Method for operating a fuel cell system, and fuel cell system

Abstract

The invention relates to a method for operating a fuel cell system (100), comprising at least one fuel cell stack (11), each of which has a cathode system (300) and an anode system (200) with an anode supply line (22), a recirculation line (21), and an anode chamber (A), and comprising a cooling circuit (400) with coolant, whereby, in order to prevent a water buildup, a fuel cell stack temperature T which is higher than the dew point temperature Ttau is set in the fuel cell stack (11) during a shutdown of the fuel cell system (100), in particular prior to an expected start under freezing conditions.

Description

[0001] Description

[0002] title

[0003] Method for operating a

[0004] The invention relates to a method for operating a fuel cell system having the features of the preamble of independent claim 1. Furthermore, the invention relates to a fuel cell system having the features of the preamble of independent claim 11.

[0005] State of the art

[0006] It is known from the prior art that fuel cell systems exist that have an anode system and a cathode system. The anode system consists of an anode supply line that supplies fuel to a fuel cell stack, and a recirculation line that recirculates anode exhaust gas to the anode supply line using a conveying unit. Furthermore, the cathode system consists of a cathode supply line, in which a compressor can be located, and a cathode outlet line through which cathode exhaust gas is conveyed from the cathode system into the exhaust system.

[0007] The waste heat from the fuel cell stack is dissipated via a cooling circuit and released into the environment via a vehicle radiator. A coolant is recirculated within the cooling circuit. The coolant is pumped through the fuel cell stack by a coolant pump. A three-way valve ensures that the vehicle radiator can be partially or completely bypassed.

[0008] If the fuel cell system is started at temperatures below freezing, local icing can occur in the fuel cell stack. The formation of ice hinders the reaction in the fuel cell stack and thus slows or prevents the fuel cell system from starting. To prevent ice formation, the fuel cell system must be conditioned accordingly. For this purpose, drying is performed when the system is shut down. This drying occurs primarily on the cathode side by conveying air, which removes water in gaseous and liquid form. The air system provides a large mass flow of air for this purpose.

[0009] Disclosure of the invention

[0010] The inventive method for operating a fuel cell system with the features according to independent claim 1 has the advantage that water accumulation in the anode compartment and / or cathode compartment can be reliably prevented. Water accumulation in the anode compartment and / or cathode compartment can be reliably prevented because the anode compartment and cathode compartment are kept above a dew point temperature Ttau during shutdown, so that no condensation of gaseous water occurs. The dew point temperature Ttau is the temperature in a water-containing gas mixture at which a gaseous water component condenses.

[0011] With the aid of the method according to the invention, freeze-starts of the fuel cell system can be advantageously reduced. The fuel cell stack has a fuel cell stack temperature T, which represents the current temperature of the fuel cell stack. The fuel cell stack can be tempered by the method according to the invention to a fuel cell temperature T that is above the dew point temperature Ttau. As a result, no or less water can condense from the anode exhaust gas in the anode compartment and / or cathode exhaust gas in the cathode compartment during shutdown of the fuel cell system. In particular, this can reduce freeze-starts of the fuel cell system, in which local icing occurs due to water accumulation in the fuel cell system and a drop in the fuel cell temperature T to the freezing point of water or a lower temperature.

[0012] It is advantageous to set the fuel cell stack temperature T to a temperature that is greater than a cooling limit temperature Tcool. If the fuel cell stack temperature T is set above a cooling limit temperature Tcool, it is ensured that the fuel cell stack has a fuel cell temperature T above the dew point temperature Ttau. By using the cooling limit temperature Tcool, the determination of the dew point temperature Ttau can be omitted, whereby the method according to the invention can be carried out even more efficiently and with less resource consumption, in particular due to a lower computational effort.

[0013] The cooling limit temperature Tcool is advantageously determined approximately by a second temperature T2, which is measured by a suitable sensor in the recirculation line. This eliminates the need to determine the dew point temperature Ttau, allowing the process to be carried out more efficiently.

[0014] It is advantageous if the fuel cell stack temperature T is set to a temperature that is greater than a first temperature T1, which is measured in the anode supply line by a suitable sensor. This ensures that the fuel cell stack, in particular the anode compartment, is the warmest area in the anode system during shutdown. If the fuel cell stack is the warmest area in the fuel cell system, no condensation of water occurs in the fuel cell stack, in particular in the anode compartment, and the discharge of water from the fuel cell stack is promoted.

[0015] The dew point temperature Ttau is advantageously determined using the first temperature T1 and the second temperature T2. By approximately determining the dew point temperature Ttau using the first temperature T1 and the second temperature T2, the dew point temperature can be continuously re-determined during shutdown, particularly when the temperature of the fuel cell system, in particular the fuel cell stack temperature T, changes, thus increasing the accuracy of the method according to the invention.

[0016] Advantageously, the first temperature T1 and the second temperature T2 are measured using temperature sensors. By using temperature sensors, the first temperature T1 and the second temperature T2 can be determined accurately and efficiently. It is advantageous to measure the dew point temperature Ttau using a sensor, particularly a dew point sensor located in the fuel cell system. This allows for an accurate and efficient determination of the dew point temperature Ttau without the need for additional conversion steps.

[0017] To adjust the fuel cell stack temperature T, the coolant in the fuel cell stack is advantageously set and / or regulated to a temperature above the dew point temperature Ttau. The coolant is a component of the fuel cell system, so that the fuel cell stack can be tempered to a fuel cell stack temperature T that is above the dew point temperature Ttau without additional components, allowing a compact and cost-effective design to be implemented.

[0018] It is advantageous if the water separator is additionally tempered so that the water separator is set to a water separator temperature TW that corresponds to the temperature Ttau or lies between a temperature Ttau and the freezing point of water.

[0019] If the water separator temperature is below or equal to the dew point temperature Ttau, the gaseous water present in the anode exhaust and / or cathode exhaust preferentially condenses at the water separator, thereby favoring the targeted removal of water through the water separator. Once the freezing point of water is reached or undershot, condensation of water in the water separator can no longer occur, so the water separator should not be deliberately set to the freezing point or below the freezing point of water.

[0020] It is advantageous if the temperature of the water separator is adjusted by a cooling unit so that the water separator is efficiently tempered.

[0021] It is advantageous if the fuel cell system has a water separator for carrying out the method and the water separator contains a cooling unit, wherein the cooling unit regulates the temperature to the water separator temperature TW. This ensures a compact design of the fuel cell system. The fuel cell system according to the invention can preferably be used for mobile applications, for example in vehicles, in particular fuel-powered vehicles. The fuel cell system according to the invention can serve as the main energy supplier for a vehicle. At the same time, however, it is also possible for the fuel cell system according to the invention to be a secondary drive and / or auxiliary drive of a vehicle, for example a hybrid vehicle. The fuel cell system according to the invention can also be used for stationary applications, for example in generators.

[0022] The fuel cell system according to the invention can comprise one or more stacks, each with a plurality of stacked fuel cells and the associated functional systems, comprising: cathode system, anode system, cooling system and an electrical system.

[0023] Description of the drawings

[0024] The fuel cell system according to the invention and the method according to the invention are explained in more detail below with reference to drawings with preferred embodiments.

[0025] They show:

[0026] Fig. 1 shows a schematic topology of a fuel cell system 100 according to a first embodiment;

[0027] Fig. 2 is a flow chart of the method according to the invention;

[0028] Figure 1 shows a schematic topology of a fuel cell system 100 with at least one fuel cell stack 11, an anode system 200, a cathode system 300 and a cooling circuit 400.

[0029] The fuel cell stack 11 has a fuel cell stack temperature T, which represents the current temperature of the fuel cell stack. The fuel cell stack temperature T can be changed; for example, the fuel cell stack temperature T can decrease when the fuel cell stack 11 is cooled. The fuel cell stack temperature T can also change due to the addition of reactants, in particular hydrogen and air.

[0030] The cathode system 300 supplies a cathode chamber K with oxygen (O2) as a reactant. Oxygen is a component of air. By supplying air to the fuel cell system 100, oxygen is made available to the system as a reactant.

[0031] A cathode supply line 31 is arranged in the cathode system 300 and leads into the fuel cell stack 11. Oxygen is supplied to the fuel cell stack 11 via the cathode supply line 31.

[0032] A cathode compressor 33 is located within the cathode supply line 31. The cathode compressor 33 conveys air into the fuel cell stack 11. The cathode compressor 33 can be used to vary the air flow rate. Increasing the power of the cathode compressor 33 results in an increased air flow rate being supplied to the fuel cell stack 11 via the cathode supply line 31. Reducing the power of the cathode compressor 33 results in a reduced air flow rate being supplied to the fuel cell stack 11.

[0033] 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.

[0034] Optionally, additional temperature sensors can be arranged in the cathode outlet line 32 and cathode supply line 31 in order to measure the temperature of the cathode exhaust gas exiting the cathode chamber K and / or to measure the temperature of the incoming air.

[0035] Optionally, an additional water separator can be arranged in the cathode outlet line 32.

[0036] The anode system 200 supplies an anode compartment A of the fuel cell stack 11 with a fuel or anode fluid, in particular hydrogen (H2), as a reactant. By supplying fuel to the anode compartment A, the fuel is made available to the fuel cell system 100 as a reactant. An anode supply line 22 is arranged in the anode system 200 and opens into the fuel cell stack 11. Fuel is supplied to the fuel cell stack 11 via the anode supply line 22.

[0037] A first temperature sensor 27 is arranged in the anode supply line 22. The first temperature sensor 27 is located upstream of the anode chamber A of the fuel cell stack 11. With the aid of the first temperature sensor 27, a first temperature T1 of the fuel and / or gas mixture that is introduced into the fuel cell stack 11 can be measured. The first temperature T1 can then be made available to the method according to the invention and used to determine the fuel cell stack temperature T in the fuel cell stack 11.

[0038] The fuel supply to the fuel cell stack 11 can be superstoichiometric, so that the anode exhaust gas still contains fuel. To make the fuel available to the anode system 200, the anode exhaust gas is recirculated from the recirculation line 21 into the anode supply line 22.

[0039] A jet pump 26 is arranged in the anode supply line 22. The jet pump is arranged between the anode supply line 22 and the recirculation line 21 and connects them.

[0040] In an alternative embodiment, the jet pump 26 can be designed as a combined valve jet pump assembly 26. A combined valve jet pump assembly typically includes a metering valve and a jet pump. The metering valve is firmly connected to the jet pump 26, for example, by means of a screw connection.

[0041] With the help of the jet pump 26, the ratio of fuel and anode exhaust gas in the anode feed line 22 downstream of the jet pump 26 is varied.

[0042] By means of the jet pump 26, an increase in the volume flow of the supplied fuel into the jet pump 26 also causes an increase in the volume flow of the anode exhaust gas into the anode supply line 22. A recirculation conveying unit 25 is optionally arranged within the recirculation line 21. The recirculation conveying unit 25 can be a compressor designed as a blower, pump, and / or compressor.

[0043] The recirculation conveying unit 25 supports recirculation of the anode exhaust gas from the recirculation line 21 into the anode supply line 22.

[0044] The recirculation feed unit 25 can be used to vary the volume flow of the anode exhaust gas flowing from the recirculation line 21 into the anode feed line 22. Increasing the power of the recirculation feed unit 25 results in an increased volume flow of anode exhaust gas being supplied to the fuel cell stack 11 via the recirculation line 21.

[0045] By reducing the power of the recirculation conveying unit 25, a reduced volume flow of anode exhaust gas is supplied to the fuel cell stack 11.

[0046] A second temperature sensor 28 is arranged in the recirculation line 21.

[0047] The second temperature sensor 28 is located downstream of the anode chamber A of the fuel cell stack 11. Using the second temperature sensor 28, a second temperature T2 of the exhaust gas discharged from the fuel cell stack 11 can be measured. The second temperature T2 can then be provided to the method according to the invention and can be used to determine the temperature in the fuel cell stack 11.

[0048] A water separator 30 is arranged within the recirculation line 21. The water separator 30 is arranged downstream of the second temperature sensor 28 in the flow direction. The water separator 30 removes water from the anode exhaust gas. The water separator 30 can be designed in various embodiments: as an automatic water separator or as a manual water separator and / or with an additional filter and / or with additional active cooling.

[0049] If the water separator 30 is designed with active cooling in an alternative embodiment, it can be actively adjusted to a water separator temperature TW that is at least equal to or below a dew point temperature Ttau. The dew point temperature Ttau corresponds to the temperature at which a gaseous water component condenses in a water-containing gas mixture, such as anode exhaust gas and / or cathode exhaust gas.

[0050] The active cooling of the water separator 30 can be achieved via a cooling unit. The cooling unit can be a low-temperature circuit present in the fuel cell system 100.

[0051] The water separator 30 is equipped with a third temperature sensor 29. The third temperature sensor 29 measures a temperature of the water separator 30, which is referred to below as the third temperature T3, so that the information about the third temperature T3 of the water separator 30 is available for controlling and / or regulating the temperature of the water separator 30 and for the method according to the invention.

[0052] It is optionally possible to arrange a bypass line 52 in the recirculation line 21. The bypass line 52 is connected to the recirculation line 21 upstream of the water separator 30 and opens into the recirculation line 21 downstream of the water separator 30. With the help of the bypass line 52, the anode exhaust gas can flow from the recirculation line 21 into the anode inlet line 22 without passing through the water separator 30.

[0053] 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 water separator 30. Gases, such as anode exhaust gas, and / or fluids, such as product water, are discharged from the anode system 200 via the anode outlet line 23.

[0054] A combined purge / drain valve 24 is arranged in the anode outlet line 23. When the combined purge / drain valve 24 is opened, anode exhaust gas and / or product water are discharged from the anode system 200. The combined valve jet pump arrangement 26 accordingly doses fuel into the anode system 200 to maintain a continuous flow rate.

[0055] In an alternative embodiment, the combined purge / drain valve 24 can also be formed into two separate components, so that one purge valve and one drain valve are present in each anode system 200. A control unit 500 is provided to regulate and control the control processes in the fuel cell system 100. This also includes the processing of at least one measurement signal for executing the method according to the invention.

[0056] The cooling circuit 400 is arranged in the fuel cell system 100. The cooling circuit 400 serves to regulate the temperature of the fuel cell stack 11 by flowing a coolant through the fuel cell stack 11 via a coolant path KM. To carry out the method according to the invention, the coolant can be set to a temperature above the dew point temperature Ttau.

[0057] By tempering the coolant to a temperature above the dew point temperature Ttau, it can be ensured that there is essentially no condensation of gaseous water contained in the anode exhaust gas and / or cathode exhaust gas within the fuel cell stack 11, in particular in the anode chamber A and / or cathode chamber K.

[0058] 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 can direct the coolant at least partially or entirely past a vehicle radiator 42 via a bypass 46.

[0059] A coolant outlet temperature sensor 44 measures the temperature of the coolant after exiting the fuel cell stack 11. Optionally, a temperature sensor can also be arranged in the cooling circuit 45, which measures the temperature of the coolant before entering the fuel cell stack 11.

[0060] In an alternative embodiment, more than one fuel cell stack 11 can be arranged in the fuel cell system 100, without the execution of the method according to the invention being restricted thereby.

[0061] The method according to the invention can be carried out in a fuel cell system 100 in several fuel cell stacks 11 in parallel or sequentially.

[0062] Figure 2 illustrates an exemplary embodiment of the method according to the invention. The method is initiated in step S100, so that step S100 represents the start. The method according to the invention is executed during the shutdown of the fuel cell system 100, so that the fuel cell system 100 is not yet in an operational stop state.

[0063] In an optional method step, it can be checked whether a freeze start is expected, so that the method according to the invention is only executed if a freeze start is expected. An expected freeze start can be determined by evaluating predetermined parameters such as the geographical location of the fuel cell system 100 and / or an expected temperature minimum and / or an ambient temperature.

[0064] In step S200, the fuel cell stack 11 is tempered to a temperature above the dew point temperature Ttau, so that the fuel cell stack temperature T is above a dew point temperature Ttau. This can be achieved by tempering the coolant in the coolant circuit 400 to a temperature above the dew point temperature Ttau, and thereby tempering the fuel cell stack 11 by the coolant to a fuel cell stack temperature T that is above the dew point temperature Ttau.

[0065] The dew point temperature Ttau can be measured directly via a dew point sensor located within the recirculation line 21. The dew point sensor is preferably located upstream of the water separator 30 in the flow direction.

[0066] The dew point temperature Ttau can be determined indirectly. Ttau can be determined indirectly by determining the first temperature T1 in the anode feed line 22 and the second temperature T2 in the recirculation line 21. A dew point temperature Ttau can be determined using the first temperature T1 and the second temperature T2 by assuming the temperature T2 as the temporary cooling limit temperature. This makes it possible to determine the dew point temperature Ttau, for example, using a Mollier h,x diagram. During shutdown of the fuel cell system 100, water is removed from the fuel cell stack 11 by evaporation at high temperatures. The anode exhaust gas and cathode gas are additionally cooled by the evaporation. As a result, the second temperature T2, which is measured in the recirculation line 21, is lower than the first temperature T1, which is measured in the anode feed line 22.

[0067] During the drying process, the second temperature T2 reaches a minimum temperature that essentially corresponds to the cooling limit temperature Tcool. The second temperature T2 essentially corresponds to the cooling limit temperature Tcool when a gradient dT2 / dt reaches ~ 0 or passes through an inflection point.

[0068] By tempering the fuel cell stack 11 to a fuel cell temperature T that is above a dew point temperature Ttau, condensation of water in the anode compartment A and / or cathode compartment B can be avoided during the drying process, so that upon restart after the operation has stopped, the reactants can flow into the anode compartment A and / or cathode compartment K without being blocked by condensate of water or ice formation.

[0069] In an alternative embodiment, it is possible to temper the fuel cell stack temperature T to a temperature above the cooling limit temperature Tcool. If the fuel cell stack temperature T is above the cooling limit temperature Tcool, condensation of water in the fuel cell stack 11 can also be prevented.

[0070] In a further alternative embodiment, it is possible to control the fuel cell stack temperature T to a temperature above or equal to the first temperature T1. If the fuel cell stack temperature T is above or equal to the first temperature T1, condensation of water in the fuel cell stack 11 can also be prevented, since the fuel cell stack 11 has the same temperature as the introduced reactants.

[0071] Additionally, it is optionally possible to temper the water separator 30 to a water separator temperature TW that is less than or equal to the dew point temperature Ttau and above the freezing point of water. By tempering the water separator 30, additional water can be removed from the anode exhaust and / or cathode exhaust.

[0072] In step S300, the method according to the invention is terminated. Once step S300 is reached, the fuel cell system 100 can enter an operational stop.

[0073] In an alternative embodiment, it is also possible to carry out the method according to the invention with more than one fuel cell stack 11.

[0074] The method according to the invention can be carried out in a fuel cell system 100 in several fuel cell stacks 11 in parallel or sequentially.

[0075] The method can further be carried out at least in part by the control unit 500 of the fuel cell system 100. A computer program in the form of code can be stored in a memory unit of the control unit 500. When the code is executed by a computing unit of the control unit 500, the program performs a method that can proceed as described above. With the aid of the control unit 500, the same advantages can be achieved that were described above in connection with the method according to the invention. These advantages are incorporated herein by reference in their entirety.

[0076] The control unit 500 may be in communication with the sensors of the fuel cell system 100 to monitor the sensor values.

[0077] The control unit 500 can control the actuators in the fuel cell system 100 in order to carry out the method accordingly.

[0078] In addition, the control unit 500 can be in communication with an external computing unit in order to outsource some process steps and / or calculations in whole or in part to the external computing unit.

[0079] 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 processing unit of the control unit 500, cause the computer to carry out the method, which can proceed as described above. Using the computer program product, the same advantages can be achieved as described above in connection with the method according to the invention and / or the

[0080] Control unit 500. These advantages are fully incorporated herein by reference.

Claims

Claims 1.) Method for operating a fuel cell system (100), with at least one fuel cell stack (11), each having a cathode system (300) and an anode system (200) with an anode feed line (22), a recirculation line (21) and an anode chamber (A) as well as a cooling circuit (400) with coolant, characterized in that in order to avoid water accumulation during the shutdown of the fuel cell system (100), in particular before an expected freezing start, a fuel cell stack temperature T is set in the fuel cell stack (11) which is greater than a dew point temperature Ttau. 2.) Method according to claim 1, characterized in that the fuel cell stack temperature T is set to a temperature which is greater than a cooling limit temperature Tcool. 3.) Method according to claim 2, characterized in that the cooling limit temperature Tcool is approximately determined by a second temperature T2 which is measured in the recirculation line (21). 4.) Method according to claim 1, characterized in that the fuel cell stack temperature T is set to a temperature which is greater than a first temperature T1 which is measured in the anode feed line (22). 5.) Method according to claim 1, characterized in that the dew point temperature Ttau is determined via the first temperature T1, which is measured in the anode feed line (22), and the second temperature T2, which is measured in the recirculation line (21). 6.) Method according to claim 3 to 5, characterized in that the first Temperature T1 and the second temperature T2 are measured using temperature sensors. Method according to claim 1, characterized in that the dew point temperature Ttau is measured via a sensor, in particular a dew point sensor which is located in the fuel cell system (100). Method according to claim 1, characterized in that in order to set the fuel cell stack temperature T in the fuel cell stack (11), the coolant is set and / or regulated to a temperature which is above the dew point temperature Ttau. Method according to claim 1, characterized in that the water separator (30) is temperature-controlled so that the water separator (30) is set to a water separator temperature TW which corresponds to a temperature Ttau or lies between a temperature Ttau and the freezing point of water. Method according to claim 9, characterized in that the temperature of the water separator (30) is set by a cooling unit.Fuel cell system (100) with a water separator for carrying out the method according to at least one of claims 1 to 10, characterized in that the water separator (30) includes a cooling unit, wherein the water separator temperature TW is tempered by the cooling unit.

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