Patent Description:
<CIT> discloses a fuel cell system provided with a first fuel cell and a second fuel cell that are arranged in series, wherein the fuel cell system is configured to increase a flow rate of air supplied to a cathode when a temperature of the second fuel cell rises, and, when a temperature of the first fuel cell rises, to either increase the flow rate of air supplied to the cathode or increase a flow rate of air in a bypass flow path, which bypasses the second fuel cell and connects to the first fuel cell, thereby adjusting the temperatures of the first and second fuel cells.

However, when implementing the configuration described above, it is necessary to detect the temperature of the first fuel cell and detect the temperature of the second fuel cell. Typically, when detecting the temperature of a fuel cell, it is a temperature at an outlet of a cathode in the fuel cell that is detected. However, arranging a sensor at a cathode outlet in the second fuel cell, i.e., between the first and second fuel cells, results in an increase in size in the system or an increase in cost.

It is accordingly an object of the present invention to provide a fuel cell system that includes fuel cells arranged in series, the fuel cell system making it possible to adjust temperatures of the fuel cells using a simple configuration.

According to one aspect of the present invention, there is provided a fuel cell system including: a fuel cell module that includes a first fuel cell and a second fuel cell arranged in series, a fuel gas circulating in a direction from the first fuel cell toward the second fuel cell, and an oxidant gas circulating in a direction from the second fuel cell toward the first fuel cell; and an oxidant gas temperature adjustment device for adjusting a temperature of the oxidant gas supplied to the fuel cell module, it being possible, at a minimum, for the first fuel cell to reform the fuel gas, wherein the fuel cell system includes a temperature control means for setting a target temperature of the oxidant gas supplied to the fuel cell module such that a temperature of the first fuel cell and a temperature of the second fuel cell do not exceed a prescribed upper-limit temperature and controlling the oxidant gas temperature adjustment device based on the target temperature, and the temperature control means calculating the target temperature T as T=Tmax-ΔT1-ΔT2 (ΔT1><NUM>), T=Tmax-ΔT2 (ΔT1≤<NUM>), or T=Tmax (ΔT2<<NUM> and ΔT1 + ΔT2<<NUM>), where T is the target temperature, Tmax is the upper-limit temperature, ΔT1 is a difference obtained by subtracting a temperature at an inlet of the first fuel cell from a temperature at an outlet thereof, and ΔT2 is a difference obtained by subtracting a temperature at an inlet of the second fuel cell from a temperature at an outlet thereof.

An embodiment of the present invention is described below with reference to the accompanying drawings.

<FIG> includes a block diagram for illustrating the present invention in a simple manner, and graphs showing temperature distributions within a fuel cell stack.

As shown in <FIG>, a fuel cell system according to the present invention is provided with a fuel cell module <NUM> in which two fuel cell stacks, specifically a first fuel cell stack <NUM> (first fuel cell, STK1) and a second fuel cell stack <NUM> (second fuel cell, STK2), are arranged in series in a gas flow path, the fuel cell module <NUM> being configured such that an anode gas (fuel gas) flows from the first fuel cell stack <NUM> to the second fuel cell stack <NUM>, and such that a cathode gas (oxidant gas) flows from the second fuel cell stack <NUM> to the first fuel cell stack <NUM>. Specifically, in the fuel cell module <NUM>, the anode gas and the cathode gas circulate in opposite directions between the first fuel cell stack <NUM> and the second fuel cell stack <NUM>; i.e., between the two stacks.

In the fuel cell module <NUM>, an anode inlet in the first fuel cell stack <NUM> serves as an inlet for the anode gas, and a cathode inlet in the second fuel cell stack <NUM> serves as an inlet for the cathode gas. An anode outlet in the first fuel cell stack <NUM> and an anode inlet in the second fuel cell stack <NUM> are connected to one another, and a cathode outlet in the second fuel cell stack <NUM> and a cathode inlet in the first fuel cell stack <NUM> are connected to one another.

Additionally, the fuel cell system according to the present invention is provided with a temperature adjustment device <NUM> (oxidant gas temperature adjustment device) for adjusting a temperature of the cathode gas supplied to the fuel cell module <NUM>; this device is described in detail later.

In the fuel cell system according to the present invention, a catalyst capable of reforming the fuel gas is arranged in the first fuel cell stack <NUM> (and in the second fuel cell stack <NUM>). The first fuel cell stack <NUM> reforms the fuel gas into a hydrogen-containing anode gas (reformed fuel), generates electric power by using the reformed anode gas and the cathode gas supplied from the second fuel cell stack <NUM>, and supplies any surplus anode gas to the second fuel cell stack <NUM>. Additionally, the cathode gas used in an electric power generation reaction is discharged to a combustor <NUM> constituting the temperature adjustment device <NUM> as a cathode off-gas.

The second fuel cell stack <NUM> generates electric power by using the anode gas supplied from the first fuel cell stack <NUM> and a cathode gas supplied from the temperature adjustment device <NUM>. The second fuel cell stack <NUM> also supplies the cathode gas used in electric power generation to the first fuel cell stack <NUM> and supplies the anode gas used in electric power generation to the combustor <NUM> as an anode off-gas.

In the first fuel cell stack <NUM> and the second fuel cell stack <NUM>, electric power generation efficiency improves as a temperature is raised. However, for heat resistance and other reasons, an upper-limit temperature (e.g., <NUM>) is set for the first fuel cell stack <NUM> and the second fuel cell stack <NUM>.

Thus, it is necessary to adjust the temperature of the cathode gas such that the temperature of the first fuel cell stack <NUM> (temperature at an outlet of a cathode in the first fuel cell stack <NUM>) and the temperature of the second fuel cell stack <NUM> (temperature at an outlet of a cathode in the second fuel cell stack <NUM>) do not exceed the aforementioned upper-limit temperature and such that the electric power generation reaction is carried out adequately.

In the present invention, taking, e.g., a temperature at an inlet of the cathode (temperature of cathode gas at the cathode inlet) of the second fuel cell stack <NUM> as a target temperature, the temperature adjustment device <NUM> is controlled according to the target temperature such that the temperatures of the first fuel cell stack <NUM> and the second fuel cell stack <NUM> do not exceed the upper-limit temperature. This makes it possible to use fewer temperature-sensing means (temperature sensor <NUM> in <FIG>) and reduce the system in size. The target temperature is computed by a target temperature computation unit <NUM> (<FIG>) that shall be described later.

The electric power generation reactions occurring in the first fuel cell stack <NUM> and the second fuel cell stack <NUM> are exothermic, but a reforming reaction occurring in the first fuel cell stack <NUM> is endothermic. The cathode gas is heated using the temperature adjustment device <NUM>, but depending on, inter alia, the magnitude of the electric power generation reactions and the reforming reaction, there are not only cases where the cathode gas is heated in the first fuel cell stack <NUM> and the second fuel cell stack <NUM> but also cases where the cathode gas is cooled. Therefore, as indicated in <FIG>, there are cases where a temperature distribution of the cathode gas attains states as in A through D as described below.

A is a case in which a load (amount of electric power generated) on the first fuel cell stack <NUM> and the second fuel cell stack <NUM> is high, an amount of heat generated through the electric power generation reactions is sufficiently greater than an amount of heat absorbed through the reforming reaction in the first fuel cell stack <NUM>, and furthermore, the temperatures (temperatures at the outlets of the cathodes) of the first fuel cell stack <NUM> and the second fuel cell stack <NUM> are greater than the temperature of the cathode gas supplied from the temperature adjustment device <NUM>. In this case, the temperature of the cathode gas supplied from the temperature adjustment device <NUM> rises commensurately with proximity to a downstream side of a cathode gas flow path and reaches a highest temperature at the cathode outlet in the first fuel cell stack <NUM>.

In the case in A, ΔT1><NUM> and ΔT2><NUM>, where ΔT1 is a difference obtained by subtracting the temperature at the inlet of the first fuel cell stack <NUM> from the temperature at the outlet thereof, and ΔT2 is a difference obtained by subtracting the temperature at the inlet of the second fuel cell stack <NUM> from the temperature at the outlet thereof.

Therefore, the target temperature computation unit <NUM> (<FIG>) calculates a target temperature T according to T=Tmax-ΔT1-ΔT2, where T is the target temperature and Tmax is the upper-limit temperature, thereby making it possible to set the temperature at the outlet of the cathode in the first fuel cell stack <NUM>, which is the highest temperature, to the upper-limit temperature Tmax.

B is a case in which the load (amount of electric power generated) on the first fuel cell stack <NUM> and the second fuel cell stack <NUM> is lower than that in the case of A, e.g., in which the amount of heat absorbed through the reforming reaction in the first fuel cell stack <NUM> and the amount of heat generated through the electric power generation reactions are approximately equal. In this case, the temperature of the second fuel cell stack <NUM> is greater than the temperature of the cathode gas supplied from the temperature adjustment device <NUM>, but the temperature of the first fuel cell stack <NUM> is equal to or less than the temperature of the cathode gas discharged from the second fuel cell stack <NUM>. Thus, the temperature of the cathode gas reaches a highest temperature at the cathode outlet in the second fuel cell stack <NUM>. In this circumstance, ΔT1≤<NUM> and ΔT2><NUM>.

Therefore, the target temperature computation unit <NUM> (<FIG>) calculates the target temperature T according to T=Tmax-ΔT2, thereby making it possible to set the temperature at the outlet of the cathode in the second fuel cell stack <NUM> (i.e., the temperature at the inlet of the cathode in the first fuel cell stack <NUM>), which is the highest temperature, to the upper-limit temperature Tmax.

C is a case in which the load (amount of electric power generated) on the first fuel cell stack <NUM> and the second fuel cell stack <NUM> abruptly decreases and the temperature of the second fuel cell stack <NUM> falls below the temperature of the cathode gas supplied from the temperature adjustment device <NUM>. In this case, a temperature distribution in which ΔT2<<NUM> and ΔT1+ΔT2<<NUM> can be produced. This includes cases in which ΔT1 satisfies the relationship ΔT1<<NUM>, or cases in which ΔT1 satisfies the relationship ΔT1><NUM> and the absolute value thereof is less than ΔT2. In the case in C, the temperature of the cathode gas reaches a highest temperature at the cathode inlet in the second fuel cell stack <NUM>.

Therefore, the target temperature computation unit <NUM> (<FIG>) calculates the target temperature T according to T=Tmax, thereby making it possible to set the temperature at the inlet of the cathode in the second fuel cell stack <NUM>, which is the highest temperature, to the upper-limit temperature Tmax.

D is a scenario similar to that in C, occurring in a case in which, for example, a heat capacity of the first fuel cell stack <NUM> is greater than a heat capacity of the second fuel cell stack <NUM> and there is little reduction in the temperature of the first fuel cell stack <NUM>. In this case, a temperature distribution in which ΔT2<<NUM> but ΔT1+ΔT2≥<NUM> is produced. In the case in D, the temperature at the outlet of the cathode in the first fuel cell stack <NUM> is a highest temperature.

Therefore, the target temperature computation unit <NUM> (<FIG>) calculates the target temperature T according to T=Tmax-ΔT1-ΔT2, thereby making it possible to set the temperature at the outlet of the cathode in the first fuel cell stack <NUM>, which is the highest temperature, to the upper-limit temperature Tmax. Because ΔT1><NUM>, the conditions for setting the target temperature T in D are the same as in A.

ΔT1 and ΔT2 can be calculated as shall be described later. Therefore, the position having the highest temperature in the first fuel cell stack <NUM> and the second fuel cell stack <NUM> is assessed by calculating ΔT1 and ΔT2 in accordance with, inter alia, the load on the first fuel cell stack <NUM> and the second fuel cell stack <NUM>, and an equation for computing the target temperature T is switched such that the temperature at the aforementioned position can be set to the upper-limit temperature Tmax. This makes it possible to prevent excessive temperatures and realize an increase in temperature in the first fuel cell stack <NUM> and the second fuel cell stack <NUM>.

As described above, the target temperature computation unit <NUM> (<FIG>) is capable of calculating the target temperature T according to the circumstances, as indicated in formula <NUM> below.

It is also possible to employ the temperature at the outlet of the cathode (the temperature of the cathode gas at the cathode) in the first fuel cell stack <NUM> as the target temperature T. In this case, the target temperature computation unit <NUM> (<FIG>) is capable of calculating the target temperature T according to the circumstances, as indicated in formula <NUM> below, with reference to <FIG>.

<FIG> is a block diagram showing a principal configuration of the fuel cell system according to the present embodiment.

The fuel cell system according to the present embodiment is configured from a fuel supply sub-system for supplying the anode gas to the fuel cell module <NUM>, an air supply sub-system for supplying air (cathode gas) to the fuel cell module <NUM>, a combustion sub-system for combusting the anode off-gas (anode gas) and the cathode off-gas (cathode gas) discharged from the fuel cell module <NUM>, a drive sub-system for extracting electric power from the fuel cell module <NUM> and obtaining motive power, and a control sub-system for controlling the entire system. The fuel cell system is mainly mounted in a vehicle (electric-powered vehicle).

The fuel supply sub-system includes a tank <NUM> (TANK), an injector <NUM> (INJ1), and an injector <NUM> (INJ2). The air supply sub-system includes a blower <NUM> (BLW), a bypass valve <NUM> (BYP VAL), a heat exchanger <NUM> (HEX), and a variable valve <NUM> (VAL). The combustion sub-system includes the combustor <NUM> (CMB). The bypass valve <NUM>, the heat exchanger <NUM>, the combustor <NUM>, and the injector <NUM> constitute the temperature adjustment device <NUM>.

The drive sub-system includes a DC/DC converter <NUM> (CONV), a battery <NUM> (BATT), and a drive motor <NUM> (M). The control sub-system includes a control unit <NUM> (CONT) for controlling the entire system.

The first fuel cell stack <NUM> and the second fuel cell stack <NUM> constituting the fuel cell module <NUM> are solid oxide fuel cells (SOFCs), are each provided with an electrolyte layer formed from a solid oxide such as a ceramic material, and are formed by stacking cells obtained by sandwiching the electrolyte layer between an anode (fuel electrode) by which the anode gas (reformed gas) is supplied and a cathode (air electrode) by which oxygen-containing air serving as the cathode gas (oxidant gas) is supplied.

A catalyst for reforming fuel for reformation, which is supplied from the injector <NUM> side, into the hydrogen-containing anode gas (reformed fuel gas) is arranged in the electrolyte layer of the first fuel cell stack <NUM> (and that of the second fuel cell stack <NUM>).

In the first fuel cell stack <NUM>, hydrogen included in the anode gas and oxygen in the cathode gas are reacted to generate electric power, any surplus anode gas is supplied to the second fuel cell stack <NUM>, and the cathode off-gas is discharged to the combustor <NUM>.

In the second fuel cell stack <NUM>, the anode gas supplied from the first fuel cell stack <NUM> and the cathode gas supplied from the blower <NUM> via the temperature adjustment device <NUM> are reacted to generate electric power, any surplus cathode gas is supplied to the first fuel cell stack <NUM>, and the anode off-gas is discharged to the combustor <NUM>.

The anode includes not only an anode electrode in the first fuel cell stack <NUM> and the second fuel cell stack <NUM> but also an internal flow path for supplying the anode gas to the anode electrode and an internal flow path for discharging post-reaction anode off-gas in the anode electrode. Similarly, the cathode includes not only a cathode electrode in the first fuel cell stack <NUM> and the second fuel cell stack <NUM> but also an internal flow path for supplying the cathode gas to the cathode electrode and an internal flow path for discharging post-reaction cathode off-gas in the cathode electrode.

In the fuel supply sub-system, the tank <NUM> stores fuel (gas) composed of methane, or a natural gas mainly composed of methane, under high pressure. The fuel is supplied to the injector <NUM> as fuel for reformation, and also is supplied to the injector <NUM> as additional fuel for combustion.

The fuel supply sub-system has a fuel flow path <NUM> (main flow path) for supplying the fuel for reformation from the tank <NUM> to the anode in the first fuel cell stack <NUM> via the injector <NUM>.

Additionally, a sub-flow path (not shown) branches from the fuel flow path <NUM> and connects to the injector <NUM>.

The injectors <NUM>, <NUM> are each provided with: a nozzle body (not shown) into which the fuel is introduced under pressure; a plunger rod (not shown) biased in a direction for closing a fuel injection orifice (not shown), which is located at a distal end of the nozzle body; and a solenoid (not shown) for causing the plunger rod to move in a direction opposing the biasing direction.

In the injectors <NUM>, <NUM>, applying a command signal (electric current) to the solenoid results in the solenoid being driven to cause the plunger rod to move in the opposing direction, whereby the plunger rod opens the fuel injection orifice, and the fuel is injected. Stopping the command signal (electric current) stops the driving of the solenoid, whereby the plunger rod moves due to biasing force and closes the fuel injection orifice, and injection of the fuel is stopped.

Additionally, in the injectors <NUM>, <NUM>, a duty ratio at which the fuel injection orifice is opened/closed depends on a duty ratio at which the command signal (electric current) is turned on/off. Thus, the injectors <NUM>, <NUM> are capable of adjusting a flow rate of fuel being injected by adjusting the duty ratio of the command signal (electric current).

The air supply sub-system has an air flow path <NUM> for supplying the cathode gas (air) to the cathode of the second fuel cell stack <NUM>. The blower <NUM>, the bypass valve <NUM>, and the heat exchanger <NUM> are arranged in the air flow path <NUM> in the stated order from the upstream side.

The blower <NUM> (oxidant gas supply source) takes in outside air and supplies the air (cathode gas) to the air flow path <NUM>, etc..

The heat exchanger <NUM> communicates with the cathode of the second fuel cell stack <NUM> via the air flow path <NUM>, carries out heat exchange (heating) of the cathode gas by using combustion gas discharged from the combustor <NUM>, and supplies the heat-exchanged cathode gas to the cathode of the second fuel cell stack <NUM>. The heat-exchanged combustion gas is discharged to outside.

The bypass valve <NUM> (flow rate adjustment means) is arranged at a position in the air flow path <NUM> that is upstream from the heat exchanger <NUM>. An upstream side of the bypass valve <NUM> is connected to the blower <NUM>, and a downstream side of the bypass valve <NUM> is connected to the air flow path <NUM> (heat exchanger <NUM>) and a bypass flow path <NUM>. Adjusting an opening degree of the bypass valve <NUM> adjusts a proportion of flow rates of the cathode gas (air) in the air flow path <NUM> and the bypass flow path <NUM>. The bypass flow path <NUM> bypasses the heat exchanger <NUM> and merges with the air flow path <NUM> at a position in the air flow path <NUM> that is between the heat exchanger <NUM> and the cathode of the second fuel cell stack <NUM>.

Additionally, a supply path <NUM> branches from a position in the air flow path <NUM> that is upstream from the bypass valve <NUM>. The supply path <NUM> merges with the fuel flow path <NUM> at a position in the fuel flow path <NUM> that is between the first fuel cell stack <NUM> and the injector <NUM>.

The variable valve <NUM> is arranged in the supply path <NUM> and adjusts the flow rate of air (oxygen) circulating through the supply path <NUM>. When the air (oxygen) is supplied to the anode of the first fuel cell stack <NUM>, a partial oxidation reaction (heat generation reaction) is caused to occur between the anode gas and the oxygen via a catalyst arranged in the first fuel cell stack <NUM> (this catalyst optionally being the same as the catalyst that carries out the reformation reaction), and the temperature of the first fuel cell stack <NUM> can be raised. This makes it possible to avoid a circumstance in which a partial-oxidation-reaction air flow rate control unit <NUM>, which shall be described later, reduces the temperature of the first fuel cell stack <NUM> below a lower-limit value for a temperature at which the reformation reaction can be carried out. In this procedure, the blower <NUM> functions as an oxygen supply source.

The supply path <NUM> furthermore can also be connected to the anode inlet in the second fuel cell stack <NUM> and can enable a partial oxidation reaction in the second fuel cell stack <NUM> as well. When the partial oxidation reaction is unnecessary in the first fuel cell stack <NUM> and the second fuel cell stack <NUM>, the supply path <NUM> and the variable valve <NUM> (as well as the partial-oxidation-reaction air flow rate control unit <NUM> that shall be described later) can be omitted.

In the combustion sub-system, the combustor <NUM> communicates with the anode outlet in the first fuel cell stack <NUM> and the cathode outlet in the second fuel cell stack <NUM>. A mixed gas of the anode off-gas and the cathode off-gas is introduced into the combustor <NUM>. The combustor <NUM> catalytically combusts the mixed gas and produces the combustion gas. The combustor <NUM> is provided with a catalyst (not shown) for carrying out the catalytic combustion, and a heater (not shown) for warming the catalyst (not shown) until the fuel reaches a temperature at which combustion is possible. Additional fuel is also supplied to the combustor <NUM> from the injector <NUM>. Thus, the combustor <NUM> prompts combustion of the additional fuel while combusting the mixed gas and combusting the combustion gas, thereby making it possible to further increase the temperature of the combustion gas.

In the temperature adjustment device <NUM> (bypass valve <NUM>, heat exchanger <NUM>, combustor <NUM>, and injector <NUM>), supplying additional fuel to the combustor <NUM> and raising the temperature of the combustion gas makes it possible to raise the temperature of the cathode gas circulating through the heat exchanger <NUM>, or increasing/reducing the flow rate of the cathode gas flowing through the bypass flow path <NUM> makes it possible to reduce/increase the temperature of the cathode gas supplied to the fuel cell module <NUM>.

The temperature sensor <NUM> detects the temperature at the inlet of the cathode in the second fuel cell stack <NUM>. The temperature sensor <NUM> can be disposed at the cathode outlet in the first fuel cell stack <NUM>.

A flow rate sensor <NUM> detects a flow rate of the air (cathode gas) taken in by the blower <NUM>.

A first voltage sensor <NUM> detects an output voltage of the first fuel cell stack <NUM>.

A second voltage sensor <NUM> detects an output voltage of the second fuel cell stack <NUM>.

The first fuel cell stack <NUM> and the second fuel cell stack <NUM> are electrically connected in series.

A current sensor <NUM> detects an output current flowing between the first fuel cell stack <NUM> and second fuel cell stack <NUM>, which are connected in series, and the DC/DC converter <NUM>.

The DC/DC converter <NUM> is connected to a series circuit including the first fuel cell stack <NUM> and the second fuel cell stack <NUM>, the DC/DC converter <NUM> boosting an output voltage of the series circuit and supplying electric power to the battery <NUM> or the drive motor <NUM>.

It is also possible to extract electric power from the first fuel cell stack <NUM> and the second fuel cell stack <NUM> individually, without the first fuel cell stack <NUM> and the second fuel cell stack <NUM> being electrically connected. In this case, it is possible to prepare two DC/DC converters <NUM>, one of the DC/DC converters <NUM> being connected to the first fuel cell stack <NUM> and the other of the DC/DC converters <NUM> being connected to the second fuel cell stack <NUM>, and to furthermore adopt a configuration in which the two DC/DC converters <NUM> are connected to the battery <NUM> and the drive motor <NUM>.

The battery <NUM> is capable of charging using electric power supplied from the DC/DC converter <NUM> and supplying electric power to the drive motor <NUM>.

The drive motor <NUM> serves as a motive power source for a vehicle and is connected to the battery <NUM> and the DC/DC converter <NUM> via an inverter (not shown). While the vehicle is decelerating, the drive motor <NUM> generates regenerative electric power, the drive motor <NUM> being capable of charging the battery <NUM> using the regenerative electric power.

The control unit <NUM> is configured from: a general-purpose electronic circuit including a microcomputer, a microprocessor, and a CPU; and peripheral accessories. The control unit <NUM> executes a specific program, thereby executing a process for controlling the fuel cell system.

The control unit <NUM> includes an extracted-electric-power control unit <NUM> (POWER CONT), an air flow rate control unit <NUM> (AIRFLOW CONT), the partial-oxidation-reaction air flow rate control unit <NUM> (POXAIRFLOW CONT), a fuel flow rate control unit <NUM> (FUELFLOW CONT), and an air temperature control unit <NUM> (AIRTEMP CONT). The control unit <NUM> is also provided with a computation unit (not shown) for calculating a target extracted electric power (POWER) based on a state of charge (SOC) of the battery <NUM> and a drive request (accelerator position) from a driver.

The control unit <NUM> is capable of controlling startup and stopping of the fuel cell system, but these controls are prior-art features and therefore are not described here.

The extracted-electric-power control unit <NUM>, upon receiving input of information pertaining to a target extracted electric power, outputs a command signal to the DC/DC converter <NUM> such that electric power extracted from the fuel cell module <NUM> reaches the target extracted electric power. The electric power extracted from the fuel cell module <NUM> is supplied to the battery <NUM> or the drive motor <NUM>.

<FIG> is a map showing a relationship between a target air flow rate and the target extracted electric power. <FIG> is a map showing a relationship between a target fuel flow rate and the target extracted electric power. <FIG> is a map showing a relationship between a target partial-oxidation-reaction air flow rate and the target extracted electric power.

The air flow rate control unit <NUM>, upon receiving input of information pertaining to the target extracted electric power, outputs, to the blower <NUM>, a command signal for attaining a target air flow rate. The target air flow rate is an air flow rate with which it is possible to ensure both an oxygen amount required for the fuel cell module <NUM> to generate the target extracted electric power and an oxygen amount required for the anode off-gas discharged from the fuel cell module <NUM> to be combusted using the combustor <NUM>.

The relationship between the target extracted electric power and the target air flow rate is set in advance according to experimentation or theoretical investigation. The air flow rate control unit <NUM> can thereby be provided with a map in which the target extracted electric power (POWER) is employed as an input value and the target air flow rate (AIRFLOW) is employed as an output value, as shown in <FIG>. This makes it possible for the air flow rate control unit <NUM> to determine a target air flow rate that corresponds to the target extracted electric power.

The fuel flow rate control unit <NUM>, upon receiving input of information pertaining to the target extracted electric power, outputs, to the injector <NUM>, a command signal for attaining a target fuel flow rate. The target fuel flow rate is a fuel flow rate that ensures a fuel usage ratio with which it is possible to stably generate electric power in the fuel cell module <NUM> in order to generate the target extracted electric power.

The relationship between the target extracted electric power and the target fuel flow rate is set in advance according to experimentation or theoretical investigation. The fuel flow rate control unit <NUM> can thereby be provided with a map in which the target extracted electric power (POWER) is employed as an input value and the target fuel flow rate (FUELFLOW) is employed as an output value, as shown in <FIG>. This makes it possible for the fuel flow rate control unit <NUM> to determine a target fuel flow rate that corresponds to the target extracted electric power.

The partial-oxidation-reaction air flow rate control unit <NUM>, upon receiving input of information pertaining to the target extracted electric power, outputs, to the variable valve <NUM>, a command signal for reaching an opening degree in the variable valve <NUM> such that a target partial-oxidation-reaction air flow rate is attained. The target partial-oxidation-reaction air flow rate is a flow rate of air supplied to the first fuel cell stack <NUM> such that the temperature of the first fuel cell stack <NUM> does not become too low (e.g., such that the aforementioned temperature does not fall below a lower-limit temperature at which the reformation reaction can be carried out) based on the target extracted electric power.

The relationship between the target extracted electric power and the target partial-oxidation-reaction air flow rate is set in advance according to experimentation or theoretical investigation. The partial-oxidation-reaction air flow rate control unit <NUM> can thereby be provided with a map in which the target extracted electric power (POWER) is employed as an input value and the target partial-oxidation-reaction air flow rate (POXAIRFLOW) is employed as an output value, as shown in <FIG>. This makes it possible for the partial-oxidation-reaction air flow rate control unit <NUM> to determine a target partial-oxidation-reaction air flow rate that corresponds to the target extracted electric power.

The air temperature control unit <NUM> (temperature control means) includes a first temperature difference computation unit <NUM> (STK1ΔT1 COMP), a second temperature difference computation unit <NUM> (STK2ΔT2 COMP), the target temperature computation unit <NUM> (AIRTEMP COMP), and a drive control unit <NUM> (DRIVE CONT).

The first temperature difference computation unit <NUM>, upon receiving input of various information pertaining to the target fuel flow rate, the target partial-oxidation-reaction air flow rate, the flow rate of air detected by the flow rate sensor <NUM>, the voltage sensed by the first voltage sensor <NUM>, the voltage sensed by the second voltage sensor <NUM>, and the electric current detected by the current sensor <NUM>, calculates, as in formula <NUM> below, a difference ΔT1 obtained by subtracting the temperature at the inlet of the cathode in the first fuel cell stack <NUM> from the temperature at the outlet of the cathode therein.

In the formula, Pp<NUM> (W) is the amount of heat generated through generation of electric power in the first fuel cell stack <NUM>. Pp<NUM> is calculated using a map in which an amount of heat generated that corresponds to the electric power generated in the first fuel cell stack <NUM> is obtained in advance through experimentation or theoretical investigations (map in which the amount of electric power generated is employed as an input value and the amount of heat generated is employed as an output value). The amount of electric power generated in the first fuel cell stack <NUM> is obtained by multiplying the voltage sensed by the first voltage sensor <NUM> (more precisely, the amount of reduction from an open voltage of the first fuel cell stack <NUM>) and the electric current detected by the current sensor <NUM>.

Pr<NUM> (W) is the amount of heat absorbed through the reformation reaction occurring in the first fuel cell stack <NUM>. A reaction formula for the reformation reaction is CH<NUM>+<NUM><NUM>O→<NUM><NUM>+CO<NUM>-<NUM> (kJ/mol) and is proportional to a CH<NUM> flow rate (mol/s). Additionally, in the partial oxidation reaction, <NUM> mol of CH<NUM> is eliminated per mole of oxygen. Thus, the flow rate of CH<NUM> after the partial oxidation reaction is a difference obtained by subtracting double a flow rate of oxygen within the target partial-oxidation-reaction air flow rate (i.e., the flow rate of CH<NUM> eliminated in the partial oxidation reaction) from the flow rate of CH<NUM> before the partial oxidation reaction.

Therefore, the first temperature difference computation unit <NUM> calculates Pr1 according to, e.g., the formula (amount of heat absorbed through reformation reaction per mole of fuel for reformation (<NUM> (kJ)))×((target fuel flow rate (mol/s))-(oxygen flow rate (mol/s) within target partial-oxidation-reaction air flow rate)×<NUM>). The oxygen flow rate is computed as being <NUM>% of the air flow rate.

However, it is not necessarily the case that all of the fuel for reformation will be reformed; an amount that is reformed can change depending on a reaction speed of the reformation reaction. Thus, because the reformation amount can also vary in accordance with a pressure or flow rate of the fuel for reformation, it is possible for these amounts to be detected and Pr corrected according to the results therefrom, or for a composition of the anode gas at the anode inlet and anode outlet in the first fuel cell stack <NUM> to be detected and Pr corrected according to the results therefrom.

Pd<NUM> (W) is a fixed amount of heat released to the outside from elsewhere besides the anode gas and the cathode gas in the first fuel cell stack <NUM>. Pd1 is calculated using a map obtained in advance through experimentation or theoretical investigation.

Po1 is an amount of heat generated when the partial oxidation reaction is carried out in the first fuel cell stack <NUM>. A reaction formula for the partial oxidation reaction is 2CH<NUM>+O<NUM>→CO+<NUM><NUM>+<NUM> (kJ/mol) and is proportional to an oxygen flow rate (mol/s). Thus, the first temperature difference computation unit <NUM> calculates Po1 according to, e.g., the formula (amount of heat generated through partial oxidation reaction per mole of oxygen eliminated (<NUM> (kJ)))×(oxygen flow rate (mol/s) within target partial-oxidation-reaction air flow rate).

σ [kg/m<NUM>] is an air density, and c (kJ/kg·°C) is a specific heat of the air. Fixed values derived in advance through experimentation or theoretical investigation are used for these variables with respect to the air supplied to the fuel cell module <NUM>. It is also possible to detect a pressure and temperature of the air and compute the aforementioned variables through estimation.

Q (m<NUM>/s) is the flow rate of the air taken in by the blower <NUM>. A value sensed by the flow rate sensor <NUM> is used for this variable.

Upon calculating ΔT1, the first temperature difference computation unit <NUM> outputs ΔT1 to the target temperature computation unit <NUM>.

The second temperature difference computation unit <NUM>, upon receiving input of the same information as that inputted to the first temperature difference computation unit <NUM>, calculates, as in formula <NUM> below, a difference ΔT2 obtained by subtracting the temperature at the inlet of the cathode in the second fuel cell stack <NUM> from the temperature at the outlet of the cathode therein, in the same manner as with ΔT1.

Pp<NUM> (W) is the amount of heat generated through generation of electric power in the second fuel cell stack <NUM>. Pp<NUM> is calculated through the same method as Pp<NUM>. Pd<NUM> (W) is a fixed amount of heat released to the outside from elsewhere besides the anode gas and the cathode gas in the second fuel cell stack <NUM>. Pd2 is calculated through the same method as Pd1.

When the reformation reaction is not carried out in the second fuel cell stack <NUM>, Pr2 (W) is zero, but when the reformation reaction is carried out, Pr2 is calculated through the same method as Pr1. When the partial oxidation reaction is not carried out in the second fuel cell stack <NUM>, Po2 (W) is zero, but when the partial oxidation reaction is carried out, Po2 is calculated through the same method as Po1.

Upon calculating ΔT2, the second temperature difference computation unit <NUM> outputs ΔT2 to the target temperature computation unit <NUM>.

Formulas <NUM> and <NUM> above depend on the target extracted electric power. Therefore, the first temperature difference computation unit <NUM> can be provided with a map in which the target extracted electric power is employed as an input value and ΔT1 is employed as an output value, whereby the first temperature difference computation unit <NUM> can calculate ΔT1 that corresponds to the target extracted electric power. Similarly, the second temperature difference computation unit <NUM> can be provided with a map in which the target extracted electric power is employed as an input value and ΔT2 is employed as an output value, whereby the second temperature difference computation unit <NUM> can calculate ΔT2 corresponding to the target extracted electric power.

The target temperature computation unit <NUM>, upon receiving input of information pertaining to ΔT1 and ΔT2, calculates the target temperature T of the air (cathode gas) in accordance with formula <NUM> (or <NUM>) above and outputs the target temperature T to the drive control unit <NUM>.

Information pertaining to the target temperature T and information pertaining to the temperature sensed by the temperature sensor <NUM> are inputted to the drive control unit <NUM>. The drive control unit <NUM> then outputs a command signal to the injector <NUM> and the bypass valve <NUM> such that, for example, the temperature of the cathode gas reaches the target temperature T through PI control using, e.g., a difference obtained by subtracting the sensed temperature from the target temperature T. For example, when the sensed temperature is lower than the target temperature T, a command signal for increasing an amount of additional fuel (raising a duty ratio) is outputted to the injector <NUM> (additional fuel supply means) in order to raise the temperature of the cathode gas. Conversely, when the sensed temperature is greater than the target temperature T, a command signal for controlling an opening degree is outputted to the bypass valve <NUM> in order to increase the flow rate of the cathode gas bypassing the heat exchanger <NUM> and reduce the temperature of the cathode gas. Either of the command signal to the injector <NUM> and the command signal to the bypass valve <NUM> can be outputted alone, or both of these command signals can be outputted simultaneously.

<FIG> is a control flow chart for the air temperature control unit <NUM>. A flow of controls in the air temperature control unit <NUM> is described below.

First, in step S1, the first temperature difference computation unit <NUM> calculates the difference ΔT1 obtained by subtracting the temperature at the inlet of the cathode in the first fuel cell stack <NUM> from the temperature at the outlet of the cathode therein.

In step S2, the second temperature difference computation unit <NUM> calculates the difference ΔT2 obtained by subtracting the temperature at the inlet of the cathode in the second fuel cell stack <NUM> from the temperature at the outlet of the cathode therein. Steps S1 and S2 also can be performed in opposite order or can be performed simultaneously.

In step S3, the target temperature computation unit <NUM> assesses whether ΔT2<<NUM> and ΔT1+ΔT2<<NUM>. If the result of assessment is YES, the process advances to step S4; if the result of assessment is NO, the process advances to step S5.

In step S4, the target temperature computation unit <NUM> assesses that the cathode inlet (temperature of the cathode gas at the inlet) in the second fuel cell stack <NUM> of the fuel cell module <NUM> has the highest temperature, calculates the target temperature T according to T=Tmax, and outputs the target temperature T to the drive control unit <NUM>.

In step S5, the target temperature computation unit <NUM> assesses whether ΔT1><NUM>. If the result of assessment is YES, the process advances to step S6; if the result of assessment is NO, the process advances to step S7.

In step S6, the target temperature computation unit <NUM> assesses that the cathode outlet (temperature of the cathode gas at the outlet) in the first fuel cell stack <NUM> of the fuel cell module <NUM> has the highest temperature, calculates the target temperature T according to T=Tmax-ΔT1-ΔT2, and outputs the target temperature T to the drive control unit <NUM>.

In step S7, the target temperature computation unit <NUM> assesses that the cathode outlet (temperature of the cathode gas at the outlet) in the second fuel cell stack <NUM> of the fuel cell module <NUM> has the highest temperature, calculates the target temperature T according to T=Tmax-ΔT2, and outputs the target temperature T to the drive control unit <NUM>.

The air temperature control unit <NUM> repeats the aforementioned steps S1 to S7 for as long as the fuel cell module <NUM> continues generating electric power.

As described above, the temperature sensor <NUM> is arranged at the cathode outlet in the first fuel cell stack <NUM>, and the target temperature can be calculated according to the same control flow even when the temperature at the outlet of the cathode is employed as the target temperature. However, the target temperature computation unit <NUM> calculates the target temperature in accordance with formula <NUM> in that instance.

Specifically, in step S4, the target temperature computation unit <NUM> calculates the target temperature T according to T=Tmax+ΔT1+ΔT2. Additionally, in step S6, the target temperature computation unit <NUM> calculates the target temperature T according to T=Tmax. Furthermore, in step S7, the target temperature computation unit <NUM> calculates the target temperature T according to T=Tmax+ΔT1.

As described above, the fuel cell system according to the present embodiment is a fuel cell system including: a fuel cell module <NUM> that includes a first fuel cell (first fuel cell stack <NUM>) and a second fuel cell (second fuel cell stack <NUM>) arranged in series, a fuel gas (anode gas) circulating in a direction from the first fuel cell toward the second fuel cell, and an oxidant gas (cathode gas) circulating in a direction from the second fuel cell (second fuel cell stack <NUM>) toward the first fuel cell (first fuel cell stack <NUM>); and an oxidant gas temperature adjustment device (temperature adjustment device <NUM>) for adjusting a temperature of the oxidant gas (cathode gas) supplied to the fuel cell module <NUM>, it being possible, at a minimum, for the first fuel cell (first fuel cell stack <NUM>) to reform the fuel gas (anode gas), wherein the fuel cell system includes a temperature control means (air temperature control unit <NUM>) for setting a target temperature of the oxidant gas (cathode gas) supplied to the fuel cell module <NUM> such that a temperature of the first fuel cell (first fuel cell stack <NUM>) and a temperature of the second fuel cell (second fuel cell stack <NUM>) do not exceed a prescribed upper-limit temperature and controlling the oxidant gas temperature adjustment device (temperature adjustment device <NUM>) based on the target temperature, and the temperature control means (air temperature control unit <NUM>) calculates the target temperature T as T=Tmax-ΔT1-ΔT2 (ΔT1><NUM>), T=Tmax-ΔT2 (ΔT1≤<NUM>), or T=Tmax (ΔT2<<NUM> and ΔT1+ ΔT2<<NUM>), where T is the target temperature, Tmax is the upper-limit temperature, ΔT1 is a difference obtained by subtracting a temperature at an inlet of the first fuel cell (first fuel cell stack <NUM>) from a temperature at an outlet thereof, and ΔT2 is a difference obtained by subtracting a temperature at an inlet of the second fuel cell (second fuel cell stack <NUM>) from a temperature at an outlet thereof.

According to the above configuration, it is possible, even when a position having a highest temperature in the fuel cell module <NUM> is the cathode outlet in the first fuel cell (first fuel cell stack <NUM>) or the cathode outlet in the second fuel cell (second fuel cell stack <NUM>), to identify the position having the highest temperature by calculating ΔT1 and ΔT2 and to calculate the target temperature T corresponding thereto. It is therefore possible, even in the fuel cell module <NUM> that includes at least the first fuel cell (first fuel cell stack <NUM>) in which a reformation reaction is carried out, to realize a control for generating electric power in a state in which a high temperature is maintained but does not exceed an upper-limit temperature by using a simple configuration.

Additionally, according to the above configuration, it is possible, even when the position having the highest temperature in the fuel cell module <NUM> is the cathode inlet in the second fuel cell (second fuel cell stack <NUM>), to identify that the position having the highest temperature is the aforementioned inlet by calculating ΔT1 and ΔT2 and to calculate the target temperature T corresponding thereto. It is therefore possible, even in the fuel cell module <NUM> that includes at least the first fuel cell (first fuel cell stack <NUM>) in which the reformation reaction is carried out, to realize a control for generating electric power in a state in which a high temperature is maintained but does not exceed an upper-limit temperature by using a simple configuration.

In the present embodiment, the temperature control means (air temperature control unit <NUM>): calculates ΔT1 based on an amount of heat generated in association with generation of electric power in the first fuel cell (first fuel cell stack <NUM>), an amount of heat absorbed in the reformation reaction of the fuel gas (anode gas) supplied to the first fuel cell (first fuel cell stack <NUM>), an amount of heat released to outside of the first fuel cell (first fuel cell stack <NUM>), and a flow rate of the oxidant gas (cathode gas) supplied to the first fuel cell (first fuel cell stack <NUM>); and calculates ΔT2 based on an amount of heat generated in association with generation of electric power in the second fuel cell (second fuel cell stack <NUM>), an amount of heat released to outside of the second fuel cell (second fuel cell stack <NUM>), and a flow rate of the oxidant gas (cathode gas) supplied to the second fuel cell (second fuel cell stack <NUM>).

According to the above configuration, it is possible to calculate ΔT1 and ΔT2 without actually detecting a temperature.

In the present embodiment: the fuel cell system is provided with an oxygen supply source (blower <NUM>) for supplying oxygen to an anode in the first fuel cell (first fuel cell stack <NUM>); at least the first fuel cell (first fuel cell stack <NUM>) is configured to be capable of carrying out a partial oxidation reaction using the oxygen and the fuel gas (anode gas); and the temperature control means (air temperature control unit <NUM>) calculates ΔT1 based on an amount of heat generated in association with generation of electric power in the first fuel cell (first fuel cell stack <NUM>), an amount of heat generated through the partial oxidation reaction, an amount of heat absorbed in a reformation reaction according to a difference obtained by subtracting a flow rate of the fuel gas (anode gas) lost due to the partial oxidation reaction from a flow rate of the fuel gas (anode gas) supplied to the first fuel cell (first fuel cell stack <NUM>), an amount of heat released to outside of the first fuel cell (first fuel cell stack <NUM>), and a flow rate of the oxidant gas (cathode gas) supplied to the first fuel cell (first fuel cell stack <NUM>), and calculates ΔT2 based on an amount of heat generated in association with generation of electric power in the second fuel cell (second fuel cell stack <NUM>), an amount of heat released to outside of the second fuel cell (second fuel cell stack <NUM>), and a flow rate of the oxidant gas (cathode gas) supplied to the second fuel cell (second fuel cell stack <NUM>).

According to the above configuration, it is possible, even when the partial oxidation reaction is executed in the first fuel cell (first fuel cell stack <NUM>), to calculate ΔT1 and ΔT2 without actually detecting a temperature.

In the present embodiment: the oxidant gas temperature adjustment device (temperature adjustment device <NUM>) is provided with a combustor <NUM> for mixing and starting combustion of a fuel off-gas (anode off-gas) and an oxidant off-gas (cathode off-gas) that are discharged from the fuel cell module <NUM>, a heat exchanger <NUM> for carrying out heat exchange between a combustion gas discharged from the combustor <NUM> and the oxidant gas (cathode gas) supplied to the fuel cell module <NUM>, and an additional fuel supply means (injector <NUM>) for supplying additional fuel to the combustor <NUM>; and the temperature control means (air temperature control unit <NUM>) controls the additional fuel supply means (injector <NUM>) such that the oxidant gas (cathode gas) reaches the target temperature.

A temperature of air taken in from the outside is equivalent to an outside air temperature. It is necessary to heat the air because the temperature of the air is low enough to hinder generation of electric power when the air is directly supplied to the fuel cell module <NUM> (SOFCs). However, it is also necessary to introduce fuel into the fuel cell module <NUM> more abundantly than is required for electric power generation, and a surplus is left over. Thus, the oxidant gas temperature adjustment device (temperature adjustment device <NUM>) carries out heat exchange between the high-temperature combustion gas, which is obtained through combustion of the remaining fuel (anode off-gas) and high-temperature air (cathode off-gas) discharged from the fuel cell module <NUM> by using the combustor <NUM>, and the air (cathode gas) supplied to the fuel cell module <NUM>. Due to the oxidant gas temperature adjustment device (temperature adjustment device <NUM>) supplying the air (cathode gas) heated through the heat exchange to the fuel cell module <NUM>, a highly efficient fuel cell system is produced.

Moreover, when the temperature of the heated air (cathode gas) does not reach the target temperature, the additional fuel is supplied to the combustor <NUM> to raise the temperature of the fuel gas and an amount of air (cathode gas) subjected to heat exchange is increased, thereby making it possible to cause the temperature of the air (cathode gas) to reach the target temperature.

In the present embodiment: the fuel cell system includes an oxidant gas supply source (blower <NUM>) for supplying the oxidant gas (cathode gas) to the fuel cell module <NUM>; the oxidant gas temperature adjustment device (temperature adjustment device <NUM>) is provided with a combustor <NUM> for mixing and starting combustion of a fuel off-gas (anode off-gas) and an oxidant off-gas (cathode off-gas) that are discharged from the fuel cell module <NUM>, a heat exchanger <NUM> for connecting the oxidant gas supply source (blower <NUM>) and the fuel cell module <NUM> and carrying out heat exchange between a combustion gas discharged from the combustor <NUM> and the oxidant gas (cathode gas) supplied to the fuel cell module <NUM>, a bypass flow path <NUM> for supplying the oxidant gas (cathode gas) from the oxidant gas supply source (blower <NUM>) to the fuel cell module <NUM> while bypassing the heat exchanger <NUM>, a flow rate adjustment means (bypass valve <NUM>) for adjusting a proportion of flow rates of the oxidant gas (cathode gas) in the heat exchanger <NUM> and the bypass flow path <NUM>, and an additional fuel supply means (injector <NUM>) for supplying additional fuel to the combustor <NUM>; and the temperature control means (air temperature control unit <NUM>) controls the flow rate adjustment means (bypass valve <NUM>) and/or the additional fuel supply means (injector <NUM>) such that the oxidant gas (cathode gas) reaches the target temperature.

Claim 1:
A fuel cell system comprising:
a fuel cell module (<NUM>) including a first fuel cell (<NUM>) and a second fuel cell (<NUM>) arranged in series such that a fuel gas circulates in a direction from the first fuel cell (<NUM>) toward the second fuel cell (<NUM>) and an oxidant gas circulates in a direction from the second fuel cell (<NUM>) toward the first fuel cell (<NUM>); and
an oxidant gas temperature adjustment device (<NUM>) for adjusting a temperature of the oxidant gas supplied to the fuel cell module (<NUM>),
the first fuel cell (<NUM>) being at least configured to reform the fuel gas,
characterized in that
the fuel cell system includes a temperature control means (<NUM>) for setting a target temperature of the oxidant gas supplied to the fuel cell module (<NUM>) such that a temperature of the first fuel cell (<NUM>) and a temperature of the second fuel cell (<NUM>) do not exceed a prescribed upper-limit temperature and controlling the oxidant gas temperature adjustment device (<NUM>) based on the target temperature, and
the temperature control means (<NUM>) calculates the target temperature T as <MAT> <MAT>
or <MAT>
wherein T is the target temperature, Tmax is an upper-limit temperature, ΔT1 is a difference obtained by subtracting a temperature at an inlet of the first fuel cell (<NUM>) from a temperature at an outlet thereof, and ΔT2 is a difference obtained by subtracting a temperature at an inlet of the second fuel cell (<NUM>) from a temperature at an outlet thereof.