Methane production method and methane production system

Abstract

To control a methane production system so as to maintain operation with high efficiency.SOLUTION: A methane production method includes the steps of: producing hydrogen by water electrolysis in a water electrolysis apparatus using supplied electric energy; synthesizing methane by reacting the produced hydrogen and carbon dioxide in a methane synthesis apparatus, and transmitting reaction heat generated when the methane is synthesized to the water electrolysis apparatus; and adjusting an amount of electric energy supplied to the water electrolysis apparatus so that the total of an excess amount of heat generated by self heat-generation from the water electrolysis apparatus and an amount of heat energy transmitted to the water electrolysis apparatus from the methane synthesis device becomes equal to an amount of heat energy required for a heat electrolysis reaction in the water electrolysis apparatus.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present disclosure relates to a method and system for producing methane. [Background technology]

[0002] Patent Document 1 describes a methane synthesis device in which a water electrolysis section, a Sabatier reaction section, a carbon dioxide supply section, and a hydrogen gas supply section are stacked and integrated. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2019-89713 A Summary of the Invention [Problem to be solved by the invention]

[0004] Since methane synthesis is an exothermic reaction and water electrolysis is an endothermic reaction, water electrolysis is sometimes performed using the reaction heat generated during methane synthesis to increase the efficiency of the entire system (see, for example, Patent Document 1).

[0005] However, if the amount of heat absorbed by the water electrolysis device changes due to aging of the device or changes in the surrounding environment, or if the reaction heat during methane synthesis changes, the water electrolysis device may experience an excess or deficiency of heat, resulting in poor efficiency.

[0006] In consideration of the above, the present disclosure has an object to control a methane production system so as to maintain highly efficient operation. [Means for solving the problem]

[0007] A first aspect is a methane production method comprising: generating hydrogen by water electrolysis in a water electrolysis device using supplied electric energy; reacting the generated hydrogen with carbon dioxide in a methane synthesis device to synthesize methane and conducting reaction heat generated during the synthesis of methane to the water electrolysis device; and adjusting the amount of electric energy supplied to the water electrolysis device so that the sum of an amount of surplus heat generated by self-heating in the water electrolysis device and an amount of thermal energy conducted from the methane synthesis device to the water electrolysis device is equal to an amount of thermal energy required for the water electrolysis reaction in the water electrolysis device.

[0008] In the methane production method of the first aspect, the amount of electrical energy supplied to the water electrolysis device is adjusted so that the sum of the amount of excess heat generated by the water electrolysis device due to self-heating and the amount of thermal energy conducted to the water electrolysis device from the methane synthesis device is equal to the amount of thermal energy required for the water electrolysis reaction in the water electrolysis device, so that it is possible to prevent the water electrolysis device from experiencing a heat surplus or shortage compared to a case in which the amount of electrical energy supplied to the water electrolysis device is constant, thereby making it possible to control the methane production system so as to maintain highly efficient operation.

[0009] In a second aspect, in the first aspect, in the step of adjusting the amount of electric energy supplied to the water electrolysis device, an amount of thermal energy required for the water electrolysis reaction in the water electrolysis device is calculated based on a temperature in the water electrolysis device, and an amount of thermal energy conducted from the methane synthesis device to the water electrolysis device is calculated based on the temperature in the methane synthesis device.

[0010] This makes it possible to adjust the amount of electrical energy supplied to the water electrolysis device according to the temperature in the water electrolysis device and the temperature in the methane synthesis device.

[0011] In a third aspect, in the first aspect, in the step of adjusting the amount of electric energy supplied to the water electrolysis device, the amount of electric energy supplied to the water electrolysis device is adjusted when an absolute value of a value obtained by subtracting both an amount of surplus heat generated by self-heating in the water electrolysis device and an amount of thermal energy conducted from the methane synthesis device to the water electrolysis device from an amount of thermal energy required for a water electrolysis reaction in the water electrolysis device is greater than a predetermined value.

[0012] In this manner, by adjusting the amount of electric energy supplied to the water electrolysis device, it is possible to prevent the water electrolysis device from becoming overheated or underheated, thereby enabling the methane production system to be controlled so as to maintain highly efficient operation.

[0013] In a fourth aspect, in the first aspect, an amount of surplus heat generated by self-heating from the water electrolysis device is calculated as a difference between an amount of self-heating of the water electrolysis device and an amount of thermal energy required to keep the water electrolysis device warm, and an amount of thermal energy conducted from the methane synthesis device to the water electrolysis device is calculated as a difference between an amount of heat generated by the methane synthesis device and an amount of thermal energy required to keep the methane synthesis device warm.

[0014] In this way, by adjusting the amount of electrical energy supplied to the water electrolysis device in consideration of the amount of thermal energy required to keep the water electrolysis device warm and the amount of thermal energy required to keep the methane synthesis device warm, it is possible to prevent the water electrolysis device from becoming overheated or underheated, thereby enabling the methane production system to be controlled so as to maintain highly efficient operation.

[0015] A fifth aspect is a methane production system comprising: a water electrolysis device that produces hydrogen by water electrolysis using supplied electric energy; and a methane synthesis device that reacts the hydrogen produced in the water electrolysis device with carbon dioxide to synthesize methane, wherein the water electrolysis device comprises a control device that controls the amount of electric energy supplied to the water electrolysis device so that the sum of surplus heat generated by self-heating from the water electrolysis device and the amount of thermal energy conducted from the methane synthesis device to the water electrolysis device is equal to the amount of thermal energy required for the water electrolysis reaction in the water electrolysis device.

[0016] In the methane production system of the fifth aspect, the amount of electrical energy supplied to the water electrolysis device is adjusted so that the sum of the excess heat generated by the water electrolysis device due to self-heating and the amount of thermal energy conducted to the water electrolysis device from the methane synthesis device is equal to the amount of thermal energy required for the water electrolysis reaction in the water electrolysis device, so that it is possible to prevent the water electrolysis device from becoming overheated or underheated compared to when the amount of electrical energy supplied to the water electrolysis device is constant. This makes it possible to control the methane production system so that it can be maintained in a highly efficient operation. Effect of the Invention

[0017] According to the present disclosure, it is possible to control a methane production system so as to maintain highly efficient operation. [Brief description of the drawings]

[0018] [Figure 1] FIG. 1 is a configuration diagram showing a methane production system according to this embodiment. [Diagram 2] FIG. 2 is a block diagram showing the hardware configuration of the control device of the methane production system of this embodiment. [Diagram 3] FIG. 3 is a flowchart showing an example of the power adjustment process in the methane production system of this embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Hereinafter, an example of an embodiment of the present disclosure will be described in detail with reference to the drawings.

[0020] FIG. 1 shows a methane production system 12 as an example to which the methane production method of the first embodiment is applied.

[0021] The methane production system 12 includes a water electrolysis device 14, a methane synthesis device 16, a heat transfer body 18 that transfers reaction heat generated in the methane synthesis device 16 to the water electrolysis device 14, and a control device 20 that controls the water electrolysis device 14, the methane synthesis device 16, etc.

[0022] A water supply source is connected to the water electrolysis device 14. The water electrolysis device 14 is also provided with a power adjustment device 22 and a temperature sensor 24. The power adjustment device 22 is configured to adjust the amount of power, which is the amount of electrical energy supplied to the water electrolysis device 14. The temperature sensor 24 detects the temperature T 1 The device is configured to detect

[0023] In the water electrolysis device 14, water (H 2 O) to hydrogen (H 2 ) and oxygen (O 2 ) and H 2 O→H 2 +(1 / 2)O 2 ΔH 0 =285.8 kJ / mol (standard condition: 25°C, 1 atm) (1) This water electrolysis reaction requires an energy supply of 285.8 kJ per mole of water to be decomposed, and this energy is usually supplied as electrical energy. Note that a part of the supplied energy can be supplied as thermal energy. The water electrolysis device 14 is configured so that electrical energy is added by a power adjustment device 22.

[0024] That is, the water electrolysis device 14 is configured to generate hydrogen by water electrolysis using the electric energy supplied by the power conditioning device 22 .

[0025] The hydrogen produced in the water electrolysis unit 14 is sent to the methane synthesis unit 16 .

[0026] A carbon dioxide supply source is connected to the methane synthesis device 16. The methane synthesis device 16 is also provided with a temperature sensor 28. The temperature sensor 28 detects the temperature T 2 (the catalyst temperature).

[0027] The methane synthesis unit 16 has a catalyst inside, and produces methane (CH) by a methane synthesis reaction as shown in the following formula (2). 4 ) and water. 4H 2 +CO 2 →CH 4 +2H 2 O ΔH 0 =-165kJ / mol(Standard condition: 25℃, 1atm) (2) This methane synthesis reaction is an exothermic reaction of 165 kJ per mole of methane, and a part or all of the generated thermal energy is made to act on the water electrolysis device 14 via a heat transfer medium or other heat transfer body 18, and is configured to be used for the water electrolysis reaction.

[0028] That is, the methane synthesis unit 16 synthesizes hydrogen from the hydrogen produced in the water electrolysis unit 14 and carbon dioxide (CO 2) to synthesize methane. The methane production system 12 is configured to transfer reaction heat generated when synthesizing methane in the methane synthesis device 16 to the water electrolysis device 14 via a heat transfer body 18. Here, the heat transfer body 18 may transfer heat by any of conductive heat, convective heat, and radiant heat. The heat transfer body 18 is not particularly limited as long as it is a material with excellent heat transfer properties, and a material with high thermal conductivity is preferable. The heat transfer body 18 may be in any of solid, liquid, and gas forms.

[0029] A pressure sensor 32, a temperature sensor 34, and a gas flow rate sensor 36 are provided on the gas outlet side of the methane synthesis unit 16.

[0030] The pressure sensor 32 detects the pressure p of the outlet gas at the gas outlet of the methane synthesis unit 16. 1 The temperature sensor 34 is configured to be able to detect the dew point temperature Td of the outlet gas at the gas outlet of the methane synthesis device 16. The gas flow rate sensor 36 is configured to be able to detect the flow rate of the outlet gas (the amount of gas flowing per unit time) f 1 The device is configured to detect

[0031] The pressure p detected by the pressure sensor 32 1 , the temperatures T 1 , T 2 , Td, the flow rate of gas detected by the gas flow sensor 36 f 1 are sent to the control device 20, respectively.

[0032] In the control device 20, the pressure p of the outlet gas detected by the pressure sensor 32 1 , the flow rate f of the outlet gas detected by the gas flow sensor 36 1 , the temperatures T 1 , T 2 , Td to control the power adjustment device 22 and the like.

[0033] FIG. 2 shows a block diagram of the hardware configuration of the control device 20. As shown in FIG.

[0034] The control device 20 includes a computer 40. The computer 40 includes a processor 42, a memory 44, a storage 46, an input device 48, an output device 50, a storage medium reading device 52, and a communication I / F (Interface) 54. Each of these elements is connected to each other via a bus 56 so as to be able to communicate with each other.

[0035] The storage 46 stores a power adjustment program for executing a power adjustment process for adjusting the amount of power supplied to the water electrolysis device 14, which will be described in detail later. The processor 42 is capable of executing various programs and controlling each element. Specifically, the processor 42 reads a program from the storage 46 and executes the program using the memory 44 as a working area. That is, the processor 42 controls each element and performs various arithmetic processing in accordance with the program stored in the storage 46.

[0036] The memory 44 can temporarily store programs and various data as a working area.

[0037] The storage 46 is, for example, a Read Only Memory (ROM), a Hard Disc Drive (HDD), a Solid State Drive (SSD), etc., and stores various programs and various data. These programs include not only application programs such as the power adjustment program described above, but also an operating system.

[0038] The input device 48 is a device for performing various inputs to the computer 40. The input device 48 includes operation switches, operation buttons, etc., and may also include pointing devices such as a keyboard and a mouse used in a personal computer, etc.

[0039] The output device 50 is a device for outputting various information from the computer 40, and includes, for example, a display, an indicator lamp, a speaker, etc. A touch panel display can also be used as the output device 50, in which case the touch panel display also functions as the input device 48.

[0040] The storage medium reader 52 is a device that reads data stored in various storage media and writes data to the storage media. Examples of storage media include a CD (Compact Disc)-ROM, a DVD (Digital Versatile Disc)-ROM, a Blu-ray disc, and a USB (Universal Serial Bus) memory.

[0041] The communication I / F 54 is an interface for communicating with other devices, and is compliant with standards such as Ethernet (registered trademark) and FDDI (Fiber Distributed Data Interface).

[0042] In this embodiment, the communication I / F 54 can control the power conditioning device 22, the temperature sensors 24, 28, 34, the pressure sensor 32, the gas flow sensor 36, etc., and obtain the various states of these devices by communicating with these devices. This communication may be wireless or wired.

[0043] Furthermore, the storage 46 stores the amount of thermal energy Q required for keeping the water electrolysis device 14 warm. 1 , the amount of heat energy required to keep the methane synthesis unit 16 warm Q 2 The data stored in the memory include data showing the relationship between temperature and the reaction heat of the water electrolysis reaction, data showing the relationship between temperature and the reaction heat of the Sabatier reaction, trial calculation data for calculating the self-heating amount Qinner in the water electrolysis apparatus 14 described later, and trial calculation data for calculating the heat generation amount Qsab in the methane synthesis apparatus 16.

[0044] Next, a method for controlling the methane production system 12 of this embodiment will be described.

[0045] As described above, the temperature detected by the temperature sensor 24 is T 1 The temperature detected by the temperature sensor 28 is T 2 , the flow rate of the outlet gas detected by the gas flow sensor 36 is f 1 , the dew point temperature of the outlet gas detected by the temperature sensor 34 is Td, the pressure of the outlet gas detected by the pressure sensor 32 is p 1 Let us assume that.

[0046] Here, the initial value before the start of water electrolysis by the water electrolysis device 14, the amount of thermal energy required to keep the water electrolysis device 14 warm, is Q 1 The amount of heat energy required to keep the methane synthesis unit 16 warm is Q 2 , the known amount of heat energy produced by the Sabatier reaction is Q 3 Let us assume that.

[0047] The reaction heat (amount of thermal energy) TΔS required for the water electrolysis reaction is given by the following equation, where enthalpy change is ΔH and Gibbs energy change is ΔG: TΔS=ΔH-ΔG (3) Here, T is the absolute temperature and ΔS is the entropy change.

[0048] The self-heating amount Qinner of the water electrolysis device 14 is expressed as the current density d 1 (A / cm 2 ), the total area of ​​the water electrolysis cell is M (cm 2 ), resistivity is ρ 1 (Ω cm 2 ), then Qinner=d 1 2 ρ 1 M (4) It can be shown that:

[0049] In addition, the flow rate f of hydrogen gas generated in the water electrolysis device 14 3 If Faraday's constant F = 96485 (C / mol) and standard gas volume V = 22.4 (NL / mol), then f3 =60Md 1 V / (2F) (5) It can be shown that:

[0050] The saturated water vapor pressure e at the gas outlet dew point temperature Td is calculated using the Wagner equation: e = Pc exp [(Ax + Bx 2 +Cx 3 +Dx 6 ) / (1-x)] (6) It can be shown that:

[0051] Where: If the critical pressure Pc = 22120 kPa and the critical temperature Tc = 647.3 K, x=1-(Td+273.15) / Tc A=-7.76451 B=1.45838 C=-2.7758 D=-1.23303 It can be shown that:

[0052] In addition, the moisture content of the outlet gas f 2 is the pressure p of the outlet gas detected by the pressure sensor 32 using equation (6). 1 and the flow rate f of the outlet gas detected by the gas flow sensor 36. 1 Using f 2 =ef 1 / p 1 (7) It can be shown that:

[0053] The methane conversion rate η is calculated by the flow rate of the outlet gas f 1 Using the above formulas (5) to (7), η = (5-4(f 1 -f 3 ) / f 2 ) / 4=(5-4(p 1 -e)f 1 / f 2 ) / 4 (8) It can be shown that:

[0054] Therefore, the calorific value Qsab of the methane synthesis unit 16 is calculated by multiplying the flow rate f of the hydrogen gas generated in the water electrolysis unit 14 by the above-mentioned 3 , the temperature T detected by the temperature sensor 28 2 The known amount of heat energy Q of the reaction heat of the Sabatier reaction based on 3 , and methane conversion rate η, Qsab=f 3 Q 3 η=Md 1 Q 3 η / (8F) (9) It can be shown that:

[0055] Therefore, it is preferable to control the above formula (3) so as to satisfy the following formula (10). TΔS=Qinner+Qsab=d 1 2 ρ 1 M+Qsab (10)

[0056] In addition, the amount of heat energy required to keep the water electrolysis device 14 warm is Q 1 The amount of heat energy required to keep the methane synthesis unit 16 warm is Q 2 Further, by controlling so as to satisfy the following formula (11), the control can be performed more efficiently. TΔS=Qinner+Qsab-Q 1 -Q 2 =d 1 2 ρ 1 M+Qsab-Q 1 -Q 2 (11)

[0057] That is, the control device 20 controls the current density d so that the sum of the surplus heat generated by the water electrolysis device 14 due to self-heating and the amount of thermal energy conducted from the methane synthesis device 16 to the water electrolysis device 14 becomes equal to the amount of thermal energy TΔS required for the water electrolysis reaction in the water electrolysis device 14. 1 and controls the power adjustment device 22 to adjust the amount of power supplied to the water electrolysis device 14. This makes it possible to control the methane production system 12 so as to maintain high-efficiency operation.

[0058] The amount of excess heat generated by the water electrolysis device 14 due to self-heating is calculated by subtracting the amount of self-heating Qinner from the amount of thermal energy required to keep the water electrolysis device 14 warm. 1 The difference between and, i.e., Qinner-Q 1 The amount of thermal energy transferred from the methane synthesis device 16 to the water electrolysis device 14 is calculated by subtracting the amount of heat generated by the methane synthesis device 16, Qsab, from the amount of thermal energy required to keep the methane synthesis device 16 warm, Q 2 and the difference, i.e., Qsab-Q 2 It is calculated as follows.

[0059] 3 shows an example of a flow chart illustrating the control process of the control device 20 in this embodiment. This power adjustment process is executed by the control device 20, for example, by adjusting the current density d at a predetermined timing. The control device 20 operates the methane production system 12 using data stored as initial values ​​from a power adjustment program.

[0060] The control device 20 first acquires measurement data to be used in the power adjustment process after a predetermined time has elapsed since the start of the control process. The measurement data includes the current density d 1 , the temperature T detected by the temperature sensor 24 1 , the temperature T detected by the temperature sensor 28 2 , the flow rate of the outlet gas f detected by the gas flow sensor 36 1 , the dew point temperature Td of the outlet gas detected by the temperature sensor 34 and the pressure p of the outlet gas detected by the pressure sensor 32 1 Includes:

[0061] In the methane production system 12, methane is produced by the reaction shown in formula (11) above, and the reaction heat generated at this time is used by the water electrolysis device 14 via the heat transfer body 18.

[0062] Then, in step S11, the control device 20 calculates the amount of thermal energy T required for the water electrolysis reaction in the water electrolysis device 14. 1From ΔS, the amount of excess heat Qinner-Q generated by self-heating from the water electrolysis device 14 is 1 and the amount of thermal energy Qsab-Q transferred from the methane synthesis device to the water electrolysis device 14. 2 It is determined whether the absolute value of the difference between the two is greater than a predetermined value a, which is a preset threshold value.

[0063] That is, |T 1 ΔS-Qinner-Qsab+Q 1 +Q 2 |>a(kW) (12) Calculate whether or not the following is satisfied.

[0064] In step S11, the amount of thermal energy T required for the water electrolysis reaction in the water electrolysis device 14 is calculated based on the equation (12). 1 From ΔS, the amount of excess heat Qinner-Q generated by self-heating from the water electrolysis device 14 is 1 and the amount of thermal energy Qsab-Q transferred from the methane synthesis device to the water electrolysis device 14. 2 If it is determined that the absolute value of the difference between the two is greater than a predetermined value a, which is a preset threshold, the process proceeds to step S12, and d 1 That is, d is calculated by the following formula (13). 1 Calculate.

number

[0065] Then, the process proceeds to step S13, and the control device 20 calculates the current density d by multiplying the current density d calculated in step S12 by d 1 That is, the control device 20 changes the d calculated by the formula (13) to 1 Based on this, the amount of power supplied to the water electrolysis device 14 is adjusted by the power adjustment device 22.

[0066] Then, the process proceeds to step S14 and waits.1 After a predetermined time has elapsed, the process proceeds to step S11.

[0067] In step S11, the formula (12) is not satisfied, that is, the amount of thermal energy T required for the water electrolysis reaction in the water electrolysis device 14 is 1 From ΔS, the amount of excess heat Qinner-Q generated by self-heating from the water electrolysis device 14 is 1 and the amount of thermal energy Qsab-Q transferred from the methane synthesis device to the water electrolysis device 14. 2 If it is determined that the absolute value of the difference between the two is equal to or less than the predetermined value a, which is a preset threshold value, the process proceeds to step S14.

[0068] That is, |T 1 ΔS-Qinner-Qsab+Q 1 +Q 2 |≦a(kW) (14) In this case, the process proceeds to step S14 and waits for a predetermined time, i.e., the current density d is maintained, and then the process proceeds to step S11.

[0069] As described above, in the water electrolysis device 14, the reaction heat generated when synthesizing methane in the methane synthesis device 16 is conducted to the water electrolysis device 14. Then, the control device 20 controls the power adjustment device 22 to adjust the surplus heat Qinner-Q generated by the self-heating of the water electrolysis device 14. 1 and the amount of thermal energy Qsab-Q transferred from the methane synthesis device to the water electrolysis device 14. 2 The current density d is changed by controlling the current or voltage supplied to the water electrolysis device 14 so that the sum of these is equal to the amount of thermal energy TΔS required for the water electrolysis reaction in the water electrolysis device 14, thereby adjusting the amount of electrical energy supplied to the water electrolysis device 14. As a result, the resistivity ρ 1Even if the temperature rises or the catalyst used in the methane synthesis unit 16 deteriorates, it is possible to prevent the water electrolysis unit 14 from becoming overheated or underheated, and it is possible to control the methane production system 12 so as to maintain highly efficient operation.

[0070] (Modification) Next, a modification of the power adjustment process of the control device 20 will be described. In the power adjustment process of this modification, the amount of thermal energy Q required to keep the water electrolysis device 14 warm is 1 and the amount of heat energy required to keep the methane synthesis unit 16 warm Q 2 is not used.

[0071] That is, the control device 20 performs control in such a way that the above formula (10) satisfies the following formula (15). TΔS=Qinner+Qsab=d 1 2 ρ 1 M+Qsab (15)

[0072] and, TΔS>Qinner+Qsab=d 1 2 ρ 1 M+Qsab (16) In this case, the control device 20 determines the current density d 1 By increasing the amount of electric power supplied to the water electrolysis device 14, the amount of self-heating in the water electrolysis device 14 is increased, and control is performed so as to satisfy the formula (15).

[0073] In other words, when the sum of the self-heating amount Qinner of the water electrolysis device 14 and the heat generation amount Qsab of the methane synthesis device 16 is smaller than the amount of thermal energy TΔS required for the water electrolysis reaction in the water electrolysis device 14, the control device 20 controls the current density d 1 Increase (make higher) and adjust to increase the amount of self-heating.

[0074] Also, TΔS <Qinner+Qsab=d 12 ρ 1 M+Qsab (17) In this case, the control device 20 determines the current density d 1 By lowering the value of , the amount of electric power supplied to the water electrolysis device 14 is reduced, thereby reducing the amount of self-heating in the water electrolysis device 14, and control is performed so as to satisfy the formula (15).

[0075] In other words, when the sum of the self-heating amount Qinner of the water electrolysis device 14 and the heat generation amount Qsab of the methane synthesis device 16 is larger than the amount of thermal energy TΔS required for the water electrolysis reaction in the water electrolysis device 14, the control device 20 controls the current density d 1 Adjust it to lower (or reduce) the amount of self-heating.

[0076] In this manner, by adjusting the amount of self-heating in the water electrolysis device 14, the resistivity ρ 1 Even if the temperature rises or the catalyst used in the methane synthesis unit 16 deteriorates, it is possible to prevent the water electrolysis unit 14 from becoming overheated or underheated, and it is possible to control the methane production system 12 so as to maintain highly efficient operation. [Explanation of symbols]

[0077] 12 Methane production system 14 Water electrolysis equipment 16 Methane synthesis unit 18 Heat Transfer Material 20 Control device 22 Power regulator 24, 28, 34 Temperature Sensor 32 Pressure Sensor 36 Gas flow sensor

Claims

1. generating hydrogen by water electrolysis in a water electrolysis device using the supplied electrical energy; a step of reacting the generated hydrogen with carbon dioxide in a methane synthesis device to synthesize methane, and conducting reaction heat generated during the synthesis of methane to the water electrolysis device; adjusting the amount of electric energy supplied to the water electrolysis device so that the sum of an amount of surplus heat generated by self-heating from the water electrolysis device and an amount of thermal energy conducted from the methane synthesis device to the water electrolysis device becomes equal to an amount of thermal energy required for a water electrolysis reaction in the water electrolysis device; A method for producing methane comprising:

2. 2. The methane production method according to claim 1, wherein in the step of adjusting the amount of electric energy supplied to the water electrolysis device, an amount of thermal energy required for a water electrolysis reaction in the water electrolysis device is calculated based on a temperature in the water electrolysis device, and an amount of thermal energy conducted from the methane synthesis device to the water electrolysis device is calculated based on the temperature in the methane synthesis device.

3. In the step of adjusting the amount of electric energy supplied to the water electrolysis apparatus, 2. The methane production method according to claim 1, wherein an amount of electric energy supplied to the water electrolysis device is adjusted when an absolute value of a difference between an amount of heat required for a water electrolysis reaction in the water electrolysis device and both an amount of surplus heat generated by self-heating in the water electrolysis device and an amount of heat energy conducted from the methane synthesis device to the water electrolysis device is greater than a predetermined value.

4. 2. The methane production method according to claim 1, wherein an amount of surplus heat generated by self-heating from the water electrolysis device is calculated as a difference between an amount of self-heating of the water electrolysis device and an amount of thermal energy required to keep the water electrolysis device warm, and an amount of thermal energy conducted from the methane synthesis device to the water electrolysis device is calculated as a difference between an amount of heat generated by the methane synthesis device and an amount of thermal energy required to keep the methane synthesis device warm.

5. a water electrolysis device that generates hydrogen by water electrolysis using the supplied electrical energy; a methane synthesis device that synthesizes methane by reacting hydrogen generated in the water electrolysis device with carbon dioxide, a control device that controls the water electrolysis device to conduct reaction heat generated when synthesizing methane in the methane synthesis device to the water electrolysis device, and adjusts the amount of electric energy supplied to the water electrolysis device so that the sum of an amount of surplus heat generated by self-heating in the water electrolysis device and an amount of thermal energy conducted from the methane synthesis device to the water electrolysis device becomes equal to an amount of thermal energy required for a water electrolysis reaction in the water electrolysis device; A methane production system comprising:

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