WO2009024205A1

WO2009024205A1 - Method and apparatus for diagnosis of a separator module in a fuel cell system

Info

Publication number: WO2009024205A1
Application number: PCT/EP2008/005195
Authority: WO
Inventors: Armin Frank; Thomas Hagmans; Cosimo S. Mazzotta; Helmut Müller; Uwe Pasera
Original assignee: Daimler Ag; Ford Global Technologie, Llc
Priority date: 2007-08-22
Filing date: 2008-06-26
Publication date: 2009-02-26
Also published as: DE102007039558A1

Abstract

The invention relates to a method and an apparatus for diagnosis of a separator module (34) in a fuel cell system (1), with the filling level of the separator module (34) being detected by at least one sensor (5), with emptying of the separator module (34) being initiated when an upper switching point (F_max) is overshot, and with emptying of the separator module (34) being ended when a lower switching point (F_min) is undershot, and in which the filling level is predicted as a function of at least one current physical state variable of the fuel cell system (1), and the predicted value of the filling level is compared with the detected value of the filling level for a plausibility check.

Description

Daimler AG and

Ford Global Technologies, LLC

Method and apparatus for diagnosis of a separator module in a fuel cell system

The invention relates to a method and an apparatus for diagnosis of a separator module in a fuel cell system.

A fuel cell system comprises a fuel cell or a plurality of fuel cells which are connected in series and/or in parallel to form a fuel cell stack. For the vehicle industry, proton exchange membrane fuel cells (PEM-FC) are known in particular, at the moment, in which hydrogen is used as a fuel. However, it is also known for methane, methanol or glucose solution to be used as the fuel. The fuel, in particular hydrogen, is fed into the fuel cell stack at an inlet on an anode side of the fuel cell or of the fuel cell stack. The anode offgases from the fuel cell or the fuel cell stack emerge at an outlet and, when using hydrogen, are composed inter alia of unused hydrogen and water. The unused fuel can be made available again at the inlet, via a recirculation circuit.

During operation of a fuel cell system, in particular a PEM fuel cell system, so-called product water is released at the cathode. In addition, however, water is also released at the anode, in particular in the anode offgas. If the system is designed with a recirculation circuit or an anode circuit which is closed in some other way, then the water which is created must be removed from the anode circuit from time to time. For this purpose, it is known for a separator module to be arranged in the anode circuit. The separator module comprises a sensor for detection of a filling level, a so- called level sensor. The separator module is in this case operated such that, when an upper switching point is exceeded, that is to say when a maximum filling level is exceeded, emptying of the separator module is initiated. Emptying is ended as soon as a lower switching point is undershot, that is to say a minimum filling level has been detected. In order to detect the two filling levels, the level sensor has at least two sensors or the like. For example, it is known for a float switch to be used as a level sensor, with two or more switching points. A float switch such as this is, however, very unreliable and/or can be operated only with a large amount of complexity, with adequate reliability.

It is known from DE 102 33 039 Al for only a lower switching point to be detected by a sensor, with a valve being opened again in order to empty the separator once a maximum time interval has elapsed after closing. The maximum time interval is in this case determined on the basis of statistical variables, such as the volume of a separator container, an inlet flow rate and an outlet flow rate.

Errors in the determination of the upper and/or the lower switching point can cause irreparable damage to the fuel cell system. For example, if the lower switching point is detected too late, then this can lead to a fuel gas, for example hydrogen, escaping into the environment. If the separator module is emptied too late, on the other hand, then this can lead to the liquid which has been created in the separator module running back into the anode circuit, and this can result in overheating of the system.

It is therefore known when using one or more sensors by means of which two or more switching points are detected for a plausibility check to be carried out such that a fault is signalled when a plurality of switching points are active at the same time.

It is also known for a cell voltage monitoring system or the like to be used to diagnose the system. Diagnostic hardware which is required for this purpose is, however, extremely complex and therefore generally costly.

The object of the invention is to provide a method and an apparatus by means of which faults in detection of a filling level in a separator module can be reliably identified.

This object is achieved by a method and an apparatus for diagnosis of a separator module in a fuel cell system, with the filling level of the separator module being detected by at least one sensor, the filling level additionally being predicted as a function of at least one current physical state variable of the fuel cell system, and the predicted value of the filling level being compared with the detected value of the filling level for a plausibility check.

In other words, the filling level is on the one hand detected directly by a sensor, and on the other hand is observed by a virtual sensor. Current system states and therefore dynamic influencing variables can be taken into account by prediction of the filling level as a function of current variables. For this purpose, the apparatus according to the invention has at least one sensor for detection of the filling level of the separator module, and at least one control and/or computation unit with means by which at least one current physical state variable of a system, in particular of a fuel cell system, can be detected, the filling level can be determined as a function of the at least one physical state variable, and the determined value can be compared with the detected value. The fuel cell system may in this case be mapped as a model in the control and/or computation unit. In other refinements, dependencies of physical state variables can be taken into account without an associated model. The state variables can in this case be detected by sensors which are already provided in the system, and can be made available via control inputs to the control and/or computation unit. In this case, the control inputs are means for detection of the state variables for the purposes of the invention.

In a development of the invention, in the event of a discrepancy between the detected value and the predicted value which exceeds a defined tolerance band, a maintenance action is initiated. The tolerance band may in this case be suitably defined. Furthermore, the tolerance band may be made variable, in which case, for example, great accuracy may be required, depending on specific state variables. By way of example, the apparatus is coupled to and/or designed with - signal and/or indication means, by which a fault or an irregularity can be indicated. In this case, different warning signals can be used so that, for example, just a visual signal is emitted in the event of minor discrepancies while, in contrast, an audible continuous signal is additionally emitted in the event of more major discrepancies, so that the need for immediate action is signalled to a user. In one development of the invention, the upper switching point is determined as a function of at least one state variable of the system, from a group comprising: the temperature of a coolant at a coolant inlet, the temperature of the coolant at a coolant outlet, the temperature of the fuel at an anode inlet, the temperature of the fuel in a fuel reservoir, the current drawn from a fuel cell stack, and the environmental temperature of the system. In this case, by way of example, the following dependencies of the filling level of a separator in an anode circuit can be taken into account: the dependency on the temperature difference between the temperature of a coolant at an outlet of a fuel cell stack and the temperature of the fuel in a fuel reservoir; the dependency on a current which is drawn from the fuel cell stack; the dependency on a system temperature change at a coolant inlet when the system is started; the dependency on temperature differences between an environmental temperature and system temperatures; the dependency on the temperature difference between the temperature at the coolant outlet and at an anode inlet.

Depending on the system design, a dependency of the filling level on further variables and/or on further relationships may be of interest.

In one refinement of the invention, the lower switching point is determined on the basis of a determination and/or a prediction of an amount of liquid which has flowed away. In this case, it is possible, instead of using a physical sensor for detection of the lower switching point, to determine or to observe this exclusively from other variables. In one exemplary embodiment, the lower switching point is determined, for example, as a function of a flow coefficient (Kv value) and/or as a function of the pressure ratio across a separator outlet flow.

In a further refinement of the invention, a prediction of the filling level is adapted to measured values. Particularly if the filling level is determined as a function of various influences, contradictory predictions may in some cases be made. In this case, it is possible to differently weight the relevance of individual relationships. The weighting can then be varied again as a function of state variables. If model- based observation of the filling level is used, then the theoretical model can be adapted to reality.

Further advantages of the invention will become evident from the following description of one exemplary embodiment of the invention, which is illustrated schematically in the drawings. The same reference symbols are used for identical or similar components in the drawings. All of the features and/or advantages which are evident from the claims, the description or the drawings, including design details, spatial arrangements and method steps, may be significant to the invention both in their own right and in widely different combinations. In the figures:

Figure 1 shows a block diagram of a fuel cell system according to the invention and

Figure 2 shows a schematic illustration of a separator module.

Figure 1 shows, schematically, a block diagram of a fuel cell system 1. The fuel cell system 1 comprises a fuel cell stack 10 which is formed from a plurality of fuel cells, which are electrically connected in series and/or in parallel. Anode sides of individual fuel cells of fuel cell stack 10 provide the anode side 103 of the fuel cell stack 10. In the same way, the cathode sides of the fuel cells provide the cathode side 102 of the fuel cell stack 10. Cathode and anode circuits 2, 3, which are illustrated in a simplified form, are arranged respectively on the cathode side 102 and on the anode side 103. Furthermore, a cooling circuit 4 is provided, with an inlet 41 and an outlet 42. The anode circuit 3, which is illustrated in a simplified form, comprises a fuel reservoir 30, with the fuel, for example hydrogen, being supplied via an inlet 31 to the fuel cell stack 10. An anode offgas is dissipated via an outlet 32 from the fuel cell stack 10. In the illustrated exemplary embodiment, a recirculation unit 33 is provided, by means of which at least some of the anode offgas can be fed back to the inlet 31 again. In other refinements, an anode circuit 3 can be closed at a different point, for example by feeding back at least some of the anode offgas to the fuel reservoir 30. A separator module 34 is provided in the anode circuit 3, by means of which water or some other condensate which is produced in the anode circuit 3 is removed from time to time.

Figure 2 shows, schematically, a corresponding separator module 34. In this case, the separator module 34 is arranged between the anode circuit 3 and a cathode off-gas line 2a, with the separator module 34 being emptied into the cathode off-gas line 2a. The separator module 34 comprises a separator container 340, a filter 342 and a valve 343. The separator module 34, in particular the valve 343, is operated or regulated by means of a control unit 6. The control unit 6 comprises an input 60 for a measurement signal from a filling level sensor 5, as well as a plurality of inputs 61, by means of which further state variables or system variables can be supplied to the control unit 6. The control unit 6 also comprises an outlet 62 for operating the separator valve 343. In the illustrated exemplary embodiment, separator water is collected in the separated container 340. The separator module 34 is in this case operated such that the filling level is regulated between a lower value F_min and an upper value F_max. When the filling level reaches the lower switching point F_min, then emptying of the separator container 340 is ended by closing the separator valve 343. The separator valve 343 is opened when the filling level reaches the upper switching point F__max.

In the illustrated exemplary embodiment, the filling level of the water which has been collected is on the one hand detected by means of the filling level sensor 5. The illustrated filling level sensor 5 is in this case arranged such that the lower switching point can be detected. On the other hand, the filling level is observed.

According to the invention, the control unit 6 determines the upper switching point or maximum filling level F_Max, from which emptying of the separator container 340 is intended to start, as a function of various state variables which are supplied to the control unit 6 at the control inputs 61. The state variables are, for example, the temperature T_KM_in of a coolant at the inlet 41 as shown in Figure 1, the temperature T_KM_out of the coolant at the outlet 42, the temperature of the fuel, in particular of the hydrogen, T_H2 _in at the inlet 31, the temperature T_Tank of the fuel at the fuel reservoir 30, the instantaneous current I_BZ drawn from the fuel cell stack 10, and/or the environmental temperature T_UM of the system.

By way of example, these variables can be used to increase the filling level in the separator module 34 as a function of the temperature difference T_KM_out - T_Tank between the temperature of the coolant at the outlet 42 and the temperature T_Tank in the fuel reservoir 30, thus making it possible to take account of mixing of the very cold or relatively cold hydrogen from the reservoir 30 with very moist anode offgas in the anode circuit 3. Furthermore, a relationship is possible between the filling level and a current I_BZ drawn from the fuel cell. Furthermore, the determined variables make it possible to take account of the filling level as a function of the rate of change of the system temperature d (T_KM_in) /dt on starting the system, taking account of the fact that water is stored in membranes in the fuel cell stack 10 when the temperature of the fuel cell stack 10 is relatively cold, and is emitted as a result of a temperature increase. Alternatively or additionally, it is also possible to take account of the relationship between the filling level and the temperature difference between the environmental temperature T_UM and various system temperatures. The temperature difference T_KM_out - T_H2_in between the temperature of the coolant at the outlet 41 and the temperature of the fuel at the inlet 31 can be taken into account as a further influence on the filling level.

As soon as the control unit 6 has observed that an upper switching point has been reached, the separator valve 343 can be opened, and the separator container 340 can be appropriately emptied.

In the illustrated exemplary embodiment, the minimum filling level F_min or lower switching point is detected by the filling level sensor 5. In addition, the reaching of the lower switching point is predicted by the control unit 6. In order to determine a decrease in the filling level and the reaching of the lower switching point, an outlet flow rate can be taken into account as a function of a flow factor (Kv value) , including characteristics of the valve 343 and of the lines and of the filter 342, and/or as a function of the pressure ratio, for example of the system pressure in the anode circuit 3 and/or in the separator container 340 in comparison to the environmental pressure.

The value, detected by the sensor 5, of the filling level is compared with the value, predicted by the control unit 6, of the filling level, for a plausibility check. If the two values differ from one another, then this indicates faulty operation of the separator module 34. This fault can be signalled by suitable means which are not illustrated. This makes it possible to avoid consequential damage to the fuel cell system by timely maintenance and/or replacement of individual components or of the separator module 34.

Possible faults in the separator module 34 may in this case result from various causes. If the illustrated sensor 5 for detection of the lower switching point F_min indicates that this switching point is being reached earlier than the observed reaching of the switching point, then this may on the one hand be caused by a fault in the sensor 5 itself, for example by mechanical sticking of a float body at the switching point. In addition, however, it is also possible for the separator valve 343 to be leaky, for example, because of mechanical sticking on the separator valve 343 or because of dirt deposits. Furthermore, the separator module 34 may have developed a leak elsewhere.

If, in contrast, the illustrated sensor 5 for detection of the lower switching point F_min indicates that this switching point is being reached later, than the observed reaching of the switching point, then this can likewise be caused by a fault in the sensor 5 itself. Furthermore, however, it is also possible for the separator valve 343 to have become blocked, for example because of dirt deposits.

Alternatively or additionally, the sensor 5 can be arranged such that an upper switching point F_max can be detected by the sensor. In this case, it is likewise possible to observe the switching point being reached excessively early and excessively late.

In another refinement of the invention, the filling level sensor is arranged in a range between the upper and the lower switching point. In this case, it is possible to predict not only a time at which the associated filling level will be exceeded but also a time at which the associated filling level will be undershot. An arrangement such as this is possible, for example, to detect not only leakage by means of the filling level being undershot too early, but also blocking of an outlet flow, as a result of the filling level being exceeded too early.

Claims

Daimler AG andFord Global Technologies, LLCPatent Claims

1. Method for diagnosis of a separator module (34) in a fuel cell system (1), with the filling level of the separator module (34) being detected by at least one sensor (5), with emptying of the separator module (34) being initiated when an upper switching point (F_max) is overshot, and with emptying of the separator module (34) being ended when a lower switching point (F_min) is undershot, characterized in that the filling level is predicted as a function of at least one current physical state variable of the fuel cell system (1), and the predicted value of the filling level is compared with the detected value of the filling level for a plausibility check.

2. Method according to Claim 1, characterized in that, in the event of a discrepancy between the detected value and the predicted value which exceeds a defined tolerance band, a maintenance action is initiated.

3. Method according to Claim 1 or 2, characterized in that a rise in the filling level and/or an upper switching point (F_max) is predicted as a function of at least one state variable of the fuel cell system (1) from a group comprising: the temperature (T KM_in) of a coolant at a coolant inlet (41), the temperature (T_KM_out) of the coolant at a coolant outlet (42), the temperature of the fuel (T_H2_in) at an anode inlet (31), the temperature (T_Tank) of the fuel in a fuel reservoir (30), the current (I_BZ) drawn from a fuel cell stack (10), and the environmental temperature (T_UM) of the system.

4. Method according to one of Claims 1, 2, or 3, characterized in that a fall in the filling level and/or a lower switching point (F_min) are/is predicted on the basis of a determination and/or a prediction of an amount of liquid which has flowed away.

5. Method according to one of Claims 1 to 4, characterized in that a prediction of the filling level is adapted to measured values .

6. Apparatus for diagnosis of a separator module (34) in a fuel cell system (1) comprising at least one sensor (5) for detection of the filling level of the separator module (34) and at least one control and/or computation unit (6) with means by which emptying of the separator module (34) can be initiated when an upper switching point (F_max) is overshot, and emptying of the separator module (34) can be ended when a lower switching point

(F_min) is undershot, characterized in that the control and/or computation unit (6) has means by which at least one current physical state variable of the fuel cell system (1) can be detected, the filling level can be predicted as a function of the at least one physical state variable, and the predicted value of the filling level can be compared with the detected value of the filling level, for a plausibility check.

7. Apparatus according to Claim 6, characterized in that the control and/or computation unit (6) has an output for operating a signal and/or indication means, such that a maintenance action can be initiated in the event of discrepancy between the detected value and the predicted value which exceeds a defined tolerance band.

8. Apparatus according to Claim 6 or 7, characterized in that the control and/or computation unit (6) has means by which an increase in the filling level and/or an upper switching point (F_max) can be predicted as a function of at least one state variable of the system, from a group comprising: the temperature (T_KM_in) of a coolant at a coolant inlet (41), the temperature (T_KM_out) of the coolant at a coolant outlet (42), the temperature of the fuel (T_H2_in) at an anode inlet (31) , the temperature (T_Tank) of the fuel in a fuel reservoir (30), the current (I_BZ) drawn from a fuel cell stack (10), and the environmental temperature (T_UM) of the system.

9. Apparatus according to one of Claims 6 to 8, characterized in that the control and/or computation unit (6) has means by which a fall in the filling level and/or a lower switching point (F_min) can be predicted on the basis of a determination and/or a prediction of an amount of liquid which has flowed away.

10. Apparatus according to one of Claims 6 to 9, characterized in that a prediction of the filling level can be adapted to measured values.