Control method for starting a fuel cell system

Abstract

The present invention relates to a control method for starting a fuel cell system after a stop, and has the following steps: an anode supply gas is supplied to the anode supply area (12), and at least temporarily, the supply of the cathode supply gas to the cathode supply area (14) is blocked and / or throttled. According to the present invention, at least a temporary block and / or throttle of the cathode supply gas is performed or continued until the allowable threshold voltage (Uth) is reached or the no-load voltage (Uoc) drops below it; and only after the no-load voltage (Uoc) has dropped to the allowable threshold voltage (Uth) or below it, the switching device (20) of the electrical connection between the fuel cell unit (10) and the DC voltage converter (30) is closed.

Description

Technical Field

[0001] The present invention relates to a control method for starting a fuel cell system after stopping, a control device corresponding thereto, and a computer programming product.

Background Art

[0002] For example, in stationary applications or in a drive train having a large electric drive for commercial vehicles, in a fuel cell system for supplying a high-voltage output to a power consumption unit, usually, the output voltage on the fuel cell stack side is raised or boosted by a DC voltage converter to the voltage level of the high-voltage supply for the power consumption unit.

[0003] Under full load operation or within a preset operation range, for efficiency, the voltage difference to be converted by the DC voltage conversion is designed to be low. In order to keep the voltage difference to be converted low, the output voltage of the fuel cell stack must be designed to be relatively high under load or close to the supply voltage to the power consumption unit. In that case, in the case of a fuel cell system for supplying high voltage, the maximum allowable operating voltage of the DC voltage converter becomes a technical constraint in the design of the lowest possible voltage difference that can be efficiently converted. In the semiconductor-based power electronics of the voltage converter, the maximum operating voltage allowed by the current state of the art is approximately 1200V.

[0004] For example, when the fuel cell stack is disconnected from an electrical load, such as at the start of a fuel cell system, the fuel cell stack may generate an unloaded voltage that exceeds the output voltage under load and, in particular, exceeds the maximum operating voltage of the DC voltage converter. If such a state occurs during the start-up of the fuel cell system, as soon as the electrical connection between the fuel cell stack and the DC voltage converter is closed, the DC voltage converter will be damaged. Accordingly, there is a need for a technique to avoid overvoltage damage to the DC voltage converter during the start-up of a fuel cell system in the high-voltage region.

[0005] In the prior art, when the no-load voltage becomes very high, a fuel cell system is known in which an artificial load or a dummy load is connected to the fuel cell stack to reduce it and avoid the adverse effect on the service life of the cell due to the too-high voltage. A problem that can arise in this connection is when all the cells of the stack are not yet receiving an equal or sufficient supply of fuel gas at the start of the fuel cell system, especially when there is air in the system after a relatively long shutdown, i.e., when there is oxygen at both the cathode and the anode. When a load is applied in such a state where the same high no-load voltage can be generated, a polarity reversal or a reversal of the voltage potential may occur in individual cells under a lack of fuel. Even if it occurs for a short time, it is much more harmful to the service life of the cell than a temporarily high no-load voltage. Accordingly, there is a need for a technique to surely suppress the potential reversal in such types of cells. Summary of the Invention Problems to be Solved by the Invention

[0006] An object of the present invention is to provide a technique for starting a fuel cell system that enables the output voltage of a fuel cell stack under a load to be designed even higher, and in particular, enables the output to be designed even higher, and in this connection, the no-load voltage of the fuel cell stack can exceed the maximum operating voltage technically restricted by the DC-DC converter. Further, an object of the present invention is to provide a technique for starting a fuel cell system that can not only avoid overvoltage damage in the DC-DC converter but also surely suppress the occurrence of harmful potential reversals in the cells. Means for Solving the Problems

[0007] The above problems are solved by a control method having the constituent elements of claim 1. Other constituent elements and details of the present invention will become apparent from the dependent claims, the detailed description of the invention, and the drawings.

[0008] According to the present invention, a control method for starting a fuel cell system after stoppage is contemplated. At this time, the system typically includes at least one fuel cell stack having a cathode supply area for supplying a cathode supply gas, a cathode discharge area for discharging a cathode discharge gas, an anode supply area for supplying an anode supply gas, and an anode discharge area for discharging an anode discharge gas; a DC voltage converter for voltage increase between the output voltage of the fuel cell stack on the low voltage side and the supply voltage of the high voltage supply line on the high voltage side; and a switch for opening and closing the electrical connection between the fuel cell stack and the DC voltage converter.

[0009] The control method according to the present invention is characterized by having the following steps: supplying an anode supply gas to the anode supply area; at least temporarily shutting off and / or throttling the cathode supply gas to the cathode supply area; monitoring the rising no-load voltage of the fuel cell stack under an open switch of the electrical connection. On the premise that the no-load voltage exceeds a preset allowable threshold voltage for the DC voltage converter, the control method according to the present invention particularly includes the step of executing and / or continuing at least a temporary shut-off and / or throttling of the cathode supply gas until the allowable threshold voltage or lower until the no-load voltage decreases; and further includes the step of closing the switch of the electrical connection after the no-load voltage has decreased to the allowable threshold voltage or lower.

[0010] Thus, the present invention contemplates for the first time a solution method in which oxygen depletion is intended during startup of the fuel cell system, thereby causing a decrease in the no-load voltage, and then the fuel cell stack is connected to the DC voltage converter.

[0011] Furthermore, the finding on which the present invention is based is that air enters the system during the stop of the fuel cell system, or the existing air in the cathode region also diffuses or disperses into the anode region. Accordingly, at the time of a new start of the fuel cell system after the stop, oxygen derived from the air remaining in the fuel cell stack always exists, and this oxygen can be used for the start of the electrochemical reaction until it is gradually consumed. Based on the interruption of the supply of new oxygen, or based on a clear decrease in the supply of oxidizing gas relative to the supply of combustion gas, oxygen depletion or exhaustion occurs in the fuel cell stack accordingly, and the throughput of the electrochemical reaction in the cell decreases, and thus the no-load voltage decreases.

[0012] One advantage of the present invention is that by such a measure, the no-load voltage can be accurately decreased before being connected to the DC voltage converter after rising at the start of the fuel cell system by controlling it by oxygen limitation, whereby no-load voltage damage in the DC voltage converter is prevented.

[0013] Another advantage of the present invention is that supersaturation of the combustion gas on the anode side is achieved rather than oxygen existing on both the cathode side and the anode side. Thereby, it is possible to prevent the potential reversal from occurring immediately in the cell when the fuel cell stack is connected to a load under an uneven combustion gas supply as in the prior art.

[0014] In a preferred embodiment of the present invention, the continuation of at least a temporary interruption and / or throttling of the cathode supply gas can include an interruption and / or throttling starting after the supply of the anode supply gas. In this embodiment, the supply of oxidizing gas or air is not started from the beginning until the fuel cell stack is electrically connected. In that way, the fuel cell system can be started as quickly as possible and put into operation.

[0015] In an alternative preferred embodiment of the present invention, the execution of at least a temporary interruption and / or throttling of the cathode supply gas can include an interruption and / or throttling that starts immediately when the no-load voltage exceeds the allowable threshold voltage. This embodiment can be selected for system designs where the no-load voltage only occasionally exceeds a critical value. At this time, it is guaranteed that intervention in the gas supply to throttle occurs only when required for overvoltage protection purposes.

[0016] In a preferred embodiment of the present invention that further expands on this, at least a temporary interruption and / or throttling of the cathode supply gas can include a change in the throttling rate as a reaction to monitoring the no-load voltage with respect to the allowable threshold voltage. This embodiment enables a more accurate metering of oxygen depletion in the cell, thereby enabling, on the one hand, ensuring the electrochemical reaction from the start and, on the other hand, controlling oxygen depletion better according to the trend of the no-load voltage in subsequent processes.

[0017] In a specific preferred embodiment of the present invention, the interruption and / or throttling of the cathode supply gas can include an increase in the throttling rate as a reaction to an excess of the no-load voltage with respect to the allowable threshold voltage. In this embodiment, initially, a relatively weak throttling of the oxidizing gas can be implemented, a stronger throttling can be implemented when a critical voltage value occurs, or a gradual change can be implemented depending on the rate of increase of the no-load voltage.

[0018] In another preferred embodiment of the present invention, the DC voltage converter can have a capacitor configured to be controllably charged and discharged, or can be connected to a capacitor; furthermore, the method has the following step: the capacitor is pre-charged before the switch of the electrical connection is closed. In this way, the capacitor provides an electrical capacitance that can be charged and discharged through the DC voltage converter. Such a charging process and discharging process are configured to be controllable in particular.

[0019] In the above preferred embodiment of the present invention, the control method can have the following steps with respect to the capacitor: After the no-load voltage falls below the allowable threshold voltage, precharging of the capacitor is started. At this time, the capacitor can have an electrical resistor, also called a precharge resistor, whereby a high current, in particular a current peak, can be avoided or at least smoothed during charging and discharging of the capacitor. When the fuel cell unit is supplied to the anode, the voltage immediately rises. Subsequently, after waiting until it falls below the threshold value, precharging of the capacitor is performed. Following the precharging, the switching device is closed. In this embodiment, the use of the capacitor can be applied only when the no-load voltage reaches a critical voltage value.

[0020] In the above preferred embodiment of the present invention, for precharging the capacitor in the high-voltage supply line, the high-voltage side of the DC voltage converter can be connected to the capacitor.

[0021] In another case, in the above preferred embodiment of the present invention, for precharging the capacitor in the fuel cell stack, the low-voltage side of the DC voltage converter can be connected to the capacitor.

[0022] In a preferred embodiment of the present invention, the allowable threshold voltage may be predetermined with respect to the maximum operating voltage of the DC voltage converter technically defined on the low-voltage side.

[0023] At this time, the allowable threshold voltage may be predetermined to be lower than or equal to the maximum operating voltage of the semiconductor circuit based on silicon carbide (SiC) of the DC voltage converter, and / or may be preset within the range of 700V to 900V, preferably around about 800V.

[0024] As an alternative, the upper threshold voltage may be lower than or equal to the maximum operating voltage of a gallium nitride (GaN)-based semiconductor circuit of the DC / DC converter, may be pre-determined, and / or may be preset within a range of 1100V to 1300V, preferably around about 1200V.

[0025] A control device for a fuel cell system and a computer program product are also objects of the present invention, by which each step of the control method according to the present invention can be embodied when the fuel cell system is started.

[0026] Accordingly, the control device and the computer program product bring the same advantages as those described in detail with respect to the control method according to the present invention.

[0027] Other advantages, components, and specific matters of the present invention will become apparent from the following description that describes embodiments of the present invention in detail with reference to the drawings.

Brief Description of the Drawings

[0028]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0029] FIG. 1 schematically shows as a block diagram a section of a fuel cell system that is important for a control method. The fuel cell system includes a fuel cell unit 10 having a fuel cell stack with an anode section and a cathode section, not shown in detail. The anode section of each fuel cell stack is connected to an anode supply area 12 for supplying an anode supply gas that supplies fuel gas (Fx), particularly hydrogen, in a known manner. The cathode section of each fuel cell stack 10 is connected to a cathode supply area 14 for supplying a cathode supply gas that supplies oxidizing gas (Ox), particularly oxygen-containing air. Further, each cathode section is connected to a cathode discharge area for discharging cathode exhaust gas, and each anode section is connected to the cathode discharge area for discharging cathode exhaust gas (not shown in detail). The fuel cell unit 10 is designed in this embodiment for large-scale power generation, for example, for stationary or semi-stationary or marine applications, i.e., for autonomous supply to a structure, or for driving a ship, and for driving other large commercial vehicles including an in-vehicle electrical system and a power consumption unit. The fuel cell unit 10 may be designed to match a main operating point in a high-voltage range, such as about 750 V or about 1100 V, under load.

[0030] The fuel cell unit 10 serves to generate an electrical output for the high-voltage supply area 40 of the fuel cell system. The high-voltage supply area 40 supplies a DC current output having a high supply voltage in power consumption units such as a drive train, an in-vehicle electrical system, and a supply network of a structure. In the illustrated embodiment, for example, a first load 51 representing an electric drive device and a second load 52 representing, for example, air conditioning equipment are connected to the high-voltage supply area 40 to take in the generated output from the fuel cell unit 10. Further, the high-voltage supply area 40 is connected to a battery storage 44 that balances fluctuations in dynamic load requirements between generation and consumption to stabilize the voltage level of the supply voltage.

[0031] A DC voltage converter 30 is provided between the fuel cell unit 10 and the high-voltage supply area 40. The DC voltage converter 30 serves as a voltage booster that raises the output voltage of the fuel cell unit 10, which varies depending on the load, to the supply voltage. For this purpose, the DC voltage converter 30 has a low-voltage side to which the output generated by the fuel cell unit 10 is supplied and a high-voltage side that transfers this output to the high-voltage supply area 40 with the boosted voltage. As an alternative, an embodiment of a "buck-boost" can also be considered as the DC voltage converter 30, in which case similar voltages are applied on both sides. Such an embodiment is also included in the present invention. Further, the DC voltage converter 30 includes a capacitor 33, and its capacitance contributes to smoothing the output-dependent fluctuations of the output voltage of the fuel cell unit 10 with respect to the supply voltage of the high-voltage supply area 40 or to compensating for the voltage peaks and current peaks between the low-voltage side and the high-voltage side that may occur, for example, during the switching process in the electrical connection of the fuel cell system.

[0032] Furthermore, the DC voltage converter 30 includes power electronics having transistors fabricated from a semiconductor made of silicon carbide (SiC) or gallium nitride (GaN) for variable voltage conversion. The maximum allowable operating voltage Umax that can be applied to the power electronics without damaging the circuit is technically defined by the maximum cut-off voltage between the band gaps of such semiconductor materials. At the current state of the art, the maximum allowable voltages with semiconductor technologies of silicon carbide (SiC) and gallium nitride (GaN) are approximately 800 V or 1200 V.

[0033] The electrical connection between the fuel cell unit 10 and the DC voltage converter 30 can be disconnected by the switching device 20. The switching device 20 is manufactured, for example, as a protection switch by a switching technology suitable for high voltage. The switching device 20 is disconnected particularly during the stop of the fuel cell system when the fuel cell unit 10 is in a standby state without gas supply. Accordingly, the switching device 20 must be closed again when the fuel cell system is restarted after being stopped, and thereafter, the output of the generated fuel cell unit 10 can be supplied to the first load 51 and the second load 52.

[0034] Figure 2 shows, in two graphs, a comparison between parameters that occur when there is air in the anode supply area (left) and when there is no air when the control method according to the present invention is applied (right) during the start-up of the fuel cell stack.

[0035] Depending on the construction form of the wiring technology, sealing technology, and valve technology in the fuel cell system, after the fuel cell system has been stopped for at least several hours, particularly after a stop of approximately one day, it is conceivable that air has entered the entire fuel cell unit 10 or diffused through the diaphragm. In other words, this means that air is not only in the cathode area of the fuel cell stack but also diffused in the anode area.

[0036] When a new start of the fuel cell system is performed, in order to start an electrochemical reaction in the cell and initiate output generation, a combustion gas is supplied through the anode supply area 12, and an oxidizing gas is supplied through the cathode supply area 14. In the conventional operation mode, the start is accordingly performed. At this time, even when there is still air supplied to the cathode side and the anode side of the cell, a high no-load voltage Uoc is generated relatively early, and this state is apparent from the upper voltage curve for the stack voltage Us and the lower group of voltage curves for the individual cell voltages Uc in the left graph of Figure 2.

[0037] When the no-load voltage Uoc of the fuel cell unit 10 or the fuel cell stack exceeds a pre-determined threshold voltage Uth of, for example, 400V, voltage limitation (so-called "voltage clipping") is executed, and the original stack current Is is discharged from the fuel cell stack. Such a stack current Is plotted by the curve in the central region of the graph is discharged by an artificial load (so-called "dummy load") or a resistor, and for this purpose, it is electrically connected to the fuel cell stack, or discharged by connection to a DC voltage converter 30 that discharges the output so as to be the load of the fuel cell system. Such a process is controlled to meet the purpose, and long-term damage to the cells due to a high no-load voltage Uoc is avoided. As a result of the output discharge of the control formula by the stack current Is, in the illustrated transition, the no-load voltage Uoc decreases from about 600V to the threshold voltage 400V, or sometimes lower than that.

[0038] During startup, combustion gas is not yet fully supplied to all cells of the fuel cell stack, or not evenly diffused, so in some cells, air may still be present on both the cathode side and the anode side. In this state, which can be regarded as partial fuel shortage, there is a risk that potential reversal will occur in the corresponding cells, causing irreversible damage to the cell membranes under the electrical load requirements imposed for voltage limitation. At this time, the wide variation in the lower group of the voltage curves of the individual cell voltages Uc is an indicator of non-uniform fuel gas distribution in the fuel cell stack, and the voltage curve having a relatively flat transition or the lowest voltage peak can be specifically attributed to the cells that have a risk of fuel shortage and accompanying potential reversal when a load requirement is imposed.

[0039] In the graph on the right, the corresponding parameters of the stack voltage Us, the individual cell voltages Uc, and the stack current Is that appear when the control method according to the present invention is implemented are plotted against the same time axis. When the fuel cell system is newly started and the supply of the combustion gas is started via the anode supply region 12, in this embodiment, in both cases (the left and right sides of FIG. 2), the supply of the oxidizing gas via the cathode supply region 14 is blocked for the time being. The right side of FIG. 2 shows a situation where there is no oxygen on either the anode side or the cathode side of the fuel cell unit 10. Therefore, the voltage starts to rise only when the supply starts on the cathode side. Voltage limitation is performed in this case. In the graph on the right side of FIG. 2, as is clear from referring to the upper voltage curve for the stack voltage Us and the lower group of voltage curves for the individual cell voltages Uc, here, due to the lack of oxygen at the anode, a slightly lower value of the open-circuit voltage Uoc occurs, and the variation in the lower group of voltage curves is significantly reduced. The reason for the latter is that there is a more uniform fuel gas distribution situation in the fuel cell stack. The gas supply of only the combustion gas or at least most of the combustion gas precedes the oxidizing gas in terms of time, so that the saturation of the combustion gas on the anode side is forced (both cases shown in FIG. 2). As a result of supplying oxygen to the fuel cell stack on the right side of FIG. 2, the current increases.

[0040] FIG. 3 shows three stages of the control method of the present invention at the time of starting the fuel cell system. Here, in the lower region, the group of the cell voltages Uc of the individual cells and the stack current Is are plotted against the time axis, and in the upper region, the pressure Pf of the supplied combustion gas and the pressure Po of the oxidizing gas supplied by the fuel cell stack become clear.

[0041] Stage 1 substantially corresponds to the process of the control method described in relation to FIG. 2. However, due to different designs of the fuel cell system, the no-load voltage value is higher than that in FIG. 1. Along with the pressure Pf of the opened fuel gas supply, in the system of FIG. 2, the no-load voltage Uoc rises to over 1000V, and the operating voltage Umax of the attached DC-DC converter 30 that is maximally allowed on the low voltage side is approximately 1000V, and the pre-determined threshold voltage Uth in this regard is defined as about 800V.

[0042] After a starting time of approximately 10 seconds on the time axis, the fuel gas supply is started and Stage 1 begins. Subsequently, due to the increase in the electrochemical reaction in the cell, while consuming the oxygen in the fuel cell stack, the no-load voltage Uoc rapidly rises. Without discharging the output, the no-load voltage Uoc remains at a high level of 1000V - 800V over the time period of Stage 1. Based on the consumption of oxygen, the electrochemical reaction in the cell decreases, and thereby, during a standby time of up to 60 seconds, the no-load voltage Uoc drops to about 800V. This standby time requires a certain delay when starting the fuel cell system. During this time, although the cell remains at a high no-load voltage Uoc, considering the risk, compared to the potential reversal of the potential that directly leads to irreversible damage of the cell, which can be avoided in the long term, this is a minor potential obstacle.

[0043] After the cell voltage UC drops from the level of the no-load voltage Uoc to the threshold voltage Uth, in the subsequent optional Stage 2, charging of the capacitor 33 of the DC-DC converter 30 is further started. By using the capacitor 33 together with the resistor for the charging and discharging process, the generation of the voltage peak on the low voltage side of the DC-DC converter 30 during the switching process for electrical connection can be mitigated, and accordingly, the capacitor 33 can be protected.

[0044] At the end of stage 2, as illustrated by the curve Po where the oxidation gas pressure rises vertically, the oxidation supply unit is opened, and immediately thereafter, the switching device 20 is operated to connect the fuel cell unit 10 to the DC voltage converter 30. At this time, a new rise in the no-load voltage Uoc of the fuel cell unit 10 occurs, but by causing a prior decrease to a certain voltage difference lower than the maximum allowable operating voltage Umax of the DC voltage converter 30, and by controlling each process of oxidation gas supply and closing of the switching device 20 quickly and continuously, such a rise can be controlled.

[0045] Depending on the configuration and control of the charging circuit of the capacitor 33, particularly when the capacitor 33 is charged by the stack current Is discharged from the fuel cell stack of the fuel cell unit 10 via the charging circuit on the low-current side of the DC voltage converter 30, the no-load voltage Uoc can thereby be further decreased. The capacitor 33 is particularly arranged on the low-voltage side. However, arrangement on the high-voltage side is also conceivable, in which case pre-charging is ensured by appropriate wiring. Due to the resulting even higher voltage difference, when the switching device 20 is then closed, an excess of the maximum allowable operating voltage Umax of the DC voltage converter 30 can be prevented with even higher certainty.

[0046] Stage 3 substantially corresponds to the operating state of a fully started fuel cell system with load capacity, and in the subsequent process, at least the pressure Pf of the combustion gas supply in the fuel cell unit 10 is variably adjusted depending on the total output required by the first load 51 and the second load 52 from the high-pressure supply area 40. In this way, with the variable voltage conversion support of the DC voltage converter 30 and the battery storage 40, the operating point of the output to be generated becomes possible at a unified voltage level.

[0047] In one embodiment, for the step of pre-charging the capacitor 33 of the DC voltage converter 30 in the control method, instead of connecting to the low-voltage side, the connection between the high-voltage side and the capacitor 33 is intended. According to the configuration of the corresponding charging circuit, the capacitor 33 is charged from the high-voltage supply area 40 or the battery reservoir 44 instead of from the fuel cell unit 10, so that the charging process can be carried out regardless of the closing of the switching device 20. Charging on the high-voltage side can be carried out more controllably and / or more quickly than from the side of the fuel cell unit 10.

[0048] In an alternative embodiment, the pre-determined allowable threshold voltage Uth for the DC voltage converter 30 is not designed to be less than the technically defined maximum operating voltage Umax of the semiconductor circuit of the power electronics of the DC voltage converter 30, but the allowable threshold voltage Uth substantially corresponds to the maximum operating voltage Umax.

[0049] In one embodiment related to device engineering, the control method is implemented by a control device (not shown) defined to direct or control each method step with the corresponding system components of the fuel cell system, namely the fuel cell unit 10 including the anode supply area 12 and the cathode supply area 14, the switching device 20, the DC voltage converter 30 including the capacitor 33 and its charging circuit, the high-voltage supply area 40 including the battery reservoir 44, and the loads 51, 52, in particular actuators, sensors, and / or at least their circuits connected by signal.

[0050] In an alternative embodiment related to device engineering, the control method is implemented by an external control device (not shown) in the system environment of the fuel cell system, which is indirectly or directly connected to the corresponding system components via at least the signal connections listed above. The control device or control method defined as above is embodied by a computer program in the hardware of the external control device.

[0051] The above description of the embodiments has described the present invention only within the framework of examples. (Other possible items) (Item 1) In a control method for starting a fuel cell system after stopping, the fuel cell system includes: A fuel cell unit (10) having a fuel cell stack having a cathode supply area (14) for supplying a cathode supply gas, a cathode discharge area for discharging a cathode discharge gas, an anode supply area (12) for supplying an anode supply gas, and an anode discharge area for discharging an anode discharge gas, a DC voltage converter (30) for voltage boost between the output voltage of the fuel cell unit (10) on the low voltage side and the supply voltage of a high voltage supply area (40) on the high voltage side, and a switching device (20) for opening and closing an electrical connection between the fuel cell unit (10) and the DC voltage converter (30), and the following steps: Supplying an anode supply gas to the anode supply area (12); At least temporarily blocking and / or throttling the cathode supply gas to the cathode supply area (14); Monitoring the rising no-load voltage (Uoc) of the fuel cell stack under the switching device (20) with the electrical connection open; and When the no-load voltage (Uoc) exceeds a pre-determined allowable threshold voltage (Uth) for the DC voltage converter (30); Executing and / or continuing at least a temporary block and / or throttle of the cathode supply gas until the no-load voltage (Uoc) drops to or below the allowable threshold voltage (Uth); and Closing the switching device (20) of the electrical connection after the no-load voltage (Uoc) has dropped to or below the allowable threshold voltage (Uth). A control method for starting a fuel cell system after stopping, having the above steps. (Item 2) The continuation of at least a temporary interruption and / or throttling of the cathode supply gas includes an interruption and / or throttling that starts after the supply of the anode supply gas, and is a control method for starting the fuel cell system according to Item 1. (Item 3) The execution of at least a temporary interruption and / or throttling of the cathode supply gas includes an interruption and / or throttling that starts immediately when the no-load voltage (Uoc) exceeds the allowable threshold voltage (Uth), and is a control method for starting the fuel cell system according to Item 1. (Item 4) At least a temporary interruption and / or throttling of the cathode supply gas includes an increase in the throttling rate as a reaction to the excess of the no-load voltage (Uoc) over the allowable threshold voltage (Uth), and is a control method for starting the fuel cell system according to any one of Items 1 to 3. (Item 5) At least a temporary interruption and / or throttling of the cathode supply gas includes a change in the throttling rate as a reaction to the monitoring of the no-load voltage (Uoc) with respect to the allowable threshold voltage (Uth), and is a control method for starting the fuel cell system according to any one of Items 1 to 4. (Item 6) The DC voltage converter (30) has a capacitor (33) configured to be controllably charged and discharged, or may be connected to the capacitor (33). Further, the method includes the following steps: The capacitor (33) is pre-charged before the switching device (20) of the electrical connection is closed. And is a control method for starting the fuel cell system according to any one of Items 1 to 5. (Item 7) The following steps: After the no-load voltage (Uoc) falls below the allowable threshold voltage (Uth), the pre-charging of the capacitor (33) is started. And further has a control method for starting the fuel cell system according to Item 6. (Item 8) The following steps: The low-voltage side of the DC voltage converter (30) is connected to the capacitor (33) to pre-charge the capacitor (33) in the fuel cell unit (10). A control method for starting the fuel cell system according to item 6 or 7, further comprising: (Item 9) The following steps The high-voltage side of the DC voltage converter (30) is connected to the capacitor (33) to pre-charge the capacitor (33) in the high-voltage supply area (40). A control method for starting the fuel cell system according to item 6 or 7, further comprising: (Item 10) The allowable threshold voltage (Uth) is predetermined with respect to the maximum operating voltage (Umax) of the DC voltage converter (30) technically defined on the low-voltage side. A control method for starting the fuel cell system according to any one of items 1 to 9. (Item 11) The allowable threshold voltage (Uth) is predetermined to be lower than or equal to the maximum operating voltage (Umax) of a semiconductor circuit based on silicon carbide (SiC) of the DC voltage converter (30), and / or within the range of 700V to 900V, preferably around about 800V. A control method for starting the fuel cell system according to item 10. (Item 12) The allowable threshold voltage (Uth) is predetermined to be lower than or equal to the maximum operating voltage (Umax) of a semiconductor circuit based on gallium nitride (GaN) of the DC / DC converter, and / or within the range of 1100V to 1300V, preferably around about 1200V. A control method for starting the fuel cell system according to item 10. (Item 13) A control device for controlling the fuel cell system and for executing each step of the control method according to any one of items 1 to 12. (Item 14) A computer program including a command that, when a program is executed by a computer or a control device, instructs the computer or control device to implement the control method according to any one of Items 1 to 12.

Explanation of Signs

[0052] 10 Fuel cell unit 12 Anode supply area 14 Cathode supply area 20 Switching device 30 DC voltage converter 33 Capacitor 40 High-voltage supply area 44 Battery storage 51 First load 52 Second load Uoc Open-circuit voltage Uth Threshold voltage Umax Maximum allowable operating voltage Uc Cell voltage Us Stack voltage Is Stack current Pf Combustion gas supply pressure Po Oxidizing gas supply pressure

Claims

1. In a control method for starting a fuel cell system after stoppage, the fuel cell system includes a fuel cell unit having a fuel cell stack having a cathode supply area for supplying a cathode supply gas, a cathode discharge area for discharging a cathode discharge gas, an anode supply area for supplying an anode supply gas, and an anode discharge area for discharging an anode discharge gas, a DC voltage converter for voltage boost between the output voltage of the fuel cell unit on the low voltage side and the supply voltage of a high voltage supply area on the high voltage side, and a switching device for opening and closing an electrical connection between the fuel cell unit and the DC voltage converter, and the following steps supplying an anode supply gas to the anode supply area, at least temporarily blocking and / or throttling the cathode supply gas to the cathode supply area, and / or monitoring the rising no-load voltage of the fuel cell stack under the switching device with the electrical connection open, and when the no-load voltage exceeds a pre-determined allowable threshold voltage for the DC voltage converter, executing and / or continuing at least a temporary block and / or throttle of the cathode supply gas until the no-load voltage drops to or below the allowable threshold voltage, and closing the switching device of the electrical connection after the no-load voltage has dropped to or below the allowable threshold voltage, A control method for starting a fuel cell system after stoppage having the above steps.

2. The control method for starting the fuel cell system according to claim 1, wherein the continuation of at least a temporary block and / or throttle of the cathode supply gas includes a block and / or throttle starting after the supply of the anode supply gas.

3. The control method for starting the fuel cell system according to claim 1, wherein the execution of at least a temporary block and / or throttle of the cathode supply gas includes a block and / or throttle starting immediately when the no-load voltage exceeds the allowable threshold voltage.

4. The control method for starting the fuel cell system according to claim 1, wherein at least a temporary block and / or throttle of the cathode supply gas includes increasing the throttle rate as a reaction to the excess of the no-load voltage with respect to the allowable threshold voltage.

5. The control method for starting the fuel cell system according to claim 1 includes at least a temporary interruption and / or throttling of the cathode supply gas, including a change in the throttling rate as a reaction to the monitoring of the no-load voltage with respect to the allowable threshold voltage.

6. The DC voltage converter has a capacitor configured to be controllably charged and discharged, or may be connected to a capacitor, and further the control method includes the following steps: The capacitor is pre-charged before the switching device of the electrical connection is closed. The control method for starting the fuel cell system according to claim 1, which has this.

7. The following steps: The pre-charging of the capacitor is started after the no-load voltage falls below the allowable threshold voltage. The control method for starting the fuel cell system according to claim 6, which further has this.

8. The following steps: The low-voltage side of the DC voltage converter is connected to the capacitor to pre-charge the capacitor in the fuel cell unit. The control method for starting the fuel cell system according to claim 6, which further has this.

9. The following steps: The high-voltage side of the DC voltage converter is connected to the capacitor to pre-charge the capacitor in the high-voltage supply area. The control method for starting the fuel cell system according to claim 6, which further has this.

10. The allowable threshold voltage is pre-determined with respect to the maximum operating voltage of the DC voltage converter technically defined on the low-voltage side. The control method for starting the fuel cell system according to claim 1.

11. The allowable threshold voltage is pre-determined to be lower than or equal to the maximum operating voltage of the semiconductor circuit based on silicon carbide of the DC voltage converter, and / or is preset within the range of 700V to 900V, preferably around about 800V. The control method for starting the fuel cell system according to claim 10.

12. The allowable threshold voltage is pre-determined to be lower than or equal to the maximum operating voltage of the semiconductor circuit based on gallium nitride of the DC / DC converter, and / or is preset within the range of 1100V to 1300V, preferably around about 1200V. The control method for starting the fuel cell system according to claim 10.

13. A control device for controlling a fuel cell system and for executing each step of the control method according to any one of claims 1 to 12.

14. A computer program including commands that, when executed by a computer or a control device, direct it to implement the control method according to any one of claims 1 to 12.

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