DC converter is determined by the switching transistor's duty cycle. R ds（on） The on-resistance of the switching transistor is denoted as . P r This refers to conduction loss.

6. The DC boost method for a flow battery system with isolation function as described in claim 1, characterized in that, The first mathematical model is: in, Pb For fuel cell stack losses, Ub This is the stack voltage.

7. The DC boost method for a flow battery system with isolation function as described in claim 1, characterized in that, The second mathematical model is: in, PDC To bidirectionally isolate DC / DC converter losses, Ub This is the stack voltage.

8. The DC boost method for a flow battery system with isolation function as described in claim 1, characterized in that, In step S3, the third mathematical model is expressed as: in, n The number of series-connected fuel cells. P b For fuel cell stack losses, U b This is the fuel cell stack voltage. U DC for n The target voltage of the energy storage module after the fuel cells are connected in series P DC / DC This is to isolate the losses of the bidirectional DC / DC converter.

9. A DC boost method for a flow battery system with isolation function as described in claim 1, characterized in that, When the flow battery system is operating normally, the bidirectional isolated DC / DC converter does not perform a boost function; when any battery stack fails and is automatically short-circuited, the output voltage of the remaining battery stacks is U. DC / n’ , n’ The number of remaining non-faulty battery stacks is used to determine the DC boost converter based on the output voltage of the remaining battery stacks.

10. A flow battery system with isolation function, wherein the flow battery system can implement the DC boost method for a flow battery system with isolation function as described in any one of claims 1-9, characterized in that, The flow battery system includes a transformer, a bidirectional AC / DC converter, an energy storage module, a positive electrode pump, and a negative electrode pump, wherein: The output terminal of the transformer is connected to the grid connection point, and its input terminal is connected to the output terminal of the bidirectional AC / DC converter. The energy storage module includes multiple series-connected fuel cells. The cathode and anode of each fuel cell are connected to the input terminals of a corresponding bidirectional isolated DC / DC converter, and the cathode of each fuel cell is grounded. The output terminals of the multiple bidirectional isolated DC / DC converters are connected in series and then connected to the input terminal of the bidirectional AC / DC converter. Each fuel cell includes a first input terminal and a second input terminal. The first input terminal is connected to the positive electrode pump through a positive common pipe, and the second input terminal is connected to the negative electrode pump through a negative common pipe. When the flow battery is working, the input and output terminals of the bidirectional isolated DC / DC converters are electrically isolated, so that the cathode and anode of the fuel cell are electrically disconnected from the series connection points of the multiple bidirectional isolated DC / DC converters.

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