Method for operating a fuel cell system, controller

Abstract

The invention relates to a method for operating a fuel cell system (1) comprising an anode subsystem (3) for supplying hydrogen gas to at least one stack (2) for generating a stack current, wherein hydrogen gas is delivered into an anode gas, which is enriched with nitrogen gas during operation, and the nitrogen gas concentration in the anode gas is controlled by temporarily expelling the anode gas from the anode subsystem (3) in a purge process through an operable purge valve (14) and replacing it by delivering fresh hydrogen gas, characterized by the following steps: (a) determining the hydrogen gas concentration in the anode gas, (b) inferring the actual anode stoichiometry from the hydrogen gas concentration and the stack current, (c) comparing the actual anode stoichiometry with a target anode stoichiometry, (d) indirectly adjusting the hydrogen gas concentration by changing the duration and / or frequency of the purge process when the actual anode stoichiometry deviates from the target anode stoichiometry. The invention also relates to a controller for carrying out the method steps.

Description

Technical Field

[0001] This invention relates to a method for operating a fuel cell system according to the preamble of claim 1. The invention also relates to a controller for performing the steps of the method of the invention. Background Technology

[0002] Fuel cells are used to electrochemically convert hydrogen and oxygen into electricity, heat, and water. At the heart of a fuel cell is the membrane electrode assembly (MEA). The MEA consists of a membrane that separates the anode and cathode sides from each other. The entire fuel cell system is thus divided into an anode subsystem and a cathode subsystem. During operation, oxygen is supplied to the fuel cell in the form of air through the cathode subsystem, and hydrogen is supplied through the anode subsystem. During operation, hydrogen and oxygen react to produce water, releasing energy in the process. To scale the energy output, individual fuel cells are typically stacked into what is known as a fuel cell stack.

[0003] Since the anode gas exiting the fuel cell stack typically still contains unconsumed hydrogen, it is recycled within the anode subsystem and supplied back to the anode side of the stack. Over time, the recycled anode gas becomes enriched with nitrogen and water, where the water can be in liquid or gaseous form. The liquid water is typically removed using a water separator. A water separator usually consists of a container in which the separated water is collected in liquid form. This container is emptied by opening a valve, known as a vent valve. The water flowing out of the vent valve merges with the exhaust gas from the cathode subsystem and is discharged from the fuel cell system.

[0004] The nitrogen in the anode gas exists in gaseous form and cannot be effectively separated from hydrogen. To remove nitrogen from the anode subsystem, the anode gas is discharged during a purging process via a so-called purge valve, either inside or downstream of the water separator. The anode gas, thus separated and containing nitrogen and hydrogen, is then combined with exhaust gas from the cathode subsystem and, if necessary, with separated water, and discharged from the fuel cell system. The purging process is variable and directly controllable in terms of duration and frequency. A disadvantage of this purging process is the loss of hydrogen that could otherwise be used as fuel.

[0005] The anode gas separated during purging is replaced within the anode subsystem by supplying fresh hydrogen. However, this fresh hydrogen stored in tanks is generally contaminated with nitrogen. The quality of hydrogen used in fuel cell vehicles is specified in SAE J2719 and ISO 14687:2019. These standards specify a maximum nitrogen concentration of 300 ppm for fresh hydrogen. Therefore, supplying fresh hydrogen to the anode subsystem is always accompanied by a nitrogen supply.

[0006] Nitrogen is an inert gas in fuel cells that reduces cell voltage and leads to efficiency losses. Furthermore, increased nitrogen concentration is accompanied by decreased hydrogen concentration, which can cause localized undersupply within the fuel cell or stack, particularly damaging the membrane and consequently shortening its lifespan. This must always be considered when operating a fuel cell system. Therefore, during operation, the amount of hydrogen supplied to the anode side is typically more than the amount required to draw current for the reaction. The ratio of the amount of hydrogen supplied to the anode side to the amount of hydrogen required for the reaction is called the anode stoichiometry, or simply stoichiometry when considering the anode side in isolation. Particular attention must be paid to determining the anode stoichiometry; both excessively high and excessively low anode stoichiometry can damage the stack or accelerate its aging.

[0007] Therefore, the present invention relates to the fine adjustment of the hydrogen concentration in the anode gas and the related adjustment of the anode stoichiometry.

[0008] To address this task, a method having the features of claim 1 is proposed. Advantageous extensions of the invention can be derived from the dependent claims. Furthermore, a controller for performing the steps of the method is also provided. Summary of the Invention

[0009] This invention proposes a method for operating a fuel cell system comprising an anode subsystem for supplying hydrogen to at least one fuel cell stack to generate stack current, wherein hydrogen is supplied to an anode gas enriched with nitrogen during operation. The nitrogen concentration in the anode gas is controlled by temporarily venting the anode gas from the anode subsystem during a purging process via a controllable purge valve and replacing it with fresh hydrogen. According to the invention, the following steps are performed: (a) Determine the hydrogen concentration in the anode gas. (b) The actual anode stoichiometry is deduced from the hydrogen concentration and the stack current. (c) Compare the actual anode stoichiometry with the target anode stoichiometry. (d) When the actual anode stoichiometry deviates from the target anode stoichiometry, the hydrogen concentration is indirectly adjusted by changing the duration and / or frequency of the purging process.

[0010] The proposed method adjusts the hydrogen concentration by specifically controlling the purging process, thereby adhering to a pre-defined target anode stoichiometry. The determination of the hydrogen concentration in the anode gas, as described in step (a), can be performed by calculation or sensing. Step (b) utilizes the relationship between hydrogen concentration, stack current, and the actual anode stoichiometry. This relationship needs to be determined empirically. Anode stoichiometry Defined as , in, m H2, tats This is the actual amount of hydrogen supplied to the anode side. m H2, st This refers to the amount of hydrogen required for complete combustion at a predetermined stack current. The former is typically set by adjusting the anode gas mass flow rate. Because the anode gas is continuously enriched with nitrogen, and consequently the hydrogen concentration gradually decreases, the actual amount of hydrogen delivered to the anode side varies. Therefore, the actual anode stoichiometry also continuously deviates from the target anode stoichiometry as the nitrogen enrichment in the anode gas increases. The degree of this deviation at a specific time point is determined in step (c).

[0011] The hydrogen concentration regulation described in step (d) is achieved through an active purging process, more specifically by varying its frequency and / or duration, to create capacity for fresh hydrogen in the anode gas by purging nitrogen from the fuel cell system. This allows for comprehensive control of the nitrogen concentration in the anode gas at all times, preventing the negative consequences of excessive nitrogen concentration. Furthermore, it enables the fuel cell system to operate continuously at the desired operating point, resulting in a better match between the controlled state and the actual operating state. A beneficial effect is that the fuel cell can be regulated to a specific operating point with greater precision.

[0012] Furthermore, this invention proposes that, in order to perform method step (a), the nitrogen concentration in the anode gas is balanced as the sum of the maximum possible nitrogen content in fresh hydrogen and the nitrogen content transferred within at least one fuel cell stack through a diffusion process, and the minimum present hydrogen concentration is estimated by calculation based on the nitrogen concentration in the anode gas thus determined. This preferred embodiment is advantageous because it allows for a worst-case scenario-based estimation of the hydrogen concentration towards the safety side without the need for additional components such as measuring devices. By estimating towards the safety side, uncertainties in determining state variables, such as tolerance ranges, do not have a negative impact. This preferred embodiment is based on pure simulation observation and can also be well combined with other methods as an upper bound for the nitrogen content in the anode gas or a lower bound for the hydrogen content.

[0013] Furthermore, this invention proposes that, in order to perform method step (a), the hydrogen concentration is determined at the input and / or output of at least one fuel cell stack using a sensing device. Typically, such a sensing device is already installed for monitoring operation, thus utilizing existing data. Moreover, measurements generally provide values ​​that best reflect reality, and are therefore more accurate than calculations.

[0014] In an extended embodiment of the invention, it is proposed that, in order to perform method step (a), the composition of the anode gas is modeled and the hydrogen concentration is determined by calculation using a data model. This preferred embodiment, which can be used alone or in combination with other methods for determining hydrogen concentration, generates a system state image that is very close to reality without requiring additional sensors. Particularly when combined with determining hydrogen concentration via sensors, operational faults can be identified by the discrepancy between model predictions and measured values.

[0015] Furthermore, the present invention proposes that, in order to perform method step (a), the pressure loss within a component of the anode subsystem, such as at least one fuel cell stack or water separator, is determined based on the fuel cell stack current, and the hydrogen concentration is inferred from there, preferably using a family of characteristic curves pre-established individually for the system. This preferred embodiment utilizes the relationship between pressure loss in a component and fuel cell stack current. This relationship needs to be determined pre-defined individually for the system. Advantageously, once determined and recorded in the characteristic curves, this relationship can be used at any time during operation to determine the hydrogen concentration. This eliminates the need for computationally intensive modeling. To improve the accuracy of this preferred embodiment, temperature signals from appropriate sensing devices at at least one location in the anode circuit are preferably used.

[0016] Furthermore, this invention proposes that, in order to perform method step (a), the hydrogen concentration is determined in advance by a family of characteristic curves established for each system, based on a pre-given stack current and a pre-given purge valve switching frequency. This family of characteristic curves shows the trend of hydrogen concentration in the anode gas as a function of stack current for different purge valve switching frequencies. This implementation is advantageous because the hydrogen concentration is directly derived from two parameters that can be freely adjusted during operation. Complex purge valve switching strategies can also be stored in the family of characteristic curves. The characteristic curves are readily available during operation, and the required hydrogen concentration can be quickly provided without modeling or computational overhead.

[0017] Furthermore, this invention proposes that, in order to perform method step (b), the actual anode stoichiometry is inferred from the stack current using a family of characteristic curves pre-established individually for the system. This family of characteristic curves illustrates the relationship between the stack current and the actual anode stoichiometry for different hydrogen concentrations in the anode gas. Performing step (b) using this family of characteristic curves is advantageous because these curves, once initially determined, can be used at any time during operation and do not depend on other components or computationally intensive modeling. Preferably, the stack current is compared with the actual anode stoichiometry... l IST The target anode stoichiometry can also be mapped onto the associated family of characteristic curves. l SOLL In order to find the difference lIST – l SOLL The form directly determines the comparison required in step (c).

[0018] Furthermore, this invention proposes that, in method step (d), when the actual anodic stoichiometry is determined to exceed the target anodic stoichiometry, the frequency and / or duration of the purging process should be reduced. This is based on the following causal relationship, wherein the target anodic stoichiometry will be used as a causal factor hereinafter. l SOLL The defined system state is simply referred to as the target state and will be determined by the actual anodic stoichiometry. l IST The defined system state is simply referred to as the actual state: if l IST > l SOLL Therefore, according to the aforementioned definition of anodic stoichiometry, we can conclude that: in, This indicates the amount of hydrogen required under a given current draw, which is the same under both the actual and target conditions. and These represent the actual amount of hydrogen delivered under the actual condition or the target condition, respectively. Therefore, we have... , This results in the actual requirement of more hydrogen available for combustion than the target requirement. This situation is corrected by reducing the duration and / or frequency of the purging process, thereby removing less nitrogen from the fuel cell system. As a result, the nitrogen concentration in the anode gas increases, crowding out the hydrogen concentration, and the amount of hydrogen delivered to the anode side decreases.

[0019] This leads to the conclusion that less hydrogen is delivered under actual conditions than under target conditions. In this case, it is inferred that the nitrogen content increases, which is expelled from the fuel cell system by increasing the duration and / or frequency of the purging process.

[0020] Similarly, it is proposed that in method step (d), when the actual anodic stoichiometry is determined to be lower than the target anodic stoichiometry, the frequency and / or duration of the purging process should be increased. This applies in this case. l IST < l SOLL Furthermore, the aforementioned causal chain is applied with the opposite operator: , This results in less hydrogen being delivered under actual conditions than under target conditions. In this case, it is inferred that the nitrogen content increases, which is expelled from the fuel cell system by increasing the duration and / or frequency of the purging process. Through these two illustrated preferred embodiments, the regulation of the hydrogen concentration in the anode gas is based on a linearly deducible causal relationship.

[0021] Furthermore, this invention proposes a controller configured to perform the steps of the method of the invention. This controller preferably stores a family of characteristic curves individually derived for each system, analyzes and evaluates measurement data, and / or compares the actual air ratio with a target air ratio. Since fuel cell systems are typically equipped with controllers that provide extensive control over the controllable components of the fuel cell system, it is preferable to place the control of the method of the invention within this controller. Attached Figure Description

[0022] A preferred embodiment of the invention is shown in more detail below with the aid of the accompanying drawings.

[0023] Figure 1 This is a schematic diagram of a fuel cell system.

[0024] Figure 2 This is a schematic diagram of the method flow of the present invention. Detailed Implementation

[0025] Figure 1 A schematic diagram of a fuel cell system 1 is shown as an example, the core component of which is the stack 2. The system includes an anode side 5 belonging to an anode subsystem 3 and a cathode side 6 belonging to a cathode subsystem 4. Hydrogen is supplied to the stack 2 through the anode subsystem 3. The hydrogen reacts with air supplied at the cathode side 6 within the stack 2 to produce water. During this process, water and nitrogen diffuse to the anode side 5, and the anode gas is correspondingly enriched. The anode gas enriched with water and nitrogen and containing unconsumed hydrogen is discharged from the stack 2 for recycling. Water is separated from the anode gas in a water separator 8, which is vented by temporarily opening a vent valve 9. The separated water is then introduced into an exhaust path 10 located in the cathode subsystem 4 and leaves the fuel cell system 1 through an exhaust device 11 along with the air discharged from the cathode side 6.

[0026] Meanwhile, the anode gas, with a significant amount of water removed, is discharged from the water separator 8 and recirculated via the jet pump 12. Alternatively, an active pump module or a combination of both can be used instead of the jet pump 12. In the jet pump 12, fresh hydrogen from the storage tank 7 is supplied to the anode gas. The hydrogen stored in the storage tank 7 is contaminated with nitrogen, typically containing a maximum nitrogen concentration of 300 ppm. This supply is made via a controllable hydrogen metering valve 13.

[0027] Due to diffusion processes within stack 2 and contamination from fresh hydrogen, the anode gas remains nitrogen-rich. To remove this nitrogen from fuel cell system 1, a so-called purging process is implemented. During purging, anode gas is discharged either inside or downstream of the water separator by temporarily opening purging valve 14. The anode gas discharged through purging valve 14 is directed into exhaust gas path 10 of cathode subsystem 4 and separated by exhaust gas device 11. The hydrogen concentration in the exhaust gas is monitored here by measuring device 15 to prevent explosive hydrogen-air mixtures. Inside anode subsystem 3, the anode gas discharged during purging is compensated by supplying fresh hydrogen via hydrogen metering valve 13. The duration and frequency of the purging process are variable and controllable.

[0028] During operation, correctly adjusting the hydrogen concentration in the anode gas and thus the anode stoichiometry is of particular importance. It especially affects the lifespan of the fuel cell within stack 2. In the method of this invention, the hydrogen concentration in the anode gas is controlled by adjusting the frequency and / or duration of the purging process. Figure 2 The present invention method is illustrated by way of example. Here, in the first method step S1, the hydrogen concentration in the anode gas is determined. This is preferably performed by a corresponding sensing device 16 at the input and / or output end of the anode side 5 of the fuel cell stack 2. In other preferred embodiments, method step S1 is performed by balancing the nitrogen mass flow rate entering the anode gas and by estimating the at least present hydrogen concentration obtained by modeling the anode gas composition or otherwise.

[0029] In the subsequent method step S2, the actual anode stoichiometry on the anode side 5 within the fuel cell stack 2 is deduced from the hydrogen concentration in the anode gas. In a preferred embodiment, the actual anode stoichiometry is deduced from the fuel cell current using a family of characteristic curves pre-established for each system, which shows the relationship between the fuel cell current and the actual anode stoichiometry for different hydrogen concentrations in the anode gas.

[0030] In the next method step S3, the actual anode stoichiometry obtained in method step S2 will be compared with the target anode stoichiometry. The target anode stoichiometry is derived from the target operating point of the fuel cell system, which defines the control or regulation target.

[0031] If the actual anode stoichiometry is greater than the target anode stoichiometry, the hydrogen concentration in the anode gas will be higher than its target value. In this case, the frequency and / or duration of the purging process are reduced in method step S4.1. As a result, the nitrogen concentration in the anode gas increases, and the hydrogen concentration decreases.

[0032] Conversely, if the actual anode stoichiometry is less than the target anode stoichiometry, the hydrogen concentration in the anode gas will be lower than its target value. In this case, the frequency and / or duration of the purging process are increased in method step S4.2. As a result, the nitrogen concentration in the anode gas decreases, and the hydrogen concentration increases.

Claims

1. A method for operating a fuel cell system (1), said fuel cell system comprising an anode subsystem (3) for supplying hydrogen to at least one fuel cell stack (2) to generate a fuel cell stack current, wherein, Hydrogen is supplied to the anode gas, which becomes enriched with nitrogen during operation, and the nitrogen concentration in the anode gas is controlled by temporarily venting the anode gas from the anode subsystem (3) during purging via a controllable purge valve (14) and replacing it with fresh hydrogen, characterized by having the following steps: (a) Determine the hydrogen concentration in the anode gas. (b) The actual anode stoichiometry is deduced from the hydrogen concentration and the stack current. (c) Compare the actual anode stoichiometry with the target anode stoichiometry. (d) When the actual anode stoichiometry deviates from the target anode stoichiometry, the hydrogen concentration is indirectly adjusted by changing the duration and / or frequency of the purging process.

2. The method according to claim 1, characterized in that, In order to perform step (a) of the method, the nitrogen concentration in the anode gas is balanced as the sum of the maximum possible nitrogen content in the fresh hydrogen gas and the nitrogen content transferred in the at least one stack through the diffusion process, and the minimum hydrogen concentration is estimated by calculation based on the nitrogen concentration in the anode gas thus determined.

3. The method according to any one of the preceding claims, characterized in that, In order to perform method step (a), the hydrogen concentration is determined at the input and / or output of the at least one stack (2) by means of a sensing device (16).

4. The method according to any one of the preceding claims, characterized in that, In order to perform method step (a), the composition of the anode gas is modeled, and the hydrogen concentration is determined by calculation using the model.

5. The method according to any one of the preceding claims, characterized in that, In order to perform step (a) of the method, the pressure loss inside a component of the anode subsystem, such as at least one stack (2) or water separator (8), is determined based on the stack current, and the hydrogen concentration is inferred from there, wherein a family of characteristic curves established in advance for each system is preferably used.

6. The method according to any one of the preceding claims, characterized in that, In order to perform step (a) of the method, the hydrogen concentration is determined by a family of characteristic curves established in advance for the system based on a pre-given stack current and a pre-given purge valve (14) switching frequency. The family of characteristic curves shows the trend of hydrogen concentration in the anode gas with the stack current for different switching frequencies of the purge valve (14).

7. The method according to any one of the preceding claims, characterized in that, To perform step (b), the actual anode stoichiometry is inferred from the stack current using a family of characteristic curves established in advance for each system individually. This family of characteristic curves shows the relationship between the stack current and the actual anode stoichiometry for different hydrogen concentrations in the anode gas.

8. The method according to any one of the preceding claims, characterized in that, In method step (d), when it is determined that the actual anodic stoichiometry exceeds the target anodic stoichiometry, the frequency and / or duration of the purging process are reduced.

9. The method according to any one of the preceding claims, characterized in that, In method step (d), when it is determined that the actual anodic stoichiometry is lower than the target anodic stoichiometry, the frequency and / or duration of the purging process are increased.

10. A controller configured to perform the steps of the method of the present invention.

Images