CN114667620A

CN114667620A - Method for operating and designing a fuel cell system

Info

Publication number: CN114667620A
Application number: CN202080076513.4A
Authority: CN
Inventors: S·比尔; A·克诺普
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2019-10-30
Filing date: 2020-09-18
Publication date: 2022-06-24
Also published as: DE102019216712A1; WO2021083579A1; EP4052317A1; JP2022552867A

Abstract

The invention relates to a method for operating a fuel cell system having a fuel cell unit to which a cathode gas, such as air, is supplied on the cathode input side by means of an electrically driven gas supply device (30) which is designed as a turbomachine, the operating region of which can be described in a characteristic map having a surge limit and a choke limit. In order to extend the service life of the fuel cell system, in a defined operating region of the electrically driven gas supply device (30) when the electrically driven gas supply device (30) is in operation, an undesired pumping event is deliberately allowed away from the surge limit.

Description

Method for operating and designing a fuel cell system

Technical Field

The invention relates to a method for operating a fuel cell system having a fuel cell unit to which a cathode gas, for example air, is supplied on the cathode input side by means of an electrically driven gas supply device, which is embodied as a turbomachine, the operating region of which can be shown in a characteristic diagram having a surge limit and a choke limit. Furthermore, the invention relates to such a fuel cell system. The invention further relates to a method for designing such a fuel cell system.

Background

From german patent document DE 102009029837B 4, a method for operating a fuel cell system and a fuel cell system are known, which comprise: 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 adapted so as to measure a pumping indicator for an initiated pumping state known from which the initiated pumping state occurred prior to a pumping event in the pumping compression device, wherein the initiated pumping state is detected thereby: the mass flow and/or the outlet pressure of the air compressor are monitored for characteristic fluctuations or oscillations, wherein the occurrence of pumping events during the operation of the fuel cell system is countered, wherein aging-related effects, such as wear and tear over the service life of the air compressor, are taken into account, from which they are known to influence the characteristic map arrangement of the surge limit.

Disclosure of Invention

The object of the invention is to extend the service life of a fuel cell system having a fuel cell unit to which a cathode gas, for example air, is supplied on the cathode input side by means of an electrically driven gas supply device, which is embodied as a turbomachine, the operating region of which can be shown in a characteristic diagram having a surge limit and a choke limit.

In a method for operating a fuel cell system having a fuel cell unit to which a cathode gas, for example air, is supplied on the cathode input side by means of an electrically driven gas supply device, which is embodied as a turbomachine, the operating region of which can be represented in a characteristic diagram having a surge limit and a blockage limit, the object is achieved by: in the operation of the electrically driven gas supply, undesirable pumping events are deliberately allowed away from the surge limit in a defined operating region of the electrically driven gas supply. The general idea of "resisting the occurrence of pumping events during operation of the fuel cell system" as described, for example, in the initially filed german patent document DE 102009029837B 4 is objected here. In tests and investigations carried out within the scope of the present invention, it was found that in certain operating regions of electrically driven gas supplies, undesired pumping events do not necessarily lead to damage of the fuel cell system, in particular of the electrically driven gas supplies, per se. This intentional allowance for the pumping event can advantageously be extended in idle operation or idle operation of the fuel cell system, so that the amount of cathode gas, in particular air, required for the electrochemical reaction in the fuel cell unit is delivered precisely by the gas supply device.

This, in turn, prevents an undesirable complete drying due to overfeed fuel cell system air. This in turn retards the aging of the fuel cell unit.

A preferred embodiment of the method is characterized in that in the lower working area of the electrically driven gas supply, an undesired pumping operation of the electrically driven gas supply is intentionally allowed away from the surge limit. The term "lower working range" relates to a family of characteristic curves of the electrically driven gas supply device. The characteristic map relates, for example, to a cartesian representation in which the mass flow through the electrically driven gas supply is plotted on the x-axis in suitable units of measure. On the y-axis of the characteristic map, for example, a pressure ratio is plotted, which is generated during operation of the fuel cell system by the electrically driven gas supply. In the lower region of the characteristic diagram, the pressure ratio and the volume flow are relatively low.

A further preferred embodiment of the method is characterized in that, in the operation of the electrically driven gas supply, in the event of a pressure ratio which is less than a critical pressure ratio, an undesired pumping event is deliberately allowed away from the surge limit. The pressure ratio can be sensed relatively simply with a pressure sensor that may already be present. Thus, a measure which is simple to implement in terms of control technology is provided, with which the service life of the fuel cell system can be effectively extended.

A further preferred embodiment of the method is characterized in that, in the operation of the electrically driven gas supply, in the event of a pressure ratio greater than the critical pressure ratio, undesired pumping events are not permitted in themselves away from the surge limit. When the critical pressure ratio is exceeded during operation of the fuel cell system, conventional measures can be used to avoid undesirable pumping events in such situations. For this purpose, for example, the rotational speed of the gas supply, which is, for example, electrically driven, can be reduced or the bypass can be opened.

A further preferred embodiment of the method is characterized in that, in operation of the electrically driven gas supply device, in the event of a pressure ratio which is greater than a critical pressure ratio, an undesired pumping event is sensed perceptually, in particular acoustically. For the purpose of sensorial sensing, for example, a structure-borne sound sensor may be used. For making acoustic measurements, a microphone may be used. Alternatively or additionally, the current of the electric drive of the electrically driven gas supply can be measured.

A further preferred embodiment of the method is characterized in that the critical pressure ratio lies between 1 and 2. In tests and studies carried out within the scope of the present invention, a value of 1.5 for the critical pressure ratio proved to be advantageous.

In a fuel cell system having a fuel cell unit to which a cathode gas, for example air, is supplied on the cathode input side by means of an electrically driven gas supply device, which is embodied as a fluid machine, the operating region of which can be shown in a characteristic diagram having a surge limit and a choke limit, the above object is alternatively or additionally solved by: the axial bearing system of the electrically driven gas supply is sufficiently robust to design the electrically driven gas supply with respect to the pumping events which are deliberately allowed according to the method described above. Advantageously, the axial bearing system comprises a dynamic air bearing. The dynamic air bearing comprises at least one air bearing, also referred to as a diaphragm bearing, with which the electric motor drive of the gas supply device is axially supported. A sufficiently robust design is therefore advantageous, in particular in relation to axial bearing systems, since strong axial force fluctuations which occur in the operation of the fuel cell system in the electrically driven gas supply occur in the pumping operation if pumping events are deliberately allowed. In addition, during the pumping operation, it is particularly advantageous to note that the acoustic effects which can be produced do not have an adverse effect. This means in particular that the acoustic effect is not allowed to be audible to the vehicle user. For this purpose, the operation of the gas supply of the fuel cell system, in particular, driven by an electric motor, can be monitored perceptually, in particular acoustically. For this purpose, an acceleration sensor may be used. In this way, pumping operations in the upper characteristic field can also be safely detected and avoided.

In a method for designing the aforementioned fuel cell system, the above-mentioned object is alternatively or additionally solved by: sensing and storing on the test stand during a test stand operation of the fuel cell system: when a pumping event occurs in the operation of an electrically driven gas supply. The sensed and stored values, such as pressure ratio, rotational speed and mass flow provided by the electrically driven gas supply, are sensed and stored in a suitable controller of the test stand. These values may be used during fuel cell system operation to identify and evaluate pumping events. The costly sensor system itself can be dispensed with at low cost.

A preferred embodiment of the method is characterized in that the following is sensed acoustically on the test bench during the test bench operation of the fuel cell system: when a pumping event occurs in the operation of an electrically driven gas supply. The pumping event may then be stored along with the measured pressure ratio, mass flow, rotational speed, etc.

The invention also relates to a test bench, on which the method for designing a fuel cell system is carried out. For example, the test stand is equipped with at least one acoustic measuring device in order to sense pumping events during operation of the fuel cell system.

Drawings

Further advantages, features and details of the invention emerge from the following description, in which different embodiments are explained in detail with reference to the figures. The figures show:

fig. 1 shows a compression device with a housing in a side view on a test stand which is only schematically illustrated;

fig. 2 shows a cartesian diagram in which a characteristic map of a gas supply of a fuel cell system is shown;

fig. 3 shows a schematic view of a fuel cell system with a gas supply; and

fig. 4 shows a flow chart for elucidating the claimed method.

Detailed Description

The fuel cell system 1 is schematically shown in fig. 3. Fuel cell systems are known per se, for example, from german patent application DE 102012224052 a 1. The fuel cell system 1 includes a fuel cell unit 3, which is indicated only by a dashed rectangle. The fuel cell unit 3 comprises at least one stack 2, which is alternatively shown with a valve symbol.

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

The cooling air supplied via the cooling air path 19 is used, for example, to cool an air bearing with which the shaft of the air compressor 5 is rotatably supported. The cooling air mass flow 7 is a loss in the compressed air mass flow 6, since the branched-off cooling air mass flow 7 can no longer be used in the stack 2 of the fuel cell unit 3.

Since the cooling air mass flow 7 is provided for internal cooling by the air compressor device 5, energy, in particular electrical energy, is necessary in order to generate the cooling air mass flow. This energy has an adverse effect on the overall efficiency of the electric drive of the motor vehicle which is driven by the fuel cell system 1.

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 chemical reaction energy of fuel and oxidant supplied through a fuel supply line, not shown, into electric energy.

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

The exhaust gas mass flow 10 is discharged via an exhaust gas turbine device 11 to an exhaust gas outlet 12, which is indicated by an arrow. The air compression device 5 is arranged in an air supply line 8. An 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.

The shaft can be electrically driven by an electric motor 14. The exhaust-gas turbine device 11 serves to assist the electric motor 14 in driving the air compressor 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.

Furthermore, the fuel cell system 1 comprises a bypass line 13 in which a bypass valve 16 is arranged. A bypass air mass flow 17 can be discharged via a bypass line 13 with a bypass valve 16, bypassing the stack 2 of the fuel cell unit 3, into the exhaust gas line 9 in order to reduce the pressure of the air supply line 8. This is advantageous, for example, in order to cause a pressure reduction in the air mass flow supplied to the fuel cell unit 3 via the air supply line 8.

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

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

Fig. 1 shows an embodiment of a compression device in a side view. The compressor device 30, also referred to as compressor, comprises a housing 31 in which a shaft 32, also referred to as rotor, for example a compressor shaft 32, is rotatably mounted. Air is delivered to the fuel stack of the fuel cell system by a compressor device or compressor 30, as shown in fig. 3.

The housing 31 of the compression device 30 includes a housing volute 35. Structure-borne sound sensor 33 is mounted on housing volute 35. Structure-borne sound sensor 33 is connected to control device 34 via a control line indicated by a dashed line.

A test stand 37 with a controller 38 is schematically shown in fig. 1. The test stand 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 comprises at least one air bearing, also referred to as a film bearing. Furthermore, the shaft 32 is mounted radially in the housing 31 by means of bearings, which are preferably likewise embodied as film bearings or air bearings.

A cartesian plot having an x-axis 41 and a y-axis 42 is shown in fig. 2. On the x-axis 41, a pressure ratio is shown, which is obtained with the gas supply device 5; 30 is provided in the fuel cell system 1. The corresponding mass flow is plotted on the x-axis 41 in suitable units of measure.

The gas supply device 5, which is preferably embodied as a radial compressor, is shown in a cartesian diagram; 30, and 40. In characteristic diagram 40, line 43 is the surge limit of the radial compressor. Curve 44 is the occlusion boundary of the radial compression device.

The further curve 45 is a line with constant rotational speed, wherein the curve designated by reference numeral 45 describes the maximum permissible rotational speed during operation of the radial compressor. An island 46 with constant efficiency is plotted in the characteristic diagram 40.

The maximum mass flow of a radial compressor is generally limited by the cross-section at the inlet of the compressor. If the air in the inlet of the compression device reaches sonic velocity, no further increase in throughput can be achieved. This is also referred to as the occlusion boundary 44.

The surge limit 43 delimits the left-hand characteristic diagram edge of the characteristic diagram 40. In the case of a volume flow that is too small and a pressure ratio that is too high, the flow is decoupled from the compressor blades. Thereby, the conveying process is interrupted. The air flows backwards through the compression device until a stable pressure ratio occurs again, together with a positive volumetric flow. The pressure builds up again. This process repeats in rapid succession. The noise generated in this case leads to the name "pump".

In the context of the present invention, the operation of the compressor on the side of the surge limit 43 is investigated. In the case of a fuel cell system having a radial compressor for air supply, operating restrictions arise due to the surge limit 43. These operational limitations apply primarily when the air supply of the fuel cell system has a dynamic air bearing.

Dynamic air bearings require a minimum rotational speed for their function. Only at a rotational speed of the order of approximately twenty thousand revolutions per minute is a sufficiently loadable air cushion formed in order to carry the weight of the compressor rotor on the one hand and to compensate for accelerations, for example by disadvantageous path excitations, on the other hand. In the characteristic diagram 40 in fig. 2, this dependence is highlighted by a point which shows the lower working range of the compression device.

The previously explained limitation results in the fuel cell system being operated at idle and delivering more air than is necessary for the electrochemical reaction in the fuel cell unit. In principle, the necessary air lambda value lies between 1.6 and 2.0, although the fuel cell unit is operated over-stoichiometrically. Due to the above-mentioned limitations of the air supply system, it is possible to generate an air λ value in the range of 5.0.

Without additional measures, the fuel cell system operated in this way is completely dry, since the air supplied displaces more water than is produced by the electrochemical reaction in the fuel cell unit. Thereby, two adverse effects are produced. The water-related transport of protons through the membrane of the fuel cell unit is deteriorated and the ageing of the fuel cell unit is improved.

For this reason, it is proposed within the scope of the invention to allow an undesired pumping operation in the lower working range 48 of the family of compression device characteristics 40, in order to avoid the above-mentioned disadvantageous effects. For this purpose, the existing axial bearing system (29 in fig. 1) must be designed to be sufficiently robust, since strong fluctuations in the axial forces occur during pumping.

During the pumping operation, it is to be noted that the acoustic effect produced has no adverse effect on the vehicle user. It may therefore be necessary to monitor the pumping operation by, for example, structure-borne sound measurement. For this purpose, an acceleration sensor is preferably used. In this way, pumping operations in the upper characteristic field can also be safely detected and thus avoided.

Microphones may also be used for pumping detection due to acoustic effects involved. A further possibility consists in evaluating the current required at the electric motor drive of the compressor. The pumping operation produces torque fluctuations that can be measured as current fluctuations.

The critical speed ratio D is indicated in FIG. 2_k. Critical pressure ratio D_kAbout 1.5. At a critical pressure ratio D_kIn the following, inherently undesirable pumping events are allowed in the operation of the electrically driven gas supply. At the critical pressure ratio D_kAbove, then undesirable pumping events at that time are prevented.

Fig. 4 shows a corresponding flow chart with rectangles 51 to 53, diamond 54 and arrows 55 to 58. The rectangle 51 symbolically shows the operation of the fuel cell system. Rectangle 52 shows: checking during operation of the fuel cell system: whether a pumping event has occurred. Check in diamond 54: whether the critical pressure ratio D is exceeded_k. If the pressure ratio D is not exceeded_kThen, by means of arrow 56: allowing the pumping operation. If the critical pressure ratio D is exceeded_kThen, as indicated by arrow 57 and rectangle 53: pumping is prevented.

In rectangle 53, for example, surge boundary pre-identification can be performed. When the surge limit is detected, suitable control electronics prevent the rotational speed of the compressor from increasing further. Alternatively or additionally, the compressor rotational speed can be reduced during operation of the compressor if a surge margin is pre-identified or if the bypass 16 is opened.

Claims

1. Method for operating a fuel cell system (1) having a fuel cell unit (3) to which a cathode gas (21), for example air, is supplied on the cathode input side (20) by means of an electrically driven gas supply device (5; 30) which is designed as a turbomachine, the operating region of which can be described in a characteristic diagram (40) having a surge limit (43) and a choke limit (44), characterized in that, in a defined operating region (48) of the electrically driven gas supply device (5; 30), on the side of the surge limit (43), undesired pumping events are deliberately allowed in operation of the electrically driven gas supply device (5; 30).

2. Method according to claim 1, characterized in that in a lower working area (48) of the electrically driven gas supply (5; 30) an undesired pumping operation of the electrically driven gas supply (5; 30) is deliberately allowed on the side of the surge limit (43).

3. Method according to any of the preceding claims, characterized in that, when the pressure ratio is smaller than the critical pressure ratio (D)_k) On the side of the surge boundary (43), the surge limit (43) is deliberately allowed to rise above the electrically driven gas supply (5; 30) is itself an undesirable pumping event.

4. Method according to claim 3, characterized in that the pressure ratio is greater than the critical pressure ratio (D)_k) On the side of the surge boundary (43), the supply of gas to the electrically driven gas supply device (5; 30) is itself an undesirable pumping event.

5. Method according to claim 4, characterized in that the pressure ratio is greater than the critical pressure ratio (D)_k) In the case of a sensor, in particular acoustically, the sensor is arranged in the electrically driven gas supply device (5; 30) is itself an undesirable pumping event.

6. Method according to any one of claims 3-5, wherein the critical pressure ratio (D)_k) Between 1 and 2.

7. Fuel cell system (1) having a fuel cell unit (3) to which a cathode gas (21), for example air, is supplied on a cathode input side (20) by means of an electrically driven gas supply device (5; 30) which is embodied as a turbomachine whose operating region can be described in a characteristic diagram (40) having a surge limit (43) and a choke limit (44), characterized in that an axial bearing system (29) of the electrically driven gas supply device (5; 30) is designed to be sufficiently robust with respect to pumping events of the electrically driven gas supply device (5; 30) which are deliberately allowed according to the method according to one of claims 1 to 6.

8. Method for designing a fuel cell system (1) according to claim 7, characterized in that on a test stand (37) in a test stand operation of the fuel cell system (1) is sensed and stored: when a pumping event occurs during operation of the electrically driven gas supply (5; 30).

9. The method according to claim 8, characterized in that on the test stand (37) in the test stand operation of the fuel cell system (1) is acoustically sensed: when a pumping event occurs during operation of the electrically driven gas supply (5; 30).