Method for adjusting relative humidity in a fuel cell system

Abstract

The presented invention relates to a method (100) for adjusting a relative humidity in a fuel cell system, the method (100) comprising: - operating (101) the fuel cell system in a control mode in which the relative humidity is adjusted depending on an operating state of the fuel cell system, - determining (103) the relative humidity in an anode subsystem of the fuel cell system continuously and in real time by means of a humidity sensor arranged in the anode subsystem, - comparing (105) the determined relative humidity with a predetermined lower limit value and a predetermined upper limit value, and - in the event that a determined value of the relative humidity is greater than the upper limit value or less than the lower limit value, operating (107) the fuel cell system in a closed control mode until a determined value of the relative humidity is greater than the lower limit value and less than the upper limit value.

Description

[0001] The presented invention relates to a method for adjusting a relative humidity in a fuel cell system according to the appended claim. State of the art

[0002] PEM fuel cells are an attractive technology for a CO2-free energy economy and, due to their low operating temperature, are well suited for mobile drive systems.

[0003] The membrane of a PEM fuel cell is proton-conducting. The proton conductivity depends strongly on the membrane humidity.

[0004] A membrane humidity as high as possible along the entire fuel cell stack is essential for stable operation of the fuel cell system.

[0005] However, an accumulation of liquid water can block channels in the fuel cells of the fuel cell stack and lead to reactant depletion. Optimal water management is therefore important for efficient operation of the fuel cell system.

[0006] In a fuel cell reaction, hydrogen reacts with oxygen to form water. The water is produced at the cathode. Electroosmotic forces cause both protons and water molecules to migrate from the anode to the cathode. Water is also transported through the membrane by diffusion.

[0007] Dehumidification of the anode can be counteracted by suitable operating conditions, the design of the fuel cells, for example by hydrophobization, and systemic measures such as recirculation of the anode gas mixture and humidification of the cathode supply air.

[0008] Typically, the relative humidity in a fuel cell system is controlled cathode-based. For this purpose, setpoints for cathode pressure and stoichiometry are specified in the control unit.

[0009] Fuel cell systems are also known in which the cathode inlet humidity is increased using an external humidifier. These humidifiers can usually be controlled via a bypass.

[0010] In some fuel cell systems, a cathode inlet humidity sensor is also used to regulate the cathode humidity.

[0011] For example, water management can be handled by a control unit using two applicable maps for cathode pressure and cathode stoichiometry. The calibration is based on a suitable combination of pressure and stoichiometry depending on the operating point and temperature. However, this only allows for stationary water management.

[0012] Measurements have shown that water management and thus pressure and stoichiometry must be temporarily adjusted during load jumps and low-load operation. Disclosure of the invention

[0013] The invention presented serves in particular to ensure robust operation of a fuel cell system.

[0014] A method for adjusting relative humidity in a fuel cell system is thus presented.

[0015] The presented method comprises operating the fuel cell system in a control mode in which the relative humidity is adjusted depending on an operating state of the fuel cell system, determining the relative humidity in an anode subsystem of the fuel cell system continuously and in real time by means of a humidity sensor arranged in the anode subsystem, comparing the determined relative humidity with a predetermined lower limit value and a predetermined upper limit value, and, in the event that a determined value of the relative humidity is greater than the upper limit value or less than the lower limit value, operating the fuel cell system in a closed-loop control mode until a determined value of the relative humidity is greater than the lower limit value and less than the upper limit value.

[0016] The presented method is based on an approach for controlling the water management of fuel cell systems using a so-called "open-loop control with constraint feedback loop." The anode moisture content is continuously monitored. If a limit (constraint) is violated, the controller switches to a so-called "closed-loop operation" until the constraints are no longer violated.

[0017] In the following, the terms closed control operation and closed loop are used synonymously.

[0018] In the following, the terms open loop and control mode are used synonymously.

[0019] In the following, the terms limit and constraint are used synonymously.

[0020] Accordingly, the closed-loop control mode provides a fallback position in case problems arise during control operation. Control operation can be configured freely, i.e., without strict limitations due to extreme conditions.

[0021] Closed loop control is particularly advantageous in dynamic operation with large load changes and ensures that the operating parameters are always within a permissible range.

[0022] The presented method improves the control of humidity in the fuel cell stack. Setting suitable water management by expanding the open-loop control while taking constraints into account represents a significant improvement, especially for dynamic operation of the fuel cell system. Accordingly, the presented method enables an increase in the service life of the fuel cell system, even under critical requirements or extreme operating conditions.

[0023] Furthermore, the presented method enables an increase in performance, since the open loop feedforward control can be made more progressive without, however, taking a greater risk with regard to the lifetime of the fuel cell system, since the method is able to return the fuel cell system to a permissible, low-aging state when critical system states occur.

[0024] Furthermore, the presented method offers advantages in the application of new applications. The presented method also makes changes to load spectra less critical.

[0025] PEM fuel cell technology is suitable for a wide range of applications, including automotive and non-automotive, and is highly scalable in terms of performance. However, this requires that the operating strategy is always implemented appropriately for the specific application. Critical or damaging conditions of the fuel cell stack due to flooding or severe drying out are reliably avoided by the presented process, even under unexpected operating conditions.

[0026] The anode humidity generally reacts significantly faster to changes in the stack's moisture balance than the cathode humidity. Accordingly, a rapid change in the anode humidity leading to drying out can be interpreted as an indication of excessive drying out of the fuel cell stack. If the anode humidity falls below a minimum level, a current drying strategy can be aborted by a control device of the fuel cell system.

[0027] The presented method is based on measuring relative humidity or moisture in the anode subsystem of the fuel cell system. For this purpose, a humidity sensor can be provided at the anode outlet after a water separator or at the anode inlet after a mixing point for fresh hydrogen and recirculated hydrogen. In principle, the humidity sensor can be located anywhere in the anode subsystem.

[0028] During quasi-steady-state operation, the cathode pressure and cathode stoichiometry are determined based on a specified cathode outlet humidity. During load changes and in the low-load range, the cathode outlet humidity must be adjusted over time until optimal water management in the stack is achieved. With the presented method, the target cathode outlet humidity can be controlled based on the measured anode humidity.

[0029] The anode humidity is very dependent on the cathode humidity, since the water is only produced on the cathode side and the anode can therefore only be humidified by transmembrane water transport.

[0030] The anode moisture can be measured in the anode loop after the condensate separator or before entering the fuel cell stack. The advantage of measuring the moisture in the anode subsystem compared to a closed cathode moisture control system, which is feasible within a certain operating range, is the expected robustness of the moisture measurement signal.

[0031] The cathode exit humidity often contains liquid water, which can severely impair humidity measurements due to droplet impact. The cathode inlet humidity also does not provide any information about the internal condition of the fuel cell stack with regard to its water balance. A high cathode exit humidity is often not a sufficient criterion for a targeted water balance of the membrane or the entire fuel cell stack.

[0032] During a load drop, i.e., a negative load step, excess water must be removed from the fuel cell stack. To do this, the target cathode outlet humidity can be set to a lower value, e.g., 0.8. The low cathode outlet humidity can then be maintained until the measured anode humidity drops below a specified limit, e.g., 0.7. Once the limit is reached, the cathode outlet humidity can be reset to the steady-state target value, e.g., 1.4.

[0033] During a positive load step, the fuel cell stack should be temporarily humidified to a higher level if necessary. To do this, the cathode outlet humidity can be set to a higher target value, e.g., 1.6, if the measured anode humidity falls below a certain limit, e.g., 0.7. As soon as the limit is exceeded again, the cathode outlet humidity is reset to the steady-state target value, e.g., 1.4. If the anode humidity does not fall below the anode humidity limit during the load step, the target cathode outlet humidity is not adjusted but remains at the steady-state target value.

[0034] During partial load operation, the low air mass flow means there is a risk that liquid water will collect in the fuel cell stack, which will compromise the supply of reactants. To avoid this, the target cathode outlet humidity can be reduced, for example, resulting in an increase in the air mass flow. There are two ways to do this. Firstly, a permanent reduction in the target cathode outlet humidity during partial load operation. The target cathode outlet humidity is regulated accordingly so that the measured anode humidity reaches and maintains a set value of, for example, 0.8. Secondly, an intermittent reduction in the target cathode outlet humidity during partial load operation, in which the target cathode outlet humidity is reduced to a predetermined value of, for example, 0.7. As soon as the measured anode humidity reaches a certain limit, for example,If the measured anode moisture content falls below 0.7, the target cathode outlet moisture content is reset to the steady-state target value of, for example, 1.4. After a specified time, the procedure is repeated if the measured anode moisture content exceeds the specified limit.

[0035] Continuous control of the target cathode humidity based on the measured anode humidity is only possible to a limited extent, since the anode leak activity during normal operation is usually above 1 (with condensation), and the sensors therefore cannot detect changes in humidity. However, relatively dry operation of the fuel cell stack can be easily detected with an anode humidity sensor.

[0036] Further advantages, features, and details of the invention will become apparent from the following description, in which exemplary embodiments of the invention are described in detail with reference to the drawings. The features mentioned in the claims and in the description may be essential to the invention individually or in any combination.

[0037] It shows schematically: Fig. 1 a schematic representation of a possible design of the presented method.

[0038] In Fig. 1 shows a method 100 for adjusting a relative humidity in a fuel cell system.

[0039] The presented method 100 comprises a control step 101, in which the fuel cell system is operated in a control mode in which the relative humidity is adjusted depending on an operating state of the fuel cell system, a determination step 103, in which a relative humidity in an anode subsystem of the fuel cell system is determined continuously and in real time by means of a humidity sensor arranged in the anode subsystem, a calibration step 105, in which the determined relative humidity is compared with a predetermined lower limit value and a predetermined upper limit value, and, in the event that a determined value of the relative humidity is greater than the upper limit value or less than the lower limit value, a control step 107, in which the fuel cell system is operated in a closed-loop control mode,until a determined value of the relative humidity is greater than the lower limit and less than the upper limit.

Claims

[1] A method (100) for adjusting a relative humidity in a fuel cell system, the method (100) comprising: - operating (101) the fuel cell system in a control mode in which the relative humidity is adjusted depending on an operating state of the fuel cell system, - determining (103) the relative humidity in an anode subsystem of the fuel cell system continuously and in real time by means of a humidity sensor arranged in the anode subsystem, - comparing (105) the determined relative humidity with a predetermined lower limit value and a predetermined upper limit value, and - in the event that a determined value of the relative humidity is greater than the upper limit value or less than the lower limit value, operating (107) the fuel cell system in a closed control mode until a determined value of the relative humidity is greater than the lower limit value and less than the upper limit value.

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