METHOD FOR MONITORING A BATTERY POWER PLANT

Description

[0001] The invention relates to a battery power plant with a plurality of separate battery energy storage units or battery modules, which are electrically interconnected to absorb or release electrical energy. The invention relates to a battery power plant with battery energy storage units designed as redox flow batteries.

[0002] Battery power plants with a multitude of separate battery energy storage units, also referred to as battery modules, are known from the prior art. For example, WO 2014 / 170373 A2 discloses a battery power plant with several battery strings connected in parallel, each battery string comprising several DC battery modules connected in series. Depending on the application, the battery modules in a battery power plant can be connected in various ways, both in series and in parallel.

[0003] Furthermore, TROVÖ ANDREA ET AL: "Multichannel Electromechanical Impedance Spectroscopy and equivalent Circuit synthesis of a large-scale Vanadium redox flow battery" discloses process steps for carrying out multichannel electrochemical impedance spectroscopy and equivalent circuit analysis of a system with multiple vanadium redox flow batteries.

[0004] To ensure the long-term, uninterrupted operation of such a battery power plant, it is advantageous if the individual battery energy storage units can be monitored in order to detect a malfunction or failure of the battery energy storage units at an early stage.

[0005] US patent 2018 / 0175429 A1 discloses a system and method for detecting faults in redox flow batteries. The faults to be detected are leaks in the electrolyte tanks, which are detected using capacitive sensors.

[0006] KR 10-2019-0072790 A discloses a device and a method for determining the service life of a battery. This method employs an impedance measurement technique known as EIS (Electrochemical Impedance Spectroscopy). To determine the impedance spectrum, a special device is connected to the battery's electrical terminals. This device includes, among other things, a waveform generator, a measuring circuit, and a microcontroller.

[0007] The inventors recognized that the method known from KR 10-2019-0072790 A is, in principle, suitable for monitoring the condition of the redox flow batteries in a battery storage system. However, the following difficulties arise. Connecting the measuring device to the individual batteries is very complex, considering that such a battery storage system typically comprises a large number of batteries. Another possibility would be to equip each battery with a corresponding measuring device, which would essentially form an integral part of the respective battery storage system. This solution would significantly increase the production costs of the battery storage system.

[0008] The object of the invention is to provide a method for monitoring the battery energy storage of a battery power plant which at least partially overcomes the aforementioned disadvantages.

[0009] The problem is solved according to the invention by an embodiment according to the independent claim. Further advantageous embodiments of the present invention are found in the dependent claims.

[0010] The invention will be explained below with the aid of figures. The figures show, in detail: Fig. 1 Redox flow battery module Fig. 2 Electrical structure of a battery power plant Fig. 3 Signal waveforms of the excitation current Fig. 4 Nyquist diagram Fig. 5 Flowchart of the process according to the invention

[0011] Figure 1Figure 1 on the left shows a schematic representation of a redox flow battery module. The battery module is labeled 1. It comprises a cell array, labeled 2, and a reservoir, labeled 3. The cell array 2 is an arrangement of multiple redox flow cells, which can be arranged in any configuration. For example, it could be a single cell stack (i.e., several redox flow cells connected in series), several stacks connected in series, several stacks connected in parallel, or a combination of series and parallel connections of several stacks. The reservoir 3 serves to store the electrolyte and supply the cell array 2 with electrolyte. With few exceptions, the reservoir 3 comprises at least two tanks, a pipe system for connecting the tanks to the cell array 2, and pumps for circulating the electrolyte.. Figure 1 The figure shows two separate pumps. The electrolyte could just as easily be pumped using a double-head pump, i.e., two pumps driven by a common motor. The tank assembly 3 is designed to supply all cells of the cell arrangement 2 with electrolyte. Thus, when the pumps are circulating the electrolyte, all cells of the cell arrangement 2 are supplied with it.

[0012] The battery module 1 optionally includes at least one temperature sensor, which is arranged to detect the electrolyte temperature. Figure 1 Two such sensors are shown, one of which is labelled 4. The temperature sensors 4 are, in the embodiment according to Figure 1 They are arranged in the tank assembly 3. However, they could just as well be arranged at any other suitable location in the battery module 1 where they can detect an electrolyte temperature.

[0013] The battery module 1 further comprises at least one additional sensor. This sensor is designed to measure the so-called open-circuit voltage (OCV). The OCV value is a measure of the battery module's state of charge (SoC). The sensor for measuring the open-circuit voltage is designated 6. Another optional sensor is designed to measure the terminal voltage of the cell array 2 and thus also of the battery module 1. During charging or discharging of the battery module 1, the terminal voltage differs from the open-circuit voltage by the voltage drop across the internal resistance of the cell array 2. The sensor for measuring the terminal voltage is designated 5. Another optional sensor is designed to measure the current through the battery module 1. The sensor for measuring the current through the battery module 1 is designated 7.Optionally, battery module 1 also includes an evaluation unit, which is designated 8.

[0014] Figure 2This shows a possible implementation of the electrical structure of a battery power plant in a highly simplified representation. On the left, battery strings with a series connection of battery modules are indicated. A battery string can comprise a single battery module or several battery modules connected in series. Each battery string is surrounded by a dashed rectangle. Each battery module of a battery string can optionally be switched in or out of the battery string using a pair of switches. Each battery string is connected to a DC-DC converter. One of the DC-DC converters is labeled 9. Each battery string optionally includes a sensor for measuring the current flowing through the battery string. One of these sensors is labeled 7. Since the current flowing through the battery string must also flow through each battery module of the same battery string, the current can be measured as shown in the diagram. Figure 2 as shown, measured with a sensor 7 outside the battery modules 1, or, as shown in Figure 1 As shown, the current is measured using sensors 7 within the battery modules 1. Several battery strings are connected to each other via DC busbars, thus forming a battery string group. The DC-DC converters are arranged between the corresponding DC busbar and the battery strings. Each battery string group is connected to the AC busbars of the battery power plant via a DC-AC converter. One of the DC-AC converters is designated 11. Figure 2Three battery strings form a battery string group. The number of battery strings per battery string group can be arbitrary and depends solely on the capacity of the DC-AC converters used and the nominal power of the battery strings. The AC busbars are connected to the transmission grid via a transformer. If a battery string group comprises only one string, the corresponding DC-DC converter 9 can be omitted.

[0015] The DC-DC converters 9 and the DC-AC inverters 11 serve to supply current to the battery modules 1 connected to them in order to charge or discharge the battery modules 1.

[0016] The right side of Figure 2Figure 1 shows an example of a control structure suitable for such a battery power plant. Each battery string has its own controller, one of which is designated 10. Each battery string group, in turn, has its own controller, one of which is designated 12. The central controller belonging to the battery power plant is designated 13. The subordinate controllers 10 and 12 can be implemented separately or integrated into the control unit belonging to the central controller. Equally, the subordinate controllers 10 and 12 could be integrated, and the central controller 13 could be implemented separately. If a battery string group comprises only one string, then this battery string group only requires one controller; that is, one of the associated controllers 10 or 12 can be omitted.

[0017] The inventors realized that, using the battery power plant elements described in the preceding sections, a rudimentary condition monitoring system could be implemented using an impedance measurement method. This means that the condition of the battery modules could be monitored without having to connect a special device to them.

[0018] If this rudimentary condition monitoring detects an anomaly in one or more battery modules of a battery string, either maintenance of the affected battery modules or a more detailed investigation of their condition can be carried out.

[0019] To describe the method according to the invention, the basic steps typically performed to determine impedance spectra are first explained. In impedance spectroscopy, the electrical arrangement is considered a system whose characteristics are to be determined. This is done by excitation with a defined input signal, while the output signal is measured as the system response. This can be achieved by injecting a current I while simultaneously measuring the voltage V across the electrical arrangement. The injected current I serves as the excitation signal for the electrical system. The voltage V represents the response signal of the electrical system. Conversely, the system can also be excited by applying a voltage V while the current I is measured as the system response.The procedure is described below using the first variant as an example, although the relationships also apply to the second variant. Fundamentally, spectroscopy involves exciting the system with signals of different frequencies, which, along with the corresponding response signal, are recorded with time resolution. From the excitation and output signals, the system's impedances at the frequencies used for excitation can then be calculated using mathematical methods.

[0020] In impedance spectroscopy, there are generally several possible signal waveforms with which the system can be excited. Sinusoidal signals are typically used, as in the upper part of Figure 3As shown, when the system is excited with such a signal, the impedance can be calculated at precisely the frequency of the sine wave. The signal is typically applied over several periods. To determine impedances at other frequencies and thus an impedance spectrum, measurements at these frequencies must subsequently be performed individually and sequentially. An alternative approach is to apply signals containing components of multiple frequencies. For this purpose, a signal shape that follows a so-called rectangular function, as shown in the lower part of [reference], is suitable. Figure 3As depicted. Unlike a sine wave, a square wave contains additional frequency components at the odd harmonics of the fundamental frequency, in addition to the fundamental frequency. If a system is excited with a square wave, both the excitation and response signals contain signal components at multiple frequencies. Thus, not only can the impedance value corresponding to the fundamental frequency of the square wave be determined, but also further impedance values ​​corresponding to the higher orders of the fundamental frequency. In practice, the number of higher orders that can be detected is limited by the finite slope of the square wave of the excitation current, the finite sampling rate of current and voltage, and superimposed noise signals.

[0021] In principle, even an excitation signal with an arbitrarily long duration can be used, i.e., a non-periodic signal. According to Fourier theory, such a signal can be considered a superposition of periodic functions. The fundamental frequency is then given by 1 / T (or 2π / T), where T is the time duration of the signal. In practice, however, the frequencies in Figure 3 The signal waveforms shown are used. Excitation with individual rectangular pulses or step functions is also practical. For periodic signals, the fundamental frequency is naturally given by 1 / T₀ (or 2π / T₀), where T₀ is the period.

[0022] The result of impedance spectroscopy is the impedance Z of the electrical arrangement as a function of the frequency f or the angular frequency ω = 2πf: Z(w). Here, Z(w) is a function in the space of imaginary numbers and contains information about the magnitude and phase of the impedances. With pure-frequency excitation using a sinusoidal signal, ω is directly derived from the frequency of the excitation current. The magnitude of the impedance at this frequency is calculated as the quotient of the magnitudes of the sinusoidal voltage signal and current signal, while the phase is calculated as the difference between the phases of the two signals. For an excitation current that includes higher frequencies in addition to the fundamental frequency, both the measured excitation current I and the measured response signal V are subjected to a Fourier transform, yielding F{I}(w) and F{V}(w). The desired impedance function Z(w) is then given by Z(ω) = F{V}(ω) / F{I}(ω).It is clear that Z(w) is only defined for those frequencies ω for which F{I}(ω) is not zero. Furthermore, it should be noted that for the impedance spectroscopy of batteries, the time-resolved response signal of the terminal voltage must be corrected for the open-circuit voltage, resulting in the voltage drop across the battery's internal resistance, i.e., V = V terminal - V open-circuit voltage.

[0023] Z(w) can be advantageously represented in the form of a so-called Nyquist diagram. In this diagram, the real part of Z(w) is plotted in the x-direction and the negative imaginary part of Z(w) in the y-direction. The unit of Z(w) is the ohm.

[0024] Figure 4 The upper part shows a typical Nyquist diagram of a redox flow battery in qualitative form. The lower part of Figure 4A simplified equivalent circuit diagram is shown, which is used for interpreting the Nyquist diagram shown above. Two resistance values ​​can be derived from the diagram: Rs and Rct. Rs is interpreted as the static internal resistance component, which is given, for example, by the resistance of the contacts and the leads, while Rct describes the component resulting from the kinetics of charge transfer between the electrode and the electrolyte. The equivalent circuit diagram makes it clear that Rs contributes to the total resistance regardless of the frequency, while the contribution of Rct is reduced at high frequencies by the parallel double-layer capacitance and therefore only contributes significantly at low frequencies. Rs can usually be roughly read from the Nyquist diagram at frequencies of approximately 20 kHz, while Rs + Rct can be plotted at frequencies around 1 Hz.Due to the parasitic influence of a series inductance (not shown in the diagram), e.g., from the connecting leads, the Nyquist diagram is shifted towards the positive imaginary axis (i.e., downwards). This also changes the x-intercept. Therefore, Rs can only be determined graphically approximately. More accurate results can be obtained by mathematically fitting an equivalent circuit model. In the following, the sum Rs + Rct is referred to as the "total internal resistance," even though this does not exactly correspond to the DC resistance.

[0025] The inventors recognized that a time-varying excitation current I can be generated in a battery power plant using the DC-DC converter 9 or the DA-AC converter 11. This current can be used to excite the associated battery modules for rudimentary impedance spectroscopy. The rise time of the excitation current is limited by the power electronics, meaning that signals of acceptable quality can only be generated up to a certain upper frequency limit. When using an excitation signal with various frequency components, such as a square wave, the number of calculable frequency components above the fundamental frequency is also limited by the signal quality and the accuracy of the measuring device. Therefore, even if a high-quality impedance spectrum up to high frequencies in the kHz range cannot be achieved, information about the behavior at low frequencies is still available with sufficient quality.This is sufficient to determine the sum Rs + Rct. Since degradation processes in redox flow batteries often lead to increased contact resistances or impaired charge transfer processes, an increased value for the sum Rs + Rct is then observed, which can be determined using the method according to the invention. Therefore, the method according to the invention is suitable for identifying battery modules that suffer from the aforementioned problems. Further, more complex investigations can then be carried out on the battery modules thus identified in order to resolve the problems.

[0026] The inventors realized that determining a single Z(w) value in the frequency range ≤ 20 Hz is essentially sufficient to obtain an initial indication of potential problems with the battery modules under investigation. If the total internal resistance of a battery module increases, the entire flank of the Nyquist graph marked with the dashed ellipse shifts to higher abscissa values ​​(Re{Z(w)}), so that increased abscissa values ​​can also be detected at any frequency within this range.

[0027] Naturally, multiple Z(w) values ​​can also be determined at different low frequencies (≤ 20 Hz) to increase the significance of the measurement. In this case, the determined impedances can also be fitted to a suitable model before comparison with the predefined limit described below to further enhance the significance of the measurement.

[0028] As mentioned above, the method according to the invention is described below and in the claims for the case where a current signal is used as excitation. However, this is not to be considered limiting and also includes the case where a voltage signal is used as excitation.

[0029] The steps of the inventive method are described in Figure 5 The procedure comprises at least the following steps: S1: Generation of a time-varying current I with a fundamental frequency ω, with which at least one battery module 1 is excited to perform impedance spectroscopy; S2: Time-resolved acquisition of the excitation current I and a response voltage V; S3: Calculation of the impedance Z(w); S4: Initiation of maintenance work on the at least one battery module 1 if Re{Z(w)} exceeds a predefined limit.

[0030] In step S1, the current I is generated using a DC-DC converter 9 or, if the associated battery string group comprises only one battery string, using an AC-DC converter 11, where the fundamental frequency of the generated time-varying current is f=ω / 2π ≤ 20 Hz.

[0031] The predefined limit value is set such that Re{Z(w)} for f=ω / 2π ≤ 20 Hz of any "healthy" battery module of the battery power plant lies below the limit value. For this purpose, a sample of "healthy" battery modules can be measured using impedance spectroscopy. The limit value is then chosen so that all Re{Z(w)} for f=ω / 2π ≤ 20 Hz of the measured battery modules lie significantly below the limit value (i.e., with respect to the measurement accuracy and the variance of the Re{Z(w)} of the measured battery modules).

[0032] In the following, the term "converter" refers to both a DC-DC converter 9 and an AC-DC converter 11. Such a converter approximates a time-varying current waveform on the DC side using discrete stages. Therefore, a sinusoidal current waveform according to the upper part of Figure 3 They do not represent a single frequency. Therefore, it is recommended in every case to apply a Fourier transform to the signals acquired in step S2, even if a sine wave signal is used.

[0033] If the open-circuit voltage of the battery module remains constant or can be assumed to be constant during the execution of the method according to the invention, then it does not need to be measured during the execution of the method according to the invention, because it only changes the DC component of the impedance and thus does not contribute to Re{Z(w)} for ω ≠ 0. However, if the open-circuit voltage changes over time during the execution of the method according to the invention, it should be measured with temporal resolution in step S2 and used in step S3 for the calculation of Z(w) as described above (i.e., V = Vclamp - Vocv).

[0034] Since the internal resistances of the at least one investigated battery module depend on the electrolyte temperature and the electrolyte flow rate through the cell arrangement, it is advantageous to record these parameters during the execution of the method according to the invention and to include them in step S4. It is particularly advantageous if the state of charge (SoC) is also recorded and included in step S4. This can be achieved, for example, by predefining a function of limit values ​​instead of a single limit value for the real part of the impedance, where this function depends on temperature, flow rate, and SoC. These parameters can also be used to computationally correct the determined total internal resistance (Re{Z(w)}) and to make the initiation of maintenance work dependent on the corrected total internal resistance.

[0035] The speed or power consumption of the pumps can also be used as a measure of the electrolyte flow rate through the cell arrangement.

[0036] With the method according to the invention, all battery modules of a battery string can be monitored simultaneously, since the excitation current generated by the inverter flows through all modules of the battery string. There are various ways to acquire and evaluate the excitation current and the response voltage. One possibility is to acquire these two quantities at the string level. For this purpose, the Figure 2The current sensor 7 (shown) and a voltage sensor (not shown) are used. Typically, suitable inverters include an integrated voltage sensor. Controllers 10 and 12 are used to evaluate and calculate the impedance Z(w). This allows the impedance of the entire battery string to be determined.

[0037] A second possibility is to measure the two aforementioned parameters at the battery module level. For this purpose, the parameters in Figure 1 The sensors 5 and 7 shown are used. For the evaluation or calculation of the impedance Z(ω), the evaluation unit 8 can then be used, for example.

[0038] Another possibility is that the excitation current is at the string level, i.e., with sensor 7 from Figure 2 , and the response voltage of the battery modules at module level with sensor 5 from Figure 1The signals must be recorded. In this case, synchronization of the recorded measurement signals must be ensured. Controllers 10 and 12 or evaluation unit 8 are suitable for evaluating or calculating the impedance Z(w).

[0039] The impedances of individual battery modules can be determined using the latter two methods.

[0040] The open-circuit voltage, if it is to be measured, is measured at the battery module level using sensor 6. Synchronization of the measured open-circuit voltage with the other signals is only necessary if the open-circuit voltage is to be measured with temporal resolution. Otherwise, it is typically only used to determine the state of charge (SoC).

[0041] If the monitored battery string is in Figure 2If the illustrated pairs of switches are included, then any subset of the battery modules belonging to the battery string can also be connected to the inverter and monitored using the method according to the invention.

[0042] The data obtained using the method according to the invention can be stored and managed in the central control system of the battery power plant.

[0043] Several options are available for maintenance work in step S4. For example, the battery module(s) in question could be subjected to complete impedance spectroscopy using a suitable device to verify the data obtained with the method according to the invention and to obtain further information, e.g., the separate values ​​of Rs and Rct. This would allow for a more precise diagnosis of the condition and any potential problems of the battery module. Furthermore, it would be possible to replace the cell arrangements in the battery modules in question and then further examine the removed cell arrangements and, if necessary, repair or dispose of them.

[0044] The method according to the invention enables monitoring of the battery modules using the components (inverters 9 and 11, sensors 5, 6, 7) that are already present in a conventional battery power plant. Therefore, the method according to the invention requires no or only minimal additional hardware. Reference symbol list

[0045] 1 Battery module 2 Cell array 3 Refueling system 4 Temperature sensor 5 Terminal voltage sensor 6 Open-circuit voltage sensor 7 Current through a battery string sensor 8 Evaluation unit 9 DC-DC converter 10 Battery string control 11 DC-AC inverter 12 Battery string control 13 Central control unit of the battery power plant

Claims

1. Method for monitoring a battery power plant comprising a plurality of battery modules (1) which are designed as a redox flow battery and each comprise a cell arrangement (2), a tank device (3) for storing an electrolyte and pumps for supplying the electrolyte, and wherein one or more battery modules (1) electrically connected to one another in a series connection form a battery string, and wherein the battery power plant comprises a converter (9, 11) and a DC side of the converter (9, 11) is connected to the battery string such that the converter (9, 11) can feed a current into the battery string in order to charge or discharge the associated battery modules (1), and wherein the method comprises the following steps: S1: generating a time-varying excitation current I with a base frequency f, by which at least one battery module (1) is excited to perform an impedance spectroscopy; S2: time-resolved detecting the excitation current I and a response voltage V; S3: calculating the impedance Z(w), wherein ω = 2πf; characterized in that the method comprises the following step: S4: initiating maintenance work on the at least one battery module (1) if Re{Z(w)} exceeds a predefined limit value, and wherein in step S1 the excitation current I is generated by use of the converter (9, 11), wherein the base frequency of the generated time-varying excitation current is f ≤ 20 Hz.

2. Method according to claim 1, wherein a temperature of the electrolyte and a flow rate of the electrolyte through the cell arrangement (2) are detected during the execution of the method in the at least one battery module (1), and wherein the temperature and the flow rate are included in step S4.

3. Method according to claim 1 or 2, wherein a state of charge is detected in the at least one battery module (1) during the execution of the method, and wherein the state of charge is included in step S4.

4. Method according to any one of claims 1 to 3, wherein in step S1 all battery modules (1) of the battery string are excited with the excitation current I to perform an impedance spectroscopy.

5. Method according to any one of the preceding claims, wherein the at least one battery module (1) comprises an evaluation device (8), a sensor (5) for measuring the terminal voltage VKlemm, a sensor for measuring the open-circuit voltage VOCV and a sensor (7) for measuring the excitation current I flowing through the at least one battery module (1), and wherein V = VKlemm - VOCV, and wherein step S3 is carried out by the evaluation device (8).