Method for operating a fuel cell system, evaluation unit for a fuel cell system

Abstract

The invention relates to a method for operating a fuel cell system having at least one fuel cell which is supplied with hydrogen via an anode path and with oxygen via a cathode path, wherein anode offgas which is discharged from the fuel cell is recirculated, whereas a portion of the anode offgas is introduced from the anode path into an offgas path which conducts cathode offgas from time to time by way of discharge, wherein the hydrogen concentration of the offgas is measured in the offgas path by means of a hydrogen sensor. According to the invention, the hydrogen concentration and / or the nitrogen concentration of the anode gas in the anode path before the last discharge is calculated on the basis of the measured hydrogen concentration, the amount of gas introduced into the offgas path from the cathode path and from the anode path and the amount of hydrogen freshly supplied to the anode path. The invention also relates to an evaluation unit for a fuel cell system for carrying out the inventive method.

Description

Technical Field

[0001] This invention relates to a method for operating a fuel cell system having at least one fuel cell. Furthermore, the invention relates to an evaluation unit for a fuel cell system by means of which the method according to the invention can be implemented. Background Technology

[0002] Fuel cells use oxygen to convert hydrogen into electricity, generating waste heat and water in the process. For this purpose, fuel cells have a membrane electrode assembly (MEA), which is supplied with hydrogen through the anode path and oxygen through the cathode path. Hydrogen is typically stored in a tank, while oxygen can be extracted from ambient air.

[0003] In practical applications, multiple such fuel cells are arranged into a fuel cell stack or "stacking" to increase the generated voltage. To supply hydrogen and air to each individual fuel cell, the fuel cell stack is traversed by supply channels. Additional channels running through the fuel cell stack are used to remove depleted anode exhaust gas and depleted humid air emitted from the fuel cells.

[0004] On the system side, a scheme is developed to supply hydrogen to the fuel cell, in which hydrogen-containing anode waste gas is re-supplyed to the fuel cell using a gas delivery device. This process is called recirculation. The gas delivery device can be a jet pump or a hybrid solution consisting of a jet pump and a blower.

[0005] Recycled anode exhaust gas can contain nitrogen that diffuses from the cathode side to the anode side. This results in a decrease in battery voltage because nitrogen, used for the electrochemical reactions occurring in the fuel cell, is an inert gas. If it is present at very high concentrations, it can also damage the battery because it is no longer being supplied with sufficient hydrogen.

[0006] Therefore, to reduce nitrogen concentration, the recirculation chamber is periodically purged. This process is called "purge." A portion of the anode exhaust gas is removed from the recirculation chamber through a purge valve and replaced with fresh hydrogen. However, too frequent purging reduces the efficiency of the fuel cell system because hydrogen is also removed as nitrogen. Therefore, understanding the nitrogen concentration is important for optimizing anode exhaust gas removal in terms of system efficiency while minimizing damage to the battery.

[0007] Hydrogen sensors are known to be used in the prior art. These sensors are arranged in the anode path to provide reliable measurements of hydrogen concentration. The nitrogen concentration can then be deduced from these measurements. However, the use of such sensors is complex and expensive, especially as sealing problems can arise in the interface area. Nevertheless, it is common practice to install at least one hydrogen sensor in the exhaust path of a fuel cell system. The exhaust path carries depleted, moist air (cathode exhaust) and anode exhaust, which is periodically removed from the recirculation chamber by means of venting. Therefore, the exhaust gas consists of a hydrogen-nitrogen-vapor mixture. The hydrogen sensor arranged in the exhaust path measures whether a non-hazardous hydrogen concentration has been maintained at any given time. Summary of the Invention

[0008] The objective of this invention is to provide a method for operating a fuel cell system, wherein the nitrogen concentration in the anode path is monitored. This monitoring should be performed as efficiently as possible and using existing methods.

[0009] To address this task, a method according to the present invention is proposed. Advantageous improvements to the invention can be derived from the specification. Furthermore, an evaluation unit for a fuel cell system is proposed, by means of which the method can be implemented.

[0010] A method for operating a fuel cell system is proposed, the fuel cell system having at least one fuel cell, the fuel cell being supplied with hydrogen through an anode path and oxygen through a cathode path. Here, anode exhaust gas discharged from the fuel cell is recirculated. However, a portion of the anode exhaust gas is periodically introduced from the anode path into an exhaust gas path that guides the cathode exhaust gas through a purging process. In the exhaust gas path, the hydrogen concentration of the exhaust gas is measured by means of a hydrogen sensor. According to the invention, the hydrogen concentration and / or nitrogen concentration of the anode gas in the anode path before the last purging is calculated based on the measured hydrogen concentration, the amount of gas introduced from the cathode path and the anode path into the exhaust gas path, and the amount of fresh hydrogen supplied to the anode path.

[0011] If the hydrogen concentration is known, the nitrogen concentration can be derived from it. Therefore, the nitrogen concentration in the anode path can be monitored directly or at least indirectly using the proposed method. Knowing the nitrogen concentration allows for timely removal of nitrogen to prevent damage to the fuel cell. Furthermore, removal can be minimized to optimize system efficiency.

[0012] According to the proposed method, the hydrogen and / or nitrogen concentrations of the anode gas are monitored by means of a hydrogen sensor installed in the exhaust gas path of the system. Since the hydrogen sensor is already present, no additional sensing device is required to implement this method, thus making it relatively simple to implement.

[0013] The measurement data attached to the hydrogen sensor, used to monitor the hydrogen and / or nitrogen concentrations in the anode path, is generally known or can be easily derived from known parameters. This is particularly relevant regarding the amount of gas introduced into the exhaust path and / or the amount of fresh hydrogen supplied to the anode path.

[0014] Preferably, the hydrogen concentration of the anode gas is first calculated. Then, the nitrogen concentration of the anode gas is determined given the hydrogen concentration. In other words, the nitrogen concentration is derived from the hydrogen concentration of the anode gas. Therefore, the nitrogen concentration is determined indirectly.

[0015] Additional steps can be performed before calculating the hydrogen concentration of the anode gas. These steps will be described in detail below.

[0016] Preferably, before calculating the hydrogen concentration, the following steps are performed: First, the total molar flow rate is calculated from the amount of gas introduced into the exhaust gas path. Then, based on the calculated total molar flow rate and the measured hydrogen concentration of the exhaust gas in the exhaust gas path, the hydrogen quantity can be determined by time integration. This determined hydrogen quantity can then be used as the basis for calculating the hydrogen concentration of the anode gas. This is because it is preferable to calculate the hydrogen concentration of the anode gas by dividing the amount of hydrogen in the exhaust gas by the amount of gas introduced from the anode path into the exhaust gas path.

[0017] To calculate the total molar flow rate, it is preferable to determine the molar flow rate of the cathode exhaust gas and the molar flow rate of the anode exhaust gas introduced into the exhaust gas path from the anode path beforehand. Therefore, the total molar flow rate consists of two separate molar flow rates of the cathode and anode exhaust gases introduced into the exhaust gas path.

[0018] In the system, the molar flow rate of the cathode exhaust gas is known by understanding the characteristic curve family of the cathode-side air compressor and different air quality measurements, as well as by understanding the amount of oxygen converted, air pressure, and relative air humidity. The molar flow rate of the anode exhaust gas can then be determined. Preferably, to determine the molar flow rate of the anode exhaust gas, the amount of gas introduced from the anode path into the exhaust gas path through purging is divided by the duration of the purging process.

[0019] This is based on the premise that the amount of gas introduced from the anode path into the exhaust gas path is known. Therefore, in an improved embodiment of the invention, it is proposed that the amount of gas introduced from the anode path into the exhaust gas path through purging is determined by the amount of hydrogen freshly supplied to the anode path and the amount of hydrogen converted under constant anode pressure. The amount of converted hydrogen is derived from the amount of hydrogen required to generate current or from the stacked current, i.e., preferably over the duration of the purging process. This amount of hydrogen is subtracted from the amount of hydrogen freshly supplied to the anode path during the purging process. Therefore, the amount of gas removed through purging can be deduced from the amount of freshly supplied hydrogen. This assumption presupposes that the anode pressure remains unchanged during the integration time period.

[0020] Furthermore, an evaluation unit for a fuel cell system is proposed, by means of which the method according to the invention can be implemented. The evaluation unit is connected to a hydrogen sensor arranged in the exhaust gas path of the fuel cell system in a data transmission manner. Thus, the evaluation unit is provided with the measurement data required for implementing the method. If other information is needed, this information can also be transmitted to the evaluation unit. Attached Figure Description

[0021] The method and its advantages according to the present invention will now be described in detail with reference to the accompanying drawings.

[0022] Should Figure 1 A preferred flow diagram of the method according to the present invention is shown below. Detailed Implementation

[0023] The flowchart of the method according to the invention, exemplarily illustrated in the block diagram, includes multiple method steps that can be performed sequentially or in parallel. A fundamental method step is the measurement of the hydrogen concentration in the exhaust gas in the exhaust gas path, denoted here as method step 10. Based on this measurement and a pre-determined total molar flow rate in the exhaust gas path, the hydrogen quantity is determined by time integration in method step 20. The determination of the total molar flow rate includes method steps 11 and 16, wherein the molar flow rate of the cathode exhaust gas is first determined (method step 11) and added to the molar flow rate of the anode exhaust gas introduced into the exhaust gas path (method step 16). The molar flow rate of the cathode exhaust gas can be assumed to be known, while the molar flow rate of the anode exhaust gas must also be determined. For this purpose, the amount of anode exhaust gas introduced into the exhaust gas path by means of removal is divided by the duration of the removal process (method step 15). The amount of anode exhaust gas introduced into the exhaust gas path by means of removal is pre-determined in method step 14. Method steps 12 and 13 are performed prior to this step, which include integrating the amount of fresh hydrogen supplied to the anode path over the duration of the purge process (method step 12) and subtracting the amount of hydrogen consumed during the purge process to generate current (method step 13). This assumes that the anode pressure remains unchanged during the integration period.

[0024] Finally, in method step 30, the hydrogen concentration in the anode path can be calculated based on the amount of gas introduced from the anode path into the exhaust gas path obtained in method step 14 and the amount of hydrogen in the exhaust gas obtained in method step 20. For this purpose, the amount of hydrogen obtained in method step 20 is divided by the amount of gas obtained in method step 14.

Claims

1. A method for operating a fuel cell system, the fuel cell system having at least one fuel cell, the fuel cell being supplied with hydrogen via an anode path and oxygen via a cathode path, wherein, The anode exhaust gas discharged from the fuel cell is recirculated; however, a portion of the anode exhaust gas is periodically introduced from the anode path into the exhaust gas path that guides the cathode exhaust gas, wherein the hydrogen concentration of the exhaust gas is measured in the exhaust gas path by means of a hydrogen sensor. The invention is characterized by calculating the hydrogen concentration and / or nitrogen concentration of the anode gas in the anode path before the last purging based on the measured hydrogen concentration, the amount of gas introduced from the cathode path and the anode path into the exhaust gas path, and the amount of fresh hydrogen supplied to the anode path; calculating the total molar flow rate from the amount of gas introduced into the exhaust gas path; and performing hydrogen quantity determination by time integration based on the calculated total molar flow rate and the measured hydrogen concentration of the exhaust gas in the exhaust gas path.

2. The method according to claim 1, Its features are, Calculate the hydrogen concentration of the anode gas and, given the hydrogen concentration, determine the nitrogen concentration of the anode gas.

3. The method according to claim 1, Its features are, The hydrogen concentration of the anode gas is calculated by dividing the amount of hydrogen in the exhaust gas by the amount of gas introduced from the anode path into the exhaust gas path.

4. The method according to any one of claims 1 to 3, Its features are, To calculate the total molar flow rate, the molar flow rate of the cathode exhaust gas and the molar flow rate of the anode exhaust gas introduced into the exhaust gas path from the anode path through elimination are determined in advance.

5. The method according to claim 4, Its features are, To determine the molar flow rate of the anode exhaust gas, the amount of gas introduced from the anode path into the exhaust gas path through removal is divided by the duration of the removal process.

6. The method according to any one of claims 1 to 3, Its features are, The amount of gas introduced into the exhaust gas path from the anode path by means of the amount of fresh hydrogen supplied to the anode path and the amount of hydrogen converted under constant anode pressure is determined.

7. An evaluation unit for a fuel cell system, the evaluation unit being used to implement the method according to any one of claims 1 to 6, wherein, The evaluation unit is connected to a hydrogen sensor via data transmission, and the hydrogen sensor is arranged in the exhaust gas path of the fuel cell system.

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