CN118198438A - Fuel cell system and method for operating a fuel cell system - Google Patents

Abstract

The present invention relates to a fuel cell system (100) for converting energy. A fuel cell system (100) comprises: a fuel cell stack comprising an anode subsystem (111); a hydrogen metering valve (101) configured for supplying hydrogen into the anode subsystem (111); a calculation unit (123); an anode inlet sensor (113) configured for detecting relative humidity and temperature at an anode inlet of the anode subsystem (111), the calculation unit (123) being configured for operating the fuel cell system (100) in steady state operation, determining an anode stoichiometry in the anode subsystem (111) from a relation between humidity compared to a humidity reference value determined by the anode inlet sensor (113), a hydrogen mass flow metered into the anode subsystem (111) by the hydrogen metering valve (101), and a current provided by the fuel cell stack, and adjusting the fuel cell system (100) according to the determined anode stoichiometry.

Description

Fuel cell system and method for operating a fuel cell system

Technical Field

The invention relates to a fuel cell system and a method for operating a fuel cell system.

Background technique

Hydrogen-based polymer electrolyte membrane (PEM) fuel cells are seen as a mobility concept of the future, since they emit only water as exhaust gas and enable fast refueling times.

On the anode side, fuel cells are typically operated in a stoichiometrically tilted manner. Operation, ie supplying more hydrogen than is required for the electrochemical reaction, can thereby avoid serious and harmful undersupply.

In order to minimize hydrogen losses at the same time, the anode exhaust gas is recirculated and supplied to the anode again in a mixed state with fresh hydrogen.

Furthermore, the water diffused from the cathode side to the anode can be recirculated in vapor form in order to ensure sufficient humidification of the membrane at the anode inlet.

Not only in operation, but also in the stopped state, nitrogen diffuses through the membrane from the cathode side to the anode side, whereby nitrogen enrichment occurs in the anode circuit and the required recirculation power increases. At the same time, the hydrogen partial pressure decreases, and thus the local maximum hydrogen diffusion flow through the gas diffusion layer to the anode catalyst layer (ACL) decreases. As a result, hydrogen deficiency can occur locally, which leads to irreversible damage to the ACL.

The nitrogen transfer through the membrane is difficult to estimate since it depends strongly on the operating point and the aging state of the membrane.

Excessive enrichment can be avoided by "purging" via appropriate purge valves, i.e. a flushing process for removing the anode gas. In this way, nitrogen-containing gases are removed from the anode circuit and fresh hydrogen is added in a metered manner, thereby reducing the relative nitrogen content.

Liquid water that accumulates during operation of the fuel cell system can also be discharged from the fuel cell system via a separate discharge valve, a so-called “drain valve”, or together with the gas mixture via a common purge/drain valve.

Typically, for cost reasons, valves for flushing and/or draining the fluid are designed as switching valves which are opened and closed cyclically.

Since not only nitrogen transfer, but also water transport can vary over the lifetime of the system, an adaptive operating strategy is advantageous.

In order to determine the stoichiometry in the anode subsystem of a fuel cell system, the measurement of the volume flow or mass flow circulating in the anode subsystem is not sufficient, since the nitrogen content and the hydrogen content can vary.

In principle, the gas composition can be measured directly by means of corresponding sensor devices. However, the sensors required for this are expensive and require the protection of a large structural volume. Therefore, a method using fewer, cheaper and/or smaller sensors is advantageous for the mobile use of fuel cell systems.

Summary of the invention

In the context of the proposed invention, a fuel cell system and a method for operating a fuel cell system are proposed. Further features and details of the invention are apparent from the preferred embodiments, the description and the drawings. Features and details described in the context of the method according to the invention are of course also applicable in the context of the fuel cell system according to the invention, and vice versa, so that reference is always made to one another or can be made to one another with regard to the disclosure of the individual inventive aspects.

The proposed invention is used, inter alia, to provide a cost-effective and fuel-efficient fuel cell system.

Therefore, according to a first aspect of the proposed invention, a fuel cell system for converting energy is proposed.

The proposed fuel cell system includes a fuel cell stack, a hydrogen metering valve, a computing unit and an anode inlet sensor, wherein the fuel cell stack includes an anode subsystem, the hydrogen metering valve is configured to supply hydrogen into the anode subsystem, and the anode inlet sensor is configured to detect relative humidity and temperature at the anode inlet of the anode subsystem.

The computing unit is configured to operate the fuel cell system under steady-state operation, and thereby determine the anode stoichiometry in the anode subsystem based on the relationship between the humidity determined by the anode inlet sensor and a humidity reference value, the hydrogen mass flow metered into the anode subsystem by the hydrogen metering valve, and the current provided by the fuel cell stack, and to adjust the fuel cell system based on the determined anode stoichiometry.

In the context of the proposed invention, a computing unit is to be understood as a computer, a processor, a sub-processor, a controller or any other programmable circuit.

The proposed invention is based on the principle of detecting a decrease in the measured relative humidity at the anode input relative to a humidity reference value, which decrease varies as a function of the metering of fresh, dry hydrogen into the anode subsystem.

The metered hydrogen mass flow can be determined very accurately since it is a pure substance. Accordingly, the metered hydrogen mass flow can be determined, for example, by means of a mass flow sensor or a mathematical model of the hydrogen metering valve.

In summary, the recirculation ratio between the recirculation mass flow and the metered hydrogen and the resulting hydrogen mass flow through the anode can be determined in this way. A comparison with the current intensity and thus with the hydrogen consumption yields the anode stoichiometry.

To calculate the recirculation ratio, the following mathematical relationship applies:

Anode stoichiometry λ _an is defined as the supplied hydrogen mass flow at the anode inlet The mass flow of hydrogen consumed in the fuel cell reaction/> The ratio between:

The consumed hydrogen mass flow is obtained from the number of cells in the stack n _zellen , the Faraday constant F, the current intensity I and the molar mass of hydrogen _MH2 :

That is, for a given stack and known current density, the consumed hydrogen mass flow can be directly calculated. The supplied hydrogen mass flow It can be determined in the following way: the substance content x _H2O of water vapor can be determined respectively from the measured relative humidity. Here, an ideal gas mixture is assumed and in the simplest case, any nitrogen in the anode circuit is ignored. If the nitrogen content is additionally measured or estimated with the help of a mathematical model, this substance content can also be taken into account in equation (3) in order to more accurately determine the hydrogen content. Therefore, the substance content x _H2 of hydrogen can be determined at two sensor positions. For this purpose, in addition, a saturated vapor pressure p ^sat related to the temperature at the sensor is required. This temperature measurement is carried out directly by the sensor, and the saturated vapor pressure can be calculated from this, for example, by the Antoine equation. In the prior art, the total pressure p is measured at one or more positions in the anode circuit in order to keep the pressure difference of the diaphragm relative to the cathode side low by means of anode pressure regulation.

Hydrogen mass flow supplied to the stack On the one hand, hydrogen supplied by HGI and on the other hand by the mass flow of hydrogen in the recycle composition:

The HGI mass flow can be determined directly and consists of pure hydrogen. It is either measured with the aid of a sensor or calculated using a model equation with the aid of the opening angle of the HGI.

For the hydrogen mass flow in the recycle The substance content of hydrogen in the recirculation x _H2,rezi (HS2) and the substance content of hydrogen before the anode inlet x _H2,an,in (HS1) are taken into account, which are determined with the aid of a humidity sensor (see equation (3)).

Depend on

inferred

And therefore

In summary, by substituting equations (2), (4) and (7) into equation (1) we obtain:

It can be provided that the humidity reference value is 100% or a value ascertained by an anode outlet sensor which is configured to detect the relative humidity and the temperature at the anode outlet of the anode subsystem.

Due to the predetermined humidity reference value of 100%, which is based on the assumption that an environment completely saturated with water vapor is present at the anode outlet, a second sensor for determining the humidity reference value can be omitted.

By means of a second humidity sensor, for example at the anode outlet, the anode stoichiometry present in the anode subsystem can be determined particularly accurately.

Furthermore, it can be provided that the computing unit is configured to infer from the current provided by the fuel cell stack the amount of hydrogen provided for providing this current and to determine the anode stoichiometry as the ratio between the hydrogen flowing into the anode subsystem and the hydrogen flowing out of the anode subsystem.

From the current supplied by the fuel cell stack, the amount of hydrogen consumed by the fuel cell stack can be inferred, so that the amount of hydrogen flowing out of the anode subsystem can be inferred given the amount of hydrogen supplied to the anode subsystem.

Furthermore, it can be provided that the fuel cell system comprises a mass flow sensor for ascertaining the mass flow of hydrogen metered into the anode subsystem.

The mass flow sensor measures the hydrogen mass flow metered into the anode subsystem precisely and in a manner dependent on the operating point, so that the anode stoichiometry can be determined correspondingly precisely and in a manner dependent on the operating point.

Furthermore, it can be provided that the computing unit is configured to determine, by means of a flow through the hydrogen metering valve, A mathematical model determines the hydrogen mass flow metered into the anode subsystem.

By determining the hydrogen mass flow metered into the anode subsystem with the aid of a mathematical model, a mass flow sensor for determining the hydrogen mass flow metered into the anode subsystem can be omitted, so that installation space is saved and the reliability of the fuel cell system is improved.

Furthermore, it can be provided that the computing unit is configured to adjust a recirculation rate and/or a rotational speed of a gas supply unit of the fuel cell system as a function of the ascertained anode stoichiometry during operation of the fuel cell system.

If the anode stoichiometry present in the anode subsystem is known, the fuel cell system can be adjusted accordingly. For this purpose, in particular the recirculation rate and/or the rotational speed of the gas supply unit of the fuel cell system can be adjusted so that, for example, the current is provided particularly fuel-efficiently.

Furthermore, it can be provided that the fuel cell system comprises a pressure sensor which is configured to ascertain the course of the pressure prevailing in the anode subsystem.

The computing unit is configured to carry out a plausibility check on the anode stoichiometry ascertained as a function of the pressure profile ascertained by the pressure sensor.

Furthermore, it can be provided that the fuel cell system comprises a gas supply unit which is configured to supply air to the fuel cell system, and that the computing unit is configured to determine a current consumed by the gas supply unit and to plausibly check the ascertained anode stoichiometry as a function of the current consumed by the gas supply unit.

By comparing the anode stoichiometry determined by means of the anode inlet sensor provided according to the invention with the current consumed by the gas supply unit, the determined anode stoichiometry can be approved, for example, if the comparison yields a value within a predefined tolerance range, or an error message can be output if the comparison yields a value outside the predefined tolerance range. The current can be measured by means of a current sensor or determined by means of a mathematical model.

Furthermore, it can be provided that the fuel cell system comprises a gas supply unit which is configured to supply air to the fuel cell system, and the computing unit is configured to determine the pressure of a gas mass flow provided by the gas supply unit and to perform a plausibility check on the determined anode stoichiometry as a function of the pressure of the gas mass flow.

By comparing the anode stoichiometry determined by means of the anode inlet sensor provided according to the invention with the pressure of the gas mass flow provided by the gas supply unit, the determined anode stoichiometry can be accepted, for example, if the comparison yields a value within a predefined tolerance range, or an error message can be output if the comparison yields a value outside the predefined tolerance range. The pressure of the gas mass flow can be measured by means of a pressure sensor or determined by means of a mathematical model.

According to a second aspect, the proposed invention relates to a method for operating one possible embodiment of the proposed fuel cell system.

The proposed method includes: operating the fuel cell system in steady-state operation, determining the anode stoichiometry in the anode subsystem based on the relationship between the humidity compared to a humidity reference value determined by an anode inlet sensor, the mass flow of hydrogen metered into the anode subsystem by a hydrogen metering valve, and the current provided by the fuel cell stack, and adjusting the fuel cell system based on the determined anode stoichiometry.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages, features and details of the invention emerge from the following description, in which various exemplary embodiments are described in detail with reference to the drawings. The features proposed according to the invention may each be essential to the invention individually or in any combination.

The accompanying drawings show:

FIG. 1 shows a schematic diagram of a possible configuration of the proposed fuel cell system,

FIG. 2 shows a possible configuration of the proposed method.

Detailed ways

A fuel cell system 100 is shown in Figure 1. The fuel cell system 100 comprises a hydrogen metering valve 101 for metering hydrogen flowing from a fuel tank 103 via a pressure reducer 105 into the anode subsystem of a fuel cell stack.

An anode inlet sensor 113 for measuring the humidity is arranged downstream of the introduction point 107 , at which the recirculation blower 109 introduces a recirculation mass flow into the anode subsystem 111 .

An optional anode outlet sensor 117 for measuring humidity is arranged between the condensate separator 115 and the recirculation blower 109 .

To flush the anode subsystem 111 , the fuel cell system 100 includes a purge valve 119 .

In order to discharge liquid water, the fuel cell system 100 includes a drain valve 121 .

Furthermore, the fuel cell system 100 comprises a computing unit 123 .

The calculation unit 123 is configured to operate the fuel cell system 100 under steady-state operation, and to determine the anode stoichiometry in the anode subsystem 111 based on the relationship between the humidity compared to the humidity reference value determined by the anode inlet sensor 113, the hydrogen mass flow metered into the anode subsystem 111 by the hydrogen metering valve 101, and the current provided by the fuel cell stack, and to adjust the fuel cell system 100 based on the determined anode stoichiometry.

For example, the computing unit 123 may be configured to activate the purge valve 119 and/or reduce the activity of the recirculation blower 109 when the anode stoichiometry is less than a predetermined threshold value.

FIG. 2 shows a method 200 for operating the fuel cell system 100 according to FIG. 1 .

The method 200 includes: an operation step 201, in which the fuel cell system 100 is maintained in steady-state operation, an acquisition step 203, in which the anode stoichiometry in the anode subsystem 111 of the fuel cell system 100 is obtained based on the relationship between the humidity obtained by the anode inlet sensor 113 and the humidity reference value, the hydrogen mass flow metered into the anode subsystem 111 by the hydrogen metering valve 101 and the current provided by the fuel cell stack, and an adjustment step 205, in which the fuel cell system 100 is adjusted according to the obtained anode stoichiometry.

Claims (10)

1. A fuel cell system (100) for converting energy,

Wherein the fuel cell system (100) includes:

a fuel cell stack, wherein the fuel cell stack comprises an anode subsystem (111),

A hydrogen metering valve (101) configured for supplying hydrogen into the anode subsystem (111),

A calculation unit (123),

An anode inlet sensor (113) configured for detecting relative humidity and temperature at an anode inlet of the anode subsystem (111),

Wherein the computing unit (123) is configured for,

The fuel cell system (100) is operated in steady-state operation, and the anode stoichiometry in the anode subsystem (111) is determined from the relationship between the humidity of the reference value compared to the humidity determined by the anode inlet sensor (113), the hydrogen mass flow metered into the anode subsystem (111) by the hydrogen metering valve (101), and the current supplied by the fuel cell stack, and the fuel cell system (100) is set according to the determined anode stoichiometry.

2. The fuel cell system (100) according to claim 1,

It is characterized in that the method comprises the steps of,

The humidity reference value is 100% or a value obtained by an anode outlet sensor (117) configured for detecting relative humidity and temperature at an anode outlet of the anode subsystem (111).

3. The fuel cell system (100) according to claim 1 or 2,

It is characterized in that the method comprises the steps of,

The calculation unit (123) is configured for deducing the amount of hydrogen provided for providing the current from the current provided through the fuel cell stack (100) and determining the anode stoichiometry as the ratio between hydrogen flowing into the anode subsystem (111) and hydrogen flowing out of the anode subsystem (111).

4. The fuel cell system (100) according to any one of the preceding claims,

It is characterized in that the method comprises the steps of,

The fuel cell system (100) comprises a mass flow sensor for determining a hydrogen mass flow metered into the anode subsystem (111).

5. The fuel cell system (100) according to any one of claims 1 to 3,

It is characterized in that the method comprises the steps of,

The calculation unit (123) is configured for determining a hydrogen mass flow metered into the anode subsystem (111) by means of a mathematical model of the flow through of the hydrogen metering valve (101).

6. The fuel cell system (100) according to any one of the preceding claims,

It is characterized in that the method comprises the steps of,

The calculation unit (123) is configured to set a recirculation rate and/or a rotational speed of a gas supply unit of the fuel cell system (100) in accordance with the determined anode stoichiometry during operation of the fuel cell system (100).

7. The fuel cell system (100) according to any one of the preceding claims,

It is characterized in that the method comprises the steps of,

The fuel cell system (100) comprises a pressure sensor configured for determining a course of pressure present in the anode sub-system (111),

The calculation unit (123) is configured for plausibility checking of the determined anode stoichiometry as a function of the pressure change determined by the pressure sensor.

8. The fuel cell system (100) according to any one of the preceding claims,

It is characterized in that the method comprises the steps of,

The fuel cell system (100) comprises a gas supply unit configured for supplying air to the fuel cell system (100),

The computing unit (123) is configured to determine the current consumed by the gas supply unit and to perform a plausibility check on the determined anode stoichiometry as a function of the current consumed by the gas supply unit.

9. The fuel cell system (100) according to any one of the preceding claims,

It is characterized in that the method comprises the steps of,

The fuel cell system (100) comprises a gas supply unit configured for supplying air to the fuel cell system (100),

The calculation unit is configured for determining a pressure of the gas mass flow supplied by the gas supply unit and for plausibility checking the determined anode stoichiometry as a function of the pressure of the gas mass flow.

10. A method (200) for operating a fuel cell system (100) according to any one of claims 1 to 9,

Wherein the method (200) comprises:

Operating (201) the fuel cell system (100) in steady state operation,

-Determining (203) an anode stoichiometry in the anode subsystem (111) from a relation between humidity of a relative humidity reference value determined by an anode inlet sensor (113), a mass flow of hydrogen metered into the anode subsystem (111) by the hydrogen metering valve (101), and a current provided by the fuel cell stack,

The fuel cell system (100) is adjusted (205) based on the determined anode stoichiometry.