JP2006202543A - Operation method of fuel cell system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent the overflow or re-freezing of water caused by the condensation of defrosted water or produced water in power generation in the starting of a fuel cell from a below-freezing temperature. <P>SOLUTION: A controller 27 estimates the amount of ice in a fuel cell stack 4 based on the lower temperature (representative temperature) of either the cooling liquid inlet temperature of the fuel cell stack 4 measured by a temperature sensor 23 and the outlet temperature measured by a temperature sensor 21. After the starting of the defrosting of the fuel cell stack by an electric heater 26, timing for starting of the takeout of a warming-up current from the fuel cell stack 4 is quickened with an increase in ice amount. The warming-up current is decreased with an increase in the ice amount. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a fuel cell system, and more particularly to a method for operating a fuel cell system with improved startability at low temperatures.

  In a fuel cell, a fuel gas such as hydrogen gas and an oxidizing gas containing oxygen are electrochemically reacted through an electrolyte, and electric energy is directly taken out between electrodes provided on both surfaces of the electrolyte. In particular, a polymer electrolyte fuel cell using a polymer electrolyte has attracted attention as a power source for electric vehicles because of its low operating temperature and easy handling. That is, a fuel cell vehicle is equipped with a hydrogen storage device such as a high-pressure hydrogen tank, a liquid hydrogen tank, or a hydrogen storage alloy tank in the vehicle, and reacts by supplying hydrogen supplied therefrom and air containing oxygen to the fuel cell. This is the ultimate clean vehicle that drives the motor connected to the drive wheels with the electric energy extracted from the fuel cell, and the only exhaust material is water.

  In order for the solid polymer electrolyte of the solid polymer fuel cell to exhibit sufficient hydrogen ion conductivity, a sufficient water content is required. For this reason, humidification is performed on the fuel gas or oxidant gas supplied to the fuel cell.

When such a fuel cell is left below freezing point, moisture in the electrolyte membrane and the catalyst layer is frozen. As a conventional technique for starting a frozen fuel cell, a heating medium heated by an electric heater, a hydrogen combustor, or the like is circulated through the fuel cell, and when the fuel cell temperature exceeds a set temperature, humidification of the supply gas is started. The operation method is known (for example, Patent Document 1).
JP 2004-103367 A (page 5, FIG. 2)

  However, in the above conventional operation method, when the fuel cell is frozen, the fuel cell is warmed up by the heater. Therefore, when the frozen fuel cell is thawed, the dissolved water stays inside and overflows. There has been a problem that the operation cannot be continued because the gas path of the fuel cell is blocked by the occurrence of a state or the re-freezing of the dissolved water.

  In order to further promote the release of dissolved moisture from the fuel cell, it is effective to raise the temperature to such an extent that the moisture becomes a gas. It takes time to raise the temperature to a certain temperature. In a moving body such as a vehicle, heater power is supplied from a battery. However, if the heater is continuously consumed by the heater, the battery power is exhausted and cannot be started.

  In order to solve the above problems, the present invention estimates the amount of ice inside the fuel cell based on the state of the fuel cell when the fuel cell system is started, and determines the amount of ice in the fuel cell according to the estimated value of the amount of ice. This is a method for operating a fuel cell system whose main point is to change the operating conditions of the system.

  According to the present invention, when the fuel cell system is started, the amount of ice in the fuel fuel cell is estimated to change the operating condition of the fuel cell system. There is an effect that it is possible to prevent the generation of a large amount of water and the retention or retention of moisture in the fuel cell from being frozen to make it impossible to continue power generation.

  Next, embodiments of the present invention will be described in detail with reference to the drawings.

[Configuration of Example]
FIG. 1 is a system configuration diagram showing a configuration example of a fuel cell system to which an operation method of a fuel cell system according to the present invention is applied.

  In FIG. 1, the fuel cell system includes a fuel cell stack (also simply referred to as a fuel cell or a stack) 4 including a solid polymer electrolyte. The high-pressure hydrogen stored in the hydrogen tank 2 is reduced to the operating pressure of the fuel cell by the hydrogen pressure adjusting valve 3 and supplied to the hydrogen electrode of the fuel cell stack 4.

  Air as an oxidant is supplied to the fuel cell stack 4 from an air supply device 1 such as a compressor. The fuel cell stack 4 generates electricity by causing an electrochemical reaction between hydrogen and oxygen in the air via an electrolyte, and converts the generated power by the power converter 9 and supplies it to the load device 10 or the secondary battery 13.

  The fuel cell stack 4 has a configuration having a hydrogen circulation path. Hydrogen exceeding the amount necessary for power generation is supplied to the hydrogen electrode inlet of the fuel cell stack 4, and unreacted hydrogen is discharged from the hydrogen electrode outlet of the fuel cell stack 4. This discharged hydrogen is returned to the hydrogen electrode inlet of the fuel cell stack 4 by the hydrogen circulation path 25 and the hydrogen circulation pump 5 and reused (referred to as “circulated hydrogen”). The hydrogen electrode of the fuel cell stack 4 is supplied with a mixed hydrogen of hydrogen supplied from the hydrogen tank 2 and circulating hydrogen. The circulating hydrogen contains a large amount of water vapor, and is mixed with the dry hydrogen in the hydrogen tank 2 to humidify the hydrogen supplied to the hydrogen electrode of the fuel cell. By supplying this mixed hydrogen, the solid polymer electrolyte of the fuel cell stack 4 can be sufficiently humidified.

  In the present embodiment, although not particularly limited, an electric heater 26 is provided on the surface of the fuel cell stack 4 as a thawing means for the fuel cell stack 4. When the fuel cell stack 4 is activated from the frozen state, the electric heater 26 generates heat by the power of the secondary battery 13 and defrosts the fuel cell stack 4.

  In this embodiment, the power converter 9 includes a DC / DC converter and a DC / AC inverter. The DC / DC converter converts the generated voltage of the fuel cell stack 4 into the charging voltage of the secondary battery 13 or the input voltage of the DC / AC inverter. The DC / AC inverter converts input DC power into AC power and supplies AC power to the load device 10. When the fuel cell system is applied to a vehicle, the load device 10 is a drive motor for driving the vehicle.

  In this embodiment, a power extraction amount is set in the power converter 9 so that a load current is extracted from the fuel cell stack 4. The voltage sensor 6 is a sensor that measures the voltage of the fuel cell stack 4, and the current sensor 7 is a current sensor that measures a generated current flowing from the fuel cell stack 4 toward the power converter 9.

  The air pressure adjusting valve 11 is a valve for adjusting the pressure of the air electrode, and the purge valve 12 is a valve for discharging impurities such as water and nitrogen accumulated in the hydrogen circulation system.

  The pressure sensor 16 and the temperature sensor 17 measure the pressure and temperature at the hydrogen electrode inlet, respectively. The pressure sensor 18 and the temperature sensor 19 measure the pressure and temperature at the air electrode inlet, respectively.

  The coolant pump 20 is a pump that circulates coolant, and the temperature sensors 21 and 23 are sensors that measure the coolant temperature at the outlet and inlet of the fuel cell, respectively.

  The current sensor 14 and the voltage sensor 15 are sensors for measuring the current and voltage of the secondary battery 13 respectively, and the temperature sensor 24 is a sensor for measuring the temperature in the vicinity of the secondary battery 13.

  Further, a controller 27 is provided for controlling the fuel cell system and realizing the operating method of the fuel cell system according to the present invention. Although not particularly limited, in this embodiment, the controller 27 includes a microprocessor including a CPU, a ROM that stores programs and control data, a working RAM, and an input / output port.

  Voltage sensors 6 and 14, current sensors 7 and 15, cell voltage sensor 8, pressure sensors 16 and 16, and temperature sensors 21, 22, 23, and 24 are connected to input ports of the controller 27. The output port of the controller 27 includes an air supply device 1, a hydrogen pressure adjustment valve 3, a hydrogen circulation pump 5, a power conversion device 9, an air pressure adjustment valve 11, a purge valve 12, a coolant pump 20, and an electric heater 26. It is connected.

  The controller 27 inputs detection signals from the sensors connected to these input ports, recognizes the states of the fuel cell stack 4 and the fuel cell system, controls the entire fuel cell system, and controls the state of the fuel cell stack 4. Based on the above, the amount of ice inside the fuel cell stack 4 is estimated, and control is performed to change the operating conditions of the fuel cell system according to the estimated value of the amount of ice.

[Description of operation]
Next, the operation of the controller in this embodiment will be described with reference to the control flowchart of FIG. First, in step (hereinafter abbreviated as “S”) 10, the coolant temperature values at the stack inlet and the stack outlet, which are detected values of the temperature sensors 23 and 21, are read. In S12, the two temperature values at the inlet and the outlet are read. In comparison, the lower one is set as the representative temperature. Next, in S14, it is determined whether or not the fuel cell stack 4 is frozen by determining whether or not the representative temperature is lower than a predetermined value (for example, 0 ° C.).

  In this embodiment, at this stage, the coolant pump 20 is still stopped, and it is assumed that the coolant does not flow through the fuel cell stack 4. Therefore, in order to estimate the stack temperature, it is determined whether the lower one of the coolant temperature at the stack inlet or the stack outlet is equal to or lower than a predetermined value.

  When the coolant does not flow when the fuel cell system is stopped, the movement of the coolant can be ignored inside and outside the stack. For this reason, the temperature of the coolant in the stack inlet coolant pipe that is in contact with the outside air and the temperature of the coolant pipe in the stack outlet outlet cools faster than the stack internal temperature. In this embodiment, using such a phenomenon, whether or not the stack is frozen is determined by checking whether the lower one of the coolant temperature at the stack inlet and the stack outlet is lower than a predetermined value without flowing the stack coolant. I tried to do it.

  If it is not determined in S14 that the stack is frozen, the process proceeds to S38, and a normal fuel cell is started that starts the fuel cell without thawing.

  If it is determined in S14 that the stack is frozen, the process proceeds to S16, and an estimated value is obtained by estimating the amount of ice in the fuel cell stack 4 from the representative temperature.

  In S16, first, the amount of ice inside the stack is obtained as a default value from the amount of water remaining in the stack after the normal stop with reference to the lower one of the coolant temperature at the stack inlet and the stack outlet. In this embodiment, the amount of water remaining in the stack after the normal stop is based on data estimated in advance through experiments. In addition, the relationship between the lower one of the coolant temperature at the stack inlet and the stack outlet and the amount of water freeze is also used based on experimentally estimated data.

  FIG. 3 is an example of a table value for estimating the amount of ice. Since there is a sensor error in the vicinity of 0 ° C., the amount of ice is gradually increased. The amount of ice settled down to a certain value when the temperature was below a certain level. In this embodiment, the estimated amount of ice is used as a default value, and the estimated amount of ice is corrected in S18.

  FIG. 4 is a detailed flowchart for explaining a subroutine for correcting the estimated amount of ice. In S50 of FIG. 4, it is determined whether the fuel cell outlet coolant temperature at the previous stop was equal to or lower than a predetermined value. If the temperature is equal to or lower than the predetermined value, it is determined that the fuel cell stack is frozen in a state of containing a large amount of water, and the process proceeds to S52. In S52, C1 is calculated so that the correction value C1 increases as the previous stop temperature decreases, and the process proceeds to S56. FIG. 5A is a diagram illustrating an example of a function or map used for calculating the correction value C1 in S52. If it is determined in S50 that the temperature at the previous stop exceeds a predetermined value, the correction value C1 is set to 0 at S54, and the estimated ice amount based on the temperature at the previous stop is not corrected, and the process proceeds to S56. .

  In S56 of FIG. 4, it is determined whether or not the time when the fuel cell outlet coolant temperature is equal to or lower than a predetermined value has been continued for a predetermined time or more at the previous stop. It is determined that the stack has been frozen in a state containing more moisture as the time during which the temperature has continued below the predetermined value before the previous stop is longer. When the determination in S56 is Yes, the process proceeds to S58, and the correction value C2 of the ice amount estimated value is calculated to increase as the duration time increases. If the determination in S56 is No, the process proceeds to S60, the correction value C2 is set to 0, and the process proceeds to S62.

  In S62 of FIG. 4, it is determined whether or not the water removal purge time at the previous stop was less than or equal to a predetermined value. The water blowout purge is an operation operation in which the fuel gas flow rate is increased and the water droplets are discharged out of the stack to increase the flow rate of the fuel gas in order to blow off the water droplets staying in the gas flow path or the like inside the fuel cell stack 4. It is determined that the stack has been frozen in a state of containing a lot of moisture as the time for which the purge time is continued before the previous stop is shorter. If the determination in S62 is Yes, the process proceeds to S64, and the correction value C3 is calculated so as to increase as the purge duration time decreases, and the process proceeds to S68. If the determination in S62 is No, 0 is set to the correction value C3 in S66, and the process proceeds to S68.

  In S68 of FIG. 4, the correction value C4 of the estimated ice amount is calculated to increase as the number of times that the fuel cell stack 4 is determined to have been frozen since the start of operation of the fuel cell system increases. When the catalyst layer inside the fuel cell stack 4 is frozen, the ice expands and the catalyst layer is distorted. After that, even if the ice melts, the strain is not completely restored, so that many “holes” in which moisture tends to stay in the catalyst layer are generated. Therefore, it can be determined that the fuel cell stack 4 is frozen this time as the number of times it is determined that the fuel cell stack 4 has been frozen increases, with more moisture contained in the stack. FIG. 5B is a diagram illustrating an example of a function or map used for calculating the correction value C4 in S68.

  In S70 of FIG. 4, the state in which the stack coolant temperature (representative value of the lower temperature of the stack inlet and outlet) is equal to or lower than a predetermined temperature continues for a predetermined time or more from the previous stop to the current start. It is determined whether or not. When the operation of the fuel cell is stopped in a low temperature environment, the stack internal temperature falls to the water freezing temperature because the temperature inside the stack is slower than the stack coolant temperature due to the heat capacity inside the fuel cell stack 4. Takes time. When the external temperature is around 0 ° C., the moisture inside the fuel cell freezes as the time during which the temperature inside the fuel cell continues around 0 ° C. is longer. When the external temperature is 0 ° C. or lower and the current internal temperature of the fuel cell is also 0 ° C. or lower, the longer the fuel cell internal temperature continues at around 0 ° C., the more water is present in the fuel cell. Freezing is frozen. When the water content of the liquid is large, the time during which the temperature is maintained near 0 ° C. where the solid phase and the liquid phase coexist is long.

  In consideration of such a phenomenon, if the determination in S70 is Yes, the process proceeds to S72, and the correction value C5 is calculated so as to increase as the time during which the state of being equal to or lower than the predetermined temperature continues. FIG. 5C is a diagram showing an example of a function or map used for calculating the correction value C5 in S72.

If the determination in S70 is No, 0 is set to the correction value C5 in S74, and the process proceeds to S76.

  In S76 of FIG. 4, the correction values C1, C2, C3, C4, and C5 are added to the ice amount estimated value estimated in S16 of FIG. 2 to correct the ice amount, and the process returns to the main routine.

  After correcting the amount of ice in S18 of FIG. 2, the amount of heat given from the thawing means to the fuel cell stack 4 before starting power generation in the half-thaw state is calculated based on the amount of ice in S20. FIG. 6 is a calculation table example of the power generation start heat amount calculated in S20. The larger the amount of ice, the more the amount of heat given to the fuel cell stack by the thawing means (electric heater 26) is set to start taking out the generated current. In other words, as the amount of ice in the stack is larger, the target generation current extraction start timing is made earlier, so the earlier the target generation current extraction start timing is, the less water is generated by power generation and the generation is interrupted by ice. There is an effect that it is possible to prevent water from being discharged and staying inside the stack and becoming overflowed.

  Next, in S22, the magnitude of the target warm-up current taken out from the fuel cell stack 4 when power generation is started is calculated. FIG. 7 is an example of a table for calculating the magnitude of the target warm-up current. The target warm-up current is set to a smaller value as the amount of ice increases. In S20 described above, the target generation current extraction start timing is made earlier as the amount of ice in the stack increases, so the result of S22 is that the target generation current is decreased as the target generation current extraction start timing is advanced. Therefore, it is possible to prevent the generated water from being disturbed by ice and staying in the stack without being discharged and becoming a water overflow state.

  Next, in S24, thawing of the fuel cell stack 4 is started by the thawing means. In this embodiment, an electric heater 26 is provided on the surface of the fuel cell stack 4 as a thawing means. The power source of the electric heater 26 uses the secondary battery 13 and starts defrosting the fuel cell stack 4 by starting energization of the electric heater 26 under the control of the controller 27.

  Next, in S26, the amount of heat applied from the electric heater 26 to the fuel cell stack 4 for thawing is integrated. In this embodiment, the amount of ice being thawed is estimated by estimating how much the ice has melted based on the integrated amount of heat given from the electric heater 26 to the fuel cell stack 4.

  Next, in S28, it is determined whether or not the accumulated heat amount is equal to or greater than the power generation start heat amount calculated in S20. If the accumulated heat amount is equal to or greater than the power generation start heat amount, the process proceeds to S30, and the increase / decrease timing of the fuel gas pressure is set. . When the generated current is taken out, the inside of the stack is warmed, so that the water that existed as ice becomes liquid and comes out to the gas flow path and the like. Since the water overflows in this state, the gas pressure is changed from high pressure to low pressure at once, causing a drastic change in the flow velocity so that the moisture staying inside the stack is discharged out of the stack.

  In this embodiment, a control cycle for increasing / decreasing the fuel gas pressure is provided, and the time for maintaining a high pressure is given as a percentage of the control cycle. When it is given as 100%, there is no case where the gas pressure is lowered while maintaining a high pressure. If 80% is given, for example, if the control cycle is 60 [s], a pressure that is 48 [s] higher is maintained and a pressure that is 12 [s] lower is maintained. A specific example is shown in FIG. In this embodiment, since the target operating pressure at the time of taking out the target warm-up current is high, the operating pressure is temporarily reduced to a low target pressure every control cycle. Further, as shown in FIG. 8, the control period for increasing / decreasing the fuel gas pressure is shortened as the amount of ice increases. This is so that moisture can be discharged finely.

  Next, in S32, the increase / decrease timing of the fuel gas flow rate is set. When the generated current is taken out, the inside of the stack is warmed, so that the water that existed as ice becomes liquid and comes out to the gas flow path and the like. Since the water overflows as it is, the gas flow rate is suddenly changed from a low flow rate to a high flow rate, and a sudden decrease in the flow velocity is caused to discharge the moisture remaining inside the stack to the outside of the stack.

  In this embodiment, a control cycle for increasing or decreasing the fuel gas flow rate is provided, and the time for maintaining a low flow rate is given as a percentage of the control cycle. When it is given as 100%, there is no case where the flow rate is increased while the flow rate is low. If 80% is given, for example, if the control period is 60 [s], a flow rate that is 48 [s] lower is maintained, and a flow rate that is 12 [s] higher is maintained. A specific example is shown in FIG.

  In this embodiment, since the target operating flow rate at the time of taking out the target warm-up current is low, it is temporarily increased to a high target flow rate every control cycle. Further, as shown in FIG. 10, the control period for increasing / decreasing the fuel gas flow rate is shortened as the amount of ice increases. This is so that moisture can be discharged finely.

  In S34, supply of fuel gas (hydrogen) and air is started at the fuel gas pressure increase / decrease timing set in S30 or the fuel gas flow rate increase / decrease timing set in S32, and the target warm-up from the fuel cell stack 4 to the power converter 9 is started. Start taking out current. As a result, reaction heat for power generation of the target warm-up current is generated inside the fuel cell stack 4, and warm-up can be performed from inside the stack. The electric power taken out at this time is mainly used for charging the electric power of the secondary battery 13 consumed for activation.

  Next, in S36, it is determined whether or not the fuel cell stack 4 has been warmed up. This determination is made based on, for example, whether or not the detection value of the stack outlet temperature sensor 21 has reached or exceeded a predetermined temperature (for example, 1 ° C.). If the warm-up is not completed in the determination of S36, the process returns to S34 and the extraction of the target warm-up current is continued. When the warm-up is completed in the determination of S36, the routine proceeds to normal operation.

1 is a system configuration diagram showing a configuration example of a fuel cell system to which an operation method according to the present invention is applied. 3 is a control flowchart illustrating a method for operating the fuel cell system according to the present invention. It is an example of the map which calculates the amount estimation value of the ice in a fuel cell stack with respect to the temperature of the coolant inlet / outlet of a fuel cell stack. It is a detailed flowchart explaining the ice amount correction subroutine. It is an example of the calculation map of the amount correction value of ice. It is an example of the map of the integration | desorption heat amount integrated value which starts electric power generation with respect to the quantity of ice. It is a map example of the target warm-up current with respect to the amount of ice. It is an example of the map of the fuel gas pressure control period with respect to the quantity of ice. It is a time chart example of the fuel gas pressure at the time of warming up. It is an example of the map of the fuel gas flow control cycle with respect to the quantity of ice. It is an example of a time chart of the fuel gas flow rate at the time of warming up.

Explanation of symbols

1: Air supply device 2: Hydrogen tank 3: Hydrogen pressure regulating valve 4: Fuel cell stack 5: Hydrogen circulation pump 6: Voltage sensor 7: Current sensor 8: Cell voltage sensor 9: Power converter 10: Load device 11: Air Pressure adjustment valve 12: Purge valve 13: Secondary battery 14: Voltage sensor 15: Current sensor 16, 18: Pressure sensor 17, 19: Temperature sensor 20: Coolant pumps 21-24: Temperature sensor 25: Hydrogen circuit 26: Electric heater 27: controller

Claims (11)

  A fuel characterized in that when the fuel cell system is started, the amount of ice inside the fuel cell is estimated based on the state of the fuel cell, and the operating condition of the fuel cell system is changed according to the estimated value of the amount of ice. Battery system operation method.   The operation of the fuel cell system according to claim 1, wherein the operation condition is a generation start timing of the generated current, and the generation start timing of the generated current is changed according to the estimated value of the ice amount. Method.   The operating method of the fuel cell system according to claim 2, wherein the operating condition is a generation start timing of the generated current, and the generation start timing of the generated current is advanced as the estimated value of the ice amount increases. .   4. The fuel cell system operating method according to claim 2, wherein the operating condition is a magnitude of a generated current, and the generated current is decreased as the estimated value of the amount of ice is larger. 5. .   4. The fuel cell system according to claim 2, wherein the operating condition is a number of fuel gas pressure fluctuations, and the larger the estimated value of the ice amount, the larger the number of pressure fluctuations. 5. Driving method.   4. The fuel cell system according to claim 2, wherein the operating condition is a flow rate variation number of the fuel gas, and the flow rate variation number is increased as the estimated value of the ice amount is larger. Driving method.   The fuel cell system according to any one of claims 1 to 6, wherein the estimated value of the ice amount is increased as the temperature at the previous stop of the fuel cell system is lower. Driving method.   7. The correction according to claim 1, wherein the estimated value of the ice amount is increased as the time during which the fuel cell temperature is kept below the predetermined temperature is longer. Method of operating the fuel cell system.   The method of operating a fuel cell system according to any one of claims 1 to 6, wherein the estimated value of the amount of ice increases as the number of times the fuel cell has been frozen increases. .   The fuel cell according to any one of claims 1 to 6, wherein the estimated value of the amount of ice is corrected so as to be shorter as the time of performing the water purge at the previous stop is shorter. How to operate the system.   The correction is performed so that the estimated value of the amount of ice becomes larger as the time during which the fuel cell temperature is kept below the predetermined temperature from the previous stop to the current start is longer. The operation method of the fuel cell system according to claim 6.

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