Operation device and operation method for water electrolysis system

Abstract

To solve the problem in which it was not possible to formulate operating conditions considering safety and economic efficiency according to a degradation state of a water electrolysis stack.SOLUTION: An operation device 10 for a water electrolysis system 1 has: a water electrolysis stack 21; a DC power supply 22 for supplying DC power to the water electrolysis stack 21; a current monitoring device 24 for measuring the current flowing through the water electrolysis stack; and a voltage monitoring device 23 for measuring a voltage applied to the water electrolysis stack 21. The operation device further comprises: an arithmetic unit 13 which estimates electrical resistance of the water electrolysis stack 21 which varies according to a degradation state by using a time-series current measurement value and a voltage measurement value, compares the estimated electrical resistance with a threshold value to determine degradation state of the water electrolysis stack 21, and defines the operating conditions of the water electrolysis stack 21 according to the degradation state of the water electrolysis stack 21; and a display control unit 11 which outputs the operating conditions of the water electrolysis stack 21 defined by the arithmetic unit 13 to a display device 12.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to an operation device for a water electrolysis system and an operation method for a water electrolysis system. [Background technology]

[0002] In recent years, with the increase in emissions of greenhouse gases such as carbon dioxide, environmental issues such as global warming and energy issues such as the depletion of oil resources have been attracting attention. From this perspective, hydrogen energy has been attracting attention as a clean alternative energy source. However, in order to replace existing fossil fuels with hydrogen energy, a large amount of hydrogen is required.

[0003] Water electrolysis is one method of producing hydrogen. Demonstration projects for large-scale water electrolysis are underway in an effort to curb global warming. In particular, a demonstration project for a hydrogen production system with a capacity of several tens of megawatts (MW) that uses electricity generated by offshore wind power is underway in Europe, and it is expected that water electrolysis systems will continue to grow in size in the future. When producing MW-class hydrogen using water electrolysis systems, the water electrolysis stack (electrolyzer) is characterized by its low voltage and high current characteristics of several hundred volts and several thousand amperes.

[0004] It is known that when the water electrolysis stack is shut down, the catalyst applied to the oxygen generating electrode is oxidized and reduced, accelerating electrode deterioration. Furthermore, in a multi-series / parallel water electrolysis system, the discharge of highly oxidized oxides on the electrodes can cause a large reverse current to flow through the water electrolysis stack, further accelerating electrode deterioration. Furthermore, economic losses occur when hydrogen cannot be produced while the water electrolysis stack is shut down.

[0005] On the other hand, the longer a water electrolysis stack is operated, the higher the overvoltage associated with the reaction becomes. Therefore, to produce hydrogen safely and inexpensively, it is necessary to formulate an operation plan for the water electrolysis equipment according to changes in the electrochemical characteristics. Conventionally, the operation sequence of the water electrolysis equipment is determined by measuring the voltage of the water electrolysis stack during steady-state operation and comparing it with the voltage rise of an undegraded water electrolysis stack under the same conditions.

[0006] For example, Patent Document 1 discloses a water electrolysis system that focuses on the operating conditions (voltage, current, temperature) of a water electrolytic cell, and updates a performance map, which is either a voltage map relative to current and temperature or a current map relative to voltage and temperature, based on measured current, voltage, and temperature values ​​of the water electrolytic cell to operate the water electrolytic cell. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 2023-128750 Summary of the Invention [Problem to be solved by the invention]

[0008] The main factors related to the voltage increase in the water electrolysis stack are (1) an increase in electrical resistance due to a decrease in the conductivity of the electrolyte membrane, and (2) an increase in reaction overvoltage due to catalyst detachment and a decrease in specific surface area.

[0009] The former (1) may be due to an increase in electrical resistance caused by damage such as pinholes in the electrolyte membrane. An increase in electrical resistance due to a decrease in the conductivity of the electrolyte membrane can lead to the mixing of hydrogen and oxygen due to damage to the electrolyte membrane, leading to the shutdown of the water electrolysis device. As such, depending on the extent of the increase in electrical resistance, there is a risk of serious events such as the shutdown of the water electrolysis device, so the operating conditions of the water electrolysis stack must be carefully determined. On the other hand, the latter (2) poses a low risk of leading to a serious event. The increase in reaction overvoltage due to catalyst detachment and a decrease in specific surface area is caused by catalyst deterioration and does not lead to electrolyte membrane damage. Therefore, even if there is an increase in reaction overvoltage, there is a low risk of it leading to a serious event.

[0010] In the water electrolysis system described in Patent Document 1, the state of the water electrolyzer is diagnosed based on the voltage, current, and temperature in a steady state, and the water electrolyzer is operated accordingly. Because the water electrolysis system described in Patent Document 1 diagnoses the state based on measurement results in a steady state, it does not identify the cause of the overvoltage increase, making it difficult to formulate an appropriate operation plan based on the cause of the overvoltage. For example, if an attempt is made to increase the temperature of the water electrolyzer to improve electrolysis efficiency, with an emphasis on economy, when electrical resistance is increasing due to a decrease in the conductivity of the electrolyte membrane, this could damage the electrolyte membrane and cause hydrogen and oxygen to mix.

[0011] In general, the larger the water electrolysis stack, the greater the amount of heat generated by overvoltage, and the temperature increases as the current density increases. Therefore, it is considered difficult to obtain a current density-voltage curve of the water electrolysis stack at a constant temperature in the water electrolysis system described in Patent Document 1.

[0012] Furthermore, Patent Document 1 does not specifically describe how to operate individual water electrolyzers based on the results of the diagnosis. In other words, Patent Document 1 does not disclose how to determine operating conditions that take safety and economy into consideration depending on the deterioration state of the water electrolysis stack.

[0013] The present invention has been made in view of the above circumstances, and an object of the present invention is to determine operating conditions that take safety and economy into consideration in accordance with the deterioration state of a water electrolysis stack. [Means for solving the problem]

[0014] To achieve the above object, one aspect of the present invention provides an operation device for a water electrolysis system including a water electrolysis stack, a DC power supply that supplies DC power to the water electrolysis stack to drive the water electrolysis stack, a current monitoring device that measures a current flowing through the water electrolysis stack, and a voltage monitoring device that measures a voltage applied to the water electrolysis stack. The operation device also includes a calculation unit that receives current measurements from the current monitoring device and voltage measurements from the voltage monitoring device, estimates the electrical resistance of the water electrolysis stack, which changes depending on the degradation state, using the time-series current and voltage measurements, compares the estimated electrical resistance with a threshold value for determining whether the electrical resistance is increasing, and determines the degradation state of the water electrolysis stack, and determines operation conditions for the water electrolysis stack depending on the degradation state of the water electrolysis stack, and a display control unit that outputs the operation conditions for the water electrolysis stack determined by the calculation unit to a display device. [Effects of the Invention]

[0015] According to at least one aspect of the present invention, it is possible to realize economical and highly efficient hydrogen production while ensuring safety in accordance with the deterioration state of the water electrolysis stack. Problems, configurations, and effects other than those described above will become apparent from the following description of the preferred embodiments of the invention. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a diagram showing a schematic configuration of a water electrolysis system according to a first embodiment of the present invention. [Figure 2] FIG. 1 is a diagram illustrating an example of the hardware configuration of each device constituting a water electrolysis system according to a first embodiment of the present invention. [Figure 3] FIG. 1 is a diagram showing an equivalent circuit of a water electrolysis stack according to a first embodiment of the present invention. [Figure 4] 4 is a flowchart showing an example of the procedure for formulating operating conditions performed by the operation device for the water electrolysis system according to the first embodiment of the present invention. [Figure 5]5 is a flowchart illustrating an example of a procedure for calculating the electrical resistance of the water electrolysis stack performed by the operation device of the water electrolysis system according to the first embodiment of the present invention. [Figure 6] FIG. 3 is a diagram showing an example of voltage measurement results and current measurement results (current density) of the water electrolysis system according to the first embodiment of the present invention. [Figure 7] FIG. 4 is a diagram showing an example of extracted results of voltage measurement and current measurement in a specified period of the water electrolysis stack in the first embodiment of the present invention. [Figure 8] FIG. 4 is a diagram showing an example of an electric resistance estimated from the measurement results of current and voltage during load fluctuation in the water electrolysis system according to the first embodiment of the present invention, and an example of a deterioration determination threshold value for the electric resistance. [Figure 9] FIG. 3 is a diagram showing an example of voltage measurement results (changes in voltage over time) of the water electrolysis stack in the first embodiment of the present invention. [Figure 10] 10 is a flowchart showing another example of the procedure for calculating the electrical resistance of the water electrolysis stack performed by the operation device of the water electrolysis system according to the first embodiment of the present invention. [Figure 11] FIG. 3 is a graph showing an example of changes over time in current density and voltage in the water electrolysis system according to the first embodiment of the present invention. [Figure 12] 10 is a flowchart showing an example of the procedure for formulating operating conditions performed by an operation device for a water electrolysis system according to a second embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0017] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, examples of modes for carrying out the present invention (hereinafter referred to as "embodiments") will be described with reference to the accompanying drawings. In this specification and the accompanying drawings, identical or similar components are given the same reference numerals, and redundant explanations may be omitted or only explanations focusing on the differences may be given. Furthermore, when there are multiple identical or similar components, they may be described using the same reference numerals with different subscripts. Note that when it is not necessary to distinguish between these multiple components, the subscripts may be omitted in the description. The number of each component may be singular or plural unless otherwise specified.

[0018] First Embodiment First, a water electrolysis system according to a first embodiment of the present invention will be described with reference to Figures 1 to 11. The water electrolysis system according to this embodiment is a multi-connection water electrolysis system configured by connecting a plurality of water electrolysis stacks.

[0019] [Outline of water electrolysis system] FIG. 1 is a diagram showing a schematic configuration of a water electrolysis system according to a first embodiment of the present invention. The water electrolysis system 1 shown in Fig. 1 includes an operation device 10 and a water electrolysis device 20. The operation device 10 controls and operates the water electrolysis device 20, and is mainly composed of a display control device 11, a display device 12, and a computing device 13. The water electrolysis device 20 includes multiple water electrolysis stacks 21, a DC power supply 22, multiple voltage monitoring devices 23, and multiple current monitoring devices 24.

[0020] The water electrolysis device 20 is equipped with a plurality of water electrolysis stacks 21 that electrolyze water. As an example, each water electrolysis stack 21 has a structure in which a plurality of thin components (water electrolysis cells) that electrolyze water to produce hydrogen and oxygen are stacked. A DC power supply 22 and each water electrolysis stack 21 are electrically connected in series and parallel by power cables. The DC power supply 22 is configured to supply DC power to each water electrolysis stack 21.

[0021] 1 shows a configuration in which all of the water electrolysis stacks 21 are connected in series and parallel to the DC power supply 22, but the water electrolysis stacks 21 may be connected in parallel, in series, or independently. A configuration in which the water electrolysis stacks 21 are connected independently means that the two-dimensional arrangement of water electrolysis stacks 21 is replaced with a single stack. The number of water electrolysis stacks 21 constituting the water electrolysis apparatus 20 is not limited to multiple stacks, and may be one.

[0022] The display control device 11 (an example of a display control unit) will be described. The display control device 11 has a function of issuing a current change command to the DC power supply 22 to vary the current output, and a function of issuing a start command to the voltage monitoring device 23 and the current monitoring device 24 to start measurement. The display control device 11 also has a notification function (output processing function) that receives calculation results from the calculation device 13 and notifies the driver of the calculation results. As an example of the notification function, the display device 12 displays an electric resistance and operating conditions (plan), which will be described later, on its display screen. The display control device 11 also has an input processing function that generates an input signal in response to the driver's operation of an input device 35 (see FIG. 2, which will be described later). The display control device 11 may be configured integrally with the display device 12, like a notebook PC (personal computer) or a tablet terminal.

[0023] The current change command may be either a control to increase or a control to decrease the output current of the DC power supply 22. The control to increase or decrease the output current of the water electrolysis device 20 and the amount of current change may be considered to be predetermined. However, although it depends on the capacity of the water electrolysis stack 21, when the current density of the current passing through each water electrolysis stack 21 is 0.5 A / cm 2 ~2A / cm 2 It is preferable that the range does not exceed this.

[0024] In an experiment using the water electrolysis stack 21 according to this embodiment, 2 If the current density is less than 2 A / cm, the water electrolysis stack 21 will deteriorate. 2At the above current densities, the water electrolysis stack 21 deteriorated rapidly and the electrolysis efficiency was low. However, this numerical range of current density is an example. The above notification is performed by displaying a warning or the like on the display screen of the display device 12 and ending the deterioration determination, but the notification may also be provided by an alarm sound or the like.

[0025] The display control device 11 and the arithmetic device 13 can be configured using, for example, a microcontroller or a PC.

[0026] Next, the water electrolysis stack 21 will be described. The water electrolysis stack 21 is, for example, a device made by stacking water electrolysis cells (not shown) each including a proton-permeable membrane, and is used to produce hydrogen and oxygen using water as a raw material. A preferred example of the water electrolysis stack 21 is a solid polymer electrolyte membrane (PEM)-type water electrolysis stack.

[0027] Next, the DC power supply 22 will be described. The DC power supply 22 is a device capable of supplying DC current to each of the multiple water electrolysis stacks 21. In this embodiment, the average current density is calculated as an index of the DC current. The average current density is the average value of the current density of the DC current output by each water electrolysis stack 21 in the water electrolysis system 1. The DC power supply 22 has a function of changing the output current in response to a current change command from the display control device 11. Multiple DC power supplies may be provided. For example, in FIG. 1 , a DC power supply (three in total) may be provided for each of three series lines in which multiple water electrolysis stacks 21 are connected in series. Alternatively, one DC power supply may be provided for one or more water electrolysis stacks 21.

[0028] Next, the voltage monitoring device 23 and the current monitoring device 24 will be described. The voltage monitoring device 23 is a device that measures the voltage applied to each water electrolysis stack 21. The current monitoring device 24 is a device that measures the current passing through each water electrolysis stack 21. The voltage monitoring device 23 and the current monitoring device 24 have the function of measuring the voltage value and the current value at predetermined time intervals Δt (see FIG. 7, which will be described later). The predetermined time interval Δt is preferably 4 ms or less. However, this value is based on the experimental results of a relatively small water electrolysis system and may be changed depending on the time constant of the water electrolysis stack.

[0029] The voltage monitoring device 23 and the current monitoring device 24 have a function of receiving a measurement command to start measurement from the display control device 11, and measuring the voltage applied to and the current passing through each water electrolysis stack 21 at the above-mentioned predetermined time intervals. The voltage monitoring device 23 and the current monitoring device 24 also have a transfer function of ending the measurement a predetermined time after the start of measurement, and transferring all voltage and current measurement results from the start to the end of measurement to the calculation device 13.

[0030] Next, the calculation device 13 (an example of a calculation unit) will be described. The calculation device 13 has a calculation function that receives voltage and current measurement results from the voltage monitoring device 23 and the current monitoring device 24 in chronological order and calculates the reaction electric resistance R0 and reaction capacitance C1 of the water electrolysis stack 21, which will be described later. The calculation device 13 also has a function that determines the deterioration state of the water electrolysis stack 21 based on the reaction electric resistance R0 and / or the reaction capacitance C1. Furthermore, the calculation device 13 searches for a change point in voltage and current from the voltage and current measurement results. The change point is the time when the output of the DC power supply 22 changes. This time is defined as 0 s. The calculation device 13 also has a function that formulates operating conditions (plans) based on the calculation result of the electric resistance R0. The reaction electric resistance R0 and reaction capacitance C1 will be described in detail below.

[0031] [Hardware configuration of each device that makes up the water electrolysis system] Next, the hardware configuration of each device constituting the water electrolysis system 1 will be described with reference to Fig. 2. Here, an example of the hardware configuration of the display control device 11 and the computer included in the arithmetic device 13 will be described.

[0032] FIG. 2 is a diagram showing an example of the hardware configuration of the computers provided in the display control device 11 and the arithmetic device 13. As shown in FIG. 2, each block may be selected according to the function and purpose of each device. The computer 30 may be, for example, a personal computer or a microcontroller.

[0033] The computer 30 includes a CPU (Central Processing Unit) 31, a ROM (Read Only Memory) 32, a RAM (Random Access Memory) 33, a display device 12, an input device 35, non-volatile storage 36, and a communication interface 37. The components within the computer 30 are connected via a system bus so that they can send and receive data to and from each other.

[0034] The CPU 31, ROM 32, RAM 33, and non-volatile storage 36 constitute a control unit. This control unit is used as an example of a computer that controls the operation of the display control device 11 or the arithmetic unit 13. The CPU 31 reads out software programs that realize the functions of each device from the ROM 32, and loads the programs into the RAM 33 for execution.

[0035] The ROM 32 is used as an example of a non-volatile memory (recording medium). The ROM 32 stores an OS (Operating System), various parameters, programs for operating each device, etc. The RAM 33 is used as an example of a volatile memory. Variables, parameters, etc. generated during the arithmetic processing of the CPU 31 are temporarily written to the RAM 33. Instead of the CPU 31, another processor such as an MPU (Micro Processing Unit) may be used as the arithmetic processing device.

[0036] The display device 12 is a monitor such as a liquid crystal display, and displays a GUI screen, the results of processing performed by the CPU 31, etc. The input device 35 generates an input signal in response to an operation by the driver or the like, and outputs the signal to the CPU 31. The input device 35 may be, for example, a mouse or a keyboard, and the driver or the like can operate the input device 35 to input information and instructions. The display device 12 and the input device 35 may be integrated into a touch panel. Note that the display device 12 and the input device 35 may be omitted from the arithmetic device 13.

[0037] The nonvolatile storage 36 is an example of a recording medium and is capable of storing data used by programs and data obtained by executing the programs. For example, the nonvolatile storage 36 stores information related to the operation and measurements of the water electrolysis system 1. The nonvolatile storage 36 may also store an OS or programs executed by the CPU 31. Examples of the nonvolatile storage 36 include a hard disk drive (HDD), a solid state drive (SSD), an optical or magnetic disk medium, and a semiconductor memory card. The programs may be provided to each device via a wired or wireless transmission medium such as a local area network (LAN), the Internet, or digital satellite broadcasting.

[0038] The communication interface 37 may be, for example, a network interface card (NIC). The communication interface 37 is configured to be able to transmit and receive various data to and from an external device via a communication network such as a LAN or the Internet to which a terminal is connected, or via a dedicated line. The communication means may be either wireless communication such as Wi-Fi (registered trademark) or LTE (Long Term Evolution) (registered trademark), or wired communication.

[0039] [Equivalent circuit of water electrolysis stack] Next, an equivalent circuit of the water electrolysis stack 21 will be described with reference to FIG. FIG. 3 is a diagram showing an example of an equivalent circuit of the water electrolysis stack 21. It is generally known that the equivalent circuit of the water electrolysis stack 21 alone can be expressed as a series-parallel circuit of three parameters: electrical resistance R0, reaction electrical resistance (hereinafter also referred to as "reaction resistance") R1, and reaction capacitance C1.

[0040] For example, a PEM water electrolysis stack mainly comprises an electrolyte membrane, electrodes arranged to sandwich the electrolyte membrane, and a catalyst layer interposed between the electrolyte membrane and the electrodes. The electrodes include an electrode that generates oxygen and hydrogen ions from water and an electrode that generates hydrogen from the hydrogen ions.

[0041] The electrical resistance R0 is the total resistance loss of the water electrolysis stack 21 based on the structure of the water electrolysis stack 21 (resistances of the electrolyte membrane, support material, electrodes, contact resistance, etc.). The reaction resistance R1 is the resistance loss at the interfaces between the electrolyte membrane and the two electrodes. The reaction capacitance C1 is the double layer capacitance at the interfaces between the electrolyte membrane and the two electrodes. The reaction resistance R1 and reaction capacitance C1 are indicators that reflect the state of the water electrolysis stack 21 and change depending on the deterioration state of the water electrolysis stack 21.

[0042] In this embodiment, a PEM water electrolysis stack will be described as an example, but the present invention is not limited to this example. The present invention can be applied to water electrolysis stacks of other types by applying the electrical resistance R0 of the entire water electrolysis stack, the reaction resistance R1 that changes depending on the deterioration state, and the reaction capacitance C1 to water electrolysis stacks of other types.

[0043] Changes in the parameters of the equivalent circuit cause fluctuations in the current and / or voltage during load fluctuations. Conversely, measuring the changes over time in the current and voltage during load fluctuations makes it possible to estimate the parameters of the equivalent circuit. In other words, the deterioration of the water electrolysis stack 21 can be estimated from changes in the parameters of the equivalent circuit. According to the equivalent circuit in Figure 3, the change dE in the voltage applied to the water electrolysis stack 21 is the sum of the change in voltage applied to the electrical resistance R0 and the change in voltage applied to the reaction capacitance C1 (Equation (1)).

[0044]

number

[0045] At this time, the amounts of change dE, dI, and dq of the voltage, current, and charge can be expressed by equations (2), (3), and (4), respectively, where τ is time.

[0046]

number

number

number

[0047] As shown in the following equation (5), in a short time after the current is changed from the steady state, the current I passing through the reaction resistance R1 R1 It is assumed that the value of is almost the same as in the steady state. R1 The value of is the current I passing through the reactive capacitance C1. C1 The change is small compared to the value of . Conversely, the reaction capacitance C1 causes a small change in voltage compared to the current.

number

[0048] The current I passing through the reactive capacitance C1 C1 is expressed by the following equation (6).

number

[0049] The integral over Δt is approximated to a trapezoid, and the charge dq stored in the reaction capacitance C1 is expressed by the following equation (7).

number

[0050] Therefore, the changes in voltage, current, and charge, dE, dI, and dq, can be calculated from the current and voltage measurement results. Based on the current and voltage measurement results from time (t-Δt) to time t, the estimation error μ(t-Δt, t) of the electrical resistance R0 and reaction capacitance C1 can be expressed by the following equation (8). Equation (8) shows the estimation error of the electrical resistance R0 and reaction capacitance C1 during Δt.

number

[0051] The estimation error μ(0,t) of the electrical resistance R0 and reaction capacitance C1 from the current measurement results and voltage measurement results from 0[s] to t[s] is expressed by the following equation (9). In other words, the estimation error μ(0,t) is the root mean square error between multiple Δt between 0[s] and t[s]. In the equation, i is a natural number (i=1, 2, ..., n).

number

[0052] Equation (9) is a function of electrical resistance R0 and reaction capacitance C1. By finding the minimum value of the estimation error μ(0,t) in equation (9), electrical resistance R0 and reaction capacitance C1 can be calculated. The electrical resistance R0 and reaction capacitance C1 when the estimation error μ(0,t) is at its minimum are expressed by equations (10) and (11). By partially differentiating equation (9) with respect to R0 and C1, the equations dμ / dR0=0 and dμ / dC1=0 are obtained. Solving R0 and C1 from these two equations gives equations (10) and (11).

[0053]

number

number

[0054] In one example (hereinafter referred to as "first example") of a method for estimating the electrical resistance using the above-described formulas (1) to (11), the arithmetic device 13 estimates the electrical resistance R0 (element of the equivalent circuit) of the water electrolysis stack 21, which changes depending on the degradation state, from time-series voltage measurement values ​​and current measurement values ​​received from the voltage monitoring device 23 and the current monitoring device 24. The arithmetic device 13 then determines the degradation state of the water electrolysis stack 21 based on the estimated result of the electrical resistance R0, and outputs the determination result.

[0055] By adopting such a configuration, during operation of the water electrolysis system 1 (e.g., a large-scale hydrogen production device), it is possible to quantitatively estimate the electrical resistance of the water electrolysis stack 21 and determine the deterioration state of the water electrolysis stack 21 based on the estimated electrical resistance without suddenly changing the load or stopping the water electrolysis stack 21. Then, as will be described later, it is possible to determine the operating conditions of the water electrolysis device 20 based on the deterioration state of the water electrolysis stack 21.

[0056] In the present embodiment, the DC power supply 22 changes the DC power from a steady state and supplies the DC power to the water electrolysis stack 21. The arithmetic device 13 acquires current measurement values ​​and voltage measurement values ​​measured at a predetermined time interval Δt after the DC power of the DC power supply 22 changes from the steady state, and performs a process of estimating the electric resistance R0 of the water electrolysis stack 21 at an arbitrary time t using the difference between the current measurement values ​​and the difference between the voltage measurement values ​​at the predetermined time interval Δt.

[0057] According to this embodiment configured as described above, the electrical resistance R0 (and the reaction capacitance C1) is calculated taking into account the predetermined time interval Δt, so it is possible to determine the degradation state of the water electrolysis stack 21 even if there is only a small fluctuation in the current and voltage of the water electrolysis stack 21. When determining the degradation state of the water electrolysis stack 21, the fluctuation range of the output current of the DC power supply 22 can be small. Furthermore, because the current measurement value measured at the predetermined time interval Δt is used, it is possible to deal with changes in the current over time that follow various curves, and it is possible to determine the degradation state of the water electrolysis stack 21.

[0058] [Operational condition formulation process] Next, the operation condition formulation process performed by the operation device 10 of the water electrolysis system 1 will be described with reference to FIG. FIG. 4 is a flowchart showing an example of the procedure for formulating operating conditions by the operation device 10 of the water electrolysis system 1. First, using an example (first example) of a method for estimating electrical resistance using the above-mentioned equations (1) to (11), the calculation device 13 analyzes the measurement results of the voltage monitoring device 23 and the measurement results of the current monitoring device 24 to estimate the electrical resistance R0 of the water electrolysis stack 21 (step S1).

[0059] [Example of electrical resistance calculation process] Here, an example of the electric resistance calculation process using an example (first example) of a method for estimating the electric resistance R0 of the water electrolysis stack 21 in step S1 will be described with reference to FIG. FIG. 5 is a diagram showing an example of the procedure for calculating the electric resistance R0 of the water electrolysis stack 21 by the operation device 10 of the water electrolysis system 1. First, the operation screen (display control device 11) of the display device 12 receives a start command from the operator and issues measurement commands to the voltage monitoring devices 23 and current monitoring devices 24 of the individual water electrolysis stacks 21 (step S11).

[0060] After issuing the measurement command, the display control device 11 issues a current change command to the DC power supply 22 to change the output (e.g., current density) of the DC power supply 22 (step S12). After a predetermined time has elapsed from the time (t=0) when the output of the DC power supply 22 changed, the voltage monitoring device 23 and the current monitoring device 24 finish measurement and transfer the voltage and current measurement results to the calculation device 13. The display control device 11 generates a display screen including information such as the output of the DC power supply 22, and the voltage measurement results and current measurement results for each water electrolysis stack 21, and displays the screen on the display device 12.

[0061] (How to find the change point) 6 is a diagram showing an example of voltage measurement results and current measurement results (current density) of the water electrolysis system 1. In FIG. 6, the vertical axis on the left represents current density (A / cm 2 ), the vertical axis on the right indicates voltage (V) and the horizontal axis indicates time (s).

[0062] In step S12, the arithmetic unit 13 determines the change point from the measurement results of the voltage and current, as shown in Fig. 6. There is a difference between the time when the command is issued from the display control unit 11 and the time when the DC power supply 22 receives the command and changes the output of the DC power supply 22, so it is necessary to determine the time when the output changes.

[0063] A typical method for determining a change point is to determine the upper and lower limits of the current and voltage based on the measurement results for a certain period of time before the change point, taking noise into consideration, and then set the latest time at which these upper and lower limits are not exceeded as 0 s (the change point). This is because the current and voltage outputs (measurement results) of the voltage monitoring device 23 and the current monitoring device 24 contain noise, and the current and voltage fluctuate within a certain range even in steady state. In the example shown in Figure 6, the voltage measurement result 51 and the current measurement result 52 start to change in the voltage and current values ​​at the change point. However, the method for determining a change point is not limited to the above method.

[0064] Next, the arithmetic unit 13 extracts the measurement results of the voltage and current from the change point (t=0) to the specified time. In this embodiment, the specified time is set to 10 seconds as an example, but the length of the specified time is not limited to this.

[0065] (Voltage measurement results and current measurement results) FIG. 7 is a diagram showing an example of extracted results of voltage measurement and current measurement (current density) for a specified period of time of the water electrolysis stack 21. In FIG. 7, the vertical axis on the left side represents current density (A / cm 2 7, the vertical axis on the right side represents voltage (V) and the horizontal axis represents time (s). In the example shown in Fig. 7, a voltage measurement result 61 and a current measurement result 62 are shown.

[0066] After the process of step S12, the calculation device 13 estimates the electrical resistance R0 and reaction capacitance C1 of each water electrolysis stack 21 by calculating the extracted results of the voltage and current of each water electrolysis stack 21 using the above-mentioned equations (10) and (11) (step S13). After the process of step S13, the process proceeds to step S2 in FIG. 4.

[0067] Returning to the description of the flowchart in FIG. 4 , after the processing of step S1, the arithmetic device 13 compares the electric resistance R0 estimated in step S1 with a first threshold value and determines whether the electric resistance R0 has exceeded the first threshold value (step S2). The first threshold value is a threshold value for determining whether the electric resistance R0 has increased. For example, the first threshold value is a reference value of the electric resistance when the difference between the operating time of the water electrolysis stack 21 in a rated state and the rated life reaches a certain standard (e.g., 1000 hours). FIG. 8 shows examples of the electric resistance R0 estimated from the measurement results of the current and voltage during load fluctuations in the water electrolysis system 1 and the threshold value used to determine the electric resistance R0. FIG. 8 shows an example in which the estimated electric resistance R0 exceeds the threshold value. The first threshold value and a second threshold value (described later) are determined in advance by experiments, simulations, or the like, and stored in the ROM 32 or the non-volatile storage 36.

[0068] Comparing the electrical resistance R0 with the first threshold value or the second threshold value described below corresponds to diagnosing the deterioration state of the water electrolysis stack 21. If the electrical resistance R0 is greater than the first threshold value, it is considered that the deterioration of the water electrolysis stack 21 has progressed to a certain level. If the electrical resistance R0 is equal to or less than the first threshold value (NO determination in step S2), the computing device 13 proceeds to step S3, and if the electrical resistance R0 exceeds the first threshold value (YES determination in step S2), the computing device 13 proceeds to step S4.

[0069] If the determination in step S2 is NO, the computing device 13 operates the water electrolysis stack 21 under rated conditions (step S3). Because the electrical resistance R0 of the water electrolysis stack 21 has not reached the rated life, the operation of the water electrolysis stack 21 can be continued under operating conditions (for example, rated conditions) that balance safety and economy. The rated conditions are operating conditions that are set in advance. Typically, the temperature of the water supplied to the water electrolysis stack 21 is 60°C, and the current density is 0.5 to 2.0 A / cm. 2 , flow rate is 1cc / cm 2 / min is preferable, but the rated conditions are not limited to these.

[0070] If the determination in step S2 is YES, the calculation device 13 compares the electrical resistance R0 calculated in step S1 with a second threshold value higher than the first threshold value and determines whether the electrical resistance R0 exceeds the second threshold value (step S4). The second threshold value is greater than the first threshold value. For example, the second threshold value is a reference value for electrical resistance when the operating time in the rated state of the water electrolysis stack 21 is equal to the rated life. The second threshold value does not have to be completely equal, but may be a value where the difference between the operating time in the rated state and the rated life is very small (for example, several tens of hours or several hours to allow for some leeway). If the electrical resistance R0 is greater than the second threshold value, it can be determined that the water electrolysis stack 21 has deteriorated further and has reached (or will soon reach) its rated life. If the electrical resistance R0 is equal to or less than the second threshold value (NO determination in step S4), the calculation device 13 proceeds to step S5. If the electrical resistance R0 exceeds the second threshold value (YES determination in step S4), the calculation device 13 proceeds to step S6.

[0071] If the determination in step S4 is NO, the calculation device 13 formulates safety-prioritized operating conditions and operates the water electrolysis stack 21 (step S5). Safety-prioritized operating conditions are operating conditions that suppress deterioration of the electrolyte membrane. Examples include operation of the water electrolysis stack 21 with reduced water temperature, flow rate, and hydrogen pressure, and maintenance such as replacement of components such as filters that purify water. However, safety-prioritized operating conditions are not limited to these examples. For example, if the current density of the current supplied to the water electrolysis stack 21 is high, the water temperature will rise, so it is conceivable to reduce the current density. Furthermore, if the pressure of the supplied water is high, the electrolyte membrane is likely to be damaged, so it is conceivable to reduce the water pressure.

[0072] If the determination in step S4 is YES, the computing device 13 formulates an operation plan for replacing the water electrolysis stack 21 (step S6). For example, the operation plan may include shutting down (stopping) the corresponding water electrolysis stack 21, disconnecting the deteriorated water electrolysis stack 21 from the water electrolysis system 1, and replacing the water electrolysis stack 21 with a new one. As an example, the supply of current to the water electrolysis stack 21 is stopped to reduce the hydrogen pressure, and an inert gas such as nitrogen gas is sealed in the piping to replace the hydrogen gas with the inert gas (purging operation) and remove the hydrogen gas. Thereafter, pure water is supplied to the water electrolysis stack 21, and when the water electrolysis stack 21 is filled with pure water, the supply of pure water to the water electrolysis stack 21 is stopped. Once the voltage and temperature of the water electrolysis stack 21 have decreased, the piping is disconnected and the water electrolysis stack 21 is removed.

[0073] The display control device 11 displays the operation conditions and operation plan formulated by the arithmetic device 13 in steps S3, S5, and S6 on the display device 12. The operation device 10 sets or learns appropriate operation conditions in advance through experiments or simulations, in accordance with the state of the water electrolysis stack 21 in each operation mode (in this example, rated, safety priority, and stack replacement).

[0074] The arithmetic device 13 checks whether there is a water electrolysis stack 21 for which the calculation and determination of the electric resistance R0 have not been performed. If there is a water electrolysis stack 21 for which the calculation and determination of the electric resistance R0 have not been performed, the arithmetic device 13 appropriately performs the processes of steps S1 to S6. The arithmetic device 13 calculates the electric resistance R0 of the target water electrolysis stack 21, and performs a process according to the result of comparing the electric resistance R0 with the first threshold value and / or the second threshold value. After confirming that the calculation and determination of the electric resistance R0 have been performed for all water electrolysis stacks 21, the arithmetic device 13 terminates this process.

[0075] As described above, the calculation device 13 basically formulates the operating conditions and operation plan based on the relationship between the electrical resistance and each threshold value. What to do under what conditions is decided, and once those conditions are met, the computer of the calculation device 13 formulates the operating conditions and operation plan. When formulating the operating conditions and operation plan, the calculation device 13 can use a map, model, etc. that specifies the relationship between the electrical resistance R0, the first threshold value, the second threshold value, and the operating conditions that optimize economic efficiency while satisfying safety.

[0076] However, the configuration may also be such that the operator checks the judgment result (deterioration state) of the electrical resistance R0 of the water electrolysis stack 21 on the display screen, determines the operation conditions and operation plan, and inputs the operation conditions and operation plan into the operation device 10.

[0077] Alternatively, the display control device 11 may receive the result of the judgment between the electric resistance R0 and each threshold value from the arithmetic device 13 and issue degradation state information according to the judgment result. Specifically, the display control device 11 displays the degradation state information on the display device 12 and notifies the driver. The driver may be notified by sound in addition to the display. The arithmetic device 13 may display on the display device 12 that the estimated result of the electric resistance R0 does not exceed the first threshold value, i.e., that the water electrolysis stack 21 has not reached a degradation state.

[0078] In the example shown in FIG. 5, two thresholds, a first threshold and a second threshold, are set. However, three or more thresholds may be set and compared with the electrical resistance R0, so that the operating conditions can be formulated in a stepwise manner in accordance with the deterioration state of the water electrolysis stack 21.

[0079] As described above, the operation device 10 of the water electrolysis system 1 according to the first embodiment is an operation device for a water electrolysis system including the water electrolysis stack 21, the DC power supply 22 that supplies DC power to the water electrolysis stack 21 to drive the water electrolysis stack 21, the current monitoring device 24 that measures the current flowing through the water electrolysis stack 21, and the voltage monitoring device 23 that measures the voltage applied to the water electrolysis stack 21. The operation device 10 is also equipped with: a calculation unit 13 that receives current measurement values ​​from the current monitoring device 24 and voltage measurement values ​​from the voltage monitoring device 23, estimates the electrical resistance of the water electrolysis stack 21 that changes depending on the degradation state using the time-series current measurement values ​​and voltage measurement values, determines the degradation state of the water electrolysis stack 21 by comparing the estimated electrical resistance with thresholds (first threshold and second threshold) for determining whether the electrical resistance is increasing, and determines the operation conditions of the water electrolysis stack 21 depending on the degradation state of the water electrolysis stack 21; and a display control unit 11 that outputs the operation conditions of the water electrolysis stack 21 determined by the calculation unit 13 to a display device 12.

[0080] [Advantages of the first embodiment] With the above-described configuration, the operation device 10 of the water electrolysis system 1 according to the first embodiment acquires measurement results of the current and voltage of the water electrolysis stack 21 during load fluctuations and calculates the electrical resistance of the water electrolysis stack 21. When an increase in overvoltage occurs due to an increase in the electrical resistance of the water electrolysis stack 21, the operation device 10 can determine the operating conditions of the water electrolysis stack 21 (select an operation mode) based on the relationship between the calculated electrical resistance and the first and second threshold values. As a result, the operation device 10 of the water electrolysis system 1 according to this embodiment can achieve economical and highly efficient hydrogen production while ensuring safety when an increase in overvoltage occurs in the water electrolysis stack 21.

[0081] In this embodiment, the operating conditions of the water electrolysis stack 21 can be carefully determined (operation modes can be switched) depending on the deterioration state of the water electrolysis stack 21, i.e., the value of the electrical resistance of the water electrolysis stack 21. As the operating temperature of the water electrolysis stack 21 increases, the electrolysis efficiency increases, but the deterioration of the water electrolysis stack 21 also accelerates and heat consumption due to water heating also increases. In this embodiment, by optimizing economic efficiency within a safe range, hydrogen can be produced under operating conditions that maximize profits, thereby achieving economic benefits.

[0082] [Determination of Deterioration State Using Reaction Capacitance] In the first embodiment described above, the electrical resistance R0 of the water electrolysis stack 21 is used to determine the degradation state of the water electrolysis stack 21. However, the reaction capacitance C1 may also be used to determine the degradation state of the water electrolysis stack 21. For example, the computing device 13 estimates the reaction capacitance C1 of the water electrolysis stack 21 from the current measurement value (measurement result) of the current monitoring device 24 and the voltage measurement value (measurement result) of the voltage monitoring device 23, compares the reaction capacitance C1 with a preset degradation determination threshold value (an example of a reference value for reaction capacitance), determines the degradation state of the water electrolysis stack 21, and outputs the determination result.

[0083] Reaction capacitance C1 (electric double layer) is a parameter related to the surface area of ​​the catalyst or the contact between the catalyst and the electrode (e.g., carbon paper). Catalyst degradation reduces the catalyst surface area, resulting in a lower capacitance value. Alternatively, poor contact between the catalyst and the electrode can result in a higher capacitance value. These phenomena cannot be expressed in electrical resistance, and therefore cannot be determined from the electrical resistance alone. Using reaction capacitance C1 to determine the state of deterioration of the water electrolysis stack 21 makes it possible to identify the location and cause of catalyst deterioration.

[0084] In this way, by using the reaction capacitance C1, it is possible to determine deterioration of the water electrolysis stack 21 due to a change in the state of the catalyst. For example, the computing device 13 may first determine the deterioration state of the water electrolysis stack 21 using the electrical resistance R0, and if it determines that the water electrolysis stack 21 is deteriorated, determine the deterioration of the catalyst using the reaction capacitance C1. This makes it possible to detect deterioration of the catalyst in the water electrolysis stack 21. Note that the deterioration state of the water electrolysis stack 21 may also be determined initially using only the reaction capacitance C1. For example, if only the reaction capacitance C1 is deteriorated, the risk of a serious event such as a shutdown of the water electrolysis device 20 is low, so it is possible to continue operation under rated conditions.

[0085] (Modification 1 of Deterioration State Determination) In the first embodiment, the estimated results of the equivalent circuit parameters are directly compared with a threshold value, but this is not limiting. For example, the computing device 13 may calculate the difference between the average electrical resistance, which is the average value of the electrical resistances R0 of the multiple water electrolysis stacks 21, and the electrical resistance R0 of the water electrolysis stack 21, and compare this difference with a preset reference value for the deviation of the electrical resistances to determine the degradation state of the water electrolysis stack 21. The reference value for the deviation of the electrical resistances is, for example, a reference value for the standard deviation of the electrical resistances of each water electrolysis stack 21.

[0086] By adopting this configuration, it is possible to realize a more robust determination of the deterioration state even when the measurement results contain transient abnormal values. Note that this determination method based on average electrical resistance can also be performed using reaction capacitance instead of electrical resistance.

[0087] (Modification 2 of Deterioration State Determination) Furthermore, for example, the computing device 13 may calculate information representing a change in the electrical resistance R0 of the water electrolysis stack 21 from the previously estimated electrical resistance R0 to the currently estimated electrical resistance R0, and compare the information representing the change in electrical resistance with a preset reference value for the change in electrical resistance to determine the degradation state of the water electrolysis stack 21.

[0088] For example, assume that the estimated result for the electrical resistance R0 of a specific water electrolysis stack 21 in the February inspection was 0.020 Ω, 0.021 Ω in the March inspection, 0.022 Ω in the April inspection, and 0.025 Ω in the May inspection. Since the rate of increase in the electrical resistance in the May inspection was clearly faster than the estimated results for the most recent few months, the computing device 13 issues a warning that the water electrolysis stack 21 has deteriorated.

[0089] However, in this example, because the rate of change in electrical resistance is high only in the past month, another problem other than deterioration of the water electrolysis stack 21, such as poor recent water quality management, may be considered. For example, another problem may be an abnormality in the system state of the water electrolysis system 1. One example of an abnormality in the system state is inappropriate operating conditions of the water electrolysis system 1. Therefore, the content of the alert may be "system abnormality" or "abnormal operating conditions." Note that this determination method based on the change in electrical resistance between the previous and current times can also be implemented using reaction capacitance instead of electrical resistance.

[0090] <Modification of the first embodiment> A modified example of the process for calculating the electrical resistance of the water electrolysis stack 21 by the operation device 10 of the water electrolysis system 1 will be described with reference to Figures 9 to 11. In this modified example, detailed descriptions of parts that are the same as or similar to those in the first embodiment will be omitted.

[0091] The difference between the first embodiment and this modification is that the equations (22) and (23) described below are incorporated into the calculation of the calculation device 13. This calculation method makes it possible to extract not only the electrical resistance R0 and the reaction capacitance C1, but also the reaction resistance R1. The principle is described below.

[0092] According to the equivalent circuit of the water electrolysis stack 21 (FIG. 3), the change dE1 in the voltage E1 applied to the reaction resistor R1 is expressed as the current I passing through the reaction capacitance C1 within the time dt. C1 and is expressed by equation (12).

number

[0093] Equation (12) is rewritten as equation (13), where dI R1 is the current I passing through the reactive resistor R1 within the time dt R1 is the amount of change.

number

[0094] As shown in equations (14) and (15), the reaction resistance R1 and the time constant τ of the equivalent circuit are defined.

number

number

[0095] Here, equation (13) can be rewritten as equation (16).

number

[0096] As shown in equation (17), the current I passing through the reactive resistor R1 in the equivalent circuit R1 and the current I passing through the reactive capacitance C1 C1 The sum of these is the total current I (electrolysis current).

number

[0097] Substituting equation (17) into equation (16) gives equation (18).

number

[0098] Equation (18) is a first-order linear differential equation, and its solution is expressed by equation (19).

number

[0099] Integrating equation (12) with respect to time t gives equation (20). The voltage E1 applied to the reactive resistor R1 is determined by equation (20).

number

[0100] Substituting equation (19) into equation (20) gives equation (21).

number

[0101] The voltage E applied to the water electrolysis stack 21 from time 0 to time t is expressed by equation (22).

number

[0102] The current change rate dI / dt of the DC power supply 22 can be calculated from the current value measured by the current monitoring device 24. When the sweep current change rate dI / dt is a constant v, equation (22) can be rewritten as equation (23), where E(0) is the voltage immediately before the current is changed.

number

[0103] The electrical resistance R0, reaction resistance R1, and reaction capacitance C1 can be estimated by extracting the constant v of the current change rate when the current is swept from the change in current over time and fitting the change in voltage over time with equation (23). That is, in this modification, as can be seen from equation (23), even if there is no change in the voltage applied to the water electrolysis stack 21, the parameters of the equivalent circuit can be identified if there is a change in the current. In this modification, the current change rate dI / dt is a constant, but even if the current change rate dI / dt is not a constant, the electrical resistance R0, reaction resistance R1, and reaction capacitance C1 can be estimated.

[0104] (Voltage change over time) Here, the fitting process in equation (23) will be described with reference to FIG. 9 is a diagram showing an example of voltage measurement results (change in voltage over time) of the water electrolysis stack 21 in the modified example of the first embodiment. In Fig. 9, the vertical axis represents voltage (V) and the horizontal axis represents time (s).

[0105] The above-mentioned equation (23) is rewritten as the following equation (24), where P1, P2, and τ are fitting parameters. For the experimental voltage values ​​shown as plotted points in Figure 9, P1, P2, and τ shown in equation (24) are selected so as to be closest to the experimental values, and the fitting result shown by the solid line is obtained.

number

[0106] Then, by comparing equations (23) and (24), the electrical resistance R0 and the reaction resistance R1 are obtained as shown in equations (25) and (26). In addition, the reaction capacitance C1 is derived from the time constant τ and the reaction resistance R1 as shown in equation (27).

[0107]

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number

number

[0108] [Another example of electrical resistance calculation processing] Next, another example of the electrical resistance calculation process using another example (hereinafter referred to as "second example") of the method for estimating the electrical resistance of the water electrolysis stack 21 in step S1 will be described with reference to FIG. FIG. 10 is a flowchart showing another example of the procedure for calculating the electrical resistance of the water electrolysis stack 21 by the operation device 10 of the water electrolysis system 1.

[0109] First, the operation screen (display control device 11) of the display device 12 receives a start command from the operator and issues measurement commands to the voltage monitoring devices 23 and current monitoring devices 24 of the individual water electrolysis stacks 21 (step S21).

[0110] After issuing the measurement command, the display control device 11 issues a current change command to the DC power supply 22, and linearly changes the output (e.g., current density) of the DC power supply 22 (step S22). After a predetermined time has elapsed from the time (t=0) when the output of the DC power supply 22 changes, the voltage monitoring device 23 and the current monitoring device 24 finish measurement and transfer the measurement results of the voltage and current to the calculation device 13. The processing of steps S21 and S22 is basically the same as steps S11 and S12 in FIG. 5.

[0111] The reason why "linearly" is written in step S22 is because it is assumed that dI / dt is a constant. In this embodiment, for the sake of convenience, the current I changes linearly, but in reality, the current I does not have to change linearly. In that case, the above-mentioned equation (22) holds, but because it is a nonlinear regression, equation (23) changes depending on the fitting equation.

[0112] In step S22, the arithmetic device 13 extracts the measurement results of the voltage and current from the change point (t=0) to a specified time (e.g., 10 s), as in the first embodiment. FIG. 11 shows an example of the change over time in the current density and voltage of the water electrolysis system 1. In FIG. 11, the vertical axis on the left represents the current density (A / cm 211, the vertical axis on the right side represents voltage (V) and the horizontal axis represents time (s). In FIG. 11, an example of a nearly linear voltage change over time 101 and a nearly linear current change over time 102 is shown.

[0113] The calculation device 13 estimates the electrical resistance R0, reaction resistance R1, and reaction capacitance C1 of each water electrolysis stack 21 by calculating the extracted results of the voltage and current of each water electrolysis stack 21 using the above-mentioned equation (23) (step S23). In step S23, the calculation device 13 estimates the constant v (sweep rate) of the current change rate dI / dt from the above-mentioned linear regression of the change in current over time, and then estimates the electrical resistance R0, reaction resistance R1, and reaction capacitance C1 from the change in voltage over time using equation (23). After processing step S23, the process proceeds to step S2 in FIG. 4.

[0114] In another example (second example) of the method for estimating the electrical resistance using the above-described equations (12) to (27), the DC power supply 22 changes the DC power from a steady state and supplies it to the water electrolysis stack 21. The arithmetic device 13 acquires measured current values ​​and measured voltage values ​​from the time the DC power of the DC power supply 22 changes from the steady state to an arbitrary time t, and performs a process of estimating the electrical resistance R0 of the water electrolysis stack 21 at the arbitrary time t using the measured current values ​​and measured voltage values ​​from the time when the DC power changes from the steady state (t=0) to the arbitrary time t.

[0115] As described above, the second example of the method for estimating electric resistance uses measured values ​​of current and voltage between times 0 and t to estimate (by nonlinear regression) the electric resistance R0, the reaction resistance R1, and the reaction capacitance C1 in consideration of the voltage E applied to the entire water electrolysis stack 21. Therefore, in addition to the effects of the first example of the method for estimating electric resistance using the above-described formulas (1) to (11), this example enables determination of the deterioration state that is more resistant to noise than the first example of the method for estimating electric resistance.

[0116] Note that the water electrolysis system 1 using the second example of the method for estimating electric resistance can also be applied to the degradation state determination using reaction capacitance, as well as Modifications 1 and 2 of the degradation state determination described in the first embodiment.

[0117] <Second embodiment> Next, the operation condition formulation process performed by the operation device 10 of the water electrolysis system 1 according to a second embodiment of the present invention will be described with reference to Fig. 12. In this embodiment, parts that are the same as or similar to those in the first embodiment are denoted by the same reference numerals, and detailed description thereof will be omitted.

[0118] The difference between this embodiment and the first embodiment is that the display device 12 displays the reaction overvoltage, the calculation device 13 formulates an operation plan based on the electrical resistance and the reaction overvoltage, and the reaction overvoltage is calculated by the following formula (33). In this embodiment, not only the electrical resistance R0 but also the reaction overvoltage E due to catalyst deterioration is calculated. act The principle is explained below.

[0119] The voltage E applied to the water electrolysis stack 21 in the steady state is determined by equation (28). cell is the number of stacked cells, E rev is the reversible voltage, E act is the reaction overvoltage, E ohm is the ohmic overvoltage, and E con is the concentration overpotential.

number

[0120] Each voltage will be briefly explained. Reversible voltage E rev is the voltage that represents the change in energy inside a molecule before and after the decomposition of water into hydrogen and oxygen. The standard reversible voltage, which will be described later, is the voltage that represents the change in energy inside a molecule before and after the decomposition of water into 1 atmosphere of hydrogen and 1 atmosphere of oxygen. Ohmic Overvoltage E ohm is the voltage increase due to the electrical resistance of the electrolyte membrane and the solution. Concentration overvoltage E con is an overvoltage that occurs when bubbles generated by water decomposition make it difficult for water to reach the electrolyte membrane.

[0121] Measurement results and information for each element can be obtained in the following ways: The voltage E applied to the water electrolysis stack 21 is obtained from the measurement results of the voltage monitoring device 23 (voltmeter). Number of cells stacked in the water electrolysis stack 21 N cell is obtained from the specification. The current I passing through the water electrolysis stack 21 is obtained from a current monitoring device 24 (ammeter). Reversible voltage E rev Since depends only on the temperature, it is roughly estimated from the water temperature (thermometer). There are several methods for measuring the water temperature. For example, the inlet temperature and outlet temperature of the water electrolysis stack 21 can be measured, or their average can be taken. Alternatively, if the water electrolysis stack 21 is structurally capable of having a thermometer installed inside the stack, the internal temperature can be measured directly.

[0122] Concentration overvoltage E under normal operating conditions con Since can be ignored, equation (28) can be rewritten as equation (29).

number

[0123] Ohmic Overvoltage E ohm is determined by the electrical resistance R0 and the current I passed through the water electrolysis stack 21.

number

[0124] Reversible voltage E rev is determined by the Nernst equation (31). In equation (31), two hydrogen molecules react with one oxygen molecule in a four-electron transfer. Here, E rev φ (φ is a superscript in the formula) is the standard reversible voltage, R is the gas constant, T is the electrolysis temperature, F is the Faraday constant, and P H2 is the hydrogen partial pressure on the cathode side, and P O2is the oxygen partial pressure on the anode side. The cathode side is the negative electrode where hydrogen is generated, and the anode side is the positive electrode where oxygen is generated and water is consumed. The hydrogen partial pressure is the pressure on the hydrogen side minus the water vapor pressure. The oxygen partial pressure is the pressure on the oxygen side minus the water vapor pressure.

number

[0125] Standard reversible voltage E rev φ can be estimated by the general empirical formula (32).

number

[0126] From equations (29) to (32), the reaction overvoltage E when the current flowing through the water electrolysis stack 21 is I is act can be estimated using equation (33).

number

[0127] [Operational condition formulation process] Next, the operation condition formulation process performed by the operation device 10 of the water electrolysis system 1 according to this embodiment will be described with reference to FIG. Figure 12 is a flowchart showing an example of the procedure for formulating operating conditions by the operation device 10 of the water electrolysis system 1 according to the present embodiment. In Figure 12, steps S31 and S32 are added to the procedure in Figure 4. The process in step S1 and steps S3 to S6 shown in Figure 12 is the same as the process in step S1 and steps S3 to S6 in Figure 4, and therefore detailed description thereof will be omitted.

[0128] After the process of step S1, the calculation device 13 compares the electrical resistance R0 estimated in step S1 with a first threshold value and determines whether the electrical resistance R0 exceeds the first threshold value (step S2). If the electrical resistance R0 is equal to or less than the first threshold value (NO determination in step S2), the calculation device 13 proceeds to step S31, and if the electrical resistance R0 exceeds the first threshold value (YES determination in step S2), the calculation device 13 proceeds to step S4.

[0129] If the determination in step S2 is NO, the calculation device 13 calculates the reaction overvoltage E act is compared with a preset reaction overvoltage threshold (step S31). The reaction overvoltage threshold is a reference value of the reaction overvoltage when the difference between the operation time in the rated state of the water electrolysis stack 21 and the rated life reaches a certain standard (for example, 1000 hours). This reaction overvoltage threshold is a criterion for determining that the electrolysis efficiency is low. If the reaction overvoltage is equal to or less than this threshold, the electrolysis efficiency is within the allowable range. If the reaction overvoltage exceeds this threshold, the electrolysis efficiency has fallen outside the allowable range. The calculation device 13 calculates the reaction overvoltage E act If the reaction overvoltage threshold value E is equal to or less than the reaction overvoltage threshold value (NO in step S31), the process proceeds to step S3. act If exceeds the reaction overvoltage threshold (YES in step S31), the process proceeds to step S32.

[0130] If the determination in step S31 is YES, the calculation device 13 calculates the reaction overvoltage E act In this embodiment, regardless of the determination result of step S2, the operating temperature of the water electrolysis stack 21 is determined based on the reaction voltage E act You can also calculate the reaction overvoltage E act is greater than the reaction overpotential threshold, it is considered that the electrolysis efficiency of the water electrolysis stack 21 is lower than expected. Therefore, the computing device 13 determines operating conditions that prioritize economic efficiency, since the risk of a serious event such as a shutdown of the water electrolysis device 20 is extremely low. Here, the electrolysis efficiency is improved by operating the water electrolysis stack 21 at a higher temperature.

[0131] In the water electrolysis stack 21, if the current density is constant, the reaction overpotential decreases linearly as the operating temperature increases. For example, the reaction overpotential before and after a temperature change can be expressed by equation (34). E1_T is the reaction overpotential after the operating temperature change, E1_T0 is the reaction overpotential before the operating temperature change, and k is a fitting parameter (k<0).

number

[0132] If there is a water electrolysis stack 21 for which the calculation and determination of the electric resistance R0 has not been performed, the calculation device 13 appropriately executes the processes of steps S1 to S6 and S31 to S32. After confirming that the calculation and determination of the electric resistance R0 have been performed for all the water electrolysis stacks 21, the calculation device 13 terminates this process.

[0133] [Effects of the second embodiment] With the above-described configuration, the operation device 10 of the water electrolysis system 1 according to the second embodiment compares the calculated electrical resistance with the first and second thresholds in a deterioration diagnosis of the water electrolysis stack 21 during load fluctuations, and further compares the reaction overvoltage with the reaction overvoltage threshold depending on the comparison result. The operation device 10 can then determine the operation conditions of the water electrolysis stack 21 (select an operation mode) depending on the deterioration diagnosis result. That is, in the deterioration diagnosis of the water electrolysis stack 21 during load fluctuations, the operation device 10 separates the causes of overvoltage into electrical resistance (deterioration of the electrolyte membrane) and reaction overvoltage (deterioration of the catalyst), and further determines the operation conditions of the water electrolysis stack 21 (selects an operation mode) based on the estimated results of the deterioration causes. As a result, the operation device 10 of the water electrolysis system 1 according to the second embodiment can achieve economical and highly efficient hydrogen production while ensuring safety depending on the deterioration state of the water electrolysis stack 21 (cause and state of overvoltage), as in the first embodiment.

[0134] For example, in this embodiment, even if the reaction overvoltage of the water electrolysis stack 21 increases, if the electrical resistance does not increase, the operation device 10 can operate the water electrolysis stack 21 under operating conditions that prioritize economic efficiency (raising the operating temperature).

[0135] As described above, the present invention is not limited to the above-described embodiments, and various other modifications and applications are possible without departing from the spirit of the invention as defined in the claims. For example, the above-described embodiments have been described in detail and specifically to clearly explain the present invention, and the present invention is not necessarily limited to those including all of the components described. Furthermore, it is possible to replace part of the configuration of one embodiment with a component of another embodiment. It is also possible to add a component of another embodiment to the configuration of one embodiment. It is also possible to add, replace, or delete other components from part of the configuration of each embodiment.

[0136] Furthermore, the above-described configurations, functions, processing units, etc. may be partially or entirely realized in hardware, for example, by designing them as integrated circuits, etc. As the hardware, a broad processor device such as an FPGA (Field Programmable Gate Array) or an ASIC (Application Specific Integrated Circuit) may be used.

[0137] Furthermore, each component of the operation device for a water electrolysis system according to the above-described embodiment may be implemented in any hardware as long as the respective hardware components are capable of transmitting and receiving information to and from each other via a network. Furthermore, the processing performed by a given processing unit may be implemented by a single piece of hardware, or may be implemented by distributed processing using multiple pieces of hardware. [Explanation of symbols]

[0138] 1...water electrolysis system, 10...operation device, 11...display control device (display control unit), 12...display device, 13...arithmetic device (arithmetic unit), 20...water electrolysis device, 21...water electrolysis stack, 22...DC power supply, 23...voltage monitoring device, 24...current monitoring device, C1...reaction capacitance, R0...electrical resistance, R1...reaction electrical resistance (reaction resistance), E act …Reaction overvoltage

Claims

1. An operation device for a water electrolysis system comprising: a water electrolysis stack; a DC power supply that supplies DC power to the water electrolysis stack to drive the water electrolysis stack; a current monitoring device that measures a current flowing through the water electrolysis stack; and a voltage monitoring device that measures a voltage applied to the water electrolysis stack, a calculation unit that receives current measurement values ​​from the current monitoring device and voltage measurement values ​​from the voltage monitoring device, estimates an electrical resistance of the water electrolysis stack that changes depending on a degradation state of the water electrolysis stack using the time-series current measurement values ​​and voltage measurement values, compares the estimated electrical resistance with a threshold value for determining whether the electrical resistance is increasing to determine a degradation state of the water electrolysis stack, and determines operating conditions of the water electrolysis stack depending on the degradation state of the water electrolysis stack; a display control unit that outputs the operation conditions of the water electrolysis stack determined by the calculation unit to a display device. Water electrolysis system operation equipment.

2. a plurality of the water electrolysis stacks are multi-connected; The calculation unit determines a deterioration state of each of the water electrolysis stacks, and determines operating conditions for the water electrolysis stacks according to the deterioration state of the water electrolysis stacks. An operation device for a water electrolysis system according to claim 1.

3. The calculation unit comparing the electrical resistance with a first threshold value as the threshold value, and if the electrical resistance is equal to or less than the first threshold value, setting rated conditions as operating conditions for the water electrolysis stack; If the electrical resistance exceeds a first threshold, the electrical resistance is compared with a second threshold that is greater than the first threshold, and if the electrical resistance is equal to or less than the second threshold, a safety-first operating condition is set for the water electrolysis stack. An operation device for a water electrolysis system according to claim 1 or 2.

4. the calculation unit estimates a reaction overvoltage of the water electrolysis stack, which changes depending on a degradation state, using the time-series current measurement value and the voltage measurement value; when the electrical resistance is equal to or less than a first threshold, comparing the reaction overvoltage with a reaction overvoltage threshold for determining whether the reaction overvoltage is increasing, and when the reaction overvoltage is equal to or less than the reaction overvoltage threshold, setting the rated condition as an operating condition for the water electrolysis stack; If the reaction overvoltage exceeds the reaction overvoltage threshold, the operating temperature of the water electrolysis stack is set to be increased. The water electrolysis system operation device according to claim 3 .

5. The calculation unit If the electrical resistance exceeds a first threshold, the electrical resistance is compared with a second threshold, and if the electrical resistance exceeds the second threshold, an operation plan for replacing the water electrolysis stack is formulated. The water electrolysis system operation device according to claim 3 .

6. The safety-first operating conditions are operating conditions for suppressing deterioration of the electrolyte membrane. The water electrolysis system operation device according to claim 3 .

7. The first threshold value is defined as a reference value of the electrical resistance when the difference between the operating time under rated conditions and the rated life of the water electrolysis stack reaches a certain standard. The water electrolysis system operation device according to claim 3 .

8. The second threshold value is defined as a reference value of the electrical resistance when the operation time under rated conditions of the water electrolysis stack is equal to the rated life. The water electrolysis system operation device according to claim 3 .

9. The reaction overvoltage threshold is defined as a reference value of the reaction overvoltage when the difference between the operation time under the rated state of the water electrolysis stack and the rated life reaches a certain standard. The water electrolysis system operation device according to claim 4.

10. The DC power supply varies DC power from a steady state, The calculation unit acquires the current measurement value and the voltage measurement value measured at a predetermined time interval after the DC power of the DC power supply changes from a steady state, and estimates the electrical resistance of the water electrolysis stack at an arbitrary time by using a difference between the current measurement value and the voltage measurement value at the predetermined time interval. An operation device for a water electrolysis system according to claim 1.

11. The DC power supply varies DC power from a steady state, The calculation unit acquires the current measurement value and the voltage measurement value from a time when the DC power of the DC power supply changes from a steady state to an arbitrary time, and estimates the electrical resistance of the water electrolysis stack at the arbitrary time by using the current measurement value and the voltage measurement value from a time when the DC power changes from the steady state to the arbitrary time. An operation device for a water electrolysis system according to claim 1.

12. 1. A method for operating a water electrolysis system comprising: a water electrolysis stack; a DC power supply that supplies DC power to the water electrolysis stack to drive the water electrolysis stack; a current monitoring device that measures a current flowing through the water electrolysis stack; a voltage monitoring device that measures a voltage applied to the water electrolysis stack; a calculation unit that receives a current measurement value of the current monitoring device and a voltage measurement value of the voltage monitoring device and performs calculations; and a display control unit, a process of estimating, by the calculation unit, the electrical resistance of the water electrolysis stack, which changes depending on a degradation state, using the time-series current measurement values ​​and the voltage measurement values; a process in which the calculation unit compares the estimated electrical resistance with a threshold value for determining whether the electrical resistance has increased, thereby determining a degradation state of the water electrolysis stack; determining, by the calculation unit, operation conditions for the water electrolysis stack according to a deterioration state of the water electrolysis stack; and outputting the operating conditions of the water electrolysis stack to a display device by the display control unit. How to operate a water electrolysis system.

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