Patent Description:
In a fuel cell system as described in, for example, <CIT> (<CIT>), in a condition where electric power generation is stopped in a fuel cell stack, cathode gas is intermittently supplied to the fuel cell stack, even during stop of power generation, so as to prevent the fuel cell stack from being unable to quickly respond to a request to generate electric power. In this manner, the voltage of a unit cell (which will be called "cell voltage") is kept equal to or larger than a given value.

Furthermore, in the system of <CIT>, air is intermittently supplied from an air compressor to the fuel cell stack, in the condition where power generation is stopped in the fuel cell stack, so as to prevent the maximum cell voltage from exceeding an upper-limit voltage, and curb deterioration of the fuel cell stack,.

<CIT> discloses a fuel cell system of the present invention can suppress an excessively wet or dry state of a fuel cell stack so as to thereby ensure the durability of the fuel cell stack. The fuel cell system supplies an oxidant gas with a reduced flow rate per unit time and for a long time period if the rate of voltage decrease of the stack becomes faster than a threshold rate, and supplies the oxidant gas with an increased flow rate per unit time and for a short time period if the rate of voltage decrease becomes slower than a threshold rate. <CIT> discloses a fuel cell system includes: a power supply circuit including a fuel cell and a secondary battery; an oxidant gas supply flow passage; a pump; and a control unit configured to drive the pump and dilute hydrogen retained in an cathode. The control unit is configured to stop supplying an oxidant gas to the cathode by stopping an operation of the pump such that dilution of the hydrogen retained in the cathode is stopped, while a fuel cell vehicle remains stationary after a starter switch of the fuel cell vehicle is switched from an off state to an on state, or while a load required of the power supply circuit remains smaller than a predetermined value after the starter switch of the fuel cell vehicle is switched from the off state to the on state.

However, the inventor of this application found that, in the system described in <CIT>, a length of time for which air is not supplied by the air compressor is longer than a length of time for which air is supplied by the air compressor, and the amount of air supplied during operation of the air compressor is larger than a necessary amount, resulting in a large variation in the cell voltage. When the cell voltage is large, a catalyst in the fuel cell stack may deteriorate. When the cell voltage is small, the fuel cell stack may not be able to quickly respond to a request to generate electric power.

A first aspect of the invention is defined in appended claim <NUM>.

The fuel cell system may further include a secondary battery. The controller may be configured to charge the secondary battery with regenerative power generated when driving of the compressor is stopped, at a time of switching from the supply period to the stop period. With the fuel cell system thus configured, the fuel efficiency is improved.

The compressor may be a turbo air compressor. The turbo air compressor consumes a smaller amount of electric power at the start of operation, and has better response, as compared with other air compressors. Thus, according to the fuel cell system, the supply period and the stop period can be quickly switched to each other, and the power consumption can be reduced, resulting in improved fuel efficiency.

A second aspect of the present invention is defined in appended claim <NUM>.

<FIG> shows a fuel cell system <NUM> as one embodiment of the invention. The fuel cell system <NUM> is installed on a fuel cell vehicle, for example. In this embodiment, the fuel cell system <NUM> includes a fuel cell stack <NUM>, controller <NUM>, air flow meter <NUM>, compressor <NUM>, cathode gas passage <NUM>, and anode gas passage <NUM>.

The fuel cell stack <NUM> is a polymer electrolyte fuel cell that is supplied with anode gas (e.g., hydrogen gas) and cathode gas (e.g., air) as reaction gases, to generate electric power. The fuel cell stack <NUM> is composed of a plurality of unit cells (not shown) stacked together. The anode gas is supplied from an anode gas tank (not shown), and passes through the anode gas passage <NUM>, to be supplied to an anode 100a of the fuel cell stack <NUM> and used for electrochemical reaction. A portion of the anode gas which was not used for electrochemical reaction is discharged as offgas to the outside of the fuel cell stack <NUM>. On the other hand, the cathode gas is supplied to a cathode 100c of the fuel cell stack <NUM>, through the cathode gas passage <NUM>, and used for electrochemical reaction. Oxygen that was not used for electrochemical reaction is discharged as offgas to the outside of the fuel cell stack <NUM>.

Through the cathode gas passage <NUM>, the cathode gas is supplied to and discharged from the fuel cell stack <NUM>. The cathode gas passage <NUM> includes a cathode gas supply passage <NUM> through which the cathode gas is supplied to the fuel cell stack <NUM>, a cathode gas discharge passage <NUM> through which the cathode gas is discharged from the fuel cell stack <NUM>, and a bypass passage <NUM> that communicates with the cathode gas supply passage <NUM> and the cathode gas discharge passage <NUM>.

The air flow meter <NUM>, compressor <NUM>, and a pressure gauge <NUM> are provided, in this order as viewed from the upstream side, in the cathode gas supply passage <NUM>. The air flow meter <NUM> is a device that measures the flow rate of cathode gas taken into the cathode gas supply passage <NUM>. The pressure gauge <NUM> measures the pressure at the downstream side of the compressor <NUM>. In this embodiment, the pressure gauge <NUM> is provided upstream of a portion of the cathode gas supply passage <NUM> connected to the bypass passage <NUM>, and downstream of the compressor <NUM>. However, the pressure gauge <NUM> may be provided downstream of the fuel cell stack <NUM>, in a portion of the cathode gas discharge passage <NUM> which is on the upstream side of its portion connected to the bypass passage <NUM>.

The compressor <NUM> is a member that supplies cathode gas to the fuel cell stack <NUM>. In this embodiment, a turbo air compressor is used as the compressor <NUM>. The turbo air compressor is characterized in that electric power consumption at the start of operation is smaller than those of other air compressors, and the compressor operates quickly to change its rotational speed. In this connection, a volume compressor, for example, may also be used as the compressor <NUM>.

A plurality of valves is provided in the cathode gas passage <NUM>. In this embodiment, a shut valve <NUM>, pressure regulating valve <NUM>, and bypass valve <NUM> are provided in the cathode gas passage <NUM>. The shut valve <NUM> controls the amount of cathode gas that enters the fuel cell stack <NUM>. The shut valve <NUM> is provided in the cathode gas supply passage <NUM>, and is located downstream of its portion connected to the bypass passage <NUM>, and upstream of the fuel cell stack <NUM>. The pressure regulating valve <NUM> controls the pressure of the cathode gas at the downstream side of the fuel cell stack <NUM>. The pressure regulating valve <NUM> is provided in the cathode gas discharge passage <NUM>, and is located upstream of its portion connected to the bypass passage <NUM>, and downstream of the fuel cell stack <NUM>. The bypass valve <NUM> is provided in the bypass passage <NUM>, for controlling the amount of cathode gas that passes the bypass passage <NUM>.

Electric power generated by the fuel cell stack <NUM> is stored in a secondary battery <NUM> via a DC/DC converter <NUM>. Various loads (not shown) are connected to a power supply circuit including the fuel cell stack <NUM>, DC/DC converter <NUM>, and secondary battery <NUM>. The fuel cell stack <NUM> and the secondary battery <NUM> can also supply electric power to the compressor <NUM> and various valves.

A voltage detector <NUM> detects the voltage (which will also be called "FC voltage") of the fuel cell stack <NUM>. In this embodiment, the average cell voltage is used as the FC voltage. The "average cell voltage" is a value obtained by dividing a voltage across the opposite ends of the fuel cell stack <NUM> by the number of unit cells.

The controller <NUM> is configured as a computer including a central processing unit (CPU), a memory, and an interface circuit to which the above components are connected. The controller <NUM> outputs signals for controlling start and stop of constituent components in the fuel cell system <NUM>, according to commands of an electronic control unit (ECU) <NUM>. The ECU <NUM> is a controller that controls the whole system including the fuel cell system <NUM>. For example, in the fuel cell vehicle, the ECU <NUM> performs control of the vehicle, according to a plurality of input values, such as the amount of depression of an accelerator pedal, the amount of depression of a brake pedal, and the vehicle speed. The ECU <NUM> may be included as a part of the functions of the controller <NUM>. The CPU executes control programs stored in the memory, so as to control power generation by the fuel cell system <NUM>, and implement cathode-gas intermittent supply control that will be described later.

The controller <NUM> switches the operating mode of the fuel cell stack <NUM> between a normal operating mode and a zero required output operating mode, for example. In the normal operating mode, the fuel cell system <NUM> receives a power generation request from the ECU <NUM>, and the fuel cell system <NUM> performs operation according to the required electric power. In the zero required output operating mode, the electric power which the ECU <NUM> requires the fuel cell system <NUM> to generate is equal to or smaller than a predetermined value, and the fuel cell stack <NUM> is not required to generate electric power. The controller <NUM> switches the operating mode of the fuel cell system <NUM>, from the normal operating mode to the zero required output operating mode, at the time of stop of the vehicle on which the fuel cell system <NUM> is installed, or during low-load operation, such as during traveling at a low speed. In the zero required output operating mode, the controller <NUM> causes the secondary battery to supply electric power. In the zero required output operating mode, the controller <NUM> supplies oxygen to the fuel cell stack <NUM>, to such an extent that the voltage of the fuel cell stack <NUM> falls within a predetermined range. In this connection, during operation in the zero required output operating mode, small current may be generated from the fuel cell stack <NUM>, so as to prevent the cell voltage from being equal to an open-circuit voltage. This case is also included in the zero required output operating mode. In this embodiment, the controller <NUM> controls each part of the fuel cell system <NUM>, to perform cathode-gas intermittent supply control (which will be described later), in the zero required output operating mode.

<FIG> is a flowchart of cathode-gas intermittent supply control executed by the controller <NUM>. When the controller <NUM> starts the zero required output operating mode, it starts the cathode-gas intermittent supply control. The controller <NUM> finishes control of <FIG>, when it receives a command to stop operation in the zero required output operating mode, more specifically, when the ECU <NUM> requires the fuel cell stack <NUM> to generate electric power. Under the cathode-gas intermittent supply control, the controller <NUM> stops supply of the anode gas, and places the shut valve <NUM> and the pressure regulating valve <NUM> in open states, while placing the bypass valve <NUM> in a closed state.

When the cathode-gas intermittent supply control is started, the controller <NUM> initially stops supply of cathode gas (step S110). More specifically, the controller <NUM> sets the flow rate of cathode gas supplied from the compressor <NUM> to the fuel cell stack <NUM>, to zero.

Then, the controller <NUM> determines whether the FC voltage is smaller than a target voltage V1 (step S120). The target voltage V1 is a voltage that can ensure sufficient output response, while curbing deterioration of the fuel cell stack <NUM>, and is obtained in advance by experiment or simulation. In this embodiment, the controller <NUM> stores the target voltage V1 in advance. The FC voltage is detected by the voltage detector <NUM>.

When the controller <NUM> determines that the FC voltage is equal to or larger than the target voltage V1 (step S120: NO), the control returns to step S110. On the other hand, when the controller <NUM> determines that the FC voltage is smaller than the target voltage V1 (step S120: YES), the controller <NUM> performs operation to supply cathode gas (step S130). More specifically, the controller <NUM> causes the compressor <NUM> to supply cathode gas to the fuel cell stack <NUM>. In connection with the cathode-gas intermittent supply control, the period over which the controller <NUM> causes the compressor <NUM> to supply cathode gas will be called "supply period P1", and the period over which the controller <NUM> stops supply of cathode gas will be called "stop period P2".

The flow rate of air fed by the compressor <NUM> in the supply period P1 is smaller than the flow rate of air when the fuel cell stack <NUM> is required to generate electric power. As a result, the FC voltage can be made less likely to rise rapidly. Here, the flow rate of air can be measured by the air flow meter <NUM>.

In this embodiment, the flow rate of air fed by the compressor <NUM> in the supply period P1 is equal to or larger than <NUM> NL/min. , and equal to or smaller than 30NL/min. Preferably, the flow rate is equal to or larger than <NUM> NL/min. , and is equal to or smaller than <NUM> NL/min. On the other hand, in this embodiment, the flow rate of air when the fuel cell stack <NUM> is required to generate electric power is equal to or larger than <NUM> NL/min. , and is equal to or smaller than <NUM> NL/min. In this connection, <NUM> NL/min. means that air flows in an amount of <NUM> per minute, under base conditions (pressure: <NUM> MPa, temperature: <NUM>, humidity: <NUM>%).

In this embodiment, the flow rate of air fed by the compressor <NUM> in the supply period P1 is equal to or smaller than <NUM>% of the maximum flow rate of air when the fuel cell stack <NUM> is required to generate electric power. As a result, the FC voltage can be effectively made less likely to rise rapidly, and therefore, the durability of the fuel cell stack <NUM> is improved.

After supply of cathode gas is started (step S130), the controller <NUM> determines whether the FC voltage is equal to or larger than the target voltage V1 (step S140). When the controller <NUM> determines that the FC voltage is smaller than the target voltage V1 (step S140: NO), the controller <NUM> continues supply of cathode gas (step S130). On the other hand, when the controller <NUM> determines that the FC voltage is equal to or larger than the target voltage V1 (step S140: YES), the control returns to step S110, and the controller <NUM> stops supply of cathode gas. The controller <NUM> repeats the above-described series of steps, until the zero required output operating mode ends.

The timing chart of <FIG> represents the cathode-gas intermittent supply control. In <FIG>, the horizontal axis indicates time, and the vertical axis indicates change of the FC voltage in the upper section, and the driving status of the compressor <NUM> in the lower section. In <FIG>, a period of a part of the cathode-gas intermittent supply control is indicated.

In this embodiment, from time t0 to time t1, the controller <NUM> stops supply of cathode gas to the fuel cell stack <NUM>. Namely, the controller <NUM> stops the compressor <NUM>.

Then, from time t1 to time t2, the FC voltage is smaller than the target voltage V1; therefore, the controller <NUM> supplies cathode gas to the fuel cell stack <NUM>. Namely, the controller <NUM> drives the compressor <NUM>. Here, the period from time t1 to time t2 is the supply period P1 in which the compressor <NUM> is driven.

Then, in a period from time t2 to time t3, the FC voltage is equal to or larger than the target voltage V1; therefore, the controller <NUM> stops supply of cathode gas to the fuel cell stack <NUM>. Namely, the period from time t2 to time t3 is the stop period P2 in which the compressor <NUM> is stopped.

Similarly, a period from time t3 to time t4 is the supply period P1 in which the compressor <NUM> is driven, and a period from time t4 to time t5 is the stop period P2 in which the compressor <NUM> is stopped. In this embodiment, one cycle including one supply period P1 and one stop period P2 is equal to or longer than two seconds, and is equal to or shorter than five seconds.

As described above, in the period in which the fuel cell stack <NUM> is not required to generate electric power, the controller <NUM> controls the compressor <NUM> so that the supply period P1 and the stop period P2 appear alternately. Namely, the controller <NUM> alternately performs (or switches) supply and stop of cathode gas by the compressor <NUM>. Also, as shown in <FIG>, the supply period P1 is longer than the stop period P2. Before the fuel cell system <NUM> is brought into the status shown in <FIG>, there may be a stop period P2 that is longer than the supply period P1. Namely, before the initial supply period P1 starts, there may be a stop period P2 that is longer than the supply period P1.

In the fuel cell system <NUM> of this embodiment, the supply period P1 is longer than the stop period P2, and the flow rate of air fed by the compressor <NUM> in the supply period P1 is smaller than the flow rate of air when the fuel cell stack <NUM> is required to generate electric power. Thus, according to the fuel cell system <NUM> of this embodiment, the rate of increase of the FC voltage that increases due to air fed by the compressor <NUM> can be reduced, so that the range of variation in the FC voltage can be reduced. As a result, a catalyst in the fuel cell stack <NUM> is less likely to deteriorate due to excessive increase of the FC voltage, and the fuel cell stack <NUM> can quickly respond to a request to generate electric power when there is any such request. Also, the flow rate of air fed by the compressor <NUM> in the supply period P1 is smaller than the flow rate of air when the fuel cell stack <NUM> is required to generate electric power; therefore, the fuel efficiency can be improved, as compared with the case where these flow rates are made equal to each other.

In the fuel cell system <NUM> of this embodiment, the supply period P1 is longer than the stop period P2. Therefore, the flowability of water vapor and water in the fuel cell stack <NUM> is improved, so that the environment within the fuel cell system <NUM> can be kept favorable.

In the fuel cell system <NUM> of this embodiment, the controller <NUM> drives the compressor <NUM> when the voltage of the fuel cell stack <NUM> is smaller than the predetermined target voltage V1, and stops the compressor <NUM> when the voltage of the fuel cell stack <NUM> is larger than the target voltage V1. While the target voltage V1 used in step S120 and the target voltage V1 used in step S140 may be set to different values, control can be simplified if the target voltage V1 used in step S120 and the target voltage V1 used in step S140 are set to the same value, as in this embodiment.

In the fuel cell system <NUM> of this embodiment, the turbo air compressor is used as the compressor <NUM>. With regard to the turbo air compressor, the power consumption at the start of operation is smaller, and the response is better, as compared with other types of air compressors. Thus, according to the fuel cell system <NUM>, the supply period P1 and the stop period P2 can be quickly switched, and the power consumption can be reduced, resulting in improved fuel efficiency.

The second embodiment is different from the first embodiment in that the controller <NUM> is configured to charge the secondary battery <NUM> with regenerative power generated when driving of the compressor <NUM> is stopped, at the time of switching from the supply period P1 to the stop period P2, but the first and second embodiments are identical with each other in other respects. According to the second embodiment, the secondary battery <NUM> is charged with regenerative power, so that the fuel efficiency can be improved.

<FIG> is a flowchart of cathode-gas intermittent supply control according to a third embodiment. The third embodiment is different from the first embodiment in step S150 and step S160, but is identical with the first embodiment in other respects.

In the third embodiment, when the controller <NUM> determines that the FC voltage is smaller than the target voltage V1 (step S140: NO), the controller <NUM> determines whether the FC voltage is smaller than a lower-limit voltage V2 (step S150). The lower-limit voltage V2 is a voltage at which the catalyst included in the fuel cell stack <NUM> switches between oxidation reaction and reduction reaction, for example, and is obtained in advance by experiment or simulation. In this embodiment, the controller <NUM> stores the lower-limit voltage V2 in advance. In this embodiment, the lower-limit voltage V2 is smaller than the target voltage V1.

When the controller <NUM> determines that the FC voltage is equal to or larger than the lower-limit voltage V2 (step S150: NO), the controller <NUM> continues supply of cathode gas (step S130). On the other hand, when the controller <NUM> determines that the FC voltage is smaller than the lower-limit voltage V2 (step S150: YES), the controller <NUM> performs a purge process (step S160). After the purge process (step S160), the control returns to step S110. Here, the purge process is performed so as to reduce water that exists in the cathode gas passage <NUM> within the fuel cell stack <NUM>. In this embodiment, the air is supplied from the compressor <NUM> to the fuel cell stack <NUM>, at a flow rate that is <NUM> times as large as the flow rate of air fed by the compressor <NUM> in the supply period P1. In this embodiment, the purge process is performed for several seconds.

Claim 1:
A fuel cell system comprising:
a fuel cell stack (<NUM>);
a compressor (<NUM>) configured to supply cathode gas to the fuel cell stack (<NUM>); and
a controller (<NUM>) configured to control constituent components of the fuel cell system including the compressor (<NUM>); and
a voltage detector (<NUM>) configured to detect a voltage of the fuel cell stack (<NUM>);
wherein:
the controller (<NUM>) is configured to, when the fuel cell stack (<NUM>) is not required to generate electric power, control the compressor (<NUM>) alternately to supply the cathode gas for a supply period and to stop supply of the cathode gas for a stop period, and
the controller (<NUM>) is configured to control the compressor (<NUM>) to supply the cathode gas when the voltage of the fuel cell stack (<NUM>) is smaller than a predetermined voltage, and to stop supply of the cathode gas when the voltage of the fuel cell stack (<NUM>) is larger than the predetermined voltage, the supply period being longer than the stop period, and a flow rate of the cathode gas supplied by the compressor (<NUM>) in the supply period being smaller than a flow rate in a case where the fuel cell stack (<NUM>) is required to generate electric power.