Fuel cell system

Abstract

To provide a technique for determining the optimal output current of each of a plurality of FC stacks in a FC system including the FC stacks connected in parallel.SOLUTION: A controller of a FC system disclosed in the present specification determines the latest current voltage characteristics of each FC stack based on the history of output current / voltage of each FC stack. The controller performs, for each FC stack, the following calculations: (1) determining provisional output current as parameters; (2) using the latest current voltage characteristics to calculate provisional output voltage for the provisional output current, and multiplying the provisional output current by the provisional output voltage to calculate provisional output power; and (3) multiplying provisional loss voltage, which is obtained by subtracting provisional output voltage from theoretical electromotive voltage, by the provisional output current to calculate a provisional calorific value. The controller determines target output current of each fuel cell stack such that the total output power of the plurality of FC stacks coincides with the target output power and the total calorific value of the plurality of FC stacks is minimized.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The technology disclosed in this specification relates to a fuel cell system including a plurality of fuel cell stacks. [Background technology]

[0002] Fuel cell systems are known that can obtain large output power by connecting multiple fuel cell stacks in parallel (Patent Documents 1-5). In a fuel cell system, it is desirable to adjust the output of each fuel cell stack according to the degree of deterioration of the multiple fuel cell stacks. For ease of explanation, "fuel cell" may be abbreviated as "FC" below. "Fuel cell system" will be abbreviated as "FC system," and "fuel cell stack" will be abbreviated as "FC stack."

[0003] For example, in the FC system of Patent Document 1, the sweep current of a highly deteriorated FC stack is set smaller than the sweep current of other FC stacks. The degree of deterioration is determined by the magnitude of the output current when the output voltage is kept constant. The controller determines that the lower the output current when the output voltage is kept constant, the greater the deterioration.

[0004] In the FC system of Patent Document 2, coolers are controlled so that multiple FC stacks are at the same temperature. The same amount of hydrogen gas and oxygen is supplied to each of the multiple FC stacks. The controller determines that the lower the output voltage of one of the multiple FC stacks, the greater the deterioration. The controller maintains a constant output for FC stacks with greater deterioration, and responds to fluctuations in the target output power for FC stacks with less deterioration. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-026862 [Patent Document 2] Japanese Patent Application Publication No. 2022-142978 [Patent Document 3] Japanese Patent Application Laid-Open No. 2016-174519 [Patent Document 4] Japanese Patent Publication No. 2023-074244 Summary of the Invention [Problem to be solved by the invention]

[0006] While both FC systems adjust the output of each FC stack according to the degree of deterioration, they do not determine the optimal output current of each FC stack. This specification provides a technique for determining the optimal output current of each FC stack in an FC system having multiple FC stacks. [Means for solving the problem]

[0007] It is known that the output voltage of an FC stack decreases as it deteriorates. This voltage drop is a loss that manifests itself as heat. In other words, the more deteriorated an FC stack is, the greater the amount of heat it generates. The technology disclosed in this specification focuses on the amount of heat generated by each FC stack and determines the optimal output current for each FC stack.

[0008] The FC system disclosed in this specification includes multiple FC stacks connected in parallel and a controller that controls the output current of each FC stack so that the output power of each FC stack matches a target output current. The controller stores the theoretical electromotive force of each FC stack. Note that the theoretical electromotive force is also sometimes called the theoretical electromotive force. The theoretical electromotive force (theoretical electromotive force) is determined by the structure of the FC stack. The controller determines the latest current-voltage characteristics of each FC stack based on the history of the output current and output voltage of each FC stack. For example, the history of the most recent output current and output voltage may be used as the latest current-voltage characteristics. The controller performs the following calculations for each FC stack: (1) The controller determines a provisional output current as a parameter. (2) The controller calculates a provisional output voltage for the provisional output current using the latest current-voltage characteristics, and calculates provisional output power by multiplying the provisional output current by the provisional output voltage. (3) The controller calculates a provisional heat generation amount by multiplying a provisional loss voltage, which is calculated by subtracting the provisional output voltage from the theoretical electromotive force, by the provisional output current. The controller then sweeps the provisional output current and sets the provisional output current at which the sum of the provisional output powers of the multiple FC stacks matches the target output power and the sum of the provisional heat generation amounts of the multiple FC stacks is minimum as the target output current of the fuel cell stack.

[0009] In the FC system disclosed in this specification, the optimal target output current of each FC stack is determined so as to minimize the total heat generation. The target output current is determined based on changes in the current-voltage characteristics over time. In other words, the target output current is determined to reflect the deterioration of the FC stack.

[0010] Furthermore, minimizing the total heat generation of the FC system means that the temperature rise of the FC system is minimized. As the temperature rises, it becomes necessary to lower the upper limit of the output power of the FC stack, but the FC system disclosed in this specification reduces the possibility of lowering the upper limit of the output power.

[0011] Details and further improvements of the technology disclosed in this specification are described in the following "Description of Embodiments of the Invention." [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a graph showing the relationship between the IV characteristics of an FC stack, output power, and heat generation. [Figure 2] FIG. 1 is a block diagram of an FC system according to an embodiment. [Figure 3] 10 is a graph showing an example of a change in IV characteristics. [Figure 4] 10 is a flowchart of a target output current determination process. [Figure 5] 5 is a flowchart of a target output current determination process (continuation of FIG. 4). DETAILED DESCRIPTION OF THE INVENTION

[0013] First, referring to Figure 1, we will explain the IV characteristics (current-voltage characteristics) of an FC stack and the relationship between output power and heat generation. The relationship between the output current and output voltage of an FC stack is called the IV characteristic. As is well known, the output voltage of an FC stack changes depending on the magnitude of the output current. In the example graph of Figure 1, when the output current is Ia, the output voltage is Va. The output power of an FC stack is expressed as the product of the output current and the output voltage, so when the output current is Ia, the output power Pa of the FC stack is Pa = Ia × Va.

[0014] Meanwhile, for FC stacks, the voltage (theoretical electromotive force or theoretical electromotive voltage) theoretically derived from the reaction equation between oxygen and hydrogen is known. The theoretical electromotive force Vid is a value determined by the structure of the FC stack and is calculated from the design data of the FC stack. In the graph of Figure 1, when the output current is Ia, the actual output voltage is Va, so the difference between the theoretical electromotive force Vid and the actual output voltage Va (Vid - Va) is the loss in the FC stack. This loss manifests itself as heat. In other words, an FC stack with the IV characteristics of Figure 1 outputs power Pa for an output current Ia and releases a calorific value Ha = Ca × Ia × (Vid - Va). Here, the symbol Ca is a conversion constant used to convert power loss to calorific value. The conversion constant Ca is also a value determined by the structure of the FC stack and can be determined in advance.

[0015] In this way, in an FC stack, if the IV characteristics and output current are given, the output voltage and heat generation amount can be calculated.

[0016] 2 shows a block diagram of an FC system 2 according to an embodiment. The FC system 2 includes multiple FC stacks (a first FC stack 11 and a second FC stack 21) and a controller 30. The output terminal of the first FC stack 11 is connected to a system output terminal 31 via a first converter 13. The output terminal of the second FC stack 21 is connected to the system output terminal 31 via a second converter 23. That is, the output terminals of the first and second FC stacks 11 and 21 are connected in parallel at the system output terminal 31. A load device 40 is connected to the system output terminal 31. The FC system 2 can supply the total output power of the two FC stacks 11 and 21 to the load device 40. The load device 40 is, for example, a drive device including an inverter and a motor.

[0017] The first FC stack 11 is accompanied by an auxiliary device 12, and the second FC stack 21 is accompanied by an auxiliary device 22. The auxiliary devices include all devices necessary to drive the FC stacks and a fuel tank. The auxiliary devices 12 and 22 are controlled by a controller 30. When the controller 30 increases the amount of hydrogen and oxygen supplied to the first FC stack 11 (second FC stack 21), the output power of the first FC stack 11 (second FC stack 21) increases.

[0018] The first converter 13 boosts and outputs the output voltage of the first FC stack 11. The second converter 23 boosts and outputs the output voltage of the second FC stack 21. The first converter 13 and the second converter 23 are controlled by a controller 30. When the controller 30 makes the output voltage of the first converter 13 (second converter 23) higher than the output voltage of the second converter 23 (first converter 13), the output current of the first converter 13 (second converter 23) increases and the output current of the second converter 23 (first converter 13) decreases.

[0019] The controller 30 can adjust the output current of each of the FC stacks 11 and 21 by controlling the auxiliary devices 12 and 22 and the converters 13 and 23 .

[0020] The FC system 2 includes voltage sensors 14, 24, and 34 and current sensors 15, 25, and 35. The voltage sensor 14 and current sensor 15 measure the output voltage and output current of the first FC stack 11. The voltage sensor 24 and current sensor 25 measure the output voltage and output current of the second FC stack 21. The voltage sensor 34 measures the higher of the output voltages of the first converter 13 and the second converter 23. In other words, the voltage sensor 34 measures the voltage at the system output terminal 31. The current sensor 35 measures the total output current of the first converter 13 and the second converter 23 (i.e., the sum of the output currents of the first FC stack 11 and the second FC stack 21). The measurement data of these sensors is sent to the controller 30.

[0021] The controller 30 obtains data on the target output power (i.e., the power required by the load device 40) from the load device 40. The controller 30 controls the auxiliaries 12, 22, the first converter 13, and the second converter 23 so that the total output power of the first FC stack 11 and the second FC stack 21 matches the target output power.

[0022] The IV characteristics of an FC stack change over time. Figure 3 shows an example of the change in IV characteristics. Graph IVa shows the initial IV characteristics, and graph IVb shows the IV characteristics after a certain time has passed. As shown in Figure 3, the output voltage of the FC stack for the same output current decreases over time due to deterioration of the FC stack. Note that even if the FC stack deteriorates, the theoretical electromotive voltage Vid of the FC stack does not change.

[0023] The controller 30 acquires and stores the history (i.e., past data) of the output current and output voltage of each of the first FC stack 11 and the second FC stack 21. The controller 30 identifies the latest IV characteristics (current-voltage characteristics) of each FC stack from the history of the output current and output voltage. For example, the controller 30 may use the latest history as the latest IV characteristics, or may use the average of multiple past histories as the latest IV characteristics. Furthermore, the controller 30 may estimate the latest IV characteristics from the history of multiple past IV characteristics.

[0024] If the IV characteristics change, the output power and heat generation amount will change even if the output current of the FC stack remains the same. When the target output power value is given to the controller 30 from the load device 40, the controller 30 determines the target output current of each FC stack so that the total output power of the first and second FC stacks 11, 21 matches the target output power and the total heat generation amount is minimized. The controller 30 then adjusts the output current of each FC stack so that the output current of each of the first and second FC stacks 11, 21 matches the respective target output current. As described above, the controller 30 can adjust the output current of each of the first and second FC stacks 11, 21 by controlling the auxiliaries 12, 22 and converters 13, 23.

[0025] 4 and 5 show flowcharts of the process by which the controller 30 determines the target output current. The controller 30 repeats the processing from steps S12 to S16 in Fig. 4 for each FC stack (step S17: NO).

[0026] The controller 30 executes the following process for each FC stack. The controller 30 sets the minimum output current value of the FC stack to the provisional output current, which is a parameter in the program (step S12). The controller 30 calculates the provisional output voltage for the provisional output current based on the latest IV characteristics (step S13). The controller 30 calculates the provisional output power by multiplying the provisional output current by the provisional output voltage. The controller 30 also calculates the provisional heat generation amount by multiplying the value obtained by subtracting the provisional output voltage from the theoretical electromotive voltage by the provisional output current (step S14). Note that here, it is assumed that the conversion constant Ca shown in FIG. 1 is "1".

[0027] The controller 30 increases the provisional output current by a fixed increment and repeats the processes of steps S13 and S14 until the provisional output current reaches the maximum output current of the FC stack (steps S15 and S16). Note that each time step S14 is executed, the controller 30 stores the provisional output current and the corresponding provisional output power and provisional heat generation amount.

[0028] Repeating a specific process while gradually increasing the provisional output current is referred to as "sweeping." In the flowchart of Fig. 4, the controller 30 calculates the provisional output power and the provisional heat generation amount while gradually increasing the provisional output current. The controller 30 may also calculate the provisional output power and the provisional heat generation amount while gradually decreasing the provisional output current from the maximum output current.

[0029] The controller 30 repeats the processing of steps S12 to S16 for all FC stacks (step S17: NO).

[0030] After the controller 30 has finished calculating the provisional output current / provisional output power / provisional heat generation amount for all FC stacks, it executes the processes from step S22 onwards. In step S22, the controller 30 determines the provisional output current of each FC stack when the sum of the provisional output powers of all FC stacks matches the target output power and the sum of the provisional heat generation amounts is minimum as the target output current for each FC stack. Finally, the controller 30 adjusts the output current of each FC stack so that the output current of each FC stack matches the target output current (step S23).

[0031] The FC system 2 includes two FC stacks 11, 21. In this case, step SS2 is processed as follows: The controller 30 determines a provisional output current Ia1 of the first FC stack 11. The controller 30 calculates a provisional output power Pa1 and a provisional heat generation amount Ha1 corresponding to the provisional output current Ia1. The controller 30 subtracts the provisional output power Pa1 of the first FC stack 11 from the target output power Ptotal to calculate a provisional output power Pa2 of the second FC stack 21. The controller 30 calculates a provisional output current Ia2 and a provisional heat generation amount Ha2 corresponding to the provisional output power Pa2. The controller 30 adds up the provisional heat generation amounts Ha1 and Ha2 to obtain the total provisional heat generation amount.

[0032] The controller 30 repeats the above process while gradually increasing (or gradually decreasing) the provisional output current Ia1. When the provisional output current Ia1 reaches its maximum (minimum), it identifies the provisional output currents Ia1 and Ia2 at which the total provisional heat generation amount is minimum. The controller 30 sets the identified provisional output currents Ia1 and Ia2 as the target output currents for the first FC stack 11 and the second FC stack 21, respectively. In other words, the controller 30 sweeps the provisional output current and sets the provisional output current at which the total provisional output power of the multiple FC stacks matches the target output power and the total provisional heat generation amount of the multiple FC stacks is minimum as the target output current for each FC stack.

[0033] The controller 30 repeats the above process periodically or every time the target output power is changed.

[0034] The FC system 2 of the embodiment has the following advantages. The FC system 2 determines the target output current of each FC stack so that the total heat generation amount of the multiple FC stacks (the sum of the heat generation amounts of the multiple FC stacks) is minimized. Because the heat generation amount of each FC stack is based on the latest IV characteristics, the heat generation amount reflects the deterioration of the FC stack. In other words, the FC system 2 can minimize the total heat generation amount while taking into account the deterioration of each FC stack. The FC system 2 can determine the optimal target output current that can minimize the total heat generation amount.

[0035] The amount of heat generated by the FC stack causes a rise in the temperature of the FC stack. Reducing the total amount of heat generated by the FC stack also helps to reduce the temperature rise of the FC stack. If the temperature of the FC stack exceeds a predetermined upper limit temperature, the controller must lower the upper limit of the FC stack's output. By reducing the total amount of heat generated, the FC system 2 of the embodiment ultimately reduces the temperature rise of the FC stack, making it less likely that the upper limit of output will need to be lowered.

[0036] Here are some points to note regarding the technology described in the embodiment. The FC system 2 in the embodiment includes two FC stacks. The technology disclosed in this specification is also suitable for application to an FC system having three or more FC stacks.

[0037] It is desirable that the characteristics (size and maximum output) of the multiple FC stacks are the same. When the characteristics of the multiple FC stacks are different, instead of setting the target output current so as to minimize the total sum of the provisional heat generation amounts, the controller 30 may set the target output current so as to minimize the total sum of the provisional heat generation amounts per unit volume of each FC stack. Alternatively, the controller 30 may set the target output current so as to minimize the total sum of the provisional heat generation amounts per unit heat capacity of the FC stack.

[0038] Although specific examples of the present invention have been described in detail above, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and variations of the specific examples exemplified above. The technical elements described in this specification or drawings exhibit technical utility alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. Furthermore, the technology exemplified in this specification or drawings can achieve multiple objectives simultaneously, and achieving one of these objectives alone is technically useful. [Explanation of symbols]

[0039] 2: FC system 11, 21: FC stack 12, 22: Auxiliary equipment 13, 23: Converter 14, 24, 34: Voltage sensor 15, 25, 35: Current sensor 30: Controller 31: System output terminal 40: Load device

Claims

[Claim 1] a plurality of fuel cell stacks connected in parallel; a controller that controls the output current of each of the fuel cell stacks so that the output current of each of the fuel cell stacks matches a target output current of the fuel cell stack; It is equipped with The controller A theoretical electromotive voltage of each of the fuel cell stacks is stored, Identifying the latest current-voltage characteristics of each of the fuel cell stacks based on the history of the output current and output voltage of each of the fuel cell stacks; For each of the fuel cell stacks: (1) Determine a temporary output current as a parameter, (2) calculating a provisional output voltage for the provisional output current using the current-voltage characteristic, and calculating provisional output power by multiplying the provisional output current by the provisional output voltage; (3) calculating a provisional heat generation amount by multiplying a provisional loss voltage obtained by subtracting the provisional output voltage from the theoretical electromotive voltage by the provisional output current; sweeping the provisional output current, and setting the provisional output current when the sum of the provisional output powers of the plurality of fuel cell stacks matches the target output power and the sum of the provisional heat generation amounts of the plurality of fuel cell stacks is minimum as the target output current of the fuel cell stack; Fuel cell system.

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