JP2014035820A - Stop method of fuel cell system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a stop method of a fuel cell system which allows for enhancement of power generation stability while minimizing deterioration of fuel cells, by suppressing the freezing of water produced in fuel cells during start-up.SOLUTION: The stop method of a fuel cell system includes a lowest temperature estimation step for estimating whether or not the lowest outside air temperature goes below a predetermined temperature, a temperature detection step for detecting the stack temperature while power generation of a fuel cell is stopping, a freeze expectation step performing freeze expectation in a fuel cell when the stack temperature is lower than a freeze expectation temperature for expecting that there is a risk of freeze in a fuel cell, and a freeze protection power generation step performing freeze protection power generation by a fuel cell, when freeze expectation is performed in the freeze expectation step.

Description

  The present invention relates to a method for stopping a fuel cell system.

  In a fuel cell mounted on a fuel cell vehicle, a membrane electrode structure (MEA) is formed by sandwiching a solid polymer electrolyte membrane (hereinafter referred to as an electrolyte membrane) between an anode and a cathode from both sides, and this membrane electrode structure A plate-shaped unit fuel cell (hereinafter referred to as a unit cell) is configured by arranging a pair of separators on both sides of the battery, and a plurality of unit cells are stacked to form a fuel cell stack. In a fuel cell, hydrogen is supplied to the anode as anode gas (fuel) and air is supplied to the cathode as cathode gas (oxidant), so that hydrogen ions generated by the catalytic reaction at the anode pass through the electrolyte membrane. It moves to the cathode and generates electricity by causing an electrochemical reaction with oxygen in the air at the cathode. Note that water (generated water) is generated in the fuel cell with this power generation.

FIG. 6 is a graph showing changes in the stack temperature of the fuel cell with respect to time in a conventional fuel cell system.
By the way, as shown in FIG. 6, when the stack temperature Tfc falls below freezing point in the cold region of winter or the like while the fuel cell system is stopped (after time t1 in FIG. 6), the generated water staying in the fuel cell is to freeze. In this case, the supply of the anode gas and the cathode gas to the unit cell is hindered, which causes a decrease in power generation stability at the next start-up (time t2 in FIG. 6), deterioration of the fuel cell, and the like.

  Therefore, for example, Patent Document 1 discloses so-called scavenging that supplies scavenging gas (for example, air) to the reaction gas flow path and discharges the generated water in the reaction gas flow path when freezing of the generated water is expected. A configuration for performing processing is disclosed.

Japanese Patent Laying-Open No. 2005-251576

  However, in the scavenging process described above, it is difficult to completely discharge the generated water in the fuel cell. In this case, the generated water that cannot be exhausted by the scavenging process and remains in the fuel cell is then frozen in the fuel cell, thereby improving the power generation stability at the next start-up and suppressing the deterioration of the fuel cell. There is a problem that is difficult.

  Therefore, the present invention has been made in view of the above-described circumstances, and suppresses freezing of generated water in the fuel cell at the time of start-up, thereby improving power generation stability and suppressing deterioration of the fuel cell. An object of the present invention is to provide a method for stopping a fuel cell system capable of performing the above.

  To achieve the above object, the invention described in claim 1 is a fuel including a fuel cell (for example, the fuel cell 2 in the embodiment) in which fuel is generated by supplying fuel to the anode and oxidant to the cathode, respectively. A method for stopping a battery system (for example, the fuel cell system 1 in the embodiment), wherein the minimum temperature of the outside air temperature (for example, the outside air temperature Tout in the embodiment) is equal to or lower than a predetermined temperature (for example, the predetermined temperature TL in the embodiment). A minimum temperature estimation step (e.g., step S <b> 1 in the embodiment) for estimating whether or not to occur, and a stack temperature (e.g., stack temperature Tfc in the embodiment) during power generation stop of the fuel cell, or a stack temperature decrease rate (e.g., , A temperature detection step (for example, step S in the embodiment) for detecting the temperature decrease rate Tq in the embodiment. And the stack temperature or the stack temperature decrease rate is lower than the expected freezing temperature (for example, the predicted freezing temperature Ti in the embodiment) or the expected freezing temperature decrease rate at which there is a risk of freezing in the fuel cell. A freezing prediction step (for example, step S2 in the embodiment) for performing freezing prediction in the fuel cell, and freezing for performing freezing preventive power generation by the fuel cell when freezing prediction is performed in the freezing prediction step. And a preventive power generation process (for example, step S3 to step 8 in the embodiment).

  In the invention described in claim 2, in the freeze prevention power generation step, the target temperature rise is reached within a predetermined time based on the stack temperature and the target temperature rise (for example, the target temperature Ts in the embodiment). A possible current value is set.

  In the invention described in claim 3, before the freeze prevention power generation step, the number of times that the stack temperature falls below the freezing temperature for determining freezing in the fuel cell (for example, the freezing temperature Tc in the embodiment) is grasped. It has a freeze number grasping step (for example, step S12 in the embodiment), and freeze prevention power generation is performed when it is determined that the stack temperature is lower than the freeze temperature by a predetermined number of times or more.

  In the invention described in claim 4, before the freeze-preventing power generation step, the interval at which the stack temperature falls below the freezing temperature for determining freezing in the fuel cell (for example, the freezing temperature Tc in the embodiment) is grasped. A freeze frequency grasping step is provided, and freeze prevention power generation is performed when it is determined that the interval at which the stack temperature falls below the freeze temperature is equal to or less than a predetermined interval.

  The invention described in claim 5 includes a frozen area estimation step for estimating whether or not the frozen area in the plane of the fuel cell is equal to or larger than a predetermined area, and the predicted freezing area is equal to or larger than the predetermined area In addition, freeze prevention power generation is performed.

  In the invention described in claim 6, after the freeze prevention power generation step, the oxidant discharged from the cathode is allowed to flow through the oxidant circulation path (for example, the cathode gas circulation path 65 in the embodiment) of the cathode. It has a cathode gas circulation processing step of returning to the inlet side and performing cathode gas circulation power generation.

  In the invention described in claim 7, when the fuel cell is started after freeze-preventing power generation, the SOC grasping step for grasping the remaining capacity of the power storage device, and when the remaining capacity is a predetermined value or more, the fuel cell And a SOC reduction step of reducing the remaining capacity of the power storage device by discharging the power of the power storage device before power generation.

  According to the first aspect of the present invention, when freezing is predicted, freezing prevention power generation is performed and the fuel cell is warmed, so that the inside of the fuel cell can be warmed before the fuel cell is frozen. Thereby, freezing of generated water in the fuel cell can be prevented, and power generation stability at the next start-up can be improved and deterioration of the fuel cell can be suppressed.

  According to the second aspect of the invention, during freeze-prevention power generation, the temperature is raised to the target temperature rise within a predetermined time, so that it is possible to suppress deterioration in NV performance and merchandise due to power generation after the fuel cell is stopped. In addition, it is possible to extend the life of the fuel cell by suppressing the deterioration of the fuel cell.

  According to the third aspect of the present invention, the number of freeze prevention power generations can be minimized by performing the freeze prevention power generation of the fuel cells when the number of freezes of the fuel cell is a predetermined number or more. Therefore, it is possible to suppress an increase in fuel consumption due to freeze-prevention power generation after improving power generation stability at the next start-up and suppressing deterioration of the fuel cell.

  According to the fourth aspect of the present invention, the number of freeze-prevention power generations can be minimized by performing the freeze-prevention power generation of the fuel cell when the fuel cell freeze frequency is equal to or less than a predetermined interval. Therefore, it is possible to suppress an increase in fuel consumption due to freeze-prevention power generation after improving power generation stability at the next start-up and suppressing deterioration of the fuel cell.

  According to the invention described in claim 5, since freeze-prevention power generation can be suppressed to a minimum, after the improvement of power generation stability at the next start-up and the suppression of deterioration of the fuel cell, the freeze-prevention power generation is performed. Increase in fuel consumption can be suppressed.

By the way, in the conventional fuel cell system, due to the permeation (crossover) of the cathode gas from the cathode to the anode in the soak, the anode side and the cathode side are each filled with an oxidizing agent (for example, air) at the next start-up. There is a case where the system is started from a state where it is operated (so-called air-air start).
Here, the air-air start-up will be described. FIG. 7 is a diagram illustrating an example of changes in the potentials on the inlet side and the outlet side in each system of the anode and the cathode when the air-air activation of the fuel cell system.
As shown in FIG. 7, when hydrogen is supplied to the anode as the anode gas at the start of air-air, the inlet side of the anode is quickly filled with fuel (for example, hydrogen), but the outlet side is on the inlet side. Compared to the fuel, it takes a long time to replace it. Therefore, on the anode side, there is a concentration gradient in which the hydrogen concentration decreases from the inlet side to the outlet side in the unit cell surface.

In this case, on the anode entrance side, the anode potential (inlet side) decreases to zero due to hydrogen ionization (H ₂ → 2H ⁺ + 2e ⁻ ). On the cathode entrance side, an electrochemical reaction (O ₂ + 4H ⁺ + 4e ⁻ → 2H) is caused by hydrogen ions that have moved from the anode to the cathode through the electrolyte membrane, oxygen that is present in the cathode system, and electrons. _The cathode potential (inlet side) is maintained at approximately 1 V by the theoretical electromotive force (= 1.23 V) of ₂ O).
Therefore, the potential difference between the inlet side of the anode and the inlet side of the cathode is approximately 1V.
On the other hand, when air remains on the outlet side of the anode (when the replacement with hydrogen is incomplete), the anode side and the cathode side are filled with air. Should be 0V.
However, in a fuel cell, the potential difference (for example, 1V) is made equal in the unit cell surface due to the influence of a conductive member (for example, a separator or the like), so that the cathode potential (outlet side) is higher than that at the inlet side. It becomes a high potential (for example, 2V).

  When the outlet side of the unit cell is at a high potential, the carbon of the catalyst (for example, platinum-supporting carbon etc.) is oxidized and converted to CO2, or platinum is eluted, leading to deterioration of the electrolyte membrane. is there.

  Therefore, according to the invention described in claim 6, since the cathode gas circulation power generation is performed after the completion of the freeze prevention power generation, the power generation of the fuel cell is stopped after the cathode system is made a nitrogen-rich atmosphere. become. For this reason, it is possible to prevent air-to-air activation at the next activation, and to reliably achieve power generation stability at the next activation and suppression of deterioration of the fuel cell.

  According to the seventh aspect of the present invention, since the amount of charge by the discharge process at the next start-up can be secured, the fuel cell system can be started without any problem.

1 is a schematic configuration diagram of a fuel cell system according to an embodiment of the present invention. It is a flowchart for demonstrating the stop method of the fuel cell system in 1st Embodiment. It is a graph which shows the change of stack temperature with respect to time in the soak of a fuel cell system. It is a flowchart for demonstrating the stop method of the fuel cell system in 2nd Embodiment. It is a graph which shows the change of stack temperature with respect to time in the soak of a fuel cell system. It is a graph which shows the change of the stack temperature of the fuel cell with respect to time in the soak of the conventional fuel cell system. It is a figure which shows the example of the change of the electric potential of the inlet side in each system of an anode and a cathode at the time of the air-air start-up of a fuel cell system.

Next, embodiments of the present invention will be described with reference to the drawings.
(First embodiment)
(Fuel cell system)
FIG. 1 is a schematic configuration diagram of a fuel cell system.
As shown in FIG. 1, a fuel cell system 1 of this embodiment is mounted on, for example, a fuel cell vehicle. A fuel cell stack 2 (hereinafter referred to as a fuel cell 2) and a cathode gas ( A cathode gas supply means 3 for supplying air as an oxidant), an anode gas supply means 4 for supplying hydrogen as an anode gas (fuel), and an ISU 5 for comprehensively controlling these components. , Mainly. The fuel cell system 1 is also provided with an outside air temperature sensor 21 that detects the outside air temperature Tout. The outside air temperature sensor 21 outputs an electric signal corresponding to the detected outside air temperature Tout toward the ISU 5.

  The fuel cell 2 generates power by an electrochemical reaction between an anode gas and a cathode gas, and includes a solid polymer electrolyte membrane. The electrolyte membrane is sandwiched between the anode and the cathode to form a membrane electrode structure (MEA). A pair of separators are arranged on both sides of the MEA to form a cell, and a plurality of the cells are stacked. Thus, the fuel cell 2 is configured.

  The fuel cell 2 is provided with a temperature sensor 22 that detects a temperature in the fuel cell 2 (hereinafter referred to as a stack temperature Tfc). The temperature sensor 22 outputs an electrical signal corresponding to the detected stack temperature Tfc to the ISU 5.

(Cathode gas supply means)
The cathode gas supply means 3 includes a cathode gas supply channel 31 through which the cathode gas flows toward the cathode of the fuel cell 2. An air compressor 32 for supplying cathode gas to the fuel cell 2 is connected to the upstream side of the cathode gas supply channel 31.
Further, the downstream side of the cathode gas supply channel 31 is connected to the cathode gas inlet sealing valve 34 and then connected to the cathode channel 33 facing the cathode on the inlet side of the fuel cell 2.

Connected to the outlet side of the cathode channel 33 is a cathode offgas discharge channel 35 through which the cathode offgas supplied to the power generation by the fuel cell 2 and the generated water generated by the fuel cell 2 by power generation and condensation flow. .
The cathode offgas discharge passage 35 is connected to the cathode offgas outlet sealing valve 36 and then connected to the dilution BOX 75 and the like on the downstream side. Each of the sealing valves 34 and 36 is an electromagnetically driven sealing valve, and is configured so that the cathode gas can be sealed between the sealing valves 34 and 36, that is, in the cathode flow path 33.

  The cathode gas delivered by the air compressor 32 passes through the cathode gas supply channel 31 and is then supplied to the cathode channel 33 of the fuel cell 2. In the cathode channel 33, oxygen in the cathode gas is supplied to the power generation as an oxidant and then discharged from the fuel cell 2 to the cathode offgas discharge channel 35 as the cathode offgas. The cathode offgas discharged to the cathode offgas discharge channel 35 passes through the dilution BOX 75 and is then exhausted outside the vehicle.

  Further, a fuel cell bypass channel 37 branched from the cathode gas supply channel 31 is provided upstream of the cathode gas inlet sealing valve 34 in the cathode gas supply channel 31. The fuel cell bypass passage 37 is connected to the cathode offgas outlet passage 35 downstream of the cathode offgas outlet sealing valve 36 via a bypass valve 38.

A scavenging flow path 61 branched from the cathode gas supply flow path 31 is provided upstream of the fuel cell bypass flow path 37 in the cathode gas supply flow path 31. The scavenging flow path 61 is connected to an anode gas supply flow path 42 described later via a scavenging valve 62. As a result, the cathode gas can be introduced into the anode gas supply channel 42 via the scavenging channel 61.
A dilution passage 63 branched from the scavenging passage 61 is provided on the upstream side of the scavenging valve 62 in the scavenging passage 61. The dilution flow path 63 is connected to the dilution BOX 75 via an open / close valve (not shown).

A cathode gas circulation passage (oxidant circulation passage) 65 branched from the cathode offgas discharge passage 35 is provided on the cathode offgas discharge passage 35 upstream of the cathode offgas outlet sealing valve 36. The cathode gas circulation channel 65 is connected to the downstream side of the cathode gas inlet sealing valve 34 in the cathode gas supply channel 31 via a cathode gas circulation pump 66.
The cathode gas circulation pump 66 is normally stopped and is driven at the time of cathode gas circulation power generation after, for example, a system switch (for example, ignition) of the fuel cell system 1 is turned off. The cathode gas circulation pump 66 circulates the cathode off-gas discharged from the cathode flow path 33 of the fuel cell 2, mixes it with the cathode gas sent from the air compressor 32, and supplies the cathode gas again to the cathode of the fuel cell 2. .

(Anode gas supply means)
The anode gas supply means 4 includes a hydrogen tank 41 (H2 tank) filled with anode gas. An anode gas supply channel 42 for supplying anode gas to the fuel cell 2 is connected to the hydrogen tank 41. The anode gas supply channel 42 is connected to the gas supply valve 43, the shutoff valve 44, and the fuel injector 45 in this order from the upstream side, and is connected to the anode channel 46 facing the anode on the inlet side of the fuel cell 2.

  The drive of the fuel injector 45 is controlled by an output signal from the ISU 5 so that anode gas is intermittently supplied toward the anode flow path 46 at a predetermined cycle.

  Connected to the outlet side of the anode channel 46 is an anode off-gas channel 47 through which anode off-gas used for power generation in the fuel cell 2 flows. The anode off gas passage 47 is connected to the downstream side of the fuel injector 45 in the anode gas supply passage 42.

  The anode off gas channel 47 is provided with a purge channel 72 having a purge valve 71 and a drain channel 74 having a drain valve 73 via a gas-liquid separator (not shown). The gas-liquid separator collects moisture contained in the anode off-gas discharged from the anode flow path 46 of the fuel cell 2 and separates it into anode off-gas and moisture.

The purge valve 71 is opened when a predetermined condition is satisfied, so that the anode off-gas separated by the gas-liquid separator and impurities contained in the anode off-gas are discharged to the dilution BOX 75 through the purge channel 72. Is done.
The drain valve 73 is opened when a predetermined amount of water accumulates in the gas-liquid separator, so that the water accumulated in the gas-liquid separator is discharged to the dilution BOX 75 through the drain flow path 74.

  The dilution BOX 75 is provided with a retention chamber in which the anode off gas introduced from the purge flow path 72 and the drain flow path 74 stays, and a discharge flow path 76 is connected to the retention chamber. That is, the anode off-gas is diluted with the cathode off-gas in the staying chamber and then discharged from the discharge passage 76 to the outside of the vehicle. The dilution BOX 75 is supplied with a cathode off gas based on the concentration of the anode off gas introduced from the purge flow path 72.

(ISU)
The ISU 5 mainly includes a cathode gas circulation processing unit 51, a winter season determination unit 52, a freezing prediction unit 53, and a freezing prevention power generation processing unit 54.
The cathode gas circulation processing means 51 executes the cathode gas circulation power generation when it is determined that the fuel cell system 1 is turned off, and when freeze prevention power generation described later is performed. Cathode gas circulation power generation is one of oxygen-deficient power generation that consumes oxygen remaining in the cathode system of the fuel cell 2 to lower the oxygen concentration, and brings the oxygen concentration in the cathode system close to zero, which is rich in nitrogen. It is processing to be the environment. This prevents the cathode side from becoming a high potential state at the next start-up of the fuel cell system 1 and prevents the electrolyte membrane of the fuel cell 2 from deteriorating. A specific method of cathode gas circulation power generation will be described later.

  For example, when the minimum temperature of the outside air temperature Tout detected by the outside air temperature sensor 21 is equal to or lower than a predetermined temperature TL (for example, 0 ° C. + α) (Tout ≦ TL), the winter season determination unit 52 determines that it is a winter season. Note that the winter determination method is not limited to the case where the outside air temperature sensor 21 is used, and the outside air temperature is predicted based on position information by GPS, date, and time information, and the outside air temperature predicted value is higher than the predetermined temperature TL. If it is low, it may be determined that it is winter.

  The freezing predicting means 53 predicts whether or not the generated water in the fuel cell 2 may freeze based on the stack temperature Tfc of the fuel cell 2 detected by the temperature sensor 22 when the fuel cell system 1 is stopped. Do. Specifically, the freezing prediction means 53 stores the predicted freezing temperature Ti, and performs freezing prediction when the stack temperature Tfc is lower than the predicted freezing temperature Ti.

  The freeze prevention power generation processing means 54 executes a freeze prevention power generation process for warming the inside of the fuel cell 2 by power generation when the freeze prediction means 53 makes a prediction of freezing.

(How to stop the fuel cell system)
Next, as a method for stopping the fuel cell system 1 described above, RTC control after the system switch OFF operation (control performed by temporarily starting the ISU 5 every predetermined time) will be described. FIG. 2 is a flowchart for explaining a method of stopping the fuel cell system according to this embodiment, and FIG. 3 is a graph showing a change in stack temperature with respect to time during soaking.
First, as shown in FIG. 3, the stack temperature Tfc of the fuel cell 2 is increased with time after the fuel cell system 1 is turned off (time t0 in FIG. 3) and the cathode gas circulation power generation is performed as a stop process. It goes down.

Therefore, in the present embodiment, as shown in FIGS. 2 and 3, in step S <b> 1, it is determined whether or not it is winter (minimum temperature estimation step). Specifically, the winter season determination unit 52 determines whether or not the outside air temperature Tout detected by the outside air temperature sensor 21 is equal to or lower than a predetermined temperature TL (Tout ≦ TL).
When the determination result of step S1 is “YES” (when Tout ≦ TL), it is determined that it is winter season and the process proceeds to step S2.
On the other hand, when the determination result of step S1 is “NO” (when Tout> TL), it is determined that it is not winter, that is, the outside air temperature Tout is sufficiently high and there is no possibility of freezing, and this routine is terminated.

Next, in step S2, it is determined whether or not there is a risk of freezing in the fuel cell 2 (temperature detection step and freezing prediction step). Specifically, the freeze prediction means 53 determines whether or not the stack temperature Tfc of the fuel cell 2 is lower than the expected freeze temperature Ti (Tfc <Ti).
When the determination result in step S3 is “YES” (when Tfc <Ti), freezing in the fuel cell 2 is predicted, and the process proceeds to step S3 (for example, time t3 in FIG. 3).
On the other hand, when the determination result of step S3 is “NO” (when Tfc ≧ Ti), it is determined that the stack temperature Tfc is sufficiently high and there is no possibility of freezing, and this routine is terminated.

In step S3, freeze prevention power generation is started by the freeze prevention power generation processing means 54 (freeze prevention power generation step).
First, in step S4, the freeze prevention power generation processing means 54 sets the target temperature Ts of the fuel cell 2 after the freeze prevention power generation process based on the outside air temperature Tout. Specifically, the freeze prevention power generation processing unit 54 uses, for example, a map showing the relationship between the outside air temperature Tout and the target temperature Ts, and sets the target temperature Ts so that the target temperature Ts decreases as the outside air temperature Tout increases. Set. The target temperature Ts is preferably set to a temperature (for example, about 40 ° C.) at which the fuel cell 2 does not react after the temperature rise (after freeze prevention power generation). This is because when the temperature of the fuel cell 2 is increased to a high temperature (for example, about 80 ° C.) by freeze-prevention power generation, the amount of reaction of the fuel cell 2 increases after the temperature increase until the stack temperature Tfc decreases again. This is because the battery 2 may be deteriorated.
The target temperature Ts can also be set based on GPS position information, date, and time information. In this case, a future outside air temperature predicted value is estimated based on the position information, date, and time information, and when an increase in the outside air temperature Tout is estimated, the target temperature Ts is set to be lowered.

  Next, in step S5, the freeze prevention power generation processing means 54, based on the stack temperature Tfc and the target temperature Ts, the target power generation amount that can reach the target temperature Ts within a predetermined time (between times t3 and t4 in the figure). Set the (current value). Then, based on the target power generation amount set by the freeze prevention power generation processing means 54, the anode gas and the cathode gas are supplied to the fuel cell 2 at desired target values (pressure, flow rate, etc.), so that the freeze prevention power generation is performed. Executed. As a result, as shown in FIG. 3, the fuel cell 2 is warmed up by power generation, and the stack temperature Tfc rises. The predetermined time is set from the viewpoints of merchantability, NV performance, suppression of deterioration of the fuel cell 2, and the like (for example, about 3 minutes).

  Here, in the freeze prevention power generation of the present embodiment, the SOC (not shown) of the power storage device (not shown) is adjusted before and after the freeze prevention power generation by adjusting the stoichiometry of the anode and the cathode or reducing the pressure of each reaction gas. It is preferable to control the power generation of the fuel cell 2 so that the remaining capacity does not change. That is, the power generation amount of the fuel cell 2 is defined as the gross output W1, and the power consumption of the auxiliary equipment (for example, the air compressor 32 for supplying the cathode gas to the cathode of the fuel cell 2) indispensable for obtaining the gross output W1. Is W2, and when the power obtained by subtracting the power consumption W2 of the auxiliary machine from the gross output W1 of the fuel cell is defined as the net output W3 of the fuel cell (W3 = W1-W2), the net output W3 is zero. The power generation of the fuel cell 2 is controlled.

Next, as shown in FIGS. 2 and 3, in step S <b> 6, the freeze prevention power generation processing unit 54 determines whether or not freeze prevention power generation is complete. Specifically, the freeze prevention power generation processing means 54 determines whether or not the stack temperature Tfc has reached the target temperature Ts.
When the determination result of step S6 is “YES” (when Tfc ≧ Ts), it is determined that the fuel cell 2 has been sufficiently warmed up, and the process proceeds to step S7.
On the other hand, when the determination result of step S6 is “NO” (when Tfc <Ts), it is determined that the target temperature Ts has not yet been reached, and the process proceeds to step S8.

In step S8, the freeze prevention power generation processing means 54 feeds back the current stack temperature Tfc to change the target power generation amount so that the target temperature Ts is reached in the remaining time of the predetermined time. Thereafter, the routine of steps S6 and S8 is repeated until the fuel cell 2 reaches the target temperature Ts within a predetermined time.
On the other hand, in step S7, freeze prevention power generation is terminated (for example, time t4 in FIG. 3).

  Thereafter, in step S9, the cathode gas circulation processing means 51 performs cathode gas circulation power generation as oxygen-deficient power generation. Specifically, in the cathode gas circulation power generation, the cathode off-gas outlet sealing valve 36 is closed, and the bypass valve 38 of the fuel cell bypass passage 37 is opened while the cathode gas inlet sealing valve 34 is opened. In this state, the air compressor 32 is continuously driven and the cathode gas circulation pump 66 is driven. As a result, the cathode off-gas discharged from the cathode flow path 33 of the fuel cell 2 circulates, mixes with part of the cathode gas delivered from the air compressor 32, and is supplied again to the cathode of the fuel cell 2. As a result, the oxygen remaining in the cathode offgas decreases while the cathode offgas discharged from the cathode of the fuel cell 2 is circulated through the cathode flow path 33 to generate power, so that the oxygen concentration in the cathode offgas is reduced. Can be reduced.

Whether the cathode gas circulation power generation is completed is determined by, for example, detecting the oxygen concentration of the cathode off-gas discharged from the fuel cell 2 with an oxygen sensor (not shown), and the oxygen concentration has reached a predetermined value or less. Sometimes it may be determined that the cathode gas circulation power generation has been completed, or it may be determined that the cathode gas circulation power generation has been completed when the duration of the cathode gas circulation power generation reaches a predetermined time set in advance. Further, the completion of cathode gas circulation power generation may be determined based on the integrated current value from the start of cathode gas circulation power generation. That is, since the amount of oxygen in the cathode system with the cathode offgas outlet sealing valve 36 closed can be grasped to some extent from the piping volume in the cathode system, etc., the integrated current that can consume the grasped amount of oxygen. Whether the cathode gas circulation power generation is complete or not can be determined depending on whether or not the value is reached.
Thus, when the cathode gas circulation power generation is completed, the atmosphere inside the cathode system of the fuel cell 2 becomes a nitrogen-rich atmosphere.
Thus, this routine is finished.

  The routine described above is repeated until the next start-up of the fuel cell system 1. In the graph shown in FIG. 3, the above-described freeze prevention power generation is performed twice until the next start-up of the fuel cell system 1 (for example, times t <b> 3 to t <b> 4 and t <b> 5 to t <b> 6 in FIG. 3).

As described above, in this embodiment, when it is determined that there is a risk of freezing in the fuel cell 2, freeze prevention power generation is performed during the soak, and the fuel cell 2 is warmed to freeze the inside of the fuel cell 2. The fuel cell 2 can be warmed before the operation. Thereby, freezing of the generated water in the fuel cell 2 can be prevented, and the power generation stability at the next startup can be improved and the deterioration of the fuel cell 2 can be suppressed.
In addition, during freeze prevention power generation, the temperature is raised to the target temperature Ts within a predetermined time, so that it is possible to suppress the deterioration of NV performance and merchantability due to power generation after the fuel cell 2 is stopped, and to suppress deterioration of the fuel cell. The life of the fuel cell 2 can be extended.

  Furthermore, in the present embodiment, the cathode gas circulation power generation is performed after the fuel cell system 1 is turned off and after the freeze prevention power generation is completed. Power generation will be stopped. Therefore, it is possible to prevent air-air activation at the next activation, and to reliably achieve power generation stability at the next activation and suppression of deterioration of the fuel cell 2.

(Second Embodiment)
Next, a second embodiment of the present invention will be described. FIG. 4 is a flowchart for explaining a stopping method of the fuel cell system in the second embodiment, and FIG. 5 is a graph showing a change in stack temperature with respect to time during soaking of the fuel cell system. In the following description, the description of the same parts as those in the first embodiment described above will be omitted.
As shown in FIGS. 4 and 5, in step S <b> 11, it is determined whether or not it is the winter season as in step S <b> 1 described above.
If the determination result in step S11 is “YES”, the process proceeds to step S12. If the determination result in step S11 is “NO”, this routine ends.

  Next, in step S12, it is determined whether or not the number of times of freezing in the fuel cell 2 is greater than or equal to a predetermined number of times per year (freezing number grasping step). Specifically, when the stack temperature Tfc of the fuel cell 2 falls below the freezing temperature Tc, it is estimated that the inside of the fuel cell 2 is frozen, and the number of times of freezing of the fuel cell 2 per year is counted. Then, based on the counted number of times of freezing, it is determined whether or not freeze prevention power generation is necessary.

When the determination result in step S12 is “YES” (when the number of freezing is equal to or greater than a predetermined number), for example, in the second half of winter, the inside of the fuel cell 2 has already been frozen many times, and freeze prevention power generation is necessary. Determine and proceed to step S13.
On the other hand, when the determination result in step S12 is “NO” (when the number of times of freezing is less than a predetermined value), for example, in the first half of winter, the freezing in the fuel cell 2 has not been experienced so much, and freeze prevention power generation is necessary. It is determined that there is no, and this routine is terminated.

  Next, in step S13, the freeze prediction means 53 determines whether or not the temperature decrease rate Tq per unit time of the stack temperature Tfc is equal to or higher than a predetermined rate (predicted freeze temperature decrease rate). That is, as shown by the broken line in FIG. 5, when the temperature decrease rate Tq during the soak is a normal speed (less than a predetermined speed), such as in the case other than the winter season, It can be determined that there is no risk of freezing before or during the daytime. On the other hand, as shown by the solid line in FIG. 5, when the temperature decrease rate Tq is equal to or higher than a predetermined rate (for example, at night), it is determined that there is a risk of freezing at the next start-up or the next temperature rise, Make freezing predictions.

When the determination result in step S13 is “YES” (when the temperature decrease rate Tq is equal to or higher than the predetermined rate), the stack temperature Tfc is rapidly decreased, so the fuel cell 2 is predicted to freeze, and the process proceeds to step S14.
On the other hand, when the determination result in step S13 is “NO” (when the temperature decrease rate Tq is less than the predetermined rate), it is determined that there is no possibility of freezing, and this routine is terminated.

  Subsequently, in step S14, an estimated rate of decrease in the future stack temperature Tfc is estimated based on the outside air temperature Tout, position information, date, time information, and the like. For example, when the outside temperature Tout tends to increase in the future, such as in the morning or during the daytime (after time t8 in the figure), the estimated decrease rate is low, and when the outside temperature Tout tends to decrease in the future, such as at night ( In the figure, before time t7), the estimated decrease rate is estimated to be high.

Next, in step S15, the necessity of freeze prevention power generation is determined based on the estimated decrease rate estimated in step S14 described above.
If the determination result in step S15 is “YES” (for example, if the estimated decrease rate is greater than or equal to a predetermined estimated decrease rate threshold), it is determined that there is a risk of freezing, and the process proceeds to step S16.
When the determination result in step S15 is “NO” (for example, when the estimated decrease rate is less than a predetermined estimated decrease rate threshold), it is determined that there is no possibility of freezing, and this routine is terminated.

Subsequently, in step S16, freeze prevention power generation is started by the same method as in step S3 described above (time t7 in FIG. 5).
Thereafter, in step S17, the end of freeze prevention power generation is determined. If the determination result is "YES", the process proceeds to step S18, and if "NO", the routine in step S17 is repeated.

In step S18, after the freeze prevention power generation is completed (time t8 in FIG. 5), in step S19, cathode gas circulation power generation is performed as oxygen-deficient power generation.
Thus, this routine is finished.

  According to the present embodiment, the same effect as that of the first embodiment described above is achieved, and the freeze preventive power generation is performed by performing the freeze preventive power generation of the fuel cell 2 when the number of freezes of the fuel cell 2 is equal to or greater than the predetermined number. Can be minimized. Therefore, it is possible to suppress an increase in fuel consumption due to freeze-prevention power generation while improving power generation stability at the next startup and suppressing deterioration of the fuel cell 2.

The technical scope of the present invention is not limited to the above-described embodiments, and includes those in which various modifications are made to the above-described embodiments without departing from the spirit of the present invention. In other words, the configuration described in the above-described embodiment is merely an example, and can be changed as appropriate.
For example, in the first embodiment described above, the configuration in which the freezing prediction is performed based on the stack temperature Tfc has been described. However, the configuration is not limited thereto, and the freezing prediction is performed based on the temperature decrease rate Tq of the stack temperature Tfc. I do not care. Similarly, in the second embodiment, a configuration may be adopted in which freezing is predicted based on the stack temperature Tfc.

Furthermore, in the above-described embodiment, the configuration in which cathode gas circulation power generation is performed as oxygen-deficient power generation has been described. However, the present invention is not limited thereto, and the same effect can be achieved by oxygen lean power generation, discharge power generation, and the like. The oxygen-lean power generation means that the cathode off-gas outlet sealing valve 36 is opened and the cathode gas circulation pump 66 is not driven, and the amount of air supplied to the fuel cell 2 is smaller than that during normal power generation. It is a method of generating electricity. That is, oxygen-lean power generation is a power generation method in which the cathode stoichiometry is lower than normal power generation. The discharge power generation is a method of discharging the power of the fuel cell 2 to a battery, a motor, various auxiliary machines, and the like.
Moreover, although embodiment mentioned above demonstrated the structure which performs winter season determination when the outside temperature Tout detected by the outside temperature sensor 21 is below predetermined temperature TL, it is not restricted to this.

  Furthermore, in the second embodiment described above, freeze prevention power generation is performed when the number of freezes per year is equal to or greater than a predetermined number. However, the present invention is not limited to this, and freeze prevention is based on the number of freezes accumulated in the fuel cell system 1. You may make it the structure which generates electric power. Further, it is possible to adopt a configuration in which freeze prevention power generation is performed when the freezing frequency in the fuel cell 2 (the interval from the previous freezing time to the current freezing time) is equal to or lower than a predetermined frequency (freezing frequency grasping step). .

Further, the freezing area (degree of freezing) in the surface of the fuel cell 2 is estimated based on the impedance when the fuel cell 2 is frozen, and freeze prevention power generation is performed when the estimated frozen area is equal to or greater than a predetermined value. It does not matter (freezing area estimation step). In this case, it can be estimated that the higher the impedance, the larger the freezing area in the fuel cell 2.
According to this configuration, freeze-prevention power generation can be suppressed to a minimum, so that power generation stability at the next start-up is improved and fuel cell deterioration is suppressed, and fuel consumption increase due to freeze-prevention power generation is suppressed. it can.

By the way, in the fuel cell system 1, there is a case where a discharge process for charging the power storage device with the power generated by the fuel cell 2 at the time of initial power generation immediately after startup (before transition to normal power generation) may be performed. In this case, it is necessary for the power storage device to stop the fuel cell system 1 in a state in which the capacity charged in the power storage device in the discharge process performed at the next startup is ensured as a free capacity.
However, in the present embodiment, the SOC of the battery may increase before and after the stop of the fuel cell system 1 due to the above-described freeze prevention power generation, cathode gas circulation power generation, etc., and the free capacity for charging by the discharge process at the next start-up May not be secured in the power storage device.

Therefore, when starting the fuel cell system 1 after performing freeze-preventive power generation, an SOC grasping step for grasping the SOC of the power storage device, and an SOC reducing step for reducing the SOC when the grasped SOC is equal to or greater than a predetermined value; It is preferable to carry out. Specifically, when the determined SOC is greater than or equal to a predetermined value, that is, when the remaining capacity of the power storage device is less than the capacity charged to the power storage device by the discharge process, the power of the power storage device is discharged due to consumption of auxiliary equipment, etc. To do.
According to this configuration, it is possible to secure the amount of charge by the discharge process at the next start-up, so that the fuel cell system 1 can be started up without any problem.

  Further, in the above-described embodiment, the configuration in which the cathode gas circulation power generation is performed after the fuel cell system 1 is stopped and the freeze prevention power generation is completed has been described. However, the cathode gas circulation power generation may not be performed.

  In addition, it is possible to appropriately replace the components in the above-described embodiments with known components without departing from the spirit of the present invention.

DESCRIPTION OF SYMBOLS 1 ... Fuel cell system 2 ... Fuel cell stack (fuel cell) 65 ... Cathode gas circulation channel (oxidant circulation channel) Tc ... Freezing temperature Tfc ... Stack temperature Ti ... Freezing expected temperature TL ... Predetermined temperature Tout ... Outside temperature Tq ... Decrease rate (stack temperature decrease rate) Ts ... Target temperature (target temperature rise)

Claims (7)

A method for stopping a fuel cell system including a fuel cell that generates power by supplying fuel to an anode and supplying an oxidant to a cathode,
A minimum temperature estimation step of estimating whether or not the minimum temperature of the outside air temperature is equal to or lower than a predetermined temperature;
A temperature detection step of detecting a stack temperature during power generation stop of the fuel cell, or a stack temperature decrease rate; and
Freezing prediction for predicting freezing in the fuel cell when the stack temperature or the stack temperature decreasing rate is lower than the predicted freezing temperature or the predicted freezing temperature decreasing rate at which there is a risk of freezing in the fuel cell Process,
A freeze prevention power generation step of performing freeze prevention power generation by the fuel cell when a freeze prediction is performed in the freeze prediction step.   2. The fuel cell according to claim 1, wherein in the freeze prevention power generation step, a current value that can reach the target temperature rise within a predetermined time is set based on the stack temperature and the target temperature rise. How to stop the system. Before the freeze prevention power generation step, it has a freeze number grasping step for grasping the number of times that the stack temperature has fallen below the freezing temperature for determining freezing in the fuel cell,
3. The fuel cell system stop method according to claim 1, wherein freeze prevention power generation is performed when it is determined that the stack temperature is lower than the freezing temperature by a predetermined number of times or more. Before the freeze prevention power generation step, it has a freezing frequency grasping step for grasping an interval at which the stack temperature falls below a freezing temperature for determining freezing in the fuel cell,
The fuel according to any one of claims 1 to 3, wherein freeze prevention power generation is performed when it is determined that an interval at which the stack temperature falls below the freezing temperature is equal to or less than a predetermined interval. How to stop the battery system. A frozen area estimation step for estimating whether or not a frozen area in a plane of the fuel cell is equal to or larger than a predetermined area;
The method for stopping a fuel cell system according to any one of claims 1 to 4, wherein freeze prevention power generation is performed when the expected freezing area is equal to or greater than the predetermined area.   After the freeze prevention power generation step, a cathode gas circulation processing step of performing cathode gas circulation power generation by returning the oxidant discharged from the cathode to the inlet side of the cathode through the oxidant circulation path. The method for stopping a fuel cell system according to any one of claims 1 to 5, wherein: An SOC grasping step for grasping the remaining capacity of the power storage device when the fuel cell is started after the freeze prevention power generation is performed;
An SOC reduction step of reducing the remaining capacity of the power storage device by discharging the power of the power storage device before power generation of the fuel cell when the remaining capacity is greater than or equal to a predetermined value. The method for stopping a fuel cell system according to any one of claims 1 to 6.

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