Method for controlling the temperature of a fuel cell system, and fuel cell system for an aircraft propulsion system

Abstract

The invention relates to a method (100) for controlling the temperature of a fuel cell system (10) of an aircraft propulsion system (2) of an aircraft (1), and to a fuel cell system (10) for an aircraft propulsion system (2), or for an aircraft (1) comprising an aircraft propulsion system (2). The fuel cell system (10) comprises a temperature-control system (20) and a separate heating system (50) in order to enable the fuel cell system (10) to be heated even at very low system temperatures (T) which are below a first threshold value (T1). The heating system (50) can be connected, when required, to the temperature-control system (20) of the fuel cell system (10).

Description

[0001] Method for temperature control of a fuel cell system and fuel cell system for a flight propulsion system

[0002] The invention relates to a method for temperature control of a fuel cell system according to the preamble of claim 1 and a fuel cell system for an aircraft propulsion system according to the preamble of claim 7.

[0003] Fuel cell systems for air propulsion include at least one fuel cell, which may be part of a fuel cell stack. Furthermore, fuel cell systems include electrical components such as the fuel cell control unit, valves, fans, and sensors to operate and control the fuel cell system.

[0004] In extremely cold environmental conditions, the fuel cell system of an air propulsion system and the electrical components of the fuel cell system may no longer function, or no longer function properly, at very low temperatures.

[0005] For example, German patent DE 102005031 087 A1 proposes integrating a catalytic burner into the fuel cell system and using its waste heat to heat the fuel cell system. The burner directly heats a heat transfer medium, which also heats the fuel cell. While this creates a compact fuel cell system that can be started over a wide temperature range, the burner increases the weight of the temperature control system, making the fuel cell system only conditionally suitable for aviation applications due to its high operating weight.

[0006] In contrast, an object of the invention is to provide a method for temperature control of a fuel cell system for an aircraft propulsion system, with which the fuel cell system can be operated safely even at very low temperatures. A further object of the invention is to provide a fuel cell system for an aircraft propulsion system that can be used flexibly in a wider ambient temperature range and is subject to less stringent certification requirements.

[0007] This problem is solved according to the invention by a method with the features of claim 1. Furthermore, the problem is solved by a fuel cell system with the features of claim 7.

[0008] The method according to the invention is provided for temperature control of a fuel cell system of an aircraft propulsion system, wherein the fuel cell system comprises at least one fuel cell and a temperature control system with a heat exchanger and an electric heater, wherein the heat exchanger can temperature control a first heat transfer medium circulating through the fuel cell and the electric heater, and wherein the electric heater can heat the first heat transfer medium. The method solves the problem by the following steps: determining a system temperature of the fuel cell system with a system temperature sensor, connecting a separate heating system to the heat exchanger to supply a hot, second heat transfer medium when the system temperature is below a first threshold value, and heating the first heat transfer medium with the heating system at least until the system temperature is above the first threshold value.

[0009] The system temperature can be the temperature measured at the fuel cell. The system temperature can be determined at multiple points. If several temperature sensors are provided, the system temperature can be the lowest measured temperature. Very low temperatures are defined as those at which other heat generators of the fuel cell system, such as the electric heater, are no longer functional or only partially functional. The first threshold can be between the lower and upper limits of -25°C and -15°C, preferably between -23°C and -18°C. The upper limit of the first threshold can be -15°C, -16°C, -17°C, -18°C, or -19°C. The lower limit of the first threshold can be -25°C, -24°C, -23°C, -22°C, or -21°C. A first threshold of -20°C is particularly preferred. The first threshold can be a temperature below which safe operation of the electric heater is no longer possible.The first threshold value can depend on the electric heater and / or the heating system. Connection can be achieved by attaching hoses and / or pipes to a first interface of the heat exchanger. The hoses and / or pipes can be housed as separate components within the heating system. The heating system can, in particular, heat a second heat transfer medium and transfer heat to the first heat transfer medium via the connection to the heat exchanger. The second heat transfer medium is advantageously a mixture of air and a combustion gas, for example, hydrogen, as explained in more detail below.

[0010] Further advantages and features will become apparent from the following description of some preferred embodiments of the method and the dependent claims.

[0011] The process can preferably be further developed by the following step: heating the first heat transfer medium with the electric heater as soon as the system temperature exceeds the first threshold and / or when the system temperature reaches a second threshold that is higher than the first. This advantageously increases the system temperature even more rapidly. Furthermore, the electric heater can advantageously enable more direct control of the system temperature. The heating is preferably carried out until the system temperature reaches an optimal operating temperature for the fuel cell.

[0012] The process can be further improved by connecting the heating system to a hydrogen tank of the fuel cell system. This allows the heating system to advantageously use the stored hydrogen as fuel.

[0013] The process can be advantageously further developed by the following step:

[0014] Controlling the supply of hydrogen from the hydrogen tank to the heating system with a control unit. Using a control unit allows fluctuations in the pressure and temperature of the compressed hydrogen within the lines to be kept constant, and leaks to be reduced or prevented. Furthermore, the volumetric flow rate of the hydrogen supplying the heating system can be controlled. The control unit can be a hydrogen tank control unit, a drive control unit, and / or an aircraft control unit. The control unit can control at least one tank valve. Controlling the hydrogen supply can include vaporizing the hydrogen for use in the hydrogen tank and / or in a vaporizer located in the line. Furthermore, controlling the hydrogen supply can include vaporizing a fresh air supply to produce a combustible gas mixture.This control can include adjusting a fan to supply fresh air to a hydrogen burner.

[0015] In an advantageous further development of the method, the following step can be carried out: When the system temperature reaches a third threshold that is equal to or higher than the first threshold, and in particular when heating by the electric heater is already taking place: heating is stopped and the heating system is subsequently decoupled from the fuel cell system. This advantageously returns the fuel cell system to its original state, so that the aircraft or the propulsion system is again available for flight. The third threshold can be below the optimal operating temperature of the fuel cell, and it can be provided, in particular, that when the third threshold is reached, only the electric heater continues to heat the first heat transfer medium. The third threshold can coincide with the first threshold.In this case, the heating of the first heat transfer medium by the heating system is replaced by the heating of the first heat transfer medium by the electric heater.

[0016] In a further embodiment of the method, the following step can be carried out: controlling a cooling airflow from an adjustable cooling air supply with a control unit to control the heat transfer to the first heat transfer medium at the heat exchanger. Another aspect of the invention relates to a fuel cell system for an aircraft propulsion system or for an aircraft with an aircraft propulsion system. The fuel cell system according to the invention comprises at least one fuel cell, in particular at least one fuel cell stack with the at least one fuel cell, and a temperature control system for temperature control of the at least one fuel cell. The temperature control system includes a heat exchanger that can transfer heat to the fuel cell via a first heat transfer medium and, in particular, can dissipate heat from the fuel cell.The fuel cell system also includes at least one system temperature sensor for determining the system temperature of the fuel cell system.

[0017] The problem is solved by the fuel cell system according to claim 7 in that the heat exchanger has a first interface that allows connection to a separate heating system for supplying a hot, second heat transfer medium, in particular heated air, into the heat exchanger in order to heat the first heat transfer medium, and that the heating system is connected to the first interface at a system temperature below a first threshold value and enables the heating of the at least one fuel cell via the first heat transfer medium and the second heat transfer medium.

[0018] Decoupling the primary heat transfer medium from the separate heating system allows for a flexible increase in the usable temperature range and simultaneously makes the fuel cell system suitable for flight without increasing certification requirements. Connecting a separate heating system enables operation of the fuel cell system even at very low temperatures, particularly below the first threshold defined above. Because the heating system can supply heat to the fuel cell system via an additional heat transfer medium, it is advantageously decoupled from the fuel cell system. This reduces the certification requirements for the heating system, significantly lowering its cost. Furthermore, this can advantageously reduce the weight of the fuel cell system, especially during flight when the heating system is not carried on board.Furthermore, this can also save space in ground-based gas turbines with fuel cell propulsion and advantageously only needs to be connected to the fuel cell system when required. This also allows a large number of aircraft engines to be heated with one and the same heating system, which further reduces the production costs for the fuel cell system.

[0019] The at least one fuel cell can be integrated into a fuel cell stack with a plurality of fuel cells. The technical features mentioned herein relating to a single fuel cell can also be applied to the plurality of fuel cells. The fuel cell can be a polymer electrolyte fuel cell. The first heat transfer medium can be water or a coolant. The first heat transfer medium can be the system water of the fuel cell. The heating system can preferably be a ground-based heating system. The heating system can be designed to be connected to the heat exchanger. The first interface can preferably be designed as a coupling, in particular as a quick-release and / or plug-in coupling, to ensure a secure connection of the lines at the interface. In particular, the heating system can be connected to the first interface via flexible lines.The heat exchanger can be an air-to-liquid heat exchanger, in particular an air-to-water heat exchanger. The heat exchanger can be integrated into an outer wall of the gas turbine and / or the aircraft and cooled by an air inlet. The heat exchanger can be directly connected to the at least one fuel cell or to the fuel cell stack. Alternatively, the heat exchanger can be integrated into the at least one fuel cell or the fuel cell stack. The first heat transfer medium can be circulated, for example, by a heat transfer medium pump. For this purpose, the heat transfer medium pump can be connected to an electric heater, which will be described in more detail below. Additionally or alternatively, the heat transfer medium pump can be connected to the heat exchanger.The heat transfer medium can be pumped to or from the at least one fuel cell and / or to or from the fuel cell stack. The fuel cell system can include the heating system. The heat exchanger can be at least partially integrated with the at least one fuel cell and / or the fuel cell stack. The heating system can include a hydrogen burner that generates heat by burning hydrogen. The hydrogen can be supplied from an external hydrogen tank via an external connection. The first interface can be located in an external housing of the aircraft propulsion system and / or the aircraft. This makes connecting the heating system particularly simple and robust. Furthermore, the external housing provides advantageous protection for the internal components of the fuel cell system.

[0020] Further advantages and features will result from the following description of some preferred embodiments of the fuel cell system and the dependent claims.

[0021] According to an advantageous embodiment of the invention, the temperature control system can include an electric heater that enables the heating of the first heat transfer medium and, in particular, can be designed for heating the at least one fuel cell at a system temperature above a second threshold value. An electric heater allows the fuel cell system to be designed with even greater flexibility. The electric heater can, for example, be supplied with electrical energy by connecting to a ground power grid. The electric heater can be designed for operation during the start-up process of the gas turbine and / or the aircraft.

[0022] According to a preferred embodiment of the invention, in which at least one fuel cell is connected to and supplied with hydrogen from an internal hydrogen tank, the hydrogen tank can be connected to a second interface that can be connected to the heating system and supply a hydrogen burner in the heating system with hydrogen for combustion and heat generation. This advantageously increases the flexibility of the fuel cell system, as the hydrogen does not need to be stored on-site. This, in particular, relieves the burden on local infrastructure and eliminates the need for stationary hydrogen storage tanks. The heating system can advantageously be coupled to the hydrogen tank of the fuel cell system and thus receive the fuel for the hydrogen burner of the heating system.According to a particularly preferred embodiment, the second interface can be located in an external housing of the aircraft engine and / or the aircraft itself. This allows for a simple connection between the heating system and the hydrogen tank. At the same time, the hydrogen tank is advantageously and flexibly always located on-site and is also protected from the elements.

[0023] In a further development, the second interface can include a control valve to introduce hydrogen into the heating system, and in particular, the control valve can be controlled by a control unit of the hydrogen tank. This allows hydrogen to be advantageously supplied to the heating system from the hydrogen tank as needed. If the control valve is controlled by the hydrogen tank's control unit, an additional safety feature is advantageously introduced into the system. Alternatively or additionally, a connection of the control valve to the aircraft's or propulsion system's control computer can be omitted, thus further increasing the mobility of the fuel cell system. Alternatively or additionally, the second interface can also include a pressure relief valve to release any parasitic hydrogen escaping from at least one fuel cell into the heating system.This advantageously prevents overpressure in the hydrogen tank. Furthermore, it advantageously creates a way to utilize even parasitic hydrogen.

[0024] In a further embodiment of the invention, the heating system comprises a hydrogen burner and, in particular, a flexible connecting line for connecting the hydrogen burner to the first interface of the fuel cell system. A separate heating system with a hydrogen burner allows the fuel cell system to be heated advantageously and quickly at very low temperatures, thus enabling the fuel cell system to be brought into a combustion mode as quickly as possible, in which the fuel cell generates electricity. Furthermore, several fuel cell systems can advantageously be heated by a single heating system. In an advantageous further development of the heating system according to the invention, the heating system can be designed to be mobile relative to the fuel cell system and, in particular, includes a transport device. This advantageously facilitates handling. The transport device can, for example, include rollers.

[0025] Preferably, the heating system may include a fan for supplying combustion air. This allows the combustion air to be easily supplied to a burner in the heating system, particularly the hydrogen burner, and the heated air can then be provided to the temperature control system or the heat exchanger of the fuel cell system.

[0026] Furthermore, the fan can be connected to an air inlet of the at least one fuel cell, in particular of the at least one fuel cell stack, preferably via a third interface. This allows an additional component of the fuel cell system to be used advantageously, and no additional air supply needs to be provided to the heating system, thus further reducing the heating system's costs.

[0027] In a further embodiment of the invention, the first and / or second interface may include a quick-connect fitting, in particular a quick-connect fitting designed as a snap-in and / or plug-in coupling. This advantageously makes handling and connecting the heating system to the heat exchanger safer and significantly easier.

[0028] Another aspect of the invention relates to an aircraft propulsion system with a fuel cell system, which can be designed as described above and performs a method as described above.

[0029] The invention is explained in more detail with reference to the following drawings and some preferred embodiments of the invention.

[0030] Fig. 1 shows an embodiment of a device according to the invention.

[0031] Fuel cell system Fig. 2 shows a flowchart of an embodiment of a method according to the invention.

[0032] Fig. 1 shows a schematic representation of an aircraft 1 with a propulsion system 2, in which an embodiment of a fuel cell system 10 according to the invention is arranged. The aircraft 1 is represented schematically by dashed lines and the propulsion system 2 by dashed lines. The aircraft 1 and the propulsion system 2 can each have an outer housing 3 or a common outer housing 3. A common outer housing 3 would be, for example, an engine pylon to which the propulsion system 2 is attached to the aircraft 1, or an outer housing section in which an integrated propulsion system is embedded.

[0033] The fuel cell system 10 is arranged in the aircraft propulsion system 2 and / or in the aircraft 1. The fuel cell system 10 can comprise at least one fuel cell stack 20 with at least one fuel cell 21. It is not strictly necessary for the fuel cells 21 to be arranged in a stack, although this is common practice. The fuel cell 21 can be designed as a polymer electrolyte membrane (PEM) fuel cell. In the present embodiment, the fuel cell 21 is supplied with hydrogen 22 as fuel. The hydrogen 22 is transported from a hydrogen tank 60 to the fuel cell 21 via a line 24. A hydrogen pump (not shown) can be used for this purpose, for example. To supply the fuel cell system 10 with sufficient catalyst medium, an air inlet 23 can be provided through which air is drawn in from the environment and supplied to the fuel cell 21.The fuel cell stack 20 is being pumped. The lines of the air system are shown with dash-dot-dot lines.

[0034] To determine the system temperature T of the fuel cell 21 and / or the fuel cell stack 20, a system temperature sensor 25 can be provided, which can determine the temperature at the fuel cell(s). For the inventive method 100 described in Fig. 2, the lowest temperature of the fuel cells is used in the present embodiment. The fuel cell system 10 also has a temperature control system 30, which can temperature the fuel cell 21 or the fuel cell stack 20 with a first heat transfer medium 41. This can mean that the temperature control system 30 can cool or heat the at least one fuel cell 21 or the fuel cell stack 20 as required. For this purpose, the heat transfer medium 41 is heated or cooled to a suitable temperature in the temperature control system 30 so that combustion in the fuel cell 21 can take place at the most ideal temperature possible.The heat transfer medium 41 can be passed through the fuel cell stack 20 and / or the fuel cell 21, where it can absorb or release heat. The temperature control system 30 can, for example, include an electric heater 31 that heats the heat transfer medium 41 to the appropriate temperature. Furthermore, the temperature control system 30 can include a heat exchanger 40 that can serve as a heat source or heat sink and can heat or cool the heat transfer medium 41. The heat required for heating can be provided, for example, from a liquid circuit or another waste heat source, such as an exhaust duct, of the aircraft engine 2. For cooling, the heat exchanger 40 can, for example, be arranged at least partially in a flow channel 4, in particular a bypass channel of the aircraft engine 2.The heat transfer medium 41 is preferably conveyed by means of a heat transfer medium pump 32 to or from the at least one fuel cell 21 or to or from the fuel cell stack 20 to and from the electric heater 31 and / or the heat exchanger 40. If the first heat transfer medium 41 is a coolant, the heat transfer medium pump 32 can be configured as a coolant pump 32. Instead of a single heat exchanger 40, several heat exchangers can also be arranged at different locations in the aircraft propulsion system 2.

[0035] The invention addresses the problem that the temperature control system of the fuel cell system, in very cold environments where temperatures fall below a first threshold value T1, particularly below -20°C, may no longer be sufficiently powerful or, to be sufficiently powerful, must be dimensioned so large that the cost and weight of the fuel cell system would no longer be economical. To solve these problems, the fuel cell system 10 is designed according to the invention such that the temperature control system 30 can be coupled to a separate, particularly mobile, heating system 50 connected to the fuel cell system 10. For this purpose, the temperature control system 30 and the heating system 50 have a first interface 42 through which a second heat transfer medium 51 can be introduced into the heat exchanger 40 to transfer heat to the first heat transfer medium 41 and thus provide heat for the fuel cell 21.to provide the fuel cell stack 20. For this purpose, for example, the flow channel 4 of the flight propulsion 2 can be used, through which the hot second heat transfer medium 51 is directed into the flow channel 4, which is connected to the heat exchanger. Alternatively, the second heat transfer medium 51 can also be directed directly through the heat exchanger 40.

[0036] The second heat transfer medium 51 can, for example, be heated in a hydrogen burner 53 of the heating system 50, which is preferably mobile and, furthermore, preferably external. The second heat transfer medium 51 can, for example, be a combusted mixture of air from an air inlet 23 and hydrogen. For this purpose, the heating system 50 can have a second interface 52 that can provide a hydrogen supply, particularly from the hydrogen tank 60. The second interface 52 can have an actively controllable control valve or be designed as such, wherein the control valve can particularly preferably be controlled by the control unit 61 of the hydrogen tank 60. This allows, on the one hand, the hydrogen supply to the heating system 50 to be precisely adjusted, and on the other hand, any hydrogen detected as having escaped parasitically from the hydrogen tank 60 can be discharged into the heating system 50 and used there.In order to also or conversely be able to use passively parasitic hydrogen from the fuel cell 21, the second interface 52 can alternatively or additionally have a pressure relief valve or be designed as a pressure relief valve.

[0037] The second heat transfer medium 51 can preferably be air, which can particularly preferably be supplied from an existing air inlet 23 for the fuel cell stack 20 or the fuel cell 21. For this purpose, the heating system 50 can be configured to be connectable to the air inlet 23 via a third interface 55. The heating system 50 also has a connecting line 56 for connecting the hydrogen burner 53 to the first interface 42 of the fuel cell system 10, wherein the connecting line 56 is in particular configured as a hot air line.

[0038] To connect the heating system 50 to the fuel cell system 10, it is mobile and equipped with a propulsion device 57, which may include wheels and / or be motorized. The heating system 50 is moved into position for connection to the fuel cell system 10, and then the connecting cable 56 is coupled to the first interface 42.

[0039] It may also be provided that the heating system 50 has its own air inlet 58. For example, the separate air inlet 58 may be arranged in an outer housing 59 of the heating system 50.

[0040] To supply the air, a fan 54 can be provided, which can be located in the heating system 50 and can therefore also be mobile. The fan 54 can supply air from an air inlet 23 or the heating system 50's own air inlet 58 to the hydrogen burner 53.

[0041] Actuators 43 can be arranged in the flow channel 4 to monitor and control the cooling or heating airflow of the second heat transfer medium 51 to the heat exchanger 40, in particular by means of a control unit 44. Actuators can also be provided in the first interface, which can be controlled by the control unit 44 in order to monitor the heating airflow and to allow for fine adjustment.

[0042] Furthermore, the electric heater 31 can, for example, be supplied with electrical energy via a power connection 71 to a ground power network 70, thereby enabling the fuel cell system 10 to be operated even more flexibly on the ground until it has been heated to an operating temperature of at least 0°C and can then commence actual operation. This also further reduces the weight of the fuel cell system 10 during operation, as the weight can be advantageously saved for a correspondingly powerful battery.

[0043] The heat transfer medium pump 32 can also be supplied with electrical energy, for example, via a power connection 71 to a ground power network 70.

[0044] An embodiment of a method 100 according to the invention will now be described with reference to the flowchart shown in Fig. 2.

[0045] First, the system temperature T of the fuel cell system 10 is determined using the system temperature sensor 25. If the system temperature T is below a first threshold value Ti, the separate heating system 50 is connected to the heat exchanger 40 to supply a hot, second heat transfer medium 51. The first heat transfer medium 41 is then heated by the heating system 50 at least until the system temperature T is above the first threshold value Ti. This allows the system temperature T of the fuel cell system 10 to be raised above the first threshold value Ti, so that the electrical components, such as the electric heater 31, can be used. To supply the hydrogen burner 53 with hydrogen, the heating system 50 is connected to a hydrogen tank 60 of the fuel cell system 10 before heating 103. A connecting line is then attached to a second interface 52.To control the hydrogen supply, the hydrogen supply is controlled by a control unit 61 of the hydrogen tank 60, in particular by adjusting the control valve in the second interface 52.

[0046] In the embodiment of method 100, the first heat transfer medium 41 is heated 104 by the electric heater 31 as soon as the system temperature T is above the first threshold value Ti and / or when the system temperature T reaches a second threshold value T2, which is higher than the first threshold value Ti. After a third threshold value Ts has been reached, which makes it possible to heat the fuel cell 21 only with the electric heater 31, the heating 103 is stopped 107 and subsequently the heating system 50 is decoupled from the fuel cell system 10 108.

[0047] As an alternative to early decoupling 108 of the heating system 50, its heat supply can also be influenced by controlling a cooling airflow to the heat exchanger 40, thus additionally controlling heat transfer to the first heat transfer medium 41 and, in particular, preventing overheating of the first heat transfer medium 41. Reference numerals

[0048] 1 airplane

[0049] 2 Flight propulsion

[0050] 3 outer casings

[0051] 4 Flow channel, bypass channel

[0052] 10 Fuel cell systems

[0053] 20 fuel cell stacks

[0054] 21 Fuel cell

[0055] 22 Hydrogen

[0056] 23 Air intake

[0057] 24 lines

[0058] 25 System temperature sensor

[0059] 30 temperature control system

[0060] 31 electric heaters

[0061] 32 Heat transfer medium pump / Coolant pump

[0062] 40 heat exchangers

[0063] 41 first heat transfer medium

[0064] 42 first interface

[0065] 43 actuating elements

[0066] 44 Control unit

[0067] 50 Heating system

[0068] 51 second heat transfer medium

[0069] 52 second interface

[0070] 53 hydrogen burners

[0071] 54 Fan

[0072] 55 third interface 56 connection cable

[0073] 57 rolls

[0074] 58 Air intake of the heating system

[0075] 59 External casing of the heating system

[0076] 60 hydrogen tank

[0077] 61 Control unit

[0078] 70 Ground power grid

[0079] 71 Power connection

[0080] 101 Determining a system temperature

[0081] 102 Connecting a separate heating system

[0082] 103 Heating the first heat transfer medium

[0083] 104 Heating the first heat transfer medium

[0084] 105 Connecting the heating system to a hydrogen tank 106 Controlling a hydrogen supply

[0085] 107 Ending the heating and subsequent

[0086] 108 Decoupling the heating system

[0087] 109 Controlling a cooling airflow

Claims

Patent claims 1. Method (100) for temperature control of a fuel cell system (10) of an aircraft propulsion system (2) of an aircraft (1), wherein the fuel cell system (10) comprises at least a fuel cell (21), a temperature control system (30) with a heat exchanger (40) and an electric heater (31), wherein the heat exchanger (40) can temper a first heat transfer medium (41) circulating through the fuel cell (21) and the electric heater (31), wherein the electric heater (31) can heat the first heat transfer medium (41), characterized by the steps: Determining (101) a system temperature (T) of the fuel cell system (10) using a system temperature sensor (25), Connecting (102) a separate heating system (50) to the heat exchanger (40) for the supply of a hot, second heat transfer medium (51) when the system temperature (T) is below a first threshold (Ti), in particular wherein the first threshold (Ti) is between -25°C and -15°C, preferably wherein the first threshold (Ti) is -20°C, and Heating (103) the first heat transfer medium (41) with the heating system (50) at least until the system temperature (T) is above the first threshold (Ti).

2. The method according to claim 1, characterized by the step: Heating (104) of the first heat transfer medium (41) by the electric heater (31) as soon as the system temperature (T) is above the first threshold (Ti) and / or when the system temperature reaches a second threshold (T2) that is higher than the first threshold (Ti).

3. Method according to one of the preceding claims, characterized by the step: connecting (105) the heating system (50) to a hydrogen tank (60) of the fuel cell system (10).

4. The method of claim 3, characterized by the step: Control (106) of a hydrogen supply from the hydrogen tank (60) to the heating system (50) with a control unit (61).

5. Method according to any of the preceding claims, characterized by the step: When the system temperature (T) reaches a third threshold (Ts) that is equal to or higher than the first threshold (Ti), and in particular when heating (104) by the electric heater is already taking place: Stop (107) the heating (103) and then decouple (108) the heating system (50) from the fuel cell system (10).

6. Method according to one of the preceding claims, characterized by the step: Control (109) a cooling air flow from an adjustable cooling air supply (43) with a control unit (44) to control the heat transfer to the first heat transfer medium (41) at the heat exchanger (40).

7. Fuel cell system (10) for an aircraft propulsion system (2), or for an aircraft (1) with an aircraft propulsion system (2), comprising at least one fuel cell (21), in particular at least one fuel cell stack (20) with at least one fuel cell (21 ), a temperature control system (30) for temperature control of the at least one fuel cell (21), wherein the temperature control system (30) has a heat exchanger (40) which can transfer heat to the fuel cell (21) via a first heat transfer medium (41), and in particular can remove heat from the fuel cell (21), and at least one system temperature sensor (25) for determining (101) a system temperature (T) of the fuel cell system (10), characterized in that the heat exchanger (40) has a first interface (42) which allows connection to a separate heating system (50) for supplying a hot, second heat transfer medium (51), in particular heated air, into the heat exchanger (40) in order to heat the first heat transfer medium (41), and that the heating system (50) is connected to the first interface (42) at a system temperature (T) below a first threshold value (Ti) and enables the heating of at least one fuel cell (21) via the first heat transfer medium (41) and the second heat transfer medium (51).

8. Fuel cell system according to claim 7, comprising the heating system (50), characterized in that that the temperature control system (30) has an electric heater (31) which enables the heating of the first heat transfer medium (41), and which is designed in particular for heating the at least one fuel cell (21) at a system temperature (T) above a second threshold value (T2).

9. Fuel cell system according to one of claims 7 or 8, wherein the at least one fuel cell (21) is connected to a hydrogen tank (60) in the flight propulsion (2) and / or in the aircraft (1) and can be supplied with hydrogen (22) from the hydrogen tank (60), characterized in that that the hydrogen tank (60) is connected to a second interface (52) which can be connected to the heating system (50) and can supply a hydrogen burner (53) in the heating system (50) with hydrogen for combustion of the hydrogen and generation of heat.

10. Fuel cell system according to claim 9, characterized in that the second interface (52) comprises a control valve (52) to enable hydrogen to be introduced into the heating system (50), in particular wherein the control valve (52) is controlled by a control unit (61) of the hydrogen tank (60).

11. Fuel cell system according to one of claims 7 to 10, characterized in that the heating system (50) has a hydrogen burner (53), and a particularly flexible connecting line (56) for connecting the hydrogen burner (53) to the first interface (42) of the fuel cell system (10).

12. Fuel cell system according to claim 11, characterized in that the heating system (50) is mobile relative to the fuel cell system (10) and in particular comprises a propulsion device (57).

13. Fuel cell system according to one of claims 7 to 12, characterized in that that the heating system (50) has a fan (54) for supplying combustion air, in particular that the fan (54) is preferably designed to be connectable to the air inlet (23) via a third interface (55).

14. Fuel cell system according to one of claims 7 to 13, characterized in that, that the first interface (42) includes a quick-connect coupling, in particular that the quick-connect coupling is designed as a snap-on and / or plug-in coupling, and / or that the second interface (52) includes a quick connection, in particular that the quick connection is designed as a latching and / or plug-in coupling.

15. Aircraft propulsion (2) for an aircraft (1) comprising a fuel cell system (10) according to any one of claims 7 to 14 for carrying out a method according to any one of claims 1 to 6.

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