JP7594993B2 - Fuel Cell Systems - Google Patents

Description

The present invention relates to a fuel cell system.

To determine the state of the fuel cell system, there are fuel cell systems that determine whether there is an abnormality based on the voltage threshold of the fuel cell. The fuel cell system can determine whether to start up or not based on the result of the determination for a certain voltage threshold.

A fuel cell system is disclosed that diagnoses whether or not it is possible to continue starting the fuel cell system based on the result of determining whether or not the voltage of the fuel cell reaches or exceeds a predetermined voltage threshold immediately after fuel gas and oxidant gas are supplied to the fuel cell (see, for example, Patent Document 1).

There is also a fuel cell system that includes a boost converter (DC-DC converter) that converts the voltage of the fuel cell to a predetermined voltage, and a power storage device connected between the boost converter and a load. The fuel cell system sets the number of fuel cell cells so that the voltage of the fuel cell is higher than the voltage of the power storage device when power generation by the fuel cell is stopped, and makes it possible to charge the power generated by the fuel cell when power generation is stopped to the power storage device through a path using a diode provided in the boost converter, etc., thereby ensuring a sufficient number of fuel cell cells and optimizing the output of the fuel cell while preventing a high potential of the fuel cell through a passive step-down operation (hereinafter also referred to as "passive step-down") without operating the step-up converter.

JP 2007-165082 A

When passive step-down is performed in a fuel cell system using a boost converter, the lower the voltage of the power storage device connected to the secondary side of the boost converter, the larger the potential difference between the fuel cell and the power storage device, and a large current flows due to the passive step-down operation. The amount of oxidant gas supplied to the fuel cell is controlled so that it reacts with hydrogen adequately for the target power generation, but the large potential difference between the fuel cell and the power storage device causes a shortage of oxidant gas for the target power generation (hereinafter referred to as an "air shortage state"). In other words, passive step-down causes a phenomenon in which power greater than the target power generation of the fuel cell is unintentionally taken out of the fuel cell, resulting in an air shortage state and a deterioration in the power-current characteristics of the fuel cell. For example, when a fuel cell system is used in an industrial vehicle such as a forklift, if a larger output than normal is required, such as when the vehicle is continuously climbing a slope, the amount of power taken out of the power storage device increases, causing the SOC (State of Charge) of the power storage device to decrease and the voltage of the power storage device to decrease. This can cause the power-voltage characteristics (PV characteristics) of the fuel cell to deteriorate, and the power storage device to enter an over-discharge state, causing the fuel cell system to shut down. One aspect of the present invention aims to provide a fuel cell system that can extend the operating time of the fuel cell system in response to a power shortage in the power storage device.

One aspect of the fuel cell system according to the present invention is a fuel cell system including a fuel cell stack that generates electric power from a fuel gas and an oxidant gas, a power storage device, a boost converter that is electrically connected to the fuel cell stack and the power storage device and converts the voltage of the fuel cell stack to a predetermined voltage, and a control unit that controls the boost converter and the fuel cell stack, and the control unit increases the amount of the oxidant gas supplied to the fuel cell stack when a predetermined condition is satisfied in a state in which the amount of power generated by the fuel cell stack is lower than the amount of power required by a load.

As a result, when the amount of power generated by the fuel cell stack is lower than the amount of power required by the load, if certain conditions are met, it is determined that passive step-down is occurring, and the amount of oxidant gas supplied to the fuel cell stack is increased to eliminate the air shortage condition, thereby improving the PV characteristics of the fuel cell stack. By improving the PV characteristics of the fuel cell stack, the amount of power generated by the fuel cell stack increases, and by reducing the amount of power taken from the power storage device, the operating time (life extension) of the fuel cell system can be extended in the event of a power shortage in the power storage device.

The specified conditions are that the voltage of the fuel cell stack is greater than the voltage of the power storage device, the current flowing through the boost converter is equal to or greater than 0, and the duty ratio of the boost converter is 0. As a result, when passive step-down is occurring, which refers to a state in which current passively flows through the DCDC converter even though the DCDC converter is not operating, the amount of oxidant gas supplied to the fuel cell stack can be increased to increase the amount of power generated by the fuel cell stack, suppressing the power drawn from the power storage device and extending the operating time (life extension) of the fuel cell system in the event of a power shortage in the power storage device.

In addition, when the temperature of a component constituting the fuel cell system reaches or exceeds a threshold value, the control unit returns the amount of oxidant gas supplied to the fuel cell stack to the original state before the increase, or reduces the amount supplied. This makes it possible to return the operating state of each component operating the fuel cell system to a normal state, preventing an increase in temperature and suppressing breakdowns of the fuel cell system due to overheating.

The present invention makes it possible to extend the operating time of a fuel cell system when there is a power shortage in the power storage device.

1 is a diagram showing an example of a fuel cell system according to an embodiment of the present invention. FIG. 4 is a diagram showing the relationship between voltage and power of a fuel cell and a power storage device. FIG. 4 is a diagram showing an example of transition of the control state of the fuel cell system. FIG. 4 is a diagram showing an example of a fuel cell control process executed by a control unit of the fuel cell system. FIG. 11 is a diagram showing an example of a fuel cell system according to another embodiment.

The following describes the embodiment in detail with reference to the drawings.

Figure 1 shows an example of a fuel cell system according to an embodiment of the present invention.

The fuel cell system 1 shown in FIG. 1 is mounted on a vehicle Ve, such as an industrial vehicle such as a forklift or an electric vehicle, and supplies power to a load Lo. The load Lo may be a system power source for supplying power to a driving motor, electrical components, a computer, memory, etc. The fuel cell system 1 may also be applied to stationary emergency generators, etc., in addition to vehicles.

The fuel cell system 1 also includes a fuel cell FC, a hydrogen tank Ht, a compressor Cp, current detection units Si0 and Si4, voltage detection units V1, V2, and V3, a DCDC converter Cnv, a power storage device S, a control unit Cnt, and a temperature sensor Ts.

The DCDC converter Cnv is an example of a boost converter. The DCDC converter Cnv is connected between the fuel cell FC and the power storage device S. The DCDC converter Cnv includes reactors Re1, Re2, and Re3, current detection units Si1, Si2, and Si3, six switching elements Q1 to Q6, six diodes D1 to D6, and a capacitor Co.

The hydrogen tank Ht supplies hydrogen (anode gas), which serves as fuel gas for power generation, to the anode of the fuel cell FC under the control of the control unit Cnt. The compressor Cp supplies air containing oxygen (cathode gas), which serves as oxidant gas, to the cathode of the fuel cell FC under the control of the control unit Cnt.

The fuel cell FC generates electrical energy through a chemical reaction between hydrogen, which is supplied to the fuel cell FC from the hydrogen tank Ht through an injector, and oxygen in the air, which is supplied to the fuel cell FC from the atmosphere through a compressor Cp. In other words, the fuel cell FC generates electricity through a chemical reaction between hydrogen and oxygen.

The fuel cell FC is connected to six switching elements Q1 to Q6 and six diodes D1 to D6 via reactors Re1, Re2, and Re3 and current detection units Si1, Si2, and Si3. The current detection unit Si0 is connected between the fuel cell FC and the reactors Re1, Re2, and Re3.

MOSFETs (metal-oxide-semiconductor field-effect transistors) are used as the switching elements Q1 to Q6. However, IGBTs (insulated gate bipolar transistors) or the like may also be used as the switching elements. The six diodes D1 to D6 are parasitic diodes of the six switching elements (MOSFETs) Q1 to Q6, respectively.

Switching element Q1 constituting the u-phase upper arm and switching element Q2 constituting the u-phase lower arm are connected in series between the positive bus bar Lp and the negative bus bar Ln. Switching element Q3 constituting the v-phase upper arm and switching element Q4 constituting the v-phase lower arm are connected in series between the positive bus bar Lp and the negative bus bar Ln. Switching element Q5 constituting the w-phase upper arm and switching element Q6 constituting the w-phase lower arm are connected in series between the positive bus bar Lp and the negative bus bar Ln.

The positive busbar Lp and the negative busbar Ln are connected to a storage device S via a capacitor Co.

By configuring the DCDC converter Cnv with a MOSFET and a parasitic diode of the MOSFET, the configuration of the DCDC converter Cnv can be made simpler than when it is configured with a mechanical switch, etc., and the fuel cell system 1 can be made smaller.

The path through which the power supplied from the fuel cell FC passes through the diodes D1, D3, and D5, or through D2, D4, and D6, is hereinafter referred to as the "first path." Also, the path through which the power supplied from the fuel cell FC passes through the switching elements Q1, Q3, and Q5, or through Q2, Q4, and Q6, is hereinafter referred to as the "second path."

The switching element Q1 constituting the u-phase upper arm, the switching element Q3 constituting the v-phase upper arm, and the switching element Q5 constituting the w-phase upper arm are connected to the load Lo via the positive bus Lp. The switching element Q2 constituting the u-phase lower arm, the switching element Q4 constituting the v-phase lower arm, and the switching element Q6 constituting the w-phase lower arm are connected to the load Lo via the negative bus Ln.

The switching operation of the switching elements Q1 to Q6 that make up the upper and lower arms allows the DC voltage, which is the voltage of the storage device S, to be supplied to the load Lo. The load Lo is, for example, a vehicle drive motor or a load handling motor.

The control unit Cnt is connected to the gate terminals of each of the switching elements Q1 to Q6. The control unit Cnt controls the switching elements Q1 to Q6 of the DCDC converter Cnv to perform switching operations based on a pulse pattern that is a control signal.

The DCDC converter Cnv converts the voltage of the fuel cell FC into a constant voltage by turning on and off the switching elements Q1 to Q6 according to the input control signal, and outputs it to the load Lo and the storage device S.

The reactor Re1 is connected between the current detection unit Si0 and the switching element Q1-switching element Q2. The reactor Re2 is connected between the current detection unit Si0 and the switching element Q3-switching element Q4. The reactor Re3 is connected between the current detection unit Si0 and the switching element Q5-switching element Q6.

Current detection unit Si1 is connected between reactor Re1 and switching element Q1 and switching element Q2. Current detection unit Si2 is connected between reactor Re2 and switching element Q3 and switching element Q4. Current detection unit Si3 is connected between reactor Re3 and switching element Q5 and switching element Q6.

The fuel cell FC is a fuel cell stack made up of multiple fuel cells connected in series, and generates electricity by utilizing an electrochemical reaction between hydrogen and oxygen (oxidant gas) supplied under the control of the control unit Cnt. The power storage device S is made up of a lithium ion capacitor or the like, and is connected between the DCDC converter Cnv and the load Lo.

When the power output from the DCDC converter Cnv is greater than the power required by the load Lo, the surplus power is supplied to the storage device S, and the storage device S is charged. When the power output from the DCDC converter Cnv is less than the power required by the load Lo, the shortage power is supplied from the storage device S to the load Lo. When regenerative power is supplied from the load Lo to the storage device S, the storage device S is charged. Note that the storage device S is not limited to a lithium ion capacitor, as long as it is a storage device (such as a lithium ion battery) that can be charged and discharged.

The current detection unit Si0 detects the current flowing from the fuel cell FC to the DCDC converter Cnv and outputs the detected current to the control unit Cnt.

The current detection unit (current detection means) Si1 detects the current flowing through the DCDC converter Cnv via the switching element Q1 or switching element Q2, and outputs the detected current to the control unit Cnt.

The current detection unit (current detection means) Si2 detects the current flowing through the DCDC converter Cnv via the switching element Q3 or the switching element Q4, and outputs the detected current to the control unit Cnt.

The current detection unit (current detection means) Si3 detects the current flowing through the DCDC converter Cnv via the switching element Q5 or the switching element Q6, and outputs the detected current to the control unit Cnt.

The current detection unit Si4 detects the current flowing from the fuel cell FC or the power storage device S to the load, i.e., the current required by the load, and outputs the detected current to the control unit Cnt.

The voltage detection unit V1 detects the voltage of the fuel cell FC and outputs the detected voltage to the control unit Cnt. The voltage detection unit V2 detects the voltage of the storage device S and outputs the detected voltage to the control unit Cnt. The voltage detection unit V3 detects the voltage (hereinafter also referred to as the "system voltage") that the load Lo requests from the fuel cell system 1 and outputs the detected voltage to the control unit Cnt. The method of detecting the voltage of the fuel cell FC may be to directly measure the voltage across both ends of the fuel cell FC, or to measure the voltage of one representative cell among the fuel cells that make up the fuel cell FC and multiply it by the number of stacked fuel cell cells. Alternatively, the representative cell voltage itself may be used. In other words, any method is acceptable as long as the voltage of the fuel cell can be measured with sufficient accuracy. The same applies to the detection of the voltage of the storage device S or the load Lo.

The temperature sensor Ts measures the temperature of the components that make up the fuel cell system 1 (hereinafter also referred to as "component temperature"). For example, the temperature sensor Ts measures the temperature of the components of the fuel cell Fc based on the water temperature of the liquid (cooling water) that cools the fuel cell Fc.

The temperature sensor Ts may be disposed near the fuel cell Fc to measure the temperature of the fuel cell Fc components. The temperature sensor Ts may also measure the temperature of the fuel cell Fc components based on the temperature of the gas that cools the fuel cell Fc. The temperature sensor Ts may also measure the temperatures of the components that make up the fuel cell system 1, such as the power storage device S, the DCDC converter Cnv, and the control unit Cnt.

The control unit Cnt is configured with a CPU (Central Processing Unit) or a programmable device (such as an FPGA (Field Programmable Gate Array) or a PLD (Programmable Logic Device)) and outputs a control signal. The control unit Cnt controls the operation of the compressor Cp and the DCDC converter Cnv to control the amount of power generated (electricity) of the fuel cell FC. That is, the more the amount of fuel (hydrogen) supplied from the hydrogen tank Ht to the fuel cell FC and the amount of air (oxygen) supplied from the compressor Cp increases, the more the amount of power generated by the fuel cell FC increases. Also, the more the amount of fuel (hydrogen) supplied from the hydrogen tank Ht to the fuel cell FC and the amount of air (oxygen) supplied from the compressor Cp decreases, the more the amount of power generated by the fuel cell FC decreases.

The control unit Cnt may gradually increase or decrease the amount of fuel and air supplied to the fuel cell FC. Furthermore, when the control unit Cnt sets the amount of fuel and air supplied to the fuel cell FC to zero, power generation by the fuel cell FC stops after a predetermined time has elapsed, and the amount of power generated by the fuel cell FC becomes zero. Furthermore, the more the current output from the fuel cell FC increases, the more the voltage of the fuel cell FC (fuel cell) decreases, and the more the current output from the fuel cell FC decreases, the more the voltage of the fuel cell FC (fuel cell) increases.

The control unit Cnt controls the operation of the DC-DC converter Cnv so that the fuel cell FC outputs a current corresponding to the power required by the load Lo or the storage device S, and so that the voltage of the fuel cell FC does not exceed a threshold. The process of adjusting the current flowing through the DC-DC converter Cnv so that the voltage of the fuel cell FC does not exceed the threshold and deteriorate the fuel cell FC is called high-potential avoidance process. The duty ratio of the control signal increases as the power required by the load Lo or the storage device S increases, and the duty ratio of the control signal decreases as the power required by the load Lo or the storage device S decreases. The threshold is the voltage of the fuel cell FC when there is a risk of deterioration of the fuel cell FC, and when the voltage of the fuel cell FC is about to exceed the threshold, the decrease in the duty ratio of the control signal is limited. In the fuel cell system 1, in order to avoid high potential, the voltage per cell of the fuel cell FC is set not to be equal to or higher than a first threshold. This first threshold voltage is hereinafter referred to as the "high potential avoidance voltage".

In this embodiment, in order to create a low-cost fuel cell system 1, the number of cells constituting the fuel cell FC is reduced, and the voltage of the fuel cell FC is boosted by a boost-type DCDC converter Cnv. The voltage of the fuel cell FC is set to satisfy the following formula 1. In the following formula 1, the "voltage of the fuel cell FC" refers to the voltage across the fuel cell FC.

Fuel cell FC voltage≦high potential avoidance voltage×number of cells (Equation 1)
Because the DCDC converter Cnv is a step-up type, during normal operation, i.e., when current is being drawn by the load Lo, the number of cells is adjusted so that the voltage of the fuel cell FC is smaller than the voltage of the power storage device S so that the voltage of the fuel cell FC does not overlap with the voltage of the power storage device S. However, if the voltage of the fuel cell FC is made too small, the number of cells becomes too small. As a result, the output of the fuel cell FC becomes insufficient, and the fuel cell system does not function.

Therefore, in this embodiment, in order to avoid high potential and improve the output of the fuel cell system 1, the fuel cell system 1 sets the voltage of the fuel cell FC to be higher than the voltage of the power storage device S when power generation by the fuel cell FC is stopped. Specifically, when the load Lo or the requested power from the power storage device S is no longer present, power generation by the fuel cell FC is stopped, and the number of cells is determined so that the value of the high potential avoidance voltage x the number of cells is higher than the voltage of the power storage device S. Note that the fuel cell system 1 sets the voltage of the fuel cell FC lower than the voltage of the power storage device S while the fuel cell FC is generating power. This clarifies the timing at which the DCDC converter Cnv should operate, making it easier to control the DCDC converter Cnv.

When power generation is stopped, the control unit Cnt makes it possible for the power generated by the fuel cell FC to be charged to the storage device via at least one of the first path (diodes D1, D3, D5) or the second path (switching elements Q1, Q3, Q5).

In this embodiment, the power generated by the fuel cell FC when power generation is stopped refers to the power generated by the reaction of hydrogen and oxygen remaining in the fuel cell system 1, among the hydrogen and oxygen whose supply has been reduced to zero based on the control of the control unit Cnt. Furthermore, stopping power generation by the fuel cell FC does not only mean a case where the amount of power generated by the fuel cell FC becomes completely zero, but also includes a case where power generation by the fuel cell FC is suppressed and the amount of power generated by the fuel cell FC becomes infinitely close to zero. Therefore, when power generation is stopped, the current of the fuel cell FC can be charged in the power storage device S by flowing current through the first path (diodes D1, D3, D5).

In addition, in the fuel cell system 1, the voltage of the fuel cell FC is set not to exceed a second threshold value so that the storage device S is not overcharged. This second threshold voltage is hereinafter referred to as the "power generation stop voltage". The power generation stop voltage is a value specific to the fuel cell system 1. For example, the voltage at a predetermined percentage (remaining amount of stored electricity) of the SOC (State Of Charge) of the storage device S is used as the power generation stop voltage. When the voltage of the storage device S exceeds (falls below) the power generation stop voltage, power generation by the fuel cell FC is stopped and the entire fuel cell system 1 is shut down. The number of cells in the fuel cell FC is determined by the following formula 2. The "high potential avoidance voltage" in the following formula 2 is the voltage across the fuel cell FC.

Number of cells × high potential avoidance voltage = power generation stop voltage + Vf ... formula 2
Vf is a threshold voltage (rise voltage) for the loss caused by diodes D1, D3, and D5.

When the number of cells in the fuel cell FC is set so that Equation 2 holds, the number of cells in the fuel cell FC can be set large, and as a result, the output of the fuel cell FC can be increased. Furthermore, in a voltage (region) where the voltage of the fuel cell FC is smaller than the voltage of the storage device S, the voltage of the fuel cell FC can be reduced by passively stepping down the voltage (hereinafter also referred to as "passive step-down") to avoid high potential by simply allowing current to flow without operating the boost converter. In other words, passive step-down is performed by setting the duty ratio of the control signals of the switching elements Q1 and Q2 constituting the u phase, the switching elements Q3 and Q4 constituting the v phase, and the switching elements Q5 and Q6 constituting the w phase to 0.

When current is drawn by the load Lo, the voltage of the fuel cell FC drops, so the DCDC converter Cnv boosts the voltage of the fuel cell FC to output the desired voltage. When boosting the voltage, the DCDC converter Cnv turns off all of the switching elements Q1, Q3, and Q5 of the upper arm, and turns on and off the switching elements Q2, Q4, and Q6 of the lower arm with a phase shift.

When no current is being drawn by the load Lo, the control unit Cnt stops power generation by the fuel cell FC. However, even if power generation by the fuel cell FC is stopped, if no power is being drawn by the load Lo, the voltage of the fuel cell FC increases due to the power generated by the reaction of the remaining fuel (hydrogen) and air (oxygen). Then, when the following formula 3 is satisfied and the voltage of the fuel cell FC becomes higher than the voltage of the storage device S, it is no longer possible to boost the voltage by the DCDC converter Cnv. In the following formula 3, the "voltage of the fuel cell FC" refers to the voltage across the fuel cell FC.

Voltage of fuel cell FC>Voltage of power storage device S...Equation 3
Even if the state of Equation 3 occurs when power generation is stopped, by setting the number of cells to the number set by Equation 2, the current of the fuel cell FC can be charged by the power storage device S so that the current flows toward the first path by passive voltage reduction, and the state in which the cells of the fuel cell FC are equal to or lower than the high potential avoidance voltage can be maintained. When current is passed passively by passive voltage reduction, the DCDC converter Cnv turns off all of the switching elements Q1, Q3, and Q5 of the upper arm and the switching elements Q2, Q4, and Q6 of the lower arm. When the relationship of Equation 3 is established, the current generated in the fuel cell FC flows to the power storage device S through the diodes D1, D3, and D5 of the upper arm.

Therefore, by utilizing the characteristics of the upper arm diodes D1, D3, and D5, which are parasitic diodes, high potential can be avoided by passive voltage reduction. In addition, since it is possible to allow the voltage of the fuel cell FC to overlap with the voltage of the power storage device S, it is possible to increase the number of cells in the fuel cell FC and increase the output of the fuel cell FC.

Furthermore, when the number of cells satisfies Equation 2, a high potential can be avoided by passive step-down without operating (i.e., controlling) the step-up DC-DC converter Cnv, while the optimal output state of the fuel cell FC is determined. Therefore, the number of cells can be optimized without complicating the control of the fuel cell system 1.

In this way, in the fuel cell system 1 of the embodiment, when power generation by the fuel cell FC is stopped, the voltage of the fuel cell FC is made higher than the voltage of the power storage device S. Furthermore, the fuel cell system 1 is configured so that when power generation by the fuel cell FC is stopped, the power generated by the fuel cell FC can be charged to the power storage device S by at least one of the diodes D1, D3, and D5 (first path) or the switching elements Q1, Q3, and Q5 (second path).

As a result, when no current is drawn by the load Lo, the control unit Cnt can charge the current of the fuel cell FC in the storage device S by determining the number of cells so that the current flows toward the first path by passive step-down even when power generation by the fuel cell FC is stopped. This makes it possible to avoid high potential and improve the output of the fuel cell system.

The control unit Cnt also controls the DCDC converter Cnv so that the voltage of the fuel cell FC is lower than the voltage of the storage device S while the fuel cell FC is generating electricity. Specifically, the control unit Cnt switches the target power generation power in stages according to the charge amount (SOC) of the storage device S as a power generation control. When the charge amount of the storage device S decreases, power generation by the fuel cell FC is started, and the control unit Cnt controls the DCDC converter Cnv so that the target current corresponds to the required power. This makes it possible to clarify the timing at which the DCDC converter Cnv should operate, making it easier to control the DCDC converter Cnv.

The control unit Cnt also sets the lowest voltage in the power generation voltage range of the fuel cell FC (the range of the lower limit voltage and the upper limit voltage in the control of the fuel cell FC) to a voltage lower than the lowest voltage in the utilization voltage range of the storage device S (the range of voltages set from the lower limit SOC and the upper limit SOC of the storage device S). This makes it possible to utilize the performance of the storage device S as much as possible.

The switching element of the DCDC converter Cnv is configured with a MOSFET, and the diode is configured with a parasitic diode of the MOSFET. This allows the DCDC converter Cnv to be configured with a simpler structure than when it is configured with a mechanical switch, etc., and allows the fuel cell system 1 to be made smaller.

Next, the relationship between the voltage and power of the fuel cell FC and the power storage device S will be described with reference to FIG. 2. FIG. 2 is a diagram showing the relationship between the voltage and power of the fuel cell FC and the power storage device S. In FIG. 2, the dotted line Vf indicates the relationship between the voltage of the fuel cell FC and the generated power, and the dashed line Vs indicates the relationship between the voltage of the power storage device S and the discharged power. Note that the dashed line Vs indicates that the more the power storage device S discharges (the more the discharged power increases), the lower the voltage becomes. Also, the open circle is the target generated power targeted by the control unit Cnt. The threshold voltage Vt indicates the voltage at which the entire fuel cell system 1 stops.

In FIG. 2 (1), areas A1 and A2 indicate areas where passive step-down is performed, and area A3 indicates an area where step-up is performed. Area A1 indicates an area where passive step-down is performed on the low load side, and area A2 indicates an area where passive step-down is performed on the high load side.

When the power generation amount Win of the fuel cell FC is lower than the power amount Wout required by the load Lo, that is, when a high load is applied, power is drawn from the power storage device S, the SOC of the power storage device S falls, and the voltage of the power storage device S falls. When the voltage of the power storage device S falls to region A2 where the voltage of the fuel cell FC is greater than the voltage of the power storage device S, passive voltage reduction is performed.

When the region A2 occurs where passive voltage drop occurs on the high load side, the power generated by the fuel cell FC becomes large relative to the amount of oxidant gas supplied to the fuel cell FC. This is because the current (amount of charge) released from the fuel cell FC becomes large as a result of passive voltage drop. The control unit Cnt controls the compressor Cp so that the amount of air is not insufficient relative to the amount of hydrogen supplied, but the occurrence of the passive voltage drop phenomenon causes the power generated by the fuel cell FC to increase, resulting in a shortage of oxidant gas relative to the power, a so-called air shortage state, and a sudden drop in the PV characteristics of the fuel cell FC.

More specifically, if the state where only the amount of oxidant gas required for the target power generation at C in Figure 2 (2) is supplied continues, a passive voltage drop phenomenon will occur, resulting in an air shortage state in which the oxidant gas is insufficient. The two-dot chain line Vf1 shows the relationship between the voltage and power of the fuel cell FC when an air shortage state occurs at stage C. When an air shortage state occurs, as shown by the two-dot chain line Vf1, the PV characteristics of the fuel cell FC will deteriorate and the power generation amount Win will not increase any further. Meanwhile, the power drawn from the storage device S will continue, and eventually the storage device S will become over-discharged and fall below the threshold voltage Vt, causing the entire fuel cell system 1 to shut down.

In this embodiment, when passive step-down is performed on the high load side, the amount of oxidant gas supplied to the fuel cell FC is increased at stage C in FIG. 2 (3) to eliminate the air shortage state, thereby improving the PV characteristics of the fuel cell FC. The dashed line Vf2 shows the relationship between the voltage and power of the fuel cell FC when the amount of oxidant gas supplied is increased at stage C. By improving the PV characteristics of the fuel cell FC, the power generation amount Win of the fuel cell FC increases as shown by the dashed line Vf2, and by suppressing the power drawn from the power storage device S, the time until the power storage device S becomes equal to or lower than the threshold voltage Vt can be extended, and the operating time (life extension) of the fuel cell system can be extended in response to a power shortage of the power storage device.

Figure 3 is a diagram showing an example of the transition of the control state of the fuel cell system 1. The control unit Cnt of the fuel cell system 1 controls the control state of the fuel cell system 1.

Figure 3 (1) shows the control state of a conventional fuel cell system. When a forklift equipped with the fuel cell system 1 is climbing a slope, the fuel cell system 1 is required to output a higher output than normal from the load Lo (step S11). If the high output is required and as a result an output power higher than the output power of the fuel cell Fc is required, the fuel cell system 1 takes power from the power storage device S and supplies it to the load Lo in addition to the output power from the fuel cell Fc (step S12).

However, because the storage capacity of the power storage device S is limited, if power is continuously drawn from the power storage device S, the voltage of the power storage device S will drop and the power storage device S will become over-discharged. As a result, the entire fuel cell system 1 will stop operating due to a power shortage in the power storage device S (step S13).

Therefore, in this embodiment, when the amount of power generated by the fuel cell Fc is lower than the amount of power required by the load Lo, the amount of oxidant gas supplied to the fuel cell Fc is increased to extend the operating time of the fuel cell system 1 to compensate for the power shortage of the power storage device S.

Figure 3 (2) shows the control state of the fuel cell system 1 of this embodiment.

For example, when a forklift equipped with the fuel cell system 1 continues to climb a slope, the fuel cell system 1 is required to provide a higher output than normal from the load Lo (step S21). If the higher output is required and the output power required is greater than the output power of the fuel cell Fc, the fuel cell system 1 takes power from the power storage device S and supplies it to the load Lo in addition to the output power from the fuel cell Fc (step S22).

Here, the control unit Cnt determines whether or not passive step-down has been detected by satisfying certain conditions. The detection of passive step-down is determined based on whether or not the following four certain conditions are satisfied:

First condition: voltage of fuel cell Fc>voltage of power storage device S Second condition: current of DCDC converter Cnv>0
Third condition: Duty ratio of the DCDC converter Cnv=0
Fourth condition: Amount of power required by load Lo > Amount of power generated by fuel cell Fc In the first condition, the voltage of the fuel cell Fc is the voltage detected by voltage detection unit V1, and the voltage of the storage device S is the voltage detected by voltage detection unit V2.

In the second condition, the condition that the current of the DCDC converter Cnv is 0 or more may be determined by the condition that the currents of the u phase and the v phase are 0 or more. Also, the condition that the current of the DCDC converter Cnv is 0 or more may be determined by the condition that the currents of the u phase, the v phase, and the w phase are 0 or more.

The third condition, that the duty ratio of the DC-DC converter Cnv is 0, is a state in which no switching operation is performed by the switching elements Q1 to Q6, and may be determined by the condition that the duty ratios of the u phase and the v phase are 0. The condition that the duty ratio of the DC-DC converter is 0 may also be determined by the condition that the duty ratios of the u phase, the v phase, and the w phase are 0. The currents of the u phase, the v phase, and the w phase are currents detected by the current detection units Si1, Si2, and Si3, respectively.

When the second and third conditions are met, a current flows passively through the DCDC converter Cnv via the diodes D1 to D6, even though no switching operation is being performed by the switching elements Q1 to Q6 of the DCDC converter Cnv.

In the fourth condition, the voltage of the load Lo is the system voltage detected by the voltage detection unit V3, and the voltage of the fuel cell Fc is the voltage detected by the voltage detection unit V1.

When a passive pressure drop is detected (step S23), the control unit Cnt controls the compressor Cp to increase the amount of oxidant gas supplied to the fuel cell Fc (step S24). Increasing the amount of oxidant gas (oxygen) supplied from the compressor Cp to the fuel cell Fc increases the amount of electricity generated by the fuel cell FC. In other words, when the amount of oxidant gas is increased, it reacts with the unreacted fuel (hydrogen) supplied from the hydrogen tank Ht, and as a result, the amount of electricity generated by the fuel cell Fc can be increased. In other words, by increasing the amount of electricity generated by the fuel cell Fc, the amount of electricity taken out from the power storage device S can be suppressed, and the operating time of the fuel cell system 1 as a whole can be extended.

Increasing the supply of oxidant gas to increase the amount of power generated by the fuel cell Fc results in an increase in the temperature of the components that make up the fuel cell system 1 (step S31). The control unit Cnt determines whether the component temperature has reached or exceeded a threshold value. As the component temperature threshold value, for example, the usable temperature range of each component that makes up the fuel cell system 1 is set.

When the component temperature reaches or exceeds the threshold value (step S32), the compressor Cp is controlled to return the amount of oxidant gas supplied to the fuel cell Fc to its original state before the increase (step S25). By returning (reducing) the amount of oxidant gas (oxygen) supplied from the compressor Cp to the fuel cell Fc, the operating state of each component operating the fuel cell system 1 can be returned to a normal state, preventing a rise in temperature and suppressing failure of the fuel cell system 1 due to overheating.

In addition, if the amount of oxidant gas supplied to the fuel cell Fc is returned to its original state before the increase, the amount of power generated by the fuel cell FC will decrease, and the amount of power supplied to the fuel cell system 1 as a whole will be insufficient. Therefore, in addition to the output power from the fuel cell Fc, power is taken from the power storage device S and supplied to the load Lo (step S33).

If power continues to be drawn from the storage device S after that, the voltage of the storage device S drops and the storage device S becomes over-discharged. As a result, the entire fuel cell system 1 is shut down due to a power shortage in the storage device S (step S34).

As described above, when a passive voltage drop is detected, the amount of power generated by the fuel cell Fc is increased by increasing the amount of oxidant gas supplied to the fuel cell FC. This reduces the amount of power drawn from the power storage device S, and enables the operating time of the fuel cell system 1 as a whole to be extended.

Next, the fuel cell control process executed by the fuel cell system 1 will be described with reference to FIG. 4. FIG. 4 is a diagram showing an example of the fuel cell control process executed by the control unit Cnt of the fuel cell system 1.

First, when a high output is requested from the load Lo (step S41), the control unit Cnt draws power from the storage device S (step S42).

After power is taken out from the power storage device S, the control unit Cnt determines whether or not passive step-down has been detected (step S43).

When a passive voltage drop is not detected (step S43: NO), power continues to be taken out from the power storage device S. When a passive voltage drop is detected (step S43: YES), the control unit Cnt controls the compressor Cp to increase the amount of oxidant gas supplied to the fuel cell FC (step S44).

As a result of increasing the amount of oxidant gas supplied and increasing the amount of power generated by the fuel cell FC, it is determined whether the temperature of the components constituting the fuel cell system 1 is equal to or higher than a threshold value (step S45). If the component temperature is below the threshold value (step S45: NO), the control unit Cnt continues to increase the amount of oxidant gas supplied. If the component temperature is equal to or higher than the threshold value (step S45: YES), the control unit Cnt controls the compressor Cp to return the amount of oxidant gas supplied to the original state before the increase in step S44 (step S46).

If the amount of oxidant gas supplied to the fuel cell Fc is returned to its original state before the increase, the amount of power generated by the fuel cell FC will decrease, and the amount of power supplied to the fuel cell system 1 as a whole will be insufficient. Therefore, the control unit Cnt takes power from the power storage device S and supplies it to the load Lo in addition to the output power from the fuel cell Fc (step S47).

The control unit Cnt determines whether the SOC of the power storage device S has become equal to or lower than a threshold value as a result of the power being drawn from the power storage device S (step S48). The SOC of the power storage device S is set to a value that is necessary and sufficient for operating the fuel cell system 1. If the SOC of the power storage device S is greater than the threshold value (step S48: NO), the control unit Cnt continues drawing power from the power storage device S. If the SOC of the power storage device S is equal to or lower than the threshold value (step S48: YES), the control unit Cnt stops the operation of the fuel cell system 1 for the safety of the fuel cell system 1 (step S49). This ends the fuel cell control process executed by the fuel cell system 1.

<Other embodiments>
The fuel cell system 1 may be a DC-DC converter as shown in Fig. 5. In this case, there is no second path having a switching element, but when the boost operation by the DC-DC converter is stopped, a current flows through a first path having a diode, and a high potential of the fuel cell FC is avoided.

The specified conditions are not limited to those described in the above embodiment. For example, the third condition, judging whether the duty ratio of the DCDC converter Cnv is 0 or not, may be omitted. The fourth condition, judging whether the amount of power required by the load Lo is greater than the amount of power generated by the fuel cell Fc, may be omitted. In the above embodiment, when current is passed by passive step-down, current is passed to the diodes D1, D3, and D5 of the upper arm, but this is not limited to the above.

The control unit Cnt may turn on (closed state) the switching elements Q1, Q3, and Q5 when it detects that the voltage of the fuel cell FC detected by the voltage detection unit V1 is higher than the voltage of the storage device S detected by the voltage detection unit V2. As a result, even if the voltage of the fuel cell FC is higher than the voltage of the storage device S (the state of formula 3 above), the current generated in the fuel cell FC flows through the switching elements Q1, Q3, and Q5 to the storage device S. This eliminates power loss due to the diodes, and allows the power output by the fuel cell FC to be efficiently charged to the storage device S.

The control unit Cnt may also be capable of detecting the ON-OFF state (open/closed state) of the switching elements Q1 to Q6. When the control unit Cnt detects that a current is flowing through the first path or the second path when the ON-OFF operation of the switching elements Q1 to Q6 is not being performed, it sets the switching elements Q1 to Q6 to the OFF state (closed state). As a result, even if the voltage of the fuel cell FC is greater than the voltage of the storage device S (the state of formula 3 above), the current generated in the fuel cell FC flows through the switching elements Q1, Q3, and Q5 to the storage device S. This eliminates power loss due to the diodes, and allows the power output by the fuel cell FC to be efficiently charged to the storage device S.

1 Fuel cell system Ve Vehicle Lo Load FC Fuel cell Si0 Current detection unit V1, V2 Voltage detection unit Co Capacitors Re1, Re2, Re3 Reactors Si1, Si2, Si3 Current detection unit Cnv DCDC converter S Storage device Cnt Control unit Q1 to Q6 Switching elements D1 to D6 Diode Cp Compressor Ht Hydrogen tank

Claims (2)

a fuel cell stack that generates electric power from a fuel gas and an oxidant gas;
A power storage device;
a boost converter electrically connected to the fuel cell stack and the power storage device, and configured to convert a voltage of the fuel cell stack into a predetermined voltage;
a control unit that controls the boost converter and the fuel cell stack;
A fuel cell system comprising:
The control unit is
increasing an amount of the oxidant gas supplied to the fuel cell stack when a predetermined condition is satisfied in a state in which an amount of power generated by the fuel cell stack is lower than an amount of power required by a load;
The predetermined condition is:
the voltage of the fuel cell stack is greater than the voltage of the power storage device,
the current through the boost converter is greater than zero;
A fuel cell system according to claim 1, wherein the duty ratio of the boost converter is 0. 2. The fuel cell system according to claim 1 ,
The control unit is
A fuel cell system characterized in that, when the temperature of a component constituting the fuel cell system reaches or exceeds a threshold value, the fuel cell stack returns the amount of oxidant gas supplied to its original state before the increase, or reduces the amount supplied.

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