FUEL CELL SYSTEM AND METHOD FOR A FUEL CELL SYSTEM

Description

[0001] The present invention relates to a fuel cell system and a method for ensuring the load point of fuel cell stacks of a fuel cell system of a fuel cell system when a fault occurs, in particular a grid fault of an electrical output grid system.

[0002] State-of-the-art applications include feeding electrical energy generated by SOFC fuel cell stacks (SOFC stands for "Solid Oxide Fuel Cell") into an AC and three-phase power grid.

[0003] The performance dynamics of such fuel cell stacks are low. There are several reasons for this, including the risk of cell cracking due to steep temperature gradients that can occur during load surges. Therefore, these fuel cell stacks cannot follow rapid electrical load changes.

[0004] In addition to the fuel cell stacks themselves, the fuel cell system as a whole can also be damaged by a sudden load shedding. In this case, the fuel gas cannot be converted into electrical energy, leading to excessive heating of the oxidation catalyst in the fuel cell system, for example, and, in the worst case, to complete destruction.

[0005] The risk of not converting the fuel gas energy into electrical energy also exists in the event of a grid failure. Grid stability requirements stipulated by local and so-called "grid codes" also require a special electrical design for these situations (e.g., so-called low-voltage ride-through).

[0006] Further fuel cell systems and methods for ensuring the load point of fuel cell stacks of a fuel cell system of such fuel cell systems are known, for example, from DE 102013207349 A1, EP 1968142 B1, DE 10106219 A1, EP 3333951 A1 and DE 102019111462 A1.

[0007] It is an object of the present invention to provide a fuel cell system with a simple, cost-effective and optimized electrical design such that a fault, in particular a mains fault, in particular a power failure, does not result in damage to the fuel cell system.

[0008] The above object is achieved by a fuel cell system having the features of claim 1 and a method having the features of claim 10. Further features and details of the invention emerge from the subclaims, the description, and the drawings. Features and details described in connection with the fuel cell system according to the invention naturally also apply in connection with the method according to the invention, and vice versa, so that with regard to the disclosure of the individual aspects of the invention, reference is or can always be made to each other.

[0009] According to the invention, a fuel cell system is provided with a fuel cell system. The fuel cell system comprises a plurality of fuel cell stacks, wherein the fuel cell system is electrically connected or connectable to an electrical output network system comprising an electrical output network for supplying an electrical output network with electrical energy generated by the operation of the fuel cell stacks.The fuel cell system further comprises an electrical power resistance network, which comprises an electrical power resistance for ensuring the load point of the fuel cell stacks in the event of a fault, in particular a network fault of the electrical output network system, in particular of the electrical output network, wherein the fuel cell system is electrically connected to the electrical power resistance network for supplying the electrical power resistance network with electrical energy generated by the operation of the fuel cell stacks.

[0010] Accordingly, the invention provides a fuel cell system in which the fuel cell stacks do not have to leave their operating point or load point in the event of a fault, in particular a grid fault such as a power failure. At least the fuel cell stacks do not have to change their load point beyond the amount predetermined by their power dynamics, which can still be regarded as harmless with regard to the risk of possible damage to the fuel cell system. A grid fault is characterized by the fact that only a reduced amount or, in particular in the event of a power failure, no electrical energy can flow into the electrical output grid. Such a power failure can occur, for example, in the event of a short circuit outside the fuel cell system.The electrical energy generated by the fuel cell stacks is fed into the electrical output grid during normal operation, so it is referred to herein as the output grid. Alternatively, it can also be referred to, for example, as the feed-in grid. If a grid fault prevents the generated power from being fed into the electrical output grid, particularly an AC and / or three-phase grid, the excess energy can be fed into the electrical power resistor. Once the grid fault is resolved, the grid voltage returns, and the electrical energy can flow back into the electrical output grid instead of into the electrical power resistor.

[0011] The fuel cell system according to the invention relates in particular to the load point protection of fuel cell stacks of a fuel cell system of a fuel cell system in the event of a grid failure of an electrical output network, but can also advantageously be used for load point protection in off-grid systems. All features described in connection with the occurrence of a grid failure are therefore also advantageously applicable to off-grid systems.

[0012] The fuel cell stacks can, in particular, be solid oxide fuel cell stacks. Thus, the fuel cell system can be, in particular, a solid oxide fuel cell system or a solid oxide electrolyzer cell system (also known as an SOFC system). Furthermore, the individual fuel cell stacks can be electrically connected in parallel, in particular via the electrical (voltage) intermediate circuit explained in more detail later. Alternatively, it is also possible to electrically connect individual or all of the fuel cell stacks in series.

[0013] It is preferred that the electrical power resistor is designed to convert electrical energy into waste heat. In this way, the excess electrical energy that cannot be dissipated from the fuel cell system via the electrical output network in the event of a power failure can be advantageously removed from the fuel cell system, namely as waste heat. Waste heat therefore means, in particular, that the heat converted from electrical energy into thermal energy is dissipated. In particular, the waste heat is dissipated outside the fuel cell stacks and the fuel cell system. The waste heat is therefore preferably not fed to the fuel cell stacks, which could increase their efficiency but would be ineffective because more electrical energy would then have to be converted into waste heat.

[0014] Furthermore, it is preferred that the electrical power resistor be arranged outside the fuel cell stacks. As previously mentioned, this is advantageous so that the heat generated by the electrical power resistor is not used in the fuel cell stacks. Instead, installation space outside the fuel cell system with the fuel cell stacks can advantageously be used for the electrical power resistor network. Optional cooling can also be better positioned and dimensioned there, in terms of installation space.

[0015] It is also preferred that the fuel cell system comprises a cooling unit for cooling the electrical power resistor. The cooling unit can be an air and / or water cooling unit, i.e., it can be configured for cooling by means of air, in particular ambient air, and / or water. Within the scope of the invention, water is advantageously understood to be any coolant, which is in particular liquid. The air can be circulated, for example, by means of a fan. The water can be actively cooled by means of a refrigeration unit. This allows a particularly high amount of electrical energy to be dissipated to the electrical power resistor without damaging it, which is particularly advantageous in the event of power outages when no electrical energy can be dissipated via the electrical output network. In principle, it can also be advantageous if cooling occurs through natural convention.

[0016] It is also advantageous if the electrical power resistor network comprises power electronics configured to activate the electrical power resistor when a grid fault occurs. Activating the electrical power resistor means supplying electrical energy generated by the fuel cell system to the electrical power resistor, so that the resistor dissipates the electrical energy, in particular converting it into waste heat or, in other words, converting and dissipating thermal energy.

[0017] It is particularly preferred that the power electronics are configured to continuously measure a voltage in an electrical intermediate circuit between the fuel cell system and the electrical output network system and to activate the electrical power resistor when a voltage increase occurs in the electrical intermediate circuit. The electrical intermediate circuit can in particular be a DC intermediate circuit. The power electronics can thus very quickly determine the fault based on a voltage measurement and any voltage increase that may occur. The electrical power resistor can therefore be activated with minimal time delay compared to the occurrence of the fault in order to avoid damage to the fuel cell system. The voltage increase required to activate the power electronics can in particular be predefined.The voltage increase can be predefined, in particular, as an absolute maximum voltage value and / or as a voltage change within a specified time period. Consequently, the electrical power resistor can be activated, in particular, when a continuously measured actual voltage exceeds a predefined maximum absolute voltage value or experiences a specified voltage change within a specified time period.

[0018] Advantageously, the power electronics also includes a controller for regulating the electrical energy supplied to the electrical power resistor depending on the voltage measured in the electrical intermediate circuit. This advantageously allows the electrical energy to be dissipated to the electrical power resistor only to the extent required by the extent of the fault, particularly if there is no total failure or power outage. It is also easy to determine when the fault has been rectified and the electrical output network system is available again.

[0019] Although the fuel cell system advantageously remains at its operating point when a grid failure occurs, it is also advantageous if the fuel cell system is configured to shut down a load point of the fuel cell stacks in operation when the failure, in particular a grid failure, occurs. In other words, the operating point of the fuel cell stacks can be lowered when the failure, in particular a grid failure, occurs. This makes it possible to reduce the electrical energy produced by the fuel cell stacks in the fuel cell system in order to relieve the electrical power resistance and reduce the resource consumption of the fuel cell stacks. Here, too, the aforementioned controller or another controller can be used to regulate the load points of the fuel cell stacks depending on the voltage measured, in particular by the power electronics.The power dynamics of the fuel cell stacks can be taken into account, ensuring that the fuel cell stacks are not shut down beyond their power dynamics at their load points, thus avoiding damage to the fuel cell stacks. The power dynamics indicate how quickly a fuel cell stack can be shut down under its load without causing damage. It is therefore an operating specification and can be specified, for example, by current change rates.

[0020] Furthermore, it is provided that the electrical power resistance network is arranged in an electrical intermediate circuit between DC / DC converters of the fuel cell stacks and a DC / AC converter of the electrical output network system, wherein the DC / AC converter is bidirectional and is connected to the electrical output network, so that electrical energy can be discharged from the electrical intermediate circuit to the electrical output network and fed from the electrical output network into the electrical intermediate circuit.

[0021] In this respect, the aforementioned measurement of the voltage in this electrical intermediate circuit, in particular the DC intermediate circuit, can be carried out. The voltage in the electrical intermediate circuit is advantageously regulated to a constant voltage by the electrical output network system. Advantageously, the at least one DC / AC converter therefore has an operating mode of constant voltage maintenance in the electrical intermediate circuit. The electrical power resistor network, the DC / DC converters of the fuel cell stacks, and the DC / AC converter can be arranged in parallel. It is possible for each fuel cell stack to be assigned a DC / DC converter or, in other words, to be electrically connected to it. It is also possible for multiple electrical output networks to be provided, wherein each electrical output network can have a DC / AC converter. The DC / AC converter is intended to be bidirectional.In principle, two DC / AC converters connected in series can also be provided, which can be unidirectional but operate in opposite directions. This allows electrical energy to be drawn from the electrical output network as needed, particularly to supply a peripheral electrical system during the heating and cooling phases of the fuel cell stacks for the operation of the fuel cell system.

[0022] It is further advantageous if the fuel cell system comprises an electrical system peripheral network comprising system peripherals for supporting operation of the fuel cell system, wherein the fuel cell system is electrically connected to the electrical system peripheral network for supplying the electrical system peripheral network with the electrical energy generated by the operation of the fuel cell stacks. The electrical system peripheral network can also be connected to the electrical intermediate circuit and, in particular, connected in parallel to the electrical power resistor network, the fuel cell system, and the electrical output network system. The system peripherals are also referred to as the "balance of plant." In particular, the system peripherals can comprise several or all components of the fuel cell system, excluding the fuel cell stacks themselves.These components can include, for example, pumps, sensors, heat exchangers, seals, compressors, recirculation fans, charge air coolers, and humidifiers. These are supplied with electrical energy for their operation by the plant's peripheral network, so that these components, in turn, can utilize the proper operation of the fuel cell stacks to generate electrical energy. Accordingly, the aforementioned bidirectionality of the DC / AC converter in the electrical output network system is advantageous when starting up and shutting down, or in other words, warming up and cooling down, the fuel cell stacks, since the fuel cell stacks themselves typically generate no or insufficient electrical energy to reliably supply the plant's peripherals with electrical energy. Although an electrical storage system is optionally provided, it is a cost-intensive investment due to the high capacity required.

[0023] The present invention also provides a method for ensuring the load point of fuel cell stacks of a fuel cell system of a fuel cell plant according to the invention when a fault occurs, in particular a grid fault of an electrical output grid system that is supplied with electrical energy generated by the operation of the fuel cell stacks. The method comprises the following steps: Occurrence of the fault, in particular the network fault of the electrical output network system, detection of the fault that has occurred, in particular the network fault, and conversion of electrical energy in the fuel cell system between the fuel cell system and the electrical output network system into waste heat by means of an electrical power resistor.

[0024] Thus, a method according to the invention brings with it the same advantages as have been explained in detail with reference to the fuel cell system according to the invention.

[0025] In particular, the fuel cell system according to the invention can be configured or designed to carry out the method according to the invention. Conversely, the method according to the invention can be carried out in or by a fuel cell system according to the invention.

[0026] Advantageously, the fault that has occurred, in particular the grid fault, can be detected by measuring a voltage increase in an electrical intermediate circuit between the fuel cell system and the electrical output grid system.

[0027] Furthermore, the electrical energy converted by the electrical power resistor can advantageously be controlled as a function of the voltage measured in the electrical intermediate circuit.

[0028] Finally, the load points of the fuel cell stacks in operation can advantageously be reduced after the fault, particularly the grid fault, has been detected, within the scope of the power dynamics available from the fuel cell stacks. If a power resistor is designed to be sufficiently large, it may also be advantageous to keep the load point unchanged.

[0029] Further advantages, features, and details of the invention will become apparent from the following description, in which exemplary embodiments are described in detail with reference to the drawings. They show schematically: Fig. 1 a circuit diagram of a fuel cell system according to the invention, Fig. 2 a section of the circuit diagram of the fuel cell system from Fig. 1 , Fig. 3 shows a power curve in the fuel cell system Fig. 1, and Fig. 4 a circuit diagram of an alternative fuel cell system according to the invention. Identical or functionally equivalent elements are in the Figures 1 to 4 each designated by the same reference symbol.

[0030] Figure 1 shows a circuit diagram of a fuel cell system 1 according to an embodiment of the invention. The fuel cell system 1 comprises a plurality of fuel cell stacks 10, each of which is connected in parallel to an electrical intermediate circuit 40. As Fig. 4 as an alternative to the fuel cell system 1 from Fig. 1 shows, the fuel cell stacks 10 can alternatively also be connected in series with one another.

[0031] In addition to the fuel cell stacks 10, an electrical output network system 20, an electrical system peripheral network 30 and an electrical power resistance network 50 are each connected in parallel to each other and to the fuel cell stacks 10 by means of the electrical intermediate circuit 40.

[0032] Figure 2 shows the Fig. 1 section of the fuel cell system 1 marked with the letter A of the Fig. 1 , which there comprises, for example, ten fuel cell stacks 10, whereby alternatively more or fewer fuel cell stacks 10 may be present. Fig. 2 shows only three fuel cell stacks 10 as an example in a detailed view, whereby the other fuel cell stacks 10 of the Fig. 1 are not shown here or can alternatively be omitted.

[0033] The fuel cell stacks 10 are part of a fuel cell system 12, which also includes DC / DC converters 14, with each fuel cell stack 10 being assigned a DC / DC converter 14. By means of one of the DC / DC converters 14, each of the fuel cell stacks 10 is electrically connected to the electrical intermediate circuit 40, which in this case is a DC intermediate circuit (DC stands for "direct current").

[0034] The electrical output network system 20, in turn, has a DC / AC converter 22 connected to an electrical output network 24. The DC / AC converter 22 is designed as a bidirectional DC / AC converter 22 (AC stands for "alternating current"). Depending on the operating direction of the DC / AC converter 22, electrical energy, or in other words, electrical current, can be discharged from the electrical intermediate circuit 40 to the electrical output network 24 or fed from the electrical output network 24 into the electrical intermediate circuit 40.

[0035] The electrical system peripheral network 30, in turn, includes system peripherals 34, such as pumps, sensors, heat exchangers, seals, compressors, recirculation fans, charge air coolers, and / or humidifiers to support the operation of the fuel cell system 12. The system peripherals 34 are connected to the electrical intermediate circuit 40 via a converter 32. The converter 32 is embodied here as a DC / AC converter, but can alternatively also be embodied as a DC / DC converter.

[0036] The electrical power resistor network 50 comprises an electrical power resistor 52 and power electronics 54, which are electrically connected to the electrical intermediate circuit 40. The power electronics 54, in turn, can comprise various components, such as, in particular, a voltmeter for measuring the voltage in the electrical intermediate circuit 40 and / or a controller for regulating the electrical energy supplied to the electrical power resistor 52 from the electrical intermediate circuit 40.

[0037] Figure 3 shows the sequence of an exemplary illustrated method 100 according to an embodiment of the invention based on a fuel cell system power curve 120 of the power in kW over time of the fuel cell system 12 and a system peripheral network power curve of the power in kW over time of the system peripheral network 30.

[0038] In a first step 102 of the method, the fuel cell system 12 is operated at full power and, for example, delivers 60 kW (see fuel cell system power curve 120 in the time range of the first step 102). Part of this power of the fuel cell system 12 is provided to the system's electrical peripheral network 30, as can be seen from the system's peripheral network power curve 130.

[0039] Now, it is assumed that a grid disturbance 104 occurs as a second step of the method 100, in particular a grid failure, in the electrical output grid 24, so that the electrical output grid 24 can no longer draw electrical energy from the electrical intermediate circuit 40. This occurrence of the grid disturbance 104 can advantageously be detected by the power electronics 54, which, using its voltmeter, detects a voltage in the electrical intermediate circuit 40 that exceeds a predefined maximum voltage for a grid disturbance 104, so that the grid disturbance 104 is detected.

[0040] In a third step 106 of method 100, the electrical power resistor 52 is then activated by the power electronics 54. The electrical energy in the electrical intermediate circuit 40, which is further generated by the fuel cell system 12 and fed into the electrical intermediate circuit 40, is then supplied to the electrical power resistor 52, which converts it into waste heat. This conversion occurs outside the fuel cell system 12. The electrical power resistor 52 is advantageously cooled during its operation, particularly advantageously by water cooling.

[0041] Parallel to the third step 106 or subsequently thereto, a fourth step 108 of the method 100 is initiated, in which the load points of the operating fuel cell stacks 10 are reduced after the grid fault 104 is detected within the scope of the power dynamics available from the fuel cell stacks 10. As a result, the electrical energy generated by them decreases over time, as can be seen from the fuel cell system power curve 120 in the temporal range of the third step 106. Thus, less electrical energy needs to be dissipated from the electrical intermediate circuit 40 to the electrical power resistor 52.

[0042] When the grid fault 104 is remedied, the grid return 110 of the electrical output grid 24 occurs at a certain point in time as the fifth step of the method 100. The electrical output grid 24 is now again available for feeding in electrical energy from the electrical intermediate circuit 40.

[0043] Accordingly, according to a sixth step 112 of the method 100, the electrical energy is fed back to the electrical output network 24 instead of to the electrical power resistor 52. The electrical power resistor 52 can be switched off by the power electronics 54 in this respect. The detection of the return of the grid 110 can also be carried out by voltage measurement by the power electronics 54. Parallel to the sixth step 112 or subsequently thereto, a seventh step 114 of the method 100 is executed, in which the load points of the fuel cell stacks 10 in operation are ramped up again after the return of the grid 110 is detected, within the scope of the power dynamics available from the fuel cell stacks 10.

[0044] Figure 4shows an alternative embodiment of a fuel cell system 1, in which the fuel cell stacks 10 of the fuel cell system 12 are interconnected in series, for example. In addition, a common DC / AC converter 22 is used for the electrical output network 24 and the system peripherals 34.

[0045] Most importantly, the electrical power resistor 52 is not located in an electrical intermediate circuit 40, but rather between the DC terminals of the fuel cell stack 10 and the DC / AC converter 22. The power electronics 54, which controls and / or regulates the electrical power resistor 52, is advantageously part of the DC / AC converter 22.

[0046] The electrical power resistor 52 can therefore be connected either directly to the DC terminals of the fuel cell stack 10 or to an electrical intermediate circuit 40, as in Fig. 1 and 2shown. A control loop can be implemented on a separate control unit with transistor switches for the electrical power resistor or on a central control unit.

[0047] The above explanations of the embodiments describe the present invention exclusively by way of examples. List of reference symbols

[0048] 1 Fuel cell plant 10 Fuel cell stack 12 Fuel cell system 14 DC / DC converter 20 Electrical output grid system 22 DC / AC converter (of the electrical output grid system) 24 Electrical output grid 30 Electrical system peripheral network 32 Converter (of the electrical system peripheral network) 34 System peripherals 40 Electrical intermediate circuit 50 Electrical power resistor network 52 Electrical power resistor 54 Power electronics 100 Procedure 102 First step 104 Grid failure 106 Third step 108 Fourth step 110 Grid return 112 Sixth step 114 Seventh step 120 Fuel cell system power curve 130 Plant peripheral network power curve

Claims

1. A fuel cell plant (1) having a fuel cell system (12) comprising a plurality of fuel cell stacks (10), the fuel cell system (12) being electrically connected or connectable to an electrical output network (20) for supplying an electrical output network (24) with electrical energy generated by the operation of the fuel cell stacks (10), wherein the fuel cell plant (1) further comprises an electrical power resistor network (50) comprising an electrical power resistor (52) for load protection of the fuel cell stacks (10) in the event of a fault, in particular a network fault (104) of the electrical output network system (20), wherein the fuel cell system (12) is electrically connected to the electrical power resistor network (50) for supplying the electrical power resistor network (50) with electrical energy generated by the operation of the fuel cell stacks (10), characterised in that the electrical power resistor network (50) is arranged in an electrical intermediate circuit (40) between DC / DC converters (14) of the fuel cell stacks (10) and a DC / AC converter (22) of the electrical output network system (20), wherein the DC / AC converter (22) has a bidirectional design and is connected to the electrical output network (24), so that electrical energy can be discharged from the electrical intermediate circuit (40) to the electrical output network (24) and fed from the electrical output network (24) into the electrical intermediate circuit (40).

2. Fuel cell system (1) according to claim 1, characterised in that the electrical power resistor (52) is set up to convert electrical energy into waste heat.

3. Fuel cell system (1) according to claim 1 or 2, characterised in that the electrical power resistor (52) is arranged outside the fuel cell stacks (12).

4. Fuel cell system (1) according to one of the preceding claims, characterised in that the fuel cell system (1) comprises a cooling unit for cooling the electrical power resistor (52).

5. Fuel cell system (1) according to one of the preceding claims, characterised in that the electrical power resistor network (50) comprises power electronics (54) which are set up to activate the electrical power resistor (52) when the network fault (104) occurs.

6. Fuel cell system (1) according to claim 5, characterised in that the power electronics (54) are set up to continuously measure a voltage in an electrical intermediate circuit (40) between the fuel cell system (12) and the electrical output network system (20) and to activate the electrical power resistor (52) when a voltage rise occurs in the electrical intermediate circuit (40).

7. Fuel cell system (1) according to claim 6, characterised in that the power electronics (54) has a controller for regulating the electrical energy supplied to the electrical power resistor (52) as a function of the voltage measured in the electrical intermediate circuit (40).

8. Fuel cell system (1) according to one of the preceding claims, characterised in that the fuel cell system (12) is set up to shut down a load point of the fuel cell stacks (10) in operation when the fault occurs.

9. Fuel cell system (1) according to one of the preceding claims, characterised in that the fuel cell system (1) comprises an electrical system peripheral network (30) comprising system peripherals (34) for supporting an operation of the fuel cell system (12), wherein the fuel cell system (12) is electrically connected to the electrical system peripheral network (30) for supplying the electrical system peripheral network (30) with the electrical energy generated by the operation of the fuel cell stacks (10).

10. Method (100) for load point protection of fuel cell stacks (10) of a fuel cell system (12) of a fuel cell plant (1) according to one of claims 1 to 9 upon occurrence of a fault of an electrical output network system (20) which is supplied with electrical energy generated by the operation of the fuel cell stacks (10), the method (100) being characterised by the following steps: - occurrence of the fault, in particular the mains fault (104) of the electrical output mains system (20), - detecting the fault that has occurred, in particular the mains fault (104), and - converting electrical energy in the fuel cell system (1) between the fuel cell system (12) and the electrical output grid system (20) into waste heat by means of an electrical power resistor (52).

11. Method (100) according to claim 10, characterised in that the fault that has occurred, in particular the network fault (104), is detected by measuring a voltage rise in an electrical intermediate circuit (40) between the fuel cell system (12) and the electrical output network system (20).

12. Method (100) according to claim 11, characterised in that the electrical energy converted by the electrical power resistor (52) is regulated as a function of the voltage measured in the electrical intermediate circuit (40).

13. Method (100) according to one of claims 10 to 12, characterised in that the load points of the fuel cell stacks (10) in operation are reduced within the scope of the power dynamics available on the part of the fuel cell stacks (10) after the fault, in particular the grid fault (104), has been detected.