Fuel cell system

Abstract

Provided is a fuel cell system capable of suppressing moisture intrusion into a fuel cell. A hydrogen supply path 34 supplies hydrogen gas to a stack 31. One end of the hydrogen supply path 34 is connected to a fuel part 2. The other end of the hydrogen supply path 34 is connected to a flow path inlet of a separator of an anode electrode 31A of the stack 31. A hydrogen circulation path 35 returns hydrogen off-gas discharged from the stack 31 to the hydrogen supply path 34. One end of the hydrogen circulation path 35 is connected to a flow path outlet of the separator of the anode electrode 31A of the stack 31. The other end of the hydrogen circulation path 35 is connected to the hydrogen supply path 34 at a connection position P1. The hydrogen supply path 34 includes a gas-liquid separator 37. The gas-liquid separator 37 is arranged between the connection position P1 and the stack 31. The gas-liquid separator 37 separates dew condensation water generated by the confluence of the hydrogen gas and the hydrogen off-gas.

Description

Fuel cell system

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

[0002] In the fuel cell system described in Patent Document 1, the reaction off-gas discharged from the fuel cell is merged with the anode gas supplied from the hydrogen supply system. The reaction off-gas discharged from the fuel cell is wet. The reaction off-gas passes through a gas-liquid separator before merging with the anode gas. The gas-liquid separator separates moisture from the reaction gas. The reaction gas from which moisture has been separated is sent to an ejector and merged with the anode gas.

[0003] Japanese Patent Application Laid-Open No. 2006-134743

[0004] When the ambient temperature is low, the temperature of the anode gas becomes low. When the reaction off-gas merges with the low-temperature anode gas, dew water is generated due to the temperature difference. If the dew water invades the stack of the fuel cell, the flow path of the fuel gas in the stack may be blocked by the dew water. In this case, the fuel cell may not be able to generate electricity stably.

[0005] An object of the present invention is to provide a fuel cell system capable of suppressing the intrusion of moisture into the fuel cell.

[0006] According to one aspect of the present invention, there is provided a fuel cell system including: a fuel cell that generates electricity by reacting a fuel gas and an oxidant and can supply power to an electrical load; a first flow path that connects a fuel supply source and the fuel cell and supplies the fuel gas discharged from the fuel supply source to the fuel cell; a second flow path that connects the fuel cell and the first flow path and returns the fuel off-gas, which is the fuel gas discharged from the fuel cell, to the first flow path; and a first gas-liquid separator disposed in the second flow path and separating moisture contained in the fuel off-gas, the fuel cell system being characterized in that it includes a second gas-liquid separator disposed in the first flow path and separating moisture contained in the fuel off-gas that has passed through the second flow path and the fuel gas flowing through the first flow path.

[0007] The fuel off-gas discharged from the fuel cell flows into the first channel through the second channel. When the temperature outside the fuel cell is low, the temperature of the fuel gas introduced into the first channel from the fuel supply source is also low. In this case, when the fuel off-gas that has passed through the second channel merges with the fuel gas, condensation water is generated due to the temperature difference. Since the second gas-liquid separator is located in the first channel, the generated condensation water can be separated from the fuel off-gas. Therefore, the fuel cell system can suppress the intrusion of moisture into the fuel cell.

[0008] This is a block diagram of fuel cell system 1. This is a cross-sectional view showing the schematic configuration of the gas-liquid separator 37. This is a flowchart of the pump control process. This is a block diagram of a modified example of fuel cell system 1. This is a cross-sectional view showing the schematic configuration of a gas-liquid separator 137, which is a modified example of the gas-liquid separator 37. This is a flowchart of a modified example of the pump control process.

[0009] One embodiment of the present invention is described below. The referenced drawings are used to illustrate the technical features that the present invention may adopt. That is, the configurations, controls, etc., shown in the drawings are not intended to limit the invention to those shown, but are merely illustrative examples.

[0010] The fuel cell system 1 shown in Figure 1 can generate electricity by reacting a fuel with an oxidizer and supply power to an electrical load 4. An example of fuel is hydrogen. An example of an oxidizer is oxygen from the atmosphere.

[0011] The fuel cell system 1 comprises a fuel unit 2 and a power generation unit 3. The fuel unit 2 comprises a plurality of hydrogen supply sources 21 and a regulator 22. The hydrogen supply sources 21 have hydrogen storage alloys. Hydrogen storage alloys are materials that can absorb hydrogen into the gaps created by the atoms of the metal and release the absorbed hydrogen by heating. Each of the plurality of hydrogen supply sources 21 is connected to the regulator 22 via a shut-off valve. The regulator 22 adjusts the pressure of the hydrogen supplied to the power generation unit 3 to a predetermined pressure.

[0012] The power generation unit 3 includes a stack 31, an air supply passage 32, an air discharge passage 33, a hydrogen supply passage 34, a hydrogen circulation passage 35, gas-liquid separators 36 and 37, and a circulation pump 38. The power generation unit 3 further includes a check valve 39, drainage passages 51 and 52, drain valves 46 and 47, an exhaust gas passage 53, a purge valve 48, a hydrogen dilution unit 49, a control unit 40, and a temperature sensor 45.

[0013] The stack 31 is a structure formed by stacking multiple flat unit cells. Each unit cell consists of a solid polymer electrolyte membrane with plate-shaped electrodes on each side, and the solid polymer electrolyte membrane and electrodes sandwiched between separators placed on each side. The anode electrode 31A and the cathode electrode 31C are plate-shaped electrodes. The separators have channels through which hydrogen gas or air flows.

[0014] The hydrogen gas supplied from the fuel unit 2 flows through the channel 31B of the separator located at the anode electrode 31A and comes into contact with the anode electrode 31A. Hydrogen gas refers to gaseous hydrogen molecules. At the anode electrode 31A, the hydrogen gas releases electrons and becomes hydrogen ions. The hydrogen ions move to the cathode electrode 31C by passing through the solid polymer electrolyte membrane. The electrons move from the anode electrode 31A to the cathode electrode 31C by passing through the electrical load 4 connected to the anode electrode 31A and the cathode electrode 31C.

[0015] Air enters the separator channel 31D, which is located at the cathode electrode 31C, via the air supply channel 32. The air flowing through the separator channel 31D comes into contact with the cathode electrode 31C. Oxygen in the air receives electrons that have moved to the cathode electrode 31C and becomes oxygen ions. These oxygen ions combine with hydrogen ions that have passed through the solid polymer electrolyte membrane to produce water. The unit cell generates electricity as electrons move from the anode electrode 31A through the electrical load 4 to the cathode electrode 31C. The generated water is discharged to the outside along with atmospheric off-gas, which is discharged from the air discharge channel 33 connected to the outlet of the separator channel 31D. Atmospheric off-gas is air that flows through the separator channel 31D and is discharged outside the stack 31 from the outlet of channel 31D.

[0016] The hydrogen supply passage 34 is a hydrogen gas flow path for supplying hydrogen gas from the fuel unit 2 to the stack 31. The hydrogen supply passage 34 includes a first supply passage 34A, a second supply passage 34B, and a third supply passage 34C. One end of the first supply passage 34A is connected to the regulator 22 of the fuel unit 2. The other end of the first supply passage 34A is connected to one end of the second supply passage 34B. The connection point P1 is defined as the position where the other end of the first supply passage 34A and one end of the second supply passage 34B connect.

[0017] The other end of the second supply channel 34B is connected to the gas inlet 62 of the gas-liquid separator 37. One end of the third supply channel 34C is connected to the gas outlet 63 of the gas-liquid separator 37. The other end of the third supply channel 34C is connected to the inlet of the flow path 31B of the separator at the anode electrode 31A of the stack 31. The hydrogen gas supplied from the fuel section 2 flows through the hydrogen supply channels 34 in the order of the first supply channel 34A, the second supply channel 34B, and the third supply channel 34C, and is supplied to the stack 31. The gas-liquid separator 37 will be described later.

[0018] The hydrogen supply channel 34 connects to the hydrogen circulation channel 35 at connection point P1. The hydrogen circulation channel 35 is a flow path for returning the hydrogen off-gas discharged from the stack 31 back to the hydrogen supply channel 34. The hydrogen off-gas is the hydrogen gas that flows through the separator's flow path 31B and is discharged outside the stack 31 from the outlet of flow path 31B. The hydrogen off-gas contains excess hydrogen gas resulting from a chemical reaction with oxygen, water adhering to the separator's flow path 31B, and water vapor from the vaporized water. The hydrogen off-gas and the water suspended in the hydrogen gas are referred to as moisture.

[0019] Some of the water generated at the cathode electrode 31C passes through the solid polymer electrolyte membrane and moves to the anode electrode 31A. The water that moves to the anode electrode 31A adheres to the separator's channel 31B. More hydrogen gas is supplied to the stack 31 than is consumed in the chemical reaction, resulting in a surplus. The surplus hydrogen gas scrapes off the water adhering to channel 31B and is discharged into the hydrogen circulation path 35.

[0020] The hydrogen circulation path 35 includes a first circulation path 35A, a second circulation path 35B, and a third circulation path 35C. One end of the first circulation path 35A is connected to the outlet of the separator's flow path 31B. The other end of the first circulation path 35A is connected to the gas inlet 62 of the gas-liquid separator 36. One end of the second circulation path 35B is connected to the gas outlet 63 of the gas-liquid separator 36. The other end of the second circulation path 35B is connected to the circulation pump 38.

[0021] One end of the third circulation path 35C is connected to the circulation pump 38. The other end of the third circulation path 35C is connected to the hydrogen supply path 34 at connection point P1. At connection point P1, the other end of the first supply path 34A, one end of the second supply path 34B, and the other end of the third circulation path 35C are connected to each other by a joint. The hydrogen off-gas discharged from the stack 31 flows through the hydrogen circulation paths 35 in the order of the first circulation path 35A, the second circulation path 35B, and the third circulation path 35C, and is returned to the hydrogen supply path 34.

[0022] The gas-liquid separator 36 is a device for separating moisture contained in the hydrogen off-gas from the hydrogen off-gas and discharging it to the outside of the fuel cell system 1. As shown in Figure 2, the gas-liquid separator 36 comprises a separation container 61 and a partition 65. The separation container 61 has a roughly rectangular cross-section and includes a top plate 61A, a bottom plate 61B, a left plate 61C, a right plate 61D, a front plate, a rear plate, and an inclined plate 61E. The top plate 61A has a gas inlet 62 located near the left end. The other end of the first circulation path 35A is connected to the gas inlet 62.

[0023] The top plate 61A has a gas outlet 63 located near its right edge. One end of the second circulation path 35B is connected to the gas outlet 63. The left plate 61C and the right plate 61D extend downward from the left and right edges of the top plate 61A, respectively. The right plate 61D extends further downward than the left plate 61C.

[0024] The bottom plate 61B extends from the lower end of the right plate 61D toward the left, for approximately half the length of the top plate 61A. The bottom plate 61B has a drain opening 64 located near its right end. A drain channel 51 is connected to the drain opening 64. The inclined plate 61E connects the lower end of the left plate 61C to the left end of the bottom plate 61B. The inclined plate 61E slopes downward and to the right from the left plate 61C toward the bottom plate 61B.

[0025] The front plate has its outer edge connected to the front ends of the top plate 61A, bottom plate 61B, left plate 61C, right plate 61D, and inclined plate 61E, closing the front of the separation container 61. The rear plate has its outer edge connected to the rear ends of the top plate 61A, bottom plate 61B, left plate 61C, right plate 61D, and inclined plate 61E, closing the rear of the separation container 61. The partition 65 is positioned inside the separation container 61. The partition 65 extends in a block shape from approximately the center of the top plate 61A toward the bottom plate 61B.

[0026] The space inside the separation container 61 includes a first region 66, a second region 67, and a third region 68. The first region 66 is the area between the partition 65 and the left plate 61C. The gas inlet 62 is connected to the first region 66. The second region 67 is the area between the partition 65 and the bottom plate 61B and the inclined plate 61E. The drain port 64 is connected to the second region 67. The third region 68 is the area between the partition 65 and the right plate 61D. The gas outlet 63 is connected to the third region 68. The first region 66 and the third region 68 are connected via the second region 67.

[0027] The hydrogen off-gas discharged from the stack 31 flows through the first circulation path 35A and enters the first region 66 of the gas-liquid separator 36 from the gas inlet 62. In the first region 66, the hydrogen off-gas flows downwards within the separation container 61 and enters the second region 67. In the second region 67, the hydrogen off-gas flows to the right within the separation container 61 and enters the third region 68. In the third region 68, the hydrogen off-gas flows upwards within the separation container 61 and is discharged from the gas outlet 63 to the second circulation path 35B. In other words, as shown by the arrow 69 in the figure, the hydrogen off-gas flows through the separation container 61 following a path that moves downwards in the first region 66, makes a U-turn in the second region 67, and moves upwards in the third region 68.

[0028] As the hydrogen off-gas flows to the right through the second region 67, it is pulled to the right as it passes through the first region 66, causing it to flow diagonally downward to the right. Therefore, the hydrogen off-gas is likely to collide with the left side of the partition 65 located on the right side within the first region 66. The moisture contained in the hydrogen off-gas adheres to the left side of the partition 65 upon collision and is separated from the hydrogen off-gas.

[0029] As the hydrogen off-gas enters the second region 67 from the first region 66 and heads toward the third region 68, the flow makes a U-turn. The water contained in the hydrogen off-gas is separated from the hydrogen off-gas due to the difference in specific gravity between hydrogen gas and water. The separated water falls onto the inclined plate 61E and accumulates at the bottom of the separation container 61.

[0030] As shown in Figure 1, the drain channel 51 connected to the drain port 64 of the bottom plate 61B is equipped with a drain valve 46. The drain valve 46 is opened and closed by the control unit 40. Thus, the water accumulated in the separation container 61 is discharged to the outside of the power generation unit 3. The circulation pump 38 circulates hydrogen off-gas in the hydrogen circulation channel 35. The control unit 40 can control the drive of the circulation pump 38 and change the amount of hydrogen off-gas circulating.

[0031] A check valve 39 and a gas-liquid separator 37 are located in the hydrogen supply passage 34. The check valve 39 is located in the first supply passage 34A. The check valve 39 prevents hydrogen off-gas, which has flowed through the hydrogen circulation passage 35 and entered the hydrogen supply passage 34 from connection point P1, from flowing back towards the fuel section 2.

[0032] The gas-liquid separator 37 is positioned between the connection point P1 and the stack 31. The hydrogen gas supplied from the fuel unit 2 merges with the hydrogen off-gas returned from the hydrogen circulation path 35 at the connection point P1. Therefore, hydrogen gas and hydrogen off-gas flow through the second supply path 34B and the third supply path 34C. The gas-liquid separator 37 separates the water contained in the hydrogen gas and hydrogen off-gas. Note that when the hydrogen off-gas and hydrogen gas merge at the connection point P1, condensation may occur in the water contained in the hydrogen off-gas due to the temperature difference. The gas-liquid separator 37 separates the water, including condensed water, contained in the hydrogen gas and hydrogen off-gas from the hydrogen gas and hydrogen off-gas.

[0033] The chemical reaction in stack 31 is exothermic. The temperature of the hydrogen off-gas immediately after it is discharged from stack 31 is higher than the temperature outside the fuel cell system 1, i.e., the ambient temperature. The hydrogen off-gas flowing through the hydrogen circulation path 35 is affected by the ambient temperature and its temperature decreases. Therefore, the temperature of the hydrogen off-gas when it reaches connection point P1 is lower than the temperature immediately after it is discharged from stack 31. However, the temperature of the hydrogen off-gas when it reaches connection point P1 is higher than the ambient temperature.

[0034] The hydrogen supply source 21 of the fuel unit 2 is affected by the ambient temperature. The temperature of the hydrogen gas supplied from the fuel unit 2 is the same as or slightly higher than the ambient temperature, but lower than the temperature at which the hydrogen off-gas reaches the connection point P1.

[0035] At connection point P1, when hydrogen off-gas and hydrogen gas at a lower temperature than the hydrogen off-gas merge, the hydrogen off-gas is cooled by the hydrogen gas. The amount of water vapor contained in the hydrogen off-gas condenses to an amount exceeding the saturation water vapor amount at the temperature cooled by the hydrogen gas. The gas-liquid separator 37 is placed between connection point P1 and the stack 31 to separate the moisture, including the condensed water produced by the condensation, from the hydrogen gas and hydrogen off-gas.

[0036] The structure of the gas-liquid separator 37 is the same as that of the gas-liquid separator 36. Therefore, the second supply channel 34B is connected to the gas inlet 62 of the gas-liquid separator 37. The third supply channel 34C is connected to the gas outlet 63 of the gas-liquid separator 37. The drain channel 52 is connected to the drain port 64 of the gas-liquid separator 37.

[0037] The drain channel 52 is equipped with a drain valve 47. The drain valve 47 is opened and closed by the control unit 40. As a result, the water accumulated in the separation container 61 of the gas-liquid separator 37 is discharged to the outside of the power generation unit 3.

[0038] The exhaust gas passage 53 is connected to the hydrogen circulation passage 35 at connection point P2 of the second circulation passage 35B. The exhaust gas passage 53 is a passage for discharging hydrogen off-gas to the outside. A purge valve 48 and a hydrogen dilution unit 49 are located in the exhaust gas passage 53.

[0039] The purge valve 48 is opened and closed at predetermined timings by the control unit 40. The hydrogen dilution unit 49 dilutes the hydrogen contained in the hydrogen off-gas, reducing its concentration to a level that does not cause a chemical reaction. The hydrogen off-gas is diluted by the hydrogen dilution unit 49 at predetermined timings and discharged to the outside of the power generation unit 3.

[0040] The control unit 40 includes a CPU 41, a ROM 42, and a RAM 43. The CPU 41 controls the fuel cell system 1. The ROM 42 is a non-volatile memory such as an EEPROM. The ROM 42 stores the operation program of the fuel cell system 1, etc. The operation program includes pump control processing. The RAM 43 temporarily stores various data generated by the calculation processing of the CPU 41.

[0041] The control unit 40 controls the operation of the circulation pump 38, the hydrogen dilution unit 49, the drain valves 46 and 47, and the purge valve 48. The temperature sensor 45 detects the temperature outside the fuel cell system 1, i.e., the ambient temperature, and transmits the detection result to the control unit 40.

[0042] Next, a method for suppressing condensation of hydrogen off-gas by changing the flow rate of hydrogen off-gas according to the ambient temperature will be explained. When hydrogen off-gas merges with hydrogen gas at connection point P1, hydrogen off-gas is cooled by hydrogen gas, which is at a lower temperature than hydrogen off-gas. The lower the temperature of the hydrogen gas, the lower the temperature of the hydrogen off-gas when it is cooled. The lower the temperature when cooled, the less saturated water vapor the hydrogen off-gas can hold, making it more susceptible to condensation. Therefore, when the ambient temperature is below the dew point temperature, the control unit 40 controls the circulation pump 38 to increase the relative ratio of hydrogen off-gas to hydrogen gas, thereby suppressing condensation.

[0043] As shown in Figure 3, when the fuel cell system 1 is started up, the CPU 41 of the control unit 40 reads and executes the pump control processing program from the ROM 42. The CPU 41 controls the drive of the circulation pump 38 so that the amount of hydrogen off-gas flowing through the hydrogen circulation path 35 by the circulation pump 38 becomes the first flow amount (S1).

[0044] The first flow rate is a preset flow rate of the hydrogen off-gas such that the amount of hydrogen gas supplied from the fuel section 2 and the amount of hydrogen off-gas flowing through the hydrogen circulation path 35 become a first ratio at the connection position P1. The hydrogen off-gas merges with the hydrogen gas at the connection position P1 and is cooled by the hydrogen gas. The first ratio is the ratio of the hydrogen gas to the hydrogen off-gas that can maintain a temperature at which the hydrogen off-gas is unlikely to condense even when cooled by the hydrogen gas when the temperature of the hydrogen gas is higher than the dew point temperature. When the amount of hydrogen off-gas relative to the amount of hydrogen gas increases, the temperature of the hydrogen off-gas merged with the hydrogen gas at the connection position P1 becomes higher than when the amount of hydrogen off-gas relative to the amount of hydrogen gas does not increase. Since the saturated water vapor amount of the hydrogen off-gas increases as the temperature gets higher, the hydrogen off-gas becomes less likely to condense.

[0045] Incidentally, if the temperatures of the hydrogen gas and the hydrogen off-gas are high, the solid polymer electrolyte membrane of the stack 31 may dry out. Therefore, in the first ratio, when the temperature of the hydrogen gas is higher than the dew point temperature, a ratio is preset such that the hydrogen off-gas does not condense even when cooled by the hydrogen gas and can prevent the drying of the solid polymer electrolyte membrane. The dew point temperature is, for example, 0°C.

[0046] The CPU 41 acquires the ambient temperature based on the detection result of the temperature sensor 45 (S2). The CPU 41 determines whether the ambient temperature is below the dew point temperature (S3). Since the temperature of the hydrogen gas is affected by the ambient temperature, the determination in S3 is made assuming the ambient temperature is the temperature of the hydrogen gas. When it is determined that the ambient temperature is higher than the dew point temperature (S3: NO), the CPU 41 returns the process to S2 and repeats the acquisition of the ambient temperature. In this case, the CPU 41 maintains the driving of the circulation pump 38 so that the hydrogen off-gas flows through the hydrogen circulation path 35 at the first flow rate.

[0047] When it is determined that the ambient temperature is below the dew point temperature (S3: YES), the CPU 41 controls the driving of the circulation pump 38 so that the flow rate of the hydrogen off-gas becomes the second flow rate (S4). The second flow rate is a preset flow rate of the hydrogen off-gas such that the amount of hydrogen gas and the amount of hydrogen off-gas become a second ratio at the connection position P1. The second flow rate increases the flow rate of the hydrogen off-gas compared to the first flow rate.

[0048] The second ratio is the ratio of hydrogen gas to hydrogen off-gas that can maintain a temperature at which the hydrogen off-gas is unlikely to condense even when cooled by the hydrogen gas when the temperature of the hydrogen gas is below the dew point temperature. If the hydrogen off-gas is merged into the hydrogen gas below the dew point temperature while maintaining the first ratio, the amount of saturated water vapor in the hydrogen off-gas will be less than when the hydrogen off-gas is merged into the hydrogen gas at a temperature higher than the dew point temperature. Therefore, in this case, the hydrogen off-gas is more likely to condense. The second ratio is a ratio in which the amount of hydrogen off-gas relative to the amount of hydrogen gas is increased compared to the first ratio.

[0049] With the second ratio, even when the hydrogen gas and the hydrogen off-gas below the dew point temperature merge at the connection position P1, the temperature of the hydrogen off-gas is unlikely to decrease compared to the temperature of the hydrogen off-gas before merging. Therefore, when the temperature of the hydrogen gas is below the dew point temperature, by setting the flow rate of the hydrogen off-gas to the second flow rate, the hydrogen off-gas is made less likely to condense.

[0050] The CPU 41 acquires the ambient temperature based on the detection result of the temperature sensor 45 (S5). The CPU 41 determines whether the ambient temperature is below the dew point temperature (S6). If it is determined that the ambient temperature is below the dew point temperature (S6: NO), the CPU 41 returns the process to S5 and repeats the acquisition of the ambient temperature. In this case, the CPU 41 maintains the drive of the circulation pump 38 so that the hydrogen off-gas flows at the second flow rate. If it is determined that the ambient temperature is higher than the dew point temperature (S6: YES), the CPU 41 returns the process to S1 and controls the flow rate of the hydrogen off-gas flowing through the hydrogen circulation path 35 to the first flow rate (S1).

[0051] As described above, the hydrogen off-gas discharged from the stack 31 passes through the hydrogen circulation path 35 and flows into the hydrogen supply path 34. When the ambient temperature of the fuel cell system 1 is low, the temperature of the hydrogen gas introduced from the fuel section 2 into the hydrogen supply path 34 is low. In this case, when the hydrogen off-gas passing through the hydrogen circulation path 35 merges with the hydrogen gas, dew water is generated due to the temperature difference. Since the gas-liquid separator 37 is arranged in the hydrogen supply path 34, the moisture including the generated dew water can be separated from the hydrogen off-gas. Therefore, the fuel cell system 1 can suppress the intrusion of moisture into the stack 31.

[0052] A circulation pump 38 located in the hydrogen circulation path 35 returns the hydrogen off-gas to the hydrogen supply path 34. The gas-liquid separator 37 is located downstream of the circulation pump 38 in the direction in which the hydrogen off-gas flows through the hydrogen circulation path 35. Therefore, the hydrogen off-gas reliably passes through the gas-liquid separator 37, and moisture, including condensation water generated by its merging with hydrogen gas, can be separated. Thus, the fuel cell system 1 can suppress the intrusion of moisture into the stack 31.

[0053] The hydrogen off-gas that has flowed through the hydrogen circulation path 35 merges with the hydrogen gas flowing through the hydrogen supply path 34 at the connection point P1 between the hydrogen supply path 34 and the hydrogen circulation path 35. The gas-liquid separator 37 is positioned between the connection point P1 and the stack 31. Therefore, the gas-liquid separator 37 can separate the hydrogen off-gas and the water containing condensation generated at the merging of the hydrogen off-gas and the hydrogen gas before they flow into the stack 31.

[0054] The temperature of the hydrogen gas supplied from the fuel unit 2 is adjusted to the ambient temperature of the fuel cell system 1. When the detected temperature drops, the temperature of the hydrogen gas is low. In this case, the hydrogen off-gas is more likely to condense due to its merging with the hydrogen gas. Based on the detected temperature, the fuel cell system 1 can change the flow rate of the hydrogen off-gas that has passed through the gas-liquid separator 36 toward the gas-liquid separator 37. By increasing the flow rate, the hydrogen off-gas is less likely to cool down upon merging with the hydrogen gas, and therefore less likely to condense. Thus, the fuel cell system 1 can suppress the intrusion of moisture into the stack 31.

[0055] The control unit 40 can change the flow rate of hydrogen off-gas through the hydrogen circulation path 35 by controlling the circulation pump 38 according to the temperature detected by the temperature sensor 45. As a result, the fuel cell system 1 can reduce the amount of condensation water generated and suppress the intrusion of moisture into the stack 31.

[0056] When the detected temperature decreases, the control unit 40 controls the circulation pump 38 to increase the flow rate of hydrogen off-gas. When the flow rate of hydrogen off-gas increases, the amount of hydrogen off-gas relative to the amount of hydrogen gas increases when it passes through the hydrogen circulation path 35 and merges with the hydrogen gas. As a result, the temperature of the hydrogen off-gas does not decrease as easily as when the flow rate is low. The higher the temperature of the hydrogen off-gas, the greater the amount of saturated water vapor, so the amount of moisture contained in the hydrogen off-gas that condenses decreases. Therefore, the fuel cell system 1 can suppress the generation of condensation water and prevent moisture from entering the stack 31.

[0057] In the above embodiment, hydrogen is an example of the fuel of the present invention. Oxygen is an example of the oxidizing agent of the present invention. Stack 31 is an example of the fuel cell of the present invention. Fuel section 2 is an example of the fuel supply source of the present invention. Hydrogen supply path 34 is an example of the first flow path of the present invention. Hydrogen circulation path 35 is an example of the second flow path of the present invention. Gas-liquid separator 36 is an example of the first gas-liquid separator of the present invention. Gas-liquid separator 37 is an example of the second gas-liquid separator of the present invention. Connection position P1 is an example of the connection position of the present invention.

[0058] The present invention is not limited to the above-described embodiment and various modifications are possible. For example, as shown in Figure 4, the hydrogen gas flowing through the hydrogen supply passage 34 and the hydrogen off-gas flowing through the hydrogen circulation passage 35 may be connected within the gas-liquid separator 137. The gas-liquid separator 137 is positioned at the connection position P1 in this embodiment in place of the joint connecting the hydrogen supply passage 34 and the hydrogen circulation passage 35.

[0059] The hydrogen supply line 34 includes a first supply line 34A and a second supply line 134B. The gas-liquid separator 137 is connected to the other end of the first supply line 34A of the hydrogen supply line 34, one end of the second supply line 134B, one end of the third circulation line 35C of the hydrogen circulation line 35, and the drainage line 52. The other end of the second supply line 134B is connected to the inlet of the flow path 31B of the separator at the anode pole 31A of the stack 31.

[0060] As shown in Figure 5, the gas-liquid separator 137 has a gas inlet 162 and a gas inlet 262 located near the left end of the top plate 71A of the separation container 161. The other end of the third circulation path 35C is connected to the gas inlet 162. The other end of the first supply path 34A is connected to the gas inlet 262. In addition, one end of the second supply path 134B is connected to the gas outlet 63.

[0061] The hydrogen off-gas flowing through the hydrogen circulation path 35 enters the first region 66 of the gas-liquid separator 137 from the gas inlet 162. The hydrogen gas supplied from the fuel unit 2 enters the first region 66 of the gas-liquid separator 137 from the gas inlet 262. The hydrogen gas and hydrogen off-gas merge in the first region 166. As shown by the arrow 169 in the figure, the hydrogen gas and hydrogen off-gas merge in the first region 166 and proceed downwards, make a U-turn in the second region 67, and proceed upwards in the third region 68. The water contained in the hydrogen gas and hydrogen off-gas is separated due to the difference in specific gravity between hydrogen and water.

[0062] The configuration of the other parts of the gas-liquid separator 137 is the same as the configuration of the other parts of the gas-liquid separator 37 in this embodiment. In this way, the hydrogen off-gas that has flowed through the hydrogen circulation path 35 merges with the hydrogen gas flowing through the hydrogen supply path 34 in the gas-liquid separator 137. Therefore, the gas-liquid separator 137 can immediately separate the condensation water generated by the merging of the hydrogen off-gas and hydrogen gas from the hydrogen off-gas and hydrogen gas.

[0063] Furthermore, in the pump control process, the fuel cell system 1 may reduce the flow rate of the circulation pump 38 if the temperature detected by the temperature sensor 45 falls below the dew temperature. As shown in Figure 6, the CPU 41 executes process S14 instead of S4 of the pump control process. That is, if the ambient temperature is higher than the dew temperature (S3: NO), the CPU 41 controls the drive of the circulation pump 38 so that the hydrogen off-gas flows at a first flow rate (S1). Then, if the ambient temperature is below the dew temperature (S3: YES / S6: YES), the CPU 41 controls the drive of the circulation pump 38 so that the hydrogen off-gas flows at a third flow rate (S14).

[0064] The third flow rate is a predetermined hydrogen off-gas flow rate such that the amount of hydrogen gas supplied from the fuel unit 2 and the amount of hydrogen off-gas flowing through the hydrogen circulation path 35 are in a third ratio at connection point P1. The third flow rate is a decrease in the amount of hydrogen off-gas flowing compared to the first flow rate.

[0065] The third ratio is the ratio of hydrogen gas to hydrogen off-gas in which the amount of hydrogen off-gas containing condensation water is reduced so that condensation is less likely to occur even when the hydrogen off-gas is cooled by the hydrogen gas when the temperature of the hydrogen gas is below the dew point. By reducing the flow rate when the detection temperature decreases, the total amount of water contained in the hydrogen off-gas at connection point P1 can be reduced. If the total amount of water contained in the hydrogen off-gas is reduced, the amount of condensation water generated when it merges with the combustion gas will be relatively small. As a result, the water containing condensation water is more reliably separated from the hydrogen off-gas by the gas-liquid separator 137. Therefore, the fuel cell system 1 can suppress the intrusion of water into the stack 31. Note that the processes S1 to S3, S5 and S6 in the modified pump control process are the same as the processes in this embodiment.

[0066] The circulation pump 38 is optional. The hydrogen dilution unit 49 is also optional. In this case, the hydrogen off-gas may be released into the atmosphere and diluted with the atmosphere. The structure of the gas-liquid separators 36 and 37 can be modified as appropriate. For example, the gas-liquid separator may be T-shaped, with an inlet for hydrogen gas or hydrogen off-gas on the side and an outlet on the top. The hydrogen gas or hydrogen off-gas may enter the gas-liquid separator from the side and separate the water by colliding with the T-shaped wall.

[0067] 1 Fuel cell system 2 Fuel unit 3 Power generation unit 4 Electrical load 31 Stack 36, 37, 137 Gas-liquid separator 38 Circulation pump 40 Control unit 41 CPU 45 Temperature sensor P1 Connection position

Claims

1. A fuel cell system comprising: a fuel cell capable of generating electricity by reacting a fuel gas and an oxidizer and supplying power to an electrical load; a first flow path connecting a fuel supply source and the fuel cell, and supplying the fuel gas discharged from the fuel supply source to the fuel cell; a second flow path connected to the fuel cell and the first flow path, and returning the fuel off gas, which is the fuel gas discharged from the fuel cell, to the first flow path; and a first gas-liquid separator disposed in the second flow path for separating moisture contained in the fuel off gas, wherein the fuel cell system further comprises a second gas-liquid separator disposed in the first flow path for separating the fuel off gas that has passed through the second flow path and moisture contained in the fuel gas flowing through the first flow path.

2. The fuel cell system according to claim 1, comprising a circulation pump arranged in the second flow path for returning the fuel off-gas in the second flow path back to the first flow path, wherein the second gas-liquid separator is arranged downstream of the circulation pump in the direction in which the fuel off-gas flows through the second flow path.

3. The fuel cell system according to claim 2, characterized in that the second gas-liquid separator is positioned between the connection point between the first flow path and the second flow path and the fuel cell.

4. The fuel cell system according to claim 2, characterized in that the second gas-liquid separator is connected to the first flow path and the second flow path, respectively.

5. The fuel cell system according to claim 3 or 4, further comprising a temperature sensor for detecting the temperature outside the fuel cell, wherein the flow rate of the fuel off-gas that has passed through the first gas-liquid separator toward the second gas-liquid separator is changed based on the temperature detected by the temperature sensor.

6. The fuel cell system according to claim 5, comprising a control unit for controlling the drive of the circulation pump, wherein the control unit controls the drive of the circulation pump according to the temperature detected by the temperature sensor and changes the flow rate of the fuel off-gas.

7. The fuel cell system according to claim 6, characterized in that the control unit controls the circulation pump to increase the flow rate of the fuel off-gas when the detected temperature decreases.

8. The fuel cell system according to claim 6, characterized in that the control unit controls the circulation pump to reduce the amount of fuel off-gas flowing when the detected temperature decreases.

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