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

Abstract

The invention relates to a method for preventing a water buildup in a fuel cell system (100), said fuel cell system comprising at least one fuel cell stack (11), each of which has a cathode system (300) and an anode system (200) with an anode inlet line (22), a recirculation line (21), and an anode chamber (A), and comprising a cooling circuit (400) with coolant and a water separator (30), wherein during a shutdown of the fuel cell system (100), in particular prior to an expected start under freezing conditions, at least one flushing process is carried out if the water separator temperature (Tw) of the water separator (30) is below a dew point temperature Ttau, said flushing process involving a flushing of the fuel cell stack (11).

Description

[0001]

[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 10.

[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 can be 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, during an operational shutdown of the fuel cell system, in particular before an expected freezing start, at least one purging process is performed when the water separator temperature Tw of the water separator is below a dew point temperature Ttau. The purging process includes purging the fuel cell stack so that no condensation of gaseous water occurs in the fuel cell stack.

[0011] By means of the method according to the invention, freeze starts and degradations of the fuel cell system due to water accumulation in the fuel cell system can be advantageously reduced, since at a water separator temperature Tw of the water separator below the dew point temperature Ttau, water is specifically condensed out and thus carried out of the fuel cell system.

[0012] Optionally, it is advantageous to continuously cool the water separator to a temperature below or equal to the dew point temperature Ttau using the outside temperature, i.e., the temperature prevailing in the open environment of the fuel cell system. This eliminates the need for additional active cooling of the water separator, making the process cost-effective and eliminating the need for additional components.

[0013] Advantageously, in the method according to the invention, the rinsing of a cathode system and / or the rinsing of an anode system can take place in the rinsing process, so that, depending on requirements, only the anode system or only the cathode system or both the anode system and the cathode system are rinsed, which in turn increases the efficiency of the method.

[0014] During the purging process, the cathode system and / or the anode system are advantageously purged with at least one reactant. By using reactants, preferably air and / or hydrogen, gases or gas mixtures that are also required for the operation of the fuel cell system can be used, eliminating the need to use new gases or gas mixtures for the process or to provide additional tanks for storing additional gases in the fuel cell system.

[0015] The rinsing of the cathode system and the rinsing of the anode system can advantageously be carried out in parallel or sequentially, so that the process is carried out in an optimized manner as required.

[0016] It is advantageous if the purging process is started as soon as it is determined that the water separator temperature Tw is below a fuel cell stack temperature T. If the water separator temperature Tw is below the fuel cell temperature T, gaseous water preferentially condenses in the water separator, so that water accumulation in the fuel cell stack can be avoided.

[0017] It is advantageous if the rinsing process is started as soon as it is determined that the water separator temperature Tw is above the freezing point of water. If the water separator temperature Tw is above the freezing point of water, it can be ensured that the method according to the invention can be carried out efficiently and that the condensation of gaseous water in the water separator can take place.

[0018] Advantageously, the purging process is carried out after a predetermined period of time during the operational shutdown of the fuel cell system. This allows the method according to the invention to be carried out efficiently and specifically, and unnecessary purging processes can be avoided. Particularly when purging with the reactant hydrogen, a purging process after a predetermined period of time has a cost-optimizing effect. An expected freeze-start can advantageously be determined by evaluating information on predetermined parameters such as geographical location during the operational shutdown and / or an expected temperature minimum and / or evaluating information on an ambient temperature during the operational shutdown. Increasing the number of parameters used increases the accuracy of the prediction of an expected freeze-start and ensures that the method according to the invention can be used reliably.

[0019] The dew point temperature Ttau can advantageously be determined using a first temperature T1, which corresponds to the temperature of the reactant introduced into the fuel cell stack, and a second temperature T2, which corresponds to the temperature of the gas mixture discharged into the fuel cell stack. This allows for an accurate and timely determination of the dew point temperature Ttau.

[0020] It is advantageous if the dew point temperature Ttau is measured using a sensor, especially a dew point sensor. By measuring the dew point Ttau using a dew point sensor, a second sensor is eliminated, allowing the fuel cell system to have a compact design and minimize measurement uncertainties.

[0021] It is advantageous if the fuel cell system according to the invention has at least one sensor that is configured to measure a temperature in the water separator and at least one sensor that is configured to measure a temperature in the fuel cell stack, so that an accurate determination of temperatures required to carry out the method according to the invention is enabled.

[0022] The fuel cell system according to the invention advantageously comprises a cooling unit configured to regulate the water separator to a temperature that lies between a temperature in the fuel cell stack and a temperature at which water freezes. This allows the water separator to be specifically adjusted to a suitable temperature. It is advantageous if the fuel cell system comprises a water separator for carrying out the method, and the water separator includes a cooling unit. If the water separator temperature Tw of the water separator is adjusted by a cooling unit, the water separator can be specifically adjusted to a suitable temperature.

[0023] The fuel cell system according to the invention can preferably be used for mobile applications, for example, in vehicles, especially 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 conceivable that the fuel cell system according to the invention can be a power take-off and / or auxiliary drive of a vehicle, for example, a hybrid vehicle.

[0024] 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.

[0025] 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.

[0026] They show:

[0027] Fig. 1 shows a schematic topology of a fuel cell system according to a first embodiment;

[0028] Fig. 2 is a flow chart of the method according to the invention;

[0029] Figure 1 shows a schematic topology of a fuel cell system 100 comprising at least one fuel cell stack 11, an anode system 200, a cathode system 300, and a cooling circuit 400. 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 it 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] In an alternative embodiment, in the cathode outlet line

[0036] A water separator may be arranged in the cathode inlet line 31 and / or a temperature sensor may be arranged in the cathode outlet line 32. The water separator in the cathode outlet line may also have a temperature sensor. In an alternative embodiment, a temperature sensor may be arranged in the cathode inlet line 31 and / or a temperature sensor may be arranged in the cathode outlet line 32.

[0037] 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.

[0038] An anode supply line 22 is arranged in the anode system 200 and leads into the fuel cell stack 11. Fuel is supplied to the fuel cell stack 11 via the anode supply line 22.

[0039] 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. Using the first temperature sensor 27, a first temperature T1 of the fuel and / or gas mixture introduced into the fuel cell stack 11 can be measured. The first temperature T1 can then be provided to the method according to the invention and used to determine the fuel cell stack temperature T in the fuel cell stack 11.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] When the volume flow of the fuel supplied into the jet pump 26 increases, the volume flow of the anode exhaust gas into the anode feed line 22 also increases.

[0045] A recirculation conveying unit 25 is optionally arranged within the recirculation line 21. The recirculation conveying unit 25 can be a compressor configured as a blower, pump, and / or compressor.

[0046] The recirculation conveying unit 25 supports recirculation of the anode exhaust gas from the recirculation line 21 into the anode supply line 22.

[0047] 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.

[0048] 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.

[0049] A second temperature sensor 28 is arranged in the recirculation line 21.

[0050] 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.

[0051] A water separator 30 is arranged within the recirculation line 21.

[0052] 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.

[0053] 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 out of a water-containing gas mixture, such as anode exhaust gas and / or cathode exhaust gas.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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. 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.

[0058] 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 the anode system 200.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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. In an alternative embodiment, more than one fuel cell stack 11 can also be arranged in the fuel cell system 100, without this limiting the implementation of the method according to the invention.

[0064] 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.

[0065] Figure 2 shows an embodiment of the method according to the invention.

[0066] The method is initiated in step S100, so that step S100 represents the start. The method according to the invention is executed after the fuel cell system 100 has been shut down, so that the fuel cell system 100 is in an operational stop state.

[0067] 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.

[0068] In step S200, a specified period of time is waited for. When the specified period of time has elapsed, step 300 is executed. Upon expiration of the specified period of time, the water separator 30 can cool down to a dew point temperature Ttau or to a temperature below the dew point temperature Ttau.

[0069] In step S300, a check is made as to whether the water separator 30 has a temperature that corresponds at least to the dew point temperature Ttau or has a temperature that is below the dew point temperature Ttau. In order to determine the temperature of the water separator 30, the water separator temperature Tw is measured by the third temperature sensor 29. The water separator temperature Tw is then compared with the dew point temperature Ttau. If the water separator temperature Tw is above the dew point temperature Ttau, step S200 is executed again. Optionally, a fixed number of repetitions can be specified, in which step S300 is returned to step S200.When the specified number of repetitions is reached, it can be assumed that the water separator temperature Tw will only reach a dew point temperature Ttau with a low probability or that a temperature below the dew point temperature Ttau will be reached.

[0070] In addition, it is optionally possible to temper the water separator 30 to a temperature that is at least equal to or below the dew point temperature Ttau.

[0071] 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.

[0072] The dew point temperature Ttau can be determined indirectly. The indirect determination of Ttau can also be performed in an upstream step during the shutdown of the fuel cell system 100 during a drying process of the fuel cell stack 11. The first temperature T1 can be determined in the anode supply line 22. The second temperature T2 can be determined 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 allows the dew point temperature Ttau to be determined, for example, using a Mollier h,x diagram.

[0073] During the shutdown of the fuel cell system 100, water is removed from the fuel cell stack 11 by evaporation at high temperatures. The evaporation further cools the anode exhaust gas and cathode gas. As a result, the second temperature T2, measured in the recirculation line 21, is lower than the first temperature T1, measured in the anode supply line 22. 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.

[0074] It is optionally possible to check in an additional method step whether the water separator temperature Tw is below the fuel cell stack temperature T. If the fuel cell stack temperature T is above the water separator temperature Tw, no further measures are necessary. If the fuel cell stack temperature T is below the water separator temperature Tw, the fuel cell stack 11 can be tempered accordingly so that the fuel cell stack temperature T is above the water separator temperature Tw. This can prevent condensate from forming in the fuel cell stack 11, and the purging process taking place in step S400 can optimally remove gaseous water from the fuel cell stack 11.

[0075] It is also possible to check in an additional method step whether the water separator temperature Tw is above the freezing point of water. If the water separator temperature Tw is below the freezing point of water, the method according to the invention is terminated with step S500, and step S400, in which the rinsing process is carried out, is not performed.

[0076] In step S400, the purging process takes place, wherein the purging process comprises purging the fuel cell 100. The purging process may comprise purging the anode system 200 and / or the cathode system 300, so that the purging process involves purging a cathode system 300 and / or purging an anode system 200.

[0077] During the purging process, the cathode system 300 and / or the anode system 200 can be purged with at least one reactant. Preferably, the anode system 200 is purged with fuel as the reactant, and the cathode system with air as the reactant. It is also possible to remove gaseous water from the anode exhaust gas in the anode system by recirculating the anode exhaust gas via the recirculation line 21 into the anode inlet line 22. In this process, the gaseous water is removed from the anode exhaust gas via the water separator 30, which is arranged in the recirculation line 21.

[0078] The rinsing process in step S400 can be performed in parallel in the anode system 200 and the cathode system 300. The rinsing process in step S400 can also be performed sequentially for the anode system 200 and the cathode system 300.

[0079] In step S500, the method according to the invention is terminated.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] The control unit 500 may be in communication with the sensors of the fuel cell system 100 to monitor the sensor values.

[0084] The control unit 500 can control the actuators in the fuel cell system 100 in order to carry out the method accordingly.

[0085] Furthermore, the control unit 500 can be in communication with an external computing unit in order to outsource some method steps and / or calculations entirely or partially to the external computing unit. 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 computing unit of the control unit 500, cause the computer to perform the method, which can proceed as described above. With the aid of the computer program product, the same advantages can be achieved that were described above in connection with the method according to the invention and / or the control unit 500 according to the invention. These advantages are fully incorporated herein by reference.

Claims

Claims 1 .) Method for preventing water accumulation in a fuel cell system (100), wherein the fuel cell system has at least one fuel cell stack (11), each of which has a cathode system (300) and an anode system (200) with an anode inlet line (22), a recirculation line (21) and an anode chamber A, a cooling circuit (400) with coolant and a water separator (30), characterized in that during an operational stop of the fuel cell system (100), in particular before an expected freeze-up start, at least one flushing process is carried out if the water separator temperature Tw of the water separator (30) is below a dew point temperature Ttau, wherein the flushing process comprises flushing the fuel cell stack (11). 2.) Method according to claim 1, characterized in that the rinsing process involves rinsing the cathode system (300) and / or rinsing the anode system (200). 3.) Method according to claim 1, characterized in that in the rinsing process the cathode system (300) and / or the anode system (200) are rinsed with at least one reactant. 4.) Method according to claim 1, characterized in that in the rinsing process the rinsing of the cathode system (300) and the rinsing of the anode system (200) can take place in parallel or sequentially. 5.) Method according to claim 1, characterized in that the flushing process is started as soon as a. it is determined that the water separator temperature Tw is below a fuel cell stack temperature T and b. It is determined that the water separator temperature Tw is above the freezing point of water 6.) Method according to claim 1, characterized in that the purging process is carried out after a predetermined period of time during the operation stop of the fuel cell system (100). 7.) Method according to claim 1, characterized in that an expected freezing start is determined by evaluating information on predetermined parameters such as geographical location during the operational stoppage and / or an expected temperature minimum and / or evaluating information on an ambient temperature during the operational stoppage. 8.) Method according to claim 1, characterized in that the dew point temperature Ttau is determined via a first temperature T1, which corresponds to the temperature of the reactant introduced into the fuel cell stack, and a second temperature T2, which corresponds to the temperature of the gas mixture discharged into the fuel cell stack (11). 9.) Method according to claim 1, characterized in that the dew point temperature Ttau is measured via a sensor, in particular a dew point sensor. 10.) Fuel cell system (100) with at least one fuel cell stack (11), each having a cathode system (300) and an anode system (200), as well as a cooling circuit (400) with coolant and a water separator (30), for carrying out the method according to claim 1, characterized in that the fuel cell system (100) comprises: At least one sensor (29) configured to measure a temperature in the water separator (30) At least one sensor configured to measure a temperature in the fuel cell stack (11) A cooling unit configured to temper the water separator (29) to a temperature that lies between a temperature in the fuel cell stack (11) and a temperature at which water freezes.

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