JP2014022299A - Fuel cell system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fuel cell system which, when starting a stack 11, heats the stack 11 so that its temperature becomes even and can rise stably.SOLUTION: At a self-heating step in a startup mode in which a stack 11 starts up, an initial air flow rate F0 is set and an output current I is increased so that the output current of the stack 11 becomes a maximum current value for the air flow rate. Then, when the maximum current value is reached, the air flow rate is incremented by an amount equal to one step (Fs), and the output current of the stack 11 is controlled so that a maximum current value is obtained for the new air flow rate. This operation is repeated, and when the output current, temperature and air flow rate of the stack 11 thereby reach prescribed values, the self-heating step is terminated and an operation mode is entered into. Thus, since variation in the internal temperature of the stack 11 can be suppressed, the problems of the stack 11 such as damage or degradation caused by a variation in temperature can be avoided.

Description

  The present invention relates to a fuel cell system, and more particularly to a technique for equalizing the temperature of the entire stack at the start of the fuel cell stack.

  For example, since a solid oxide fuel cell (SOFC) has a high operating temperature, it is necessary to raise the temperature of a fuel cell stack (hereinafter abbreviated as “stack”) using a combustion burner during startup. As a conventional example of a fuel cell system that performs such temperature increase, for example, one described in Patent Document 1 is known. In Patent Document 1, the combustion gas of the combustion burner is supplied to the stack at the start of the stack to raise the temperature of the stack, and the stack temperature is measured using a temperature sensor installed at an appropriate position of the stack. Power generation starts after reaching the power generation possible temperature. Then, the temperature rises due to self-heating of the stack, and when the output current reaches a predetermined value and the stack temperature reaches a preset threshold temperature, the combustion burner is stopped and the supply of combustion gas is stopped.

  In such a conventional fuel cell system, when the combustion gas flow rate and the power generation output are increased in order to shorten the startup time, the temperature variation in the stack increases. In particular, in the case of a type having a small stack heat capacity, such as an in-vehicle fuel cell, the temperature distribution of the oxidizing gas temperature supplied into the fuel cell tends to increase from upstream to downstream. The cell temperature distribution is likely to fluctuate due to the flow rate change. For this reason, the variation in temperature in the fuel cell is further increased, and this variation causes a problem that the stack is damaged.

JP 2009-146647 A

  As described above, in the conventional example disclosed in Patent Document 1, when the stack is heated to a predetermined temperature by a preset sequence and then heated by self-heating of the stack, the stack temperature and the output current of the stack are Since it is detected that a certain threshold value has been exceeded and the starting combustion burner is stopped, in the case of a stack with a small heat capacity and a high output density, the temperature in the stack tends to vary, and it tends to become unstable. For this reason, when the variation in the temperature in the stack increases, there is a problem that a large thermal stress is applied to the stack, leading to deterioration or damage.

  The present invention has been made to solve such a conventional problem. The object of the present invention is to maintain a uniform temperature in the stack even when a stack having a low heat capacity and a high output density is rapidly started. An object of the present invention is to provide a fuel cell system that can be heated and stably heated.

  In order to achieve the above object, according to the present invention, in the self-heating step during the start-up mode of the fuel cell, the oxidizing gas flow rate supplied to the cathode is set to an initial oxidizing gas flow rate that is smaller than the flow rate in the operation mode. Based on the first detected temperature, the output current of the fuel cell is adjusted so that the output current of the fuel cell becomes the maximum output current at the initial oxidizing gas flow rate. Then, after the output current reaches the maximum output current, the flow rate of the oxidizing gas supplied to the cathode is increased, and the output current is controlled again so as to become the maximum output current for the new oxidizing gas flow rate. Repeat until the battery output reaches the target output.

  In the fuel cell system according to the present invention, in the self-heating step during the start-up mode of the fuel cell, the flow rate of the oxidizing gas supplied to the cathode is set to a flow rate smaller than the oxidizing gas flow rate when determining the operation mode start, Since the output current value is controlled to be a value corresponding to the oxidant gas flow rate while gradually increasing the oxidant gas flow rate, the balance between the heating / cooling performance of the cell by the oxidant gas and the cell heat generation accompanying power generation is stable. Therefore, it is possible to suppress the temperature variation of the entire fuel cell, and to make the fuel cell temperature uniform.

It is a block diagram which shows the structure of the fuel cell system which concerns on 1st, 2nd embodiment of this invention. It is a flowchart which shows the process sequence of the fuel cell system which concerns on 1st Embodiment of this invention. It is a characteristic view which shows the change of the air flow rate, electric current, and each temperature at the time of starting mode of the fuel cell system which concerns on 1st Embodiment of this invention. It is a characteristic view which shows the change of the air flow rate, electric current, and each temperature in the self-heating step of the fuel cell system according to the first embodiment of the present invention. It is a flowchart which shows the process sequence of the fuel cell system which concerns on 2nd Embodiment of this invention. It is a characteristic view which shows the change of the air flow rate, electric current, and each temperature in the self-heating step of the fuel cell system according to the second embodiment of the present invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[Description of First Embodiment]
FIG. 1 is a block diagram showing the configuration of the fuel cell system according to the first embodiment of the present invention. As shown in FIG. 1, the fuel cell system 100 supplies a fuel cell stack 11 (hereinafter abbreviated as “stack 11”) having a cathode 11a and an anode 11b, and air (oxidizing gas) to the cathode 11a. An air blower 12, a heat exchanger 13 for heating air sent from the air blower 12, and a first fuel pump for supplying fuel (reformed gas) such as hydrocarbon fuel to the anode 11 b of the stack 11 14. An evaporator and a fuel reformer are mounted on the inlet side of the anode 11b, but are not shown in FIG. In this embodiment, an example in which air is used as the oxidizing gas will be described. However, the present invention is not limited to this, and a gas containing oxygen can be used.

  An upstream side of the cathode 11a is provided with a start-up combustor (combustor) 21 that generates combustion gas for raising the temperature of the cathode 11a when the stack 11 is started. Part of the air sent from the air blower 12 is branched and supplied by the valve 22, and further fuel is supplied from the second fuel pump 23. In the start-up combustor 21, combustion is performed using fuel and air, and combustion gas is supplied to the cathode 11 a to heat the cathode 11 a when the stack 11 is started.

  Here, the starting combustor 21, the air blower 12, the valve 22, and the second fuel pump 23 constitute an oxidant supply means. That is, the oxidant supply means is provided on the upstream side of the cathode 11a, and the air blower 12 that outputs air (oxidizing gas) and the start-up combustor that heats the air sent from the air blower 12 when the stack 11 is started. 21 and has a function of supplying air to the cathode 11a.

  An inlet temperature sensor (first temperature detecting means) 41a is provided on the cathode 11a inlet side (air introduction side) of the stack 11, and an outlet temperature sensor (first air detection side) is provided on the outlet side (air discharge side). 2 temperature detecting means) 41b is provided. Further, the stack 11 is provided with a current / voltage sensor (current detection means) 42 for detecting the output current I and the output voltage V. In the present embodiment, an example in which the inlet temperature sensor 41a is used as the first temperature detecting means will be described. However, the present invention is not limited to this, and for example, the temperature is applied to a portion having the smallest heat capacity in the stack 11. It is also possible to provide a sensor and use this as the first temperature detecting means.

  Further, the air blower 12, the second fuel pump 23, the valve 22, the inlet temperature sensor 41 a, the outlet temperature sensor 41 b, and the current / voltage sensor 42 are each connected to a control unit (control means) 31. The control unit 31 includes an inlet temperature Ti (first detected temperature) detected by the inlet temperature sensor 41a, an outlet temperature To (second detected temperature) detected by the outlet temperature sensor 41b, and a current / voltage sensor. Based on the output current I and output voltage V detected at 42, the flow rate of air sent from the air blower 12, the amount of fuel output from the second fuel pump 23, and the opening of the valve 22 are controlled, and the stack 11 is activated. In the self-heating step, the temperature of the entire stack 11 is controlled to be uniform. In addition, the control part 31 can be comprised as an integrated computer which consists of memory | storage means, such as central processing unit (CPU), RAM, ROM, a hard disk, for example.

  Next, the processing operation at the start of power generation by the stack 11 in the fuel cell system 100 according to the first embodiment will be described with reference to the flowchart shown in FIG. In the present embodiment, when starting the stack 11, a start mode for increasing the temperature of the stack 11 to an operation mode start determination temperature Td described later is executed, and the temperature of the stack 11 has reached this temperature Td. Later, the operation mode is switched to a mode for supplying power to the load. The startup mode is divided into a heating and heating step in which the temperature of the stack 11 is raised by the startup combustor 21 and a self-heating step in which the stack 11 is heated by generating power in the stack 11. And the flowchart shown in FIG. 2 has shown about the process until it transfers to the operation mode after it transfers to a self-heating step from a heating temperature rising step in the starting mode.

  Details will be described below. First, in step S11 of FIG. 2, the control unit 31 determines the operation mode start determination temperature Td as a condition for shifting from the start mode to the operation mode, and the output current of the stack 11 at this time, that is, the operation mode start determination. A current Id (target current), an air flow rate at this time, that is, an operation mode start determination flow rate Fd, and an upper limit value ηcu of the oxygen utilization rate are set. The air flow rate Fd can be obtained from the current Id and the upper limit value ηcu of the oxygen utilization rate. This process is acquired based on an output request from the host system, or is set by an operation input. Here, the oxygen utilization rate is the ratio of the amount of oxygen used for power generation out of the oxygen contained in the air supplied to the cathode.

  Next, in step S12, the control unit 31 sets the upper limit temperature Tu of the stack 11, the start temperature Th of the self-heating step, and the upper limit value −Z (negative value) of the inlet temperature change rate of the stack 11. Each of these numerical values can be acquired from a setting input by the operator or from a host system. In the present embodiment, the upper limit temperature Tu of the stack 11 is set to a constant value. It is also possible to change the upper limit temperature Tu.

  In step S13, the control unit 31 sets the initial air flow rate F0 (initial oxidizing gas flow rate) in the self-heating step and the current I0 (where I0 <Id) at this time, the air flow rate increase amount Fs in one step, A current change amount Is and a time interval ti for determining increase / decrease in current are set. Each of these numerical values can be acquired from a setting input by the operator or from a host system. In the present embodiment, an example in which the air flow rate increase amount Fs is set to a constant value will be described. However, the air flow rate increase amount Fs can be changed in conjunction with the air flow rate F, the maximum output current, or the like. For example, the air flow rate increase amount Fs at the current step can be set to be smaller in proportion to the maximum output current at the flow rate at the previous step.

  In step S14, the control unit 31 compares the inlet temperature Ti (first detection temperature) of the stack 11 with the self-heating step start temperature Th set in the process of step S12. That is, it is determined whether or not “Ti> Th”. If it is determined that “Ti> Th” is not satisfied (NO in step S14), the heating temperature raising step is continued in step S25. That is, when the inlet temperature Ti has not reached the self-heating step start temperature Th, it is not possible to shift to the self-heating step, so the heating temperature increasing mode is continued and the process of heating the stack 11 is continued.

  On the other hand, if it is determined that “Ti> Th” (YES in step S14), in step S15, the control unit 31 switches the combustion gas supplied to the cathode 11a to air, and the air flow rate F is the above-described value. And an output current of the stack 11 is set to I0. This process can be performed by adjusting the opening degree of the valve 22 shown in FIG. That is, when the inlet temperature Ti of the stack 11 reaches the self-heating step start temperature Th set in the process of step S12, the stack 11 has reached a temperature at which power can be generated. The air heated by the exchanger 13 is introduced into the cathode 11a to start power generation. This power generation is performed to raise the temperature of the stack 11 by heat generation, and does not supply power to the external load.

  In step S16, the control unit 31 compares the inlet temperature Ti of the stack 11 with the upper limit temperature Tu set in the process of step S12, and determines whether “Ti <Tu”. If it is determined that “Ti <Tu” is not satisfied (NO in step S16), the control unit 31 decreases the output current I of the stack 11 in step S17. In this process, when the inlet temperature Ti of the stack 11 exceeds the upper limit temperature Tu, the inlet temperature Ti needs to be lower than the upper limit temperature Tu. Therefore, by reducing the output current I, the temperature of the inlet temperature Ti is reduced. Suppresses the rise. In actual processing, the output current I of the stack 11 is controlled to decrease by Is, which is the change amount for each step, so that the output current I decreases. When the process of step S17 ends, the process proceeds to step S22.

  On the other hand, when it is determined that “Ti <Tu” is satisfied (YES in step S16), in step S18, the control unit 31 calculates a change rate (time differential value) “dTi / dt” of the inlet temperature Ti. Then, it is determined whether or not the rate of change “dTi / dt” is greater than the upper limit value “−Z” (negative value) of the rate of change set in the process of step S12. If it is determined that the value is small (NO in step S18), the process proceeds to step S19. If it is determined that the value is large (YES in step S18), the process proceeds to step S20. That is, when the inlet temperature Ti is increasing or gradually decreasing with the passage of time, the determination at step S18 is YES, and the process proceeds to step S20. When the inlet temperature Ti is rapidly decreasing (inlet temperature) If the decrease rate is large), the determination is NO and the process proceeds to step S19.

  For example, immediately after the fluid to be introduced into the cathode 11a is switched from combustion gas to air, the inlet temperature Ti rapidly decreases (since the rate of decrease of the inlet temperature Ti increases). By increasing the output current I in the process of S19, the heat generation amount of the stack 11 is increased, and the rapid decrease of the inlet temperature Ti is prevented. Thereafter, the process proceeds to step S22.

  In step S20, the control unit 31 determines whether or not the air flow rate F has reached the operation mode start determination flow rate Fd set in the process of step S11, and the current I is set in the process of step S11. It is determined whether or not the start determination current Id has been reached, and it is further determined whether or not the inlet temperature Ti has reached the operation mode start determination temperature Td set in the process of step S11. If at least one of these determination results is not satisfied (NO in step S20), the control unit 31 increases the output current I in step S21. Thereafter, the process proceeds to step S22.

  On the other hand, when all the conditions shown in step S20 are satisfied, that is, when F ≧ Fd, I ≧ Id, and Ti ≧ Td are satisfied (YES in step S20), the present process is terminated and the operation mode is shifted to. To do.

  In step S22, the control unit 31 detects the output current I of the stack 11 and obtains the oxygen utilization rate ηc. Here, the oxygen utilization rate ηc is the ratio of the amount of oxygen used for power generation out of the oxygen contained in the air supplied to the cathode, and the total amount of oxygen is the flow rate of air delivered from the air blower 12. Since the oxygen amount used for power generation can be obtained from the output current I, the oxygen utilization rate ηc can be calculated based on these. When the fluid supplied to the cathode 11a is air or a mixed gas of air and combustion gas, the partial pressure of oxygen in the gas is estimated based on the combustion state and flow rate of the start-up combustor 21, and Based on this, the air flow rate F can be corrected. It is also possible to set the utilization rate upper limit value ηcu in conjunction with the air flow rate supplied to the cathode 11a.

  In step S23, the control unit 31 determines whether or not the oxygen utilization rate ηc obtained in step S22 is larger than the utilization upper limit value ηcu set in step S11, and “ηc> ηcu” is satisfied. If not (NO in step S23), the process returns to step S16. If “ηc> ηcu” (YES in step S23), the process proceeds to step S24.

  That is, the oxygen utilization rate ηc does not reach the utilization rate upper limit value ηcu (when NO is determined in step S23), the larger output current at the air flow rate F (initially F0) at this point. Since I can be obtained (the maximum output current has not been reached), the process returns to step S16.

  On the other hand, when the oxygen utilization rate ηc has reached the utilization rate upper limit value ηcu (when it is determined YES in step S23), the output current I obtained with the air flow rate F at this time reaches the maximum output current. Since the output current I cannot be increased any further, in step S24, the control unit 31 sets the air flow rate F supplied to the cathode 11a for one step set in the process of step S13. The air flow rate increase amount Fs is increased, and the process returns to step S16. As a result, the air flow rate F supplied to the cathode 11a becomes “F + Fs”, and the processing from step S16 is repeated again.

  That is, by setting the air flow rate F and repeating the processes in steps S16 to S21, the output current I of the stack 11 is controlled to be the maximum output current at the air flow rate F, and the output current I is the maximum output. When the current is reached, the air flow rate F is increased by Fs. That is, F = F + Fs. Then, by performing the same processing as described above for the new air flow rate F, the output current I is controlled to be the maximum output current for the new air flow rate F.

  By repeating this control, the output current I of the stack 11 eventually reaches the operation mode start determination current Id, the air flow rate F reaches Fd, and the temperature Ti reaches Td. Can be met. That is, it is determined YES in step S20. Then, this process is complete | finished and it transfers to the operation mode.

  To summarize the processing shown in FIG. 2, the start-up mode is started, and the start-up combustor 21 is stopped when the inlet temperature Ti reaches the self-heat generation step start temperature Th by the first heating and heating step. Then, the temperature shifts to the self-heating step. At this time, the flow rate of air supplied to the stack 11 is set to F0 lower than the flow rate at the start of the operation mode, and power generation is started. The current I is controlled.

  Specifically, in the process of step S16, when the inlet temperature Ti exceeds the upper limit temperature Tu, control is performed so as to decrease the current I. In the process of step S18, when the inlet temperature Ti decreases rapidly, Further, in step S20, the air flow rate F, the output current I, and the inlet temperature Ti are changed to the air flow rate Fd, the output current Id, and the inlet temperature Td when the operation mode is shifted. The output current I is controlled so as to reach it. When these conditions are satisfied, the conditions for shifting to the operation mode are satisfied, so the self-heating step is terminated and the operation mode is switched from the start mode.

  By executing such processing, the air flow rate F supplied to the cathode 11a and the output current I of the stack 11 can be stably increased while the inlet temperature Ti of the stack 11 is stabilized, and the air When the flow rate F, the inlet temperature Ti, and the output current I reach Fd, Td, and Id, respectively, the processing of this embodiment can be terminated and the operation mode can be shifted. Therefore, when the stack 11 is activated, the temperature variation in the stack 11 can be suppressed, and a desired output can be obtained in a short time.

  Next, with reference to the timing chart shown in FIG. 3, changes in the air flow rate, the output current, and each temperature by executing the process shown in the flowchart of FIG. 2 will be described. FIG. 3 is a characteristic diagram showing changes in the air flow rate, the output current, and each temperature in the start mode performed after starting the stack 11, and the air flow rate F (combustion gas and heat exchange) supplied to the cathode 11a. The air heated by the vessel 13), the air temperature Tg supplied to the cathode 11a, the inlet temperature Ti of the stack 11, and the outlet temperature To of the stack 11 are shown.

  When the stack 11 is activated at time t0 shown in FIG. 3, a heating temperature raising step is first started to heat the stack 11. Specifically, air is supplied from the air blower 12 shown in FIG. 1 to the startup combustor 21, fuel is supplied from the second fuel pump 23 to the startup combustor 21, and the valve 22 is controlled. The start-up combustor 21 is combusted, and the combustion gas output from the start-up combustor 21 is supplied to the cathode 11 a of the stack 11.

  Then, the air temperature Tg, the inlet temperature Ti, and the outlet temperature To gradually increase. At this time, the air temperature Tg is the highest, and then the inlet temperature Ti and the outlet temperature To decrease in this order. When the inlet temperature Ti reaches the self-heating step start temperature Th at time th0, the heating temperature raising step is switched to the self-heating step.

  When the self-heating step is started, the supply of combustion gas by the start-up combustor 21 is stopped. Although the first embodiment shows an example in which the start-up combustor 21 is stopped, the start-up combustor 21 may not be stopped completely, and the supply amount of the combustion gas may be reduced. Accordingly, as shown in FIG. 3, the air flow rate F supplied to the stack 11 rapidly decreases at time th0. At this time, the air flow rate F is set to be smaller than the flow rate Fd at the start of the operation mode. Specifically, the initial air flow rate F0 (F0 <Fd) in the self-heating step described above is set.

  As the combustion gas supply is stopped, the air temperature Tg supplied to the stack 11 rapidly decreases, and the inlet temperature Ti of the stack 11 slightly decreases. Then, by performing the process of FIG. 2 described above, the air flow rate F is gradually increased so that the inlet temperature Ti is maintained at a substantially constant temperature, and the air flow rate F reaches the operation mode start determination flow rate Fd, In addition, at the time td when the output current I reaches the operation mode start determination current Id, the self-heating step is ended and the operation mode is shifted.

  FIG. 4 is a characteristic diagram showing in detail changes in the inlet temperature Ti, the air temperature Tg, the air flow rate F, and the output current I in the self-heating step. Times th0 and td shown in FIG. 4 correspond to times th0 and td shown in FIG. 3, respectively.

  When the inlet temperature Ti reaches the self-heating step start temperature Th at time th0 shown in FIG. 4, the air flow rate F is set to F0, and the output current I is set to I0. At this time, the air (oxidizing gas) supplied to the stack 11 is switched from the combustion gas output from the startup combustor 21 to the air heated by the heat exchanger 13.

  Further, after the time th0 shown in FIG. 4, the rate of decrease “−dTi / dt” of the inlet temperature Ti becomes larger than the inlet temperature change rate upper limit (−Z) (see step S18 in FIG. 2). At time th1, the current I is increased by the change amount Is for one step.

  At time th2, since the current I has reached the maximum current value Imax (F0) determined by the flow rate F0, the air flow rate F is increased by an increase amount Fs corresponding to one step (see step S21 in FIG. 2).

  At time th3, since the inlet temperature Ti has exceeded the preset upper limit temperature Tu, the current I is decreased by the change amount Is for one step (see step S17 in FIG. 2).

  At time th4, the inlet temperature Ti is lower than the upper limit temperature Tu and the current I has not reached Id. Therefore, the current I is increased by the change amount Is for one step.

  At time td, the air flow rate F reaches the operation mode start determination flow rate Fd, the current I reaches the operation mode start determination current Id, and the inlet temperature Ti reaches the operation mode start determination temperature Td. And the operation mode is shifted to (see step S20 in FIG. 2).

  As understood from the characteristic curves of the inlet temperature Ti, the air flow rate F, and the output current I shown in FIG. 4, when the self-heating step is being executed, the air flow rate F supplied to the cathode 11a is stepped. As the output current I fluctuates up and down along with this, the output current I rises stepwise as a whole (may be temporarily lowered). Further, the inlet temperature Ti does not fluctuate greatly and is maintained at a substantially constant value. Further, as shown between th0 and td in FIG. 3, the outlet temperature To gradually increases without significant fluctuation. Therefore, it can be seen that the temperature of the entire stack 11 does not vary greatly and the temperature is substantially uniform during the period from the execution of the self-heating step until the operation mode is reached. That is, when the stack 11 is activated, the operation mode is controlled so as to reach the operation mode while suppressing variations in temperature in the stack 11.

  Thus, in the fuel cell system 100 according to the present embodiment, when the heating temperature raising step is completed in the startup mode in which the stack 11 is started, the air flow rate F supplied to the cathode 11a is changed to the initial air flow rate F0. Based on the temperature of the stack 11, control is performed so that the maximum current value for the initial air flow rate F0 is output. When the maximum current value is obtained, the air flow rate F is increased and output again. Control is performed so that the maximum current value is output by increasing the current. Therefore, it is possible to shift to the operation mode by reaching the target output (operation mode start determination current Id) while suppressing the temperature variation in the stack 11 and stably increasing the stack temperature.

  In particular, in order to perform rapid start-up, when the flow rate of the combustion gas for stack heating is larger than the air flow rate at the time of determining the operation mode, the temperature variation from upstream to downstream of the combustion gas supplied into the cathode 11a The output current can be quickly increased and the mode can be shifted to the start-up mode while suppressing the above.

  In addition, the stack configuration in which flat cells are stacked has the advantage that the cell integration density per stack volume can be increased, while the generated voltage in one stacked cell is operated at a constant level. In this case, there is a disadvantage that the generated current density varies depending on the temperature variation in the cell plane depending on the air flow, and this current density variation tends to further increase the temperature variation in the cell plane. In this embodiment, even a high power density flat plate stack can be activated while suppressing temperature variations in the cell.

  Furthermore, when the stack 11 is frequently started and stopped, such as when a fuel cell is mounted on the vehicle, the temperature distribution in the cell is not constant due to the elapsed time from the previous stop (when there is a large variation) However, according to the present invention, it is possible to quickly shift the stack 11 to the operation mode without increasing or destabilizing the variation in the in-cell temperature.

  As a result, it is possible to avoid the problem that the stack 11 is damaged or deteriorated due to variations in temperature generated in the stack 11, and to reach the target output in a short time.

  In the process of step S23 in FIG. 2, the oxygen utilization rate ηc is calculated based on the output current I of the stack 11 and the air flow rate F supplied to the cathode 11a, and the oxygen utilization rate ηc is a preset utilization rate. When the value exceeds the upper limit value ηcu, it is determined that the output current I of the stack 11 has reached the maximum output current, so that the maximum output current can be detected with high accuracy, and the air supplied to the cathode 11a. The timing for increasing the flow rate can be set accurately. In addition, rapid start-up can be performed while suppressing deterioration of the cathode electrode material, performance deterioration, and separation due to insufficient oxygen partial pressure on the downstream side of the air flow in the cathode 11a.

  Further, in the process of step S16 in FIG. 2, when the inlet temperature Ti of the cathode 11a exceeds the upper limit temperature Tu, the output current I is reduced, so that an abnormal increase in the inlet temperature Ti can be suppressed, and consequently The temperature variation of the entire stack 11 can be reduced. Furthermore, the cell temperature distribution can be stabilized even in a flat-plate stack structure in which high power density can be expected.

  Further, in the process of step S18 in FIG. 2, when the rate of decrease of the inlet temperature Ti of the cathode 11a is larger than the inlet temperature change rate upper limit value (−Z), the output current I of the stack 11 is increased. In the case where the inlet temperature Ti rapidly decreases as in the case of a system configuration in which the air supplied into the cathode 11a is switched from the combustion gas output from the start-up combustor 21 to the air output from the blower. Therefore, this temperature decrease can be suppressed, and as a result, the temperature variation of the entire stack 11 can be reduced. As a result, it is possible to construct a sequence in which combustion gas and air are not mixed and introduced into the cathode 11a, so that the air flow path structure in the stack 11 can be reduced in size and the air flow rate control is simplified. Can be

  Further, since the current is controlled while increasing the flow rate of air supplied to the cathode 11a by an increment (Fs) for each step, it is possible to suppress a large change in the air flow rate F during power generation, and to the cathode 11a stably. The supplied air flow rate F can be increased, and the output current I can be increased to the operation mode start determination current Id. For this reason, it is possible to stabilize the cell temperature distribution determined by the heating / cooling performance of the cell by the air flow and the heat generation balance of the cell.

  In the description of the present embodiment, the outlet temperature To detected by the outlet temperature sensor 41b is used. However, since this value is not used for control, this measurement is not essential.

[Description of Second Embodiment]
Next, a second embodiment of the present invention will be described. The system configuration is the same as that shown in FIG. In the fuel cell system according to the second embodiment, the inlet temperature Ti of the stack 11 is measured, the outlet temperature To is measured, and the time change rate (time differential value) of these temperature differences is calculated. When it is larger than a preset threshold value (“temperature difference change rate upper limit value Y” described later), the output current I of the stack 11 is controlled to decrease.

  Further, when the process proceeds from the heating temperature raising step to the self-heating step, the start-up combustor 21 is not stopped, but is adjusted by adjusting the opening degree of the valve 22 to be sent from the start-up combustor 21. Air mixed with combustion gas and air sent via the heat exchanger 13 is supplied to the cathode 11a, and the start-up combustor 21 is stopped after a predetermined time has elapsed (time th2 in FIG. 6 described later). Let

  By performing such control, when the temperature on the downstream side of the stack 11 has not risen, the upstream temperature suddenly rises and the temperature distribution changes greatly, or the temperature on the downstream side is delayed. Prevents thermal runaway.

  Hereinafter, the processing operation at the start of power generation of the stack 11 in the fuel cell system 100 according to the second embodiment will be described with reference to the flowchart shown in FIG.

  First, in step S51, the controller 31 determines the operation mode start determination temperature Td, which is a condition for shifting from the start mode to the operation mode, the output current Id, the air flow rate Fd, and the oxygen utilization rate at this time. Set the upper limit value ηcu. The air flow rate Fd can be obtained from the current Id and the upper limit value ηcu of the oxygen utilization rate, as in the first embodiment. This process is acquired based on an output request from the host system, or is set by an operation input.

  Next, in step S52, the control unit 31 sets the first upper limit temperature Tu1 and the second upper limit temperature Tu2 that are the upper limit temperatures of the stack 11, the self-heating step start temperature Th, and the upper limit value of the inlet temperature change rate of the stack 11 − Z (negative value) and upper limit value Y of temperature difference change rate are set. The temperature difference change rate is the time change rate of the difference between the inlet temperature Ti measured by the inlet temperature sensor 41a and the outlet temperature To measured by the outlet temperature sensor 41b. Each of these numerical values can be acquired from a setting input by the operator or from a host system.

  In step S53, the controller 31 determines the initial air flow rate F0 of the self-heating step, the output current I0 at this time, the air flow rate increase amount Fs of 1 step, the current change amount Is of 1 step, and the current increase / decrease time. Set the interval ti. Each of these numerical values can be acquired from a setting input by the operator or from a host system. The time interval ti is set corresponding to the calculation cycle of this flowchart.

  In step S54, the control unit 31 calculates an average temperature Tav between the inlet temperature Ti (first detected temperature) of the stack 11 and the outlet temperature To, and further, the average temperature Tav and the self-heating set in the process of step S52. The step start temperature Th is compared. That is, it is determined whether or not “Tav> Th”. If it is determined that “Tav> Th” is not satisfied (NO in step S54), the heating temperature raising mode is continued in step S67. That is, when the average temperature Tav has not reached the self-heating step start temperature Th, it is not possible to shift to the self-heating step, so the heating temperature increasing mode is continued and the process of heating the stack 11 is continued.

  On the other hand, when it is determined that “Tav> Th” is satisfied (YES in step S54), in step S55, the control unit 31 burns the fluid supplied to the cathode 11a from the start-up combustor 21. Switch to a mixture of gas and air. At this time, the combustion gas flow rate is F0b, and the air flow rate is F0a. Note that F0a + F0b = F0. Further, the output current is set to I0. Here, the combustion gas flow rate F0b is the flow rate of the combustion gas output from the start-up combustor 21 shown in FIG. 1, and the air flow rate F0a is the cathode 11a after being heated by the heat exchanger 13 shown in FIG. It is the air flow rate supplied to. The output current I0 is a current output from the stack 11. This process can be performed by adjusting the opening degree of the valve 22 shown in FIG.

  That is, when the average temperature Tav of the stack 11 reaches the self-heating step start temperature Th set in the process of step S52, the stack 11 has reached a temperature at which power can be generated, and is sent from the air blower 12. A part of the air is heated by the start-up combustor 21, and the rest is introduced into the cathode 11 a as air heated by the heat exchanger 13 to start power generation.

  In step S56, the control unit 31 compares the inlet temperature Ti of the stack 11 with the upper limit temperature Tu (indicating either Tu1 or Tu2) set in the process of step S52, and whether or not “Ti <Tu”. Determine whether. If it is determined that “Ti <Tu” is not satisfied (NO in step S56), the control unit 31 decreases the output current I of the stack 11 in step S57. In this process, when the inlet temperature Ti of the stack 11 exceeds the upper limit temperature Tu, the inlet temperature Ti needs to be lower than the upper limit temperature Tu. Therefore, by reducing the output current I, the temperature of the inlet temperature Ti is reduced. Suppresses the rise. In actual processing, the output current I of the stack 11 is controlled to decrease by Is, which is the change amount for each step, so that the output current I decreases. When the process of step S57 ends, the process proceeds to step S64.

  Here, as described above, the upper limit temperature Tu is set so as to change depending on the elapsed time after the self-heating step is started. Specifically, the upper limit temperature Tu is set to the first upper limit temperature Tu1 within the predetermined time τ1 after the start of the self-heating step, and after the predetermined time τ1 has elapsed, the upper limit temperature Tu is set to the second upper limit temperature Tu. The upper limit temperature Tu2 (where Tu2> Tu1) is set (see FIG. 6). The predetermined time τ1 is a time until the supply of the combustion gas by the start-up combustor 21 is stopped, and is set in advance by an operator input or the like.

  On the other hand, when it is determined that “Ti <Tu” in the process of step S56 (YES in step S56), in step S58, the control unit 31 determines the rate of change (time differential value) of the inlet temperature Ti. dTi / dt "is calculated, and it is determined whether or not the rate of change" dTi / dt "is larger than the upper limit value" -Z "(negative value) of the rate of change set in the process of step S52. If it is determined that the value is small (NO in step S58), the process proceeds to step S59. If it is determined that the value is large (YES in step S58), the process proceeds to step S60. That is, when the inlet temperature Ti is increasing or gradually decreasing with the passage of time, the determination at step S58 is YES, and the process proceeds to step S60. When the inlet temperature Ti is decreasing rapidly (inlet temperature) If the decrease rate is large), the determination is NO and the process proceeds to step S59.

  For example, immediately after the air introduced into the cathode 11a is switched from the combustion gas to the mixture of the combustion gas and the air, the inlet temperature Ti rapidly decreases (since the rate of decrease of the inlet temperature Ti increases), step S58. NO is determined, and the amount of heat generated in the stack 11 is increased by increasing the output current I in the process of step S59, thereby preventing a rapid decrease in the inlet temperature Ti. Thereafter, the process proceeds to step S64.

  In step S60, the control unit 31 calculates the time change rate “dΔT / dt” of the temperature difference ΔT between the inlet temperature Ti and the outlet temperature To, and sets the upper limit value Y of the temperature difference change rate set in the process of step S52. Compare. That is, it is determined whether or not “dΔT / dt <Y”. If it is determined that “dΔT / dt <Y” is not satisfied (NO in step S60), the process proceeds to step S61. If it is determined that “dΔT / dt <Y” is satisfied (step S60). YES), the process proceeds to step S62.

  In step S <b> 61, the control unit 31 reduces the heat generation amount of the stack 11 by reducing the output current I, and reduces the temperature difference in the stack 11. That is, “dΔT / dt <Y” means that the difference between the inlet temperature Ti and the outlet temperature To of the stack 11 is increasing. Therefore, in order to suppress the temperature variation in the stack 11, the output current is reduced. Decrease. Thereafter, the process proceeds to step S64.

  In step S62, the control unit 31 determines whether or not the air flow rate F has reached the operation mode start determination flow rate Fd set in the process of step S51, and the current I is set to the operation mode set in the process of step S51. It is determined whether or not the start determination current Id has been reached. Further, it is determined whether or not the inlet temperature Ti has reached the operation mode start determination temperature Td set in the process of step S51. If at least one of these determination results is not satisfied (NO in step S62), the control unit 31 increases the output current I in step S63. Thereafter, the process proceeds to step S64.

  On the other hand, if all the conditions shown in step S62 are satisfied, that is, if F ≧ Fd, I ≧ Id, and Tav ≧ Td are satisfied (YES in step S62), this process is terminated.

  In step S64, the control unit 31 detects the output current I of the stack 11 and obtains the oxygen utilization rate ηc. Here, the oxygen utilization rate ηc is the ratio of the amount of oxygen used for power generation out of the oxygen contained in the air supplied to the cathode, and the total amount of oxygen is the flow rate of air delivered from the air blower 12. Since the oxygen amount used for power generation can be obtained from the output current I, the oxygen utilization rate ηc can be calculated based on these.

  In step S65, the control unit 31 determines whether or not the oxygen utilization rate ηc determined in the process of step S64 is larger than the utilization rate upper limit value ηcu set in the process of step S51, and “ηc> ηcu” is satisfied. If not (NO in step S65), the process returns to step S56. If “ηc> ηcu” (YES in step S65), the process proceeds to step S66.

  That is, if the oxygen utilization rate ηc does not reach the utilization rate upper limit ηcu (when NO is determined in step S65), more output is obtained at the air flow rate F (initially F0) at this point. Since the current I can be obtained (the maximum output current has not been reached), the process returns to step S56.

  On the other hand, when the oxygen utilization rate ηc has reached the utilization rate upper limit value ηcu (when YES is determined in step S65), the output current I obtained at the air flow rate F at this time reaches the maximum output current. Since the output current I cannot be increased any further, in step S66, the control unit 31 sets the air flow rate F supplied to the cathode 11a for one step set in the process of step S53. The air flow rate increase amount Fs is increased, and the process returns to step S56. As a result, the air flow rate F supplied to the cathode 11a becomes “F + Fs”, and the processing from step S56 is repeated again.

  That is, by setting the air flow rate F and repeating the processes of steps S56 to S63, the output current I of the stack 11 is controlled to be the maximum output current at the air flow rate F, and the output current I is the maximum output. When the current is reached, the air flow rate F is increased by Fs. That is, F = F + Fs. Then, by performing the same processing as described above for the new air flow rate F, the output current I is controlled to be the maximum output current for the new air flow rate F.

  By repeating this control, the output current I of the stack 11 eventually reaches the operation mode start determination current Id, the air flow rate F reaches the operation mode start determination flow rate Fd, and the temperature Tav becomes the operation mode start determination temperature. Since Td is reached, the operation mode start condition can be satisfied. That is, it is determined YES in step S62. Then, this process is complete | finished and it transfers to the operation mode.

  In the process shown in the flowchart of FIG. 5, when the average temperature Tav reaches the self-heating step start temperature Th by the heating heating step that is started first and the heating heating step that is executed first, the temperature is raised by the self-heating step. Transition. Then, without stopping the start-up combustor 21, the combustion gas sent from the start-up combustor 21 and air are mixed, and the mixed air is supplied to the cathode 11a.

  At this time, the air flow F supplied to the cathode 11a is set to F0 (= F0a + F0b) lower than the flow at the start of the operation mode, and power generation is started, so that the maximum current for the air flow F0 can be obtained. The output current I of the stack 11 is controlled.

  Specifically, in the process of step S56, when the inlet temperature Ti exceeds the upper limit temperature Tu, control is performed so as to decrease the current I, and in the process of step S58, when the inlet temperature Ti decreases rapidly. The current I is controlled to increase. In the process of step S60, when the change rate of the temperature difference between the inlet and the outlet is large, the current I is controlled to decrease. Further, in the process of step S62, the air flow rate is controlled. The current I is controlled so that the F, the output current I, and the average temperature Tav reach the air flow rate Fd, the output current Id, and the average temperature Td at the start of operation. When these conditions are satisfied, the start condition for the operation mode is satisfied, so the self-heating step is terminated and the operation mode is switched from the start mode.

  By executing such processing, the air flow rate F supplied to the cathode 11a and the output current I of the stack 11 can be stably increased while maintaining the inlet temperature Ti of the stack 11 at a substantially constant temperature. Can do. Therefore, when the stack 11 is activated, the temperature variation in the stack 11 can be suppressed, and a desired output can be obtained in a short time.

  Next, changes in each temperature, air flow rate, and output current after the transition from the heating temperature raising step to the self-heating step will be described with reference to FIG. FIG. 6 shows an inlet temperature Ti, an outlet temperature To, an average temperature Tav of the inlet temperature Ti and the outlet temperature To, an air temperature Tg, an air flow rate F (including combustion gas), and an output current I in the self-heating step. It is a characteristic view which shows the change of in detail. A time th0 shown in FIG. 6 indicates a time when the heating temperature raising step is shifted to the self-heating step, and a time td indicates a time when the start mode is shifted to the operation mode. That is, it corresponds to the times th0 and td of FIG. 3 shown in the first embodiment.

  When the average temperature Tav in the stack 11 reaches the self-heating step start temperature Th at time th0, the heating temperature rising step shifts to the self-heating step, the air flow rate F becomes F0 (= F0a + F0b), and the output current I is The power generation is started by adjusting to I0. Note that this power generation does not supply power to the load, but is power generation for raising the temperature of the stack 11.

  In the second embodiment, the start-up combustor 21 is stopped at time th2 shown in FIG. 6, the supply of the combustion gas is stopped, and only the air heated by the heat exchanger 13 is supplied to the cathode 11a. At this time, the upper limit temperature Tu of the stack 11 is changed from the first upper limit temperature Tu1 to the second upper limit temperature Tu2 (where Tu2> Tu1).

  At time th1 shown in FIG. 6, since the inlet temperature Ti has exceeded the first upper limit temperature Tu1, the current I is decreased by the change amount Is for one step. At time th2, the supply of combustion gas is stopped, and the upper limit temperature is changed from the first upper limit temperature Tu1 to the second upper limit temperature Tu2.

  At time th3, the change rate “dΔT / dt” of the temperature difference exceeds the upper limit value Y of the temperature difference change rate (see step S60 in FIG. 5), so the output current I is decreased by the change amount Is for one step. .

  At time td, the air flow rate F reaches the operation mode start determination flow rate Fd, the output current I reaches the operation mode start determination current Id, and the inlet temperature Ti reaches the operation mode start determination temperature Td. Exit the mode and enter the operation mode.

  As understood from the characteristic curves of the inlet temperature Ti, the outlet temperature To, the average temperature Tav, the air flow rate F, and the current I shown in FIG. 6, when the self-heating step is executed, the cathode 11a The supplied air flow rate F rises stepwise (increase Fs), and the output current I rises up and down along with this, and rises stepwise overall (temporarily down). doing). Further, the inlet temperature Ti and the outlet temperature To do not change greatly, and the difference between them does not increase. Therefore, it can be seen that the temperature of the entire stack 11 does not vary greatly and the temperature is substantially uniform during the period from the execution of the self-heating step until the operation mode is reached. That is, when the stack 11 is activated, the operation mode is controlled so as to reach the operation mode while suppressing variations in temperature in the stack 11.

  Thus, in the fuel cell system 100 according to the second embodiment, the inlet temperature Ti and the outlet temperature To of the stack 11 are measured. Then, the temperature difference ΔT is obtained, and the rate of change of the temperature difference ΔT is further obtained. When the rate of change is large (when the upper limit value Y of the temperature difference rate of change is exceeded), the output current I of the stack 11 is obtained. Decrease. Therefore, when the temperature variation in the stack 11 becomes large, this can be suppressed. As a result, the temperature variation of the entire stack 11 can be reduced. In addition, even in a flat-stacked stack configuration where high power density can be expected, it is easy to grasp the internal temperature distribution state of the cell from upstream to downstream of the air flow, and local temperature rise due to local current density is suppressed. Meanwhile, the stack 11 can be activated quickly.

  Further, in the heating and heating step during the start-up mode, the combustion gas is supplied from the upstream side of the cathode 11a, so that the upstream side of the cathode 11a has a higher temperature than the downstream side. On the other hand, in the operation mode, since air having a low temperature is supplied from the upstream side of the cathode 11a, the temperature on the downstream side of the cathode 11a is higher than that on the upstream side. Therefore, by monitoring the temperature difference ΔT between the inlet temperature Ti and the outlet temperature To, it is possible to grasp the temperature distribution in the cell more accurately, suppress abnormal local heat generation, and increase the current in a shorter time. It becomes possible.

  Furthermore, in the second embodiment, since the temperature difference generated in the stack 11 is the temperature difference between the inlet temperature Ti and the outlet temperature To of the cathode 11a, current control based on the temperature variation of the entire stack 11 becomes possible.

  Although the fuel cell system of the present invention has been described based on the illustrated embodiment, the present invention is not limited to this, and the configuration of each part is replaced with an arbitrary configuration having the same function. Can do.

  For example, in the above-described embodiment, an example in which the inlet temperature sensor 41a provided at the inlet of the cathode 11a is used as the first temperature detecting means and the outlet temperature sensor 41b provided at the outlet of the cathode 11a is used as the second temperature detecting means. However, the installation position of the first and second temperature detection means is not limited to this, and it is sufficient that the second temperature detection means is provided on the downstream side of the first temperature detection means.

  Furthermore, in the above-described embodiment, the example in which the inlet temperature sensor 41a and the outlet temperature sensor 41b are provided has been described. However, if the process of step S60 in FIG. 5 is not employed, only the inlet temperature sensor 41a may be provided. The outlet temperature sensor 41b is not necessary.

  INDUSTRIAL APPLICABILITY The present invention can be used to suppress temperature variations when starting a fuel cell and start up in a short time.

DESCRIPTION OF SYMBOLS 11 Fuel cell stack 11a Cathode 11b Anode 12 Air blower 13 Heat exchanger 14 1st fuel pump 21 Start-up combustor 22 Valve 23 2nd fuel pump 31 Control part 41a Inlet temperature sensor (1st temperature detection means)
41b Outlet temperature sensor (second temperature detection means)
42 Current / voltage sensor (current detection means)
100 Fuel cell system

Claims (7)

A fuel cell in which reformed gas is supplied to the anode and oxidizing gas is supplied to the cathode to generate electricity;
An oxidizing gas supply means provided on the upstream side of the cathode, for supplying an oxidizing gas to the cathode;
First temperature detection means for detecting a temperature at a desired point in the cathode as a first detection temperature;
Current detecting means for detecting an output current of the fuel cell;
Control means for controlling the flow rate of the oxidizing gas supplied to the cathode, the oxidizing gas temperature, and the output current of the fuel cell,
When the fuel cell is started, the control means
A heating temperature raising step for raising the temperature of the fuel cell by supplying heated oxidizing gas to the cathode, and when the oxidizing gas is heated to a predetermined temperature by the heating temperature raising step, heating is stopped and the fuel is In the start-up mode, the fuel cell is heated to an operable temperature, and then the current output from the fuel cell is loaded. Control to switch to the operation mode supplied to the
In the self-heating step, an oxidizing gas flow rate supplied to the cathode is set to an initial oxidizing gas flow rate that is smaller than an oxidizing gas flow rate when the operation mode is shifted to, and based on the first detected temperature of the cathode, Adjust the output current of the fuel cell, and control the output current of the fuel cell to be the maximum output current at the initial oxidizing gas flow rate,
After the output current reaches the maximum output current, the flow rate of the oxidizing gas supplied to the cathode is increased, and the output current is controlled again so as to become the maximum output current for the new oxidizing gas flow rate. The fuel cell system is characterized in that the output current of the battery is repeated until the target current is reached, and when the target current is reached, the operation mode is shifted.   In the self-heating step, the control means calculates the oxidizing gas flow rate required by the cathode based on the output current of the fuel cell, and further supplies the oxidizing gas amount required by the cathode and the cathode. The oxygen utilization rate is calculated based on the amount of oxidizing gas, and when the oxygen utilization rate exceeds a preset utilization rate upper limit value, it is determined that the maximum output current for the oxidizing gas flow rate has been reached. The fuel cell system according to claim 1.   2. The control unit according to claim 1, wherein, in the self-heating step, the control unit controls the output current of the fuel cell to decrease when the first detected temperature exceeds a preset upper limit temperature. Or the fuel cell system in any one of Claim 2.   In the self-heating step, the control means controls the output current of the fuel cell to increase when the decrease rate of the first detected temperature is larger than a preset threshold value. The fuel cell system according to any one of claims 1 to 3. The fuel cell further comprises second temperature detection means provided on the cathode downstream side of the first temperature detection means and detecting the fuel cell temperature at this point as the second detection temperature,
In the self-heating step, the control means acquires the second detection temperature in addition to the first detection temperature of the fuel cell, and calculates a time change rate of a difference between the first detection temperature and the second detection temperature. 5. The calculation according to claim 1, wherein the output current of the fuel cell is controlled to decrease when the rate of time change is greater than a preset threshold value. 6. The fuel cell system described.   In the self-heating step, the control means sets an increase amount for one step of the oxidizing gas flow rate, and when increasing the oxidizing gas flow rate, the control means increases the increment amount for the one step. It controls, The fuel cell system of any one of Claims 1-5 characterized by the above-mentioned.   The first temperature detecting means is provided in the vicinity of the oxidizing gas inlet of the fuel cell to detect the cathode inlet temperature, and the second temperature detecting means is provided in the vicinity of the oxidizing gas outlet of the fuel cell to the cathode. The fuel cell system according to any one of claims 1 to 6, wherein an outlet temperature of the fuel cell is detected.

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