Fuel cell system

Abstract

To further improve the efficiency of fuel cell systems. [Solution] The fuel cell system includes a power generation module which includes a fuel cell that generates electricity when supplied with fuel and a combustion unit which burns unused fuel that was not used in the fuel cell; a fuel supply system which supplies fuel to the fuel cell; a voltage sensor which detects the voltage of the fuel cell; and a control unit which maintains a constant current drawn from the fuel cell and gradually increases the fuel utilization rate while monitoring the voltage detected by the voltage sensor, and performs a first control which determines a set fuel utilization rate based on the fact that the amount of decrease in the voltage detected by the voltage sensor per unit time falls below a predetermined amount.

Description

Technical Field

[0001] The present specification discloses a fuel cell system.

Background Art

[0002] Conventionally, as this type of fuel cell system, there has been proposed one that diagnoses the deterioration state of a predetermined portion of a cell stack based on the amount of change (rate of change) in the output voltage of the cell stack when: detecting a stable temperature state of the predetermined portion of the cell stack, and changing the output current of the cell stack, the supply amount of fuel gas, and the supply amount of oxidant gas at a constant ratio during the stable state, or changing the output current of the cell stack while keeping the supply amount of fuel gas and the supply amount of oxidant gas constant (see, for example, Patent Document 1).

Prior Art Literature

Patent Literature

[0003]

Patent Document 1

Summary of the Invention

Problem to be Solved by the Invention

[0004] In the fuel cell system described above, although it is described that the deterioration state of the cell stack is diagnosed based on the amount of change (rate of change) in the output voltage of the cell stack, no mention is made at all about improving the efficiency of the system.

[0005] The main object of the present disclosure is to further improve the efficiency of a fuel cell system.

Means for Solving the Problem

[0006] The present disclosure adopts the following means to achieve the above-mentioned main object.

[0007] The first fuel cell system of this disclosure comprises a power generation module including a fuel cell that generates electricity upon receiving a fuel supply and a combustion unit that burns unused fuel not used by the fuel cell; a fuel supply system that supplies fuel to the fuel cell; a voltage sensor that detects the voltage of the fuel cell; and a control unit that maintains a constant current drawn from the fuel cell and gradually increases the fuel utilization rate while monitoring the voltage detected by the voltage sensor, and performs a first control that determines a fuel utilization rate to be set based on the fact that the amount of decrease in the voltage detected by the voltage sensor per unit time falls below a predetermined amount.

[0008] In the first fuel cell system of this disclosure, a first control is performed in which the current drawn from the fuel cell is kept constant, and the fuel utilization rate is gradually increased while monitoring the voltage detected by a voltage sensor, and determining the fuel utilization rate to be set based on the fact that the amount of decrease in the voltage detected by the voltage sensor per unit time falls below a predetermined amount. When a fuel shortage occurs in the fuel cell, the voltage of the fuel cell drops sharply, so by monitoring this, it is possible to set the highest possible fuel utilization rate within the range in which a fuel shortage does not occur in the fuel cell. As a result, the efficiency of the fuel cell system can be further improved.

[0009] The second fuel cell system of this disclosure comprises a power generation module including a fuel cell that generates electricity upon receiving a fuel supply and a combustion unit that burns unused fuel not used by the fuel cell; a fuel supply system that supplies fuel to the fuel cell; and a control unit that performs a second control that calculates the power generation efficiency before and after the change in the fuel utilization rate while changing the fuel utilization rate, compares the two power generation efficiencies, and transitions to the fuel utilization rate with the higher power generation efficiency.

[0010] In the second fuel cell system of this disclosure, the power generation efficiency is calculated before and after the change in fuel utilization rate while the fuel utilization rate is changed, and the power generation efficiency of both is compared. A second control is then performed to transition to the fuel utilization rate that has the higher power generation efficiency. By transitioning the fuel utilization rate to the one with the higher power generation efficiency, the efficiency of the fuel cell system can be further improved. [Brief explanation of the drawing]

[0011] [Figure 1] This is a schematic diagram of the fuel cell system of this embodiment. [Figure 2] This is a schematic diagram illustrating the configuration of each power generation module and auxiliary equipment in a fuel cell system. [Figure 3] This is a schematic diagram of the power generation module. [Figure 4] This flowchart shows an example of a fuel utilization rate adjustment control routine. [Figure 5] This flowchart shows an example of a fuel utilization rate adjustment control routine. [Figure 6] This diagram illustrates how the output current, output voltage, fuel flow rate, and power generation efficiency change over time as the set utilization rate is increased. [Figure 7] This is an explanatory diagram showing the timing of the efficiency evaluation. [Figure 8] This diagram illustrates how the optimal utilization rate is set for each power generation module HM1 and HM2 based on efficiency evaluation. [Figure 9] This is an explanatory diagram showing the relationship between the set utilization rate and power generation efficiency for each power generation module HM1 and HM2. [Modes for carrying out the invention]

[0012] The forms for implementing this disclosure will be explained with reference to the drawings.

[0013] FIG. 1 is a schematic configuration diagram of a fuel cell system 10 according to the present embodiment, FIG. 2 is a schematic configuration diagram of each power generation module 20 and auxiliary equipment 30 included in the fuel cell system 10, and FIG. 3 is a schematic configuration diagram of the power generation module 20.

[0014] As shown in FIG. 1, the fuel cell system 10 of the embodiment includes a plurality of power generation units 11 and an integrated control device 100 that manages the plurality of power generation units 11.

[0015] As shown in FIG. 1, each of the plurality of power generation units 11 includes a power generation module 20 including a fuel cell stack 21, various auxiliary equipment 30 necessary for operating the fuel cell stack 21, and a module control device 90 that controls the various auxiliary equipment 30.

[0016] As shown in FIG. 3, the power generation module 20 includes a combustor 22 and heat exchangers 23, 24 in addition to the fuel cell stack 21, and these are housed in a heat-insulating module case 29. As shown in FIG. 2, the various auxiliary equipment 30 includes a fuel supply system 40, an air supply system 50, a circulation system 60, an exhaust heat recovery system 70, and the like, and the fuel supply system 40, the air supply system 50, and the circulation system 60 are provided for each power generation module 20.

[0017] The fuel cell stack 21 includes a plurality of solid oxide single cells each including an electrolyte, a fuel electrode disposed on one surface side of the electrolyte, and an oxidizer electrode disposed on the other surface side of the electrolyte. Since the fuel cell stack 21 operates in a high-temperature environment of, for example, 600 to 800°C, the electrolyte, the fuel electrode, and the oxidizer electrode are made of ceramic materials. Further, a cermet of a metal having a catalytic action such as nickel and ceramics is used for the fuel electrode. Each fuel cell stack 21 generates power through a reaction between hydrogen contained in fuel gas and oxygen contained in oxidant gas. A temperature sensor 94 is installed in the vicinity of the fuel cell stack 21. The temperature sensor 94 detects a temperature correlated with the temperature of the fuel cell stack 21 (stack temperature T4).

[0018] As shown in Figure 3, a fuel gas supply pipe 21a is connected to the fuel electrode inlet of the fuel cell stack 21, and a heat exchanger 23 is installed in the fuel gas supply pipe 21a to exchange heat between the fuel gas flowing through the fuel gas supply pipe 21a and the fuel off-gas discharged from the fuel cell stack 21. In addition, an oxidizer gas supply pipe 21b is connected to the oxidizer electrode inlet of the fuel cell stack 21, and a heat exchanger 24 is installed in the oxidizer gas supply pipe 21b to exchange heat between the oxidizer gas flowing through the oxidizer gas supply pipe 21b and the combustion exhaust gas discharged from the combustor 22.

[0019] Hydrogen gas supplied by the fuel supply system 40 is introduced as fuel gas to the fuel electrode of the fuel cell stack 21 via the fuel gas supply pipe 21a, and air supplied by the air supply system 50 is introduced as oxidant gas to the oxidant electrode of the fuel cell stack 21 via the oxidant gas supply pipe 21b. Then, oxide ions (O) are introduced into the oxidant electrode. 2-) is generated, and the oxide ions permeate the electrolyte and react with hydrogen at the fuel electrode, thereby obtaining electrical energy. The fuel off-gas (residual fuel gas) that has not been used in the electrochemical reaction (power generation) at the fuel electrode of each unit cell undergoes heat exchange with the fuel gas (hydrogen gas) supplied from the fuel supply system 40 to the fuel electrode in the heat exchanger 23, and is then discharged to the outside of the module case 29. Then, the fuel off-gas discharged to the outside of the module case 29 is supplied to the circulation system 60 through the fuel off-gas pipe 61, and after passing through the circulation system 60, is supplied to the combustor 22. In addition, the oxidant off-gas (residual oxidant gas) that has not been used in the electrochemical reaction (power generation) at the oxidant electrode of each unit cell is directly supplied to the combustor 22. The fuel off-gas introduced into the combustor 22 is a flammable gas containing hydrogen, and is mixed with the oxidant off-gas containing oxygen introduced into the combustor 22. When the mixed gas is combusted in the combustor 22, the fuel cell stack 21 is maintained at an appropriate temperature by the heat of combustion. The combustor 22 is provided with an ignition device 22f for igniting the mixed gas introduced into the combustor 22 during startup processing, and a temperature sensor 95 for detecting the internal temperature of the combustor 22 (combustion portion temperature T6). Furthermore, in the combustor 22, combustion exhaust gas is generated by the combustion of the mixed gas, the combustion exhaust gas undergoes heat exchange with the oxidant gas (air) supplied from the air supply system 50 to the oxidant electrode in the heat exchanger 24, and is then supplied to the exhaust heat recovery system 70 through the combustion exhaust gas pipe 71. Then, after the exhaust heat is recovered by the exhaust heat recovery system 70, the combustion exhaust gas is discharged to the outside air.

[0020] Each fuel supply system 40 includes a branch pipe 41 that branches off from the other end of a hydrogen supply pipe 31, one end of which is connected to a hydrogen supply source such as a hydrogen tank, to the corresponding power generation module 20, and a hydrogen blower 42 installed in the branch pipe 41. By operating the hydrogen blower 42, hydrogen gas from the hydrogen supply source is pressurized and supplied to the power generation module 20 as fuel gas. Since a hydrogen blower 42 is installed in each branch pipe 41, the amount of fuel gas supplied to each power generation module 20 can be controlled by individually controlling each hydrogen blower 42. In addition, the hydrogen supply pipe 31 is equipped with an on / off valve 32 (double valve) and a negative pressure prevention valve (not shown), and each branch pipe 41 is equipped with a governor 43 (pressure equalizing valve) and a fuel flow meter 44 in addition to the hydrogen blower 42. The fuel flow meter 44 detects the flow rate (fuel flow rate Fg) of the fuel gas flowing through the branch pipe 41 per unit time. The fuel gas introduced into the power generation module 20 is heated in the heat exchanger 23 through heat exchange with the fuel off-gas, and then supplied to the fuel electrode of the fuel cell stack 21.

[0021] Each air supply system 50 includes an air supply pipe 51 connected to the corresponding power generation module 20, a filter 53 provided at the inlet of the air supply pipe 51, and an air blower 52 installed on the air supply pipe 51. By operating the air blower 52, air is drawn in from the filter 53 and the drawn air is pressurized and supplied to the power generation module 20 as an oxidizer gas. Since an air blower 52 is installed on each air supply pipe 51, the amount of air supplied to each power generation module 20 can be controlled by individually controlling each air blower 52. In addition, an air flow meter 54 is installed on each air supply pipe 51. The air flow meter 54 detects the flow rate (air flow rate Fa) of the air flowing through the air supply pipe 51 per unit time. The air introduced into the power generation module 20 is heated by heat exchange with combustion exhaust gas in the heat exchanger 24 and then supplied to the oxidizer electrode of the fuel cell stack 21.

[0022] Each circulation system 60 includes a fuel off-gas pipe 61 connected to a corresponding power generation module 20, a condenser 62 installed in the fuel off-gas pipe 61, and a combustion gas pipe 63 that branches off from downstream of the condenser 62 in the fuel off-gas pipe 61 and is connected to the combustor 22 of the corresponding power generation module 20. Furthermore, the circulation system 60 also includes a recirculation pipe 64 that branches off from downstream of the condenser 62 in the fuel off-gas pipe 61 and is connected between the hydrogen blower 42 and the governor 43 in the corresponding branch pipe 41, and an orifice 65 formed in the recirculation pipe 64.

[0023] Fuel off-gas discharged from the fuel electrode outlet of the fuel cell stack 21 of the corresponding power generation module 20 passes through the fuel off-gas piping 61, where the water vapor contained in the fuel off-gas is condensed in the condenser 62 through heat exchange with the cooling water. The fuel off-gas that has passed through the condenser 62 is then distributed to the combustion gas piping 63 and the recirculation piping 64. The fuel off-gas distributed to the combustion gas piping 63 is supplied to the combustor 22 of the corresponding power generation module 20. Meanwhile, the fuel off-gas distributed to the recirculation piping 64 is drawn into the branch pipe 41 by the negative pressure generated by the operation of the hydrogen blower 42, introduced into the power generation module 20, and supplied to the fuel electrode of the fuel cell stack 21. An orifice 65 is formed in the recirculation piping 64, but a solenoid valve may be provided instead of the orifice 65.

[0024] Furthermore, the condensed water obtained by condensing water vapor in each condenser 62 is stored in a condensed water tank 67 via a condensed water pipe 66 that branches off from the fuel off-gas pipe 61. A drain valve 68 is installed in the condensed water tank 67, and the condensed water stored in the condensed water tank 67 is discharged to the outside via the drain valve 68.

[0025] The waste heat recovery system 70 includes a heat exchanger 72 connected to each combustion exhaust gas pipe 71. The heat from the combustion exhaust gas is recovered in the heat exchanger 72 through heat exchange with a heat exchange medium, and the recovered heat is supplied to heat utilization equipment installed in factories, etc.

[0026] Each fuel cell stack 21 of the power generation module 20 is connected in series to a single power conditioner 15, and the DC power generated in each fuel cell stack 21 is converted by the power conditioner 15 and supplied to the load L. A voltage sensor 91 is installed between the output terminals of each fuel cell stack 21 of the power generation module 20 to detect the output voltage of the fuel cell stack 21. In addition, a voltage sensor 92 is installed between one terminal (fuel electrode terminal) of the fuel cell stack 21 located at one end of the series-connected fuel cell stack 21 and the other terminal (oxidizer electrode terminal) of the fuel cell stack 21 located at the other end to detect the total voltage (total voltage Vt) of each fuel cell stack 21. Furthermore, a current sensor 93 is installed in the power line connecting each fuel cell stack 21 in series to detect the current flowing through the power line.

[0027] The power conditioner 15 has a DC / DC converter and an inverter, and converts the DC power from each fuel cell stack 21 into AC power at a voltage that can be connected to the grid power supply (for example, AC 200V) and outputs it. A power supply board (not shown) is connected to the power conditioner 15. The power supply board converts the power from each fuel cell stack 21 into DC power at a voltage suitable for driving various auxiliary equipment 30, module control device 90, and integrated control device 100, and supplies them to each. In addition, the auxiliary equipment room where the power conditioner 15 and power supply board are located is equipped with a cooling fan (not shown) and a ventilation fan to cool the power conditioner 15 and power supply board.

[0028] Each module control device 90, although not shown in the diagram, is configured as a microprocessor centered around a CPU. In addition to the CPU, it includes a ROM for storing processing programs, a RAM for temporarily storing data, an EEPROM as non-volatile memory, input / output ports, and communication ports. Each module control device 90 receives inputs via its input ports, including the stack temperature T4 from a temperature sensor 94 installed near the fuel cell stack 21 of the corresponding power generation module 20, the combustion section temperature T6 from a temperature sensor 95 installed in the combustor 22 of the corresponding power generation module 20, the output voltage V from a voltage sensor 91 attached between the output terminals of the fuel cell stack 21 of the corresponding power generation module 20, the fuel flow rate Fg from a fuel flow meter 44 installed in the branch pipe 41 of the corresponding fuel supply system 40, and the air flow rate Fa from an air flow meter 54 installed in the air supply pipe 51 of the corresponding air supply system 50. Furthermore, each module control device 90 outputs control signals to the hydrogen blower 42 of the corresponding fuel supply system 40 and control signals to the air blower 52 of the corresponding air supply system 50 via its output ports.

[0029] The integrated control unit 100 is configured as a microprocessor centered around a CPU 101. In addition to the CPU 101, it includes a ROM 102 for storing processing programs, a RAM 103 for temporarily storing data, an EEPROM 104 as non-volatile memory, a timer (not shown), input / output ports, and communication ports (not shown). The integrated control unit 100 receives inputs such as the total voltage Vt detected by the voltage sensor 92 and the output current I from the current sensor 93 via its input ports. The integrated control unit 100 also outputs control signals to the power conditioner 15 (DC / DC converter, inverter) and control signals to the on / off valve 32 via its output ports. Furthermore, the integrated control unit 100 is connected to each module control unit 90 via a communication bus 12, and they exchange control signals and data with each other.

[0030] Next, the operation of the fuel cell system 10 of this embodiment, configured in this way, will be described. When a request to start the fuel cell system 10 is received from a higher-level system, each module control device 90 performs a start process according to instructions from the integrated control device 100. In the start process, each module control device 90 sequentially controls the auxiliary equipment 30 to perform a purging process of the combustor 22, and then supplies fuel gas (hydrogen gas) and air to the combustor 22, performing a warm-up process to warm up the fuel cell stack 21 by burning the fuel gas in the combustor 22. Then, when the warm-up of each fuel cell stack 21 is complete and it is ready to generate power, each module control device 90 moves on to the power generation process. As described above, the fuel cell stack 21 of each power generation module 20 is connected in series with respect to the load L, and current flows to all fuel cell stacks 21 when the current sweep starts. For this reason, each module control device 90 needs to start the power generation process of the fuel cell stack 21 simultaneously. Therefore, the transition to power generation is initiated by an instruction from the integrated control device 100 after the integrated control device 100 has confirmed that all fuel cell stacks 21 have completed warming up and reached a state where power generation is possible.

[0031] The integrated control unit 100 sets a target current Itag based on the power requirements of the system, instructs each module control unit 90 to perform power generation processing based on the target current Itag, and controls the power conditioner 15 so that a current corresponding to the target current Itag is drawn from each fuel cell stack 21. Each module control unit 90 controls the fuel flow rate and air flow rate based on the target current Itag. Specifically, the fuel flow rate is controlled by setting the target fuel flow rate Fgtag to a flow rate obtained by multiplying the fuel flow rate required for the output of the target current Itag by the reciprocal of the fuel utilization rate (the set utilization rate Uf converted to a decimal), and then controlling the fuel pump 35 by feedback control so that the fuel flow rate Fg detected by the fuel flow meter 38 matches the target fuel flow rate Fgtag. The fuel utilization rate is the ratio of the amount of fuel used for power generation to the amount of fuel supplied to the fuel electrodes. The set utilization rate Uf is set individually in each module control unit 90. Air flow rate control is performed by setting the target air flow rate Fatag to the air flow rate required for the reaction with the fuel in the fuel cell stack 21 multiplied by the reciprocal of the air utilization rate Ua (converted to a decimal), and then controlling the air pump 53 by feedback control so that the air flow rate Fa detected by the air flow meter 54 matches the target air flow rate Fatag. The air utilization rate Ua is the ratio of the amount of air used for power generation to the amount of air supplied to the oxidizer electrode.

[0032] If a system shutdown is requested during power generation, each module control device 90 performs a shutdown process based on instructions from the integrated control device 100. During the shutdown process, each module control device 90 controls the hydrogen blower 42 so that fuel gas is supplied at a flow rate that does not cause oxidative degradation of the electrodes of the fuel cell stack 21, and controls the air blower 52 so that air is supplied at a flow rate necessary for cooling the fuel cell stack 21. When the stack temperature T4 from the temperature sensor 94 drops below a predetermined temperature, each module control device 90 stops the supply of fuel gas and air.

[0033] Next, we will explain the process for adjusting the fuel utilization rate. Figures 4 and 5 are flowcharts showing an example of a fuel utilization rate adjustment control routine. This routine is executed for each module control device 90 after the system has started up.

[0034] When the fuel utilization rate adjustment control routine is executed, each module control device 90 (CPU) first waits for rated operation to begin after startup (step S100). Once rated operation has started, the integrated control device 100 (CPU 101) controls the power conditioner 15 (DC / DC converter, inverter) so that the current drawn from each fuel cell stack 21 remains constant. After determining that rated operation has started after startup, each module control device 90 determines whether this is the first startup (step S102).

[0035] When each module control device 90 determines that this is the first startup, it acquires the amount of fluctuation (voltage fluctuation ΔV) of the output voltage V detected by the voltage sensor 91 per unit time (e.g., 300 sec) (step S104), and determines whether the acquired voltage fluctuation ΔV is less than a negative threshold α (step S106). Here, the threshold α is a threshold used to determine whether there is a shortage in the supply of fuel (hydrogen) to the fuel cell stack 21, and is predetermined by experiments or analyses. Since the threshold α is a negative value, the determination of whether the voltage fluctuation ΔV is less than the threshold α means determining whether the output voltage V has decreased by a predetermined amount (absolute value of threshold α) during a unit time.

[0036] When each module control device 90 determines that the voltage fluctuation ΔV is not less than the threshold α, it increases the set utilization rate Uf by a predetermined ratio (step S108) and returns to step S104. That is, each module control device 90 repeats the process of increasing the set utilization rate Uf by a predetermined ratio (e.g., 0.5%) every predetermined time (e.g., 10 min) until the voltage fluctuation ΔV becomes less than the negative threshold α in step S106. When the set utilization rate Uf is increased by the predetermined ratio, the target fuel flow rate Fgtag is set to decrease by an amount multiplied by the reciprocal of the predetermined ratio. This makes it possible to improve power generation efficiency with a smaller fuel supply. In the process of repeating the process, when each module control device 90 determines in step S106 that the voltage fluctuation ΔV is less than the threshold α, it decreases the set utilization rate Uf by a predetermined ratio, that is, returns it to the previous set utilization rate Uf (step S110), and proceeds to step S200.

[0037] Figure 6 is an explanatory diagram showing the time changes in output current I, output voltage V, fuel flow rate Fg, and power generation efficiency when the set utilization rate Uf is increased. As shown in the figure, the set utilization rate Uf is increased by a predetermined ratio at predetermined time intervals while keeping the output current I constant. As a result, the power generation efficiency increases as the set utilization rate Uf increases. On the other hand, if the set utilization rate Uf is increased too much, a phenomenon occurs where the output voltage V drops sharply due to a fuel shortage in the fuel cell stack 21. For this reason, when the voltage fluctuation amount ΔV falls below a negative threshold α and the output voltage V drops by more than a predetermined amount per unit time (time t0), the increase in the set utilization rate Uf is stopped and it is returned to the previous set utilization rate Uf. This makes it possible to set the set utilization rate Uf as high as possible without causing a fuel shortage, and thus optimize the power generation efficiency.

[0038] If the CPU 101 determines in step S102 that this is not the first boot but a subsequent boot, it reads the setting utilization rate Uf stored in the EEPROM (step S112) and proceeds to step S200.

[0039] Next, each module control device 90 determines whether or not the efficiency determination condition has been met (step S200). Here, the efficiency determination condition is a condition that is met, for example, when a predetermined time T has elapsed after the execution of the process of increasing the set utilization rate Uf in steps S104 to S108, or when a predetermined time T has elapsed since the execution of the previous efficiency determination, as shown in Figure 7. The predetermined time T is the waiting time required for the operating state to stabilize after changing the set utilization rate Uf. If each module control device 90 determines that the efficiency determination condition has not been met, it proceeds to step S226.

[0040] On the other hand, when each module control device 90 determines that the efficiency determination condition has been met, it increases the set utilization rate Uf by a predetermined ratio (step S202), and then calculates the power generation efficiency η after the increase in the set utilization rate using the following equation (1) (step S204). In equation (1), "V" is the output voltage, "I" is the output current, and "Fg" is the fuel flow rate. Also, "k" is a coefficient. In this embodiment, the power generation efficiency η is calculated as the average value over a predetermined period (e.g., 5 min) after a predetermined time (e.g., 5 min) has elapsed since the set utilization rate Uf was changed.

[0041] η = (V × I) / (k·Fg) …(1)

[0042] Next, each module control device 90 determines whether the power generation efficiency η before the increase in the set utilization rate has already been calculated (step S206). If each module control device 90 determines that the power generation efficiency η before the increase in the set utilization rate has not been calculated, it returns to the set utilization rate Uf before the increase (step S208) and calculates the power generation efficiency η before the increase in the set utilization rate using equation (1) (step S210). Then, each module control device 90 determines whether the power generation efficiency η after the increase in the set utilization rate is less than or equal to the power generation efficiency η before the increase in the set utilization rate (step S212). If each module control device 90 determines that the power generation efficiency η after the increase in the set utilization rate is less than or equal to the power generation efficiency η before the increase in the set utilization rate, it proceeds to step S220. On the other hand, if each module control device 90 determines that the power generation efficiency η after increasing the set utilization rate is greater than the power generation efficiency η before increasing the set utilization rate, it sets the increased set utilization rate Uf in step S202 (step S214), returns to step S202, and repeats the process of steps S202 and S204, where the set utilization rate Uf is increased by a predetermined ratio and the power generation efficiency η at that time is calculated, before proceeding to step S206. Here, since the power generation efficiency η after increasing the set utilization rate calculated in the most recent step S204 is the power generation efficiency η before increasing the set utilization rate, it is determined that the power generation efficiency η before increasing the set utilization rate has already been calculated. Next, each module control device 90 determines whether the power generation efficiency η after increasing the set utilization rate is less than or equal to the power generation efficiency η before increasing the set utilization rate (step S216). If each module control device 90 determines that the power generation efficiency η after increasing the set utilization rate is greater than the power generation efficiency η before increasing the set utilization rate, it returns to step S202 and repeats the process of steps S202 to S216. In other words, each module control device 90 increases the set utilization rate Uf by a predetermined ratio at predetermined intervals until the power generation efficiency η after the increase in the set utilization rate is less than or equal to the power generation efficiency η before the increase in the set utilization rate. On the other hand, when each module control device 90 determines that the power generation efficiency η after the increase in the set utilization rate is less than or equal to the power generation efficiency η before the increase in the set utilization rate, it sets the previous (before increase) set utilization rate Uf (step S218) and proceeds to step S220.

[0043] Next, each module control device 90 reduces the set utilization rate Uf by a predetermined ratio (step S220), and then calculates the power generation efficiency η after the reduction in the set utilization rate using equation (1) (step S222). Subsequently, each module control device 90 determines whether the power generation efficiency η after the reduction in the set utilization rate is less than or equal to the power generation efficiency η before the reduction in the set utilization rate (step S224). If each module control device 90 determines that the power generation efficiency η after the reduction in the set utilization rate is greater than the power generation efficiency η before the reduction in the set utilization rate, it returns to step S220 and repeats the process from steps S220 to S224. Each module control device 90 reduces the set utilization rate Uf by a predetermined ratio at predetermined time intervals until the power generation efficiency η after the reduction in the set utilization rate is less than or equal to the power generation efficiency η before the reduction in the set utilization rate. On the other hand, if each module control device 90 determines that the power generation efficiency η after the reduction in the set utilization rate is less than or equal to the power generation efficiency η before the reduction in the set utilization rate, it sets the previous (before reduction) set utilization rate Uf (step S226) and proceeds to step S228.

[0044] As shown in Figure 8, each module control device 90 changes the set utilization rate Uf for each power generation module 20 (for example, HM1 and HM2 in the figure) by a predetermined ratio, calculates the power generation efficiency η before and after the change, and transitions to the set utilization rate Uf that yields the higher power generation efficiency η. As a result, as shown in Figure 9, it is possible to find the set utilization rates Uf1 and Uf2 that yield the optimal power generation efficiency η for each power generation module 20 (for example, HM1 and HM2 in the figure). This makes it possible to further improve the efficiency of the fuel cell system 10. As described above, fuel flow rate control is performed by setting a target fuel flow rate Fgtag by multiplying the fuel flow rate required for power generation in the fuel cell stack 21 by the reciprocal of the set utilization rate Uf (converted to a decimal), and then using feedback control based on the deviation between the set target fuel flow rate Fgtag and the fuel flow rate Fg detected by the fuel flow meter 44. Since the detected value of the fuel flow meter 44 usually contains errors, the set utilization rate Uf does not match the actual fuel utilization rate (actual utilization rate). However, by adjusting the set utilization rate Uf so that the optimal power generation efficiency η is obtained for each power generation module 20, the actual utilization rate of each power generation module 20 can be adjusted to the optimal utilization rate.

[0045] Next, each module control device 90 determines whether the system has stopped operating (step S228). If it determines that the system has not stopped operating, it obtains the change in output voltage V per unit time (e.g., 300 sec) (voltage fluctuation ΔV) detected by the voltage sensor 91 (step S230), and determines whether the obtained voltage fluctuation ΔV is less than a negative threshold α (step S232). If each module control device 90 determines that the voltage fluctuation ΔV is less than the threshold α, it determines that there is a fuel shortage in the fuel cell stack 21, reduces the set utilization rate Uf by a predetermined ratio (step S234), and returns to step S200. On the other hand, if each module control device 90 determines that the voltage fluctuation ΔV is not less than the threshold α, it skips step S234 and returns to step S200. As a result of the efficiency determination causing a change in the set utilization rate Uf, if a fuel shortage occurs in the fuel cell stack 21, the set utilization rate Uf can be reduced to resolve the fuel shortage, thereby preventing malfunctions in the fuel cell stack 21 caused by the fuel shortage.

[0046] In step S228, if each module control device 90 determines that the system has stopped, it saves the current set utilization rate Uf to the EEPROM (step S236) and terminates this routine. As a result, when the system is started again, in step S102, each module control device 90 determines that this is not the first time the system is started, and uses the set utilization rate Uf saved in the EEPROM to set the target fuel flow rate Fgtag and control the fuel flow rate. Although the processing in steps S104 to S110 is performed when the system is first started, it may also be performed each time the system is started, or each time the number of starts reaches a predetermined number.

[0047] In the embodiment described above, the fuel supply system 40 supplies hydrogen gas as fuel gas to the fuel electrode of the fuel cell stack 21. However, the raw fuel gas, such as natural gas or LPG, may be reformed into a fuel gas containing hydrogen gas and supplied to the fuel electrode of the fuel cell stack 21. In this case, the fuel supply system may include a gas pump that pressurizes the raw fuel gas to the branch pipe 41, a desulfurizer that removes sulfur components from the raw fuel gas, an evaporator that receives water (reformed water) to generate steam in the module case 29, and a reformer that uses the steam from the evaporator to reform the raw fuel gas into fuel gas.

[0048] In the embodiment described above, the fuel cell system 10 comprises a plurality of fuel cell stacks 21 connected in series, but it may also comprise a single fuel cell stack 21.

[0049] In the embodiments described above, each fuel cell stack 21 is configured to perform a power generation operation by reacting hydrogen with oxygen contained in the air. However, the fuel cell stack 21 may be a reversible solid oxide cell stack and may include an FC mode in which power generation is performed, and an EC mode in which hydrogen is produced by high-temperature steam electrolysis while power is supplied from a power source. As a power source, grid power, renewable energy such as solar power generation equipment, or storage batteries can be used.

[0050] The above describes the forms for implementing this disclosure using embodiments, but this disclosure is not limited in any way to these embodiments, and can of course be implemented in various forms without departing from the gist of this disclosure. [Industrial applicability]

[0051] This disclosure can be used in industries such as the manufacturing of fuel cell systems. [Explanation of symbols]

[0052] 10 Fuel cell system, 20 Power generation module, 21 Fuel cell stack, 22 Combustor (combustion section), 40 Fuel supply system, 90 Module control unit (control section), 91 Voltage sensor.

Claims

1. A power generation module including a fuel cell that generates electricity by receiving a fuel supply, and a combustion unit that burns unused fuel not used by the fuel cell, A fuel supply system that supplies fuel to the aforementioned fuel cell, A voltage sensor for detecting the voltage of the fuel cell, A control unit that performs a first control, which maintains a constant current drawn from the fuel cell and gradually increases the fuel utilization rate while monitoring the voltage detected by the voltage sensor, and determines a set fuel utilization rate based on the fact that the amount of decrease in the voltage detected by the voltage sensor per unit time falls below a predetermined amount. A fuel cell system equipped with the following features.

2. A fuel cell system according to claim 1, The control unit, as the first control, increases the fuel utilization rate by a predetermined ratio at predetermined intervals until the amount of voltage per unit time detected by the voltage sensor falls below a predetermined amount, and when the amount of voltage per unit time detected by the voltage sensor falls below a predetermined amount, it decreases the fuel utilization rate by a predetermined ratio. Fuel cell system.

3. A power generation module including a fuel cell that generates electricity by receiving a fuel supply, and a combustion unit that burns unused fuel not used by the fuel cell, A fuel supply system that supplies fuel to the aforementioned fuel cell, A control unit performs a second control that changes the fuel utilization rate, calculates the power generation efficiency before and after the change in fuel utilization rate, compares the two power generation efficiencies, and transitions to the fuel utilization rate with the higher power generation efficiency. A fuel cell system equipped with the following features.

4. A fuel cell system according to claim 3, The fuel cell is equipped with a voltage sensor that detects the voltage of the fuel cell, When the amount of voltage per unit time that is detected by the voltage sensor falls below a predetermined amount, the control unit reduces the fuel utilization rate regardless of the second control. Fuel cell system.

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