Methods for measuring the pollutant load of a fuel cell and for operating a fuel cell

Abstract

Method for measuring the pollutant load of a fuel cell (3) operated in a vehicle (2) for the provision of electrical drive power using ambient air, wherein the amount of air flowing into the fuel cell (3) is determined at short intervals during operation, wherein the location of the vehicle (2) is recorded at each time, wherein the location is divided into one of several categories, wherein each of the categories is assigned an average pollutant load, and wherein the instantaneous pollutant load for the fuel cell (3) is determined from the amount of air flowing in and according to the local category, and the total pollutant load is added up.

Description

The invention relates to a method for measuring the pollutant emissions of a fuel cell operated with ambient air in a vehicle to provide electric drive power. Furthermore, the invention relates to a method for operating a fuel cell in a vehicle with cyclic regeneration of the fuel cell. From US patent 2006 / 0166051 A1, which represents the closest prior art, it is known to condition or regenerate a fuel cell by cyclically loading and unloading it at optimal temperature and humidity. Such a process can be carried out, for example, in a workshop as needed. It can help to reduce reversible degradation of the fuel cell, thus making more fuel cell power available again. Furthermore, such a process, which proposes operating the fuel cell with improved air in the workshop for regeneration, is known from US 2011 / 008686 A1. In both documents, conditioning or regeneration takes place during a workshop visit. Therefore, the frequency of this process is essentially determined by the service intervals specified by the vehicle manufacturer. However, this can be problematic depending on the environmental pollution to which the fuel cell or the vehicle with the fuel cell is exposed. In areas with high pollution, the regeneration may be performed too late to allow for complete regeneration. Conversely, in areas with low pollution, for example, if the vehicle is primarily operated in rural areas, unnecessarily frequent regeneration may occur, which represents a significant disadvantage in terms of effort and associated costs. From the prior art, specifically DE 10 2010 048 254 A1, it is also known to recondition a fuel cell. For this purpose, it is determined whether reconditioning is desired. If so, various system parameters are queried. If all parameters are within a range suitable for reconditioning, the process is carried out. This procedure is comparatively complex and, while it allows reconditioning during operation, it does so only under very specific conditions. This represents a significant disadvantage, particularly when used in a vehicle and the associated dynamic power demands on the fuel cell. The object of the present invention is to provide an improved method for operating a fuel cell and a method for detecting the pollutant load of a fuel cell. According to the invention, this problem is solved by the methods with the features in claim 1 and claim 4. Advantageous embodiments and further developments of the respective methods are described in the dependent claims. The inventive method according to claim 1 is concerned with measuring the pollutant load of a fuel cell operated with ambient air in a vehicle for the provision of electric drive power. The inventive method utilizes a measurement of the amount of air flowing into the fuel cell, which is determined at short intervals during operation. Such an air volume measurement is usually performed anyway for controlling the fuel cell. For example, a measurement can be taken at intervals of approximately one minute. If air is used in the system for other purposes, such as purging a fuel cell housing, for the air bearing of a flow compressor, for a system bypass, or similar, the required air volume is typically known or can at least be estimated computationally.This allows for a simple and efficient determination of the actual airflow to the fuel cell. Since modern vehicles typically have navigation systems installed, a GPS module or similar navigation module, which uses various satellite data or terrestrial data such as radio networks for location tracking, can determine the vehicle's location at the time of each airflow measurement. From this location, it is then possible to deduce the average concentration of pollutants in the air. For this purpose, a data collection from the Jülich Research Centre within the framework of the "ALASKA" project can be used, for example, according to a beneficial further development of the idea (D. Klemp, R. Wegener, R. Dubus, L. Karadurmus, N. Kille, Z. Tan: Distribution of trace gases with adverse effects on fuel cells. In: Energie & Umwelt / Energy & Environment, 539, 2021, 107-112. - ISSN 1866-1793). Within the framework of this project, all road sections were classified into different categories. Four categories emerged as relevant, each exhibiting typical pollutant concentrations in the air. These categories are: 1. Motorways, 2. Motorway tunnels, 3. Federal and state roads, 4. Main roads, secondary roads, and other tunnels. For each of these categories, an average pollutant concentration is then calculated with regard to the relevant air pollutants, in particular nitrogen oxides, sulfur dioxide, and ammonia. If, in the inventive method, the respective measurement point of the air volume is classified into the corresponding category, then a corresponding concentration of the pollutant, as is typical for a road of this category, can be directly assigned to the air volume. The air volumes can then be added together between the individual measurement times in conjunction with the pollutant quantities typically occurring there, so that ultimately the estimated pollutant quantity for the respective pollutant can be easily determined. The method according to the invention thus enables the quantity of the pollutant present to be recorded and thus makes it possible to determine how much pollutant the fuel cell has “received”, for example since the last regeneration or since its first commissioning. The inventive method for operating a fuel cell in a vehicle with cyclic regeneration of the fuel cell provides that the cyclic regeneration is carried out during a workshop visit as soon as a pollutant load, detected according to the method described above, exceeds a predetermined limit value. Critical limit values, above which the performance of the fuel cell decreases due to contamination with air pollutants in the cathode area, can be determined, for example, through simulations, laboratory tests, or field trials. In this way, a limit value can be reliably established above which regeneration of the fuel cell is necessary. Using the inventive method described above, it is then possible to estimate how much pollutant has entered the fuel cell.As soon as the level of the detected pollutant entering the fuel cell exceeds the predefined limit, a warning message is triggered, indicating that the vehicle should be taken to the nearest workshop for regeneration. This warning can be displayed on the dashboard, for example, or, particularly in fleet vehicles, transmitted to a central control room via radio or other telecommunications, allowing fleet management to efficiently schedule workshop visits. The regeneration of the fuel cell in the workshop is then carried out by operating the fuel cell with clean air, for which, in particular, air filtered via an activated carbon filter can be provided. This air is typically free of nitrogen oxides, ammonia, and sulfur dioxide, thus enabling regeneration. The cell is then operated at an operating temperature and humidity level, both of which are higher than during normal operation. This is followed by operation with a cyclically changing current density for a certain operating period, preferably approximately two to five hours. During this operation, the current density is varied, for example, in three stages, namely with 0.01 A / cm², 0.7 A / cm², and 1.5 A / cm² for 180 seconds each, according to an advantageous embodiment of the idea.This leads to a regeneration of the fuel cell, particularly when operating with clean air, increased humidification and increased operating temperature, so that after regeneration it has a higher performance than before. In order to document and verify this, according to a further advantageous embodiment of the method according to the invention, it can be provided that a voltage-current characteristic curve of the fuel cell is recorded before and after regeneration in order to check and document the changes. By combining the two methods according to the invention, regeneration can be carried out whenever this is sensible and necessary based on the determined pollutant load of the fuel cell. This ensures that regeneration occurs only when necessary, but not when unnecessary, thus enabling the ideal performance of the fuel cell with minimal effort in terms of costs and service time. Further advantageous embodiments of the two methods according to the invention also result from the exemplary embodiment, which is described in more detail below with reference to the figures. Figure 1 shows a basic representation of a fuel cell system in a motor vehicle; Figure 2 shows a diagram of a pollutant exposure test with subsequent regeneration at 10 ppm NO in the intake air; Figure 3 shows a diagram of a pollutant exposure test with subsequent regeneration at 5 ppm NH3 in the intake air; and Figure 4 shows the ul characteristics of the fuel cell, each measured at the times marked 1 to 4 in Figure 3. Figure 1 shows a basic fuel cell system 1 in a similarly basic representation of a vehicle 2. The core of the fuel cell system 1 is a fuel cell 3, which is constructed as a stack of individual PEM cells, a so-called fuel cell stack. A cathode compartment 4 and an anode compartment 5 are shown here by way of example. Ambient air is supplied to the cathode compartment 4 via an air conveying system 6, a charge air cooler 7, and a humidifier 8. The unused exhaust air passes through the humidifier 8 to transfer its moisture to the supply air before flowing into the environment. Hydrogen (H2), for example from a pressurized gas storage tank, is supplied to the anode compartment 5. In this highly simplified embodiment, any unused hydrogen escapes from the anode compartment 5 into the environment.This setup is shown in a highly simplified form, but it is familiar to experts in various versions with additional components such as turbines, a system bypass, hydrogen recirculation, and the like. The previously mentioned charge air cooler 7 can, for example, be cooled by a liquid cooling medium in a cooling circuit indicated as 9. Part of this cooling circuit can also include a cooling heat exchanger 10 for cooling the fuel cell 3, i.e., for dissipating the waste heat generated in the fuel cell 3. This allows the operating temperature of the fuel cell 3 to be adjusted. Furthermore, a bypass 11 with a corresponding valve assembly can be provided around the humidifier 8. This can be located, as indicated here, on the exhaust air side, but also on the supply air side. The bypass allows adjustment of how much moisture enters the humidifier and thus ultimately how humidified the supply air flowing to the fuel cell 3 or the cathode chamber 5 actually is. All of this is generally known to the expert, so only a rough outline of this exemplary setup is provided here. To operate the fuel cell 3, ambient air is typically drawn in from the vicinity of the vehicle 2. Depending on the location where the vehicle is driven, this ambient air can be contaminated with varying concentrations of typical air pollutants. These are generally pollutants such as NO, NO2, NH3, SO2, and similar substances. Over time, the ingress of such pollutants into the cathode chamber 4 of the fuel cell leads to their accumulation and thus to a reduction in the fuel cell 3's performance. To counteract this performance degradation, the fuel cell 3 can be regenerated cyclically. For example, operating the fuel cell with clean air, which can be provided in a workshop, particularly by temporarily installing an activated carbon filter, can already have a beneficial effect.Depending on the pollutant, this effect can be further supported by increasing the operating temperature and humidification of the fuel cell 3 by setting a higher humidification and by enabling a higher operating temperature of the fuel cell 3 by reducing the cooling via the cooling heat exchanger 10. In such an operating environment, the current densities occurring in fuel cell 3 can be cyclically varied by applying a suitable load. In particular, it has proven effective to change the current densities in three stages to 0.01, 0.7, and 1.5 A / cm², with a dwell time of approximately 180 seconds in each stage. In tests conducted by the inventors, such operation over a period of two to five hours has led to a significant improvement in the performance of fuel cell 3. This is illustrated below using two diagrams in Figures 2 and 3. Both diagrams show the average cell voltage on the y-axis and time on the x-axis. The operation is carried out as follows: in the experimental setup shown in Figure 2, a static operation with a pollutant concentration of 10 ppm NO is performed for approximately two hours, followed by a dynamic operation for approximately five hours with the same pollutant-laden air. This is carried out analogously in Figure 3, where NH3 with a concentration of 5 ppm in the air is used as the pollutant. From approximately 7.5 hours onwards, static operation with pollutant-free air is resumed. It can be seen that a corresponding recovery occurs with nitrogen oxide as the pollutant in Figure 2, while this is practically not the case with NH3 in Figure 3.The actual regeneration then takes place over a period of slightly more than five hours in the experiments shown, during which the current density is cyclically varied as described above, while simultaneously the temperature and humidity are increased compared to normal operation. Furthermore, the pressure of the supplied air is also cyclically increased to expel water. This is carried out for five hours. An improvement is noticeable. To document such an improvement, it may also be necessary to record current-voltage characteristic curves of the fuel cell at the times labeled 1, 2, 3, and 4 in the diagram of Fig. 3. These four current-voltage characteristic curves are shown in Fig. 4. The Y-axis represents the voltage in volts, and the X-axis represents the current in A / cm². The line labeled 1 shows the beginning of the actual test, i.e., the time when the dynamic load was started for approximately five hours. At time 2, i.e., after the dynamic load, the lowest UI characteristic curve is obtained, which clearly indicates damage. Static regeneration is evidently not very effective with NH3, as the characteristic curve labeled 3 is almost identical to the one labeled 2 and, as mentioned above, shows hardly any improvement.However, the uppermost characteristic curve recorded at time 4 shows a significant improvement, which even goes beyond the initial state 1 and thus impressively demonstrates the effectiveness of the regeneration. If the procedure is carried out in the workshop, it is advisable to record the voltage characteristics of the fuel cell 3 at least before the start of the regeneration, i.e. at time 2, and after its completion at time 4, on the one hand to check the effectiveness of the regeneration and on the other hand to document it for the user of the vehicle 2.

Claims

Method for measuring the pollutant load of a fuel cell (3) operated in a vehicle (2) for the provision of electrical drive power using ambient air, wherein the amount of air flowing into the fuel cell (3) is determined at short intervals during operation, wherein the location of the vehicle (2) is recorded at each time, wherein the location is divided into one of several categories, each category being assigned an average pollutant load, and wherein the instantaneous pollutant load for the fuel cell (3) is determined from the amount of air flowing in and according to the location category, and the total pollutant load is added up. Method according to claim 1, characterized in that the pollutant loads of NO, NO2, NH3, SO2 are calculated and summed. Method for operating a fuel cell (3) in a vehicle with cyclic regeneration of the fuel cell (3), characterized in that the cyclic regeneration is carried out during a workshop visit as soon as a pollutant load detected according to a method according to one of claims 1 to 2 exceeds a predetermined limit value. Method according to claim 3, characterized in that the fuel cell (3) is operated with clean air during regeneration. Method according to claim 3 or 4, characterized in that the fuel cell (3) is operated during regeneration with an operating temperature and humidification which is greater than in normal operation. Method according to claim 3, 4 or 5, characterized in that the fuel cell (3) is operated during regeneration with a cyclic change in current density for a predetermined operating time. Method according to claim 6, characterized in that a current density of 0.01; 0.7 and 1.5 A / cm2 is maintained during a cycle for 180 s each, wherein such operation is carried out for an operating duration of two to five hours. Method according to one of claims 3 to 7, characterized in that a voltage-current characteristic curve of the fuel cell (3) is recorded before and after regeneration.

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