Method for operating a fuel cell system, control unit

Abstract

The invention relates to a method for operating a multi-stack system (1) with at least two stacks (2), each connected to a common air supply system (5) via an inlet-side valve (3) and an outlet-side valve (4), wherein at least one compressor (7) coupled to an exhaust turbine (6) is integrated into the air system (5). According to the invention, the air supply is controlled stack-specifically, wherein (a) the pressure is adjusted via the rotational speed of the at least one compressor (7) and the flow cross-section of the respective inlet-side valve (3), and (b) the mass flow is adjusted via the flow cross-section of the respective outlet-side valve (4) and the flow cross-section of a bypass valve (9) integrated into a bypass path (8) to bypass the exhaust turbine (6). The invention further relates to a control unit for a multi-stack system (1) for carrying out steps of a method according to the invention.

Description

The invention relates to a method for operating a fuel cell system having the features of the preamble of claim 1.Mobile fuel cell systems or fuel cell vehicles are preferred fields of application.Prior ArtIn vehicles whose drive energy is supplied by one or more fuel cell systems, the oxidizing agent oxygen from the ambient air is generally used to react with hydrogen to water in the fuel cells and to supply an electrical power by electrochemical conversion. The air taken from the environment is provided via an air system comprising an air compression system. This is because the electrochemical reaction in the fuel cells requires a certain air mass flow and a certain pressure level.For compressing the air, thermal turbomachines are usually used, which are driven by electric motor. In high efficiency air compression systems, a portion of the energy used for compression can be recovered by means of an exhaust air turbine. Higher system pressures can be achieved using multi-stage air compression systems.The air compressed with the aid of the air compression system is supplied to the fuel cells of a fuel cell stack, also referred to as a "stack", via an air supply path of the air system. Since the air heats up very strongly during the compression, it is cooled before it enters the fuel cell stack. For this purpose, at least one cooling device is integrated into the supply air path. For further conditioning of the air, a humidifying device may be further arranged in the supply air path. The air or exhaust air emerging from the fuel cell stack is discharged via an exhaust air path of the air system. The exhaust air turbine is integrated into the exhaust air path, if present.With the aid of such an air system, it is also possible to supply a plurality of fuel cell stacks or stacks simultaneously with air. If this is the case, this is referred to as a multi-stack system with a central air supply via an air system. This has the advantage that installation space and costs are saved. However, an increased complexity in the regulation of the air system has proven disadvantageous, since ideally the air mass flow and the pressure are set individually for the stack.The present invention is concerned with the object of simplifying the control of an air system in a multi-stack system. The control is intended to take into account limitations which are predefined, for example, by the air compression system.To achieve the object, the method having the features of claim 1 is proposed. Advantageous embodiments can be taken from the dependent claims. Furthermore, a control device for a fuel cell system for carrying out steps of the method is specified.Disclosure of the InventionA method for operating a multi-stack system having at least two stacks is proposed, which are each connected via an inlet-side valve and an outlet-side valve to a common air system for supplying air. At least one compressor coupled to an exhaust-air turbine is integrated into the air system. According to the invention, the air supply is regulated individually for the stack, wherein (a) the pressure is set via the rotational speed of the at least one compressor and the flow cross section of the respective inlet-side valve, and (b) the mass flow is set via the flow cross section of the respective outlet-side valve and the flow cross section of a bypass valve integrated into a bypass path for bypassing the exhaust air turbine.The proposed method enables covering the entire operating range of the multiple stacks and taking into account different system boundary conditions and component-specific limitations. Since each stack is connected to the common air system via an inlet-side and an outlet-side valve, pressures below a minimum pressure of the compressor and above a maximum pressure of the exhaust-air turbine can be set.Covering the entire operating range also requires only a few controller changeovers, so that the stack-specific control of pressure and mass flow in a multi-stack system is simplified. Furthermore, with the aid of the proposed method, the operation of the air system can be optimized with regard to system efficiency and hydrogen consumption.According to the proposed method, only the pressure is set via the rotational speed of the at least one compressor. In contrast, the valves mentioned are used to set the mass flow. The mass flow required by a stack can thereby be set more quickly.In order to provide pressures of different levels, the rotational speed of the at least one compressor is preferably set to the respectively highest pressure. A lower pressure required by a further stack can then be adjusted by partially closing the respective inlet-side valve. The inlet-side valve of the stack requiring the highest pressure may remain fully open.In the stack-specific pressure control via the rotational speed of the at least one compressor and the flow cross sections of the inlet-side valves, the stuffing limit of the at least one compressor may be undershot. If this is the case, the flow cross section of at least one inlet-side valve is preferably reduced. For example, the flow cross section of an inlet-side valve that is still fully open can be reduced by partial closing. This means that the inlet-side valve of the stack, which requires the highest pressure, is then also partially closed.Furthermore, it is proposed that, in order to maintain a minimum mass flow of the at least one compressor, a bypass valve is opened, which bypass valve is integrated into a bypass path for bypassing the stacks. By opening the bypass valve, the total mass flow is increased, so that the minimum mass flow is not undershot. In this way, further component-specific, in particular compressor-specific, limitations can be taken into account in the stack-specific regulation of the air supply.In carrying out the proposed method, a plurality of compressors are preferably used for air compression. The plurality of compressors may collectively form a multi-stage air compression system, each air compression stage including at least one compressor. In the case of a plurality of compressors, the rotational speeds are preferably set such that the power of the compressors is minimal. In this way, the efficiency of the system can be increased. If the air system comprises two compressors, for example, the inlet temperature at the first compressor is generally lower than at the second compressor. In order for the output to be minimal, the pressure ratio at the second compressor should be set as low as possible.Furthermore, a pilot control and / or a PID controller are preferably used for the stack-specific regulation of the air mass flow. The pilot control concerns the outlet-side valves and the bypass valve for bypassing the exhaust-air turbine. For the pilot control, a setpoint outlet pressure can be calculated on a stack-specific basis, for example on the basis of the setpoint mass flow and the characteristic curve of the respective stack. The flow cross sections of the outlet-side valves and of the bypass valve for bypassing the exhaust-air turbine are then appropriately pilot-controlled or adjusted. The bypass valve is preferably kept closed or closed in order to maximize the output of the exhaust air turbine or to minimize the electromotive drive output of the at least one compressor.Furthermore, a control device for a multi-stack system is proposed, which is configured to execute steps of a method according to the invention. With the aid of the control unit, the rotational speed of the at least one compressor and the flow cross sections of the inlet-side valves can be adjusted for stack-specific regulation of the pressure. Furthermore, with the aid of the control unit for stack-specific regulation of the mass flow, the flow cross sections of the outlet-side valves and of the bypass valve for bypassing the exhaust air turbine can be adjusted.The invention and its advantages are explained in more detail below with reference to the attached drawings. These show: FIG. 1 shows a schematic illustration of a multi-stack system having two stacks, which can be operated according to a method according to the invention, and FIG. 2 shows a diagram for the stack-specific regulation of the mass flow in a multi-stack system, which is constructed analogously to the system of FIG. 1.DETAILED DESCRIPTION OF THE DRAWINGSThe multi-stack system 1 shown in FIG. 1 comprises two stacks 2 which are supplied with air via a common air system 5. The air is supplied to the stacks 2 via an inlet air path 12 which divides into two inlet air paths 12.1, 12.2 at a point A. The air or exhaust air emerging from the stacks 2 is discharged via an exhaust air path 13, to which two separate exhaust air paths 13.1, 13.2 combine at a point B. Integrated in each of the supply air paths 12.1, 12.2 is an inlet-side valve 3. An outlet-side valve 4 is integrated in each of the exhaust air paths 13.1, 13.2.The illustrated air system 5 comprises in the present case two compressors 7 driven by electric motor, which are each coupled to an exhaust air turbine 6 for energy recuperation. For cooling the compressed air, a cooling device 14 is integrated into the air system 5 downstream of each compressor 7. Since the air is taken from the environment 15, an air filter 16 is additionally connected upstream of the first compressor 7. The exhaust air discharged via the exhaust air path 13 is discharged again to the environment 15.To reduce the pressure drop across the exhaust air turbines 6, the mass flow through the exhaust air turbines 6 can be reduced in each case with the aid of a bypass valve 9, which is integrated into a bypass path 8 for bypassing the exhaust air turbines 6. However, this also reduces the power of the exhaust air turbines 6, so that-as far as possible-the bypass valves 9 are closed.The rotational speed of the two compressors 7 can be regulated by means of a control unit (not shown). According to the method according to the invention, the pressure is set individually for the stack by means of the rotational speed of the compressors 7 and the flow cross sections of the inlet-side valves 3. For the stack-specific setting of the mass flow, with unchanged rotational speed of the compressors 7, only the flow cross sections of the outlet-side valves 4 and of the bypass valves 9 are set. The stack-specific mass flow can be set more quickly in this way.With the aid of a further bypass valve 11, which is integrated into a bypass path 10 for bypassing the stacks 2, it can be ensured that a certain minimum mass flow through the compressors 7 is not undershot.The setpoint mass flow mf1Des for the first stack 2 or the setpoint mass flow mf2Des for the second stack 2 is regulated by means of the outlet-side valves 4 and the bypass valves 9 for bypassing the exhaust air turbines 6. Since the setpoint mass flow is to be set faster than the setpoint pressure, the valves 4, 9 are used for the regulation and not the rotational speeds of the compressors 7. At this time, the higher of the two pressures is set, if possible, at the point A. The lower of the two pressures is adjusted by closing the respective inlet-side valve 3. If the greater of the two pressures is below the plugging limit of the two compressors 7, the inlet-side valve 3 of the stack 2 having the greatest desired pressure must also be closed in order to set the pressure. The respective setpoint outlet pressure of the stacks 2, which can be calculated using the setpoint mass flow and the stack characteristic curve, is used for calculating the pilot control of the outlet-side valves 4 and of the bypass valves 9 for bypassing the exhaust air turbines 6. The valves 4, 9 are pilot-controlled or adjusted accordingly. The bypass valves 9 are preferably closed in order to maximize the power of the exhaust air turbines 6 and thus minimize the necessary electromotive drive power of the compressors 7. For the mass flow control, for example, a pilot control and a PID controller which controls the mass flow can be used.The pressure drop between stack outlet and environment is calculated with the total desired mass flow (sum of mfXDes and mfByCaDes) when the bypass valves 9 are open. With the respective stack mass flow mfXDes, the pressure drop of the respective stack is calculated when the outlet-side valve 4 is open. The lower limit for the pressure pxDes at the stack inlet is in each case the sum of these two pressure differences and the ambient pressure.To control the bypass valve 11, which serves to bypass the stacks 2, the minimum mass flow mfCMinDes for the setpoint pressure max(pxDes) is preferably stored as a characteristic curve at the stack inlet. The characteristic curve can be calculated from the conditions at the stack inlet and the pump characteristic curve. If a straight line is assumed as the pump characteristic, the following results:From the difference mfCMinDes-sum(mfXDes), the setpoint mass flow mfByCaDes for the bypass path 10 for bypassing the stacks 2 is calculated. If the desired mass flow>0, this is set, for example, with a PID controller with pilot control. For pilot control, for example, the valve geometry of the bypass valve 11 integrated into the bypass path 10, the setpoint pressure at the inlet / outlet of the bypass valve 11 and the setpoint mass flow can be used.The rotational speeds nCompX of the compressors 7 are set so that the output of the compressors 7 becomes minimum. Since the inlet temperature at the second compressor 7 is generally higher than the inlet temperature at the first compressor 7, the lowest power is generally produced if the pressure ratio at the second compressor 7 is set to be as low as possible.Safety factors which set limits to the pressure ratio can be taken into account in the limit characteristic maps. The maximum and minimum pressure ratio of the first compressor 7 is stored as a function of the corrected mass flow:Here, mf is the maximum of the desired mass flow sum(mfXDes) and the minimum mass flow mfCMinDes.The total desired compression pressure ratio PIDes is limited to values between PI1_min*PI2_min_ and PI1_max*PI2_max_→PIDesLim (see FIG. 2 ). The values min_ and max_ are derived from the values of the first compressor 7.Smaller pressure conditions can be set only by closing the inlet-side valves 3 of the stacks 2. Larger pressure ratios cannot be set. The minimum and maximum pressure ratio of the second compressor 7 is stored in the control unit as a characteristic diagram of the form PI2_min or PI2_max=f(mf*sqrt(T_in / Tref),p_out). The characteristic map can be iteratively calculated from the characteristic map of the form Plmin2or PImax2=f(mf*sqrt(T_in / Tref ), p_in). With p_out=PIDesLim*pC1 _in, T_in=TC2_in, and mf=max(sum(mfXDes), mfCMinDes), the boundaries may be read from the map.Selection of Optimum Pressure Ratio:A PID controller increases or decreases the respective setpoint inlet pressure for the control. The two pressure ratios are set with the rotational speeds of the compressors 7. The respective rotational speed is calculated via characteristic maps as a function of Plx_opt and mf_corr= mf*sqrt(TCx_in / Tref) / (pX_in / pRef) with pC2_in=Pl1_opt*pC1_in+dp.In the case of unequal desired pressures, the smaller pressure is set via the respective inlet-side valve 3. At the same desired pressures, the inlet-side valves 3 of both stacks 2 are open if the compressor 7 surge limit is not reached. At the stuffing limit, both pressures are adjusted via the inlet-side valve 3.The mass flow mfCDes=max(sum(mfXDes ), mfCMinDes), at point A (see FIG. 1 ) is adjusted with the bypass valves 9 for bypassing the exhaust air turbines 6 and the outlet-side valves 4 of the stacks 2. For efficiency reasons, the bypass valves 9 are closed before the outlet-side valves 4 are closed.A second mass flow controller controls the differential mass flow mf1Desmf2Des. The two mass flow controllers can be designed, for example, as PID controllers and apply a differential pressure to the desired pressures.The control of the stack outlet pressures pxDes_out is calculated with the aid of the setpoint mass flows mfxDes and the setpoint pressures pxDesLim, limited by the minimum pressure of the exhaust air turbines 6 and the maximum pressure ratio of the compressors 7.The pressure offset of the regulators is added to the two pressures. The lower of the two pressures determines the position of the bypass valves 9 for bypassing the exhaust air turbines 6, and the higher pressure is set with the outlet-side valve 4 of the respective stack 2. If the exhaust air turbines 6 cannot set the lower pressure when the bypass valves 9 are closed, the outlet-side valves 4 of both stacks 2 set the respective pressure and the bypass valves 9 are fully closed.

Claims

Method for operating a multi-stack system (1) having at least two stacks (2), which are each connected via an inlet-side valve (3) and an outlet-side valve (4) to a common air system (5) for supplying air, wherein at least one compressor (7) coupled to an exhaust-air turbine (6) is integrated into the air system (5), characterized in that the air supply is regulated in a stack-specific manner, wherein (a) the pressure is set via the rotational speed of the at least one compressor (7) and the flow cross section of the respective inlet-side valve (3) and (b) the mass flow is set via the flow cross section of the respective outlet-side valve (4) and the flow cross section of a bypass valve (9) integrated into a bypass path (8) for bypassing the exhaust-air turbine (6).Method according to Claim 1, characterized in that, in order to provide pressures of different magnitude, the rotational speed of the at least one compressor (7) is set to the respectively highest pressure.Method according to Claim 1 or 2, characterized in that, if the plugging limit of the at least one compressor (7) is undershot, the flow cross section of at least one inlet-side valve (3), preferably of a fully open inlet-side valve (3), is reduced.Method according to one of the preceding claims, characterized in that, in order to maintain a minimum mass flow of the at least one compressor (7), a bypass valve (11) is opened, which bypass valve is integrated into a bypass path (10) for bypassing the stacks (2).Method according to one of the preceding claims, characterized in that, in the case of a plurality of compressors (7), the rotational speeds are set such that the power of the compressors (7) is in each case minimal.Method according to one of the preceding claims, characterized in that a pilot control and / or a PID controller are used for the stack-specific regulation of the mass flow.Control device for a multi-stack system (1) which is configured to carry out steps of a method according to one of the preceding claims.

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