Method for operating a fuel cell unit

Abstract

Method for operating a fuel cell unit (1) for the electrochemical generation of electrical energy comprising the steps of: passing recirculating fuel through channels (12) for recirculating fuel of a fuel cell stack (40), passing the recirculating fuel after it has been discharged from the fuel cell stack (40) through a recirculation system (65) with a recirculation line (50) and a water separator (51) and reintroducing the recirculating fuel passed through the recirculation system (65) into the channels (12) for recirculating fuel of the fuel cell stack (40) as a recirculation circuit,Supply of new fuel from a fuel storage (21) through a pressurized gas line (59) with an injector (19) into the recirculation line (50) and routing of the mixture of new fuel and recirculated fuel through a mixing recirculation line (69) and from the mixing recirculation line (69) into the channels (12) for recirculated fuel of the fuel cell stack (40), wherein water in the mixture in the mixing recirculation line (69) is detected by measuring volume flows and / or mass flows and temperatures of the recirculated fuel and new fuel, and at least one measure is carried out depending on the amount of water detected.

Description

[0001] The present invention relates to a method for operating a fuel cell unit according to the preamble of claim 1, a fuel cell unit according to the preamble of claim 12 and a fuel cell system according to the preamble of claim 14. State of the art

[0002] Fuel cell units, as galvanic cells, convert a continuously supplied fuel and oxidant into electrical energy through redox reactions at an anode and cathode. Fuel cells are used in a wide variety of stationary and mobile applications, for example, in homes without a power grid connection or in motor vehicles, in rail transport, aviation, aerospace, and shipping. A large number of fuel cells are stacked in the fuel cell unit to form a fuel cell stack. Integrated into the fuel cell stack are channels for the circulation of recirculating fuel, channels for the circulation of oxidant, and channels for the circulation of coolant.The recirculation fuel flowing through the channels is not completely consumed after flowing through, so after the recirculation fuel is discharged from the recirculation fuel channels, it is returned to the recirculation fuel channels via a recirculation line of a recirculation system. The consumed fuel, depending on the required power of the fuel cell unit, is fed into the recirculation line via an injector. The higher the power of the fuel cell unit, the greater the fuel consumption, and this consumed fuel is fed accordingly via the injector. The fuel, generally hydrogen, is stored in a compressed gas reservoir at high pressure.The recirculation fuel discharged from the fuel cell stack contains a high moisture content, so a water separator in the recirculation system mechanically removes water before the recirculation fuel is reintroduced into the fuel cell stack. The recirculation system includes a blower, a jet pump, the water separator, and drain valves.

[0003] The water separator, as a mechanical water separator, has a separation flow chamber with a flow cross-sectional area that is significantly larger than the flow cross-sectional area of ​​a discharge line or a supply line as a recirculation line for recirculation fuel. The water is separated from the recirculation fuel in the separation flow chamber due to the low flow velocity and the residence time of the recirculation fuel in the separation flow chamber. The recirculation fuel is introduced into the separation flow chamber of the water separator through an inlet opening and discharged from the separation flow chamber through an outlet opening. A collection chamber for water is formed in the water separator. If the water in the collection chamber exceeds a predetermined water level, the water is discharged into the environment via a drain valve.The water level in the plenum is measured by a sensor. However, this sensor is expensive to purchase and unreliable in operation, as the acceleration and deceleration of a vehicle with a fuel cell unit causes fluctuations in the water level in the plenum. Introducing recirculation fuel with a high mass flow of water into the recirculation fuel channels of the fuel cell stack causes damage to the fuel cells.

[0004] DE 10 2017 215 501 A1 shows a method for operating a fuel cell system, comprising a fuel cell stack, an anode supply for supplying an anode operating gas to the fuel cell stack and discharging an anode exhaust gas therefrom, wherein the anode supply has a recirculation system for returning the anode exhaust gas to the fuel cell stack. Disclosure of the inventionAdvantages of the inventionInventive

[0005] A method for operating a fuel cell unit for electrochemically generating electrical energy, comprising the steps of: passing recirculation fuel through recirculation fuel channels of a fuel cell stack, passing the recirculation fuel after being discharged from the fuel cell stack through a recirculation system having a recirculation line and a water separator, and reintroducing the recirculation fuel passed through the recirculation system into the recirculation fuel channels of the fuel cell stack as a recirculation circuit, feeding new fuel from a fuel storage unit through a compressed gas line having an injector into the recirculation line, and passing the mixture of new fuel and recirculation fuel through a mixed recirculation line and from the mixed recirculation line into the recirculation fuel channels of the fuel cell stack,wherein preferably the temperature of the recirculation fuel in the recirculation line is detected as the first temperature with a first temperature sensor before the new fuel is supplied, the temperature of the new fuel supplied by the injector is detected as the second temperature with a second temperature sensor, the temperature of the mixture of new fuel and recirculation fuel in the mixed recirculation line is detected as the third actual temperature with a third temperature sensor, a first volume flow and / or first mass flow and / or a first heat capacity flow of the recirculation fuel in the recirculation line is determined before the new fuel is supplied, a second volume flow and / or second mass flow and / or second heat capacity flow of the new fuel supplied through the compressed gas line with the injector is determined from the detected first and second temperatures,the determined first and second volume flows and / or the determined first and / or second heat capacity flows as well as the density and / or the specific heat capacity of the first volume flow of the recirculation fuel and / or the density and / or the specific heat capacity of the second volume flow of the new fuel, the target temperature of the mixture of new fuel and recirculation fuel in the mixed recirculation line is calculated as the third target temperature and is compared with the third actual temperature and at a third actual temperature which is lower than the third target temperature by a predetermined limit value,at least one measure is carried out and / or by means of the recording of volume flows and / or mass flows and / or heat capacity flows and / or temperatures and / or the specific heat capacity and / or density of the recirculation fuel and new fuel, water and / or a mass flow of water in the liquid state in the mixture in the mixed recirculation line is recorded, and at least one measure is carried out depending on the recorded water and / or recorded mass flow of water. The third target temperature is determined without taking into account the necessary latent heat of vaporization for water, solely based on the mixing of the new fuel and the recirculation fuel. The product of the specific heat capacity c and the mass m results in the heat capacity of a substance. The mass of a substance, for example, the new fuel and the recirculation fuel,results from the product of volume and density. The mass flow of a substance results from the product of volume flow and density. The mass flow of a substance can thus be calculated from the volume flow and density. The heat capacity of a substance results from the product of mass and specific heat capacity. The heat capacity flow of a substance results from the product of mass flow and specific heat capacity. The heat capacity as an extensive state variable for a mixture of different substances is the sum of the respective heat capacities of the different substances. Preferably, the second temperature sensor detects the temperature of the new fuel at a pressure of the new fuel that is substantially, in particular with a deviation of less than 30%, 20% or 10%,corresponds to the pressure of the mixture in the mixed recirculation line. If the second temperature sensor detects the temperature of the new fuel at a pressure that differs significantly from the pressure of the mixture in the mixed recirculation line, the temperature of the new fuel is calculated using appropriate gas equations at essentially the pressure of the mixture in the mixed recirculation line, and this calculated temperature is used. A new fuel is considered to be a fuel that has not yet been passed through the recirculation fuel channels of the fuel cell stack. The specified limit, for example, 0.1 °C, 0.2 °C, 0.5 °C, 1 °C, 3 °C, 5 °C, or 10 °C,is preferably stored in a control and / or regulation unit as a computing unit and was determined for the fuel cell unit using empirical and / or analytical methods and / or numerical simulation methods. The limit value is determined such that only at a critical mass flow of water supplied to the fuel cell stack through the opening of the recirculation fuel channels is the difference between the third target temperature and the third actual temperature smaller than the specified limit value.

[0006] In a further embodiment, the first volume flow of the recirculation fuel in the recirculation line is determined before the new fuel is supplied as a function of at least one parameter of at least one conveying device for the recirculation fuel. Preferably, the at least one parameter of the at least one conveying device for the recirculation fuel is the rotational speed and / or the electrical power of an electric motor for driving the at least one conveying device, and the first volume flow can be calculated using this at least one parameter and stored data, in particular empirically determined data.

[0007] In a supplementary variant, the volume flow of the recirculation fuel in the recirculation line is measured using a first volume flow sensor before the new fuel is supplied. The first volume flow sensor is formed, for example, by two pressure sensors arranged on the recirculation line with different flow cross-sectional areas, so that the volume flow can be calculated using the Bernoulli equation based on the pressure differences at the different flow cross-sectional areas and the known different flow cross-sectional areas. The first volume flow sensor is preferably designed as an ultrasonic flow sensor or a magnetic-inductive flow sensor. A second and / or third volume flow sensor is preferably designed in a manner analogous to the first volume flow sensor.

[0008] In an additional embodiment, the second volume flow of the new fuel supplied by the injector is determined as a function of at least one parameter of the injector and / or the pressure of the new fuel in the fuel reservoir and / or the pressure difference before and after the injector and / or a pressure difference of the new fuel in the compressed gas line before the mixed recirculation line.

[0009] Preferably, the volume flow of the new fuel supplied through the compressed gas line with the injector is detected by a second volume flow sensor.

[0010] In a supplementary embodiment, the predetermined limit value is determined as a function of at least one limit value parameter.

[0011] In an additional variant, the at least one limit parameter is the ambient temperature and / or the first temperature and / or the second temperature and / or the first volume flow and / or the second volume flow. For example, at low outside temperatures, the mixture in the recirculation line cools even without evaporation of water, so this must be taken into account when determining the limit value.

[0012] In a further embodiment, the at least one measure is the temporary opening of a drain valve on the water separator. A full water collection chamber in the water separator is a cause of a large mass flow of water into the mixed recirculation line, ie, the method can replace a sensor for detecting the water level in the collection chamber of the water separator.

[0013] In a further embodiment, the at least one measure is the shutdown of the fuel cell unit. An excessively high mass flow of water into the mixed recirculation line can lead to damage to the fuel cell unit, so that in the case of very high mass flows of water and a resulting very large difference between the third target temperature and the third actual temperature, shutdown of the fuel cell unit may be appropriate.

[0014] In a further embodiment, the at least one measure is the reduction of the power of the fuel cell system in order to again reach the tolerance range with regard to the mass flow of water in the mixed recirculation line.

[0015] Preferably, the at least one measure is an error message, in particular as a visual and / or acoustic warning signal.

[0016] In a supplementary variant, the process is carried out using a fuel cell unit described in this patent application.

[0017] A fuel cell unit according to the invention for the electrochemical generation of electrical energy, comprising stacked fuel cells and the stacked fuel cells forming a fuel cell stack, channels for recirculation fuel integrated into the fuel cell stack, a recirculation system with a recirculation line for recirculating the recirculation fuel discharged from the recirculation fuel channels back into the recirculation fuel channels, so that a recirculation circuit with recirculating recirculation fuel is formed in the fuel cell unit, a water separator integrated into the recirculation line for separating water from the recirculation fuel, a compressed gas line with an injector for passing new fuel from a fuel reservoir,a mixed recirculation line for passing a mixture of new fuel and recirculation fuel, and the mixed recirculation line opens into the channels for recirculation fuel of the fuel cell stack, a control and / or regulating unit, wherein a method described in this patent application can be carried out with the fuel cell unit.

[0018] In a further variant, the fuel cell unit comprises a first temperature sensor for detecting the temperature of the recirculation fuel in the recirculation line before the new fuel is supplied and comprises a second temperature sensor for detecting the temperature of the new fuel supplied by the injector.

[0019] A fuel cell system according to the invention, comprising a fuel cell unit with a fuel cell stack, a fuel storage device for fuel, a gas conveying device for conveying a gaseous oxidizing agent through channels for oxidizing agents of the fuel cell stack, wherein a method described in this patent application can be carried out with the fuel cell system.

[0020] In a supplementary embodiment, the fuel cell unit of the fuel cell system is designed as a fuel cell unit described in this patent application.

[0021] In a further variant, water in the mixture in the mixed recirculation line is detected by detecting volume flows and / or mass flows and temperatures of the recirculation fuel in the recirculation line and the new fuel in the compressed gas line, and at least one measure is carried out depending on the detected water.

[0022] Preferably, oxidant is passed through oxidant channels of the fuel cell stack.

[0023] In a further embodiment, the temperature of the recirculation fuel in the recirculation line is detected as the first temperature with the first temperature sensor before the new fuel is supplied and after it has passed through the water separator.

[0024] In a further embodiment, a third volume flow of the mixture of new fuel and recirculation fuel is determined in the mixed recirculation line. The third volume flow is detected, for example, by a third volume flow sensor. Due to the pneumatic line routing, the third volume flow corresponds to the sum of the first volume flow and the second volume flow. By determining the third volume flow, a plausibility check of the accuracy of the detected volume flows can be carried out. If a difference is exceeded as an absolute value or percentage of the difference between, on the one hand, the sum of the first and second volume flows and, on the other hand, the third volume flow, an error message can be issued and / or the process can be aborted.

[0025] In a further embodiment, at least one temperature and / or at least one volume flow and / or at least one mass flow and / or at least one heat capacity flow and / or at least one density and / or at least one specific heat capacity is determined and / or recorded by means of, preferably analytical and / or empirical, models, with a computing unit, so that preferably the computing unit optionally forms a sensor.

[0026] In a further embodiment, at least one temperature and / or at least one volume flow and / or at least one mass flow and / or at least one heat capacity flow and / or at least one density and / or at least one specific heat capacity is stored in the computing unit, in particular as an approximate value or approximate value range, and / or is calculated and / or determined, in particular as a function of at least one parameter.

[0027] Preferably, the computing unit is a control and / or regulating unit or vice versa.

[0028] In a supplementary embodiment, the fuel cell unit comprises a housing, in particular a housing with a connection plate, and preferably the fuel cell stack is enclosed by the housing.

[0029] In a supplementary variant, the recirculation system comprises a conveying device, in particular a blower and / or a jet pump, for conveying the recirculation fuel through the recirculation circuit.

[0030] Preferably, the recirculation system is designed as a structural unit.

[0031] Preferably, the components of the fuel cells are ion exchange membranes, in particular proton exchange membranes, anodes, cathodes, gas diffusion layers and bipolar plates.

[0032] Preferably, the fuel cells each comprise stacked layered components.

[0033] In a further embodiment, the recirculation system comprises at least one electrical resistance heating element, in particular a plurality of electrical resistance heating elements, for heating at least one component of the recirculation system and / or for heating the recirculation system. Thus, ice can be thawed into water using the at least one electrical resistance heating element.

[0034] The components of the recirculation system are expediently a recirculation line and / or a mixed recirculation line and / or a conveying device for recirculation fuel, in particular a blower and / or a jet pump, and / or the water separator and / or at least one drain valve and / or an injector for new fuel and / or a housing and / or a connecting plate and / or at least one connecting means.

[0035] In a further embodiment, the fuel cell unit, in particular the recirculation system, comprises an injector for supplying fuel from a fuel storage unit, in particular a compressed gas storage unit, for fuel into the mixed recirculation line as a recirculation line and / or a compressed gas line for conducting fuel from the fuel storage unit into the injector and / or from the injector into the mixed recirculation line.

[0036] In a supplementary variant, the separation of water from the recirculation fuel in the water separator can be carried out as a mechanical water separator using sedimentation and / or swirl. With separation using swirl, for example, the recirculation fuel can be set into a rotating motion in the discharge line as a cyclone, allowing water to be separated due to the resulting centrifugal forces.

[0037] In a further variant, a purge valve is incorporated into the recirculation line to release recirculation fuel into the environment. If the recirculation fuel contains a very low proportion of fuel, i.e., if the recirculation fuel contains a high concentration of other substances and / or gases than fuel, these substances and / or gases can be discharged into the environment. The recirculation fuel thus preferably comprises other substances and / or gases in addition to the pure fuel.

[0038] In an additional embodiment, the water separator comprises a drain valve for draining water from a collecting chamber of the water separator.

[0039] Preferably the fuel is hydrogen, hydrogen-rich gas, reformate gas or natural gas.

[0040] Advantageously, the fuel cells and / or components are essentially flat and / or disc-shaped.

[0041] In a complementary variant, the oxidizing agent is air with oxygen or pure oxygen.

[0042] Preferably, the fuel cell unit is a PEM fuel cell unit with PEM fuel cells or an SOFC fuel cell unit with SOFC fuel cells or an alkaline fuel cell (AFC) or a solid oxide fuel cell (SOFC).

[0043] The invention further comprises a computer program having program code means stored on a computer-readable data carrier for carrying out a method described in this patent application when the computer program is executed on a computer or a corresponding computing unit.

[0044] The invention also includes a computer program product with program code stored on a computer-readable data carrier for implementing a method described in this patent application when the computer program is executed on a computer or a corresponding computing unit. The computer-readable data carrier is, for example, a hard disk drive, in particular an HDD or SSD, a USB flash drive, a DVD (Digital Versatile Disc), CD (Compact Disk), RAM, ROM, and / or EEPROM. Short description of the drawings

[0045] In the following, exemplary embodiments of the invention are described in more detail with reference to the accompanying drawings. They show: Fig. 1 a highly simplified exploded view of a fuel cell system with components of a fuel cell, Fig. 2 a perspective view of part of a fuel cell, Fig. 3 a longitudinal section through a fuel cell, Fig. 4 a perspective view of a fuel cell stack without housing and without recirculation system, Fig. 5 a section through the fuel cell unit with housing and recirculation system, Fig. 6 a side view of the fuel cell unit with recirculation system and without optional recirculation system housing,

[0046] In the Fig. Figures 1 to 3 show the basic structure of a fuel cell 2, a PEM fuel cell 3 (polymer electrolyte fuel cell 3). The principle of fuel cells 2 is that electrical energy or electrical current is generated by means of an electrochemical reaction. Hydrogen H2 is fed to an anode 7 as a gaseous recirculation fuel, and the anode 7 forms the negative pole. A gaseous oxidant, namely air with oxygen, is fed to a cathode 8; the oxygen in the air provides the necessary gaseous oxidant. Reduction (electron absorption) takes place at the cathode 8. Oxidation, as electron release, takes place at the anode 7.

[0047] The redox equations of the electrochemical processes are: O2+ 4 H + + 4 e - → 2 H2O cathode: 2 H2 → 4 H + + 4 e - Anode: 2 H2 + O2 → 2 H2O Total reaction equation of cathode and anode:

[0048] The difference between the normal potentials of the electrode pairs under standard conditions as the reversible fuel cell voltage or open-circuit voltage of the unloaded fuel cell 2 is 1.23 V. This theoretical voltage of 1.23 V is not achieved in practice. In the idle state and at low currents, voltages above 1.0 V can be achieved, and during operation with larger currents, voltages between 0.5 V and 1.0 V are achieved. The series connection of several fuel cells 2, in particular a fuel cell unit 1 with a fuel cell stack 40 of several fuel cells 2 arranged one above the other, has a higher voltage, which corresponds to the number of fuel cells 2 multiplied by the individual voltage of each fuel cell 2.

[0049] The fuel cell 2 also includes a proton exchange membrane 5 (Proton Exchange Membrane, PEM), which is arranged between the anode 7 and the cathode 8. The anode 7 and cathode 8 are layered or disc-shaped. The PEM 5 acts as an electrolyte, catalyst support, and separator for the reaction gases. The PEM 5 also acts as an electrical insulator and prevents an electrical short circuit between the anode 7 and cathode 8. Generally, 12 µm to 150 µm thick, proton-conducting films made of perfluorinated and sulfonated polymers are used. The PEM 5 conducts the protons H + and blocks ions other than protons H + essentially, so that due to the permeability of the PEM 5 for the protons H +the charge transport can take place. The PEM 5 is essentially impermeable to the reaction gases oxygen O2 and hydrogen H2, i.e., it blocks the flow of oxygen O2 and hydrogen H2 between a gas space 31 at the anode 7 containing the recirculation fuel hydrogen H2 and the gas space 32 at the cathode 8 containing air or oxygen O2 as the oxidizing agent. The proton conductivity of the PEM 5 increases with increasing temperature and increasing water content.

[0050] On both sides of the PEM 5, each facing the gas spaces 31, 32, are the electrodes 7, 8, serving as the anode 7 and cathode 8. A unit comprising the PEM 5, anode 7, and cathode 8 is referred to as a membrane electrode assembly (MEA) 6. The electrodes 7, 8 are pressed onto the PEM 5. The electrodes 7, 8 are platinum-containing carbon particles bonded to PTFE (polytetrafluoroethylene), FEP (fluorinated ethylene-propylene copolymer), PFA (perfluoroalkoxy), PVDF (polyvinylidene fluoride), and / or PVA (polyvinyl alcohol) and hot-pressed into microporous carbon fiber, glass fiber, or plastic mats. A catalyst layer 30 (not shown) is normally applied to each of the electrodes 7, 8 on the side facing the gas spaces 31, 32.The catalyst layer 30 in the gas space 31 with recirculation fuel at the anode 7 comprises nanodispersed platinum-ruthenium on graphite soot particles bound to a binder. The catalyst layer 30 in the gas space 32 with oxidizing agent at the cathode 8 similarly comprises nanodispersed platinum. Binders used include Nafion®, a PTFE emulsion, or polyvinyl alcohol.

[0051] In contrast, the electrodes 7, 8 are made of an ionomer, for example Nafion®, platinum-containing carbon particles, and additives. These electrodes 7, 8 with the ionomer are electrically conductive due to the carbon particles and also conduct the protons H + and additionally function as a catalyst layer 30 due to the platinum-containing carbon particles. Membrane electrode assemblies 6 with these electrodes 7, 8 comprising the ionomer form membrane electrode assemblies 6 as CCM (catalyst coated membrane).

[0052] A gas diffusion layer 9 (GDL) lies on the anode 7 and the cathode 8. The gas diffusion layer 9 on the anode 7 evenly distributes the recirculation fuel from channels 12 for recirculation fuel onto the catalyst layer 30 on the anode 7. The gas diffusion layer 9 on the cathode 8 evenly distributes the oxidant from channels 13 for oxidant onto the catalyst layer 30 on the cathode 8. The GDL 9 also draws off reaction water in the opposite direction to the flow direction of the reaction gases, i.e., in one direction from the catalyst layer 30 to the channels 12, 13. Furthermore, the GDL 9 keeps the PEM 5 moist and conducts the current. The GDL 9 is constructed, for example, from a hydrophobic carbon paper and a bonded carbon powder layer.

[0053] A bipolar plate 10 rests on the GDL 9. The electrically conductive bipolar plate 10 serves as a current collector, for water drainage and for conducting the reaction gases through a channel structure 29 and / or a flow field 29, and for dissipating the waste heat, which occurs particularly during the exothermic electrochemical reaction at the cathode 8. To dissipate the waste heat, channels 14 for conducting a liquid or gaseous coolant are incorporated into the bipolar plate 10. The channel structure 29 at the gas space 31 for recirculation fuel is formed by channels 12. The channel structure 29 at the gas space 32 for oxidizing agent is formed by channels 13. Materials used for the bipolar plates 10 include, for example, metal, conductive plastics, composite materials, or graphite.The bipolar plate 10 thus comprises the three channel structures 29, formed by the channels 12, 13, and 14, for the separate passage of recirculation fuel, oxidant, and coolant. In a fuel cell unit 1 with a fuel cell stack 40 and / or a fuel cell stack 40, a plurality of fuel cells 2 are arranged in an aligned stack (Fig. Fig. 4). The fuel cells 2 and the components 5, 6, 7, 8, 9, 10 of the fuel cells 2 are layered and / or disc-shaped and span fictitious planes 37 ( Fig. 3). The components 5, 6, 7, 8, 9, 10 of the fuel cells 2 are proton exchange membranes 5, anodes 7, cathodes 8, gas diffusion layers 9, and bipolar plates 10.

[0054] In Fig. 1 shows an exploded view of two stacked fuel cells 2. A seal 11 seals the gas chambers 31, 32 in a fluid-tight manner. In a compressed gas storage 21 ( Fig. 1) Hydrogen H2 is stored as a fuel at a pressure of, for example, 350 bar to 800 bar. From the compressed gas storage 21, the fuel is led through a high-pressure line 18 to a pressure reducer 20 to reduce the pressure of the fuel in a medium-pressure line 17 from approximately 10 bar to 20 bar. From the medium-pressure line 17, the fuel is led to an injector 19. At the injector 19, the pressure of the fuel is reduced to an injection pressure between 1 bar and 3 bar. From the injector 19, the fuel is indirectly injected by means of a jet pump 62 (only in Fig. 5) a supply line 16 for recirculation fuel ( Fig. 1) and from the supply line 16 to the channels 12 for recirculation fuel, which form the channel structure 29 for recirculation fuel. The recirculation fuel thereby flows through the gas space 31 for the recirculation fuel. The gas space 31 for the recirculation fuel is formed by the channels 12 and the GDL 9 at the anode 7. After flowing through the channels 12, the fuel not consumed in the redox reaction at the anode 7 and, if applicable, water from a controlled humidification of the anode 7 are discharged from the fuel cells 2 through a discharge line 15.

[0055] A gas conveying device 22, for example designed as a blower 23 or a compressor 24, conveys air from the environment as oxidant into an oxidant supply line 25. From the supply line 25, the air is fed to the oxidant channels 13, which form a channel structure 29 on the oxidant bipolar plates 10, so that the oxidant flows through the oxidant gas space 32. The oxidant gas space 32 is formed by the channels 13 and the GDL 9 at the cathode 8. After flowing through the channels 13 and the oxidant gas space 32, the oxidant not consumed at the cathode 8 and the reaction water produced at the cathode 8 as a result of the electrochemical redox reaction are discharged from the fuel cells 2 through a discharge line 26. The oxidant discharge line 26 opens into the environment.A supply line 27 serves to supply coolant into the coolant channels 14, and a discharge line 28 serves to discharge the coolant passed through the channels 14. The supply and discharge lines 15, 16, 25, 26, 27, 28 are shown in . Fig. 1 are shown as separate lines for simplification purposes and are actually designed as aligned fluid openings (not shown) at the end region of the superimposed membrane electrode assemblies 6 near the channels 12, 13, 14. Similarly, fluid openings (not shown) are also formed on plate-shaped extensions (not shown) of the bipolar plates 10, and the fluid openings in the plate-shaped extensions of the bipolar plates 10 are aligned with the fluid openings (not shown) on the membrane electrode assemblies 6 to partially form the supply and discharge lines 15, 16, 25, 26, 27, 28. The fuel cell unit 1, together with the compressed gas reservoir 21 and the gas delivery device 22, forms a fuel cell system 4.

[0056] In the fuel cell unit 1, the fuel cells 2 are arranged between two clamping elements 33 as clamping plates 34. An upper clamping plate 35 rests on the uppermost fuel cell 2 and a lower clamping plate 36 rests on the lowest fuel cell 2. The fuel cell unit 1 comprises approximately 200 to 400 fuel cells 2, which for graphic reasons are not all shown in Fig. 4 to 5. The clamping elements 33 apply a compressive force to the fuel cells 2, i.e. the upper clamping plate 35 rests with a compressive force on the uppermost fuel cell 2 and the lower clamping plate 36 rests with a compressive force on the lowest fuel cell 2. The fuel cell stack 40 is thus clamped in order to ensure the tightness for the fuel, the oxidizing agent and the coolant, in particular due to the elastic seal 11, and also to keep the electrical contact resistance within the fuel cell stack 40 as low as possible. In order to clamp the fuel cells 2 with the clamping elements 33, four connecting devices 38 are designed as bolts 39 on the fuel cell unit 1, which are subject to tensile stress. The four bolts 39 are firmly connected to the clamping plates 34.

[0057] The fuel cell stack 40 is in a housing 42 ( Fig. 5). The housing 42 has an inner side 43 and an outer side 44. An intermediate space 41 is formed between the fuel cell stack 40 and the housing 42. The housing 42 is also formed by a connection plate 47 made of metal, in particular steel. The remaining housing 42 without the connection plate 47 is fastened to the connection plate 47 with fixing elements 48 as screws 49. An opening 45 for introducing recirculation fuel into the channels 12 for recirculation fuel is formed in the connection plate 47 and in the lower clamping plate 36. Furthermore, an opening 46 for discharging recirculation fuel from the channels 12 for recirculation fuel is formed in the connection plate 47 and in the lower clamping plate 36.Further openings (not shown) are formed in the connecting plate 47 and the lower clamping plate 36, which serves as the clamping element 33, for introducing oxidizing agent, discharging oxidizing agent, introducing coolant, and discharging coolant. Thus, a total of six openings are formed in the connecting plate 47 and the lower clamping plate 36.

[0058] The recirculation fuel, as recirculation hydrogen, is conducted through the recirculation fuel channels 12 and thus also through the recirculation fuel gas chamber 31. A recirculation line 50 serves to recirculate the recirculation fuel discharged from the recirculation fuel channels 12, i.e., the recirculation fuel discharged from the recirculation fuel channels 12 is fed back to the recirculation fuel channels 12 and thus also to the recirculation fuel gas chamber 31 via the recirculation line 50. The recirculation fuel discharge line 15 and the recirculation fuel supply line 16 thus also function as the recirculation line 50. A water separator 51 is integrated into the recirculation line 50. The water separator 51 ( Fig. 5) is thus installed in the recirculation line 50 and is also designed as a mechanical water separator 52 with a separation flow space 53. In Fig. 1, the water separator 51 is not shown. The separation flow chamber 53 has a significantly larger flow cross-sectional area than the recirculation line 50, so that the flow velocity of the recirculation fuel in the separation flow chamber 53 is significantly lower than in the recirculation line 50, and thus small water droplets in the recirculation fuel are separated in the separation flow chamber 53 by sedimentation, thus forming a mechanical water separator 52. The separation flow chamber 53 is delimited by walls 54 of the water separator 51.

[0059] A water drainage opening 57 is installed in the separation flow chamber 53 so that water separated in the separation flow chamber 53 can be drained into the environment through the water drainage opening 57. A drain valve 63 is formed on the water drainage opening 57 so that the separated water can initially be collected in the separation flow chamber 53 and then selectively released into the environment. An inlet opening 55 for introducing the recirculation fuel into the separation flow chamber 53 and an outlet opening 56 for draining the recirculation fuel from the separation flow chamber 53 are formed in the water separator 51. The recirculation line 50 opens into the inlet opening 55 and the outlet opening 56. The medium-pressure line 17 and the high-pressure line 18 are also referred to as the compressed gas line 59 for fuel.The fuel is stored in the compressed gas storage 21 under a very high pressure of, for example, 400 or 800 bar.

[0060] A recirculation system 65 of the fuel cell unit 1 thus comprises the recirculation line 50 and the water separator 51. The recirculation system 65 also comprises a conveying device 60 for the recirculation fuel in the form of a blower 61. The conveying device 60 circulates the recirculation fuel, i.e., a mixture of the recirculation fuel hydrogen, nitrogen, water vapor, and liquid water, in the circuit. The conveying device 60 is driven by an electric motor (not shown). To save electrical energy for driving the electrically operated conveying device 60, the recirculation system 65 additionally comprises the jet pump 62. With the aid of the jet pump 62, the high pressure of the recirculation fuel is utilized to circulate the recirculation fuel and thereby save electrical energy for the conveying device 60.When the electrical power delivered by the electric fuel cell unit 1 is low, only a small volume flow of hydrogen is introduced into the recirculation line 50 by means of the injector 19 and the jet pump 62, so that only a small amount of energy per unit time from the volume flow of hydrogen can be utilized to deliver the recirculation fuel. For this reason, when the electrical power delivered by the fuel cell unit 1 is low, it is necessary to additionally operate the delivery device 60 using electrical energy. On the other hand, when the electrical power delivered by the fuel cell unit 1 is high, the delivery device 60 is not required and is switched off because the volume flow of hydrogen is sufficient to deliver the recirculation fuel by means of the jet pump 62.

[0061] The drain valve 63 is formed at the water drainage opening 57 of the water separator 51. The drain valve 63 opens into a drain line 64, and the drain line 64 opens into the discharge line 26 for the oxidizing agent and water. The discharge line 26 opens into the environment. Since condensation water can also accumulate on the conveying device 60, such as the fan 61, the drain valve 63 with the drain line 64 is also formed on the conveying device 60. The drain line 64 of the drain valve 63 of the conveying device 60 also opens into the discharge line 26 for the oxidizing agent.

[0062] Electrical resistance heating elements 58 are arranged on the delivery device 60, the jet pump 62, and the drain valves 63. When the fuel cell unit 1 is started at temperatures below 0°C, the water on the delivery device 60, the jet pump 62, and the drain valves 63 freezes, rendering the recirculation system 65 inoperable and thus preventing the fuel cell unit 1 from being started. A drive system for a motor vehicle includes a battery as a buffer battery for storing electrical energy. When the fuel cell unit 1 is started at temperatures below 0°C, the electrical resistance heating elements 58 are operated using electrical energy from the battery (not shown), thereby heating the delivery device 60, the jet pump 62, and the drain valves 63, so that the frozen water contained therein thaws as ice and is present in the form of liquid water.The fuel cell unit 1 can then be started.

[0063] The recirculation system 65 comprises a connecting plate 66. The connecting plate 66 is made of metal, in particular steel or aluminum, and / or plastic, and is essentially flat and / or disc-shaped. The recirculation system 65 optionally comprises a housing 68. Fig. 5, the housing 68 shown in dashed lines encloses the components of the recirculation system 65 together with the connection plate 66. The connection plate 66 is thus also a component of the housing 58 of the recirculation system 65. The recirculation system 65 is mechanically connected and fastened to the remaining fuel cell unit 1, ie the fuel cell stack 40 to the housing 42, by contact elements 67.

[0064] The compressed gas reservoir 21, as fuel reservoir 21, stores fuel as hydrogen. The fuel in the fuel reservoir 21 is dried and contains no or essentially no water. After the fuel is passed through the medium-pressure line 17, the pressure of the fuel in the injector 19 is further reduced. The fuel is fed from the injector 19 through the low-pressure line 70 as the compressed gas line 59, indirectly through the jet pump 62, to the recirculation circuit with the recirculation line 50. In the flow direction of the recirculation fuel, downstream of the jet pump 62 and upstream of the opening 45, in this section of the recirculation line 50, as the mixed recirculation line 69, a mixture of new fuel and recirculation fuel, which was discharged from the fuel cell stack 40 through the opening 46 and has already been passed through the water separator 51, is present.

[0065] In the recirculation line 50, in the flow direction of the recirculation fuel, between the water separator 51 and the supply of the new fuel, i.e., at the jet pump 62, there is a first temperature sensor 71 for detecting the temperature of the recirculation fuel and, optionally, a first volume flow sensor 74 for detecting the volume flow of the recirculation fuel. On or in the low-pressure line 70 for the new fuel, there is a second temperature sensor 72 for detecting the temperature of the new fuel after it has passed through the injector 19 and before it is introduced into the mixed recirculation line 69. In addition, on or in the low-pressure line 70 for the new fuel, there is an optional second volume flow sensor 75 for detecting the volume flow of the new fuel after it has passed through the injector 19 and before it is introduced into the mixed recirculation line 69.A third temperature sensor 73 on the mixed recirculation line 69 detects the temperature of the mixture of new fuel and recirculation fuel in the mixed recirculation line 69. An optional third volume flow sensor 76 on the mixed recirculation line 69 detects the volume flow of the mixture of new fuel and recirculation fuel in the mixed recirculation line 69. The first, second and third volume flow sensors 74, 75, 76 are designed, for example, as an ultrasonic flow sensor, a magnetic-inductive flow sensor or a flow sensor for determining the volume flow with two pressure sensors at a taper of the recirculation line 50 and / or compressed gas line 59 and / or mixed recirculation line 59 at two different flow cross-sectional areas of the recirculation line 50 and / or compressed gas line 59 and / or mixed recirculation line 59 using the Bernoulli equation.The third volume flow sensor 76 generally serves only to perform a plausibility check of the volume flows detected by the first and second volume flow sensors 74, 75. The volume flows can also be calculated and / or determined as a function of at least one other parameter, in particular using a model, preferably based on empirical and / or analytical and / or numerical data, depending on the electrical power of the fuel cell unit 1 as a parameter.

[0066] The recirculation fuel is conveyed by the conveying device 60, i.e., the blower 61 and / or the jet pump 62, through the recirculation circuit with the recirculation line 50. Parameters of this conveying device 60, for example, the electrical power of an electric motor that drives the blower 61, are recorded in a control and / or regulating unit (not shown) as a computing unit with data lines (not shown), and from these parameters of this conveying device 60 and the known structural properties of the fuel cell unit 1, for example, the pressure drop in the recirculation circuit as a function of the required volume flow, the first volume flow is determined as a function of these parameters.

[0067] The volume flow of fuel supplied by the injector 19 is determined either by the second volume flow sensor 75 or as a function of parameters of the injector 19, for example, the open flow cross-sectional area of ​​the injector 19 and the pressure of the fuel in the compressed gas reservoir 21. The density of the fuel in the fuel reservoir 21 is known, and from the determined volume flow of fuel introduced into the recirculation line 50 through the injector 19 and the density of the fuel, the mass flow and thus also the heat capacity flow with the specific heat capacity of the new fuel can be calculated.In an analogous manner, the mass flow and thus also the heat capacity flow with the specific heat capacity of the recirculation fuel can be calculated from the determined volume flow of the recirculation fuel in the recirculation line 50 before the introduction of new fuel and the known density of the recirculation fuel. The third target temperature of the recirculation fuel after mixing the new fuel and the recirculation fuel in the mixed recirculation line 69 is calculated from the temperature of the recirculation fuel detected by the first temperature sensor 71, the mass flow and / or heat capacity flow of the recirculation fuel, the temperature of the new fuel detected by the second temperature sensor 72, and the mass flow and / or heat capacity flow of the new fuel.The second temperature detected by the second temperature sensor 72 is the relevant temperature of the new fuel at the pressure when it is introduced into the recirculation line 50 and into the mixed recirculation line 69, and the relevant second temperature is either detected directly by the second temperature sensor 72, or the relevant temperature is calculated from the detected temperature when the pressure of the new fuel is reduced between the detection of the second temperature by the second temperature sensor 72 and the introduction of the new fuel into the recirculation line 50 and into the mixed recirculation line 69. This third target temperature is determined as the average temperature based on the supplied mass flows and / or heat capacity flows of new fuel and recirculation fuel at the first and second temperatures.This calculated third target temperature is compared with the third actual temperature detected by the second temperature sensor 73.

[0068] If the third actual temperature is lower than the third target temperature by a specified limit, at least one action is carried out.

[0069] The new fuel fed into the recirculation circuit contains essentially no moisture. Moisture and possibly also water in the liquid state are present in the recirculation fuel after passing through the water separator 51 and before the new fuel is added. The water in the liquid state is present, for example, as small droplets in the form of mist and / or as water droplets on the inner wall of the recirculation line 50. After the new, dry fuel is added, the relative and / or absolute humidity of the mixture in the mixed recirculation line 69 is reduced compared to the recirculation fuel in the recirculation line 50 after the water separator 51 and before the new fuel is added. This increases the willingness of water to transform from the liquid to the gaseous state in the mixed recirculation line 69.Converting liquid water into the gaseous state (change of state) requires latent heat of vaporization, so the mixture in the mixed recirculation line 69 cools due to the evaporation of the water. This cooling due to the necessary latent heat of vaporization results in the third actual temperature being lower than the third target temperature. The greater the mass flow of water in the recirculation line 50 before the new fuel is supplied, the greater the difference between the third target temperature and the third actual temperature. A larger mass flow of the new fuel supplied increases the willingness of the water to evaporate, if water is present.

[0070] If the actual temperature is lower than the third target temperature by a predefined limit, for example, 0.1°C, 0.2°C, 0.5°C, 1°C, 3°C, 5°C, or 10°C, the proportion of water in the mixed recirculation line 69 in the recirculation line 50 is classified as critical for the operating state of the fuel cell unit 1. For example, in the water separator 51, the collection chamber for the separated water is completely filled, so the drain valve 63 is temporarily opened as a measure. Further measures when the third actual temperature falls below the predefined limit compared to the third target temperature include, for example, an error message or the shutdown of the fuel cell unit 1.

[0071] If the difference between the third target temperature and the third actual temperature is smaller than the predefined limit value, the cause may also lie in the ambient conditions. Very low outside temperatures, for example at -20 °C, can cause a sharp drop in the temperatures detected between the third temperature sensor 73 on the one hand and the first and second temperature sensors 71, 72 on the other hand due to heat conduction to the environment. The predefined limit value is therefore calculated depending on at least one limit value parameter. The limit value parameter is, for example, the ambient temperature and the temperature of the fuel cell unit 1. This can prevent an incorrectly overestimated mass flow of water due to a low outside temperature.

[0072] Overall, the method according to the invention for operating the fuel cell unit 1, the fuel cell unit 1 according to the invention, and the fuel cell system 4 according to the invention offer significant advantages. Sensors for detecting the water level in a water separator 51 are expensive to purchase and unreliable in operation. While a motor vehicle is traveling, fluctuations in the water occur in a water collection chamber in the water separator 51 due to acceleration and deceleration of the motor vehicle. Reliable detection of the water level in the water separator 51 is therefore not possible. An excessive mass flow of water in the recirculation fuel, which is introduced into the fuel cell stack 40 through the opening 45 and thus into the channels 12 for recirculation fuel, leads to damage to the fuel cell unit 1.Advantageously, this avoids the need for an expensive and unreliable sensor for detecting the water level in the water separator 51, and additionally, damage to the fuel cell unit 1 due to the introduction of an excessive mass flow of water into the channels 12 for recirculation fuel can be avoided because an excessive mass flow of water in the mixed recirculation line 69 is detected using the method. The method can also be used to test and monitor fuel cell units 1 under development in motor vehicles by detecting and recording field data from motor vehicles regarding the number of events with an excessive amount of water in the opening 45 for introducing the recirculation fuel into the fuel cell unit 1 using the method. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2017 215 501 A1

[0004]

Claims

[1] Method for operating a fuel cell unit (1) for electrochemically generating electrical energy, comprising the steps: - guiding recirculation fuel through channels (12) for recirculation fuel of a fuel cell stack (40), - guiding the recirculation fuel after being discharged from the fuel cell stack (40) through a recirculation system (65) with a recirculation line (50) and a water separator (51) and reintroducing the recirculation fuel passed through the recirculation system (65) into the channels (12) for recirculation fuel of the fuel cell stack (40) as a recirculation circuit, - supplying new fuel from a fuel reservoir (21) through a compressed gas line (59) with an injector (19) into the recirculation line (50) and passing the mixture of new fuel and recirculation fuel through a mixed recirculation line (69) and from the mixed recirculation line (69) into the channels (12) for recirculation fuel of the fuel cell stack (40), characterized by , that - the temperature of the recirculation fuel in the recirculation line (50) is detected as the first temperature by a first temperature sensor (71) before the new fuel is supplied, - the temperature of the new fuel supplied by the injector (19) is detected as a second temperature by a second temperature sensor (72), - the temperature of the mixture of new fuel and recirculation fuel in the mixed recirculation line (69) is detected as the third actual temperature by a third temperature sensor (73), - a first volume flow of the recirculation fuel in the recirculation line (50) is determined before the new fuel is supplied, - a second volume flow of the new fuel supplied through the compressed gas line (59) with the injector (19) is determined, - the target temperature of the mixture of new fuel and recirculation fuel in the mixed recirculation line (69) is calculated as a third target temperature from the detected first and second temperatures, the determined first and second volume flows and the density of the first volume flow of the recirculation fuel and the density of the second volume flow of the new fuel and is compared with the third actual temperature, and at a third actual temperature which is lower than the third target temperature by a predetermined limit value, at least one measure is carried out. [2] Method according to claim 1, characterized bythat the first volume flow of the recirculation fuel in the recirculation line is determined before the new fuel is supplied as a function of at least one parameter of at least one conveying device (60, 61, 62) for the recirculation fuel. [3] Method according to claim 1 or 2, characterized by that the volume flow of the recirculation fuel in the recirculation line is detected by a first volume flow sensor (74) before the new fuel is supplied. [4] Method according to one or more of the preceding claims, characterized bythat the second volume flow of the new fuel supplied by the injector (19) is determined as a function of at least one parameter of the injector (19) and / or the pressure of the new fuel in the fuel reservoir (21) and / or the pressure difference before and after the injector (19) and / or a pressure difference of the new fuel in the compressed gas line (59) before the mixed recirculation line (69). [5] Method according to one or more of the preceding claims, characterized by that the volume flow of the new fuel supplied through the compressed gas line (59) with the injector (19) is detected by a second volume flow sensor (75). [6] Method according to one or more of the preceding claims, characterized by that the specified limit value is determined as a function of at least one limit value parameter. [7] Method according to claim 6, characterized bythat the at least one limit parameter is the ambient temperature and / or the first temperature and / or the second temperature and / or the first volume flow and / or the second volume flow. [8] Method according to one or more of the preceding claims, characterized by that the at least one measure is the temporary opening of a drain valve (63) on the water separator (51). [9] Method according to one or more of the preceding claims, characterized by that the at least one measure is the switching off of the fuel cell unit (1). [10] Method according to one or more of the preceding claims, characterized by that the at least one measure is an error message, in particular as a visual and / or acoustic warning signal. [11] Method according to one or more of the preceding claims, characterized bythat the method is carried out with a fuel cell unit (1) according to claim 12 or 13. [12] Fuel cell unit (1) for the electrochemical generation of electrical energy, comprising - stacked fuel cells (2) and the stacked fuel cells (2) form a fuel cell stack (40), - channels (12) for recirculation fuel integrated into the fuel cell stack (40), - a recirculation system (65) with a recirculation line (50) for recirculating the recirculation fuel discharged from the recirculation fuel channels (12) back into the recirculation fuel channels (12), so that a recirculation circuit with recirculating recirculation fuel is formed in the fuel cell unit (1), - a water separator (51) integrated into the recirculation line (50) for separating water from the recirculation fuel, - a compressed gas line (59) with an injector (19) for passing new fuel from a fuel reservoir (21), - a mixed recirculation line (69) for conducting a mixture of new fuel and recirculation fuel, and the mixed recirculation line (69) opens into the channels (12) for recirculation fuel of the fuel cell stack (40), - a control and / or regulation unit, characterized by that a method according to one or more of the preceding claims can be carried out with the fuel cell unit (1). [13] Fuel cell unit according to claim 12, characterized bythat the fuel cell unit (1) comprises a first temperature sensor (71) for detecting the temperature of the recirculation fuel in the recirculation line (50) before the new fuel is supplied and a second temperature sensor (72) for detecting the temperature of the new fuel supplied by the injector (19). [14] Fuel cell system (4), comprising - a fuel cell unit (1) with a fuel cell stack (40), - a fuel storage (21) for fuel, - a gas conveying device (22, 23, 24) for conveying a gaseous oxidizing agent through channels (13) for oxidizing agents of the fuel cell stack (40), characterized by that a method according to one or more of claims 1 to 11 can be carried out with the fuel cell system. [15] Fuel cell system according to claim 14, characterized bythat the fuel cell unit (1) is designed according to claim 12 or 13.

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