Fuel cell system

Abstract

The invention relates to a fuel cell system (22a-g) with at least two interconnected fuel cell units (10a-g, 12a-g), each of which has at least one buffer energy storage device (14a-g, 16a-g), wherein the buffer energy storage devices (14a-g, 16a-g) are each configured for use in connection with the conversion of direct current generated by the respective associated fuel cell unit (10a-g, 12a-g) into alternating current. It is proposed that the fuel cell system (22a-g) has a common discharge unit (18a-g) with a switching unit (20a-g) to at least selectively discharge the buffer energy storage units (14a-g, 16a-g) of the two interconnected fuel cell units (10a-g, 12a-g), preferably all buffer energy storage units (14a-g, 16a-g) of the fuel cell system (22a-g), depending on the switching position of the switching unit (20a-g), via a, preferably common, discharge string (24a-g) of the discharge unit (18a-g).

Description

State of the art

[0001] A fuel cell system with at least two interconnected fuel cell units, each of which has at least one buffer energy storage device, has already been proposed, wherein the buffer energy storage devices are each set up for use in connection with the conversion of direct current generated by the respective associated fuel cell unit into alternating current.

[0002] Fuel cells generally have the advantage that, in their smallest configuration, they exhibit low electrical power output compared to systems with similar electrical efficiency, for example, 10 kW. This allows for flexible scaling to fuel cell systems with higher overall power output by connecting several such small units. Further intermediate stages are often introduced during this scaling process to develop practical intermediate systems—for example, 10 units of 10 kW each form a 100 kW power system. Several of these power systems can, in turn, be connected together if even higher power outputs are required. Necessary functions can be integrated within the power systems. One example of this is the conversion of the direct current (DC) supplied by the fuel cells into alternating current (AC).In the example of the 100 kW power system, the necessary DC / AC converter can be a single large DC / AC converter with a power rating of 100 kW or more, or several smaller DC / AC converters, for example, ten small DC / AC converters with a power rating of 10 kW each. The typical power system uses one large converter corresponding to the total power output of the system. A necessary aspect of converting the direct current (DC) generated by the fuel cells into alternating current (AC) is buffering or additional consumption of electrical energy to meet the power control requirements (minimum power, speed of power adjustment) stipulated by consumers or grid connection regulations (so-called grid codes). Otherwise, the fuel cells can become an undesirable limiting factor. In particular, the control speed, but also the minimum power output, represent limitations of fuel cells.To circumvent this, storage systems with buffer energy storage and / or additional consumers such as heating elements are installed. It would be desirable if the fuel cell systems could be serviced and replaced while in operation. To ensure this safely, the buffer energy storage should be able to be discharged safely. Disclosure of the invention

[0003] The invention relates to a fuel cell system with at least two interconnected fuel cell units, each of which has at least one buffer energy storage device, wherein the buffer energy storage devices are each configured for use in connection with the conversion of direct current generated by the respective associated fuel cell unit into alternating current.

[0004] It is proposed that the fuel cell system include a common discharge unit with a switch unit for selectively discharging the buffer energy storage devices of the two interconnected fuel cell units, or preferably all buffer energy storage devices of the fuel cell system, via a single, preferably common, discharge line of the discharge unit, depending on the switch position. This advantageously minimizes the number of components in the fuel cell system. Costs and / or installation space can be advantageously saved. In particular, the selective discharge capability of the individual buffer energy storage devices of the fuel cell units ensures safe maintenance and / or safe replacement, especially safe reconnection, of individual fuel cell units of the fuel cell system, even during operation.

[0005] The fuel cell system can be a power system or a system comprising multiple power systems, each containing several fuel cell units. The fuel cell system can be a solid oxide fuel cell (SOFC) system or based on another type of fuel cell technology. The buffer energy storage devices can be batteries, capacitors, or other electrical energy storage devices. Preferably, the buffer energy storage devices are responsive and / or have a sufficient storage capacity, for example, comparable to 1 F to 20 F. The buffer energy storage devices serve, in particular, to compensate for fluctuations in energy demand or energy production in the fuel cell system or in at least one fuel cell unit.In particular, buffer energy storage systems serve to compensate for the inertia of fuel cell units during load fluctuations, to avoid high wear and tear on fuel cell units due to frequent switching on, off and / or off of the fuel cell units, to improve system efficiency by supporting the start-up of fuel cell units, to bridge the start-up time of fuel cell units, to cover peak loads of fuel cell units and / or to compensate for fluctuations in the operation of fuel cell units in connection with non-constant energy sources such as wind or solar.Preferably, the buffer energy storage systems, which are designed for use in connection with the conversion of the direct current generated by the respective fuel cell unit into alternating current, serve to provide the power control capacity required by a consumer or a grid connection control system for the fuel cell system. For example, if a higher power output than that currently supplied by the fuel cell system is quickly required, the buffer energy storage systems activate and supply part of the difference until the fuel cell units, which are slow to respond, have adjusted. Conversely, if a lower power output than that currently supplied by the fuel cell system is quickly required, the buffer energy storage systems activate and absorb part of the difference until the fuel cell units, which are slow to respond, have adjusted.The terms "intended" and / or "configured" should be understood to mean, in particular, specifically programmed, designed, and / or equipped. The fact that an object is intended or configured for a specific function should be understood to mean, in particular, that the object fulfills and / or performs this specific function in at least one application and / or operating state.

[0006] The common discharge unit is assigned to at least the two interconnected fuel cell units, but can also be assigned to further fuel cell units of the fuel cell system, which are particularly interconnected with the two fuel cell units. Preferably, the respective interconnected fuel cell units with the common discharge unit do not have any further additional discharge units or are electrically connected in such a way that none of the associated energy storage devices can be electrically discharged by a further additional discharge unit. The switching unit preferably comprises one or more electrical switches. If several switches are present, the switching position of the switching unit comprises a fixed combination of the switching positions of all switches in the switching unit.The switching unit is preferably designed such that the discharge of a buffer energy storage unit can only be activated when the associated fuel cell unit is switched off. For example, a switch based on the switch-within-a-switch principle can be provided, ensuring that the common discharge unit can only be connected to the buffer energy storage unit when the associated fuel cell unit is disconnected from the power grid / deactivated. In principle, all switches of the switching unit can be designed as double-pole disconnect switches / double-pole grounding disconnect switches, particularly for safety reasons and / or if individual lines are not at the same potential. In many cases, however, especially when the potentials are identical, single-pole disconnect switches are sufficient.Typical operating voltages of the buffer discharge storage devices, particularly buffer capacitors, of the present fuel cell system are in the range between 100 V and 1000 V. The discharge circuit of the discharge unit is, in particular, a path or arrangement of components through which electrical energy can flow from the buffer energy storage device and be delivered to a load, e.g., a discharge resistor, or another destination. The discharge circuit preferably ensures that the electrical energy stored in the buffer energy storage devices is delivered safely and in a controlled manner. The discharge circuit can comprise a number of components, such as cables or other electrical conductors, that enable energy to be transferred from the buffer energy storage devices to the connected discharge resistor. Preferably, there is only one discharge circuit per switch unit / per fuel cell unit assembly.Preferably, each switch unit / each fuel cell unit assembly has only one discharge resistor / one discharge resistor unit. In particular, the discharge circuit has only one discharge resistor / one discharge resistor unit. The discharge resistor / discharge resistor unit preferably has an ohmic resistance in the range between 10 Ω and 500 Ω.

[0007] Furthermore, it is proposed that the buffer energy storage device(s) be designed as capacitors, in particular supercapacitors. This advantageously allows for a short response time. In addition, a long service life and, especially compared to batteries, low maintenance requirements and / or high temperature resistance can be advantageously achieved.

[0008] Furthermore, it is proposed that the common discharge unit has a common discharge resistor / unit through which all fuel cell units assigned to the discharge unit, in particular their buffer energy storage devices, can be discharged. This advantageously results in savings in installation space and / or weight, especially since typical individual discharge resistors are relatively large and heavy. Cost savings can also be advantageously achieved. The discharge resistor / unit is preferably a component of the, in particular common, discharge circuit of the common discharge unit. The discharge resistor / unit is designed to convert the current / electrical power flowing from the respective buffer energy storage devices into heat. This heat is then usually released into the environment, but can also be used elsewhere, e.g.,for heating a component of the fuel cell system.

[0009] If the switch unit, preferably only, has a single selector switch, in particular a rotary switch / rotary selector switch, which is configured to selectively connect only one of the fuel cell units connected to the common discharge unit to the discharge circuit of the discharge unit (depending on the selected position / rotary position), while at the same time (in this selected position / rotary position) all other fuel cell units connected to the common discharge unit are electrically disconnected from any discharge circuits of the discharge unit, in particular the single common discharge circuit, then a cost-effective, space-efficient, and / or weight-reduced design can advantageously be achieved. Advantageously, only a single switch, the selector switch, is required / sufficient for the operation of the discharge unit responsible for multiple fuel cell units.The selector switch, in particular the rotary selector switch, is preferably a mechanical switch that allows different switching positions, e.g., by turning a handle, in order to selectively connect a choice of different electrical switching contacts. The selector switch, in particular the rotary selector switch, preferably has several switching positions, each of which activates different electrical connections between different buffer energy storage devices and the discharge resistor. Furthermore, a switching position of the selector switch is also conceivable in which the discharge circuit is grounded, i.e., in particular, none of the buffer energy storage devices are electrically connected to the discharge resistor. If a buffer energy storage device is electrically connected to the discharge resistor via the switch unit, the discharge process is activated, and the electrical energy stored in the buffer energy storage device flows through the discharge resistor, e.g., to earth.The selection switch forms, in particular, a main switch for the discharge unit.

[0010] Alternatively or additionally, it is proposed that the switching unit for each fuel cell unit, and in particular for each buffer energy storage unit, has a separate individual switch which, in its closed state, connects the respective buffer energy storage unit to the discharge circuit. This advantageously allows for a simple design. The separate individual switches can be provided instead of a main switch for the discharge unit or in addition to a main switch for the discharge unit. The separate individual switches are preferably assigned to respective buffer energy storage circuits, which are designed separately from and distinct from the common discharge circuit. However, the buffer energy storage circuits advantageously each terminate in the common discharge circuit or converge to form the common charging circuit.

[0011] Furthermore, it is proposed that the separate individual switches be interlocked in such a way that only one of the separate individual switches can be closed at any given time. This advantageously prevents incorrect operation and / or short circuits, e.g., caused by the simultaneous closing of two separate individual switches or the simultaneous connection of two buffer energy storage devices to the discharge resistor. A high level of operational reliability can thus be achieved. The interlocking of the separate individual switches can be implemented mechanically, e.g., via a mechanical interlock (physical lever, rail, bolt, etc.), electrically (switch control via electrical relays, contactors, or the like), or logically / software-based, e.g., via a programmable logic controller (PLC) or electronic logic.

[0012] Alternatively, it is proposed that each of the separate individual switches be assigned a diode that allows current flow only from the associated buffer energy storage device. This advantageously eliminates the need for interlocking switches. Advantageously, closing two separate individual switches does not create a short circuit between two buffer energy storage devices. Advantageously, a simple and particularly fault-resistant design can be achieved. The diodes are preferably assigned to the respective buffer energy storage strings, each of which also has its associated separate individual switch. The diodes preferably prevent current / energy flow from the buffer energy storage strings to the respective buffer energy storage devices. In particular, a number of buffer energy storage devices corresponds to a number of diodes and / or a number of separate individual switches.

[0013] It is further proposed that, in addition to the selector switch or the separate individual switches, the switching unit includes a main switch configured to allow or prevent any current flow in the discharge circuit, depending on its switching position. This advantageously ensures that only a single switch (the main switch) needs to be operated under load. Advantageously, a grounding position for the selector switch can be omitted. Redundancy also advantageously increases safety. The main switch is preferably a component of the discharge circuit. The main switch is preferably connected in series with the discharge resistor.

[0014] Furthermore, it is proposed that the selector switch, one or more of the separate individual switches, and / or the main switch be double-pole disconnect switches. This advantageously makes the system suitable for fuel cell systems whose buffer energy storage strings are at different ground potentials. In particular, a common ground is not required. This also advantageously increases safety.

[0015] If each of the separate individual switches designed as double-pole disconnect switches is assigned a (further) diode that only allows current flow to the associated buffer energy storage device, a particularly high level of safety for electronic components of the fuel cell system and / or for operators of the fuel cell system can be advantageously achieved. The (further) diodes assigned to the separate individual switches designed as double-pole disconnect switches are preferably arranged in the negative paths of the respective buffer energy storage strings.

[0016] Additionally, it is proposed that each buffer energy storage unit be assigned a fuse, which is located in the circuit(s) between the respective buffer energy storage unit and the switch unit. This advantageously provides a high level of safety for electronic components of the fuel cell system and / or for operators of the fuel cell system. The fuses are preferably located in the respective buffer energy storage strings, particularly in the negative paths of the respective buffer energy storage strings. However, it is also conceivable to omit fuses.

[0017] The fuel cell system according to the invention is not to be limited to the application and embodiment described above. In particular, the fuel cell system according to the invention may, to fulfill a function described herein, have a different number of individual elements, components, and units than the number specified herein. Furthermore, values ​​within the specified limits of the value ranges stated in this disclosure are also to be considered disclosed and freely usable. drawing

[0018] Further advantages become apparent from the following description of the drawings. The drawings illustrate seven exemplary embodiments of the invention. The drawings, the descriptions, and the claims contain numerous features in combination. It is advantageous for those skilled in the art to also consider the features individually and combine them into meaningful further combinations.

[0019] They show: Fig. 1 A schematic representation of a fuel cell system with a common discharge unit that includes a switch unit, Fig. 2 a schematic representation of a fuel cell system with a first alternative common discharge unit comprising a first alternative switching unit, Fig. 3 a schematic representation of a fuel cell system with a second alternative common discharge unit which has a second alternative switching unit, Fig. 4 a schematic representation of a fuel cell system with a third alternative common discharge unit which includes a third alternative switching unit, Fig. 5 a schematic representation of a fuel cell system with a fourth alternative common discharge unit which includes a fourth alternative switching unit, Fig. 6 a schematic representation of a fuel cell system with a fifth alternative common discharge unit which includes a fifth alternative switch unit, and Fig. Figure 7 shows a schematic representation of a fuel cell system with a sixth alternative common discharge unit, which includes a sixth alternative switching unit. Description of the exemplary implementations

[0020] In the Fig. Figure 1 schematically depicts a fuel cell system 22a. Except for the components shown, the fuel cell system 22a corresponds to a known fuel cell system 22a and is therefore not described in further detail. The fuel cell system 22a comprises at least two interconnected fuel cell units 10a, 12a. In principle, the fuel cell system 22a could also comprise three, four, five, six, seven, eight, nine, ten, or more than ten, in particular N, fuel cell units 10a, 12a, all interconnected analogously to the one shown in the figures. For the sake of clarity, only the first two and the Nth fuel cell unit are sketched in the drawings, while the description refers only to the first two fuel cell units 10a, 12a. All information described is also applicable to the Nth fuel cell unit and all other fuel cell units between 2 and N.

[0021] Each of the fuel cell units 10a, 12a has a buffer energy storage device 14a, 16a. The buffer energy storage devices 14a, 16a are each configured as capacitors. The capacitors are each supercapacitors. Alternatively, the buffer energy storage devices 14a, 16a could also be batteries, or partially configured as capacitors, particularly supercapacitors, and partially as batteries. Buffer energy storage devices 14a, 16a configured as combined capacitor-battery arrangements are also conceivable. The buffer energy storage devices 14a, 16a are each configured for use in the conversion of direct current generated by the respective associated fuel cell unit 10a, 12a into alternating current.The buffer energy storage units 14a, 16a are each designed to increase the response time of the respective associated fuel cell units 10a, 12a by providing support in the event of short-term changes in power demand.

[0022] The fuel cell system 22a has a discharge unit 18a. The discharge unit 18a is a common discharge unit 18a for all interconnected fuel cell units 10a, 12a of the fuel cell system 22a. The discharge unit 18a is configured for electrically discharging the buffer energy storage devices 14a, 16a. The discharge unit 18a has a common discharge resistor 50a. The common discharge resistor 50a is designed to discharge all buffer energy storage devices 14a, 16a assigned to the discharge unit 18a. Depending on the switching position of a switch unit 20a of the fuel cell system 22a, each of the buffer energy storage devices 14a, 16a can be discharged via the common discharge resistor 50a. The discharge unit 18a has a common discharge circuit 24a. The discharge resistor 50a is arranged in the discharge circuit 24a. The common discharge line 24a is formed by an electrical conductor.The fuel cell units 10a and 12a each have a buffer energy storage string 44a and 46a, respectively. Each of the buffer energy storage units 14a and 16a is assigned to one of the buffer energy storage strings 44a and 46a. The buffer energy storage strings 44a and 46a are each connected by separate electrical lines. The buffer energy storage units 14a and 16a are electrically connected to the fuel cells (not shown) of the respective fuel cell units 10a and 12a via the buffer energy storage strings 44a and 46a. The buffer energy storage units 14a and 16a are also electrically connected to the discharge string 24a via the buffer energy storage strings 44a and 46a.

[0023] The fuel cell system 22a has a switching unit 20a. Depending on its switching position, the switching unit 20a is designed to selectively discharge the buffer energy storage units 14a and 16a of the interconnected fuel cell units 10a and 12a. The buffer energy storage units 14a and 16a are preferably discharged individually or can be discharged individually via the common discharge unit 18a by switching the switching unit 20a. The buffer energy storage units 14a and 16a are discharged via the common discharge line 24a of the discharge unit 18a, preferably individually.

[0024] The switch unit 20a has a single selector switch 26a. The selector switch 26a is designed as a rotary switch. The selector switch 26a is configured to selectively connect only one of the fuel cell units 10a, 12a connected to the common discharge unit 18a, in particular their buffer energy storage units 14a, 16a, to the discharge line 24a of the discharge unit 18a, while at the same time all other fuel cell units 10a, 12a connected to the common discharge unit 18a are electrically disconnected from any discharge lines 24a of the discharge unit 18a. Through the electrical connection of the buffer energy storage unit 14a, 16a to the discharge line 24a, which includes the discharge resistor 50a, the electrical charge flows from the buffer energy storage unit 14a, 16a and is consumed by the discharge resistor 50a. The fuel cell system 22a has a grounding string 48a.The selection switch 26a is configured to electrically connect the discharge resistor 50a / the discharge circuit 24a to the grounding circuit 48a in at least one switching state. In this case, neither of the buffer energy storage devices 14a, 16a will be discharged.

[0025] The fuel cell system 22a has fuses 40a and 42a. Fuses 40a and 42a are located in the buffer energy storage strings 44a and 46a of the respective fuel cell units 10a and 12a. Each buffer energy storage unit 14a and 16a is assigned one fuse 40a and 42a. In the event of a fuse failure (short circuit, overcurrent, etc.), fuses 40a and 42a are designed to interrupt a circuit of the discharge unit 18a. Fuses 40a and 42a are located between the respective assigned buffer energy storage unit 14a and 16a and the switch unit 20a.

[0026] In the Fig. Figures 2 to 7 show six further embodiments of the invention. The following descriptions and drawings are essentially limited to the differences between the embodiments, whereby, with regard to identically designated components, particularly those with the same reference numerals, reference is also generally made to the drawings and / or the description of the other embodiments, especially those of the Fig. 1, can be referenced. To distinguish the embodiments, the letter a is the reference numeral of the embodiment in the Fig. 1. In the exemplary embodiments of the Fig. In numbers 2 to 7, the letter a is replaced by the letters b to g.

[0027] The Fig. Figure 2 also schematically shows a fuel cell system 22b with a first alternative discharge unit 18b, which has a first alternative switch unit 20b. The first alternative switch unit 20b has a single selector switch 26b. In addition to the selector switch 26b, the first alternative switch unit 20b has a main switch 36b. The main switch 36b is arranged in a discharge circuit 24b of the first alternative discharge unit 18b, which has a discharge resistor 50b of the first alternative discharge unit 18b. The main switch 36b is configured to allow or prevent any current flow in the discharge circuit 24b, and thus, in particular, any discharge of buffer energy storage devices 14b, 16b of the fuel cell units 10a, 12a of the fuel cell system 22a, depending on its switching position. The design of the Fig. 1, however, is free of a main switch, in particular one assigned to the discharge line 24a.

[0028] The Fig. Figure 3 also schematically shows a fuel cell system 22c with a second alternative discharge unit 18c, which includes a second alternative switch unit 20c. The fuel cell system 22c has fuel cell units 10c, 12c, each with associated buffer energy storage devices 14c, 16c. The second alternative switch unit 20c has a separate individual switch 28c, 30c for each fuel cell unit 10c, 12c, and in particular for each buffer energy storage device 14c, 16c. In their respective closed states, the separate individual switches 28c, 30c connect the respective associated buffer energy storage device 14c, 16c to a discharge circuit 24c of the second alternative discharge unit 18c, the discharge circuit having a discharge resistor 50c. The separate individual switches 28c, 30c are assigned to buffer energy storage strings 44c, 46c of the respective fuel cell units 10c, 12c. The design of the Fig. 1, however, is free of separate individual switches. The design of the Fig. 3 is free of a main switch. Instead, the design of the Fig. 3 the separate individual switches 28c, 30c are interlocked against each other in such a way (indicated by a dashed line connecting the individual switches 28c, 30c) that only one of the separate individual switches 28c, 30c can be closed at any one time.

[0029] The Fig. Figure 4 also schematically shows a fuel cell system 22d with a third alternative discharge unit 18d, which includes a third alternative switch unit 20d. The fuel cell system 22d and the third alternative switch unit 20d of the Fig. With one exception, 4 are identical to the fuel cell system 22c and the second alternative switch unit 20c of the Fig. 3. The third alternative switch unit 20d additionally includes a main switch 36b. The main switch 36d of the configuration of the Fig. 4 is analogous to the one in connection with the Fig. The main switch 36b described in section 2 is arranged and designed accordingly. Therefore, further details are omitted here.

[0030] The Fig. Figure 5 also schematically shows a fuel cell system 22e with a fourth alternative discharge unit 18e, which includes a fourth alternative switch unit 20e. The fuel cell system 22e and the fourth alternative switch unit 20e of the Fig. With two exceptions, 5 are identical to the fuel cell system 22d and the third alternative switch unit 20d. Fig. 4. The first difference is that the separate individual switches 28e, 30e of the fourth alternative switch unit 20e are not interlocked and can be switched independently, i.e., opened and closed. Furthermore, in the fourth alternative switch unit 20e, each of the separate individual switches 28e, 30e is assigned a diode 32e, 34e. The diodes 32e, 34e each allow only one current flow from a buffer energy storage device 14e, 16e of a respective fuel cell unit 10e, 12e of the fuel cell system 22e, which is associated with the respective separate individual switch 28e, 30e. The reverse current direction of the diodes 32e, 34e runs from a common discharge circuit 24e of the fourth alternative discharge unit 18e to the buffer energy storage device 14e, 16e to which the respective diode 32e, 34e is assigned.The forward bias directions of diodes 32e and 34e run from the buffer energy storage devices 14e and 16e, to which the respective diode 32e and 34e is assigned, to the common discharge circuit 24e. Diodes 32e and 34e can each exhibit reverse voltages in the range between 600 V and 1500 V.

[0031] The design of the switch unit 20e of the Fig. 5 has a main switch 36e arranged in the discharge line 24e. Fig. Figure 6 also schematically shows a fuel cell system 22f with a fifth alternative discharge unit 18f, which includes a fifth alternative switch unit 20f. The fifth alternative switch unit 20f of the configuration of the Fig. 6 corresponds to the fourth alternative switch unit 20e of the design of the Fig. 5 with the one exception that the fifth alternative switch unit 20e does not have a main switch / is designed free of a main switch.

[0032] The Fig. Figure 7 also schematically shows a fuel cell system 22g with a sixth alternative discharge unit 18g, which includes a sixth alternative switch unit 20g. The fuel cell system 22g has fuel cell units 10g and 12g, each with associated buffer energy storage units 14g and 16g. The sixth alternative switch unit 20g has a separate individual switch 28g and 30g for each fuel cell unit 10g and 12g, and in particular for each buffer energy storage unit 14g and 16g. In their respective closed states, the separate individual switches 28g and 30g connect the respective associated buffer energy storage units 14g and 16g to a discharge line 24g of the fifth alternative discharge unit 18g, which has a discharge resistance 50g. The separate individual switches 28g, 30g are assigned to buffer energy storage strings 44g, 46g of the respective fuel cell units 10g, 12g. The separate individual switches 28g, 30g of the design of the Fig. 7 are double-pole disconnect switches. The sixth alternative switch unit 20g has, in addition to the separate individual switches 28g and 30g, a main switch 36g. The main switch 36g is arranged in the discharge circuit 24g of the sixth alternative discharge unit 18g, which has a discharge resistor 50g. The main switch 36g is configured to allow or prevent any current flow in the discharge circuit 24g, and thus, in particular, any discharge of the buffer energy storage units 14g and 16g, depending on its switching position. The main switch 36g of the configuration of the Fig. 7 is a single-pole disconnect switch, but could also be a double-pole disconnect switch.

[0033] The fuel cell units 10g, 12g of the design of the Fig. 7 are located at different mass potentials (see “GND1” to “GNDN” in Fig. 7) In all other configurations of the Fig. 1 to 6 are the respective separate individual switches 28c-f, 30c-f, the respective main switches 36b, 36d, 36e and / or the respective selector switches 26a-b, single-pole disconnect switches, and the fuel cell units 10a-f, 12a-f are at the same earth potential. Alternatively, the separate individual switches 28c-f, 30c-f, the main switches 36b, 36d, 36e and / or the selector switches 26a-b could also be used. Fig. 1 to 6 could be partially or all double-pole disconnect switches. In that case, the respective fuel cell units 10a-f and 12a-f could also be at different ground potentials.

[0034] The sixth alternative switch unit 20g has additional diodes 38g and 52g. These additional diodes are each assigned to one of the separate single switches 28g and 30g, which are configured as double-pole disconnect switches. The additional diodes 38g and 52g are each arranged in negative paths 54g and 56g from the respective buffer energy storage strings 44g and 46g of the fuel cell units 10g and 12g, which are assigned to the respective buffer energy storage units 14g and 16g. The additional diodes 38g and 52g each allow only a current flow to their associated buffer energy storage unit 14g and 16g.

Claims

[1] Fuel cell system (22a-g) with at least two interconnected fuel cell units (10a-g, 12a-g), each of which has at least one buffer energy storage device (14a-g, 16a-g), wherein the buffer energy storage devices (14a-g, 16a-g) are each configured for use in connection with the conversion of direct current generated by the respective associated fuel cell unit (10a-g, 12a-g) into alternating current, characterized by a common discharge unit (18a-g) with a switching unit (20a-g) at least to selectively discharge the buffer energy storage units (14a-g, 16a-g) of the two interconnected fuel cell units (10a-g, 12a-g), preferably all buffer energy storage units (14a-g, 16a-g) of the fuel cell system (22a-g), depending on the switching position of the switching unit (20a-g), via a, preferably common, discharge string (24a-g) of the discharge unit (18a-g). [2] Fuel cell system (22a-g) according to claim 1, characterized by, that the buffer energy storage device(s) (14a-g, 16a-g) are designed as capacitors, in particular supercapacitors. [3] Fuel cell system (22a-g) according to claim 1 or 2, characterized by , that the common discharge unit (18a-g) has a common discharge resistor (50a-g) / a common discharge resistor unit, through which all fuel cell units (10a-g, 12a-g) associated with the discharge unit (18a-g), in particular their buffer energy storage units (14a-g, 16a-g), can be discharged. [4] Fuel cell system (22a-b) according to any of the preceding claims, characterized by, that the switching unit (20a-b) has a single selection switch (26a-b), in particular a rotary switch, which is configured at least to selectively connect only one of the fuel cell units (10a-b, 12a-b) connected to the common discharge unit (18a-b), in particular their buffer energy storage units (14a-b, 16a-b), to the discharge string (24a-b) of the discharge unit (18a-b), while at the same time all other fuel cell units (10a-b, 12a-b) connected to the common discharge unit (18ab), in particular their buffer energy storage units (14a-b, 16a-b), are electrically disconnected from any discharge strings (24a-b) of the discharge unit (18a-b). [5] Fuel cell system (22c-g) according to any one of claims 1 to 3, characterized by, that the switching unit (20c-g) has a separate individual switch (28c-g, 30c-g) for each fuel cell unit (10c-g, 12c-g), in particular for each buffer energy storage unit (14c-g, 16c-g), which in the respective closed switching state connects the respective associated buffer energy storage unit (14c-g, 16c-g) to the discharge string (24c-g). [6] Fuel cell system (22c-d) according to claim 5, characterized by , that the separate individual switches (28c-d, 30c-d) are interlocked in such a way that only one of the separate individual switches (28c-d, 30c-d) can be closed at any given time. [7] Fuel cell system (22e-g) according to claim 5, characterized by , that each of the separate individual switches (28e-g, 30e-g) is assigned a diode (32e-g, 34e-g) which only allows a current flow from the associated buffer energy storage (14e-g, 16e-g). [8] Fuel cell system (22b; 22d; 22e; 22g) according to any one of claims 4 to 7, characterized by , that the switching unit (20b; 20d; 20e; 20g) has, in addition to the selection switch (26b) or in addition to the separate individual switches (28d, 30d; 28e, 30e; 28g, 30g), a main switch (36b; 36d; 36e; 36g) which is configured to allow or prevent any current flow in the discharge circuit (24b; 24d; 24e; 24g), depending on the switching position. [9] Fuel cell system (22g) according to any one of claims 4 to 8, characterized by , that the selector switch, one or more of the separate individual switches (28g, 30g) and / or the main switch (36g) are double-pole disconnect switches. [10] Fuel cell system (22g) according to claim 9, characterized by, that each of the separate individual switches (28g, 30g) designed as two-pole disconnect switches is assigned a diode (38g, 52g) which only allows a current flow to the associated buffer energy storage (14g, 16g). [11] Fuel cell system (22a-g) according to any of the preceding claims, characterized by , that each of the buffer energy storage units (14a-g, 16a-g) is assigned a fuse (40a-g, 42a-g) which is located in the circuit / in the circuits between the respective buffer energy storage unit (14a-g, 16a-g) and the switch unit (20a-g).

Images