Fuel cell system

Abstract

To update an offset correction value for a current flowing between the fuel cell stack and a DC / DC converter during intermittent power generation of a fuel cell stack, and reduce an error occurring in the offset correction value.SOLUTION: A fuel cell system FCS starts the drive of a DC / DC converter CNV when a voltage Vf exceeds a threshold Vth1 during intermittent power generation of a fuel cell stack FC, ends the drive of the DC / DC converter CNV when the voltage Vf falls below a threshold Vth2, and supplies fuel gas to the fuel cell stack FC when the voltage Vf falls below a threshold Vth3. During intermittent power generation of the fuel cell stack FC, in a period from when the voltage Vf falls below the threshold Vth2 until when the voltage Vf exceeds the threshold Vth1, the fuel cell system updates an offset correction value for a current If flowing between the fuel cell stack FC and the DC / DC converter CNV.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a fuel cell system. [Background technology]

[0002] There is a fuel cell system that updates an offset correction value of a current flowing between a fuel cell stack and a DC-DC converter while the fuel cell stack is generating intermittent power.

[0003] During intermittent power generation by the fuel cell stack, the DC-DC converter is temporarily driven and fuel gas is repeatedly supplied to the fuel cell stack in order to fluctuate the voltage of the fuel cell stack within a predetermined voltage range. That is, during intermittent power generation by the fuel cell stack, the DC-DC converter is temporarily driven, and there is a period during which current flows between the fuel cell stack and the DC-DC converter.

[0004] Therefore, in the above fuel cell system, there is a risk that the offset correction value will be updated during the period when current flows between the fuel cell stack and the DC-DC converter during intermittent power generation by the fuel cell stack. If the offset correction value is updated during the period when current flows between the fuel cell stack and the DC-DC converter, an error will occur in the offset correction value, causing a deviation between the stoichiometric ratio of the oxidant gas supplied to the fuel cell stack and the true value, which may result in a decrease in the power generation efficiency of the fuel cell stack or an abnormality such as water clogging in the fuel cell stack. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-327102 Summary of the Invention [Problem to be solved by the invention]

[0006] An object of one aspect of the present invention is to reduce errors that occur in the offset correction value when updating the offset correction value for the current flowing between the fuel cell stack and the DC-DC converter during intermittent power generation by the fuel cell stack. [Means for solving the problem]

[0007] A fuel cell system according to one embodiment of the present invention comprises a fuel cell stack, a battery, a DCDC converter provided between the fuel cell stack and the battery, a current sensor for detecting a current flowing between the fuel cell stack and the DCDC converter, a first voltage sensor for detecting a voltage of the fuel cell stack, and a control unit for starting operation of the DCDC converter when the voltage detected by the first voltage sensor exceeds a first threshold value during intermittent power generation of the fuel cell stack, terminating operation of the DCDC converter when the voltage detected by the first voltage sensor falls below a second threshold value that is smaller than the first threshold value, and supplying oxidant gas to the fuel cell stack when the voltage detected by the first voltage sensor falls below a third threshold value that is smaller than the second threshold value, and for updating an offset correction value of the current detected by the current sensor during intermittent power generation of the fuel cell stack during the period from when the voltage detected by the first voltage sensor falls below the second threshold value until the voltage detected by the first voltage sensor exceeds the first threshold value.

[0008] This allows the offset correction value to be updated during periods when no current is flowing between the fuel cell stack and the DC-DC converter during intermittent power generation by the fuel cell stack, thereby reducing errors that occur in the offset correction value.

[0009] The control unit may also be configured to update the offset correction value when the voltage detected by the first voltage sensor exceeds the first threshold during intermittent power generation of the fuel cell stack, and then start driving the DC-DC converter.

[0010] This allows the offset correction value to be updated immediately before the DC-DC converter starts to be driven, thereby improving the accuracy of correction of the current detected by the current sensor.

[0011] The fuel cell system may also include a second voltage sensor that detects the voltage of the battery, and the control unit may be configured to update the offset correction value when, during intermittent power generation by the fuel cell stack, the voltage detected by the second voltage sensor is equal to or greater than the voltage detected by the first voltage sensor during the period from when the voltage detected by the first voltage sensor falls below the second threshold to when the voltage detected by the first voltage sensor exceeds the first threshold.

[0012] This allows the offset correction value to be updated during periods when no current flows between the fuel cell stack and the DCDC converter and when no current flows from the fuel cell stack to the battery via the DCDC converter during intermittent power generation by the fuel cell stack, thereby further reducing errors that occur in the offset correction value. [Effects of the Invention]

[0013] According to the present invention, when updating the offset correction value of the current flowing between the fuel cell stack and the DC-DC converter during intermittent power generation by the fuel cell stack, it is possible to reduce errors that occur in the offset correction value. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a diagram illustrating an example of a fuel cell system according to an embodiment. [Figure 2] 4 is a flowchart showing the operation of a control unit during intermittent power generation in the first embodiment. [Figure 3] 10 is a flowchart showing the operation of a control unit during intermittent power generation in the second embodiment. [Figure 4] 11 is a flowchart showing the operation of a control unit during intermittent power generation in the third embodiment. [Figure 5]10 is a flowchart showing the operation of a control unit during intermittent power generation in the fourth embodiment. [Figure 6] FIG. 4 is a diagram showing an example of voltages detected by a voltage sensor during intermittent power generation and threshold values. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, the embodiments will be described in detail with reference to the drawings.

[0016] FIG. 1 is a diagram illustrating an example of a fuel cell system according to an embodiment.

[0017] The fuel cell system FCS shown in Fig. 1 is mounted on a vehicle such as a forklift, a towing tractor, or an automatic guided vehicle (AGV), and supplies power to a load Lo mounted on the vehicle. In this configuration, the load Lo is, for example, an inverter circuit that drives a loading device or a travel motor. The fuel cell system FCS may also be provided in a stationary generator such as an industrial stationary generator, a home stationary generator, or an emergency stationary generator. In this configuration, the load Lo is, for example, industrial machinery or a home appliance.

[0018] The fuel cell system FCS also includes a fuel cell stack FC, a current sensor Sif, a voltage sensor Svf (first voltage sensor), a current sensor Sib, a voltage sensor Svb (second voltage sensor), a DC-DC converter CNV, a battery B, and a control unit Cs.

[0019] The fuel cell system FCS also includes auxiliary equipment such as an injector (not shown) and a hydrogen circulation pump (not shown) that supply fuel gas (hydrogen gas) to the fuel cell stack FC, which is the main unit, and an air compressor (not shown) that supplies oxidant gas (air) to the fuel cell stack FC.

[0020] The fuel cell stack FC is composed of multiple fuel cell units connected in series, and generates electricity through an electrochemical reaction between hydrogen contained in fuel gas supplied from a fuel tank (not shown) via an injector and oxygen contained in oxidant gas supplied from an air compressor.

[0021] The current sensor Sif is composed of a shunt resistor, a Hall element, etc., and detects the current If flowing between the fuel cell stack FC and the DCDC converter CNV, and sends the detected current If to the DCDC converter CNV and the control unit Cs.

[0022] The voltage sensor Svf is configured with a plurality of voltage dividing resistors and the like, detects the voltage Vf of the fuel cell stack FC, and sends the detected voltage Vf to the DC-DC converter CNV and the control unit Cs.

[0023] The current sensor Sib is configured with a shunt resistor, a Hall element, etc., and detects a current Ib flowing through the battery B and sends the detected current Ib to the control unit Cs.

[0024] The voltage sensor Svb is configured with a plurality of voltage dividing resistors and the like, detects the voltage Vb of the battery B, and sends the detected voltage Vb to the control unit Cs.

[0025] The DC-DC converter CNV is provided between the fuel cell stack FC and the battery B, and includes an inductor L, a diode D, a switching element SW, a capacitor C, and a control unit Cc. The switching element SW is configured, for example, by an IGBT (Insulated Gate Bipolar Transistor), with a diode connected in parallel. The DC-DC converter CNV is not limited to the circuit configuration shown in FIG. 1, as long as it is a boost circuit including at least the diode D.

[0026] One terminal of inductor L is connected to one terminal of fuel cell stack FC, and the other terminal of inductor L is connected to the connection point between the anode terminal of diode D and the collector terminal of switching element SW. The cathode terminal of diode D is connected to one terminal of capacitor C, and the emitter terminal of switching element SW is connected to the other terminal of fuel cell stack FC and the other terminal of capacitor C. One terminal of the accessories, one terminal of load Lo, and the positive terminal of battery B are each connected to one terminal of capacitor C, and the other terminal of the accessories, the other terminal of load Lo, and the negative terminal of battery B are each connected to the other terminal of capacitor C.

[0027] The control unit Cc is configured with a microcomputer or the like and controls the operation of the switching element SW. For example, when the voltage Vf detected by the voltage sensor Svf is lower than the target output voltage sent from the control unit Cs, the control unit Cc repeatedly turns the switching element SW on and off to boost the voltage Vf to the target output voltage. Furthermore, when the voltage Vf is equal to or higher than the target output voltage, the control unit Cc keeps the switching element SW off at all times and does not boost the voltage Vf. When the switching element SW is always off (when the DC-DC converter CNV is not operating) and the voltage Vb is lower than the voltage Vf, a current flows from the fuel cell stack FC to the battery B via the DC-DC converter CNV (inductor L, diode D, and capacitor C).

[0028] Battery B is composed of a lithium-ion capacitor or the like and is connected to the connection point between the DC-DC converter CNV, the auxiliary equipment, and the load Lo. Battery B and fuel cell stack FC work together to supply power to the load Lo. For example, if the supply power corresponding to the difference between the power output from the fuel cell stack FC (DC-DC converter CNV) and the power supplied to the auxiliary equipment is greater than the power requested by the load Lo, the requested power is supplied to the load Lo, and the remaining power is supplied to battery B. When power is supplied from the fuel cell stack FC to battery B, battery B is charged, and the charging rate of battery B (the percentage [%] of the remaining capacity relative to the full charge capacity of battery B) increases. Furthermore, when regenerative power supplied from the load Lo to the fuel cell system FCS is supplied to battery B, battery B is charged, and the charging rate of battery B increases. Furthermore, when the supply power corresponding to the difference between the power output from the fuel cell stack FC (DC-DC converter CNV) and the power supplied to the auxiliary equipment is smaller than the power requested by the load Lo, the supplied power is supplied to the load Lo, and the shortfall in power is supplied from battery B to the load Lo. When power is supplied from battery B to load Lo, battery B is discharged and the charge rate of battery B decreases.

[0029] The control unit Cs is configured with a microcomputer or the like, and controls the operation of the auxiliary devices to control the power generation of the fuel cell stack FC. For example, during normal power generation by the fuel cell stack FC, the control unit Cs gradually changes the target power generation in accordance with the result of comparing the charging rate of the battery B with multiple switching thresholds, and controls the operation of the auxiliary devices using PI (Proportional-Integral) control or the like so that the power generation of the fuel cell stack FC follows the target power generation. For example, the control unit Cs calculates the stoichiometric ratio of the oxidant gas based on a current command value corresponding to the target power generation, and controls the drive of the motor of the air compressor so that the rotation speed of the motor follows the rotation speed command value corresponding to the calculated stoichiometric ratio. The stoichiometric ratio is the ratio of the flow rate of the oxidant gas actually supplied to the fuel cell stack FC to the theoretical flow rate of the oxidant gas required to output a desired current from the fuel cell stack FC.

[0030] Furthermore, during normal power generation by the fuel cell stack FC, the control unit Cs controls the power generation of the fuel cell stack FC using a corrected current If' obtained by subtracting the offset correction value from the current If, or a corrected current If' obtained by adding the current If to the offset correction value. For example, if the difference between the corrected current If' and the current command value is relatively large, the control unit Cs corrects the stoichiometric ratio so that the difference becomes zero. This reduces the effect of offset errors that occur in the detection value of the current sensor Sif, and makes it possible to bring the corrected stoichiometric ratio closer to the true value.

[0031] The offset correction value is the current detected by the current sensor Sif when no current flows between the fuel cell stack FC and the DC-DC converter CNV. If the offset correction value fluctuates with temperature changes in the fuel cell system FCS, it is desirable to periodically update the offset correction value. For example, when the control unit Cs stops driving the DC-DC converter CNV during startup of the fuel cell system FCS or during intermittent power generation by the fuel cell stack FC (described later), the control unit Cs sets the current detected by the current sensor Sif as the new offset correction value.

[0032] Furthermore, if the control unit Cs determines that the charging rate of battery B is relatively high during normal power generation by the fuel cell stack FC and that the power required by the load Lo can be met solely by the power output from battery B, it transitions from normal power generation to intermittent power generation in order to reduce fuel gas consumption. Furthermore, if the control unit Cs determines that the charging rate of battery B is relatively low during intermittent power generation by the fuel cell stack FC and that the power required cannot be met solely by the power output from battery B, it transitions from intermittent power generation to normal power generation. Note that while the fuel cell stack FC is not able to output a relatively large amount of power during intermittent power generation, it is maintained in a state in which it can return from intermittent power generation to normal power generation at any time.

[0033] Furthermore, during intermittent power generation by the fuel cell stack FC, the control unit Cs repeatedly starts and stops the operation of the DC-DC converter CNV and repeatedly supplies fuel gas to the fuel cell stack FC so that the voltage Vf detected by the voltage sensor Svf does not exceed an upper limit Vmax and so that the voltage Vf does not fall below a lower limit Vmin. For example, the upper limit Vmax is the minimum value of the voltage Vf when the catalyst in the fuel cell stack FC dissolves, and the lower limit Vmin is the maximum value of the voltage Vf when the catalyst in the fuel cell stack FC condenses.

[0034] Furthermore, during intermittent power generation by the fuel cell stack, when the voltage Vf detected by the voltage sensor Svf exceeds a threshold Vth1 (first threshold), the control unit Cs starts driving the DC-DC converter CNV, when the voltage Vf falls below a threshold Vth2 (second threshold), the control unit Cs stops driving the DC-DC converter CNV, and when the voltage Vf falls below a threshold Vth3 (third threshold), the control unit Cs supplies fuel gas to the fuel cell stack FC. Note that the threshold Vth1 is set to an arbitrary value smaller than the upper limit value Vmax, the threshold Vth2 is set to an arbitrary value smaller than the threshold Vth1, and the threshold Vth3 is set to an arbitrary value smaller than the threshold Vth2 and larger than the lower limit value Vmin. The threshold Vth1 may be set based on the responsiveness of the DC-DC converter CNV1, and the threshold Vth2 may be set based on the responsiveness of the auxiliary devices. This allows the voltage Vf to fluctuate between the threshold value Vth1 and the threshold value Vth3 during intermittent power generation by the fuel cell stack FC, thereby preventing the voltage Vf from exceeding the upper limit value Vmax or falling below the lower limit value Vmin, thereby preventing deterioration of the fuel cell stack FC.

[0035] Furthermore, during intermittent power generation of the fuel cell stack FC, the control unit Cs updates the offset correction value during the period from when the voltage Vf falls below the threshold Vth2 and stops driving the DC-DC converter CNV until the voltage Vf exceeds the threshold Vth1 and starts driving the DC-DC converter CNV. This allows the offset correction value to be updated during the period when no current flows from the fuel cell stack FC to the DC-DC converter CNV during intermittent power generation of the fuel cell stack FC, thereby reducing errors that occur in the offset correction value. In other words, when the offset correction value is updated during intermittent power generation of the fuel cell stack FC, errors that occur in the offset correction value can be reduced.

[0036] Furthermore, the control unit Cs may be configured to update the offset correction value if, during intermittent power generation by the fuel cell stack FC, the voltage Vb detected by the voltage sensor Svb is equal to or higher than the voltage Vf during the period from when the voltage Vf falls below the threshold Vth2 until when the voltage Vf exceeds the threshold Vth1. This makes it possible to update the offset correction value during the period when no current flows between the fuel cell stack FC and the DC-DC converter CNV and when no current flows from the fuel cell stack FC to the battery B via the DC-DC converter CNV during intermittent power generation by the fuel cell stack FC, thereby further reducing errors that occur in the offset correction value.

[0037] Example 1 2 is a flowchart showing the operation of the control unit Cs during intermittent power generation in Example 1. When transitioning from normal power generation to intermittent power generation, the control unit Cs starts driving the DC-DC converter CNV to cause a current to flow from the fuel cell stack FC to the DC-DC converter CNV in order to reduce the voltage of the fuel cell stack FC.

[0038] First, the control unit Cs acquires the voltage Vf detected by the voltage sensor Svf (step S1), and determines whether the voltage Vf exceeds a threshold value Vth1 (step S2), or whether the voltage Vf falls below a threshold value Vth2 (step S3), or whether the voltage Vf falls below a threshold value Vth3 (step S4).

[0039] Furthermore, if the control unit Cs determines that the voltage Vf is not higher than the threshold Vth1 or is not lower than the thresholds Vth2 and Vth3 (steps S2 to S4: No), it repeats the operations of steps S1 to S4.

[0040] Furthermore, when the control unit Cs determines that the voltage Vf has fallen below the threshold Vth2 (step S3: Yes), it terminates the operation of the DC-DC converter CNV (step S5), updates the offset correction value (step S6), and returns to the operations of steps S1 to S4. That is, the control unit Cs terminates the operation of the DC-DC converter CNV when the voltage Vf falls below the threshold Vth2 during intermittent power generation by the fuel cell stack FC. For example, if the voltage Vf detected by the voltage sensor Svf in the previous control cycle was equal to or greater than the threshold Vth2 and the voltage Vf detected by the voltage sensor Svf in the current control cycle is lower than the threshold Vth2, the control unit Cs determines that the voltage Vf has fallen below the threshold Vth2. Furthermore, when updating the offset correction value, the control unit Cs acquires the current If detected by the current sensor Sif and sets the acquired current If as the new offset correction value.

[0041] Furthermore, when the control unit Cs determines that the voltage Vf has fallen below the threshold value Vth3 (step S4: Yes), it supplies fuel gas to the fuel cell stack FC (step S7) and returns to the operations of steps S1 to S4. That is, the control unit Cs starts supplying fuel gas to the fuel cell stack FC at the timing when the voltage Vf falls below the threshold value Vth3 during intermittent power generation by the fuel cell stack FC. For example, if the voltage Vf detected by the voltage sensor Svf in the previous control cycle was equal to or greater than the threshold value Vth3 and the voltage Vf detected by the voltage sensor Svf in the current control cycle is lower than the threshold value Vth3, the control unit Cs determines that the voltage Vf has fallen below the threshold value Vth3.

[0042] Furthermore, when the control unit Cs determines that the voltage Vf has exceeded the threshold value Vth1 (step S2: Yes), it starts driving the DC-DC converter CNV (step S8) and returns to the operations of steps S1 to S4. That is, the control unit Cs starts driving the DC-DC converter CNV at the timing when the voltage Vf exceeds the threshold value Vth1 during intermittent power generation of the fuel cell stack FC. For example, if the voltage Vf detected by the voltage sensor Svf in the previous control cycle was equal to or lower than the threshold value Vth1 and the voltage Vf detected by the voltage sensor Svf in the current control cycle is higher than the threshold value Vth1, the control unit Cs determines that the voltage Vf has exceeded the threshold value Vth1.

[0043] In the first embodiment, during intermittent power generation of the fuel cell stack FC, the offset correction value is updated immediately after the drive of the DCDC converter CNV ends. Immediately after the drive of the DCDC converter CNV ends, no current flows between the fuel cell stack FC and the DCDC converter CNV. In other words, according to the first embodiment, during intermittent power generation of the fuel cell stack FC, the offset correction value can be updated during the period when no current flows between the fuel cell stack FC and the DCDC converter CNV, thereby reducing errors that occur in the offset correction value.

[0044] <Example 2> Fig. 3 is a flowchart showing the operation of the control unit Cs during intermittent power generation in Example 2. Steps S1 to S4 shown in Fig. 3 are the same as steps S1 to S4 shown in Fig. 2, and therefore a description thereof will be omitted. Furthermore, when transitioning from normal power generation to intermittent power generation, the control unit Cs starts driving the DC-DC converter CNV, thereby causing a current to flow from the fuel cell stack FC to the DC-DC converter CNV.

[0045] When the control unit Cs determines that the voltage Vf has fallen below the threshold Vth2 (step S3: Yes), it stops driving the DC-DC converter CNV (step S5) and returns to the operations of steps S1 to S4.

[0046] Furthermore, when the control unit Cs determines that the voltage Vf has fallen below the threshold Vth3 (step S4: Yes), it updates the offset correction value (step S6'), then supplies fuel gas to the fuel cell stack FC (step S7), and returns to the operations of steps S1 to S4. Note that when the control unit Cs determines that the voltage Vf has fallen below the threshold Vth3, it may start power generation in the fuel cell stack FC and then update the offset correction value.

[0047] Furthermore, when the control unit Cs determines that the voltage Vf has exceeded the threshold Vth1 (step S2: Yes), it starts driving the DC-DC converter CNV (step S8) and returns to the operations of steps S1 to S4.

[0048] In the second embodiment, during intermittent power generation of the fuel cell stack FC, the offset correction value is updated when the voltage Vf falls below the threshold Vth3. The timing when the voltage Vf falls below the threshold Vth3 is within the period from when the voltage Vf falls below the threshold Vth2 and the drive of the DC-DC converter CNV ends until when the voltage Vf exceeds the threshold Vth1 and the drive of the DC-DC converter CNV starts. Therefore, when the voltage Vf falls below the threshold Vth3, no current flows from the fuel cell stack FC to the DC-DC converter CNV. In other words, according to the second embodiment, as in the first embodiment, the offset correction value can be updated during the period when no current flows between the fuel cell stack FC and the DC-DC converter CNV during intermittent power generation of the fuel cell stack FC, thereby reducing errors that occur in the offset correction value.

[0049] Example 3 Fig. 4 is a flowchart showing the operation of the control unit Cs during intermittent power generation in Example 3. Steps S1 to S4 shown in Fig. 4 are the same as steps S1 to S4 shown in Fig. 2, and therefore a description thereof will be omitted. Furthermore, when transitioning from normal power generation to intermittent power generation, the control unit Cs starts driving the DC-DC converter CNV, thereby causing a current to flow from the fuel cell stack FC to the DC-DC converter CNV.

[0050] When the control unit Cs determines that the voltage Vf has fallen below the threshold Vth2 (step S3: Yes), it stops driving the DC-DC converter CNV (step S5) and returns to the operations of steps S1 to S4.

[0051] Furthermore, when the control unit Cs determines that the voltage Vf has fallen below the threshold Vth3 (step S4: Yes), it supplies fuel gas to the fuel cell stack FC (step S7) and returns to the operations of steps S1 to S4.

[0052] Furthermore, when the control unit Cs determines that the voltage Vf has exceeded the threshold Vth1 (step S2: Yes), it updates the offset correction value (step S6''), and then starts driving the DC-DC converter CNV (step S8), and returns to the operations of steps S1 to S4.

[0053] In the third embodiment, the offset correction value is updated when the voltage Vf exceeds the threshold Vth1 during intermittent power generation of the fuel cell stack FC. The timing at which the voltage Vf exceeds the threshold Vth1 is within the period from when the voltage Vf falls below the threshold Vth2 and the drive of the DC-DC converter CNV ends until when the voltage Vf exceeds the threshold Vth1 and the drive of the DC-DC converter CNV starts. Therefore, when the voltage Vf exceeds the threshold Vth1, no current flows from the fuel cell stack FC to the DC-DC converter CNV. In other words, according to the second embodiment, as in the first embodiment, the offset correction value can be updated during the period when no current flows between the fuel cell stack FC and the DC-DC converter CNV during intermittent power generation of the fuel cell stack FC, thereby reducing errors that occur in the offset correction value.

[0054] Furthermore, in the third embodiment, the offset correction value is updated immediately before the DC-DC converter CNV starts to operate. The current If detected by the current sensor Sif immediately before the DC-DC converter CNV starts to operate may fluctuate due to the greatest influence of the temperature rise in the fuel cell system FCS caused by the operation of the DC-DC converter CNV. Therefore, by updating the offset correction value using the current If detected by the current sensor Sif immediately before the DC-CDC converter CNV starts to operate, the correction accuracy of the current If' can be improved. That is, according to the third embodiment, the offset correction value is updated using the current If detected by the current sensor Sif immediately before the DC-CDC converter CNV starts to operate, and therefore the correction accuracy of the current If' can be improved.

[0055] Example 4 Fig. 5 is a flowchart showing the operation of the control unit Cs during intermittent power generation in Example 4. Steps S1 to S4 shown in Fig. 5 are the same as steps S1 to S4 shown in Fig. 2, and therefore a description thereof will be omitted. Furthermore, when transitioning from normal power generation to intermittent power generation, the control unit Cs starts driving the DC-DC converter CNV, thereby causing a current to flow from the fuel cell stack FC to the DC-DC converter CNV.

[0056] When the control unit Cs determines that the voltage Vf has fallen below the threshold Vth2 (step S3: Yes), it stops driving the DC-DC converter CNV (step S5) and returns to the operations of steps S1 to S4.

[0057] Furthermore, when the control unit Cs determines that the voltage Vf has fallen below the threshold Vth3 (step S4: Yes), it supplies fuel gas to the fuel cell stack FC (step S7) and returns to the operations of steps S1 to S4.

[0058] Furthermore, when the control unit Cs determines that the voltage Vf has exceeded the threshold value Vth1 (step S2: Yes), it acquires the voltage Vb detected by the voltage sensor Svb (step S9) and determines whether the voltage Vb is equal to or greater than the voltage Vf (step S10).

[0059] When the control unit Cs determines that the voltage Vb is equal to or greater than the voltage Vf (step S10: Yes), it updates the offset correction value (step S6''), then starts driving the DC-DC converter CNV (step S8), and returns to the operations of steps S1 to S4. That is, when the voltage Vb is equal to or greater than the voltage Vf, no current flows from the fuel cell stack FC to the battery B via the DC-DC converter CNV, so by updating the offset correction value in this case, it is possible to further reduce the error that occurs in the offset correction value.

[0060] On the other hand, if the control unit Cs determines that the voltage Vb is lower than the voltage Vf (step S10: No), it does not update the offset correction value, starts driving the DC-DC converter CNV (step S8), and returns to the operations of steps S1 to S4. That is, when the voltage Vb is lower than the voltage Vf, current flows from the fuel cell stack FC to the battery B via the DC-DC converter CNV, and updating the offset correction value in this case may cause an error in the offset correction value, so in this case the offset correction value is not updated.

[0061] According to the fourth embodiment, as in the first to third embodiments, the offset correction value can be updated during the period when no current flows between the fuel cell stack FC and the DC-DC converter CNV during intermittent power generation of the fuel cell stack FC, thereby reducing the error that occurs in the offset correction value.

[0062] Furthermore, according to the fourth embodiment, similar to the third embodiment, the offset correction value can be updated immediately before the DC-DC converter CNV starts to operate during intermittent power generation of the fuel cell stack FC, thereby improving the correction accuracy of the current If'.

[0063] Furthermore, according to the fourth embodiment, the offset correction value can be updated during a period when no current flows between the fuel cell stack FC and the DCDC converter CNV, and during a period when no current flows from the fuel cell stack FC to the battery B via the DCDC converter CNV, thereby further reducing the error that occurs in the offset correction value.

[0064] The operations of steps S9 and S10 shown in FIG. 5 (operations for determining that the offset correction value can be updated if the voltage Vb is equal to or greater than the voltage Vf) may be performed after determining that the voltage Vf has fallen below the threshold Vth2 in FIG. 2 (step S3: Yes), or after determining that the voltage Vf has fallen below the threshold Vth3 in FIG. 3 (step S4: Yes).

[0065] FIG. 6 is a diagram showing an example of the voltage Vf detected by the voltage sensor Svf and the threshold values ​​Vth1 to Vth3 during intermittent power generation. Note that the horizontal axis of the two-dimensional coordinate system shown in FIG. 3 represents time, and the vertical axis represents voltage. The solid line in FIG. 6 shows an example of the voltage Vf varying over time. Assume that time t0<time t1<time 2<time t3, and that at time t0, the voltage Vf is lower than the threshold value Vth1 and higher than the threshold value Vth2.

[0066] First, at time t0, the control unit Cs transitions from normal power generation to intermittent power generation and starts driving the DC-DC converter CNV. Then, current flows from the fuel cell stack FC to the DC-DC converter CNV, and the voltage Vf gradually decreases.

[0067] Next, at time t1, when the control unit Cs determines that the voltage Vf has fallen below the threshold Vth2, it stops driving the DC-DC converter CNV. Then, current stops flowing from the fuel cell stack FC to the DC-DC converter CNV, but the voltage Vf continues to gradually decrease due to natural discharge of the fuel cell stack FC. Note that, as in the first embodiment, the offset correction value may be updated at the timing when the driving of the DC-DC converter CNV is stopped.

[0068] Next, when the control unit Cs determines at time t2 that the voltage Vf has fallen below the threshold value Vth3, it supplies fuel gas to the fuel cell stack FC. Then, the voltage Vf gradually increases. This prevents the voltage Vf from falling below the lower limit value Vmin. Note that, as in the second embodiment, the offset correction value may be updated at the timing when the voltage Vf falls below the threshold value Vth3.

[0069] Next, at time t3, when the control unit Cs determines that the voltage Vf has exceeded the threshold value Vth1, it starts driving the DC-DC converter CNV. When the DC-DC converter CNV starts to be driven and current flows from the fuel cell stack FC to the DC-DC converter CNV, the voltage Vf starts to decrease again. This prevents the voltage Vf from exceeding the upper limit value Vmax. Note that, as in the third embodiment, the offset correction value may be updated at the timing when it is determined that the voltage Vf has exceeded the threshold value Vth1. Alternatively, as in the fourth embodiment, the offset correction value may be updated at the timing when it is determined that the voltage Vf has exceeded the threshold value Vth1 and that the voltage Vb is equal to or greater than the voltage Vf.

[0070] In the fuel cell system FCS of the embodiment, during intermittent power generation of the fuel cell stack FC, the offset correction value is updated not during the period from when the drive of the DCDC converter CNV is started to when the drive of the DCDC converter CNV is stopped to when the drive of the DCDC converter CNV is started, but during the period from when the drive of the DCDC converter CNV is stopped to when the drive of the DCDC converter CNV is started (for example, the period from time 2 to time t3 shown in FIG. 6). This makes it possible to update the offset correction value during the period when no current flows from the fuel cell stack FC to the DCDC converter CNV during intermittent power generation of the fuel cell stack FC, thereby reducing errors that occur in the offset correction value.

[0071] The present invention is not limited to the above-described embodiments, and various improvements and modifications can be made without departing from the spirit and scope of the present invention. [Explanation of symbols]

[0072] FCS fuel cell system FC fuel cell stack Svf, Svb voltage sensor Sif, Sib current sensors CNV DC / DC converter L inductor D diode SW Switching element Cs control section C capacitor Cc control section Lo load

Claims

1. a fuel cell stack; A battery, a DC-DC converter provided between the fuel cell stack and the battery; a current sensor that detects a current flowing between the fuel cell stack and the DC-DC converter; a first voltage sensor for detecting a voltage of the fuel cell stack; a control unit that starts driving the DC-DC converter when the voltage detected by the first voltage sensor exceeds a first threshold during intermittent power generation of the fuel cell stack, stops driving the DC-DC converter when the voltage detected by the first voltage sensor falls below a second threshold that is smaller than the first threshold, and supplies fuel gas to the fuel cell stack when the voltage detected by the first voltage sensor falls below a third threshold that is smaller than the second threshold; Equipped with The control unit updates an offset correction value of the current detected by the current sensor during the period from when the voltage detected by the first voltage sensor falls below the second threshold to when the voltage detected by the first voltage sensor exceeds the first threshold during intermittent power generation by the fuel cell stack. Fuel cell system.

2. 2. The fuel cell system according to claim 1, When the voltage detected by the first voltage sensor exceeds the first threshold during intermittent power generation by the fuel cell stack, the control unit updates the offset correction value and then starts driving the DC-DC converter. Fuel cell system.

3. 3. The fuel cell system according to claim 1, wherein: a second voltage sensor for detecting the voltage of the battery; The control unit updates the offset correction value when, during intermittent power generation by the fuel cell stack, the voltage detected by the second voltage sensor is equal to or higher than the voltage detected by the first voltage sensor during a period from when the voltage detected by the first voltage sensor falls below the second threshold to when the voltage detected by the first voltage sensor exceeds the first threshold. Fuel cell system.

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