Method for operating a fuel cell system; fuel cell system

Abstract

Method for operating a fuel cell system (100) with at least one fuel cell stack (11) comprising at least one fuel cell and comprising an anode system (200) and a cathode system (300), such that when operating the fuel cell system (100) in a first load point range comprising a low load point range and a medium load point range, an expected variance of the load points is determined and the humidity in the fuel cell stack (11) is adjusted depending on the expected variance of the load points.

Description

The invention relates to a method for operating a fuel cell system having the features of the preamble of independent claim 1.Prior ArtIt is known from the prior art that fuel cell systems exist which have an anode system and a cathode system. In this case, the anode system usually comprises an anode feed line which feeds fuel to a fuel cell stack, and a recirculation line which recirculates anode off-gas into the anode feed line by means of a conveying unit. Furthermore, the cathode system usually consists of a cathode feed line, via which air is fed to the fuel cell stack and in which a compressor can be arranged. In the cathode system, a cathode outlet line is arranged, through which cathode exhaust gas is conveyed from the cathode system into the exhaust line.The waste heat of the fuel cell stack is dissipated by means of a cooling circuit and discharged to the environment at a vehicle radiator. A coolant is recirculated in the cooling circuit. The coolant is pumped through the fuel cell stack by means of a coolant pump. A 3-way valve allows the vehicle radiator to be partially or fully bypassed.Disclosure of the InventionThe method according to the invention for operating a fuel cell system having the features according to independent claim 1 has the advantage that a more rapid load increase from a low load point range or a medium load point range can be set to a high load point range.It is advantageous if an expected variance of the load points is determined from a first load point range, which comprises a low load point range and a medium load point range, and the humidity in the fuel cell stack is set depending on the expected variance of the load points. This allows improved dynamic operation of the fuel cell system by adapting the moisture in the fuel cell system to the variance to be expected.In the case of a low variance of the load points to be expected, at which the load points to be expected lie within the first load point range, a high humidity is advantageously set in the fuel cell stack. As a result, the operation of the fuel cell system in the first load point range can take place in an optimized manner.In the case of a high variance of the load points to be expected, a low humidity in the fuel cell stack is advantageously set. This allows a quick and efficient response to a change in the power requirement of the fuel cell system. A high variance of the load points to be expected is present if the load points to be expected are within the first and the second load point range, wherein the second load point range corresponds to a high load point range.It is advantageous if at least one of the following measures is carried out in order to reduce the moisture in the fuel cell stack, since these measures reduce the moisture in the fuel cell stack without additional components having to be added to the fuel cell system, thus ensuring a compact and cost-effective construction:increasing the supply of reactants and / orreducing the pressure in the cathode system and / or anode system and / orincreasing the temperature of the fuel cell stack and / orbypassing a humidifier arranged in the cathode system via a bypass line and / orremoving gaseous and / or liquid water from the anode system via a purge valve and / or drain valve which are arranged in the anode systemIt is advantageous if at least one of the following measures is carried out in order to increase the moisture in the fuel cell stack, since these measures reduce the moisture in the fuel cell stack without additional components having to be added to the fuel cell system, thus ensuring a compact and cost-effective construction:reducing the supply of reactants and / orincreasing the pressure in the cathode system and / or anode system and / orreducing the temperature of the fuel cell stack and / orhumidifying the cathode system by the humidifier connected to the cathode supply line; and / orrecirculating anode exhaust gas from a recirculation line into an anode supply line which is arranged in the anode system, so that an increase in the humidity in the anode system is set by the recirculationIt is advantageous if a transition between the first load point range and the second load point range is determined by a dynamic limit value, wherein the dynamic limit value determines the transport-away capacity of liquid water from the fuel cell stack. The transition between the first load point range and the second load point range can be determined more accurately by the dynamic limit value.The dynamic limit value is advantageously determined via parameters in the fuel cell system, such as the flow rate of mass flows and / or pressure loss and / or moisture state and / or voltage difference across one or more fuel cells.It is advantageous if the variance of the load points to be expected is determined by evaluating geographical data and topographical data and / or navigation data, in particular the expected path profile and / or the type of road, and / or information from the previous driving behavior and / or fleet data. This data evaluation can be used to determine in an optimized manner which variance of the load points to be expected occurs in the near future during operation of the fuel cell system.A low variance of the load points to be expected is advantageously determined if the driving mode suggests a moderate increase in the power demand in the near future, in particular if an energy-saving mode is activated and / or a cruise control is activated. As a result, the variance to be expected can be determined without the additional sensors being necessary in the fuel cell system, whereby a cost-effective and space-saving design is made possible.A high variance of the load points to be expected is advantageously determined if the driving mode is expecting an increased increase or a sudden increase in the power requirement in the near future, in particular if a dynamic driving mode is activated. As a result, the variance to be expected can be determined without the additional sensors being necessary in the fuel cell system, whereby a cost-effective and space-saving design is made possible.It is advantageous to use a control device for carrying out the method according to the invention, wherein the control device is designed to determine an expected variance of the load points and to correspondingly set the humidity of the fuel cell stack to the expected variance of the load points. All necessary steps for carrying out the method according to the invention can be regulated and / or controlled centrally and in a method-optimized manner by the control unit.The fuel cell system in the sense of the invention can preferably be used for mobile applications, for example in vehicles, in particular fuel-driven vehicles.The fuel cell system in the sense of the invention can serve as a main energy supplier for a vehicle. At the same time, however, it is also possible that the fuel cell system in the sense of the invention can be a power take-off and / or auxiliary drive of a vehicle, for example a hybrid vehicle.DESCRIPTION OF THE DRAWINGSThe method according to the invention and the fuel cell system according to the invention are explained in more detail below with reference to drawings with preferred exemplary embodiments.The following are shown: FIG. 1 shows a schematic topology of a fuel cell system according to a first exemplary embodiment; FIG. 2 shows a schematic diagram with a current-voltage characteristic curve as a function of the moisture state; FIG. 3 shows a flow chart of the method according to the invention;FIG. 1 shows a schematic topology of a fuel cell system 100 having at least one fuel cell stack 11, an anode system 200, a cathode system 300 and a cooling circuit 400.A plurality of fuel cells are located in the fuel cell stack 11, which together form the fuel cell stack 11.The cathode system 300 supplies a cathode space K with oxygen (O 2) as a reactant. Oxygen forms a constituent of air. By supplying air into the fuel cell system 100, the oxygen is made available to the fuel cell system as a reactant.A cathode feed line 31 is arranged in the cathode system 300 and opens into the fuel cell stack 11. Air is supplied to the fuel cell stack 11 via the cathode supply line 31.A cathode compressor 33 is located inside the cathode feed line 31; the cathode compressor 33 conveys air into the fuel cell stack 11.The volume flow of the air can be varied with the aid of the cathode compressor 33. An increase in the power of the cathode compressor 33 has the effect that an increased volume flow of air is supplied to the fuel cell stack 11 via the cathode feed line 31. By reducing the power of the cathode compressor 33, a reduced volumetric flow of air is supplied to the fuel cell stack 11.A cathode outlet line 32 is arranged in the cathode system 300. Gases, such as cathode exhaust gas and / or fluids, such as product water, are discharged from the cathode system 300 via the cathode outlet line 32.Optionally, temperature sensors can be additionally arranged in each of the cathode outlet line 32 and cathode feed line 31 in order to measure the temperature of the cathode exhaust gas exiting from the cathode space K and / or in order to measure the temperature of the inflowing air.A humidifier 35 is arranged in the cathode system 300. The humidifier is connected to the cathode lead 31 and the cathode outlet lead 32. The humidifier 35 accumulates the air supplied to the cathode space K with water and thereby increases the humidity in the cathode system 300.Upstream of the humidifier 35, a bypass line 34 is connected to the cathode supply line 31 in the flow direction. The bypass line 34 opens into the cathode feed line 31 in the flow direction between the humidifier 35 and the cathode space K. Air can be supplied to the cathode space K through the bypass line 35 without the latter being enriched with water by the humidifier 35.The anode system 200 supplies an anode space A of the fuel cell stack 11 with a fuel or anode fluid, in particular hydrogen (H 2) as reactants. By supplying fuel into the anode space A, the fuel is made available as reactant to the fuel cell system 100.An anode feed line 22 is arranged in the anode system 200, which leads into the fuel cell stack 11. Fuel is supplied to the fuel cell stack 11 via the anode feed line 22.The supply of fuel into the fuel cell stack 11 can take place superstoichiometrically, with the result that fuel is still contained in the anode offgas. In order to make the fuel contained in the anode exhaust gas available to the anode system 200, anode exhaust gas is recirculated from a recirculation line 21 into the anode feed line 22. The recirculation line 21 is connected to the anode space A.A combined valve jet pump arrangement 26 is arranged in the anode feed line 22. The jet pump is arranged between the anode feed line 22 and the recirculation line 21 and connects these to one another.A combined valve jet pump assembly 26 typically includes a metering valve and a jet pump. The metering valve is connected firmly to the jet pump, for example, by means of a screw connection.The combined jet pump assembly 26 varies the ratio of fuel and anode exhaust gas in the anode feed line 22 downstream of the combined jet pump assembly 26.When the volume flow of the supplied fuel into the combined valve jet pump arrangement 26 is increased, the combined valve jet pump arrangement 26 likewise increases the volume flow of the anode exhaust gas from the recirculation line 21 into the anode feed line 22.A recirculation conveying unit 25 is optionally arranged within the recirculation line 21. The recirculation delivery unit 25 can be a compressor which is designed as a blower, pump and / or compressor.The recirculation delivery unit 25 assists the recirculation of the anode off-gas from the recirculation line 21 into the anode feed line 22.With the aid of the recirculation feed unit 25, the volume flow of the anode off-gas, which flows from the recirculation line 21 into the anode feed line 22, can be additionally varied. An increase in the power of the recirculation delivery unit 25 has the effect that an increased volume flow of anode exhaust gas can be supplied to the fuel cell stack 11 via the recirculation line 21. By reducing the power of the recirculation delivery unit 25, a reduced volume flow of anode exhaust gas is supplied to the fuel cell stack 11.An anode outlet line 23 is arranged in the anode system 200. The anode outlet line 23 is connected to the recirculation line 21. Via the anode outlet line 23, gases, such as anode off-gas and / or fluids, such as water, are discharged from the anode system 200.A combined purge / drain valve 24 is disposed in the anode outlet line 23. When the combined purge / drain valve 24 is opened, anode off-gas and / or water is discharged from the anode system 200. The valve jet pump assembly 26 correspondingly meters fuel into the anode system 200 to establish a continuous flow rate.In an alternative embodiment, the combined purge / drain valve 24 can also be formed in two separate components, such that a purge valve and a drain valve are present in the anode system 200, respectively.A water separator 27 is arranged in the anode system 200. The water separator 27 is arranged in the flow direction between the recirculation line 21 and the anode outlet line 23 and connects these to one another. Water is removed from the anode off-gas with the aid of the water separator 27. In various embodiments, the water separator 27 can be designed: as an automatic water separator or as a manual water separator and / or with an additional filter and / or with an additional active cooling system.A control unit 500 is provided in order to control or control all control and control processes in the fuel cell system 100.This also includes the processing of information for carrying out the method according to the invention.The controller 500 may be in communication with the sensors in the fuel cell system 100 to monitor sensor values. The control unit 500 can actuate the actuators in the fuel cell system 100 in order to correspondingly carry out the method according to the invention.In addition, the control device 500 can be in a communication connection with an external computing unit in order to transfer method steps and / or calculations wholly or partly to the external computing unit.The cooling circuit 400 is provided in the fuel cell system 100. The cooling circuit 400 serves for the temperature control of the fuel cell stack 11 in that a coolant flows through the fuel cell stack 11 through a coolant path KM.The coolant circuit 400 comprises a cooling circuit 45 in which a coolant is recirculated by means of a coolant pump 43. A 3-way valve 41 may direct the coolant at least partially or completely past a vehicle radiator 42 via a bypass 46.A coolant outlet temperature sensor 44 measures the temperature of the coolant after it exits from the fuel cell stack 11 Optionally, a temperature sensor can likewise be arranged in the cooling circuit 45 downstream of the coolant pump 43 in the flow direction.In an alternative embodiment, more than one fuel cell stack 11 can also be arranged in the fuel cell system 100, without the execution of the method according to the invention being restricted thereby.The method according to the invention can be carried out in a fuel cell system 100 having a plurality of fuel cell stacks 11 in parallel or sequentially in the fuel cell stacks 11.FIG. 2 shows an exemplary diagram with a current-voltage characteristic curve of the fuel cell system 100 as a function of the moisture state. The x-axis shows the course of the current in amperes. The voltage in volts is represented on the y-axis. The thick line represents the course of the current-voltage characteristic curve of the fuel cell system during moist operation, in which a high moisture content has been set in the fuel cell stack. The thin line represents a dry operation of the fuel cell stack, in which a low humidity has been set in the fuel cell stack.There is a low load point range at load points below 20% of the rated current strength. The average load point range is in a range of about 20% to 70% of the rated current strength at load points. Starting from a range of more than 70% of the rated current strength, the high load point range begins. The rated current intensity is the current intensity at which the fuel cell system 100 provides the maximum power during dry operation. The percentages given are based on the graph shown in FIG. 2, which may vary depending on the embodiment of the fuel cell system 100.FIG. 2 shows by way of example how the moisture content of the fuel cell system 100 has an effect on the current-voltage characteristic curve. It follows from the diagram that, in the case of a high load point range, low moisture, in the form of dry operation, proves to be advantageous for the performance of the fuel cell system 100. At high humidity in the high load point range, local undersupply of reactants may occur due to the accumulation of liquid water in the fuel cell system 100, in particular in gas diffusion layers. The gas diffusion layers are arranged between the fuel cells within the fuel cell stack 11. The gas diffusion layers serve for the efficient and uniform distribution of the reactants hydrogen and oxygen.In the case of a low load point range and a medium load point range, moist operation proves to be advantageous since the efficiency of the fuel cell system 100 is the highest at a high moisture and the ohmic losses are the smallest.It is evident from the diagram that for a change in the load requirement of the fuel cell system towards operation in the high load point range, dry operation with a low humidity should be set. Short term changes in power demand during operation of the fuel cell system from a low load point or a medium load point to a high load point require adjustment of the humidity from a wet operation to a dry operation. While the change of the load point toward a high load point range can take place in a very short time, for example in less than one second, the reduction of the moisture in the fuel cell stack requires a longer period of time, such as 10 s, for example, depending on the embodiment of the fuel cell system 100.FIG. 3 shows a flow diagram of an exemplary method according to the invention.In a step S 100, the method is started, so that step S 100 represents the start. The method according to the invention is preferably carried out during the operation of the fuel cell system 100 in the first load point range, which comprises a low and a medium load point range.In step S 200, an expected variance of the load points is determined.There is a low variance to be expected if the load points to be expected are in a first load point range. The first load point range comprises a low and a medium load point range as exemplarily shown in FIG. 2.A high variance to be expected is present if the load points to be expected are in the second load point range. The second load point range corresponds to a high load point range as shown by way of example in FIG. 2.The variance to be expected can be determined by evaluating the following parameters.the evaluation of a driving mode, wherein the activation of an energy-saving driving mode ("eco mode") and / or cruise control suggests a moderate increase in the power demand in the near future and indicates a low variance to be expected, and / or the activation of a dynamic driving mode ("sport mode") that suggests an increased increase or a sudden increase in the power demand in the near future and indicates a high variance to be expected, and / orthe evaluation of geographical data and topographical data, wherein a low-arc road and / or a low expected gradient during the operation of the fuel cell system 100 points to a low variance to be expected and a high-arc road and / or a high expected gradient during the operation of the fuel cell system 100 points to a high variance to be expected and / orthe evaluation of navigation data, in particular the expected path profile and / or the type of road, wherein a traffic guidance that changes little direction indicates a low variance to be expected and / or a traffic guidance that changes direction frequently indicates a high variance to be expected and / orthe evaluation of information from the previous driving behavior and / orfleet dataIt is optionally additionally possible to determine the transition from the first load point range to the second load point range in an optimized manner via a dynamic limit value, wherein the dynamic limit value is determined via the draining capacity of liquid water from the fuel cell system 100.The dynamic limit value is determined by means of parameters in the fuel cell system 100, such as the flow rate of mass flows and / or pressure loss and / or moisture state and / or voltage difference across one or more fuel cells. The parameters can be stored in the control device via a table or a model.A high flow rate of mass flows in the fuel cell system 100 promotes the removal of liquid water from the anode system 200 and / or cathode system 300. The mass streams can be volume streams, mole streams and mass streams. A low flow rate of mass flows in the fuel cell system 100 promotes the accumulation of liquid water and reduces the transport-away capacity of liquid water from the fuel cell system 100.If a high mass flow and additionally a high pressure loss are determined, there is a sufficient draining capacity of liquid water from the fuel cell system 100. When a high mass flow rate and, in addition, a low pressure loss are detected, the liquid water discharge capability from the fuel cell system 100 is impaired.In addition, the humidity in the fuel cell stack 11 can be measured to determine the dynamic limit value. This can be done, for example, by means of a high-frequency resistance measurement. In this case, less liquid water can be transported away in the case of a high humidity in the fuel cell stack 11 than in the case of a low humidity in the fuel cell stack 11.In addition, a voltage difference across one or more fuel cells may be determined to determine the dynamic limit value. This may be via multi-cell voltage monitoring ("MCM") and / or cell voltage monitoring ("CVM"). In the case of a high voltage difference, it is to be assumed that the transport-away capacity of liquid water is restricted. With small measured voltage differences, the transport-away capacity of liquid water is sufficiently given.If a high variance to be expected is determined in step S 200, a step S 301 is then carried out. In step S 301, a low humidity in the form of dry operation of the fuel cell system 100 is set.The setting of a low humidity in the fuel cell system 100 may be performed by the following measures:increasing the supply of reactants and / orreducing the pressure in the cathode system 300 and / or anode system 200 and / orincreasing the temperature of the fuel cell stack 11 and / orbypassing the humidifier 35 arranged in the cathode system 300 and / orremoving gaseous and / or liquid water from the anode system 200 via a purge valve and / or drain valve, which are arranged in a recirculation line 21 in the anode system 200.If a low variance to be expected is determined in step S 200, a step S 302 is then carried out. In step S 302, a high humidity is set in the fuel cell system 100, and the following measures can be taken:reducing the supply of reactants and / orincreasing the pressure in the cathode system 300 and / or anode system 200 and / orreducing the temperature of the fuel cell stack 11 and / orhumidifying the cathode system 300 by the humidifier 35 connected to the cathode supply line 31 and / orrecirculating anode exhaust gas from recirculation line 21 into anode supply line 22, which is situated in anode system 200, so that an increase in the humidity in anode system 200 is established by the recirculation.In step S400, it is checked whether an event triggering end of the method has occurred. An event triggering an end of the method may represent a shutdown of the fuel cell system 100. If no end-of-process triggering event has occurred, step S 200 is carried out again.In step S 500, the method according to the invention is ended, so that step S 500 represents the end.In an alternative exemplary embodiment, it is likewise possible to carry out the method according to the invention using more than one fuel cell stack 11.The method according to the invention can be carried out in a fuel cell system 100 having a plurality of fuel cell stacks 11 in parallel or sequentially.According to a further aspect, the invention provides a computer program product comprising instructions which, when the computer program product is executed by a computer, such as the arithmetic unit of the control unit, cause the computer to carry out the method which can be carried out as described above. With the aid of the computer program product, the same advantages as were described with the control unit according to the invention can be achieved.

Claims

Method for operating a fuel cell system (100) having at least one fuel cell stack (11), which has at least one fuel cell and which has an anode system (200) and a cathode system (300), characterized in that, during operation of the fuel cell system (100), in a first load point range which comprises a low load point range and a medium load point range, an expected variance of the load points is determined and the moisture in the fuel cell stack (11) is set as a function of the expected variance of the load points.Method according to Claim 1, characterized in that, in the case of a low variance of the load points to be expected, at which the load points to be expected lie within the first load point range, a high humidity is set in the fuel cell stack (11).Method according to Claim 1, characterized in that, in the case of a high variance of the load points to be expected, a low moisture is set in the fuel cell stack (11), wherein, in the case of a high variance of the load points to be expected, the load points to be expected lie within the first load point range and within a second load point range which corresponds to a high load point range.Method according to Claims 1 to 3, characterized in that the following measures can be carried out in order to reduce the moisture in the fuel cell stack (11): - increasing the feed of reactants and / or - reducing the pressure in the cathode system (300) and / or anode system (200) and / or - increasing the temperature of the fuel cell stack (11) and / or - bypassing a humidifier (35) which is arranged in the cathode system (300) via a bypass line (34) and / or - removing gaseous and / or liquid water from the anode system (200) via a purge valve and / or drain valve which are arranged in the anode system (200)Method according to Claims 1 to 3, characterized in that the following measures can be carried out in order to increase the moisture in the fuel cell stack (11): - reducing the supply of reactants and / or - increasing the pressure in the cathode system (300) and / or anode system (200) and / or - reducing the temperature of the fuel cell stack (11) and / or - humidifying the cathode system (300) by the humidifier (35) which is connected to the cathode supply line (31) and / or - recirculating anode off gas from a recirculation line (21) into an anode supply line (22) which is arranged in the anode system (200), with the result that an increase in the moisture in the anode system (200) is set by the recirculationMethod according to Claim 1, characterized in that a transition between the first load point range and the second load point range is determined by a dynamic limit value, the dynamic limit value determining the transport-away capacity of liquid water from the fuel cell stack (11).Method according to Claim 6, characterized in that the dynamic limit value is determined via parameters in the fuel cell system (100), such as the throughflow of mass flows and / or pressure loss and / or moisture state and / or voltage difference across one or more fuel cells.Method according to Claim 1, characterized in that the expected variance of the load points is determined by evaluating geographical data and topographical data and / or navigation data, in particular the expected path profile and / or the type of road, and / or information from the previous driving behavior and / or fleet data.Method according to Claim 2, characterized in that a low variance of the load points to be expected is determined if the driving mode suggests a moderate increase in the power demand in the near future, in particular if an energy-saving mode is activated and / or a cruise control is activated.Method according to Claim 3, characterized in that a high variance of the load points to be expected is determined if the driving mode is expected to have an increased increase or a sudden increase in the power requirement in the near future, in particular if a dynamic driving mode is activated.Fuel cell system (100) having at least one fuel cell stack (11), which has at least one fuel cell and which has an anode system (200) and a cathode system (300), having a control unit (400) for carrying out the method according to at least one of Claims 1 to 10, characterized in that the control unit (400) is designed to determine an expected variance of the load points and to set the humidity of the fuel cell stack (11) correspondingly to the expected variance of the load points.

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