Method for increasing the safety and/or the reliability of the operation of a fuel cell stack

Abstract

A method increases the safety and/or the reliability of the operation of a fuel cell stack. The method determines that the fuel cell stack is in a space with a reduced air exchange rate. The method determines consumption information with respect to an oxygen consumption of the fuel cell stack within an interval of time. The method determines, on the basis of the air exchange rate, inflow information with respect to an amount of oxygen which was supplied to the space within the interval of time. The method determines an estimated value for an oxygen content of air in the space on the basis of the consumption information and on the basis of the inflow information.

Claims

1. A method for increasing safety and/or reliability of the operation of a fuel cell stack, the method comprising: determining that the fuel cell stack is situated in a space with a reduced air exchange rate; ascertaining volume information relating to an air volume in the space; ascertaining an oxygen content difference between an oxygen content of air flowing into the space and of air flowing out of the space; ascertaining consumption information relating to an oxygen consumption of the fuel cell stack within a time interval; ascertaining, on the basis of the air exchange rate, the oxygen content difference, the volume information and a temporal length of the time interval, inflow information relating to a quantity of oxygen that has been fed to the space within the time interval; and ascertaining an estimated value for an oxygen content of air in the space on the basis of the consumption information and on the basis of the inflow information; initiating a measure in a manner dependent on the ascertained estimated value for the oxygen content if the ascertained estimated value reaches or falls below an oxygen content threshold value, wherein the measure comprises: outputting a notification to a user of the fuel cell stack; increasing the air exchange rate; and/or reducing the oxygen consumption of the fuel cell stack.

2. The method according to claim 1, further comprising: ascertaining pressure information relating to a pressure within the space relative to a pressure outside the space; and ascertaining the air exchange rate and/or the inflow information on the basis of the pressure information.

3. The method according to claim 2, wherein during the operation of the fuel cell stack, water vapor as reaction product is discharged from the fuel cell stack; the method further comprising: ascertaining a fraction of the water vapor that condenses in the space, wherein the pressure information is dependent on the fraction of the water vapor that condenses in the space.

4. The method according to claim 3, further comprising: ascertaining temperature information relating to a temperature of air in the space and/or relating to a temperature of a condensation surface of the space; ascertaining humidity information relating to a relative air humidity of the air in the space; and ascertaining the fraction of the water vapor that condenses in the space on the basis of the temperature information and on the basis of the humidity information.

5. The method according to claim 1, the method further comprising: iteratively repeating the ascertainment of consumption information, the ascertainment of inflow information and the ascertainment of the estimated value of the oxygen content at a sequence of points in time in order to ascertain estimated values of the oxygen content for the sequence of points in time.

6. The method according to claim 1, further comprising: ascertaining a fuel consumption of the fuel cell stack within the time interval; and ascertaining the consumption information relating to the oxygen consumption of the fuel cell stack on the basis of a stoichiometry of a reaction between fuel and oxygen in the fuel cell stack and on the basis of the fuel consumption of the fuel cell stack.

7. The method according to claim 1, further comprising: ascertaining, on the basis of the inflow information and on the basis of the consumption information, a change in the oxygen quantity in the space within the time interval; and updating the estimated value for the oxygen content on the basis of the change in the oxygen quantity in the space within the time interval.

8. The method according to claim 1, further comprising one or more of: (i) ascertaining movement data relating to the movement of a vehicle in which the fuel cell stack is situated; (ii) ascertaining position data relating to the position of the fuel cell stack and/or of the vehicle; (iii) ascertaining surroundings data relating to direct surroundings of the fuel cell stack and/or of the vehicle; (iv) ascertaining signal data relating to shadowing and/or attenuation of electromagnetic signals that are or have been transmitted and/or received by the vehicle; (v) ascertaining input data relating to an input by a user of the fuel cell stack and/or of the vehicle; and wherein the method further comprises determining, on the basis of the movement data, the position data, the surroundings data, the signal data and/or the input data, that the fuel cell stack is situated in a space with a reduced air exchange rate.

9. A method for increasing safety and/or reliability of operation of a fuel cell stack, the method comprising: ascertaining measurement data relating to the operation of a fuel cell stack at a sequence of points in time, the measurement data relating to a level of humidity within the fuel cell stack; detecting, on the basis of the measurement data at the sequence of points in time, an increase in air humidity of air in a surroundings of the fuel cell stack; and determining, when it has been detected that the air humidity of the air in the surroundings of the fuel cell stack has increased, that an oxygen content of air in the surroundings of the fuel cell stack has decreased; and, in response to the decrease in the oxygen content, initiating a measure for increasing the safety and/or the reliability of the operation of the fuel cell stack, wherein the measure comprises: outputting a notification to a user of the fuel cell stack; increasing an air exchange rate of the surroundings; and/or reducing an oxygen consumption of the fuel cell stack.

10. The method according to claim 9; wherein the measurement data indicate: a level, a duration and/or a frequency of recirculation of water from exhaust gases of the fuel cell stack into a reaction space of the fuel cell stack; and/or a level, a duration and/or a frequency of a humidification of anodes and/or cathodes of the fuel cell stack.

11. The method according to claim 9, further comprising: determining, on the basis of the measurement data and on the basis of characteristic data for the operation of the fuel cell stack, an estimated value for the oxygen content of the air in the surroundings of the fuel cell stack.

12. The method according to claim 11, wherein the measurement data indicate: a volume and/or mass flow of air that is fed to the fuel cell stack; a volume and/or mass flow of fuel that is fed to the fuel cell stack; an amount of electrical power that is generated by the fuel cell stack; an amount of non-reacted fuel in exhaust gases of the fuel cell stack; and/or an air ratio of the amount of air provided for a reaction of the fuel cell stack.

13. A device for a vehicle, comprising: a control unit configured to execute the acts of: determining that the fuel cell stack is situated in a space with a reduced air exchange rate; ascertaining consumption information relating to an oxygen consumption of the fuel cell stack within a time interval; ascertaining, on the basis of the air exchange rate, inflow information relating to a quantity of oxygen that has been fed to the space within the time interval; and ascertaining an estimated value for an oxygen content of air in the space on the basis of the consumption information and on the basis of the inflow information; initiating a measure in a manner dependent on the ascertained estimated value for the oxygen content if the ascertained estimated value reaches or falls below an oxygen content threshold value, wherein the measure comprises: outputting a notification to a user of the fuel cell stack; increasing the air exchange rate; and/or reducing the oxygen consumption of the fuel cell stack.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows exemplary components of a vehicle.

(2) FIG. 2 shows a closed space with a vehicle.

(3) FIG. 3 is a flow diagram of an exemplary method for operating a fuel cell system.

(4) FIG. 4 is a flow diagram of a further exemplary method for operating a fuel cell system.

DETAILED DESCRIPTION OF THE DRAWINGS

(5) As discussed in the introduction, the present document is concerned with the reliable and safe operation of a fuel cell system, in particular in a motor vehicle. In this context, FIG. 1 shows exemplary components of a vehicle 100. In particular, FIG. 1 shows a fuel cell stack 101, which typically comprises a multiplicity of fuel cells. The fuel cell stack 101 is configured to generate electrical energy for the operation of a drive motor of the vehicle 100. For this purpose, the fuel cell stack may be fed with fuel 121 from a fuel vessel 102 (in particular from a pressure vessel 102).

(6) A fuel cell of a fuel cell stack 101 comprises an anode and a cathode which are separated by an ion-selective or ion-permeable separator. The anode is supplied with fuel 121. Preferred fuels are: hydrogen, low molecular mass alcohol, biofuels, or liquefied natural gas. The cathode is supplied with oxidant 122. Preferred oxidants 122 are: air, oxygen and peroxides. The ion-selective separator may for example be in the form of a proton exchange membrane (PEM). Use is preferably made of a cation-selective polymer electrolyte membrane. Materials for such a membrane are for example: Nafion®, Flemion® and Aciplex®.

(7) The reaction products (in particular water) generated in the fuel cells of a fuel cell stack 101 can be conducted out of the fuel cell stack 101 as exhaust gases 123 via one or more exhaust-gas channels.

(8) The vehicle 100 furthermore comprises a device 110 for controlling the operation of a fuel cell system with a fuel cell stack 101. For this purpose, measurement data 111 may be ascertained, wherein the measurement data 111 may indicate values of one or more measurement variables. Exemplary measurement variables are the volume flow of fuel 121 and/or oxidant 122 into the fuel cell stack 101; the electrical power generated by the fuel cell stack 101; the amount of water within the individual fuel cells; and/or the amount of non-reacted fuel 121 in the exhaust gases 123.

(9) The device 110 is configured to vary one or more operating parameters 112 of the fuel cell stack 101 on the basis of the measurement data 111. Exemplary operating parameters 112 are the volume flow of fuel 121 and/or oxidant 122 into the fuel cell stack 101; the duration, the intensity and/or the frequency of flushing cycles for flushing the anodes of the fuel cells with fuel 121 (in order to discharge inert gas and/or condensate); here, the intensity may in particular indicate the mass flow of fuel 121 that is used for the flushing of the anodes; the air ratio, that is to say the ratio of oxidant 122 to fuel 121; through variation of the air ratio, the amount of water at the cathodes of the fuel cells of a fuel cell stack 101 can be changed; and/or a humidification and/or drying of the gas fed to the anodes and/or cathodes.

(10) The device 110 may thus ascertain the mass of fed oxidant 122 (in particular air) as measurement data 111 (by way of the parameters of an oxidant-conveying means). Furthermore, the device 110 may ascertain the consumption of fuel 121 as measurement data 111 (by way of the valve and/or injector controller of the fuel cell stack 101). Furthermore, the device 110 may ascertain the generated electrical power of the fuel cell stack 101 as measurement data 111 (in particular by way of the cell voltage and the generated electrical current). Furthermore, the amount of non-reacted fuel 121 (in particular hydrogen) and/or oxygen in the exhaust gas 123 may be ascertained as measurement data 111 by way of corresponding sensor means (for example by way of a lambda probe).

(11) The measurement data 111 may be used by the device 110 to identify a change (in particular a decrease) in the oxygen content within the oxidant 122. For example, an increase in the amount of non-reacted fuel 121 in the exhaust gases 123 (in the presence of otherwise unchanged conditions) is an indication that the oxygen content within the oxidant 122 is decreasing. In particular, a change (in particular a drop) in the oxygen content may be identified by way of the measured air ratio in the exhaust gases 123.

(12) As presented above, during the operation of a fuel cell stack 101, water is typically generated as exhaust gas 123. The operation of a fuel cell stack 101 thus typically leads, in particular in the presence of a depletion of oxygen or in the case of operation of a fuel cell stack 101 in a closed space, to an increase in the air humidity in the air surrounding the fuel cell stack 101, and thus, in effect by way of an external recirculation, to an increase in the air humidity of the gases fed to the cathodes. The increased air humidity thus typically leads to an increased humidity of the fuel cells. The increased humidity within the cells of a fuel cell stack 101 can be detected as measurement data 111.

(13) The device 110 may be configured to adjust the level of humidity of the anodes and/or cathodes of the fuel cells of a fuel cell stack 101 in each case to a particular target value, in particular by closed-loop control. For example, a recirculation of water from the reaction products to the cathodes of the fuel cells may be performed if it is identified that the level of humidity of the cathodes is too low. Alternatively or in addition, humidification of the gases fed to the anodes may be performed if it is identified that the level of humidity of the anodes is too low. It is thus possible, on the basis of the level and/or the frequency of the water recirculation and/or of the anode humidification, to infer a level of the humidity of the fuel cells of a fuel cell stack 101.

(14) Furthermore, from an increased level of humidity of the fuel cells of a fuel cell stack 101, it is possible to infer increased air humidity of the air in the surroundings of the fuel cell stack 101 and thus an oxygen depletion of the ambient air.

(15) The device 110 may thus be configured to identify or determine, on the basis of the measurement data 111 and/or on the basis of the operating parameters 112 during the operation of a fuel cell stack 101, that an oxygen depletion is present. Furthermore, the device 110 may be configured to initiate one or more measures relating to the identified oxygen depletion. Exemplary measures are: the outputting of an (acoustic and/or visual) warning notification; one or more measures for increasing the ventilation and/or the air exchange in the surroundings of the fuel cell stack 101; and/or a restriction, limitation and/or ending of the oxygen consumption by the fuel cell stack 101.

(16) An oxygen depletion may arise in particular if a vehicle 100 with a fuel cell stack 101 is situated within a closed space 200, in particular within a (relatively small) garage. FIG. 2 illustrates an exemplary vehicle 100 with a fuel cell stack 101 within a garage 200 (as an example of a closed space). A garage 200 typically permits a relatively small exchange of gas or a relatively low air exchange rate 221. Furthermore, a garage 200 typically has a relatively small volume V defined by the dimensions 211, 212, 213 of the garage 200.

(17) The device 110 may be configured to, for an operating phase of the fuel cell stack 101 for which presence at a location in a garage 200 cannot be ruled out, ascertain the oxygen concentration within the garage 200. Here, a determined air exchange rate 221 of the garage 200 may be assumed (for example a garage 200 as defined by the SAE, Society of Automotive Engineers, with an air exchange rate 221 of 0.03 air exchanges per hour).

(18) It is thus possible at all times to ascertain an estimated value for the oxygen content in the surroundings of the fuel cell stack 101 (in particular within the garage 200). If a preset oxygen content threshold value is reached or undershot, one or more of the above-stated measures can be initiated.

(19) The device 110 may be configured to determine whether or not a vehicle 100 is situated in a closed space 200 with reduced air exchange rate 221. For this purpose, it is for example possible for the movement of wheels of the vehicle 100 to be analyzed. The movement of the wheels over a distance which is not consistent with typical garage dimensions 211 (for example of 10 meters or more) may be considered as an indication that presence at a location in a garage 200 is not applicable. Alternatively or in addition, position data relating to a present position of the vehicle 100 and/or geographic information (for example digital map information relating to buildings and a road network) may be taken into consideration in order to determine whether or not a vehicle 100 is situated in a garage 200. Alternatively or in addition, the shadowing and/or attenuation of electromagnetic signals may be taken into consideration in order to determine whether or not a vehicle 100 is situated in a garage 200. Alternatively or in addition, a camera and image processing system of the vehicle 100 may be used to determine, on the basis of image data of the surroundings of the vehicle 100, whether or not the vehicle 100 is situated in a garage 200. Alternatively or in addition, on the basis of an input by a user of the vehicle 100, it can be determined whether or not the vehicle 100 is situated in a garage 200.

(20) In particular if it is determined that the vehicle 100 is situated in a space 200 with a reduced air exchange rate 221, the device 110 may be configured to ascertain the present oxygen consumption on the basis of the measurement data 111, in particular on the basis of the present fuel consumption. The ratio of oxygen to fuel 121 is in this case defined by the stoichiometry of the reactions involved, in particular 2 H.sub.2+O.sub.2.fwdarw.2H.sub.2O in the case of a fuel cell.

(21) FIG. 4 shows an exemplary method 400 for ascertaining the oxygen content in the air surrounding a fuel cell stack 101. Here, it can be assumed that the fuel 121 is provided (typically in compressed form) from a vessel 102 with a constant volume. It can thus be assumed that the volume of the container 102 remains constant (even if fuel 121 is being consumed).

(22) In a step 401, it may be determined whether or not the fuel cell stack 101 is situated in a space 200 with a reduced air exchange rate 221. If it is determined that the fuel cell stack 101 is situated in surroundings with an adequately high air exchange rate 221, then, as an estimated value O.sub.2 content (t) for the oxygen content at the time t, a base value can be assumed which is typically 21% (step 407).

(23) If, on the other hand, it is determined that the fuel cell stack 101 is situated in a space 200 with reduced air exchange rate 221, then it can be ascertained how much oxygen has been consumed in a time interval with the length Δt. The consumed oxygen volume can be ascertained on the basis of the measurement data 111. In particular, the consumed volume of fuel 121 can be ascertained (that is to say the H.sub.2 consumption in the time interval with the length Δt). Furthermore, on the basis of the stoichiometry of the reaction in the fuel cells, a factor F can be ascertained which indicates what quantity of oxygen is consumed per quantity of fuel 121. The consumed O.sub.2 volume can then be ascertained as a product of the factor F and the H.sub.2 consumption in the time interval with the length Δt (step 402).

(24) Furthermore, on the basis of the air exchange rate 221, it can be ascertained what quantity of oxygen has newly flowed into the closed space 200 during the time interval Δt. Here, an exchange of air with full oxygen content (for example with a base value of 21%) and of air with a reduced oxygen content of O.sub.2 content (t) typically takes place. From the difference between the base value (of 21%) and the present estimated value O.sub.2 content (t) of the oxygen content in the space 200, the amount of oxygen fed per unit volume of air is thus obtained.

(25) The total exchanged volume of air is dependent on the volume V of the closed space 200, on the air exchange rate 221 and on the length Δt of the time interval. On the basis of the formula presented in FIG. 4 (see step 403), it is thus possible to ascertain the O.sub.2 volume that has flowed in in the time interval Δt.

(26) The updated estimated value O.sub.2 content (t+Δt) of the oxygen content in the space 200 is then obtained on the basis of the difference between the O.sub.2 volume that has flowed in in the time interval and the O.sub.2 volume consumed in the time interval, as per the formula presented in step 404.

(27) The updated estimated value O.sub.2 content (t+Δt) can then be compared (step 405) with an oxygen content threshold value or an O.sub.2 limit value. Furthermore, in the event of the oxygen content threshold value being undershot, one or more measures may be implemented, in particular in order to prevent a hazard to a user and/or in order to permit reliable operation of the fuel cell stack 101 (step 406).

(28) In the context of the method 400, a distinction may be made as to whether the water vapor generated during the operation of a fuel cell stack 101 remains in the closed space 200 as a gaseous constituent or changes into the liquid phase as a result of condensation. For this purpose, the temperature in the surroundings of the fuel cell stack 101, in particular in the space 200, may be taken into consideration. Alternatively or in addition, it may be taken into consideration whether the space 200 has condensation points and/or condensation surfaces which could cause condensation of the generated water. Furthermore, the temperature of a condensation surface of the space 200 may be ascertained. Alternatively or in addition, the dewpoint table of water may be taken into consideration, which indicates, in a manner dependent on the air temperature and the relative air humidity, the dew point proceeding from which a condensation of water occurs on a condensation surface.

(29) If it is ascertained that the generated water remains (for the most part) in the gaseous state, the water vapor leads to an increase in the volume of gas in the closed space 200 and thus to a positive pressure in the closed space 200. On the other hand, a (predominant) condensation of water vapor leads to a reduction in volume and thus to a negative pressure in the closed space 200.

(30) It is thus possible to ascertain measurement data 111 relating to the condensation of water vapor contained in the exhaust gases 123. The measurement data 111 may indicate the air temperature in the space 200; the relative air humidity in the space 200; and/or the temperature and/or the size of one or more condensation surfaces of the space 200.

(31) It is then possible, on the basis of the measurement data 111, to ascertain pressure information relating to the pressure within the space 200 relative to the pressure of the external surroundings of the space 200. In particular, it can be ascertained whether a negative pressure, a positive pressure or no significant pressure difference prevails in the space 200 relative to the external surroundings of the space 200. The air exchange rate 221 may then be adapted in a manner dependent on the pressure data. For example, a minimal gas exchange or a relatively low air exchange rate 221 arises if no pressure difference prevails, for example if in each case half of the substance quantity of the hydrogen generated is generated as gas and the other half is generated as liquid condensate. On the other hand, in particular, a negative pressure typically leads to an increased air exchange rate 221.

(32) The device 110 may be configured to ascertain the volume V and/or the air exchange rate 221 of the respective location at which the vehicle 100 is present by means of geographical data (in particular on the basis of the present position of the vehicle 100 and on the basis of digital map information). Here, the digital map information may indicate the volume and/or the air exchange rate 221 and the position of spaces 200 with reduced air exchange rate 221.

(33) FIG. 3 shows a flow diagram of an exemplary method 300 for increasing the safety and/or the reliability of the operation of a fuel cell stack 101. The method 300 may be carried out by the control unit 110 of a fuel cell stack 101.

(34) The method 300 comprises ascertaining 301 measurement data 111 relating to the operation of a fuel cell stack 101 at a sequence of points in time. It is for example possible for measurement data 111 to be detected and evaluated periodically (for example with a frequency of 0.1 Hz, 1 Hz or more).

(35) Furthermore, the method 300 comprises determining 302, on the basis of the measurement data 111 at the sequence of points in time, that an oxygen content of air in the surroundings of the fuel cell stack 101 has changed, in particular decreased. In particular, a change in the oxygen content can be identified from the profile with respect to time of the measurement data 111. It may be possible for an estimated value for the oxygen content to be ascertained from the profile with respect to time of the measurement data 111 and on the basis of characteristic data for the operation of the fuel cell stack 101. Here, the characteristic data may indicate different estimated values of the oxygen content for different value combinations of measurement variables (for example the mass flow of air 122, the mass flow of fuel 121, the generated electrical power, the amount of non-reacted fuel 121 in the exhaust gases 123, etc.). The characteristic data may be ascertained in advance by means of tests.

(36) Preferably, for the characteristic data, different measurement values are processed in combination in order to obtain an improved estimation for the oxygen content. Here, use may be made of recursive predictor-corrector structures, such as for example Kalman filters, in particular EKF (Extended Kalman Filters) and UKF (Unscented Kalman Filters), owing to the, in part, non-linear parameter relationships. Other approximation methods (gaussian sum filters, projection filters and quadratic filters) or simulations (such as the sequential Monte Carlo methods) may be used in order to provide characteristic data which indicate different estimated values of the oxygen content for different value combinations of measurement variables.

(37) Furthermore, the method 300 comprises, in reaction to the fact that it has been determined 302 that the oxygen content has changed, initiating 303 a measure for increasing the safety and/or the reliability of the operation of the fuel cell stack 101. In particular, a warning notification may be output, and/or the oxygen consumption of the fuel cell stack 101 may be adapted.

(38) The aspects described in this document make it possible to ascertain the oxygen content of the surroundings of a fuel cell stack 101 in an efficient and precise manner. It is thus possible in a reliable manner to identify oxygen depletion and initiate or carry out suitable measures for increasing the safety, the efficiency and/or the reliability of the operation of a fuel cell stack 101.

(39) The present invention is not restricted to the exemplary embodiment shown. In particular, it is to be noted that the description and the figures are intended merely to illustrate the principle of the proposed methods, devices and systems.