Fuel cell system with improved cold start properties and processes

Abstract

Fuel cell system (1) with at least one fuel cell stack (2) which is designed to convert reactants to generate electricity, with a cold start detection device (7) for detecting a cold start state of the fuel cell stack (2), with a load (5, 10) that can be switched on to the fuel cell stack (2), and with a control device (6) configured to switch on the load (5, 10) in the cold start state of the fuel cell stack (2), wherein the supply of reactants for the fuel cell stack (2) is adapted to the switching on of the load (5, 10), where the control device (6) is designed in terms of programming and / or circuitry to vary the connected load with one or more load jumps in response to the detection of the cold start state of the fuel cell stack (2), characterized by the fact that a drive release device (8) which is configured to output a drive release signal for the fuel cell system (1), wherein the control device (6) is designed to control the load step when a drive release signal is present.

Description

[0001] The invention relates to a fuel cell system with at least one fuel cell stack, which is designed for converting reactants to generate electricity, with a cold start detection device for detecting a cold start state of the fuel cell stack, with a load that can be switched on to the fuel cell stack and with a control device which is designed to switch on the load in the cold start state of the fuel cell stack, wherein the supply of reactants to the fuel cell stack is adapted to the switching on of the load, and a corresponding method.

[0002] Fuel cell systems are used to generate electricity by electrochemically reacting a fuel, such as hydrogen, with an oxidant, such as oxygen or ambient air. Fuel cell systems typically consist of at least one fuel cell stack, which often comprises more than 100 individual fuel cells. Each fuel cell has a cathode and an anode, separated by a membrane (e.g., PEM). The electrochemical process takes place between these cathode and anode regions through the catalytic combustion of the reactants. The efficiency of this process is highly dependent on the operating conditions.An important process condition concerns the operating temperature of the fuel cells, as the electrochemical process takes place with the greatest efficiency and at the same time the highest energy yield in a temperature range of approximately 60°C to 120°C.

[0003] While ensuring optimal process conditions seems straightforward in stationary fuel cell systems, mobile fuel cell systems, such as those used as a power source for vehicles, face the challenge of keeping the system operational in all weather conditions or making it operational as quickly as possible. A common task is performing a cold start, i.e., starting the fuel cell system at a temperature below its normal operating temperature. The so-called freeze start of fuel cell systems at temperatures below 0°C is particularly critical, as a significant reduction in the performance of the fuel cell stacks is known to occur during operation in this low temperature range.

[0004] One possible solution is to supply heat externally, for example by using a burner or heater to heat the fuel cell stack. However, this approach requires an external heat source and also consumes valuable energy.

[0005] Another approach is known from German patent application DE 600 05836 T2, which proposes limiting the reactant current at at least some of the electrodes during a start-up phase in which the fuel cell stack is at a start-up temperature below the normal operating temperature. This reduction in reactant current changes the electrode potential, leading to an increase in the electrode overvoltage and consequently to an increase in the amount of heat generated in the fuel cell stack at a given operating current density. An alternative embodiment described in this patent application proposes connecting a transient electrical load to the fuel cell stack once or intermittently and—in order to achieve this reduction—not increasing the reactant supply rates.This approach also increases the rate at which the reactants are consumed, thus reducing the stoichiometry of the reactant supply and causing a shortage of reactants.

[0006] Document US 2006 / 0088738 A1 discloses a fuel cell system and a method for controlling it, proposing to connect an electrical load when the fuel cell stack is operating at low temperatures and to achieve self-heating of the fuel cell stack by drawing off a constant load. A disadvantage of this approach appears to be that the heating process increases the start-up time and consumes fuel. In a second, downstream procedure, it is proposed to heat the battery in response to an excessively low battery temperature by dynamically co-charging and discharging processes with the fuel cell stack.

[0007] The object of the present invention is to propose a fuel cell system and a corresponding method which enable an energy-efficient cold start of the fuel cell system.

[0008] This problem is solved by a fuel cell system with the features of claim 1 and by a method for cold-starting a fuel cell system with the features of claim 6. Preferred or advantageous embodiments of the invention will become apparent from the dependent claims, the following description, and the accompanying figures.

[0009] According to the invention, a fuel cell system is proposed which is preferably designed and / or suitable for mobile use, particularly in vehicles. The fuel cell system comprises one or more fuel cell stacks, each fuel cell stack having a plurality, in particular more than 100, fuel cells. Each fuel cell has an anode and a cathode region, the two regions being separated from each other by a membrane, in particular a proton exchange membrane (PEM). The fuel cell or fuel cell stack is configured to electrochemically react reactants, preferably a fuel, in particular hydrogen, which originates, for example, from a tank or a reformer, and an oxidant, in particular oxygen or ambient air, to generate electric current.

[0010] The fuel cell system includes a cold start detection device for detecting the cold start state of the fuel cell stack. The cold start state is preferably characterized by the fact that the temperature of the fuel cell stack during the cold start state is below the normal operating temperature; in particular, the cold start state is configured as a freeze-start state, such that the fuel cell stack temperature is below 0°C. In one possible embodiment of the invention, the cold start detection device can be configured as a temperature sensor that detects the temperature of the fuel cell stack or the fuel cells. Alternatively, the cold start detection device can also be configured to detect the cold start state of the fuel cell stack via a relative measurement or even an estimation.

[0011] The fuel cell stack is connected to, and / or can be connected to, a switchable load, so that the current demand on the fuel cell stack can be increased. The switchable load can be configured as desired; for example, it can be a primary load, in particular a drive motor; a secondary load, in particular a vehicle's heating or air conditioning system; or a tertiary load, such as an energy storage device, or the like. Preferably, the switchable load is selected such that the energy fed to it is not lost but can be used elsewhere.

[0012] A control device is provided for controlling the fuel cell system. This device is designed to switch on the load, at least during the cold start state of the fuel cell stack, and simultaneously adjust the supply of reactants to the fuel cell stack in response to the load being switched on. The control device can be designed as a single, centralized control unit, or alternatively, it can be organized decentrally, so that the switching on of the load and the supply of reactants are coordinated but carried out by separate control units. In particular, the control device is designed such that switching on the load does not lead to a reduction in the reactant supply, as is known from the prior art.

[0013] It is proposed that the control device be configured, either programmatically or circuit-wise, to vary the connected load with one or more load steps in response to the detection of the cold-start state of the fuel cell stack. It is thus claimed that a control-related relationship exists between the detection of the cold-start state and the variation of the connected load. This control-related relationship can be implemented, for example, as a sequence control, a regulation, or a control loop, where the detection of the cold-start state is an input variable and the variation of the connected load is an output variable. More complex control-related relationships, such as control via adaptive controllers, neural networks, etc., are also within the scope of the invention.It is essential that the load variation occurs with one or more load jumps in response to the detection of the cold start state of the fuel cell stack.

[0014] It was found that varying the applied load with one or more load steps significantly accelerates the cold start process. While applying a constant load sink results in the polarization characteristic of the fuel cell stack rising with an almost constant slope, a load step in the applied load creates a step-like progression in the polarization characteristic and thus a disproportionate increase in the power output of the fuel cell stack.

[0015] In the invention, the fuel cell system includes a drive enable device configured to output a drive enable signal for the fuel cell system. The control device is designed to control the load step when drive enable is present, i.e., during dynamic operation of the vehicle. In particular, but not limited to, it is preferred that when drive enable is present, the load step or load steps are achieved by load distribution between an energy storage device, especially a battery or a capacitor, especially a supercapacitor, and the fuel cell stack. This utilizes the fact that, through clever or adapted energy management between the energy storage device and the fuel cell stack, loads or load sinks can be varied or distributed highly dynamically.

[0016] In a preferred embodiment, the load step is designed as a reduction in the connected load and / or the current demand on the fuel cell stack. This experimentally verified preferred embodiment is surprising in that an increase in the power output of the fuel cell stack is achieved by a prior reduction in the current demand. On the one hand, this behavior seems contrary to the idea of ​​achieving self-heating of the fuel cell stack by reducing the load; on the other hand, this behavior also seems contrary to the aforementioned publication DE 600 05836 T2, since the process control proposed according to the invention does not create a shortage of reactants, but rather an oversupply of reactants. Accordingly, it is also within the scope of the invention to improve the cold-start behavior of a fuel cell system by providing an oversupply of reactants during the start-up phase.

[0017] In a further development of the invention, it is proposed that the amplitude of the load step be at least 30%, preferably at least 60%, and in particular at least 90% of the connected load and / or the current demand present at the connected load. Particularly in the latter case, the fuel cell stack is switched almost or – in another embodiment – ​​completely without load.

[0018] It has been found that a load step duration of less than 3 seconds, preferably less than 2 seconds, and particularly less than 1 second, is sufficient. The load step duration is defined, for example, as the peak width at the FWHM (Full Width Half Maximum) level. Thus, in a preferred embodiment, the load step is formed as a negative peak in the load and / or current curve, preferably with exactly one load step occurring during the cold start phase.

[0019] A further object of the invention relates to a method for cold starting, in particular freeze starting, of a fuel cell system, wherein the fuel cell system is preferably designed as just described or according to one of the preceding claims, wherein a cold start state of a fuel cell stack is first detected, an electrical load is simultaneously or subsequently connected to the fuel cell stack, and the reactant supply for this fuel cell stack is simultaneously or promptly adapted to the connected load, and wherein the load is varied with at least one load step in response to the detected cold start state.

[0020] In particular, the method is designed for the intended use of the fuel cell system described above. Consequently, the described fuel cell system is also designed to carry out the claimed method.

[0021] In a preferred implementation of the method, the load step(s) occur within five minutes, preferably three minutes, and particularly within two minutes, after the start or cold start of the fuel cell system. In an exemplary embodiment, the load step is performed between 120 and 130 seconds after the cold start.

[0022] Further features, advantages, and effects of the invention will become apparent from the following description of a preferred embodiment of the invention and the accompanying figures. These show: Fig. 1 a block diagram of a fuel cell system as an embodiment of the invention; Fig. 2 a measurement protocol to illustrate the invention.

[0023] The Fig. Figure 1 shows a fuel cell system 1, which is configured, for example, to be used as a mobile fuel cell system in a vehicle for generating propulsion energy. The fuel cell system 1 comprises one or more fuel cell stacks 2, wherein in the Fig. Figure 1 shows only a single fuel cell stack 2. The fuel cell stack 2 has a plurality of fuel cells, for example, more than 100 fuel cells are arranged in a fuel cell stack 2.

[0024] The fuel cell stack 2 has a load output 3, which is connected via a load manager 4 to one or more load sinks 5. The load sinks 5 can be configured, for example, as heating, propulsion, air conditioning, or the like. A control unit 6 is integrated to control the load manager 4 and organizes the load distribution. The control unit 6 receives as input a signal from a temperature sensor 7, which measures the temperature of the fuel cell stack 2 or the fuel cells. As an optional additional input, the control unit 6 also receives a signal from an enable module 8, which enables the fuel cell system 1 for operation.An output of the control device 6 is connected to a reactant control 9, which controls the inflow of reactants or the stoichiometry of the inflows, i.e. the proportion of reactants in the inflow streams.

[0025] As a further option, the fuel cell system 1 shows an energy storage device in the form of a battery 10, which can be connected via the load manager 4 to the load sinks 5 and / or to the output 3 of the fuel cell stack 2.

[0026] From a functional standpoint, the control device 6 is designed, both programmatically and / or circuit-wise, to support a cold start of the fuel cell system by means of an adapted start sequence. For this purpose, the control device 6 evaluates the signal from the temperature sensor 7 by comparing the measured value with a predefined value for the operating temperature of the fuel cell system 1 or for the fuel cell stack 2. If the measured value is below this operating temperature or below another freely definable limit temperature, the control device 6 executes the partial start sequence. The limit temperature is preferably defined as less than 0°C, so that the cold start is performed, in particular, as a freeze start.

[0027] In the first step of the cold start sequence, one or more load sinks 5 are connected to the output 3 of the fuel cell stack 2 by the load manager 4, causing the fuel cell stack 2 to supply power and begin to heat up as a result. Simultaneously or in conjunction with the connection of the load, the reactant control is activated, adjusting the reactant supply and / or stoichiometry to the increased load requirement. During an initial warm-up phase, lasting, for example, 120 seconds, the fuel cell stack is subjected to a constant power draw and / or current demand. In a second cold start phase, the load manager 4 is controlled by the control device to dynamically change the load and / or current demand for a short period.During this change, the control signal from the control device 6 to the reactant control 9 remains unchanged, so the reactant supply and stoichiometry also remain essentially unchanged. After the brief load step or jump in current demand, a constant load is again applied to output 3 in a third cold start phase by switching on the load sinks 5. Measurements have shown that, during the described start sequence with load step, the power output and / or the current output and / or the polarization characteristic change in steps and / or abruptly, particularly disproportionately, between the first and third cold start phases. It was clearly evident that the performance of the fuel cell stack 2 increased significantly after the dynamic load step.

[0028] When the control device 6 is connected to the release module 8, the described load step can be carried out in two operating phases. In the first alternative, the load step is performed during the self-heating process of the fuel cell stack by varying the load sinks 5 before the release module 8 transmits a drive authorization to the control device 6. In another alternative, the second cold start phase takes place after the drive authorization by the release module 8 during the dynamic operation of the vehicle. In this case, to ensure sufficient power is available for driving, the load manager 4 is controlled by the control device 6 in such a way that the loads between the fuel cell stack 2 and the battery 10 are varied, particularly dynamically.In the second alternative, it is therefore possible that the load change is implemented by briefly connecting and discharging the battery 10 to the load sinks 5 and simultaneously disconnecting or relieving the fuel cell stack 2 by the load manager 4.

[0029] The Fig. Figure 2 shows an exemplary measurement protocol of a cold start procedure with a first measurement curve 11, which represents the current at the output 3 of the fuel cell stack 2, with a second measurement curve 12, which shows the voltage or the polarization characteristic of the fuel cell stack 2 also at the output 3, and with a third measurement curve 13, which represents the power drawn (net power) of the fuel cell stack 2.

[0030] As can be seen from the graph, in an initial cold start phase I, a constant load is connected to output 3 of the fuel cell stack 2, with the measured curves of power 13, current 12, and voltage 13 rising slowly. For example, in the graph in Fig. 2. It was observed that a power increase from 18.8 kW to 22.4 kV was measured within a time interval of 80 to 120 seconds.

[0031] In the second cold start phase II, the current demand is briefly reduced (less than 1 second) from 150 amps to 10 amps and then immediately increased again to 176 amps. As a direct reaction, the power drops to near zero at the same time, while the voltage increases by approximately 50%.

[0032] In the third cold start phase III, a constant load is again applied, whereby – compared to the first cold start phase I – the output current (as shown in measurement curve 11), the output power (as shown in measurement curve 13), and the output voltage or polarization characteristic (as shown in measurement curve 12) have all increased significantly. Comparing the end of the first cold start phase I with the beginning of the third cold start phase III, it becomes clear that the performance of the fuel cell stack has increased by approximately 35% after this dynamic load change.

[0033] Ultimately, it can be stated that by introducing one or more load jumps during the freeze start or cold start of the fuel cell stack 2 using constant load sinks, the self-heating of the fuel cell stack 2 can be greatly accelerated.

Claims

[1] Fuel cell system (1) with at least one fuel cell stack (2) which is designed to convert reactants to generate electricity, with a cold start detection device (7) for detecting a cold start state of the fuel cell stack (2), with a load (5, 10) that can be switched on to the fuel cell stack (2), and with a control device (6) configured to switch on the load (5, 10) in the cold start state of the fuel cell stack (2), wherein the supply of reactants for the fuel cell stack (2) is adapted to the switching on of the load (5, 10), where the control device (6) is designed in terms of programming and / or circuitry to vary the connected load with one or more load jumps in response to the detection of the cold start state of the fuel cell stack (2), characterized by , that a drive release device (8) which is configured to output a drive release signal for the fuel cell system (1), wherein the control device (6) is designed to control the load step when a drive release signal is present. [2] Fuel cell system (1) according to claim 1, characterized by , that the load jump is formed by a reduction in the switched-on load (5) and / or the current demand. [3] Fuel cell system (1) according to claim 2, characterized by that the amplitude of the load change is at least 30%, preferably at least 60%, in particular at least 90% of the added load. [4] Fuel cell system (1) according to any one of the preceding claims, characterized by that the duration of the load jump is less than 3s, preferably less than 2s, in particular less than 1s. [5] Fuel cell system (1) according to any one of the preceding claims, characterized by, that the load jump occurs through load distribution between an energy storage device (10) and the fuel cell stack (2). [6] A method for cold-starting a fuel cell system (1), preferably according to one of the preceding claims, comprising the steps: Detection of a cold start state of a fuel cell stack (2); Connection of a load (5,10) to the fuel cell stack and adjustment of the reactant supply for the fuel cell stack (2); Load variation with at least one load jump in response to the detected cold start condition, characterized by , that the load change after a driving authorization in the dynamic operation of the fuel cell stack (2) and by a load redistribution to an energy storage device (10) takes place. [7] Method according to claim 6, characterized bythat the load change occurs within 5 min, preferably 3 min, in particular 2 min after the start or cold start of the fuel cell system (1).

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