WO2009024204A2

WO2009024204A2 - Method and apparatus for determining a switching point for filling level regulation

Info

Publication number: WO2009024204A2
Application number: PCT/EP2008/005138
Authority: WO
Inventors: Armin Frank; Cosimo S. Mazzota; Helmut Müller; Uwe Pasera
Original assignee: Daimler Ag; Ford Global Technologies, Llc
Priority date: 2007-08-22
Filing date: 2008-06-25
Publication date: 2009-02-26
Also published as: DE102007039557A1; WO2009024204A3

Abstract

The invention relates to a method and an apparatus for determining a filling level for filling level regulation of a liquid which is present in a system, with emptying being initiated when an upper switching point (F_max) is exceeded, with emptying being ended when a lower switching point (F_min) is undershot, and with at least one switching point (F_min, F_max) being predicted as a function of at least one current physical state variable of the system. The invention relates in particular to a method for determining a switching point for filling level regulation for a separator in a fuel cell system.

Description

Method and apparatus for determining a switching point for filling level regulation

The invention relates to a method and an apparatus for determining a filling level for filling level regulation. In particular, the invention relates to a method and an apparatus for determining a filling level for filling level regulation for a separator 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. At the moment, proton exchange membrane fuel cells (PEM-FC) are known in particular for the vehicle industry, with hydrogen being used as the fuel. However, it is also known for methane, methanol or glucose solution to be used as a 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 off gases from the fuel cell or the fuel cell stack emerge at an outlet and comprise unused hydrogen and water, inter alia, when using hydrogen. The unused fuel can be made available at the inlet again via a recirculation circuit.

So-called product water is released at the cathode during operation of a fuel cell system, in particular a PEM fuel cell system. In addition, however, water is also released at the anode, in particular in the anode off gas. If the system has a recirculation circuit or an anode circuit which is closed in some other way, then the water which is present 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 has a sensor for detection of a filling level, a so-called level sensor. The separator module is in this case operated such that emptying of the separator module is initiated when an upper switching point is exceeded, that is to say when a maximum filling level is exceeded. 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 comprises at least two sensors or the like. For example, it is therefore 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, often unreliable and/or can be operated with adequate reliability only with a high degree of complexity.

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 since it was closed. The maximum time interval is in this case determined on the basis of static variables, such as the volume of a separator container, an inlet flow rate and an outlet flow rate.

One object of the invention is to provide a method and an apparatus by means of which at least one switching point for filling level regulation can be detected reliably in changing conditions. This object is achieved by a method and an apparatus for determining a filling level for filling level regulation of a liquid which is present in a system, with emptying being initiated when an upper switching point or maximum filling level of the liquid is exceeded, and with emptying being ended when a lower switching point or minimum filling level of the liquid is undershot, and with at least one switching point being predicted as a function of at least one current physical state variable of the system. In other words, the switching point is not detected directly but is observed. In this case, it is possible to dispense with a measurement element such as a measurement sensor or the like for direct detection of this switching point. Dispensing with a corresponding component leads to system simplification by component reduction, to cost saving, to a reduction in the physical space required, to weight saving and/or to reduced wiring complexity. In this case, in principle, a filling level can be predicted for any system in which at least the major influencing variables relating to a filling level are known, and/or the filling level can be observed. Prediction of the switching point as a function of current variables allows current influencing variables or states of the system to be taken into account.

In this case, it is also feasible to couple a prediction to a measurement method, for example by using a Kalman filter process or the like, in order in this way to enhance the quality of the prediction and/or of the measurement.

For this purpose, an apparatus according to the invention has 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, and a switching point can be determined as a function of the at least one physical state variable. In this case, the system may be in the form of a model in the control and/or computation unit. In other embodiments, relationships between physical state variables can be taken into account without an associated model. The state variables can in this case be detected by means of sensors which are already provided in the system, and can be made available to the control and/or computation unit via control inputs. In this case, the control inputs are means for detecting the state variables for the purposes of the invention.

In one development of the invention, the amount of liquid which is present and/or will be present in a separator in an anode circuit of a fuel cell system is determined on the basis of at least one physical, dynamic state variable of the fuel cell system. In this case, it is possible to provide a separator module for a corresponding fuel cell system, with little physical space and high reliability.

In one development of the invention, an upper switching point is determined as a function of at least one state variable of the system, from a group comprising: a temperature of a coolant at a coolant inlet, a temperature of the coolant at a coolant outlet, a temperature of the fuel at an anode inlet, a temperature of the fuel in a fuel reservoir, the current which is drawn from a fuel cell stack, and an environmental temperature of the system. In this case, for example, it is possible to take account of the following relationships relating to the filling level of a separator in an anode circuit: the dependency on a temperature difference between a temperature of a coolant at an outlet of a fuel cell stack and a temperature of the fuel in a fuel reservoir; the dependency on the current drawn from the fuel cell stack; the dependency on the system temperature change at a coolant inlet on starting the system; the dependency on temperature differences between an environmental temperature and system temperatures; the dependency on the temperature difference between a temperature at the coolant outlet and at an anode inlet. Depending on the system design, the dependency of the filling level on further variables and/or on further relationships may be of interest.

In one refinement of the invention, a 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, instead of using a physical sensor to detect the lower switching point, it is possible to determine or to observe this switching point exclusively from other variables. In one exemplary embodiment, the lower switching point is, for example, determined as a function of a flow coefficient (Kv value) and/or as a function of the pressure ratio across a separator outlet flow.

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 become evident from the claims, the description or the drawings, including design details, physical arrangements and method steps, may be significant to the invention both on their own and in widely differing combinations. In this case, 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 connected electrically in series and/or in parallel. Anode sides of individual fuel cells in the 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, respectively, 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 to the fuel cell stack 10 via an inlet 31. An anode off gas is carried away out of the fuel cell stack 10 via an outlet 32. In the illustrated exemplary embodiment, a recirculation unit 33 is provided, by means of which at least a portion of the anode off gas can be fed back to the inlet 31. In other refinements, an anode circuit 3 can be connected at some other point, for example by feeding back at least some of the anode off gas into the fuel reservoir 30. A separator module 34 is provided in the anode circuit 3 and is used to separate out water or some other condensate which occurs in the anode circuit 3, from time to time.

Figure 2 shows, schematically, a co rresponding 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, water which has been separated accumulates in the separator container 340. The separator module 34 is in this case operated such that the filling level is regulated at a level between a lower value F_min and an upper value F_max. When the filling level reaches the lower switching point F_min, then the process of emptying 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 that has been separated is detected on the one hand 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 F_min 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, 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. By way of example, the state variables are a temperature T_KM_in of a coolant at the inlet 41 as shown in Figure 1, a 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 in the fuel reservoir 30, an instantaneous current I_BZ drawn from the fuel cell stack 10, and/or the environmental temperature T_UM of the system 1.

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, with this making it possible to take account of mixing of very cold or relatively cold hydrogen from the reservoir 30 with the very moist anode off gas in the anode circuit 3. Furthermore, the filling level may depend on the current I_BZ drawn from the fuel cell. Furthermore, the determined variables can be used 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 system start, taking account of the fact that, when the temperatures in the fuel cell stack 10 are relatively low, water is stored in membranes in the fuel cell stack 10, and is emitted when the temperature increases. Alternatively or additionally, it is also possible to take account of the dependency of the filling level on 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 a maximum filling level F_max or upper switching point has been reached, the separator valve 343 can be opened, and the separator container 340 can be correspondingly 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 another embodiment, a lower switching point is also observed by the control unit 6 and/or by an additional control unit. In order to determine a decrease in the filling level and therefore a 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, of the lines and of the filter 342, and/or as a function of a pressure ratio, for example a system pressure in the anode circuit 3 and/or in the separator container 340, in comparison to the environmental pressure.

An apparatus according to the invention for determining a filling level can therefore also be referred to as a "virtual" sensor or monitor, with the variables to be detected not being measured directly, in which case, instead, they can be detected on the basis of variables which are already available and/or can be measured easily, such as a temperature.

Claims

Patent Claims

1. Method for determining a filling level for filling level regulation of a liquid which is present in a system (1), with emptying being initiated when an upper switching point (F_max) is exceeded, and with emptying being ended when a lower switching point (F_min) is undershot, characterized in that at least one switching point (F_min, F_max) is predicted as a function of at least one current physical state variable of the system.

2. Method according to Claim 1, characterized in that the amount of liquid which is present and/or will be present in a separator module (34) of an anode circuit (3) of a fuel cell system (1) is determined on the basis of at least one physical state variable of the fuel cell system (1) .

3. Method according to Claim 2, characterized in that the upper switching point (F_max) is determined 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), a temperature (T_KM_out) of the coolant at a coolant outlet (42), a temperature of the fuel (T_H2_in) at an anode inlet (31), a temperature (T_Tank) of the fuel in a fuel reservoir (30), the current (I_BZ) which is drawn from a fuel cell stack (10) , and the environmental temperature (T_UM) of the system (1) .

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

5. Apparatus for determining a filling level for filling level regulation of a liquid which is present in a system (1), comprising at least one control and/or computation unit (6) by means of which emptying can be initiated when an upper switching point (F_max) is exceeded, and emptying can be ended when a lower switching point (F_min) is undershot, characterized in that the control and/or computation unit (6) has means using which at least one current physical state variable of the system can be detected and at least one switching point (F_min, F_max) can be determined as a function of the at least one physical state variable.

6. Apparatus according to Claim 5, characterized in that the control and/or computation unit (6) has at least one input (61) for a physical variable of a fuel cell system (1), such that the amount of liquid which is present and/or will be present in a separator module (34) of an anode circuit (3) of the fuel cell system (1) can be predicted by means of the control and/or computation unit (6) .

7. Apparatus according to Claim 6, characterized in that the control and/or computation unit (6) has means using which the upper switching point (F_max) can be determined 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), a temperature of the fuel (T_H2_in) at an anode inlet (31), a temperature (T_Tank) of the fuel in a fuel reservoir (30), the current (I_BZ) which is drawn from a fuel cell stack (10), and the environmental temperature (T_UM) of the system.

8. Apparatus according to one of Claims 5 to 7, characterized in that the control and/or computation unit (6) has means using which the lower switching point (F_min) can be determined on the basis of a determination and/or a prediction of an amount of liquid which has flowed away.