Fuel cell system

Abstract

To suppress an excessive decrease in a steam carbon ratio while coping with an increase in a required output, with a simple configuration.SOLUTION: A fuel cell system includes a control unit that sets a target flow rate of a raw fuel gas based on a required output, also sets a target flow rate of reforming water based on the target flow rate of the raw fuel gas so that the steam carbon ratio becomes a target ratio, and controls a raw fuel gas supply portion and a reforming water supply portion. The control unit sets the target flow rate of the raw fuel gas so that when the required output has increased, the output of the fuel cell increases according to a set value for an output increase rate; and when the output of the fuel cell is within a predetermined output range, lowers the output increase rate as compared with the case where the output is outside the predetermined output range.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a fuel cell system.

Background Art

[0002] Conventionally, as this type of fuel cell system, there has been proposed one including a vaporizer for vaporizing reforming water, a water supply pump for supplying the reforming water to the vaporizer, a reformer for steam reforming fuel gas, a fuel gas pump for supplying the fuel gas to the reformer, and a cell stack that generates electricity by oxidation and reduction of the reformed fuel gas reformed in the reformer and an oxidant (see, for example, Patent Document 1). In this system, the water supply pump is constituted by a pulsating metering pump that feeds the reforming water in a pulsating manner. Further, an upper portion inside the vaporizer is provided with a scattering space for the supplied reforming water to jet and scatter, and a lower portion inside the vaporizer is provided with a filler for promoting heat transfer and / or heat transfer fins. Thereby, since the supplied reforming water scatters and vaporizes in a wide area regardless of the supply amount of the reforming water, even if the power generation output increases and the supply amount of the reforming water suddenly increases, there is almost no time for a part of it to spread without evaporating, and a temporary decrease in the steam carbon ratio can be alleviated.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the above-described fuel cell system, fillers and heat transfer fins for promoting heat transfer are required in the vaporizer, which complicates the structure and increases the cost. Further, even if the structure of the vaporizer promotes vaporization of the reforming water, there is a limit to its vaporization if the supply amount of the reforming water suddenly increases.

[0005] The fuel cell system of the present invention mainly aims to suppress an excessive decrease in the steam carbon ratio while coping with an increase in required output with a simple configuration.

Means for Solving the Problems

[0006] The fuel cell system of the present invention has adopted the following means to achieve the above main object.

[0007] The fuel cell system of the present invention a fuel cell that generates electricity by the reaction of a fuel gas and an oxidant gas, a reforming unit that reforms a raw fuel gas with steam to generate the fuel gas, a vaporizing unit that vaporizes reformed water to generate the steam, a raw fuel gas supply unit that supplies the raw fuel gas to the reforming unit, a reformed water supply unit that supplies the reformed water to the vaporizing unit, a control unit that sets a target flow rate of the raw fuel gas based on a required output and sets a target flow rate of the reformed water based on the target flow rate of the raw fuel gas so that the steam carbon ratio becomes a target ratio, and controls the raw fuel gas supply unit and the reformed water supply unit, and the control unit sets the target flow rate of the raw fuel gas so that when the required output increases, the output of the fuel cell increases according to a set value of an output increase rate, when the output of the fuel cell is within a predetermined output range, the output increase rate is decreased compared to when it is outside the predetermined output range. This is the gist.

[0008] The fuel cell system of the present invention sets a target flow rate of the raw fuel gas based on the required output, and sets a target flow rate of the reformed water based on the target flow rate of the raw fuel gas so that the steam carbon ratio becomes the target ratio, and controls the raw fuel gas supply unit and the reformed water supply unit. When the required output increases, the target flow rate of the raw fuel gas is set so that the output of the fuel cell increases according to the set value of the output increase rate. When the output of the fuel cell is within a predetermined output range, the output increase rate is reduced compared to the case where it is outside the predetermined output range. Thereby, for fluctuations in the required output of the fuel cell outside the predetermined output range, the output response can be improved by setting the output increase rate relatively high. Further, for fluctuations in the required output of the fuel cell that straddle the predetermined output range, the output increase rate is temporarily reduced and the vaporization of the reformed water is waited for, so that an excessive decrease in the steam carbon ratio can be suppressed. As a result, with a simple configuration, it is possible to suppress an excessive decrease in the steam carbon ratio while corresponding to an increase in the required output.

[0009] In such a fuel cell system of the present invention, the control unit may set the output increase rate so that it gradually decreases as the output of the fuel cell approaches a predetermined value within the predetermined output range from the boundary of the predetermined output range, or may set the output increase rate so that it decreases stepwise when the output of the fuel cell crosses the boundary of the predetermined output range.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Embodiments for Carrying Out the Invention

[0011] Embodiments for carrying out the present invention will be described with reference to the drawings.

[0012] FIG. 1 is a schematic configuration diagram of a fuel cell system 10 according to the present embodiment. As shown in the figure, the fuel cell system 10 of the present embodiment includes a power generation module 20 including a fuel cell stack 21 that generates power by an electrochemical reaction between hydrogen in the anode gas and oxygen in the cathode gas, a raw fuel gas supply device 30 that supplies a raw fuel gas (for example, natural gas or LP gas) that is a raw material of the anode gas to the power generation module 20 via a raw fuel gas supply pipe 31, a reformed water supply device 40 that supplies reformed water necessary for reforming (steam reforming) the raw fuel gas into the anode gas to the power generation module 20, an air supply device 50 that supplies air as the cathode gas to the power generation module 20 (fuel cell stack 21), a waste heat recovery device 60 that recovers waste heat generated in the power generation module 20, a reflux device 80 that refluxes a part of the unused fuel that has not been used in the electrochemical reaction (power generation) in the fuel cell stack 21 to the raw fuel gas supply pipe 31, and a control device 100 that controls the entire system.

[0013] The power generation module 20 includes a fuel cell stack 21, a vaporizer 22, a reformer 23, a combustor 24, and two heat exchangers 26 and 27, which are housed in a module case 29 having heat insulation properties.

[0014] The fuel cell stack 21 has an electrolyte such as zirconium oxide, and an anode electrode and a cathode electrode sandwiching the electrolyte, and includes a plurality of solid oxide type single cells arranged in the left - right direction (horizontal direction). An anode gas passage (not shown) is formed in the anode electrode of each single cell. Also, a cathode gas passage (not shown) is formed in the cathode electrode of each single cell. Further, a temperature sensor 112 is installed near the fuel cell stack 21. The temperature sensor 112 detects a temperature (stack - correlated temperature) correlated with the temperature of the fuel cell stack 21.

[0015] The vaporizer 22 and the reformer 23 of the power generation module 20 are arranged at intervals above the fuel cell stack 21 in the module case 29. Also, between the fuel cell stack 21 and the vaporizer 22 and the reformer 23, a combustor 24 that generates heat necessary for the operation of the fuel cell stack 21 and the reactions in the vaporizer 22 and the reformer 23 is arranged. An ignition heater 25 is installed in the combustor 24.

[0016] The vaporizer 22 heats the raw fuel gas from the raw fuel gas supply device 30 and the reforming water from the reforming water supply device 40 by the heat from the combustor 24, pre - heats the raw fuel gas, and evaporates the reforming water to generate water vapor. The raw fuel gas pre - heated by the vaporizer 22 is mixed with water vapor, and the mixed gas flows from the vaporizer 22 into the reformer 23. Also, a temperature sensor 111 that detects the temperature of the mixed gas flowing into the reformer 23 (vaporizer temperature) is installed near the inlet of the reformer 23.

[0017] The reformer 23 has, for example, a Ru-based or Ni-based reforming catalyst filled therein, and in the presence of heat from the combustor 24, hydrogen gas and carbon monoxide are generated by the reaction (steam reforming reaction) of the mixed gas from the vaporizer 22 by the reforming catalyst. Further, the reformer 23 generates hydrogen gas and carbon dioxide by the reaction (carbon monoxide shift reaction) of carbon monoxide and steam generated in the steam reforming reaction. Thus, anode gas containing hydrogen, carbon monoxide, carbon dioxide, steam, un-reformed raw fuel gas, etc. is generated by the reformer 23. The anode gas generated by the reformer 23 flows into the anode gas passage of each single cell through the anode gas pipe 71 and is supplied to the anode electrode.

[0018] Also, air as cathode gas flows into the cathode gas passage of each single cell through the cathode gas pipe 72 and is supplied to the cathode electrode. At the cathode electrode of each single cell, oxide ions (O2 - ) are generated, and the oxide ions permeate through the electrolyte and react with hydrogen or carbon monoxide at the anode electrode to obtain electrical energy.

[0019] The anode gas (hereinafter referred to as "anode off-gas") that has not been used for the electrochemical reaction (power generation) in each single cell is supplied to the condenser 62 through the anode off-gas pipe 73, cooled by the condenser 62 to remove the steam contained in the anode off-gas, and then supplied to the combustor 24 through the anode off-gas pipe 74. A heat exchanger 26 is installed in the anode off-gas pipes 73 and 74, and the anode off-gas flowing through the anode off-gas pipe 74 (the anode off-gas after passing through the condenser 62) is heated by heat exchange with the high-temperature anode off-gas (the anode off-gas before passing through the condenser 62) flowing through the anode off-gas pipe 73 from the fuel cell stack 21 in the heat exchanger 26. Also, the cathode gas (hereinafter referred to as "cathode off-gas") that has not been used for the electrochemical reaction (power generation) in each single cell is supplied to the combustor 24 through the cathode off-gas pipe 75.

[0020] The anode off-gas flowing into the combustor 24 is a combustible gas containing fuel components such as hydrogen and carbon monoxide, and is mixed with the cathode off-gas containing oxygen flowing into the combustor 24. When the mixed gas is ignited by the ignition heater 25 and ignites in the combustor 24, heat required for the operation of the fuel cell stack 21, preheating of the raw fuel gas in the vaporizer 22, generation of steam, steam reforming reaction in the reformer 23, etc. is generated by the combustion of the mixed gas. Further, in the combustor 24, combustion exhaust gas containing unburned fuel is generated, and the combustion exhaust gas passes through the combustion exhaust gas pipe 76 and is discharged to the outside air through the heat exchanger 27 and the combustion catalyst 28. The combustion catalyst 28 is an oxidation catalyst for reburning unburned fuel in the combustion exhaust gas.

[0021] The raw fuel gas supply device 30 includes a raw fuel gas supply pipe 31 connecting the raw fuel gas supply source 1 that supplies the raw fuel gas and the vaporizer 22, and on-off valves (two-way valves) 32, 33, an orifice 34, a zero governor (equalizing valve) 35, a gas pump 36, and a desulfurizer 38 installed in the raw fuel gas supply pipe 31. The raw fuel gas is pumped (supplied) from the raw fuel supply source 1 to the vaporizer 22 through the desulfurizer 38 by operating the gas pump 36. Further, a flow rate sensor 39 for detecting the flow rate per unit time (gas flow rate Qg) of the raw fuel gas flowing through the raw fuel gas supply pipe 31 is installed between the orifice 34 and the zero governor 35 of the raw fuel gas supply pipe 31.

[0022] The reforming water supply device 40 includes a reforming water tank 42 for storing reforming water, a reforming water supply pipe 41 connecting the reforming water tank 42 and the vaporizer 22, and a reforming water pump 43 installed in the reforming water supply pipe 41. The reforming water in the reforming water tank 42 is pumped (supplied) to the vaporizer 22 by the reforming water pump 43 by operating the reforming water pump 43.

[0023] The air supply device 50 includes an air supply pipe 51 connected to a cathode gas pipe 72 installed within the module case 29, an air filter 52 provided at the inlet of the air supply pipe 51, and an air pump 53 installed in the air supply pipe 51. By operating the air pump 53, air as the cathode gas is sucked into the air supply pipe 51 through the air filter 52 and is pressure-fed (supplied) to the fuel cell stack 21 (cathode electrode) through the cathode gas pipe 72. The air flowing through the cathode gas pipe 72 is heat-exchanged with the high-temperature combustion exhaust gas flowing through the combustion exhaust gas pipe 76 in the heat exchanger 27 and is heated up.

[0024] The waste heat recovery device 60 includes a hot water storage tank 61 for storing hot water, a condenser 62 for heat-exchanging the anode off-gas flowing through the anode off-gas pipe 73 from the fuel cell stack 21 with hot water to condense the water vapor contained in the anode off-gas, a circulation pipe 63 connected to the hot water storage tank 61 and the condenser 62, and a circulation pump 64 incorporated in the circulation pipe 63. The hot water stored in the hot water storage tank 61 is introduced into the condenser 62 by operating the circulation pump 64, is heated up by heat-exchanging with the anode off-gas in the condenser 62, and then is returned to the hot water storage tank 61.

[0025] Also, a condensate pipe 44 and an anode off-gas pipe 74 are connected to the passage outlet on the anode off-gas side in the condenser 62. The condensed water obtained by condensing the water vapor in the anode off-gas through heat-exchange with the hot water from the hot water storage tank 61 is introduced into the reformed water tank 42 through the condensate pipe 44. Note that a water purifier (not shown) for purifying the condensed water that has passed through the condensate pipe 44 is installed in the reformed water tank 42. Also, as described above, the anode off-gas from which the water vapor has been removed in the condenser 62 is supplied to the combustor 24 through the anode off-gas pipe 74.

[0026] Furthermore, the exhaust heat recovery device 60 includes a radiator 65 incorporated in the circulation pipe 63, a radiator fan (electric fan) 66 that sends air to the radiator 65, and an electric heater 67 that consumes the electric power generated by the power generation module 20 to heat the hot water in the circulation pipe 63. The radiator 65 is installed so as to be positioned between the circulation pump 64 and the condenser 62 of the circulation pipe 63. The electric heater 67 is installed so as to be positioned between the radiator 65 and the circulation pump 64 of the circulation pipe 63.

[0027] The reflux device 80 includes a reflux pipe 81 that branches from the anode off-gas pipe 74 and is connected between the zero governor 35 and the gas pump 36 in the raw fuel gas supply pipe 31, a solenoid valve 82 installed in the reflux pipe 81, and an orifice 83 formed in the reflux pipe 81. The solenoid valve 82 is a normally closed on-off valve, and the reflux line of the anode off-gas from the anode off-gas pipe 74 to the raw fuel gas supply pipe 31 is blocked when the solenoid valve 82 is closed and is opened by opening the solenoid valve 82.

[0028] The input terminals of a power conditioner 90 are connected to the output terminals of a fuel cell stack 21, and the output terminals of the power conditioner 90 are connected to a power line 3 from a power system 2 to a load 4 via a relay. The power conditioner 90 has a DC / DC converter that converts the DC power output from the fuel cell stack 21 into DC power of a predetermined voltage (for example, DC250V to 300V), and an inverter that converts the converted DC power into AC power of a voltage (for example, AC200V) that can be interconnected with the power system. Thereby, it becomes possible to convert the DC power from the fuel cell stack 21 into AC power and supply it to the load 4 such as home appliances. A power supply board 91 is connected to the power conditioner 90. The power supply board 91 converts the DC power from the fuel cell stack 21 and the AC power from the power system 2 into low-voltage DC power, and supplies it to auxiliary machines such as a gas pump 36, a reformed water pump 43, an air pump 53, and a circulation pump 64, sensors such as a flow rate sensor 39, temperature sensors 111 and 112, a current sensor 113, and a voltage sensor 114, and a control device 100. Further, in an auxiliary machine room where the power conditioner 90, the power supply board 91, etc. are arranged, a cooling fan (not shown) and a ventilation fan for cooling the power conditioner 90 and the power supply board 91 are arranged. The cooling fan sends air into the heat generating parts of the power conditioner 90 and the power supply board 91. The air that has cooled the heat generating parts and has been heated is discharged into the atmosphere by the ventilation fan.

[0029] The control device 100 is configured as a microprocessor centered around the CPU 101, and in addition to the CPU 101, it includes a ROM 102 that stores a processing program, a RAM 103 that temporarily stores data, and an input / output port (not shown). Various detection signals from a current sensor 113 that detects the current (output current I) output from the fuel cell stack 21, a voltage sensor 114 that detects the voltage (output voltage V) output from the fuel cell stack 21, a flow rate sensor 39, temperature sensors 111 and 112, etc. are input via the input port. Also, various control signals from the control device 100 to the solenoids of the on-off valves 32 and 33, the pump motor of the gas pump 36, the pump motor of the reformed water pump 43, the pump motor of the air pump 53, the pump motor of the circulation pump 64, the ignition heater 25, the solenoid of the electromagnetic valve 82, etc. are output via the output port. Further, a remote controller (not shown) is connected to the control device 100 via a wireless or wired communication line. The control device 100 executes various controls based on signals from the remote controller operated by the user of the fuel cell system 10.

[0030] Next, the operation of the fuel cell system 10 configured in this way will be described. FIG. 2 is a flowchart showing an example of a power generation control routine executed by the CPU 101 of the control device 100. This routine is repeatedly executed at predetermined time intervals (for example, every few msec or every several tens of msec) when the system is started.

[0031] When the power generation control routine is executed, the CPU 101 of the control device 100 first inputs the required output (required power) Preq requested by the load 4, the output current I detected by the current sensor 113, the output voltage V detected by the voltage sensor 114, etc. (step S100). Subsequently, based on the input required output Preq and output voltage V, the required current Ireq to be output from the fuel cell stack 21 is set (step S110). Next, based on the input output current I, the output increase rate ΔI is set (step S120). Here, the output increase rate ΔI is the increase amount of the output current I (output power) allowed per unit time to mitigate the rapid increase of the output current I (output power) when the required current Ireq (required output Preq) rapidly increases, and is set so that the rate of decrease of the output voltage V accompanying the increase of the output current I does not exceed a predetermined specification upper limit. The setting of the output increase rate ΔI is performed, for example, by previously obtaining the relationship between the output increase rate ΔI and the output current I through experiments or the like, storing it in the ROM 102 as a map for setting the output increase rate, and deriving the corresponding output increase rate ΔI from the map when the output current I is given. An example of the map for setting the output increase rate is shown in FIG. 3. As shown in the figure, the output increase rate ΔI becomes the maximum speed ΔImax when the output current I is less than or equal to the first current I1 and greater than or equal to the third current I3, and is set to be lower as it is closer to the intermediate second current I2 within the output range greater than the first current I1 and less than the third current I3. Note that although the output increase rate ΔI is set as the increase rate of the output current, it may be set as the increase rate of the output power. Also, although the map for setting the output increase rate is defined as the relationship between the output current I and the output increase rate, it may be defined as the relationship between the output power and the output increase rate.

[0032] When the output increase rate ΔI is set, the smaller value between the output current I plus the output increase rate ΔI and the required current Ireq is set as the target output current Itag (step S130). That is, the target output current Itag is set so that the required current Ireq is output within a range where the increase amount of the output current I per unit time does not exceed the output increase rate ΔI. Then, based on the target output current Itag, the target gas flow rate Qgtag, which is the target flow rate of the raw fuel gas supplied by the raw fuel gas supply device 30, is set so that the fuel utilization rate Uf (the ratio of the amount of fuel gas used for power generation to the amount of fuel gas supplied to the anode) becomes the target utilization rate Uftag (step S140).

[0033] Next, based on the target gas flow rate Qgtag, the target reformed water flow rate Qwtag, which is the target flow rate of the reformed water supplied by the reformed water supply device 40, is set so that the steam-carbon ratio SC (the molar ratio of carbon contained in the hydrocarbon in the raw fuel gas to the steam added for steam reforming) in the reformer 23 becomes the target ratio SCtag (step S150). Further, based on the target air utilization rate Uatag, the target air flow rate Qatag, which is the target flow rate of the air supplied by the air supply device 50, is set so that the air utilization rate Ua becomes the target utilization rate Uatag (step S160).

[0034] After setting the target gas flow rate Qgtag, the target reformed water flow rate Qwtag, and the target air flow rate Qatag in this way, the gas pump 36 is controlled so that the raw fuel gas is supplied at the target gas flow rate Qgtag (step S170), the reformed water pump 43 is controlled so that the reformed water is supplied at the target reformed water flow rate Qwtag (step S180), and the air pump 53 is controlled so that the air is supplied at the target air flow rate Qatag (step S190), and the power generation control routine is terminated. A flow sensor 39 is installed in the raw fuel gas supply pipe 31, and the control of the gas pump 36 is performed by feedback control (for example, proportional-integral control) based on the deviation between the target gas flow rate Qgtag and the gas flow rate Qg from the flow sensor 39.

[0035] Here, consider the case where the required output Preq suddenly increases such that the fuel cell system 10 changes from operating at low output to operating at high output. When the supply amount of the reformed water increases rapidly along with the supply amount of the raw fuel gas to the vaporizer 22 in order to correspond to the required output Preq, since the heat transfer from the vaporizer 22 to the reformed water becomes rate-determining and it takes time to vaporize the reformed water, the amount of steam generated becomes insufficient with respect to the supply amount of the raw fuel gas, and a phenomenon occurs where the steam carbon ratio SC temporarily decreases. If the steam carbon ratio SC decreases too much, carbon components may deposit on the reforming catalyst and the electrodes of the fuel cell stack 21, and there is a risk that the fuel cell stack 21 and the like may malfunction. By setting the output increase rate ΔI and increasing the target output current Itag in accordance with the output increase rate ΔI in response to the sudden increase in the required output Preq, it is possible to suppress the excessive increase in the reformed water beyond the vaporization capacity of the vaporizer 22 and suppress the excessive decrease in the steam carbon ratio SC.

[0036] On the other hand, as shown by the dashed-dotted line in FIG. 3, if the output increase rate ΔI is overall decreased with respect to the output current I, the output responsiveness decreases and it becomes impossible to follow the load fluctuations well. Therefore, in the present embodiment, when the output current I is outside the predetermined output range (equal to or less than the first current I1 and equal to or greater than the third current I3), the high speed (maximum speed ΔImax) is set for the output increase rate ΔI, and when the output current I is within the predetermined output range (greater than the first current I1 and less than the third current I3), the output increase rate ΔI is made low. Thereby, when the required output Preq suddenly increases such that the operation changes from operating at low output to operating at high output across the predetermined output range, the output increase rate ΔI changes in the order of high speed, low speed, and high speed. That is, the increase amount per unit time of the raw fuel gas and the reformed water supplied to the vaporizer 22 changes in the order of large, small, and large. Thereby, after significantly increasing the reformed water supplied to the vaporizer 22 along with the sudden increase in the required output Preq, by temporarily decreasing the increase amount of the reformed water, a waiting time is ensured to wait for the vaporization of the reformed water supplied until then, so that it is possible to suppress the excessive decrease in the steam carbon ratio SC and improve the output responsiveness.

[0037] FIG. 4 is an explanatory diagram for explaining the relationship between the required output Preq and the steam carbon ratio SC when the output power P is increased to the required output Preq with the output increase rate ΔI being the maximum speed ΔImax. In the figure, SClim indicates the lower limit value within the allowable range of the steam carbon ratio SC. Also, in the figure, the solid line indicates the relationship when the output power P is increased from 0 W to the required output Preq, the dashed-dotted line indicates the relationship when the output power P is increased from 100 W to a required output Preq greater than this, the double-dashed-dotted line indicates the relationship when the output power P is increased from 300 W to a required output Preq greater than this, and the dashed line indicates the relationship when the output power P is increased from 500 W to a required output Preq greater than this. As shown in the figure, when increasing the output power P from 0 W to the required output Preq, up to around 340 W, even if the output power is increased with the output increase rate ΔI being the maximum speed ΔImax, the steam carbon ratio SC does not fall below its lower limit value SClim. Also, when increasing the output power P from 100 W to the required output Preq, up to around 410 W, even if the output power is increased with the output increase rate ΔI being the maximum speed ΔImax, the steam carbon ratio SC does not fall below its lower limit value SClim. Further, when increasing the output power P from 300 W or more to the required output Preq, even if the output increase rate ΔI is increased as the maximum speed ΔImax up to the rated output (700 W), the steam carbon ratio SC does not fall below its lower limit value SClim. Based on these relationships, by determining the relationship between the output current I (output power) and the output increase rate ΔI (output increase rate setting map), it is possible to improve the output responsiveness while suppressing an excessive decrease in the steam carbon ratio SC.

[0038] In the fuel cell system 10 of the present embodiment described above, while setting the target gas flow rate Qgtag based on the required output Preq and setting the target reformed water flow rate Qwtag based on the target gas flow rate Qgtag so that the steam carbon ratio SC becomes the target ratio SCtag to control the raw fuel gas supply device 30 and the reformed water supply device 40, when the required output Preq increases, the target gas flow rate Qgtag is set so that the output current I increases according to the output increase rate ΔI. Then, when the output current I is within a predetermined output range (equal to or higher than the first current I1 and equal to or lower than the third current I3), the output increase rate ΔI is decreased compared to the case where it is outside the predetermined output range. Thereby, for fluctuations in the required output Preq outside the predetermined output range, the output response can be improved by setting the output increase rate ΔI relatively high. Further, for fluctuations in the required output Preq that cross the predetermined output range, the output increase rate ΔI is temporarily decreased to wait for the vaporization of the reformed water, thereby suppressing an excessive decrease in the steam carbon ratio. As a result, with a simple configuration, it is possible to suppress an excessive decrease in the steam carbon ratio while corresponding to an increase in the required output.

[0039] In the above-described embodiment, the output increase rate setting map is configured such that in the output range equal to or higher than the first current I1 and less than the second current I2, the output increase rate ΔI gradually decreases as the output current I increases, and in the output range equal to or higher than the second current I2 and less than the third current I3, the output increase rate ΔI gradually increases as the output current I increases. However, as shown in the output increase rate setting map of the modification example in FIG. 5, it may be configured such that the output increase rate ΔI decreases stepwise in the output range equal to or higher than the first current I1 and less than the third current I3.

[0040] In the above-described embodiment, only one output range for decreasing the output increase rate ΔI is provided. However, as shown in the output increase rate setting map of the modification example in FIG. 6, a plurality of output ranges in which the output increase rate ΔI decreases may be provided.

[0041] The correspondence between the main elements of the embodiments and the main elements of the invention described in the column of means for solving the problems will be described. In the embodiments, the fuel cell stack 21 corresponds to the "fuel cell" of the present invention, the reformer 23 corresponds to the "reforming section", the vaporizer 22 corresponds to the "vaporizing section", the raw fuel gas supply device 30 corresponds to the "raw fuel gas supply section", the reforming water supply device 40 corresponds to the "reforming water supply section", and the control device 100 corresponds to the "control section".

[0042] Note that the correspondence between the main elements of the embodiments and the main elements of the invention described in the column of means for solving the problems is an example for specifically explaining the form for implementing the invention described in the column of means for solving the problems in the embodiments, and thus does not limit the elements of the invention described in the column of means for solving the problems. That is, the interpretation of the invention described in the column of means for solving the problems should be made based on the description in that column, and the embodiments are merely specific examples of the invention described in the column of means for solving the problems.

[0043] As described above, the embodiments for implementing the present invention have been described using the embodiments, but the present invention is not limited to such embodiments at all, and it goes without saying that the present invention can be implemented in various forms without departing from the gist of the present invention.

Industrial Applicability

[0044] The present invention can be used in the manufacturing industry of fuel cell systems and the like.

Explanation of Reference Numerals

[0045] 1. Primary fuel supply source, 2. Power system, 3. Power line, 4. Load, 10. Fuel cell system, 20. Power generation module, 21. Fuel cell stack, 22. Vaporizer, 23. Reformer, 24. Combustor, 25. Ignition heater, 26, 27. Heat exchanger, 28. Combustion catalyst, 29. Module case, 30. Primary fuel gas supply device, 31. Primary fuel gas supply pipe, 32, 33. On-off valve, 34. Orifice, 35. Zero governor, 36. Gas pump, 38. Desulfurizer, 39. Flow sensor, 40. Reformed water supply device, 41. Reformed water supply pipe, 42. Reformed water tank, 43. Reformed water pump, 44. Condensate pipe, 50. Air supply device, 51. Air supply pipe, 52. Air filter, 53. Air pump, 60. Waste heat recovery device, 61. Hot water storage tank, 62. Condenser, 63. Circulation pipe, 64. Circulation pump, 65. Radiator, 67. Electric heater, 71. Anode gas pipe, 72. Cathode gas pipe, 73, 74. Anode off-gas pipe, 75. Cathode off-gas pipe, 76. Combustion exhaust gas pipe, 78. Power conditioner, 79. Power supply board, 80. Reflux device, 81. Reflux pipe, 82. Solenoid valve, 83. Orifice, 90. Control device, 91. CPU, 92. ROM, 93. RAM, 95, 96. Temperature sensor, 97. Current sensor, 98. Voltage sensor.

Claims

1. A fuel cell that generates electricity by the reaction of a fuel gas and an oxidant gas, a reforming unit that reforms a raw fuel gas with steam to generate the fuel gas, a vaporization unit that vaporizes reformed water to generate the steam, a raw fuel gas supply unit that supplies the raw fuel gas to the reforming unit, a reformed water supply unit that supplies the reformed water to the vaporization unit, a control unit that sets a target flow rate of the raw fuel gas based on a required output and sets a target flow rate of the reformed water based on the target flow rate of the raw fuel gas so that a steam carbon ratio becomes a target ratio, and controls the raw fuel gas supply unit and the reformed water supply unit, comprising: The control unit sets the target flow rate of the raw fuel gas so that when the required output increases, the output of the fuel cell increases according to a set value of an output increase rate, and when the output of the fuel cell is within a predetermined output range, reduces the output increase rate as compared with the case where the output is outside the predetermined output range. A fuel cell system.

2. The fuel cell system according to claim 1, wherein the control unit sets the output increase rate so that the output increase rate gradually decreases as the output of the fuel cell approaches a predetermined value within the predetermined output range from the boundary of the predetermined output range. A fuel cell system.

3. The fuel cell system according to claim 1, wherein the control unit sets the output increase rate so that the output increase rate decreases stepwise when the output of the fuel cell crosses the boundary of the predetermined output range. A fuel cell system.

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