Method for operating a fuel cell system

Abstract

The invention relates to a method for operating a fuel cell system (1), the fuel cell system having a fuel cell stack (2), to which a gas mass flow containing an oxidant is supplied via a gas delivery device (3), wherein the gas delivery device (3) comprises a first compressor stage (5) having at least one motor-driven compressor (7, 8) and a second compressor stage (10), the first compressor stage having a compressor (11) driven by a turbine (12), the turbine (12) being able to generate a gas mass flow containing an oxidant with the fuel cell stack. The invention relates to a fuel cell system (2) driven by an exhaust gas mass flow, the exhaust gas mass flow comprising a cathode path with a cathode mass flow and a cathode pressure, wherein the compressor (7, 8) of the first compressor stage (5) can be driven at a variable speed, wherein the turbine (12) of the second compressor stage (10) is assigned a turbine bypass (15) with a turbine bypass valve (16), wherein the fuel cell stack (2) is assigned a stack bypass (17) with a stack bypass valve (18), wherein a post-stack valve (19) is connected downstream of the fuel cell stack (2). In order to simplify and / or improve the operation of the fuel cell system (1), during the operation of the fuel cell system (1), in the operating strategy, the control variables for cathode mass flow regulation and cathode pressure regulation are switched.

Description

Technical Field

[0001] The present invention relates to a method for operating a fuel cell system, which has a fuel cell stack (Brennstoffzellenstack), to which a gas mass flow containing an oxidant is supplied via a gas delivery device, wherein the gas delivery device includes a first compressor stage and a second compressor stage, the first compressor stage having a compressor driven by at least one electric motor (elektromotorisch), the second compressor stage having a compressor driven by a turbine (Turbine), the turbine being capable of being driven by an exhaust gas mass flow of the fuel cell stack, the exhaust gas mass flow including a cathode path having a cathode mass flow and a cathode pressure (Kathodendruck), wherein the compressor of the first compressor stage can be driven at a variable speed, wherein the turbine of the second compressor stage is assigned a turbine bypass with a turbine bypass valve, wherein the fuel cell stack is assigned a stack bypass with a stack bypass valve, wherein a stack back valve (Nachstackventil) is connected downstream of the fuel cell stack. Background Art

[0002] German published patent application DE 10 2012 224 052 A1 discloses a fuel cell system comprising a fuel cell, a compressor, a drive for an electrically driven compressor, and a regulating device, wherein the regulating device is configured to detect compressor kick vibrations of the electrically driven compressor, wherein the regulating device is configured to adapt the drive torque to a load torque acting on the drive device, wherein the drive torque of the drive device is determined based on a desired rotational speed of the compressor, wherein a desired current of the drive device is determined based on the desired rotational speed of the compressor, wherein a voltage for driving the drive device is generated based on the desired current, wherein the actual current of the drive device and the actual rotational speed of the compressor are detected, wherein the actual rotational speed of the compressor is determined based on a torque of the drive device resulting from the drive torque and the load torque, wherein the compressor kick vibrations are detected based on a change in the determined torque and based on the desired rotational speed and the desired current. Summary of the Invention

[0003] The object of the present invention is to simplify and / or improve the operation of a fuel cell system.

[0004] This object is achieved in a method for operating a fuel cell system by switching controlled variables for cathode mass flow control and cathode pressure control in an operating strategy during operation of the fuel cell system. The controlled variables are, in particular, a controllable desired speed and controllable valves. These valves include a turbine bypass valve, a stack bypass valve, and a post-stack valve. The claimed method relates to a control or regulator structure for the pressure level in a fuel cell stack and the mass flow through the fuel cell stack. The regulator structure or regulation can be implemented not only as a pressure ratio but also as a pressure (i.e., as an absolute pressure or a relative pressure). Unless otherwise specified, the term "pressure" refers to the cathode pressure in the cathode path. The pressure loss in the cathode path can be measured and / or calculated. The measured and / or calculated pressure loss, for example using a suitable model, can be taken into account in the regulation or its feedforward control. The post-stack valve is arranged between the fuel cell stack and the turbine. Optionally, a gas conditioning unit, for example in the form of a humidifier, can also be arranged between the fuel cell stack and the post-stack valve. According to an embodiment, a post-stack valve may also be connected downstream of the turbine. Cathode pressure regulation is also referred to as pressure regulation for short, and may also be implemented as pressure ratio regulation as described above.

[0005] A preferred embodiment of the method is characterized in that the turbine bypass valve remains closed in operating range A at high pressure ratios, wherein the post-stack valve is used in operating range A to adjust the cathode mass flow. The high pressure ratio is dependent on the mass flow. The dividing line between operating ranges A and B can be considered as a boundary in a Cartesian coordinate diagram, in which the pressure or pressure ratio is plotted as a function of the mass flow. At the boundary, the post-stack valve is open and the turbine bypass is closed.

[0006] A further preferred embodiment of the method is characterized in that the speed of the compressor of the first compressor stage is used to adjust the pressure ratio in operating range A. The properties of the cathode path can be controlled by means of the post-stack valve.

[0007] Another preferred embodiment of the method is characterized in that the post-stack valve remains fully open in operating range B at small pressure ratios, wherein the speed of the compressor of the first compressor stage is used in operating range B to adjust the cathode mass flow, wherein the pressure is regulated by the turbine bypass. In this case, operating range B is delimited downward by the filling limit (stopfgrenze) of the compressor of the first compressor stage. Small or low pressure ratios depend on the mass flow. The dividing line between ranges A and B can be considered as a boundary in a Cartesian coordinate diagram, in which the pressure or pressure ratio is plotted as a function of the mass flow. At the boundary, the post-stack valve is open and the turbine bypass is closed.

[0008] Another preferred embodiment of the method is characterized in that the cathode mass flow is additionally reduced in operating range C by means of a stack bypass valve, wherein the pressure is controlled via the speed as in operating range A. The mass flow is advantageously reduced by means of the post-stack valve only to such an extent that the surge limit of the two compressor stages is maintained. If the mass flow through the two compressor stages falls below the surge limit, the mass flow through the fuel cell stack can be additionally reduced by means of the stack bypass valve.

[0009] Another preferred embodiment of the method is characterized in that the switchover from operating range A to operating range B occurs when the post-stack valve is completely open or is completely opened, or when a regulator setpoint changes. A changed regulator setpoint means that when a jump in the setpoint value occurs, it can already be read from the two setpoint variables, namely the setpoint pressure and the setpoint mass flow, into which operating range the switchover occurs.

[0010] Another preferred embodiment of the method is characterized in that the switchover from operating range B to operating range A occurs when the turbine bypass valve is completely closed or completely closed or when the controller setpoint changes. A changed controller setpoint means that when the setpoint value jumps, it can already be read from the two setpoint variables, the setpoint pressure and the setpoint mass flow, into which operating range the switchover occurs.

[0011] Another preferred embodiment of the method is characterized in that a switch from operating range A to operating range C occurs when the desired mass flow falls below a minimum mass flow, wherein the switch from operating range C to operating range A occurs when the desired mass flow exceeds the minimum mass flow. The mass flow through the fuel cell stack, which is predetermined as a desired value by the control device, is referred to as the desired mass flow. The stack mass flow corresponds to the compressor mass flow minus the bypass mass flow.

[0012] Another preferred embodiment of the method is characterized by the use of a PID controller with feedforward control. Both the regulation and the feedforward control optionally take into account a plurality of input parameters, such as sensor values ​​from the fuel cell system, values ​​from a calculation model of the fuel cell control unit, environmental parameters, etc. These include, in particular, the stack temperature, the ambient pressure or the current altitude above sea level, the ambient temperature, etc.

[0013] The invention further relates to a system for operating a fuel cell system according to the above-described method.

[0014] The present invention may also relate to a vehicle having a drive device including the above fuel cell system. The present invention may also relate to a mobile application or a stationary application having the above fuel cell system.

[0015] The present invention also relates to a computer program product having a computer program with software elements for carrying out the above-described method when the computer program is executed on a programmable computer device, for example a control device of a fuel cell system. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Further advantages, features and details of the present invention emerge from the following description, in which various exemplary embodiments are described in detail with reference to the drawings.

[0017] The accompanying drawings show:

[0018] Figure 1 shows a schematic diagram of a fuel cell system having two compressor stages;

[0019] Figure 2 shows a Cartesian coordinate diagram in which cathode pressure is plotted against cathode mass flow;

[0020] Figure 3 The regulation in operating range C is shown;

[0021] Figure 4 The regulation in operating range A is shown;

[0022] Figure 5 The regulation in operating range B is shown;

[0023] Figure 6 A Cartesian coordinate diagram is shown for regulation in operating range A;

[0024] Figure 7 A Cartesian coordinate diagram is shown for regulation in operating range B;

[0025] Figure 8 A schematic diagram showing one possible variant of a PID regulator with feedforward control for operating range A; and

[0026] Figure 9 A possible variant of a PID controller with feedforward control for operating range B is shown. DETAILED DESCRIPTION

[0027] Figure 1 A fuel cell system 1 is shown having a fuel cell stack 2. An oxidant, in particular oxygen-containing air, is supplied to the fuel cell stack 2 via a gas delivery device 3 in the form of a gas mass flow 4. The gas delivery device 3 comprises a first compressor stage 5 and a second compressor stage 10.

[0028] The first compressor stage 5 comprises two compressors 7, 8 connected in parallel, which are driven by an electric motor 6. An air filter 9 is connected upstream of the first compressor stage 5.

[0029] The second compressor stage 10 comprises a compressor 11 and a turbine 12. The compressor 11 is connected to the turbine 12 in terms of drive via a shaft 13. For this purpose, the turbine 12 is driven by an exhaust gas mass flow 14 of the fuel cell stack 2.

[0030] A turbine bypass 15 with a turbine bypass valve 16 is associated with the turbine 12. A stack bypass 17 with a stack bypass valve 18 is associated with the fuel cell stack 2. A post-stack valve 19 is connected downstream of the fuel cell stack 2. The post-stack valve 19 is arranged between a gas conditioning unit 20 and the turbine 12.

[0031] The dashed rectangle indicates that a gas conditioning unit 20, for example, including a humidifier, is optional. The rectangle 21 indicates that a gas mass flow 4, in particular in the form of ambient air, is withdrawn from the environment. The exhaust gas mass flow 14 is supplied to the environment 21. A compressor bypass 22 with a compressor bypass valve 23 is associated with the compressor 11 of the second compressor stage 10.

[0032] exist Figures 2 to 8 In the following it is shown how Figure 1 The desired cathode mass flow and the desired cathode pressure are set or regulated in the fuel cell stack 2 in FIG. The setting or regulation is carried out by means of the speed of the first compressor stage 5 , which is variable via the electric motor 6 , by means of the turbine bypass valve 16 and by means of the stack bypass valve 18 .

[0033] By cleverly switching the operating strategies for cathode mass flow regulation and cathode pressure regulation, a robust control or regulation is achieved, which makes the entire operating range of the fuel cell system available. The turbine ( Figure 1 The complex control device at 12) in FIG. This reduces system costs. Furthermore, complex system measures, such as de-icing a turbine with a variable turbine geometry that is blocked by ice, are avoided.

[0034] Figure 2 A Cartesian coordinate diagram is shown having an x-axis 25 and a y-axis 26. On the x-axis 25 the cathode mass flow is plotted in suitable units of measure.

[0035] The cathode pressure is plotted in suitable pressure units or the corresponding pressure ratio on the y-axis 26. A total of three operating ranges are designated by capital letters A, B and C. The pressure ratios and pressures can be converted to one another accordingly.

[0036] The three dividing lines 27, 28, and 29 are Figure 2 Starting from the origin of the coordinate diagram in FIG. A dividing line 27 represents the boundary between operating ranges A and C. A dividing line 28 represents the boundary between operating ranges A and B. A dividing line 29 represents the lower boundary of operating range B.

[0037] The regulation in operating range A is as follows. At high pressure ratios, the turbine bypass valve 16 is closed. The characteristics of the cathode path can be adjusted using the post-stack valve 19, that is, the system characteristic curve is shifted from the marked dividing line 28 toward the surge limit. The post-stack valve 19 is used to adjust the cathode mass flow.

[0038] In an optional feedforward control, when the post-stack valve 19 is fully opened, the characteristic curve KL2 of the mass flow (see Figure 8 The rectangle 73 in FIG. 1 is stored as a function of the rotational speed. A further characteristic curve KL3 (see Figure 8 The rectangle 75 in FIG. 1 contains the percentage mass flow reduction as a function of the valve position of the post-stack valve 19 .

[0039] The speed of the first compressor stage 5 is then used to adjust the pressure ratio. In an optional feedforward control, the characteristic curve KL1 of the speed (see Figure 8 The rectangle 72 in FIG. 7 is stored as a function of the pressure ratio.

[0040] Control in operating range B is as follows. At low pressure ratios, the post-stack valve 19 is fully open. The mass flow is regulated by means of the rotational speed of the first compressor stage 5. The pressure is regulated by means of the turbine bypass valve 16. Range B is delimited downward by a dividing line 29, which represents the filling limit.

[0041] As a feedforward control of the speed in the operating range B, when the post-stack valve 19 is open, the characteristic curve KL4 (see Figure 9 The characteristic curve KL5 can be saved for the reduction of the pressure ratio (see Figure 9 94 in FIG), which has the maximum pressure ratio of the second compressor stage 10 as a function of the speed, and KL6 with the pressure ratio of the first compressor stage 5 as a function of the speed (see Figure 9 rectangle 93 in the figure), and characteristic curve KL7 (see Figure 9 ), which has the percentage pressure ratio reduction of the second compressor stage 10 as a function of the valve position of the turbine bypass valve 16 .

[0042] Regulation in range C is considered as follows. The mass flow can be reduced by means of the post-stack valve 19 only to such an extent that the surge limits of the two compressor stages 5 and 10 are maintained (range A). If the mass flow through the compressor stages 5 and 10 were to fall below the surge limit, the mass flow through the fuel cell stack 2 can be additionally reduced by means of the stack bypass valve 18.

[0043] As a feedforward control, a characteristic curve can be stored in operating range C, in which the valve position is plotted as a function of the surge-limited mass flow reduction.

[0044] Regulation in different ranges can be achieved with or without feedforward control. For example, a PID controller with feedforward control can be used as the controller.

[0045] exist Figures 3 to 5 It is shown in FIG. 1 that the proposed regulation or regulation structure with respect to the pressure level can be realized not only as a pressure ratio but also as a pressure (absolute pressure or relative pressure). Figures 3 to 5 Shows an overview of the controllers and switches. Figure 3 Regulation in operating range C is shown. Figure 4 Control in operating range A is shown. Figure 5 Control in operating range B is shown.

[0046] Rectangle 31 represents pressure regulation. Rectangle 32 represents mass flow regulation. Rectangle 33 represents a regulation section, also referred to simply as section. Arrow 34 represents the desired pressure. Arrow 35 represents the actual pressure, also referred to simply as pressure. Arrow 36 represents the desired mass flow. Arrow 37 represents the actual mass flow, also referred to simply as mass flow. Arrow 38 represents the surge limit. Arrow 39 represents the air compressor speed of the first compressor stage 5.

[0047] As in Figure 1 In FIG, reference numeral 17 represents a stack bypass. Figure 1 In the figure, reference numeral 19 represents a post-stack valve.

[0048] exist Figure 3 , arrow 41 illustrates that the turbine bypass ( Figure 1 15) is closed. Figure 4 , arrow 42 illustrates that, additionally, the stack bypass ( Figure 1 17) is closed. Figure 5 In FIG, arrow 43 illustrates that the post-stack valve 19 is open. Figure 5 In FIG, arrow 44 additionally illustrates the rotational speed of first compressor stage 5 .

[0049] When the post-stack valve 19 is fully open or fully opened, the switch from range A to range B occurs. Furthermore, the switch from range A to range B can be performed using a predetermined pressure and mass flow, for example, when the controller's desired value changes. A changed controller's desired value means that when the desired value changes suddenly, the operating range to which the switch occurs can be determined from the two desired variables—the desired pressure and the desired mass flow.

[0050] The mass flow through the fuel cell stack 2 is called the desired mass flow. The corresponding desired value is stored, for example, in the control device of the fuel cell system 1. The mass flow through the stack, which is also called the stack mass flow, is the compressor mass flow provided by the compressor stages 5 and 10 minus any bypass mass flow. When switching from range A to range B, i.e., from Figure 4 Switch to Figure 5 , the post-stack valve 19 is changed to the open state.

[0051] When the turbine bypass valve 16 is completely closed or completely closed, the switch from range B to range A is performed. In addition, the switch from range B to range A is performed with the aid of a predetermined pressure and mass flow, for example when a change in the regulator is desired. Figure 5 Switch to Figure 4 When , the turbine bypass 15 is turned into a fully closed state.

[0052] When the desired mass flow falls below a minimum mass flow, which depends on the surge limit, a switch is made from range A to range C. The activation of the mass flow regulation is performed by the stack bypass valve 18 .

[0053] The switch from range C to range A is made when the desired mass flow is above a minimum mass flow, which depends on the surge limit. The deactivation of the mass flow regulation is made by the stack bypass valve 18 .

[0054] Optionally, a corresponding hysteresis can also be used for each controller switch in order to avoid undesired switching. The hysteresis can be a time hysteresis, a mass flow hysteresis or a pressure range hysteresis.

[0055] Safety is advantageously also considered with regard to component protection or surge protection of the compressors of the compressor stages 5 and 10. For safety reasons, the switch from operating range A to operating range C is performed before the surge characteristic curve is reached.

[0056] Unlike what is shown in the drawings, the post-stack valve 19 implemented as a regulating valve may not be as shown in FIG. Figure 1 Rather than being arranged upstream of the turbine 12 as shown in FIG, it can also be arranged downstream of the turbine 12 in the direction of exhaust gas flow.

[0057] exist Figure 6 4 shows a Cartesian coordinate diagram with an x-axis 48 and a y-axis 49. On the x-axis 48 the mass flow of the first compressor stage 5 is plotted in suitable units. On the y-axis 49 the pressure or pressure ratio of the first compressor stage 5 is plotted.

[0058] By vertical arrow 50, in Figure 6 As shown in FIG, the pressure increases with the increase of the rotational speed. Figure 6It is shown in FIG that, at approximately the same pressure ratio, the mass flow is reduced by means of the post-stack valve. Figure 6 This involves regulation in range A.

[0059] Figure 7 This relates to regulation in range B. The mass flow through the second compressor stage 10 is plotted on the x-axis 54. The pressure or the pressure ratio of the second compressor stage 10 is plotted on the y-axis 55. Figure 7 The arrow 56 indicates that the speed of the second compressor stage 10 is reduced by opening the turbine bypass valve. As a result, the pressure or the pressure ratio of the second compressor stage 10 becomes lower.

[0060] exist Figure 8 A possible variant of the PID regulator with feedforward control for range A is shown in FIG. Figure 9 A possible variant of a PID regulator with feedforward control for range B is shown in .

[0061] exist Figure 8 In the diagram, the controllers or control structures are indicated by rectangles 71 to 78. Rectangle 71 represents a PID controller. Rectangle 72 represents a feedforward control with characteristic curve KL1. Rectangle 73 represents a feedforward control with characteristic curve KL2. Rectangle 74 represents the section consisting of 64 and 65. Rectangle 75 represents a feedforward control with characteristic curve KL3. Rectangle 76 represents a PID controller. Rectangles 77 and 78 represent additive relationships.

[0062] 60 represents the actual pressure. 61 represents the desired pressure. 62 and 63 represent the desired rotational speed. 64 represents the mass flow when the post-stack valve is open. 65 represents the desired mass flow limited by the kick characteristic curve. 66 represents the actual mass flow. 67 and 68 represent the valve position of the post-stack valve.

[0063] exist Figure 9 In the figure, rectangle 91 represents a PID controller. Rectangle 92 represents a feedforward control with characteristic curve KL4. Rectangle 93 represents a feedforward control with characteristic curve KL6. Rectangle 94 represents a feedforward control with characteristic curve KL5. Rectangle 95 represents a portion of the pressure ratio of the second compressor stage. Rectangle 96 represents a feedforward control with characteristic curve KL7. Rectangle 97 represents a PID controller. Rectangles 98 and 99 represent additive relationships.

[0064] 80 represents the actual mass flow. 81 represents the desired mass flow. 82 and 83 represent the desired rotational speed. 84 represents the stack temperature. 85 represents the pressure or pressure ratio of the first compressor stage. 86 represents the maximum pressure or maximum pressure ratio of the second compressor stage. 87 represents the desired pressure or desired pressure ratio. 88 represents the actual pressure or actual pressure ratio. 89 and 90 represent the valve position of the turbine bypass valve.

[0065] On the one hand, the claimed method can be used for operating strategies for vehicle drives with fuel cell systems. This can also involve commercial vehicles. However, the claimed method can also be used for mobile applications with fuel cell systems, such as construction machinery.

Claims

1. A method for operating a fuel cell system (1), said fuel cell system having a fuel cell stack (2), to which a gas mass flow containing an oxidant is supplied via a gas supply device (3), wherein: The gas delivery device (3) comprises a first compressor stage (5) and a second compressor stage (10), the first compressor stage having at least one motor-driven compressor (7, 8), the second compressor stage having a compressor (11) driven by a turbine (12), the turbine (12) being drivable with an exhaust gas mass flow of the fuel cell stack (2), the exhaust gas mass flow comprising a cathode path having a cathode mass flow and a cathode pressure, wherein the compressor (7, 8) of the first compressor stage (5) is drivable at a variable speed, wherein the turbine (12) of the second compressor stage (10) is assigned a turbine bypass (15) having a turbine bypass valve (16), wherein the fuel cell stack (2) is assigned a stack bypass (17) having a stack bypass valve (18), wherein a post-stack valve (19) is connected downstream of the fuel cell stack (2), and wherein three dividing lines are arranged from the Cartesian coordinates Starting from the origin of the diagram, the x-axis (25) of the Cartesian coordinate diagram represents the cathode mass flow and the y-axis (26) represents the cathode pressure, wherein the first dividing line (27) is the dividing line with the highest ratio of cathode pressure to cathode mass flow, the second dividing line (28) is the dividing line with the second highest ratio of cathode pressure to cathode mass flow, and the third dividing line (29) is the dividing line with the lowest ratio of cathode pressure to cathode mass flow, wherein the operating range C is limited by the first dividing line (27) and the y-axis, the operating area A is limited by the second dividing line (28) and the first dividing line (27), and the operating area B is limited by the third dividing line (29) and the second dividing line (28), wherein the turbine bypass valve (16) remains closed in the operating range A at high pressure ratios, wherein the post-stack valve (19) is used in the operating range A to adjust the cathode mass flow, wherein, In operating range C, the cathode mass flow is additionally reduced by means of the stack bypass valve (18) and / or the post-stack valve (19) remains fully open in operating range B at small pressure ratios, wherein in operating range B the speed of the compressor (7, 8) of the first compressor stage (5) is used to adjust the cathode mass flow, wherein the pressure is regulated via the turbine bypass (15).

2. The method according to claim 1, characterized in that In the operating range A, the rotational speeds of the compressors (7, 8) of the first compressor stage (5) are used to adjust the pressure ratio.

3. The method according to claim 2, characterized in that In operating range C, as in operating range A, the pressure is regulated via the rotational speed.

4. The method according to claim 1, wherein When the post-stack valve (19) is fully open or is fully opened, or in the event of a desired change in the regulator, the switch from the operating range A to the operating range B is performed.

5. The method according to claim 4, characterized in that Switching from the operating range B to the operating range A occurs when the turbine bypass valve (16) is completely closed or is completely closed, or in the event of a desired regulator change.

6. The method according to claim 5, characterized in that The switch from the operating range A to the operating range C occurs when the desired mass flow falls below a minimum mass flow, wherein the switch from the operating range C to the operating range A occurs when the desired mass flow is above the minimum mass flow.

7. The method according to any one of claims 1 to 6, characterized in that A PID regulator with feedforward control (71, 76, 91, 97) is used. 8 . A computer program product comprising a computer program having software elements for carrying out the method according to claim 1 , when the computer program is executed on a programmable computer device.

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