Method for determining the compressive tensile force acting on a fuel cell stack

Abstract

The present invention relates to a method for determining the compressive tensile force acting on a fuel cell stack due to at least one tensioning element. Thereby, the compressive tensile force is the overall tensile force compressing the fuel cell stack. This is determined according to the invention by means of acoustic measurements on vibratable sections of the tensioning elements. The subject matter of the invention also includes a data processing program for carrying out the method according to the invention along with the use of a smartphone for carrying out the method according to the invention.

Claims

1. A method for determining a compressive tensile force acting on a fuel cell stack due to at least one tensioning element, wherein the fuel cell stack has a plurality of fuel cells arranged in a stacking direction between two end plates and at least one tensioning element clamped between the end plates in the stacking direction, the method comprising: exciting a vibration of a vibratable section of the at least one tensioning element; detecting an acoustic signal emitted from the vibratable section of the at least one tensioning element; determining a fundamental frequency of the detected acoustic signal; and determining a tensile force acting on the vibratable section of the at least one tensioning element on the basis of the determined fundamental frequency, the length of the vibratable section, and the linear mass distribution of the at least one tensioning element.

2. The method according to claim 1, wherein the vibratable section of the at least one tensioning element extends from a first end plate to an opposite second end plate of the fuel cell stack.

3. The method according to claim 1, wherein the at least one tensioning element extends in a cross-section in the stacking direction essentially around a circumference of the fuel cell stack and has two vibratable sections along opposite sides of the fuel cell stack.

4. The method according to claim 1, wherein the fuel cell stack has a plurality of tensioning elements clamped in the stacking direction between the end plates, the method further comprising: determining a tensile force acting on the vibratable section of each of the tensioning elements on the basis of a respective determined fundamental frequency, a respective length of the vibratable section, and a respective linear mass distribution of the tensioning element; and determining the compressive tensile force acting on the fuel cell stack as the sum of the tensile forces determined for each vibratable section or determining the compressive tensile force distribution acting on the fuel cell stack on the basis of the tensile forces determined for each vibratable section.

5. The method according to claim 1, wherein detecting an acoustic signal is carried out with a sound transducer.

6. The method according to claim 1, wherein the linear mass distribution of the at least one tensioning element is determined on the basis of the density of the at least one tensioning element and the cross-section of the at least one tensioning element.

7. A computer program which, after being loaded into a memory of a data processing device, enables the data processing device to carry out a method for determining a compressive tensile force acting on a fuel cell stack, the method comprising: exciting a vibration of a vibratable section of the at least one tensioning element; detecting an acoustic signal emitted from the vibratable section of the at least one tensioning element; determining a fundamental frequency of the detected acoustic signal; and determining a tensile force acting on the vibratable section of the at least one tensioning element on the basis of the determined fundamental frequency, the length of the vibratable section, and the linear mass distribution of the at least one tensioning element.

8. A data processing device comprising a storage medium on which a program is stored, which enables the data processing device to carry out a method for determining a compressive tensile force acting on a fuel cell stack, the method comprising: exciting a vibration of a vibratable section of the at least one tensioning element; detecting an acoustic signal emitted from the vibratable section of the at least one tensioning element; determining a fundamental frequency of the detected acoustic signal; and determining a tensile force acting on the vibratable section of the at least one tensioning element on the basis of the determined fundamental frequency, the length of the vibratable section, and the linear mass distribution of the at least one tensioning element.

9. The data processing device according to claim 8, wherein the data processing device is a smartphone.

10. The method of claim 1 wherein the detecting an acoustic signal is performed by a smartphone.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) Embodiments of the invention are explained below in reference to the respective drawings. The following is shown:

(2) FIG. 1 is a schematic representation of a fuel cell system according to the prior art,

(3) FIG. 2 is a fuel cell stack with a plurality of tensioning elements compressing the stack;

(4) FIG. 3 illustrates the dependence of the fundamental frequency of a tensioning element on its length;

(5) FIG. 4 illustrates the dependence of the fundamental frequency of a tensioning element on the tensile force acting on it; and

(6) FIG. 5 is a flow chart of a method.

DETAILED DESCRIPTION

(7) FIG. 1 shows a fuel cell system designated in total with 100, in accordance with the prior art. The fuel cell system 100 is part of a vehicle (not shown), in particular an electric vehicle, which has an electric traction motor, which is supplied with electrical energy by the fuel cell system 100.

(8) The fuel cell system 100 comprises as core component a fuel cell stack 10, which comprises a plurality of individual cells 11, which are arranged in the form of a stack and which are formed by alternately stacked membrane electrode assemblies (MEAS) 14 and bipolar plates 15 (see detailed view). Each individual cell 11 thus respectively comprises an MEA 14 with an ion-conductive polymer electrolyte membrane not shown in more detail here and catalytic electrodes arranged thereon on both sides. These electrodes catalyze the respective partial reaction of the fuel conversion. The anode and cathode electrodes are designed as coating on the membrane and comprise a catalytic material, such as platinum, which is provided on an electrically conductive substrate material, with a large specific surface, such as a carbon-based material.

(9) As shown in the detailed view of FIG. 1, an anode chamber 12 is formed between a bipolar plate 15 and the anode and the cathode chamber 13 is formed between the cathode and the next bipolar plate 15. The bipolar plates 15 serve to supply the operating media in the anode and cathode chambers 12, 13 and further establishes the electrical connection between the individual fuel cells 11. Optionally, gas diffusion layers can be arranged between the membrane electrode assemblies 14 and the bipolar plates 15.

(10) In order to supply the fuel cell stack 10 with the operating medium, the fuel cell system 100 has an anode supply 20, on the one hand, and a cathode supply 30, on the other hand.

(11) The anode supply 20 of the fuel cell system 100 shown in FIG. 1 comprises an anode supply path 21, which serves to supply an anode operating medium (the fuel), for example, hydrogen, to the anode chambers 12 of the fuel cell stack 10. For this purpose, the anode supply path 21 connects a fuel storage tank 23 with an anode inlet of the fuel cell stack 10. The feed pressure of the anode operating medium into the anode chambers 12 of the fuel cell stack 10 is adjusted via a metering valve 27.1. The anode supply 20 also comprises an anode exhaust path 22 which discharges the anode exhaust gas from the anode chambers 12 via an anode outlet of the fuel cell stack 10.

(12) In addition, the anode supply 20 of the fuel cell system 100 shown in FIG. 1 has a recirculation line 24, which connects the anode exhaust gas path 22 with the anode supply path 21. The recirculation of fuel is a common practice in order to return the overstoichiometrically used fuel to the fuel cell stack 10. A recirculation conveying device 25, such as a recirculation fan, along with a flap valve 27.2 are arranged in the recirculation line 24.

(13) In addition, a water separator 26 is installed in the anode supply 22 of the fuel cell system, in order to discharge the product water resulting from the fuel cell reaction. A drain of the water separator can be connected to the cathode exhaust line 32, a water tank or an exhaust system.

(14) The cathode supply 30 of the fuel cell system 100 shown in FIG. 1 comprises a cathode supply path 31, which supplies an oxygen-containing cathode operating medium, in particular air taken in from the environment, to the cathode chambers 13 of the fuel cell stack 10. The cathode supply 30 also comprises a cathode exhaust path 32, which discharges the cathode exhaust gas (in particular the exhaust air) from the cathode chambers 13 of the fuel cell stack 10 and supplies it, if appropriate, to an exhaust system (not shown).

(15) For conveying and compacting the cathode operating medium, a compressor 33 is arranged in the cathode supply path 31. In the embodiment shown, the compressor 33 is designed as a compressor 33, which is mainly driven by an electric motor 34 equipped with appropriate power electronics 35.

(16) The fuel cell system 100 shown in FIG. 1 also has a humidifier module 39 arranged upstream of the compressor 33 in the cathode supply line 31. The humidifier module 39 is arranged in the cathode supply path 31 so that, on the one hand, the cathode operating gas can flow through it. On the other hand, it is arranged in the cathode exhaust path 32 such that the cathode exhaust gas can flow through it. A humidifier 39 typically comprises a plurality of water vapor permeable membranes, which are designed to be either flat or in the form of hollow fibers. In this case, the comparatively dry cathode operating gas (air) flows over one side of the membranes and the comparatively moist cathode exhaust gas (exhaust gas) flows over the other side. Driven by the higher partial pressure of water vapor in the cathode exhaust gas, water vapor is transferred across the membranes into the cathode operating gas, which is humidified in this manner.

(17) The fuel cell system 100 also has a humidifier bypass 37 connecting the cathode supply line upstream and downstream of the humidifier 39 to each other, with a flap valve arranged therein as bypass control means 38. Furthermore, flap valves 27.3 and 27.4 are arranged upstream of fuel cell stack 10 in the anode supply line 21 and downstream of the fuel cell stack 10 in the anode exhaust line 22.

(18) Various other details of anode and cathode supply 20, 30 are not shown in FIG. 1 for reasons of clarity. For example, the anode exhaust line 22 can open into the cathode exhaust line 32, such that the anode exhaust and the cathode exhaust are discharged via a common exhaust system.

(19) FIG. 2 shows a detailed representation of the fuel cell stack 10 shown in FIG. 1. The fuel cell stack has a plurality of fuel cells stacked flat on top of each other in stack direction S. In the stacking direction, the fuel cell stack 10 is bounded by a first end plate 55 and an opposite second end plate 56. In a first direction transverse to stack direction S, the fuel cell stack 10 is bounded by side panels 57. In a second direction transverse to the first direction and transverse to stack direction S, the fuel cell stack 10 is bounded by side panels 58. The fuel cell stack 10 can further comprise a plurality of mounting elements for fastening the fuel cell stack 10 to a supporting structure, for example, a car body.

(20) The fuel cell stack 10 shown in FIG. 2 is compressed over a plurality of a total of ten tensioning elements 50. Thereby, each tensioning element 50 is fixed to the first end plate 55 via a tensioning device (not shown) and to the second end plate via an additional tensioning device 54 and runs parallel to a side panel 58 of the fuel cell stack 10. Thereby, each of the tensioning elements 50 has a spacing to the side panel 58. FIG. 2 shows in detail five tensioning elements 50.1, 50.2, 50.3, 50.4 and 50.5, which run parallel to an upper side panel 58. In addition, the fuel cell stack has five additional tensioning elements 50.6, 50.7, 50.8, 50.9 and 50.0 (not shown), which run parallel to a lower side panel and of which only the tensioning devices 54 fixed to the second end plate 56 are shown in FIG. 2. The tensioning elements 50 are made of nylon and have a linear mass distribution of 0.152 kg/m.

(21) Each of the tensioning devices 54 is arranged on surfaces of the first end plate 55 facing outwards in stacking direction S and the second end plate 56. Thus, the tensioning elements 50 fixed to the tensioning devices 54 initially run along these surfaces, against which they abut, over an edge of the end plates 55, 56 and a narrow side surface of the end plates 55, 56. The end plates 55, 56 protrude approximately 2 mm beyond the side panels 58 in the second direction.

(22) As a result, each tensioning element 50 has a first fixed end 52 at an inner edge of the first end plate 55 and a second fixed end 53 at an inner edge of the second end plate 56. FIG. 2 shows an example of the first fixed end 52.1 of a first tensioning element 50.1 and the second fixed end 53.2 of a second tensioning element 50.2. Each tensioning element 50 has a vibratable section 51 between its first fixed end 52 and its second fixed end 53. FIG. 2 shows an example of the vibratable section 51.2 of a second tensioning element 50.2 and the vibratable section 51.3 of a third tensioning element 50.3. Each of these vibratable sections 51 has a length of 383 mm.

(23) In some embodiments, each vibratable section 51 of each tensioning element 50 is deflected manually in the second direction one after the other and thus excited to a transverse vibration. Since the end plates 55, 56 protrude approximately 2 mm beyond the side panels 58 in the second direction, the vibratable sections 51 of the clamped tensioning elements 50 are not in contact with the side panels 58 and can vibrate freely between the fixed ends 52, 53 with amplitudes of up to 2 mm.

(24) For each vibrating vibratable section 51, an acoustic signal is initially detected, for example, by means of the microphone of a smartphone. In a next step, a frequency spectrum is determined for each of the recorded acoustic signals by means of a suitable program and from this, or directly for each recorded acoustic signal, a fundamental frequency of the recorded acoustic signal is determined. Programs for determining the frequency spectrum and/or the fundamental frequency are freely available on the market.

(25) The following table shows for each vibratable section of each of the ten tensioning elements 50 the fundamental frequencies f.sub.1 determined in accordance with the description herein and furthermore the tensile force F.sub.zug acting on the respective vibratable section 51, which was determined from the frequency f.sub.1 of the respective vibratable section 51 according to the formula mentioned in the description:

(26) TABLE-US-00001 50.1 50.2 50.3 50.4 50.5 50.6 50.7 50.8 50.9 50.0 f.sub.1 [Hz] 188 167 173 179 173 173 170 170 179 182 F.sub.zug [kN] 3.1 2.5 2.7 2.9 2.7 2.7 2.6 2.6 2.9 2.9

(27) Under the assumption that the fuel cell stack 10 as a whole is in equilibrium of forces, the sum of the tensile forces determined for all vibratable sections 51 corresponds to the total compressive tensile force of 27.6 kN acting on the fuel cell stack 10.

(28) The fuel cell stack 10 was originally compressed with a defined pressure force of 28.5 kN and was fixed in the compressed form by means of tensioning elements 50. The method described herein thus produces results of the right order of magnitude and also indicates a decreasing compression of the fuel cell stack 10.

(29) The length deviation l of approximately 1 mm shown in FIG. 3 corresponds to a change in the fundamental frequency f of approximately 0.5 Hz. Thus, the method described herein is relatively invariant with respect to smaller measuring errors of the lengths of the vibratable sections 51. The change of only 15 N in the tensile force F acting on a vibratable section 51, shown in FIG. 4, corresponds to a change of its fundamental frequency f by 0.5 Hz as well. Thus, the method described herein is sufficiently accurate with respect to the measurement of force.

(30) In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled.