Power storage facility and method for measuring ac impedance

Abstract

A power storage facility 10A includes a power conversion device 20 provided in a main circuit L, and one or more power storage banks 50A, 50B connected to the power conversion device 20 via the main circuit L. The power storage banks 50A, 50B each have a plurality of cells 61 connected in series. An AC signal is applied to the cells 61 of the power storage banks 50A, 50B by the power conversion device 20, and the AC impedance of each of the cells 61 is measured on the basis of the response.

Description

Energy storage equipment, AC impedance measurement method

[0001] The present invention relates to a technique for measuring the AC impedance of an energy storage element.

[0002] In recent years, the introduction of renewable energy sources such as solar and wind power has expanded, and fluctuations in power generation output due to weather have become an issue. In order to stabilize the power grid against output fluctuations due to weather, it is important to ensure adjustment capacity, and one means for achieving this is energy storage facilities (e.g., energy storage systems (ESS)). Energy storage facilities have a storage bank in which multiple storage batteries are connected in series.

[0003] Since the capacity of a storage battery (hereinafter also referred to as a cell) decreases due to its own degradation, it is desirable to grasp the battery capacity and then perform optimal control. There are the following methods for grasping the battery capacity of a cell.

[0004] (1) Actual measurements using DC charging and discharging (2) Prediction using mathematical models

[0005] Method (1) involves charging and discharging a cell with DC current under specified conditions while the energy storage facility is in operation, and calculating the battery capacity from the amount of electricity [Ah] charged and discharged at that time and the voltage difference [V] between the cells before and after charging and discharging. This method requires charging and discharging an amount of electricity equivalent to at least several tens of percent of the SOC (State of Charge), which takes several hours. Furthermore, the battery capacity can be obtained only after charging and discharging an amount of electricity equivalent to several tens of percent of the SOC, but not before that.

[0006] Method (2) involves conducting cell degradation tests under various conditions (voltage, current, temperature, etc.) and expressing the data obtained in a mathematical model (such as the root law or Arrhenius law). Then, using the cell voltage, current, temperature, etc. during operation of the energy storage facility as input, the current battery capacity is predicted from the mathematical model. Unlike actual measurements using DC charging and discharging, this method allows for the prediction of battery capacity before charging and discharging. However, while the energy storage facility is in operation, it is necessary to store all of the cell voltage, current, and temperature as historical data from the start of operation, and the amount of historical data stored increases as the operation period becomes longer.

[0007] In addition to the above (1) and (2), the AC impedance method is considered a promising method for understanding the internal state of a cell. The AC impedance method is a technique for measuring the response when an AC signal is applied to a cell, and can evaluate the internal state of a cell indirectly, non-destructively, and in a short time. Patent Document 1 discloses technology related to the AC impedance method.

[0008] JP 2013-64697 A

[0009] The secondary battery system of Patent Document 1 is provided with a dedicated AC signal generating unit (boosting power supply) to apply an AC signal to the battery pack.

[0010] When the technology of Patent Document 1 is applied to a power storage facility and an AC signal generator is arranged in each power storage module of a power storage bank, there are size and cost constraints, which result in the input / output signals of the AC signal generator being unavoidably small.

[0011] Since the measurement accuracy of AC impedance depends on the levels of the input and output signals of the AC signal generator, there is a concern that the measurement accuracy may decrease.

[0012] According to one aspect of the present invention, there is provided a power storage facility including a power conversion device provided in a main circuit and one or more power storage banks connected to the power conversion device via the main circuit. The power storage banks each have a plurality of cells connected in series. The power conversion device applies an AC signal to the cells of the power storage bank, and measures the AC impedance of the cells based on the response of the power conversion device.

[0013] The inventors came up with the idea of ​​generating an AC signal in a power conversion device that is installed in a main circuit and normally charges and discharges the cells with DC, and arrived at the above configuration, which allows for improved accuracy in measuring the AC impedance of the cells.

[0014] Block diagram of the energy storage system Block diagram of the CMU (Cell Management Unit) Block diagram of the BMU (Battery Management Unit) Diagram showing the measurement principle of the AC impedance method Cole-Cole plot of the cell Graph showing the relationship between R and -X of the cell Graph showing the relationship between the cell's battery capacity and -X / R Equivalent circuit of the cell Cole-Cole plot of the cell Graph showing the relationship between SOC and OCV (Open Circuit Voltage) of the cell Flowchart of the estimation process Current waveform (waveform of the AC signal) Voltage waveform (response waveform) Lookup table Block diagram of the energy storage system

[0015] An overview of the embodiments is described below. (1) A power storage facility includes a power conversion device provided in a main circuit and one or more power storage banks connected to the power conversion device via the main circuit. The power storage banks have a plurality of cells connected in series. The power conversion device applies an AC signal to the cells of the power storage bank, and measures the AC impedance of the cells based on the response. The power conversion device has a larger power capacity than an AC signal generator dedicated to measurement, and can output an AC signal at a level suitable for measurement. Therefore, this configuration provides high accuracy in measuring AC impedance.

[0016] (2) In the power storage facility described in (1), the AC impedance of the cell may be measured based on a response from a second cycle onward of the AC signal applied to the cell by the power conversion device. This configuration allows the response to be acquired while the cell is in a stable state, thereby improving the accuracy of measuring the AC impedance.

[0017] (3) In the energy storage equipment described in (1) or (2) above, the AC signal may be a current signal having a C-rate in the range of 0.05 to 0.3 [CA]. If the C-rate is 0.05 CA or higher, an amplitude suitable for measurement can be obtained, thereby maintaining the measurement accuracy of AC impedance. Furthermore, if the C-rate is 0.3 CA or lower, there is almost no effect of heat generation due to current flow. The C-rate is the ratio of the charge / discharge current value to the battery capacity (full charge capacity), and indicates the relative magnitude of the current value. 1 C is the current value at which the battery capacity is discharged (charged) in 1 hour.

[0018] (4) In the power storage facility according to any one of (1) to (3), the AC signal may be a square wave signal. A square wave signal does not require fine-level control of the AC signal compared to a sine wave signal, making it easier to control the power conversion device.

[0019] (5) In the energy storage facility described in any one of (1) to (4) above, the power conversion device may apply the AC signal to a plurality of the energy storage banks connected in parallel to the main circuit. This configuration allows the AC impedance of cells in a plurality of energy storage banks to be measured efficiently in a short time, making it possible to monitor the states of all cells in almost real time while the energy storage facility is in operation. The techniques described in (1) to (4) above can be applied to methods for measuring the AC impedance of cells.

[0020] <Embodiment 1> 1. Structure of Energy Storage System 10A Fig. 1 is a block diagram of an energy storage system 10A. The energy storage system 10A may be connected to a power grid 1 to adjust the supply and demand of power. The energy storage system 10A is an example of a power storage facility.

[0021] The power system 1 may be provided by an electric power company, or may be an electric power system independent from the power system of the electric power company.

[0022] The energy storage system 10A includes a power conditioning system (PCS) 20 and a storage battery system 30. The PCS 20 is a device that charges and discharges the storage battery system 30, and is provided on a power line L that is a main circuit. The PCS 20 may be housed in a container of the storage battery system 30.

[0023] The PCS 20 includes a bidirectional inverter 21 and a power control unit 23. The bidirectional inverter 21 is a converter capable of both forward conversion (AC to DC) and reverse conversion (DC to AC). The bidirectional inverter 21 is an example of a power conversion device. The power control unit 23 controls the bidirectional inverter 21 based on the state of the storage battery system 30 to adjust the supply and demand of power.

[0024] The battery system 30 is composed of one or more power storage banks 50. In this example, the battery system 30 is composed of two power storage banks 50A and 50B connected in parallel. The power storage banks 50A and 50B are connected to the PCS 20 via a power line L.

[0025] The power storage banks 50A and 50B are each made up of a current sensor 51 that measures the current of the power storage bank 50, a BMU 55, and a plurality of power storage modules 60 connected in series to the power line of the power storage bank 50.

[0026] The power storage module 60 includes a plurality of cells 61 connected in series and a CMU 65. The plurality of cells 61 are fixed to a frame to form a unit. The cells 61 are chargeable and dischargeable power storage elements. Various types of cells can be used, such as lithium-ion secondary batteries (an example of a non-aqueous electrolyte secondary battery).

[0027] As shown in FIG. 2A , the CMU 65 includes a CPU 66 and a storage unit 67 , and monitors the temperature of the power storage module 60 and the voltage of each cell 61 .

[0028] A BMU 55 is provided for each of the power storage banks 50A and 50B. As shown in Fig. 2B, the BMU 55 includes a CPU 56 and a storage unit 57, and monitors the states of the power storage banks 50A and 50B.

[0029] Specifically, the BMU 55 monitors the current of the power storage banks 50A and 50B based on the measurement results of the current sensor 51. The BMU 55 is also connected to each CMU 65 via a communication line, and acquires information on the temperature of the power storage module 60 and the voltage of each cell 61 from the CMU 65 to monitor the state of each power storage module 60.

[0030] 2. Estimation of the Internal State of the Cells When utilizing the storage battery system 30 as a power supply and demand adjustment capability, the following issues arise: (1) Because the capacity of the cells 61 decreases due to degradation, it is desirable to constantly grasp the internal state of the cells 61 and perform optimal control. (2) Ideally, it would be possible to individually grasp the internal state of the multiple cells 61 connected in series and parallel and monitor all of the cells.

[0031] In this embodiment, in order to contribute to solving the above problems (1) and (2), the internal state of each cell 61 of the storage battery system 30 is estimated using an AC impedance method.

[0032] 3 is a diagram showing the measurement principle of the AC impedance method. The AC impedance method is a technique for measuring the response when an AC signal (e.g., an AC current signal or voltage signal) is applied to the cell 61, and can evaluate the internal state of the cell 61 indirectly, non-destructively, and in a short time.

[0033] When an AC signal is applied, the impedance is measured while sweeping the frequency ω, thereby making it possible to obtain a Cole-Cole plot (complex impedance locus) of the cell 61 as shown in FIG.

[0034] The Cole-Cole plot of cell 61 shows that the resistance components that contribute to the impedance differ depending on the frequency band, with ohmic resistance contributing primarily at high frequencies, charge transfer resistance at intermediate frequencies, and diffusion resistance at low frequencies. In other words, by analyzing the Cole-Cole plot, it is possible to indirectly evaluate the phenomena occurring in each part of cell 61.

[0035] In a Cole-Cole plot, the resistance component R and reactance component −X of the AC impedance Z in the diffusion region have a linear relationship, as shown in Figure 4, and the slope of the line Y is expressed as −dX / dR. The diffusion region is a frequency band of 10 to 100 mHz where the diffusion resistance becomes dominant among the resistance components contributing to the impedance.

[0036] A plurality of cells (e.g., lithium ion batteries) 61 with different battery capacities were prepared, and the AC impedance was measured when an AC signal with a frequency of 10 to 100 mHz, which corresponds to the diffusion region, was applied. The battery capacity is the capacity [Ah] that can be extracted from the fully charged cell 61, and is also called the full charge capacity.

[0037] 5A and 5B are graphs showing the measurement results of AC impedance in the diffusion region, where Fig. 5A shows the relationship between AC impedance R and −X, and Fig. 5B shows the relationship between battery capacity and −dX / dR, where R is the resistance component (real part) and X is the reactance component (imaginary part).

[0038] 5B, a correlation was observed between the battery capacity and −dX / dR. In other words, if the relationship between the battery capacity and −dX / dR is known through a preliminary test for cell 61 of energy system 10A in operation, it is possible to estimate the battery capacity of cell 61 by measuring the AC impedance of the diffusion region and calculating −dX / dR.

[0039] The reason why the battery capacity can be estimated from the −dX / dR of the diffusion region is explained below. As shown in Figure 6A, the characteristics of the AC impedance Z of a cell 61 can be expressed by an equivalent circuit consisting of an ohmic resistance R0, a charge transfer resistance R1, an electric double layer C1, and a diffusion resistance Rw.

[0040] Of these, the diffused resistance Rw has a characteristic in which the slope of the reactance component X with respect to the resistance component R is 45 degrees (straight line Y in FIG. 4) in the diffusion region of the Cole-Cole plot. However, in an actual cell 61, there are cases where this angle is greater than 45 degrees. In such cases, analysis becomes possible by adding a differential capacitance Cdiff as a capacitance component, as shown in FIG. 6B.

[0041] The differential capacity Cdiff means the differential capacity dSOC / dOCV in the SOC-OCV characteristics of the cell 61. In other words, because the AC impedance measurement itself involves charging and discharging the cell 61, the SOC and OCV are likely to increase or decrease during measurement, particularly when a low-frequency AC signal is applied (during charging and discharging over a long period of time), and as shown in FIG. 7, this effect appears as the impedance 1 / jωCdiff of the differential capacity in the diffusion region of the AC impedance characteristics.

[0042] The differential capacity Cdiff is a parameter based on the SOC-OCV characteristics, and differs between new and deteriorated products, as shown in Figure 8. When the battery capacity decreases due to deterioration, the curve representing the SOC-OCV characteristics shrinks, and the differential capacity Cdiff decreases.

[0043] As a result, the impedance 1 / jωCdiff of the differential capacitance Cdiff in the diffusion region of the AC impedance characteristic increases, and the magnitude (absolute value) of −dX / dR also increases.

[0044] As described above, since there is a correlation between the battery capacity and −dX / dR of the diffusion area, it is possible to estimate the battery capacity of the cell 61 of the energy system 10A in operation based on −dX / dR of the diffusion area.

[0045] FIG. 9 is a flowchart of the process for estimating the internal state of the cell 61. The estimation process is composed of five steps, S10 to S50. The estimation process in FIG. 9 can be executed at any timing while the energy storage system 10A is in operation. It is desirable to execute the estimation process when the storage battery system 30 is not charging or discharging DC current through the power line L. Alternatively, the estimation process may be executed while the storage battery system 30 is charging or discharging DC current.

[0046] <S10 (Application of AC signal)> When the control unit of the battery system 30 (in this example, the BMU 55 of the power storage bank 50A) detects the start of the estimation process, it instructs the PCS 20 to output an AC signal and further notifies the BMU 55 of each power storage bank 50A, 50B of the start of the estimation process.

[0047] Upon receiving the instruction, the PCS 20 uses the bidirectional inverter 21 to simultaneously apply AC signals to each of the power storage banks 50A, 50B of the battery system 30 via the power line L for DC charging and discharging. The frequency of the AC signals is not limited, but is preferably in a frequency band within the diffusion region. AC signals in the frequency band within the diffusion region can be generated relatively easily by the bidirectional inverter 21 of the PCS 20. The AC signals may have either a sine wave or a square wave waveform. If the AC signal is a square wave, it is not necessary to control the AC signal at a fine level, making it easier to control the bidirectional inverter 21.

[0048] In this embodiment, the bidirectional inverter 21 applies, as an AC signal, a square-wave current signal included in the frequency band of the diffusion region to the storage battery system 30, as shown in FIG. 10A.

[0049] The number of cycles of the AC signal to be applied may be one cycle, since it takes time for the state of the cell 61 to stabilize immediately after application, but it is preferable that the number of cycles is several (two or more).

[0050] Furthermore, if the amplitude of the applied AC signal is large, the amplitude of the response will also be large, improving measurement accuracy. On the other hand, if the amplitude is too large, the influence of heat generated by current flow will be more pronounced. The C rate of the current signal is preferably in the range of 0.05 to 0.3 [CA]. The range of 0.05 to 0.3 includes 0.05 and 0.3. The C rate of the current that can be output by the bidirectional inverter is, for example, ±1 CA.

[0051] <S20 (Measurement of Response)> While the PCS 20 is applying the AC signal, the following measurements are performed for each of the power storage banks 50A and 50B. The BMU 55 measures the AC signal (in this example, a current signal) applied to the power storage banks 50A and 50B using the current sensor 51, and stores the measurement result in the storage unit 57. Furthermore, as shown in FIG. 10B , the CMU 65 measures the voltage of each cell 61 as a response of each cell 61 to the AC signal, and stores the measurement result in the storage unit 67.

[0052] When an AC signal is applied from the PCS 20 for several cycles, it is desirable to measure data from the second cycle onwards without using data from the first cycle.

[0053] <S30 (Calculation of AC Impedance)> Subsequently, the time series data of the current and voltage measured by the BMU 55 and CMU 65 are subjected to a discrete Fourier transform.

[0054] Specifically, the Fourier coefficients Va(f) and Vb(f) of the voltage and the Fourier coefficients Ia(f) and Ib(f) of the current are calculated using the following equations.

[0055]

[0056] where f is the frequency to be extracted (specifically, the frequency band of the diffusion region), n is the number of voltage and current measurement points, t is the measurement time of the voltage and current, V(t) is the time series data of the voltage, and I(t) is the time series data of the current.

[0057] Thereafter, each CMU 65 acquires necessary data from the BMU 55 and calculates the AC impedance Z(f) of each cell 61 using the following equation: This gives the AC impedance Z(f) of the frequency band included in the diffusion region.

[0058]

[0059] R(f) is the real part of Z(f), and X(f) is the imaginary part of Z(f). Basically, AC impedance Z(f) is calculated from time series data when an AC signal of frequency f is applied.

[0060] <S40 (Calculation of -dX / dR)> The CMU 65 calculates -dX / dR for each cell 61 from the AC impedance Z(f) of the diffusion region calculated in S30.

[0061] <S50 (Estimation of Internal State)> In this embodiment, the relationship between −dX / dR in the diffusion region and the capacity maintenance rate is determined by a preliminary test, and the data is stored as a lookup table in the memory unit 67 of the CMU 65 (FIG. 11).

[0062] Therefore, the CMU 65 can estimate the capacity maintenance rate [%] of each cell 61 by referring to the lookup table shown in FIG. 11 for the −dX / dR of the diffusion region calculated in S40.

[0063] The capacity maintenance rate is the ratio of the initial battery capacity C0 to the current battery capacity Ct, and is expressed as follows: The capacity maintenance rate is also referred to as SOH (State of Health).

[0064] Capacity maintenance rate = (Ct / C0) x 100

[0065] If the characteristics of the diffusion region of AC impedance change depending on the temperature or SOC, correlation data (lookup table) between the diffusion region's -dX / dR and the capacity maintenance rate for each temperature and SOC can be acquired (e.g., stored in the storage unit 67) and the correlation data can be selected and used according to the temperature and SOC. Improved accuracy can be expected by performing this estimation in an SOC region where the dV / dQ characteristics of the cell 61 have a slope. V represents voltage [V], and Q represents quantity of electricity [Ah]. This is because the region where the SOC-OCV characteristics of the cell 61 have a slope is characterized by a large increase in dV / dQ (change in V per unit quantity of electricity Q) associated with the degradation of the cell 61 (see FIG. 8 ).

[0066] 3. Explanation of Effects The energy storage system 10A can apply an AC signal to the cell 61 using the PCS 20 and measure the AC impedance Z of the cell 61 based on the response, without providing an AC signal generator dedicated to measurement.

[0067] The bidirectional inverter 21 is a power conversion device installed in the main circuit (power line) and has a larger power capacity than an AC signal generator dedicated to measurement. Therefore, by using the PCS 20, the AC impedance Z of the cell 61 can be measured with an AC signal at a level suitable for measurement, resulting in high measurement accuracy of the AC impedance Z.

[0068] <Other Embodiments> The present invention is not limited to the embodiments described above and illustrated in the drawings, and the following embodiments, for example, are also included within the technical scope of the present invention.

[0069] (1) In the above embodiment, in S10 of the estimation process, an AC signal is applied to each cell 61 using the PCS 20. When a DC / DC converter 70 is provided in each power storage bank 50A, 50B, as in the energy storage system 10B shown in FIG. 12 , an AC signal may be applied to each cell 61 of the power storage bank 50A, 50B using the DC / DC converter 70. The DC / DC converter 70 is an example of a power conversion device provided on the main circuit (on the power line L). In FIGS. 1 and 12 , thick lines indicate the power line L, and thin lines indicate signal lines. In the above embodiment, the C rate of the current signal output by the power conversion device is set to a range of 0.05 to 0.3 CA, but the C rate may be a value outside that range.

[0070] (2) In the above embodiment, the C rate of the current signal output by the power conversion device is set to a range of 0.05 to 0.3 CA. However, the C rate may be set to a value outside this range.

[0071] (3) In the above embodiment, the AC impedance of the cell 61 is measured by the CMU 65. However, the AC impedance of the cell 61 may be measured by the BMU 55. The CMU 65 may be eliminated, and the functions of acquiring the AC impedance and estimating the capacity maintenance rate may be integrated into the BMU 55. The BMU 55 and / or the CMU 65 are "measurement units that measure the AC impedance of the cell based on the response to an AC signal."

[0072] (4) In the above embodiment, the capacity retention rate and battery capacity of the cell 61 are estimated based on the AC impedance characteristics of the cell 61. Instead of the capacity retention rate and battery capacity, the internal resistance of the cell 61 may be estimated. The capacity retention rate, battery capacity, and internal resistance are examples of the internal state of the cell 61. The purpose of measuring the AC impedance is not limited to estimating the internal state of the cell 61, and may be another purpose.

[0073] (5) In the above embodiment, the energy storage systems 10A and 10B are shown as examples of power storage equipment. However, the present technology may be applied to other power storage equipment such as a UPS, a DC power supply, or a PCS with a storage battery.

[0074] 10A, 10B Energy storage system (storage equipment) 21 Bidirectional inverter (power conversion device) 30 Battery storage system 50A, 50B Storage bank 51 Current sensor 55 BMU 60 Storage module 61 Cell 65 CMU 70 DC / DC converter (power conversion device) L Power line (main circuit)

Claims

1. An energy storage facility comprising: a power conversion device provided in a main circuit; and one or more energy storage banks connected to the power conversion device via the main circuit, wherein the energy storage banks have a plurality of cells connected in series, and wherein the energy storage facility applies an AC signal to the cells of the energy storage bank by the power conversion device, and measures the AC impedance of the cells based on the response.

2. The power storage facility according to claim 1, wherein the AC impedance of the cell is measured based on the response of the AC signal applied to the cell by the power conversion device from the second cycle onwards.

3. The power storage facility according to claim 1 or 2, wherein the AC signal is a current signal having a C rate in the range of 0.05 to 0.3 CA.

4. The power storage facility according to claim 1 or 2, wherein the AC signal is a square wave signal.

5. The power storage facility according to claim 1 or 2, wherein the power conversion device applies the AC signal to a plurality of the power storage banks connected in parallel to the main circuit.

6. A method for measuring AC impedance of a power storage facility including a power conversion device provided in a main circuit and one or more power storage banks connected to the power conversion device via the main circuit and having a plurality of cells connected in series, the method comprising: applying an AC signal to the cells of the power storage bank using the power conversion device; and measuring the AC impedance of the cells based on their responses.

Images