Fuel cell system and method for controlling such a fuel cell system

Abstract

A fuel cell system (100) is proposed. The fuel cell system (100) has at least two cell stacks (10.1, 10.2) and an auxiliary system (20) configured to control the at least two cell stacks and supply them with hydrogen and air. The auxiliary system (20) has a control unit configured to control the at least two cell stacks (10.1, 10.2) in partial load operation and in switching operation. The at least two cell stacks (10.1, 10.2) can be controlled with a time offset, in particular alternately, in switching operation.

Description

State of the art

[0001] Hydrogen-based PEM fuel cells are considered the mobility concept of the future because they emit only water as exhaust gas and enable fast refueling times.

[0002] In truck and bus systems, two or more cell stacks are often operated to meet performance requirements. There are two basic system approaches. In the first system approach, each stack has its own auxiliary components, also known as BoP components. For example, two stacks require two systems, three stacks require three, and so on. These systems can be operated largely independently of one another. The design of the BoP components is tailored to the operation of a single stack. In a second system approach, two or more stacks share common BoP components or even entire subsystems. The operation of a first stack interacts with the operation of other stacks. The design of the BoP components differs from the first system approach and must be tailored to multi-stack operation.

[0003] The second system approach, if cleverly implemented, promises synergy potential with a number of required components and synergies in operation (e.g. master-slave freeze start of two stacks - the first stack supports the second stack with heat during freeze start).

[0004] It is possible to operate two stacks with a common air system, in particular a common electrical fluid energy machine.

[0005] In multi-stack systems, part-load capability is crucial for overall system efficiency and thus hydrogen consumption. During part-load operation, PEM stacks face two competing requirements: 1. Operation at the highest possible efficiency, while simultaneously ensuring optimal humidification of the fuel cell membrane, corresponds to operation at the lowest possible cathode stoichiometry, resulting in minimal electrical fluid energy machine power. In such operation, the stoichiometry lambda can vary between 1.5 and 2. 2. Operation in which liquid water is safely removed from the stack. The accumulation of liquid water is only permissible up to a certain level. If the maximum liquid water quantity is exceeded, the cathode stoichiometry must be increased, at least temporarily. In such operation, the lambda stoichiometry can vary between 4 and 10.

[0006] These requirements can be met through switching operation, also known as toggle operation. For this purpose, the stacks are operated at energy-optimal levels at a relatively low stoichiometry (mass flow) for a larger portion of the time and at increased stoichiometry for a smaller portion of the time. This has a direct impact on the required characteristic map width of the fluid energy machine. It should be noted that the nature of these requirements can vary depending on the stack type and system architecture (e.g., with or without an external humidifier).

[0007] In terms of fluid energy machines, there is a trend toward turbocompressors. The operating range of turbocompressors is limited by surge and blocking limits. Disclosure of the invention

[0008] The requirements for the map width of the electric fluid energy machine increase due to the partial load requirements of multiple cell stacks. These map limitations make partial load operation of multiple stacks with a shared electric fluid energy machine difficult or even impossible. Without further measures, the mass flow spread increases by 100% with each additional stack that the electric fluid energy machine must supply.

[0009] The fuel cell system according to the invention and the method according to the invention for controlling such a fuel cell system make it possible to at least partially eliminate the above disadvantages.

[0010] 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.

[0011] A first aspect of the present disclosure relates to a fuel cell system. The fuel cell system, in particular for a motor vehicle, comprises at least two cell stacks, in particular PEM cell stacks, and an auxiliary system configured to control the at least two cell stacks and supply them with hydrogen and air. The auxiliary system comprises a control unit configured to control the at least two cell stacks in partial load operation and in switching operation. The at least two cell stacks can be controlled with a time offset, in particular alternately, in switching operation.

[0012] In other words, the fuel cell system has a common air path, or fluid energy machine. The fuel cell system can be referred to as a multi-stack system. The auxiliary system can be referred to as a PoB system and typically has an electric fluid energy machine, which can be designed as a turbocompressor.

[0013] In the context of the present disclosure, the terms mass flow and air mass flow are used synonymously.

[0014] Switching the air mass flow is essential in PEM cell stacks operating at partial load. However, this can be done with a time delay, particularly alternating between different cell stacks. The fuel cell system according to the invention can therefore be designed such that a switching strategy can be adjusted so that only one cell stack at a time requests increased flow. This avoids the operating condition in which multiple cell stacks require high flow rates simultaneously.

[0015] Such a fuel cell system can achieve the following advantages: A limited range of operating maps for electric fluid energy machines or turbomachines is sufficient to operate more than one cell stack at partial load. Synergies at the subsystem level are possible (reduced number of components). Partial load operation of multiple cell stacks can achieve increased overall system efficiency through further operating point modulation. Fuel cell systems without external humidifiers are possible because they tend to have a larger mass flow spread in switching mode.

[0016] Furthermore, such a fuel cell system makes it possible to keep the air mass flow in the air system constant and vary the flow only in the cell stacks. This can achieve improved NVH behavior (no or only minimal speed changes of the electric fluid energy machine during partial load operation).

[0017] A second aspect of the present disclosure relates to a method for controlling a fuel cell system according to the first aspect of the present disclosure. The method comprises the following steps: - Controlling a first cell stack of the at least two cell stacks in partial load operation, - controlling a second cell stack of the at least two cell stacks in switching mode or controlling a second cell stack of the at least two cell stacks in partial load mode, and - changing the function of at least two cell stacks after an at least indirectly specified time.

[0018] In other words, an operating strategy for a multi-stack system is described that avoids the simultaneous operation or control of two cell stacks in switching mode. This allows membrane humidity criteria to be met with the greatest possible energy efficiency, while also preventing the accumulation of excessively high amounts of droplets / liquid water.

[0019] Since the mass flow only needs to be increased for a short time, it is possible to design the operating strategy or control the fuel cell system in such a way that the air system of the auxiliary system provides an increased air flow for at most one cell stack at any given time.

[0020] It is advantageous if, in partial load operation, the auxiliary system supplies the corresponding cell stack with air using a predefined first mass flow and, in switching operation, the auxiliary system supplies the corresponding cell stack with air using a predefined second mass flow, whereby the first mass flow is smaller than the second mass flow.

[0021] In the following, embodiments of the invention are described with reference to the figures. Fig. 1 shows a flowchart of a fuel cell system according to an embodiment, Fig. 2 schematically shows a diagram of the auxiliary system map of a fuel cell system according to an embodiment, Fig. 3 schematically shows a diagram of the auxiliary system map of a fuel cell system according to an embodiment, Fig. 4 shows a flowchart of a method according to an embodiment, and Fig. 5 shows a flowchart of a method according to an embodiment.

[0022] Similar, similarly acting, identical, or equivalent elements are provided with similar or identical reference numerals in the figures. The figures are merely schematic and not to scale.

[0023] Fig. 1 shows a flow diagram of a fuel cell system 100 according to an embodiment. The fuel cell system 100 of Fig. 1 is preferably a fuel cell system of a motor vehicle. The fuel cell system 100 of Fig. 1 comprises two cell stacks 10.1, 10.2 and several valves 12.1, 12.2, 13.1, 13.2. In the multi-stack system, or the fuel cell system 100 of the Fig. 1, an air subsystem supplies two cell stacks, 10.1, 10.2, with a single electric fluid energy machine 17. The electric fluid energy machine 17 and a compressor 16 are part of the auxiliary system 20 of the fuel cell system 100. The valves 12.1 and 10.1, or 12.2 and 10.2, have two functions. First, they divide the mass flows in both cathodes of the two cell stacks. Second, they isolate the cathode from ambient oxygen when the vehicle is parked.

[0024] Furthermore, the fuel cell system 100 of the Fig. 1 a bypass valve 18 and an air gap 14.

[0025] Optionally, a toggle valve, i.e., a switching valve 19, can be provided. In other words, the switching behavior in switching mode can be structurally improved by an additional toggle valve. The switching valve 19, or toggle valve, can be characterized by enabling the smoothest possible transition between low and high mass flow in one or the other cell stack.

[0026] Fig. Figure 2 schematically shows a diagram of the auxiliary system characteristic map, in particular of the electrical fluid energy machine of the auxiliary system of a fuel cell system according to an embodiment. The vertical axis represents the pressure, and the horizontal axis represents the mass flow. The diagram of the Fig. Figure 2 shows a compressor map. The map is limited by the surge limit S on the one hand, and its operating range is limited by the blocking limit C on the other. To meet the operating requirements, a cell stack is operated between partial load operation B1.1 and switching operation B2.1. The limited map width (min and max mass flow at constant pressure ratio) is generally sufficient to meet the requirements of the cell stack.

[0027] Fig. Figure 3 schematically shows a diagram of the auxiliary system characteristic map, in particular of the electrical fluid energy machine of the auxiliary system of a fuel cell system according to an embodiment. The vertical axis represents the pressure, and the horizontal axis represents the mass flow. The diagram of the Fig. Figure 3 shows a compressor characteristic map. Operating points B1.1 and B2.1, marked with empty circles, represent the case when only one cell stack is operating. Operating points B1.2, B2.2, and B3, marked with rectangles, correspond to operating points that arise when a second cell stack is operated. If both stacks are operated simultaneously in switching mode and thus place increased mass flow demands, operating point B3 lies far outside the characteristic map or above the blocking limit. Adjusting the characteristic map towards a blocking limit C at higher mass flows generally also leads to a shift in the surge limit S, so that partial load with a single stack is no longer possible.

[0028] By means of the method according to the invention and the fuel cell system according to the invention, the operating point B3 can be avoided from occurring at all.

[0029] Fig. 4 and Fig. 5 each show a flowchart of a method according to an exemplary embodiment. In the first step S1, both cell stacks are operated in partial load mode by the control unit of the auxiliary system 20 of the fuel cell system 100. Subsequently, in a second step S2, a first cell stack 10.1 is operated in switching mode, with the second cell stack 10.2 continuing to operate in partial load mode. In a further third step S3, the two cell stacks 10.1, 10.2 now exchange their function, so that the first cell stack 10.1 is now operated in partial load mode and the second cell stack 10.2 is operated in switching mode. These steps S1 to S3 can be repeated as often as desired. Fig. 5 shows a similar process, where the method is carried out according to the flow chart of Fig.5 is configured to control a fuel cell system 100 comprising three cell stacks. Thus, in a further step S4, the second and third cell stacks are operated in partial load mode, while the first cell stack is operated in switching mode. In this case, the mass flow across all three cell stacks is preferably permanently constant, and the mass flows only change in the cell stacks. It should be noted that step S1 can be omitted while all cell stacks are operated in partial load mode.

[0030] Additionally, it should be noted that the terms "comprising" and "having" do not exclude other elements, and the indefinite articles "a" or "an" do not exclude a plurality. Furthermore, it should be noted that features and steps described with reference to one of the above embodiments may also be used in combination with other features and steps of other embodiments described above. Reference signs in the claims are not to be considered limitations.

Claims

[1] Fuel cell system (100), comprising at least two cell stacks (10.1, 10.2) and an auxiliary system (20) which is designed to control the at least two cell stacks (10.1, 10.2) and to supply them with hydrogen and air, wherein the auxiliary system (20) comprises a control unit which is configured to control the at least two cell stacks (10.1, 10.2) in a partial load operation and in a switching operation, wherein the at least two cell stacks (10.1, 10.2) can be controlled in a time-delayed, in particular alternating, manner in switching operation. [2] A method for controlling a fuel cell system (100) according to claim 1, comprising the following steps: - (S1) controlling a first cell stack (10.1) of the at least two cell stacks (10.1, 10.2) in partial load operation, - (S2) controlling a second cell stack (10.2) of the at least two cell stacks (10.1, 10.2) in switching operation or controlling a second cell stack (10.2) of the at least two cell stacks (10.1, 10.2) in partial load operation, and - (S3) after an at least indirectly predetermined time, changing the function of the at least two cell stacks (10.1, 10.2). [3] Method according to claim 2, characterized by , that in partial load operation, the auxiliary system (20) supplies the corresponding cell stack (10.1, 10.2) with air using a predefined first mass flow and in switching operation, the auxiliary system (20) supplies the corresponding cell stack (10.1, 10.2) with air using a predefined second mass flow, where the first mass flow is smaller than the second mass flow.

Images