Method for operating and designing a fuel cell system

Abstract

The present invention relates to a method for operating a fuel cell system having fuel cell units, in which a cathode gas such as air is supplied to the fuel cell on the cathode input side by means of an electrically driven gas supply device (30), the gas supply device being embodied as a fluid machine, the operating range of which can be depicted in a family of characteristic curves having a surge boundary and a choke boundary. In order to extend the service life of the fuel cell system, during operation of the electrically driven gas supply device (30), in a defined operating range of the electrically driven gas supply device (30), pumping events that are not desired per se are intentionally permitted away from the surge boundary.

Description

Field of the Invention

[0001] The present invention relates to a method for operating a fuel cell system having fuel cell units, in which a cathode gas, such as air, is supplied to the fuel cell units on the cathode input side by means of an electrically driven gas supply device, the gas supply device being embodied as a fluid machine, the operating range of which can be shown in a characteristic curve family having a surge boundary and a choke boundary. Furthermore, the present invention relates to such a fuel cell system. Furthermore, the present invention relates to a method for designing such a fuel cell system. Background Art

[0002] From German patent document DE 10 2009 029 837 B4, a method for operating a fuel cell system and a fuel cell system are known, which include: a fuel cell stack having an anode inlet and a cathode inlet; an air compression device in fluid communication with the cathode inlet; at least one sensor, which is adjusted to measure a pumping indicator for a starting pumping state, from which it is known that the starting pumping state occurs before a surge event in the pumping compression device, wherein the starting pumping state is detected thereby: monitoring the mass flow rate and / or the outlet pressure of the air compression device for characteristic fluctuations or vibrations, wherein the occurrence of a surge event during operation of the fuel cell system is to be countered, and wherein, taking into account aging-related effects such as wear over the service life of the air compression device, from which it is known that they affect the arrangement of the characteristic curve family of the surge boundary. Summary of the Invention

[0003] The object of the present invention is to extend the service life of a fuel cell system having fuel cell units, in which a cathode gas, such as air, is supplied to the fuel cell units on the cathode input side by means of an electrically driven gas supply device, the gas supply device being embodied as a fluid machine, the operating range of which can be shown in a characteristic curve family having a surge boundary and a choke boundary.

[0004] In a method for operating a fuel cell system having a fuel cell unit, a cathode gas, such as air, is supplied to the fuel cell unit at the cathode input side by means of an electrically driven gas supply device, which is embodied as a fluid machine, the operating range of which can be represented in a family of characteristic curves having a surge boundary and a choke boundary. The problem is solved in that, in a defined operating range of the electrically driven gas supply device, on the side of the surge boundary, a surge event, which is not desired per se during operation of the electrically driven gas supply device, is deliberately permitted. For example, the general view "surge events during operation of the fuel cell system are to be resisted", as commended at the beginning, is contradicted here. In tests and studies carried out within the scope of the present invention, it has been found that, in a defined operating range of the electrically driven gas supply device, a surge event that is not desired per se does not necessarily lead to damage to the fuel cell system, in particular to the electrically driven gas supply device. By this deliberate permission of surge events, the idling operation or no-load operation of the fuel cell system can be advantageously extended such that the gas supply device exactly conveys the amount of cathode gas, in particular air, required for the electrochemical reaction in the fuel cell unit.

[0005] Thereby, undesired complete drying caused by an over-supplied fuel cell system air can be prevented again. This in turn delays the aging of the fuel cell unit.

[0006] A preferred embodiment of the method is characterized in that, in the lower operating range of the electrically driven gas supply device, on the side of the surge boundary, a surge operation of the electrically driven gas supply device that is not desired per se is deliberately permitted. The term "lower operating range" relates to the family of characteristic curves of the electrically driven gas supply device. The family of characteristic curves relates, for example, to a Cartesian coordinate diagram in which the mass flow through the electrically driven gas supply device is plotted on the x-axis in a suitable unit of measurement. The pressure ratio, which is generated by the electrically driven gas supply device during operation of the fuel cell system, is plotted on the y-axis of the family of characteristic curves. In the lower region of the family of characteristic curves, the pressure ratio and the volume flow are relatively small.

[0007] Another preferred embodiment of the method is characterized in that, in the case where the pressure ratio is less than a critical pressure ratio, on the side of the surge boundary, a surge event, which is not desired per se during operation of the electrically driven gas supply device, is deliberately permitted. The pressure ratio can be sensed relatively simply with a pressure sensor that may already be present. Thus, a measure that can be simply implemented in terms of regulation technology is provided, by means of which the service life of the fuel cell system can be effectively extended.

[0008] Another preferred embodiment of the method is characterized in that, when the pressure ratio is greater than the critical pressure ratio, surging events that are not desired during the operation of the electrically driven gas supply device are not allowed on the surging boundary side. When the critical pressure ratio is exceeded during the operation of the fuel cell system, conventional measures can be used to avoid the undesired surging events in such cases. For this purpose, for example, the rotational speed of the electrically driven gas supply device can be reduced or a bypass can be opened.

[0009] Another preferred embodiment of the method is characterized in that, when the pressure ratio is greater than the critical pressure ratio, surging events that are not desired during the operation of the electrically driven gas supply device are sensed, especially acoustically. For the purpose of sensing, for example, solid acoustic sensors can be used. For acoustic measurement, microphones can be used. Alternatively or additionally, the current of the electric drive device of the electrically driven gas supply device can be measured.

[0010] Another preferred embodiment of the method is characterized in that the critical pressure ratio is between 1 and 2. In the tests and studies carried out within the scope of the present invention, a critical pressure ratio value of 1.5 has proven to be advantageous.

[0011] In a fuel cell system having a fuel cell unit, a cathode gas, such as air, is supplied to the fuel cell unit on the cathode input side by means of an electrically driven gas supply device, and the gas supply device is implemented as a fluid machine, the working area of which can be shown in a characteristic curve family having a surging boundary and a choking boundary. The above task can be solved alternatively or additionally by designing the axial bearing system of the electrically driven gas supply device to be sufficiently robust with respect to the surging events intentionally allowed in the electrically driven gas supply device according to the foregoing method. Advantageously, the axial bearing system includes a dynamic air bearing support. The dynamic air bearing support includes at least one air bearing, also known as a thin film bearing, by means of which the electric motor-driven device of the gas supply device is axially supported. Therefore, advantageously, the sufficiently robust design particularly relates to the axial bearing system because strong axial force fluctuations generated in the electrically driven gas supply device during the operation of the fuel cell system occur during surging operation when surging events are intentionally allowed. In addition, during surging operation, it should be particularly noted that the possible acoustic effects do not have an adverse impact. This particularly means that the acoustic effects are not allowed to be audible to the vehicle user. For this purpose, the operation of the fuel cell system, especially the electrically driven gas supply device, can be monitored perceptibly, especially acoustically. For this purpose, acceleration sensors can be used. With this method, surging operation in the upper characteristic curve family region can also be safely detected and avoided.

[0012] In a method for designing the aforementioned fuel cell system, the above task is alternatively or additionally solved by: sensing and storing on a test bench during the test bench operation of the fuel cell system: when a surge event occurs during the operation of the electrically driven gas supply device. Sensing and storing the sensed and stored values in a suitable controller of the test bench, for example, using the pressure ratio, rotational speed, and mass flow provided by the electrically driven gas supply device. During the operation of the fuel cell system, these values can be used to identify and evaluate surge events. The costly sensing devices themselves on the fuel cell system can be omitted at low cost.

[0013] A preferred embodiment of the method is characterized in that, on a test bench during the test bench operation of the fuel cell system, acoustically sensing: when a surge event occurs during the operation of the electrically driven gas supply device. Then, the surge event can be stored together with the measured pressure ratio, mass flow, rotational speed, etc.

[0014] Optionally, the invention also relates to a test bench on which a method for designing a fuel cell system is implemented. For example, the test bench is equipped with at least one acoustic measuring device to sense surge events during the operation of the fuel cell system. Description of the Drawings

[0015] Further advantages, features, and details of the invention result from the following description, in which different embodiments are explained in detail with reference to the drawings. The drawings show:

[0016] Figure 1 A compression device with a housing on a test bench shown in a side view, which is only schematically represented;

[0017] Figure 2 A Cartesian coordinate graph is shown in which a family of characteristic curves of the gas supply device of the fuel cell system is shown;

[0018] Figure 3 A schematic diagram of a fuel cell system with a gas supply device is shown; and

[0019] Figure 4 A flowchart for explaining the claimed method is shown. Detailed Description of the Invention

[0020] In Figure 3 a fuel cell system 1 is schematically shown. The fuel cell system itself is known, for example, from German patent application document DE 10 2012 224 052 A1. The fuel cell system 1 includes a fuel cell unit 3, which is only represented by a dashed rectangular box. The fuel cell unit 3 includes at least one fuel cell stack 2, which is alternatively shown by a valve symbol.

[0021] The air mass flow is indicated by arrow 4 and is supplied to the fuel cell unit 3 by an air supply device 5 embodied as an air compression device. The compressed air mass flow 6 is indicated by arrow 6, and a cooling air mass flow 7 branches off from the compressed air mass flow. The cooling air mass flow 7 is also only indicated by an arrow and is part of a cooling air path 19 through which cooling air is supplied to the air compression device 5 via a cooling air inlet 23.

[0022] The cooling air supplied via the cooling air path 19 is used, for example, to cool an air bearing by means of which the shaft of the air compression device 5 is rotatably supported. The cooling air mass flow 7 is a loss part of the compressed air mass flow 6 because the branched-off cooling air mass flow 7 can no longer be used in the fuel cell stack 2 of the fuel cell unit 3.

[0023] Since the cooling air mass flow 7 is provided for internal cooling by the air compression device 5, energy, in particular electrical energy, is required to generate this cooling air mass flow. This energy has an adverse effect on the overall efficiency of the electric drive of a motor vehicle driven by the fuel cell system 1.

[0024] The remaining air mass flow 6 is supplied to the fuel cell unit 3 via an air supply line 8. The fuel cell unit 3 is a primary cell unit that converts the chemical reaction energy of fuel and an oxidant supplied via a fuel supply line (not shown) into electrical energy.

[0025] The oxidant is the air supplied to the fuel cell unit 3 via the air supply line 8. Preferably, the fuel can be hydrogen or methane or methanol. Accordingly, water vapor and carbon dioxide are produced as exhaust gases. The exhaust gases are discharged via an exhaust line 9 in the form of an exhaust gas mass flow 10, as indicated by arrow 10.

[0026] The exhaust gas mass flow 10 is discharged to an exhaust gas outlet 12 via an exhaust gas turbine device 11, which is indicated by an arrow. The air compression device 5 is arranged in the air supply line 8. The exhaust gas turbine device 11 is arranged in the exhaust line 9. The air compression device 5 and the exhaust gas turbine device 11 are mechanically connected by a shaft.

[0027] This shaft can be electrically driven by an electric motor 14. The exhaust gas turbine device 11 is used to assist the electric motor 14 when driving the air compression device 5. The air compression device 5, the exhaust gas turbine device 11, the shaft, and the electric motor 14 together form a turbo-compression device 15, which is also referred to as a turbine.

[0028] Furthermore, the fuel cell system 1 includes a bypass line 13 in which a bypass valve 16 is arranged. Via the bypass line 13 with the bypass valve 16, a bypass air mass flow 17 can be discharged around the stack 2 of the fuel cell unit 3 into the exhaust gas line 9 to reduce the pressure in the air supply line 8. This is advantageous, for example, to cause a pressure drop in the air mass flow supplied to the fuel cell unit 3 via the air supply line 8.

[0029] Furthermore, the fuel cell system 1 includes an intercooler 18, which is represented by a dashed rectangle. The main task of the intercooler is to cool the air for the fuel cell unit 3. A secondary task of the intercooler 18 is to cool the compressed air mass flow 6 before the cooled air mass flow 7 branches off via the cooling air path 19.

[0030] Generally, the air supply device 5 is also referred to as a gas supply device 5. The compressed air mass flow 6 is supplied as cathode gas 21 to the fuel cell unit 3 on the cathode side 20 via the air supply line 8.

[0031] In Figure 1 a side view shows an embodiment of the compression device. The compression device 30, also referred to as a compressor, includes a housing 31 in which a shaft 32, also referred to as a rotor, for example a compressor shaft 32, is rotatably supported. Air is conveyed into the fuel stack of the fuel cell system by the compression device or the compressor 30, as shown in Figure 3 as shown.

[0032] The housing 31 of the compression device 30 includes a housing scroll 35. A structure-borne sound sensor 33 is mounted on the housing scroll 35. The structure-borne sound sensor 33 is connected to a control device 34 via a control line represented by a dashed line.

[0033] In Figure 1 a test bench 37 with a controller 38 is schematically shown. The test bench 37 is fixedly arranged on the ground 39. The shaft 32 is rotatably supported by means of an axial bearing system 29. The axial bearing system 29 includes at least one air bearing, also referred to as a foil bearing. Furthermore, the shaft 32 is radially supported in the housing 31 by means of a bearing, preferably also implemented as a foil bearing or an air bearing.

[0034] In Figure 2 a Cartesian coordinate graph with an x-axis 41 and a y-axis 42 is shown. The pressure ratio is shown on the y-axis 42, which is provided in the fuel cell system 1 with the gas supply devices 5; 30. The corresponding mass flow rate is plotted on the x-axis 41 in suitable units of measurement.

[0035] The characteristic curve family 40 of a gas supply device 5; 30, which is preferably embodied as a radial flow compression device, is shown in a Cartesian coordinate graph. In the characteristic curve family 40, the line 43 is the surge boundary of the radial flow compression device. The curve 44 is the choke boundary of the radial flow compression device.

[0036] A further curve 45 is a line with a constant rotational speed, wherein the curve marked with the reference numeral 45 describes the maximum allowable rotational speed during the operation of the radial flow compression device. An island 46 with a constant efficiency is drawn in the characteristic curve family 40.

[0037] The maximum mass flow of the radial flow compression device is generally limited by the cross-section at the inlet of the compression device. If the air at the inlet of the compression device reaches the speed of sound, a further increase in throughput can no longer be achieved. This is also referred to as the choke boundary 44.

[0038] The surge boundary 43 delimits the left characteristic curve family edge of the characteristic curve family 40. In the case where the volume flow is too small and the pressure ratio is too high, the flow detaches from the compression device blades. Thereby, the conveying process is interrupted. The air flows backward through the compression device until a stable pressure ratio together with a positive volume flow reappears. The pressure is established again. This process is repeated rapidly and continuously. The name "surge" is derived from the noise generated hereby.

[0039] In the context of the present invention, the operation of the compression device on the side of the surge boundary 43 is studied. In the case where a fuel cell system has a radial flow compression device for air supply, operation limitations are caused by the surge boundary 43. These operation limitations mainly apply when the air supply section of the fuel cell system has a dynamic air bearing.

[0040] The dynamic air bearing requires a minimum rotational speed for its function. An adequately load-bearing air cushion is formed only at a rotational speed of the order of about 20,000 revolutions per minute, so as to carry the weight of the compression device rotor on the one hand and to compensate for accelerations, for example, compensated by adverse path excitation, on the other hand. In Figure 2 the characteristic curve family 40, this correlation is highlighted by points showing the lower operating region of the compression device.

[0041] The previously described limitations result in supplying more air during the idling operation of the fuel cell system than is necessary for the electrochemical reaction in the fuel cell unit. In principle, although the fuel cell unit operates stoichiometrically, the necessary air lambda value is between 1.6 and 2.0. Due to the above limitations of the air supply system, an air lambda value in the range of 5.0 may occur.

[0042] Without additional measures, a fuel cell system operating in this way is completely dry because the water discharged by the supplied air is more than the water generated by the electrochemical reaction in the fuel cell unit. As a result, two adverse effects occur. The water-related proton transport through the fuel cell unit membrane deteriorates and the aging of the fuel cell unit is increased.

[0043] For this reason, in the scope of the present invention, it is proposed to allow the inherently undesirable surge operation in the lower operating region 48 of the characteristic curve family 40 of the compression device in order to avoid the above-mentioned adverse effects. For this purpose, the existing axial bearing system ( Figure 1 No. 29 therein) must be designed to be strong enough because strong fluctuations in the axial force occur during surging.

[0044] Attention should be paid during surge operation that the generated acoustic effects have no adverse impact on the vehicle user. Therefore, it may be necessary to monitor the surge operation by, for example, solid sound measurement. For this purpose, an acceleration sensor is preferably used. With this method, surge operation in the upper characteristic curve family region can also be safely detected and thus avoided.

[0045] Due to the acoustic effects involved, a microphone can also be used for surge detection. Another possibility lies in analyzing and evaluating the current required for the electric motor drive device of the compression device. Surge operation generates torque fluctuations that can be measured as current fluctuations.

[0046] In Figure 2 the critical pressure ratio D is marked k . The critical pressure ratio D k is approximately 1.5. Below the critical pressure ratio D k , inherently undesirable surge events are allowed during the operation of the electrically driven gas supply device. Above the critical pressure ratio D k , the then undesirable surge events are prevented.

[0047] Figure 4 The corresponding flowcharts with rectangles 51 to 53, rhombus 54, and arrows 55 to 58 are shown. Rectangle 51 symbolically shows the operation of the fuel cell system. Rectangle 52 shows: checking during the operation of the fuel cell system whether a surge event occurs. In rhombus 54, it is checked whether the critical pressure ratio D k is exceeded. If the pressure ratio D k is not exceeded, it is indicated by arrow 56 that surge operation is allowed. If the critical pressure ratio D k is exceeded, it is shown by arrows 57 and rectangle 53 that surging is prevented.

[0048] In the rectangle 53, for example, surge boundary pre-identification can be performed. When implementing surge boundary pre-identification, a suitable control electronic device prevents the compressor unit speed from increasing further. Alternatively or additionally, if surge boundary pre-identification occurs or the bypass 16 is opened, the compressor unit speed during the operation of the compressor unit can be reduced.

Claims

1. A method for operating a fuel cell system (1) having a fuel cell unit (3), wherein a cathode gas (21) is supplied to the fuel cell unit at a cathode inlet side (20) by means of an electrically driven gas supply device (5), the gas supply device being implemented as a fluid machine, the operating range of which can be described in a characteristic curve family (40) having a surge boundary (43) and a choke boundary (44), characterized in that, During idling or no-load operation of the fuel cell system (1), in a defined operating range (48) of the electrically driven gas supply device (5) on the side of the surge boundary (43), a surge event that is not desired per se during operation of the electrically driven gas supply device (5) is deliberately permitted.

2. The method according to claim 1, wherein The cathode gas (21) is air.

3. The method according to claim 1 or 2, characterized in that In a lower operating range (48) of the electrically driven gas supply device (5), a surge operation of the electrically driven gas supply device (5) that is not desired per se is deliberately permitted on the side of the surge boundary (43).

4. The method according to claim 1 or 2, characterized in that, In the case where the pressure ratio is less than the critical pressure ratio (D k ), on the side of the surge boundary (43), a surge event that is not desired per se in the operation of the electrically driven gas supply device (5) is intentionally permitted.

5. The method according to claim 4, characterized in that, At a pressure ratio greater than the critical pressure ratio (D k ), on the side of the surge boundary (43), surge events that are not desired per se during operation of the electrically driven gas supply device (5) are not permitted.

6. The method according to claim 5, characterized in that In the case where the pressure ratio is greater than the critical pressure ratio (D k ), a surge event that is not desired per se during the operation of the electrically driven gas supply device (5) is sensed.

7. The method according to claim 6, characterized in that, At a pressure ratio greater than the critical pressure ratio (D k ), acoustically sense surge events that are not desired per se during operation of the electrically driven gas supply device (5).

8. The method according to claim 4, wherein The critical pressure ratio (D k ) is between 1 and 2.

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