Electric energy system with fuel cells

Abstract

Energy system (10) for a vehicle, comprising at least one fuel cell (11); at least one HV battery (12); a diode (13) arranged between the at least one fuel cell (11) and the at least one HV battery (12), which allows current flow only in the direction from the at least one fuel cell (11) to the at least one HV battery (12);a switching element (15) arranged between the at least one fuel cell (11) and the at least one HV battery (12) and configured to reversibly interrupt or close a circuit between the fuel cell (11) and the HV battery (12), a control unit configured to control the switching element (15), and an inductor (14) arranged between the at least one fuel cell (11) and the at least one HV battery (12) and connected in series with the switching element (15), wherein the inductor (14) is a choke in the form of a storage choke designed to reach its saturation magnetization during a switching operation of the switching element (15), the choke being designed such that saturation of the core material occurs above a certain current, which is deliberately accepted.

Description

The invention relates to an electrical energy system containing a fuel cell and a method for operating an electrical energy system for a motor vehicle. In a fuel cell vehicle, it is common practice to connect the fuel cell (FC) as the energy source to the traction intermediate circuit via a direct current converter (DC / DC converter). The DC link connects the high-voltage battery to the DC link, which in turn connects the pulse inverters (PWRs) and the traction motors (electric motors). This is necessary because the voltage of the fuel cell is highly dependent on the current supplied by the fuel cell. The higher the current drawn from the fuel cell, the lower its voltage. The DC / DC converter compensates for this effect and maintains a constant output voltage for the fuel cell system (fuel cell plus DC / DC converter). Such a system is described, for example, in DE 101 33 580 A1. The fuel cell serves as an auxiliary energy source and is activated and connected to the power supply via a DC / DC converter when the required power is higher than the available power, or when the state of charge (SOC) of the HV battery falls below a critical value. Suitable DC / DC converters are known, for example, from EP 3 024 130 A1. The DC / DC converter connects the fuel cell to the HV battery and prevents current from flowing back from the HV battery to the fuel cell. To make the system more cost-effective and space-saving, the DC / DC converter can be replaced by a diode. In this case, the diode blocks current when the voltage of the high-voltage battery is higher than the voltage of the fuel cell. When the battery is under load, its voltage drops. If the voltage falls below the fuel cell voltage, the diode becomes conductive and the fuel cell assists the traction circuit. However, using a simple diode to connect the fuel cell to the other high-voltage components and the high-voltage battery has the disadvantage that the power distribution between the fuel cell and the high-voltage battery can no longer be controlled, resulting in limitations to the operating strategy. For example, it is no longer possible to conserve the fuel cell's fuel supply when, for instance, a short trip is planned, the battery's energy content is sufficient, and a charging option is available at the destination. To avoid this disadvantage, a switching element can be used in the energy system which can prevent the energy flow from the fuel cell to the traction circuit even under load, and can re-couple the systems in situations where an energy transfer from the fuel cell to the traction circuit is desired. When the fuel cell is connected to the battery circuit, the differing voltages of the fuel cell and the high-voltage battery can cause equalizing currents, which are initially very high. Furthermore, not only high amplitudes but also, in particular, high current gradients (di / dt) can occur. This can be problematic with regard to the electromagnetic compatibility (EMC) of the energy system and can adversely affect sub-components. The DE 10 2016 200 668 A1 relates to a multilevel / NPC inverter for an electric machine with two voltage sources, in which an additional non-linear component connects the neutral point in such a way that the sources can be operated individually, in series or in parallel (optionally including energy transfer / charging between the storage units). DE 198 44 750 C1 describes a grid support / UPS-like supply in which an energy storage device is coupled to a grid-supplied load via an inductive element with two coupled, counter-rotating windings (duplex choke), so that the grid and storage device are strongly decoupled and disturbances / fault currents are less likely to affect each other. DE 10 2010 030 693 A1 describes a vehicle high-voltage circuit that allows an energy storage device to be safely disconnected from the inverter intermediate circuit (DC link capacitor) via a main switch and pre-charges the intermediate circuit in a controlled manner before switching it on via a pre-charging circuit (pre-charging switch plus RL element, optionally with freewheeling diode) connected in parallel to the main switch in order to avoid inrush / equalizing currents. DE 11 2006 000 895 T5 describes a fuel cell system with a DC power bus in which a battery adapted to the fuel cell polarization characteristic (number of cells / SOC and impedance parameters) is operated directly on the bus in order to provide additional power without a DC / DC converter and to prevent harmful deep discharge / overcharging (e.g. by blocking diode and bypass switch). DE 10 2014 205 977 A1 describes a circuit arrangement for an air-independent underwater vehicle, which connects the fuel cell system and rechargeable battery via a bidirectional DC-DC converter (buck / boost) and a semiconductor switch for (possibly direct) coupling to the road network in order to efficiently manage load changes / potential load formation, enable battery charging / support in both directions and ensure uninterrupted or emergency operation in case of failures. The present invention aims to provide devices and methods that at least partially eliminate the disadvantages described. DE 10 2011 002 673 A1 discloses an energy storage system consisting of several electrically interconnected subsystems of individual energy storage devices, and a method for adding an energy storage device to an energy storage system that prevents critical balancing currents from flowing between the energy storage devices. For this purpose, the voltages of the energy storage device to be added and the energy storage system are equalized using a DC-DC converter before the respective energy storage device is connected to the energy storage system via switching elements. German patent application DE 10 2008 037 064 A1 discloses a circuit arrangement for supplying an electric drive, to which at least two electrical energy sources can be connected, wherein at least one of the at least two electrical energy sources supplies the electric drive at least temporarily by means of at least one actuator, and wherein at least one electrical energy source can be disconnected from the electric drive by means of a switch. The switch can be a diode or a power semiconductor, and the actuator can comprise an inverter bridge. From EP 2 270 966 A1, a DC / DC converter with an auxiliary converter for earth current compensation is known. The DC / DC converter converts an input DC voltage applied between two input lines into an output DC voltage applied between two output lines, wherein the input lines and the output lines are galvanically isolated from each other by capacitors connected in all AC forward and reverse paths. The auxiliary converter compensates for a current flowing through all AC forward and reverse paths by connecting a compensation current path, which runs parallel to all AC forward and reverse paths, alternately to one of the input lines or alternately to one of the output lines via two switched-mode switches.The compensation current path is connected on its side opposite the clocked switches via at least one capacitor to at least one of the output lines or an intermediate potential line carrying a potential between them, or to one of the input lines or an intermediate potential line carrying a potential between them. An inductor connected in series with the capacitor is provided in the compensation current path. The problem is solved according to the invention by a device having the features of claim 1 and a method having the features of claim 5. Embodiments and further developments of the invention are set out in the dependent claims. In the energy system according to the invention, an inductor is arranged between the fuel cell and the high-voltage battery. This inductor is connected in series with a switching element, which allows the fuel cell to be reversibly connected to and disconnected from the battery circuit. This prevents large equalizing currents from flowing between the fuel cell and the battery circuit when connecting or disconnecting the fuel cell, thus avoiding high equalizing current gradients. The invention relates to an energy system for a vehicle. The energy system comprises at least one fuel cell; at least one high-voltage battery; a diode arranged between the at least one fuel cell and the at least one high-voltage battery, which allows current flow only in the direction from the at least one fuel cell to the at least one high-voltage battery; a switching element configured to reversibly interrupt or close a circuit between the fuel cell and the high-voltage battery; a control unit configured to control the switching element; and at least one inductor arranged between the at least one fuel cell and the at least one high-voltage battery and connected in series with the switching element. According to the invention, the switching element, which is configured to reversibly interrupt or close a circuit between the fuel cell and the high-voltage battery ("switching element"), is arranged between the fuel cell and the high-voltage battery. In a further embodiment, the switching element is arranged between the fuel cell and the diode. In another embodiment, the switching element is arranged between the diode and the high-voltage battery. In one embodiment, the switching element is designed as an electromechanical switching element, for example as a contactor. In another embodiment, the switching element is designed as a semiconductor switch. In special embodiments, the switching element comprises at least one IGBT or one MOSFET. The switching element is controlled by a control unit to reversibly interrupt or close the circuit between the fuel cell and the high-voltage battery. Using the control unit, a vehicle user can connect the fuel cell to the intermediate circuit or disconnect it as needed. One of the advantages of this energy system is that the switching element allows the fuel cell and high-voltage battery to be connected or disconnected very quickly without having to temporarily affect the fuel cell's media supply or shut down or start up the fuel cell itself. In one embodiment, the energy system according to the invention additionally includes an emergency shutdown element designed to interrupt the circuit between the fuel cell and the high-voltage battery if a critical overvoltage, short circuit, or other critical condition occurs in the energy system, for example, due to a defect. The emergency shutdown element is used to interrupt the energy flow from the fuel cell to the intermediate circuit in the event of a fault. If a short circuit is generated in the intermediate circuit, for example, due to a problem with a cable or component, the high-voltage battery is typically disconnected from the intermediate circuit by contactors or fuses. The emergency shutdown element is provided to also disconnect the fuel cell from the intermediate circuit. This prevents, for example, a short-circuit current from flowing from the fuel cell into the traction circuit. In one embodiment, the emergency shutdown element comprises a fusible link or a pyrolytic disconnect element. In one embodiment of the energy system, the emergency shutdown element and the on / off switching element are combined into a single unit that integrates the functions of both elements. This unit then comprises, for example, a contactor or other electromechanical switching element and / or a semiconductor switch, which may contain, for example, power transistors such as IGBTs or MOSFETs. According to the invention, the energy system comprises at least one inductor arranged between the at least one fuel cell and the at least one high-voltage battery and connected in series with the switching element. The current rise in an inductor follows the following law: This means that the voltage difference leads to a linear increase in current. However, the slope is determined by 1 / L. This ensures a smooth and system-compatible switching-on. This means that the voltage difference leads to a linear increase in current. However, the slope is determined by 1 / L. This ensures a smooth and system-compatible switching-on. In one embodiment, the inductance is implemented as an air-core coil or by a suitably conditioned high-voltage line. For example, a very short cable run or a planar busbar system can be used. This results in low costs. According to the invention, the inductance is implemented by a choke, specifically a storage choke. It can be advantageous not to design the choke's magnetization for the maximum current of the fuel cell in order to make the choke more economical and compact. According to the invention, the choke is designed such that it reaches its saturation magnetization during a switching operation of the switching element. The choke is designed, according to the invention, such that saturation of the core material occurs above a certain current, which is intentionally accepted. This offers the further advantage of limiting the energy stored in the choke, which has a positive effect on system compatibility during load shedding. Among the advantages of the energy system according to the invention is the system-compatible connection of the fuel cell to the intermediate circuit. Other components of the intermediate circuit are not overloaded by high current gradients. Furthermore, the diode or switching element is not overloaded. In addition, the energy system exhibits improved electromagnetic compatibility (EMC). The invention also relates to a method for operating an energy system with at least one fuel cell and at least one high-voltage battery, between which a diode is connected, a switching element configured to reversibly interrupt the circuit between the fuel cell and the high-voltage battery, and an inductor arranged between the at least one fuel cell and the at least one high-voltage battery and connected in series with the switching element (15). The method comprises interrupting the circuit between the fuel cell and the high-voltage battery when a current flow from the fuel cell to the high-voltage battery is not desired; and closing the circuit between the fuel cell and the high-voltage battery when a current flow from the fuel cell to the high-voltage battery is desired. The present invention provides additional degrees of freedom for the operating strategy of the energy system, thus enabling increased efficiency. Cost advantages can be achieved by controlling which resource the vehicle is powered by at any given time. Furthermore, user comfort is increased because the resource used can be adapted to the existing infrastructure (charging and refueling). The invention is schematically illustrated with reference to embodiments in the drawings and is further described with reference to the drawings. Figure 1 shows a schematic representation of an embodiment of the energy system according to the invention with connected loads; Figure 2 shows the current waveform over time in an embodiment of the energy system according to the invention with a low series inductance when the fuel cell is switched on and the corresponding equivalent circuit diagram; Figure 3 shows the current waveform over time in an embodiment of the energy system according to the invention with an increased series inductance when the fuel cell is switched on and the corresponding equivalent circuit diagram. Fig. 1 shows a schematic representation of an embodiment of the energy system 10 according to the invention with connected loads 16, 17, 18. The energy system 10 comprises a fuel cell 11 and a high-voltage battery 12 as energy sources. These are connected via a diode 13, which allows current flow only in the direction from the fuel cell 11 to the high-voltage battery 12. A switching element 15 and an inductor 14 are arranged between the fuel cell 11 and the high-voltage battery and are connected in series. The switching element 15 is controlled by a control unit (not shown in the drawing) to selectively open or close the circuit between the fuel cell 11 and the high-voltage battery 12. The inductor 14 is designed to reduce equalizing currents and current gradients that occur during a switching operation. In the representation shown in Fig.In the variant shown, the inductor 14 is arranged directly at the positive terminal of the fuel cell 11. Pulse inverters 16 and electric motors 17 are connected to the energy system 10, as well as other HV components 18 such as auxiliary units of the fuel cell, chargers, 12 V DC / DC converters, HV heaters, electric air conditioning compressors, etc. Fig. 2 shows in its upper part the result of a simulation of the current profile iL [A] over time t [ms] in an embodiment of the energy system according to the invention during a switching operation of the fuel cell, and below it the corresponding equivalent circuit diagram. In this embodiment, the energy system has a low series inductance of only 10 nF. The inductance can be realized, for example, by a very short conductor or a planar bus system. The current reaches its final value within 0.5 µs. Fig. 3 shows in its upper part the result of a simulation of the current profile iL [A] over time t [ms] in another embodiment of the energy system according to the invention during a switching operation of the fuel cell, and below it the corresponding equivalent circuit diagram. In this embodiment, the energy system has an increased series inductance of 10 µF. The inductance can be implemented, for example, by an air-core inductor or a storage choke. The current reaches its final value within 0.5 ms; the gradient of the equalizing current is therefore 1,000 times smaller than in the embodiment shown in Fig. 2. Reference symbol list 10 Energy system 11 Fuel cell (FC) 12 High-voltage battery 13 Diode 14 Inductor 15 Switching element 16 Pulse inverter (PWR) 17 Electric motor (EM) 18 Other high-voltage components

Claims

Energy system (10) for a vehicle, comprising at least one fuel cell (11); at least one HV battery (12); a diode (13) arranged between the at least one fuel cell (11) and the at least one HV battery (12), which allows current flow only in the direction from the at least one fuel cell (11) to the at least one HV battery (12);a switching element (15) arranged between the at least one fuel cell (11) and the at least one HV battery (12) and configured to reversibly interrupt or close a circuit between the fuel cell (11) and the HV battery (12), a control unit configured to control the switching element (15), and an inductor (14) arranged between the at least one fuel cell (11) and the at least one HV battery (12) and connected in series with the switching element (15), wherein the inductor (14) is a choke in the form of a storage choke designed to reach its saturation magnetization during a switching operation of the switching element (15), the choke being designed such that saturation of the core material occurs above a certain current, which is deliberately accepted. Energy system (10) according to claim 1, wherein the switching element (15) is designed as an electromechanical switching element in the form of a contactor. Energy system (10) according to claim 1, wherein the switching element (15) is designed as a semiconductor switch. Energy system (10) according to claim 3, wherein the switching element (15) comprises at least one IGBT or one MOSFET. Method for operating an energy system (10) with at least one fuel cell (11) and at least one high-voltage battery (12) between which a diode (13) is connected, a switching element (15) configured to reversibly interrupt the circuit between the fuel cell (11) and the high-voltage battery (12), and an inductor (14) arranged between the at least one fuel cell (11) and the at least one high-voltage battery (12) and connected in series with the switching element (15), wherein the inductor (14) is a choke designed to reach its saturation magnetization during a switching operation of the switching element (15), the choke being designed such that a saturation of the core material occurs above a certain current, which is deliberately accepted, comprising interrupting the circuit between the fuel cell (11) and the high-voltage battery (12).when a current flow from the fuel cell (11) to the HV battery (12) is not desired; and closing the circuit between the fuel cell (11) and the HV battery (12) when a current flow from the fuel cell (11) to the HV battery (12) is desired.

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