Drying process for drying a fuel cell stack
The drying process for fuel cell stacks, involving blowing out the cathode subsystem and setting constant operating conditions, addresses the challenge of maintaining optimal water content and ensuring successful startup in freezing conditions, thereby enhancing the durability and efficiency of the fuel cell system.
Patent Information
- Application Number
- DE102023210611
- Authority / Receiving Office
- DE · DE
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2023-10-26
- Publication Date
- 2025-04-30
- Estimated Expiration
- Not applicable · inactive patent
AI Technical Summary
Fuel cell systems face challenges in starting up in various environmental conditions and maintaining optimal water content within the fuel cell stack to prevent freezing and damage, while also ensuring proton conductivity.
A drying process for fuel cell stacks involves blowing out the cathode subsystem with air having a high lambda value, setting constant operating conditions, and dynamically ending the drying based on cathode and anode pressure differences to maintain membrane moisture within a tolerance range.
This process minimizes drying time, ensures the fuel cell system can start successfully in freezing conditions, and prevents damage by maintaining optimal water content, thus enhancing the durability and efficiency of the fuel cell system.
Smart Images
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Abstract
Description
[0001] The presented invention relates to a drying method for drying a fuel cell stack according to the appended claims. State of the art
[0002] In fuel cell systems, the oxidizing agent oxygen from the ambient air and hydrogen as a reducing agent or fuel are usually used to react in the respective fuel cells to form water or water vapor and thus provide electrical power through electrochemical conversion.
[0003] A very challenging task for mobile fuel cell systems, for example in a vehicle, is to enable the fuel cell system to start under all globally relevant conditions and during different lengths of downtime of the vehicle, while ensuring a maximum service life or useful life of the fuel cell system.
[0004] During a freeze start, the aim is, among other things, to heat the fuel cell stack as quickly as possible above a predetermined temperature threshold of, for example, 0°C, so that water produced during operation of the fuel cell system does not freeze at critical points in the fuel cell system.
[0005] In the event of a faulty freeze start, the fuel cell stack may suffer massive irreversible damage and the fuel cell system may not be able to start, so that, for example, a corresponding vehicle must be brought to a warm environment, i.e., usually towed.
[0006] For a successful freeze start, a water content in the fuel cell stack before or at the beginning of the start is therefore crucial.
[0007] The water content in the fuel cell stack must be within a specified tolerance range so that the fuel cell stack can, on the one hand, store water accumulating during start-up in its storage-capable components, such as the membrane, gas diffusion layer, etc., without becoming blocked by freezing water, and, on the other hand, is not dried out to such an extent that no proton conductivity of the membrane is possible or the membrane is damaged due to excessively dry conditions.
[0008] Thus, the conditions and operating modes prior to startup are essential to ensure preparatory measures for restart. For this purpose, drying processes and measures are known, such as a so-called "parking purge" or blowing out a cathode subsystem for a defined period. Disclosure of the invention
[0009] The present invention proposes a drying method for drying a fuel cell stack. Further features and details of the invention are set forth in the respective subclaims, the description, and the drawings.
[0010] The invention presented serves in particular to minimize a drying time for which a drying process is carried out.
[0011] Thus, according to a first aspect of the invention presented, a drying method for drying a fuel cell stack is presented.
[0012] The drying method presented comprises blowing out a cathode subsystem of the fuel cell system with a predetermined air lambda value in order to dry the fuel cell stack, setting constant operating conditions of the fuel cell stack, determining a cathode pressure difference between a cathode inlet and a cathode outlet, determining an anode pressure difference between an anode inlet and an anode outlet, determining a cathode characteristic value based on a gradient of a course of the cathode pressure difference, determining an anode characteristic value based on a gradient of a course of the anode pressure difference, comparing the cathode characteristic value with a predetermined cathode threshold value, comparing the anode characteristic value with a predetermined anode threshold value, and terminating the blowing out of the cathode subsystem in the event thatthat the cathode characteristic falls below the cathode threshold value and / or the anode characteristic falls below the anode threshold value.
[0013] The invention presented is based on the principle that a drying process is carried out by blowing out at least the cathode subsystem of a fuel cell system. Blowing out occurs with an air mass flow that creates a high excess air in the cathode subsystem, in particular an air lambda value greater than 10.
[0014] To monitor a change in the water content in the fuel cell stack or a change in the dryness of the fuel cell stack, constant operating conditions of the fuel cell stack are set according to the invention. For this purpose, a constant air mass flow, a constant pressure level at the cathode inlet, and a constant cathode inlet temperature can be set using an air system of the fuel cell system.
[0015] Furthermore, a cooling system of the fuel cell system can set a constant coolant temperature, in particular at the coolant inlet into the fuel cell stack or a constant temperature difference between a coolant inlet and a coolant outlet of the fuel cell stack.
[0016] Furthermore, an anode subsystem can be operated at a constant operating point, so that the conditions at the anode remain constant or change only minimally. For this purpose, for example, an anode recirculation system, a purge valve or purge / drain valve, and a hydrogen dosing valve can be operated constantly, in particular, constantly, i.e., cycled at a specified tank rate.
[0017] Furthermore, an electrical subsystem of the fuel cell system can be adjusted to draw a constant current or a constant current density so that the operating point of the fuel cell system remains constant.
[0018] The respective subsystems of the fuel cell system can be set to a constant operating point simultaneously or at different times relative to the start time of the cathode subsystem purge. Of course, the subsystems can also be set to a constant operating point before the start time.
[0019] Optionally, the drying process presented can also be carried out with two or more constant operating conditions or operating points, whereby several different parameter sets are used and no evaluation of the cathode characteristic value and the anode characteristic value takes place during a transition between the respective parameter sets.
[0020] According to the invention, while constant operating conditions of the fuel cell stack are set, a cathode characteristic value is determined based on a gradient of a course of a cathode pressure difference and an anode characteristic value is determined based on a gradient of a course of the anode pressure difference.
[0021] If the cathode characteristic falls below the cathode threshold and / or the anode characteristic falls below the anode threshold, the purging of the cathode subsystem and, consequently, the drying of the fuel cell stack is terminated. Accordingly, the termination of the drying process occurs dynamically depending on the physical condition of the fuel cell stack. This ensures that the membrane moisture in the fuel cell stack remains within a specified tolerance range and, consequently, that the fuel cell system is capable of freezing starts. Degradation or damage to the fuel cell stack during a freezing start is reliably avoided or minimized, and the duration of the drying process when shutting down the fuel cell system is minimized.
[0022] In the event that the cathode characteristic value falls below the cathode threshold value and the anode characteristic value falls below the anode threshold value, a subsequent undershoot of the limit value can serve as a termination condition.
[0023] If only the cathode characteristic value falls below the cathode threshold, or only the anode characteristic value falls below the anode threshold, a predefined time can be waited for. If the other threshold is not reached, the blowout can also be terminated when the first condition is met.
[0024] It can further be provided that at least one additional characteristic value from the following list of additional characteristic values is determined and compared with a respective additional threshold value, and in the event that a predetermined minimum number of additional characteristic values, the cathode characteristic value and the anode characteristic value are below a respective threshold value, the blowing out of the cathode subsystem is terminated: voltage applied to the fuel cell stack, voltage applied to a respective fuel cell of the fuel cell stack, impedance of the fuel cell stack, impedance of a respective fuel cell of the fuel cell stack.
[0025] Additional parameters determined in addition to the anode characteristic and the cathode characteristic increase the robustness of the prediction of the time at which the blowout of the cathode subsystem can be terminated.
[0026] Respective additional key figures can, for example, be weighted and included in a comparison with a condition, such as an additional threshold value.
[0027] Alternatively or additionally, the respective additional characteristic values can be evaluated by a quality function that specifies conditions and evaluates the fulfillment of the conditions in a degree of fulfillment, for example in [%].
[0028] For example, a quality function could be formulated according to equation (1). G=(g1*krit1*b1+g2*krit2*b2+g3*krit*b3+g4*krit4*b4) / (b1+b2+b3+b4)
[0029] Here, b represents a plausible determination and availability (b=0 or b=1), meaning that only criteria that are available and plausible during the current evaluation can be applied. For example, a sensor could freeze and become implausible. If G > a threshold value Glim, the purge of the cathode subsystem is terminated immediately or after an additional time dt additional.
[0030] Further advantages, features, and details of the invention will become apparent from the following description, which describes exemplary embodiments of the invention 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.
[0031] They show: Fig. 1 a schematic representation of the presented drying process.
[0032] In Fig. 1 shows a drying method 100 for drying a fuel cell stack.
[0033] The drying method presented comprises a blow-out step 101, in which a cathode subsystem of the fuel cell system is blown out with a predetermined air lambda value in order to dry the fuel cell stack, a setting step 103, in which constant operating conditions of the fuel cell stack are set, a first determination step 105, in which a cathode pressure difference between a cathode inlet and a cathode outlet is determined, a second determination step 107, in which an anode pressure difference between an anode inlet and an anode outlet is determined, a first determination step 109, in which a cathode characteristic value is determined based on a gradient of a course of the cathode pressure difference, a second determination step 111, in which an anode characteristic value is determined based on a gradient of a course of the anode pressure difference, a first adjustment step 113,in which the cathode characteristic value is adjusted with a predetermined cathode threshold value, a second adjustment step 115 in which the anode characteristic value is adjusted with a predetermined anode threshold value, and a termination step 117 in which the blowout of the cathode subsystem is terminated in the event that the cathode characteristic value falls below the cathode threshold value and / or the anode characteristic value falls below the anode threshold value.
[0034] At the beginning of the drying process 100, the fuel cell stack is well moistened internally from previous operating states to achieve the required efficiencies. This means that the membrane, the gas diffusion layer, the channels, or the entire surfaces are well moistened or wetted with a significant amount of water. Small water droplets are also present in the channels and structures of the fuel cell system.
[0035] In particular, the water droplets result in increased flow resistances for the air flow in the cathode path but also for the gas flow in the anode path, which decrease during the drying process as the water and moisture are blown out.
[0036] In addition, the density of the gas flow along the flow length also changes over time during the drying process 100. Since moist air has a lower density than dry air, the volume flow of the moist air is larger at a constant mass flow and thus the pressure loss is higher.
[0037] During the drying process 100, the saturation limit of the air migrates toward the exit of the fuel cell stack, meaning that the incoming dry air is only saturated in the fuel cell stack in later flow sections or not at all at the end. The density increases, the volume flow decreases, and the pressure drop decreases. The aforementioned effects also occur for the anode gas in the anode path.
[0038] When the pressure differences no longer change significantly, an equilibrium state is reached and the drying process can be terminated.
[0039] When determining the anode characteristic value, a trajectory of the pressure difference curve can additionally include an optional preprocessing step in which purge / drain and hydrogen dosing processes are filtered out or ignored.
Claims
[1] A drying method (100) for drying a fuel cell stack, the drying method (100) comprising: - blowing out (101) a cathode subsystem of the fuel cell system with a predetermined air lambda value in order to dry the fuel cell stack, - setting (103) constant operating conditions of the fuel cell stack, - determining (105) a cathode pressure difference between a cathode inlet and a cathode outlet, - determining (107) an anode pressure difference between an anode inlet and an anode outlet, - determining (109) a cathode characteristic value based on a gradient of a course of the cathode pressure difference, - determining (111) an anode characteristic value based on a gradient of a curve of the anode pressure difference, - adjusting (113) the cathode characteristic value with a predetermined cathode threshold value, - adjusting (115) the anode characteristic value with a predetermined anode threshold value, - terminating (117) the purging (101) of the cathode subsystem in the event that the cathode characteristic falls below the cathode threshold value and / or the anode characteristic falls below the anode threshold value. [2] Drying method (100) according to claim 1, characterized by that at least one additional characteristic value from the following list of additional characteristic values is determined and compared with a respective additional threshold value, and in the event that a predetermined minimum number of additional characteristic values, the cathode characteristic value and the anode characteristic value are below a respective threshold value, the blowing out (101) of the cathode subsystem is terminated: voltage applied to the fuel cell stack, voltage applied to a respective fuel cell of the fuel cell stack, impedance of the fuel cell stack, impedance of a respective fuel cell of the fuel cell stack.