JP2015103329A - Solid oxide fuel cell system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a solid oxide fuel cell system in which lowering in a water level inside a water tank is suppressed.SOLUTION: A solid oxide fuel cell system 1 comprises: a solid oxide fuel cell 20; a water recovery part 36 for recovering a water content included in an exhaust gas F from the fuel cell 20; a water tank 12 for storing the recovered water of the water recovery part 36; a water level determination part 42 for determining a water level inside the water tank 12; a power consumption load 34h for consuming, in a system, power generated by the fuel cell 20; and a control part 43 for activating the power consumption load 34h so as to consume the power generated by the fuel cell 20 while maintaining the power generated by the fuel cell 20 equal to or larger than predetermined power lower than rated power, when the water level determination part 42 determines that the water level inside the water tank 12 lowers to be equal to or lower than a first predetermined water level. As a result, supplying an oxidant gas whose flow rate exceeds a flow rate necessary for power generation to the fuel cell 20 in order to cool the fuel cell 20 can be avoided, so that lowering in the water level inside the water tank 12 can be suppressed.

Description

  The present invention relates to a solid oxide fuel cell system, and more particularly to a solid oxide fuel cell system capable of suppressing a decrease in the water level in a water tank.

  In recent years, the spread of fuel cells that have high power generation efficiency and contribute to energy saving is expected as a distributed power generation device having advantages such as reduction of power transmission loss. A fuel cell introduces a hydrogen-containing gas and an oxygen-containing gas, generates power by an electrochemical reaction between hydrogen and oxygen, and generates water during power generation. When installing a fuel cell, in view of insufficient infrastructure for obtaining a hydrogen-containing gas, a reformer that reforms a hydrocarbon-based material into a hydrogen-containing gas is often provided. One of the methods for reforming a hydrocarbon-based material is steam reforming in which steam is mixed with a hydrocarbon-based material and heated to reform. If the water (steam) used in steam reforming can be maintained by water generated by power generation in the fuel cell or water discharged from the reformer, the fuel cell and the reformer are included. There is no need to introduce water from outside the system.

  When the amount of water that can be recovered in the system decreases, it is possible to promote the recovery of condensed water while maintaining various operating conditions such as prevention of overheating of the fuel cell properly so that the water can become independent. In addition to suppressing the excessive temperature rise of the fuel cell that has reduced the generated power, the oxidant gas supply amount to the fuel cell is relatively decreased to increase the proportion of water vapor contained in the exhaust from the fuel cell. In some cases, when the temperature of the fuel cell is high, the fuel cell is cooled by increasing the supply amount of the oxidant gas (see, for example, Patent Document 1).

JP 2013-73903 A (paragraphs 0011-0013 and the like)

  However, in recent years, by improving the heat insulation performance that contributes to the improvement of the power generation efficiency of the fuel cell, in the low output region, even if the generated power is reduced, heat radiation does not proceed, and the operating temperature of the fuel cell can be maintained at an appropriate temperature. It tends to be impossible. At this time, if the oxidant gas supplied to the fuel cell is increased in order to cool the fuel cell, the water discharged from the system together with the gas discharged from the fuel cell increases, and water independence can be maintained. There was a case that disappeared.

  In view of the above problems, an object of the present invention is to provide a solid oxide fuel cell system capable of suppressing a decrease in the water level in a water tank.

  In order to achieve the above object, a solid oxide fuel cell system according to a first aspect of the present invention is a solid oxide that generates electric power to be supplied to an electric power load 95 outside the system, for example, as shown in FIG. A fuel cell 20 having a shape; a water recovery unit 36 for recovering water contained in the exhaust gas F discharged from the fuel cell 20; a water tank 12 for storing water recovered by the water recovery unit 36; A water level determination unit 42 for determining the water level in the fuel cell; a power consumption load 34h for consuming electric power generated in the fuel cell 20 in the system; and the water level in the water tank 12 can be lowered to a first predetermined water level Ls or less. When the water level determination unit 42 determines that there is a power, the power consumption load 34h is set so that the power generated by the fuel cell 20 is consumed while the power generated by the fuel cell 20 is not less than a predetermined power lower than the rated power. Control unit to be operated Equipped with a 3 and. Here, the predetermined power typically has an oxidant gas flow rate in a range where water self-sustainability does not become difficult when the flow rate of the oxidant gas supplied to the fuel cell is increased for cooling the fuel cell. Any power that is equal to or higher than the recommended minimum output that is the output corresponding to the upper limit and is as close as possible to the recommended minimum output.

  With this configuration, the power generated by the fuel cell is set to a predetermined level or higher, so that it is possible to avoid supplying an oxidant gas exceeding the flow rate necessary for power generation to cool the fuel cell to the fuel cell. Thus, it is possible to suppress a decrease in the water level in the water tank. Moreover, since the power consumption load is operated, reverse power flow can be avoided.

  Moreover, the solid oxide fuel cell system according to the second aspect of the present invention is a fuel cell in the solid oxide fuel cell system 1 according to the first aspect of the present invention as shown in FIG. An off-gas combustion unit 32 that combusts the exhaust gas F discharged from 20 before being led to the water recovery unit 36; and a combustion catalyst before the combustible substance in the exhaust gas Fb discharged from the off-gas combustion unit 32 is guided to the water recovery unit 36 The exhaust gas purifying unit 34 is configured to combust with the exhaust gas purifying unit 34 having the combustion catalyst heater 34h for heating the combustion catalyst; the power consumption load is constituted by the combustion catalyst heater 34h.

  With this configuration, it is not necessary to provide a dedicated power consumption load for maintaining the generated power of the fuel cell at a predetermined level or more, and the system configuration can be simplified.

  Moreover, the solid oxide fuel cell system according to the third aspect of the present invention is, for example, as shown in FIG. 1, and the solid oxide fuel cell system according to the first aspect or the second aspect of the present invention. 1 includes power demand detection units 25 and 98 that detect the demand for power generated in the fuel cell 20; the water level determination unit 42 is when the power detected by the power demand detection units 25 and 98 is less than a predetermined power. In addition, the water level in the water tank 12 is determined to be likely to drop below the first predetermined water level Ls.

  If comprised in this way, it can avoid that the electric power generated of a fuel cell becomes less than predetermined electric power.

  Moreover, the solid oxide fuel cell system according to the fourth aspect of the present invention is, for example, as shown in FIG. 1, and the solid oxide fuel cell system according to the first aspect or the second aspect of the present invention described above. 1 includes an exhaust gas temperature detection unit 45 that directly or indirectly detects the temperature of the exhaust gas F from which water has been recovered by the water recovery unit 36; the water level determination unit 42 has a temperature detected by the exhaust gas temperature detection unit 45. When the temperature is equal to or higher than the predetermined temperature, it is determined that the water level in the water tank 12 may be lowered to the first predetermined water level Ls or lower.

  If comprised in this way, the water quantity in a water tank can be estimated, without providing the apparatus which detects the water level in a water tank.

  Further, the solid oxide fuel cell system according to the fifth aspect of the present invention is, for example, as shown in FIG. 1, and the solid oxide fuel cell system according to the first aspect or the second aspect of the present invention. 1 includes a water level detection unit 12g that detects that the water level in the water tank 12 is equal to or lower than a second predetermined water level Lc higher than the first predetermined water level Ls; the water level determination unit 42 includes a water level detection unit 42 When it is detected that 12g is equal to or lower than the second predetermined water level Lc, the water level in the water tank 12 is determined to be likely to drop below the first predetermined water level Ls. .

  If comprised in this way, the water level in a water tank can be detected directly, and it becomes possible to avoid reliably that the amount of water in a water tank is insufficient.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to avoid supplying oxidant gas exceeding the flow volume required for electric power generation to a fuel cell for cooling of a fuel cell, and can suppress the fall of the water level in a water tank. it can.

1 is a schematic system diagram of a solid oxide fuel cell system according to an embodiment of the present invention. It is a graph which shows the relationship between the electric power generated of a fuel cell, and the flow volume of oxidant gas supplied to a fuel cell when maintaining a fuel cell at a suitable operating temperature. It is a flowchart which shows the 1st example of control for maintaining water independence. It is a flowchart which shows the 2nd example of control for maintaining water independence. It is a flowchart which shows the 3rd example of control for maintaining water independence.

  Embodiments of the present invention will be described below with reference to the drawings. In the drawings, the same or similar members are denoted by the same or similar reference numerals, and redundant description is omitted.

  First, a solid oxide fuel cell system 1 according to an embodiment of the present invention will be described with reference to FIG. FIG. 1 is a schematic system diagram of a solid oxide fuel cell system 1. A solid oxide fuel cell system 1 (hereinafter simply referred to as “fuel cell system 1”) includes a reformer 11 that reforms a raw material D into a fuel gas E rich in hydrogen, and the raw material D in the reformer 11. A water tank 12 for storing reformed water W used for reforming, a solid oxide fuel cell 20 (hereinafter simply referred to as “fuel cell 20”), and a kind of off-gas discharged from the fuel cell 20. Combustion section 32 as an off-gas combustion section for burning fuel off-gas Fe, purification section 34 as an exhaust gas purification section for combusting combustible components of combustion exhaust gas Fb discharged from the combustion section 32, and heat exchange section as a water recovery section 36 and a control device 40.

  The raw material D can be made into a gas rich in hydrogen (hydrogen-rich gas) that can be used for power generation in the fuel cell 20 by reforming, and typically a hydrocarbon-based fuel is used. Specific examples include hydrocarbons, alcohols, ethers, biofuels, and these hydrocarbon fuels are derived from fossil fuels such as petroleum and coal, those derived from synthetic fuels such as synthesis gas, Those derived from biomass can be used as appropriate. Examples of hydrocarbons include methane, ethane, propane, butane, natural gas, LPG, city gas, town gas, gasoline, naphtha, kerosene, and light oil. Examples of alcohols include methanol and ethanol. Examples of ethers include dimethyl ether. Examples of biofuels include biogas, bioethanol, biodiesel, and biojet.

  The reformer 11 has a reforming catalyst that promotes reforming of the raw material D inside. In the present embodiment, at least the steam reforming catalyst is accommodated in the reformer 11. Since the steam reforming catalyst is a reforming catalyst in which the endothermic reaction is dominant, in the present embodiment, the steam reforming catalyst is configured so that heat necessary for the endothermic reaction can be obtained also from the combustion unit 32. ing. The reformer 11 includes a reforming water pipe 51 as a reforming water line that guides the reforming water W from the water tank 12 to the reformer 11, a raw material pipe 56 as a raw material line for introducing the raw material D, and a raw material D An air pipe 58 is connected as an oxidant line used for an oxidation reaction when reforming. A "... line" is a fluid flow path, typically a tube that exclusively guides the fluid, but is a space formed between objects used for other purposes. There may be.

  In the reforming water pipe 51, a reforming water pump 13 for conveying the reforming water W and a vaporizer 14 for vaporizing the reforming water W are disposed. The reformer 11 and the vaporizer 14 are typically provided adjacent to each other, but may be connected by a pipe. A raw material blower 16 serving as a raw material supply device that pumps the raw material D toward the reformer 11 is inserted and disposed in the raw material pipe 56. An air blower 18 as an oxidant supply device that pressure-feeds air A1 as an oxidant toward the reformer 11 is inserted into the air pipe 58. Although FIG. 1 shows an example in which the raw material pipe 56 is directly connected to the reformer 11, the raw material pipe 56 is connected to the vaporizer 14, and the raw material D and the reformed water W are simultaneously introduced into the vaporizer 14. It is good also as a structure which vaporizes and mixes both. The reformer 11 is configured to generate the fuel gas E by introducing the raw material D, the vaporized reforming water W, and the air A1. The reformer 11 is also connected with a fuel gas pipe 61 as a fuel gas line for deriving the generated fuel gas E.

  The water tank 12 has an effective capacity capable of storing an amount of reforming water W sufficient to continuously reform the raw material D in the reformer 11. The other end of the reforming water pipe 51 whose one end is connected to the reformer 11 is connected to the lower part of the water tank 12. An overflow pipe 12p that prevents the reforming water W stored therein from overflowing is connected to the upper side surface of the water tank 12. The lower end of the portion of the overflow pipe 12p connected to the water tank 12 is the highest water level Lh of the water tank 12. Further, the water tank 12 is provided with a water level meter 12g as a water level detection unit for detecting the water level in the water tank 12. The water level meter 12g is configured to detect a warning water level Lc that is higher than the low water level Ls and lower than the highest water level Lh. Here, the low water level Ls is a water level where it is expected that it will be difficult to maintain so-called water independence, in which the reformed water W is covered with water collected in the fuel cell system 1, and corresponds to the first water level. To do. For example, the low water level Ls can be a water level when the reformed water W having a capacity half the effective capacity of the water tank 12 is stored. The warning water level Lc is a water level for knowing that the water level in the water tank 12 is approaching the low water level Ls, and corresponds to the second water level. The water level gauge 12g typically uses an electrode rod or a float switch.

  The fuel cell 20 includes an anode 21, a cathode 22, and an electrolyte 23. The anode 21 is connected to the reformer 11 via the fuel gas pipe 61. An oxidant gas pipe 62 as an oxidant gas line for introducing an oxidant gas A2 containing oxygen is connected to the cathode 22. The oxidant gas A2 is typically air. An oxidant gas blower 28 serving as an oxidant gas supply device that pumps the oxidant gas A2 toward the cathode 22 is inserted into the oxidant gas pipe 62. The fuel cell 20 is configured to introduce the fuel gas E and the oxidant gas A2 and generate DC power by an electrochemical reaction between hydrogen in the fuel gas E and oxygen in the oxidant gas A2. Yes.

  As the fuel cell 20, a cylindrical type (including a flat plate cylindrical type) solid oxide fuel cell (SOFC) is used in the present embodiment. The fuel cell 20 has a temperature suitable for power generation (generally 700 to 1000 ° C.), and in order to maintain the temperature, an insulating material (non-heat insulating material) formed so as to have a low heat dissipation rate by an inorganic material such as silica or alumina. (Illustrated). The fuel gas E and the oxidant gas A2 supplied to the fuel cell 20 for power generation in the fuel cell 20 are not all used for power generation, but the amount corresponding to the power generation current of the fuel cell 20 is used. The Of the fuel gas E supplied to the fuel cell 20 for power generation in the fuel cell 20, the portion not used for power generation is discharged as fuel off-gas Fe. Further, of the oxidant gas A2 supplied to the fuel cell 20 for power generation in the fuel cell 20, the portion not used for power generation is discharged as the oxidant off-gas Fa.

  The combustion unit 32 communicates with the anode 21 via a fuel off gas line 63. The combustor 32 is also in communication with the cathode 22 via an oxidant offgas line 64. The combustion unit 32 is configured to burn the fuel offgas Fe introduced through the fuel offgas line 63. As the oxidant necessary for the combustion of the fuel offgas Fe, the oxidant offgas Fa introduced through the oxidant offgas line 64 is used. The combustion unit 32 is disposed adjacent to the reformer 11 so that heat can be applied to the reforming catalyst in the reformer 11. For example, in a casing (not shown) in which the reformer 11 and the fuel cell 20 are accommodated, the reformer 11 is disposed above a portion where the fuel off-gas Fe and the oxidant off-gas Fa are discharged, and the reformer The fuel off-gas Fe may be combusted in the space between the fuel cell 11 and the fuel cell 20. In this case, the space between the reformer 11 and the fuel cell 20 corresponds to the combustion unit 32, and the fuel off-gas line 63, the oxidant off-gas line 64, and the combustion unit 32 are very close to each other. The combustion unit 32 is configured to discharge the combustion exhaust gas Fb generated by the combustion of the fuel off gas Fe. A combustion exhaust gas pipe 65 as a combustion exhaust gas line through which the combustion exhaust gas Fb flows is connected to the combustion unit 32.

  The purification unit 34 communicates with the combustion unit 32 via the combustion exhaust gas pipe 65. The purifying unit 34 burns the combustible component in the combustion exhaust gas Fb by burning the combustible component in the combustion exhaust gas Fb when the combustible component in the combustion exhaust gas Fb is contained in the combustion unit 32 without combusting the combustible component in the fuel off-gas Fe. This is the part to be purified. The purification unit 34 may be arranged so that heat generated when combustible components in the combustion exhaust gas Fb are burned can be given to the reformer 11. The purification unit 34 includes a combustion catalyst (not shown) that burns combustible components in the combustion exhaust gas Fb, and a combustion catalyst heater 34h that raises the temperature of the combustion catalyst to an activation temperature. The combustion catalyst is made of a substance that promotes the oxidation of combustible components in the combustion exhaust gas Fb. For example, a noble metal catalyst such as platinum or palladium, a base metal catalyst such as manganese or iron, or the like can be used. The combustion catalyst heater 34h is configured to have a heat generation amount that can quickly raise the combustion catalyst (not shown) to the activation temperature when the fuel cell system 1 is started. The combustion catalyst heater 34h is typically configured to change the amount of heat generated by changing the duty ratio. In the present embodiment, the combustion catalyst heater 34h corresponds to the power consumption load. The purification unit 34 is configured to discharge the purified exhaust gas Fp generated by purifying the combustion exhaust gas Fb. The fuel off gas Fe, the oxidant off gas Fa, the combustion exhaust gas Fb, and the purified exhaust gas Fp that are discharged from the fuel cell 20 and flow into the heat exchanging unit 36 are collectively referred to simply as “exhaust gas F”. A purified exhaust gas pipe 66 as a purified exhaust gas line through which the purified exhaust gas Fp flows is connected to the purification unit 34.

  The heat exchange unit 36 is a part that recovers moisture and heat that the purified exhaust gas Fp has. The heat exchanging part 36 has a heat recovery fluid flow path 36x through which the heat recovery fluid C flows. The heat exchange unit 36 introduces the purified exhaust gas Fp and causes heat exchange between the introduced purified exhaust gas Fp and the heat recovery fluid C flowing in the heat recovery fluid flow path 36x, so that the enthalpy of the heat recovery fluid C is obtained. Is increased (heat recovery), and the enthalpy of the purified exhaust gas Fp is decreased. Further, the heat exchanging unit 36 is configured to generate condensed water Wc in which moisture contained in the purified exhaust gas Fp is condensed by reducing the enthalpy of the purified exhaust gas Fp. The heat exchanging unit 36 includes an out-of-system discharge gas pipe 67 as an out-of-system discharge gas line through which the out-of-system discharge gas G after the moisture and heat are recovered from the purified exhaust gas Fp, and a condensed water line for discharging the condensed water Wc. A condensate water pipe 69 is connected. The out-of-system discharge gas pipe 67 is provided with a thermometer 45 as an exhaust gas temperature detection unit that detects the temperature of the out-of-system discharge gas G discharged from the heat exchange unit 36. The other end of the condensed water pipe 69 whose one end is connected to the heat exchange unit 36 is open in the water tank 12. The condensed water pipe 69 is disposed so as to guide the condensed water Wc generated in the heat exchange unit 36 to the water tank 12. The condensed water Wc guided to the water tank 12 is used as the reformed water W through a water treatment device (not shown).

  The fuel cell system 1 is linked to the system power supply PS via the power conditioner 25. The system power source PS is a power source supplied from a commercial distribution line network owned by an electric power company. The power conditioner 25 is a device that adjusts the electric power generated by the fuel cell 20 to a stable output. The power conditioner 25 has a converter that boosts DC power generated in the fuel cell 20 and an inverter that converts DC power boosted by the converter into AC. The fuel cell system 1 connected to the system power supply PS via the power conditioner 25 is electrically connected to an external load 95 (for example, home appliances) that is a power load arranged outside the fuel cell system 1 (outside the system). Are connected to each other, and the electric power generated by the fuel cell 20 can be supplied to the external load 95. The power conditioner 25 is configured to be able to adjust the power output from the fuel cell system 1. The power conditioner 25 in the present embodiment is configured so that the output power can be instantaneously reduced. As a result, it is possible to prevent surplus power from being output (reverse power flow) toward the system power source PS without following the decrease in demand power, and a dedicated power source for consuming surplus power for this reason. It is not necessary to provide equipment (typically, a heater that radiates heat to the cooling water for cooling the fuel cell or the recovered water for heat storage). A power meter 98 that detects the power supplied from the system power supply PS is disposed on the power cable 99 upstream of the connection point with the fuel cell system 1.

  The control device 40 includes a receiving unit 41 that receives a value detected by each meter, a water level determining unit 42 that determines whether or not the water level in the water tank 12 may drop below the low water level Ls, And a control unit 43 that controls the operation of the devices constituting the fuel cell system 1. The receiving unit 41 is connected to the water level meter 12g, the thermometer 45, and the wattmeter 98 through signal cables, respectively, and is configured to receive values detected by the respective meters 12g, 45, and 98 as signals. Has been. The water level determination unit 42 is configured to determine whether or not there is a possibility that the water level in the water tank 12 is lowered below the low water level Ls based on the value received by the reception unit 41. Here, there is a possibility that the water level will drop below the low water level Ls is typically a state in which it can be predicted that the water level in the water tank 12 tends to decrease. The control unit 43 is connected to the reforming water pump 13, the raw material blower 16, the air blower 18, and the oxidant gas blower 28 by signal cables, respectively, so that the start / stop and discharge flow rate of these devices can be controlled. It is configured. The control unit 43 is connected to the power conditioner 25 through a signal cable, and is configured to adjust the output power from the fuel cell system 1 to the outside of the system and the output power to the catalyst heater 34h. ing. The control unit 43 receives a command signal related to the operation of the fuel cell system 1 via an operation panel (not shown) or remotely, and based on a sequence stored in advance, It is configured to control operation.

  With continued reference to FIG. 1, the operation of the fuel cell system 1 will be described. When the control unit 43 receives a command to start the fuel cell system 1, the control unit 43 starts the raw material blower 16 and the air blower 18 to start generating the fuel gas E necessary for power generation in the fuel cell 20. The raw material D and air A1 are supplied to the reformer 11. Then, a predetermined partial oxidation reaction is performed in the reformer 11, and the fuel gas E is generated. Next, the reforming water pump 13 is started in place of the air blower 18, and the reforming water W is supplied to the reformer 11. When the reforming water pump 13 is activated, the reforming water W stored in the water tank 12 flows through the reforming water pipe 51 and is vaporized by the vaporizer 14 on the way, and then flows into the reformer 11. In the reformer 11 into which the raw material D and the vaporized reforming water W are introduced, the steam reforming reaction of the raw material D is performed, and the generation of the fuel gas E is continued. Note that the fuel gas E may be generated by supplying the air A1 and the reforming water W to the reformer 11 in addition to the raw material D and performing the autothermal reforming reaction.

  The fuel gas E generated by the reformer 11 is supplied to the anode 21 of the fuel cell 20. Further, the control unit 43 activates the oxidant gas blower 28. As a result, the oxidant gas A <b> 2 is supplied to the cathode 22 of the fuel cell 20. In the fuel cell 20 into which the fuel gas E and the oxidant gas A2 are introduced, power generation is performed by an electrochemical reaction between hydrogen or the like in the fuel gas E and oxygen in the oxidant gas A2. The electric power generated in the fuel cell 20 is appropriately supplied to the external load 95 via the power conditioner 25. In the fuel cell 20, water is generated with a reaction for power generation. The fuel gas E and the oxidant gas A2 introduced into the fuel cell 20 are discharged as fuel offgas Fe and oxidant offgas Fa after being used for power generation, and reach the combustion unit 32. The fuel off-gas Fe is burned in the combustion section 32 using the oxidant off-gas Fa as an oxidant, and the combustion heat generated at this time is used for the steam reforming reaction in the reformer 11.

  The combustion exhaust gas Fb generated by burning the fuel off-gas Fe in the combustion unit 32 flows through the combustion exhaust gas pipe 65 and reaches the purification unit 34. The combustion exhaust gas Fb is purified by being combusted by the combustion catalyst of the purification unit 34 when combustible components are present. In the purifying unit 34, since the combustion catalyst has not reached the activation temperature at the initial start of the fuel cell system 1, the control unit 43 operates the combustion catalyst heater 34h to quickly raise the combustion catalyst to the activation temperature. When the combustion catalyst reaches the activation temperature, the control unit 43 stops energizing the combustion catalyst heater 34h. The purified exhaust gas Fp generated by purifying the combustion exhaust gas Fb in the purification unit 32 flows through the purified exhaust gas pipe 66 and reaches the heat exchange unit 36. The exhaust gas F discharged from the fuel cell 20 contains moisture. For this reason, the purified exhaust gas Fp is cooled by the heat recovery fluid C flowing through the heat recovery fluid flow path 36x in the heat exchanging portion 36, whereby the moisture in the purified exhaust gas Fp is condensed. The out-of-system discharge gas G generated by cooling and removing moisture from the purified exhaust gas Fp in the heat exchange unit 36 flows through the out-of-system exhaust pipe 67 and is discharged out of the fuel cell system 1. In the present embodiment, the thermometer 45 detects the temperature of the out-of-system discharge gas G discharged from the heat exchange unit 36 and transmits it to the receiving unit 41 of the control device 40 as needed.

  On the other hand, the condensed water Wc condensed from the purified exhaust gas Fp cooled by the heat exchange unit 36 flows through the condensed water pipe 69 and flows into the water tank 12. In this way, even if the reformed water W decreases from the water tank 12 to generate the fuel gas E, the reformed water W flows into the water tank 12 as condensed water Wc. Even if the reforming water W is not directly supplied to the water tank 12 from the outside, it is possible to avoid the depletion of the reforming water W in the water tank 12. The condensed water Wc that flows into the water tank 12 is typically reformed out of the water tank 12 by the operation of the reforming water pump 13 because water is generated with the power generation in the fuel cell 20. More than water W. When the condensed water Wc flows into the water tank 12 and exceeds the maximum water level Lh, the reformed water W exceeding the maximum water level Lh is discharged from the fuel cell system 1 via the overflow pipe 12p.

  In the fuel cell 20 during steady operation, the electric power requested by the power conditioner 25 is generated. Here, the steady operation is an operation in a state where desired generated power can be generated in the fuel cell 20, and is typically an operation when the fuel cell system 1 is in a state other than the start process and the stop process. It is. The power required by the power conditioner 25 can change depending on the situation during operation. In general, the fuel cell is supplied with hydrogen and oxygen at a flow rate higher than the flow rates of hydrogen and oxygen used when the requested power is generated by the fuel cell. The control unit 43 modifies the fuel gas E at a flow rate capable of supplying the fuel cell 20 with a flow rate that is higher by a predetermined rate than the flow rate of hydrogen used when the fuel cell 20 generates the required power. The rotational speed of the raw material blower 16 is adjusted so that it can be generated by the mass device 11. Further, the control unit 43 adjusts the rotational speed of the reforming water pump 13 so as to supply the reforming water W with the reforming water W corresponding to the generation flow rate of the fuel gas E in the reformer 11. In addition, the control unit 43 provides a flow rate of oxidant gas that can supply the fuel cell 20 with a flow rate of oxygen that is higher by a predetermined ratio than the flow rate of oxygen used when the fuel cell 20 generates the required power. The rotational speed of the oxidant gas blower 28 is adjusted so that A2 is supplied to the fuel cell 20. The DC power generated by the fuel cell 20 is boosted by the power conditioner 25, converted into AC power, and supplied to the external load 95. When the electric power supplied to the external load 95 is insufficient only by the electric power generated by the fuel cell 20, the insufficient electric power is supplied from the system power supply PS to the external load 95. The wattmeter 98 detects the power supplied from the system power supply PS to the external load 95 and transmits it to the receiving unit 41 of the control device 40 as needed.

  In general, a solid oxide fuel cell (SOFC) has a correlation between temperature and power generation efficiency, and can be operated at an appropriate temperature at which power generation efficiency is high in a predetermined temperature range determined in consideration of a lifetime or the like. preferable. If the temperature of the fuel cell 20 is excessively increased, the fuel cell 20 can be cooled by increasing the flow rate of the oxidant gas A2 supplied to the fuel cell 20. Considering that the flow rate of the oxidant gas A2 supplied to the fuel cell 20 is used for power generation in the fuel cell 20 and used for maintaining the fuel cell 20 at an appropriate temperature, conventionally, the power generation of the fuel cell Although the supply flow rate is reduced as the electric power is reduced, the fuel cell 20 with enhanced heat insulation has such characteristics that the decrease rate of the supply flow rate is reduced or the supply flow rate is increased in the low output region. It has become.

  FIG. 2 shows the power generated by the fuel cell 20 (horizontal axis) and the flow rates of the fuel gas E and oxidant gas A2 supplied to the fuel cell 20 (vertical axis) when the fuel cell 20 is maintained at an appropriate operating temperature. ) Is shown as an example. In FIG. 2, the curve Re indicates the flow rate of the fuel gas E, and the curve Rt indicates the total flow rate of the fuel gas E and the oxidant gas A2. Therefore, the amount obtained by subtracting the curve Re from the curve Rt corresponds to the flow rate of the oxidant gas A2. The portion indicated by the broken line of the curve Rt indicates the total flow rate excluding the amount supplied for cooling the fuel cell 20, in other words, the fuel gas E and the amount necessary for power generation of the fuel cell 20. The total flow rate of the oxidant gas A2 is shown. From this, the portion sandwiched between the solid line portion and the broken line portion of the curve Rt is an increase amount of the oxidant gas A2 for cooling the fuel cell 20.

  As is apparent from FIG. 2, when the output of the fuel cell 20 is high, the fuel gas E and the oxidant gas A2 supplied to the fuel cell 20 as the generated power of the fuel cell 20 decreases from the rated output Pr. The flow rate is decreasing. This is because the flow rate of the oxidant gas A2 necessary for power generation decreases as the power generated by the fuel cell 20 decreases. When the output of the fuel cell 20 is in a low output region, the flow rate of the oxidant gas A2 supplied to the fuel cell 20 does not decrease even if the generated power of the fuel cell 20 decreases. This is because although the flow rate of the oxidant gas A2 required for power generation decreases as the power generated by the fuel cell 20 decreases (see the broken line portion of the curve Rt), the surplus heat amount in the fuel cell 20 increases below a certain output. This is because the flow rate of the oxidant gas A2 required to lower (cool) the fuel cell 20 to an appropriate temperature increases. In FIG. 2, when the portion sandwiched between the solid line and the broken line of the curve Rt is seen, the fuel cell 20 out of the flow rate of the oxidant gas A2 supplied to the fuel cell 20 as the generated power of the fuel cell 20 decreases. It can be seen that the proportion occupied by the cooling increases. As will be described later, when the flow rate of the oxidant gas A2 supplied to the fuel cell 20 in the low output region increases beyond the flow rate necessary for power generation, it may be difficult for the fuel cell system 1 to self-support water. Hereinafter, the output corresponding to the flow rate of the oxidant gas A2 in a range where the water self-sustainability of the fuel cell system 1 is not difficult, in other words, the oxidant taking into account the increase in the flow rate of the oxidant gas A2 for cooling the fuel cell 20 The lowest point of the output when the flow rate of the gas A2 is in a range where water can be self-sustained is referred to as “recommended minimum output Pm”. That is, the recommended minimum output Pm is the minimum output recommended for maintaining water independence.

  The solid oxide fuel cell (SOFC) generally continues without stopping operation even when there is no power demand of the external load 95 due to its characteristics. The power demand of the external load 95 is reduced, and the power generated by the fuel cell 20 is less than the lowest point (typically an output point higher than the recommended minimum output Pm) of the outputs that do not require cooling of the fuel cell 20. When the temperature is lowered, in order to prevent an excessive temperature rise of the fuel cell 20, it is necessary to supply the oxidant gas A2 at a flow rate exceeding the amount necessary for the power generation of the fuel cell 20. However, when the generated power of the fuel cell 20 is reduced and the flow rate of the oxidant gas A2 supplied to the fuel cell 20 is increased beyond the amount necessary for power generation, the oxidant utilization rate ("in the fuel cell 20" The flow rate of the oxidant gas A2 used for power generation "/" the flow rate of the oxidant gas A2 supplied to the fuel cell 20 ") decreases. When the oxidant utilization rate decreases, the flow rate of the out-system emission gas G discharged out of the fuel cell system 1 increases. As a result, the moisture discharged outside the fuel cell system 1 without being condensed in the heat exchanging unit 36 is increased, and the flow rate of the condensed water Wc recovered in the water tank 12 is decreased. In principle, since the reformed water W is not introduced from the outside in the fuel cell system 1, if the water discharged outside the fuel cell system 1 increases, water independence cannot be maintained, and continuation of operation becomes difficult. Become. Therefore, in the fuel cell system 1, the following control is performed in order to maintain water independence.

  FIG. 3 is a flowchart showing a first example of control for maintaining water independence in the fuel cell system 1. In the following description, when referring to the configuration or characteristics of the fuel cell system 1, FIG. 1 or FIG. 2 will be referred to as appropriate. During operation, the receiving unit 41 receives power supplied from the system power source PS to the external load 95 via the wattmeter 98 as described above, and the fuel cell system 1 via the power conditioner 25. Power supplied to the external load 95 is received. When the power detected by the wattmeter 98 and the power supplied from the fuel cell system 1 to the external load 95 via the power conditioner 25 are summed, the demand power of the external load 95 can be found. Thus, in this Embodiment, the power conditioner 25 and the wattmeter 98 comprise the demand power detection part. Based on the value received by the receiving unit 41, the water level determining unit 42 estimates the power demand status of the external load 95, and the power demand generated by the fuel cell 20 is less than the predetermined power Pp (see FIG. 2). Whether or not (S11). Here, the predetermined power Pp is an electric power having an arbitrary value lower than the rated output Pr, equal to or more than the recommended minimum output Pm and as close as possible to the recommended minimum output Pm.

  In the step of determining whether or not the demand for power generated in the fuel cell 20 is less than the predetermined power Pp (S11), the water level determination unit 42 determines that the water level in the water tank 12 is low. The control unit 43 determines that there is a possibility that the power will decrease to Ls or less, and the control unit 43 performs combustion while setting the power generated by the fuel cell 20 through the power conditioner 25 as close as possible to the predetermined power Pp that is equal to or higher than the predetermined power Pp. The catalyst heater 34h is energized (S12). Here, when the demand for power generated in the fuel cell 20 is less than the predetermined power Pp, it is determined that there is a possibility that the water level in the water tank 12 is lowered to the low water level Ls or less. If it is below the recommended minimum output Pm, the flow rate of the oxidant gas A2 supplied to the fuel cell 20 that is used for cooling the fuel cell 20 increases to such an extent that it becomes difficult for the water to become self-sustaining. This is because the discharged water increases and the flow rate of the condensed water Wc recovered in the water tank 12 decreases. In the fuel cell system 1, even when the demand for power generated in the fuel cell 20 is less than the predetermined power Pp, the generated power of the fuel cell 20 is set to be equal to or higher than the predetermined power Pp. Therefore, it is possible to avoid supplying the fuel cell 20 with the oxidant gas A2 having a flow rate exceeding the flow rate required for power generation for cooling, and it is possible to suppress a decrease in the water level in the water tank 12.

  When the demand for the electric power generated in the fuel cell 20 is less than the predetermined electric power Pp, if the combustion catalyst heater 34h is energized while making the electric power generated by the fuel cell 20 equal to or higher than the predetermined electric power Pp, the electric power generated in the fuel cell 20 is reduced. Of this, surplus power (power not supplied to the external load 95) is consumed by the combustion catalyst heater 34h. Since the combustion catalyst heater 34h is exclusively used to raise the temperature of the combustion catalyst when the fuel cell system 1 is started, it is normally not energized during steady operation. Therefore, surplus power generated in the fuel cell 20 can be consumed by the combustion catalyst heater 34h and released as heat to the purified exhaust gas Fp. Further, since the combustion catalyst heater 34h can adjust the duty ratio, it can cope with a change in surplus power of the fuel cell 20.

  Thereafter, the water level determination unit 42 determines whether or not the demand for power generated in the fuel cell 20 is equal to or higher than the predetermined power Pp based on the value received by the reception unit 41 (S13). If it is not equal to or higher than the predetermined power Pp, the process returns to the step of determining whether or not the demand for power generated in the fuel cell 20 is equal to or higher than the predetermined power Pp (S13). On the other hand, when it becomes more than predetermined electric power Pp, the control part 43 interrupts | blocks the electricity supply to the combustion catalyst heater 34h (S14). From the step (S11) for determining whether or not the demand for the electric power generated in the fuel cell 20 is less than the predetermined electric power Pp once again, the control for suppressing the lowering of the water level in the water tank 12 is once completed. Repeat the flow. In the step (S11) of determining whether or not the demand for power generated in the fuel cell 20 is less than the predetermined power Pp, control for suppressing a decrease in the water level in the water tank 12 even when the demand is not less than the predetermined power Pp. The above-described flow is repeated from the step (S11) of determining whether or not the demand for the power generated in the fuel cell 20 is less than the predetermined power Pp.

  Next, another example of control for maintaining water independence will be described with reference to FIG. FIG. 4 is a flowchart showing a second example of control for maintaining water self-supporting in the fuel cell system 1. During operation, the receiving unit 41 receives the temperature of the out-of-system emission gas G from the thermometer 45 as described above. When the fuel cell system 1 is in a steady operation, the water level determination unit 42 determines the heat exchange unit 36 from the flow rate of the fuel gas E, the flow rate of the oxidant gas A2, and the flow rate of the reforming water W that are input to the fuel cell 20. The flow rate of the purified exhaust gas Fp and the water content are estimated, and based on the estimated values, the temperature of the out-of-system emission gas G that can condense only the amount of water that makes it difficult for the fuel cell system 1 to stand by itself. A lower limit value (hereinafter, this temperature is referred to as “water self-sustainable temperature”) is set (S21). Here, supplementing the relationship between the temperature of the out-of-system released gas G and water self-sustained, the larger the amount of heat taken away by the heat recovery fluid C from the purified exhaust gas Fp introduced into the heat exchanging section 36, the greater the generated flow rate of the condensed water Wc. It can be said that the temperature of the out-of-system released gas G decreases. In the present embodiment, the water independence difficult temperature corresponds to a predetermined temperature. Note that the flow rate of the fuel gas E input to the fuel cell 20 can be acquired by a flow meter (not shown) disposed downstream of the raw material blower 16 and the oxidant gas A2 input to the fuel cell 20. Can be obtained by a flow meter (not shown) disposed downstream of the oxidant gas blower 28.

  After setting the water independence difficult temperature, the water level determination unit 42 determines whether or not the temperature detected by the thermometer 45 is equal to or higher than the water independence difficult temperature (S22). When the temperature detected by the thermometer 45 is equal to or higher than the water independence temperature, the water level determination unit 42 determines that the water level in the water tank 12 may be lowered to the low water level Ls or less, and the control unit 43 The combustion catalyst heater 34h is energized through the power conditioner 25 while making the generated power of the fuel cell 20 as close as possible to the predetermined power Pp that is equal to or higher than the predetermined power Pp (S23). Normally, when the flow rate of the condensed water Wc generated by the heat exchange unit 36 decreases, the demand for the power generated in the fuel cell 20 is less than the predetermined power Pp (see FIG. 2). 1, the surplus power is consumed by the combustion catalyst heater 34h while the generated power of the fuel cell 20 is set to be equal to or higher than the predetermined power Pp. This makes it possible to avoid supplying the fuel cell 20 with the oxidant gas A2 having a flow rate that exceeds the flow rate required for power generation for cooling the fuel cell 20 as water independence becomes difficult. A drop in the water level can be suppressed.

  Thereafter, the water level determination unit 42 resets the water independence difficult temperature in accordance with the fluctuation of the generated power of the fuel cell 20 (S24). And the water level judgment part 42 judges whether the temperature detected with the thermometer 45 became less than the reset water independence difficult temperature (S25). If it is not lower than the water self-sustainable temperature, the process returns to the step of resetting the water self-sustainable temperature (S24). On the other hand, when the temperature becomes lower than the water self-sustainable temperature, the control unit 43 cuts off the power supply to the combustion catalyst heater 34h (S26). Above, the control which suppresses the fall of the water level in the water tank 12 is once complete | finished, and the above-mentioned flow is repeated from the process (S21) which sets water independence difficult temperature again. In the step of determining whether or not the temperature detected by the thermometer 45 is equal to or higher than the water self-sustainable temperature (S22), control for suppressing a decrease in the water level in the water tank 12 is performed even when the temperature is not equal to or higher than the water self-sustainable temperature. The above-described flow is repeated from the step (S21) of once ending and setting again the water independence difficult temperature.

  Next, still another example of the control for maintaining water independence will be described with reference to FIG. FIG. 5 is a flowchart showing a third example of control for maintaining water self-sustainability in the fuel cell system 1. The receiver 41 receives information on the water level in the water tank 12 from the water level meter 12g during operation. The water level determination unit 42 determines whether or not the water level meter 12g has detected that the water level in the water tank 12 is equal to or lower than the warning water level Lc (S31). When the water level in the water tank 12 is lower than or equal to the warning water level Lc, the water level determination unit 42 determines that there is a possibility that the water level in the water tank 12 will drop below the low water level Ls, and the control unit 43 performs power conditioning. The combustion catalyst heater 34h is energized while the generated power of the fuel cell 20 is made as close as possible to the predetermined power Pp equal to or higher than the predetermined power Pp via the na 25 (S32). The fact that the water level in the water tank 12 has fallen to the warning water level Lc suggests that a large amount of moisture is released out of the system and the flow rate of the condensed water Wc generated in the heat exchanging unit 36 is reduced. doing. Normally, when the flow rate of the condensed water Wc generated by the heat exchange unit 36 decreases, the demand for the power generated in the fuel cell 20 is less than the predetermined power Pp (see FIG. 2). 1, the surplus power is consumed by the combustion catalyst heater 34h while the generated power of the fuel cell 20 is set to be equal to or higher than the predetermined power Pp. This makes it possible to avoid supplying the fuel cell 20 with the oxidant gas A2 having a flow rate that exceeds the flow rate required for power generation for cooling the fuel cell 20 as water independence becomes difficult. A drop in the water level can be suppressed.

  Thereafter, the water level determination unit 42 determines whether or not the water level in the water tank 12 has exceeded the warning water level Lc based on the detection result of the water level gauge 12g received by the reception unit 41 (S33). If the warning water level Lc has not been exceeded, the process returns to the step of determining whether or not the water level in the water tank 12 has exceeded the warning water level Lc (S33). On the other hand, when the warning water level Lc is exceeded, the control unit 43 cuts off the power supply to the combustion catalyst heater 34h (S34). From the step (S31) of determining whether or not the control for suppressing the lowering of the water level in the water tank 12 is temporarily ended and it is detected again that the water level in the water tank 12 is equal to or lower than the warning water level Lc. The above flow is repeated. In the step of determining whether or not the water level in the water tank 12 is below the warning water level Lc (S31), even if the water level is not below the warning water level Lc, a decrease in the water level in the water tank 12 is suppressed. The above-described flow is repeated from the step (S31) of determining whether or not the control is once ended and it is detected again that the water level in the water tank 12 is equal to or lower than the warning water level Lc.

  As mentioned above, although the example of the control for maintaining the water independence in the fuel cell system 1 was demonstrated from the 1st example to the 3rd example, all these controls were implemented in parallel, respectively, and it was from a safety side viewpoint. The subsequent flow may be advanced in accordance with a control example in which it is determined that the water level in the water tank 12 may drop below the low water level Ls earliest. Alternatively, two of the three examples may be performed in parallel, or any one of the three examples may be performed. In the case where two or one of the three examples are selectively implemented, if the water level meter 12g and / or the thermometer 45 is not used to execute the selected control example, the fuel cell system 1 does not use the water level meter. 12g and / or the thermometer 45 may be omitted.

  In the above description, the power consumption load is the combustion catalyst heater 34h. However, devices other than the combustion catalyst heater 34h that can arbitrarily consume power in the fuel cell system 1 (for example, the fuel cell system 1 It may be an electric device such as a pump that can consume surplus power without affecting the operation.

  In the above description, the thermometer 45 directly detects the temperature of the exhaust gas in the heat exchange unit 36 by detecting the temperature of the out-of-system emission gas G. However, the heat recovery supplied to the heat exchange unit 36 is performed. The temperature of the exhaust gas in the heat exchange unit 36 may be indirectly detected by detecting the inlet temperature or the inlet / outlet temperature difference of the fluid C. Or it is good also as detecting the physical quantity which can estimate the flow volume of the condensed water Wc produced | generated by the heat exchange part 36. FIG.

DESCRIPTION OF SYMBOLS 1 Solid oxide fuel cell system 12 Water tank 12g Water level meter 20 Solid oxide fuel cell 32 Combustion part 34 Purification | purification part 34h Combustion catalyst heater 36 Heat exchange part 40 Control apparatus 41 Receiving part 42 Water level judgment part 43 Control part 45 Temperature Total 95 System load 98 Power meter F Exhaust gas Fb Combustion exhaust gas

Claims (5)

A solid oxide fuel cell for generating electric power to be supplied to a power load outside the system;
A water recovery unit for recovering moisture contained in the exhaust gas discharged from the fuel cell;
A water tank for storing the water recovered by the water recovery unit;
A water level determination unit for determining the water level in the water tank;
A power consuming load that consumes power generated in the fuel cell in the system;
When the water level determination unit determines that the water level in the water tank may drop below a first predetermined water level, the generated power of the fuel cell is set to be equal to or higher than a predetermined power lower than the rated power. And a controller that operates the power consumption load so as to consume the power generated in the fuel cell;
Solid oxide fuel cell system. An off-gas combustion section for burning the exhaust gas discharged from the fuel cell before introducing it to the water recovery section;
An exhaust gas purification unit for combusting a combustible substance in the exhaust gas discharged from the off-gas combustion unit with a combustion catalyst before introducing the combustible material to the water recovery unit, the exhaust gas purification unit having a combustion catalyst heater for heating the combustion catalyst; Comprising:
The power consumption load comprises the combustion catalyst heater;
The solid oxide fuel cell system according to claim 1. A demand power detection unit for detecting a demand for power generated in the fuel cell;
The water level determination unit determines that the water level in the water tank may drop below a first predetermined water level when the power detected by the demand power detection unit is less than the predetermined power. Configured as follows;
The solid oxide fuel cell system according to claim 1 or 2. An exhaust gas temperature detection unit that directly or indirectly detects the temperature of the exhaust gas from which moisture has been recovered by the water recovery unit;
The water level determination unit may determine that the water level in the water tank may drop below a first predetermined water level when the temperature detected by the exhaust gas temperature detection unit is equal to or higher than a predetermined temperature. Composed of;
The solid oxide fuel cell system according to claim 1 or 2. A water level detection unit for detecting that the water level in the water tank is equal to or lower than a second predetermined water level higher than the first predetermined water level;
When the water level determination unit detects that the water level detection unit is below the second predetermined water level, the water level in the water tank may drop below the first predetermined water level. Configured to judge;
The solid oxide fuel cell system according to claim 1 or 2.

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