Method of operating a hydrogen fuel cell of an aircraft during takeoff, and an aircraft comprising a controller configured to perform said method

Abstract

Method of operating a hydrogen fuel cell of an aircraft to provide a predetermined electrical power output during takeoff, comprising: - before takeoff-roll, predicting or determining an ambient temperature (TA) at an airport; - before takeoff-roll, predicting the actual obtainable temperature operating range (∆T) of the fuel cell that is expected to be obtainable at the runway of the airport during takeoff; - if ∆T is lower than a predetermined minimum threshold for which a cooling system of the aircraft was designed: - limiting the power output PFC supplied by said fuel cell by a value PFC,LIM; and - providing a supplementary power source for use during takeoff that has a maximum power output PSUP,MAX > PFC,LIM to compensate for the set limitation of the power output PFC,LIM supplied by said fuel cell to thereby guarantee that, during takeoff, a total available power output PTOT of the fuel cell and the supplementary power source together, i.e. PTOT = PFC + PSUP, is equal to or higher than a maximum power of the fuel cell PFC,MAX. The invention further relates to an aircraft comprising a controller configured to perform said method of operating the hydrogen fuel cell.

Description

[0001] Title: Method of operating a hydrogen fuel cell of an aircraft during takeoff, and an aircraft comprising a controller configured to perform said method

[0002] Descri ption:

[0003] The present invention relates to a method of operating a hydrogen fuel cell of an aircraft during takeoff, and more in particular to a method of operating a hydrogen fuel cell of an aircraft to provide a predetermined electrical power output (PFC) supplied by said fuel cell during takeoff.

[0004] The invention is further related to an aircraft comprising a controller configured to perform said method of operating the hydrogen fuel cell.

[0005] There is an ever growing need for environmentally sustainable alternatives in today's society due to the looming threat of greenhouse gasses. Many countries share the ambition to reduce CO2 emissions to nett zero by 2050.

[0006] The largest contributor to the emissions of greenhouse gasses is the transportation industry with about a quarter of the total share of emissions. The aviation industry is a major contributor to this and is in need of innovation to be in line with the UN’s goal for sustainability. Consequently, the aviation industry has been seeking alternatives to traditional fossil fuels.

[0007] Hydrogen presents a promising fuel in this regard. In theory, hydrogen fuel cell powertrains could enable zero-emission flight when the hydrogen is produced through renewable methods. Moreover, the energy density of hydrogen is relatively high. Hydrogen is also considered a versatile energy source. Unlike fossil fuels, hydrogen can be produced through various means, including electrolysis powered by renewable energy sources such as wind and solar. This versatility not only aligns with sustainability goals but also offers a degree of energy independence, reducing reliance on finite oil reserves.

[0008] However, despite its immense potential, the widespread adoption of hydrogen in aviation also faces numerous challenges. Infrastructure development, safety considerations, cost-effectiveness, and other technical challenges, are among the hurdles that must be overcome to realize hydrogen's full potential as an aviation fuel. When hydrogen is used as a fuel source in fuel cells, it undergoes a transformation into electricity. This electricity can then be used for various purposes, such as powering the propulsion system of an aircraft. During this conversion, a lot of heat is generated, reducing the efficiency of hydrogen, but also causing other challenges. Careful attention should be paid to managing the heat generated during conversion of the hydrogen. Heat exchangers, cooling systems, insulation, and thermal management techniques play vital roles in ensuring the efficient and safe utilization of hydrogen as an energy source.

[0009] Aircraft, by nature, travel between different destinations. As a result, the aircraft will be faced with a huge variety of environmental conditions. For example, the ambient temperature at an air field may vary in dependence on the graphical location (depending on the local climate), the season, the time of day (day time or night time), actual weather conditions (rain, humidity, wind, the presence of clouds, etc.), and more.

[0010] A varying ambient temperature at an air field is especially important during takeoff. After all, the takeoff of an aircraft requires the aircraft to accelerate on the limited length of the runway to reach its lift-off speed, i.e. the velocity at which the aircraft lifts off from the runway and transitions into the airborne phase of flight.

[0011] During takeoff, i.e. between takeoff-roll and reaching the lift-off speed, maximum power is delivered and the energy demand of the aircraft is therefore extremely high. Takeoff-roll is defined as the portion of the takeoff procedure during which the airplane is accelerated from a standstill to an airspeed that provides sufficient lift for it to become airborne. Consequently, also the heat generated during conversion of hydrogen into electricity reaches its maximum during takeoff. Moreover, air cooling effects are still relatively limited as the aircraft is still accelerating, also taking into account the possibility that hot tarmac of the runway is emitting heat, resulting in hot air in an area above the tarmac where the airplane passes during takeoff.

[0012] It is common practice that an aircraft is designed to deal with the most extreme use case it may encounter. For a hydrogen powered aircraft, the most extreme use case would be during takeoff at a runway in hot conditions, e.g. a maximum power delivery during 30 seconds, while taking-off on a runway having hot tarmac of about 50 °C. As such extreme conditions are only happening very occasionally, it would result in an aircraft having a cooling system with a high cooling capacity that is basically over-dimensioned for other use cases. After all, during the rest of the flight, and even during the relatively high energy demanding takeoff at an airfield in more moderate conditions, the cooling capacity demands on the cooling system will be much lower. For example in the Middle East region, an ambient temperature TA of about 40 °C is no exception. On the other hand, in northern Europe, a maximum ambient temperature TA of about 30 °C on a hot summer day is more realistic.

[0013] In order to prevent accelerated aging, or even damaging, of the hydrogen fuel cell, it is important that the operating temperature of the fuel cell stays below a predetermined temperature limit. However, an over-dimensioned cooling system is also undesirable, because it adds to the weight of the aircraft, and may take up valuable space as well. There is a need to allow the hydrogen based aircraft to operate in all use conditions, including the most extreme use cases it may encounter, while also mitigating the adverse effects of an over-dimensioned cooling system.

[0014] European patent application EP 3 950 509 A1 , which is considered the closest prior art, relates to aircraft equipped with at least one fuel cell-based propulsion system and a cooling circuit in which coolant is intended to circulate for regulating the temperature of the fuel cell. More in particular, it relates to fuel cell temperature management for events wherein the fuel cell power system experiences highly demanding electrical energy situations or limited cooling situations. Such events may be manoeuvres of the aircraft that require extra electrical power, such as takeoff phases. Relative to EP 3 950 509 A1 , at least the characterizing features are novel.

[0015] US 2022 / 0348311 A1 is acknowledged as further prior art. It discloses a drive system of an aircraft, comprising a hydrogen fuel cell, and an electrical energy store that is configured to provide additional drive power for takeoff and landing. In the event of failure of the electrical energy store, the fuel cell can be supplied with an additional hydrogen mass flow from an additional hydrogen tank and with an additional air or oxygen mass flow, in order to compensate at least partially for the loss of the drive power provided by the electrical energy store for landing the aircraft.

[0016] An objective of the present invention is to provide a method of operating a hydrogen fuel cell of an aircraft during takeoff, that is improved relative to the prior art, and wherein at least one of the above stated problems is obviated or alleviated. Said objective is achieved with the method of operating a hydrogen fuel cell of an aircraft to provide a predetermined electrical power output (PFC) supplied by said fuel cell during takeoff according to claim 1 , said method comprising:

[0017] - before takeoff-roll, predicting or determining an ambient temperature (TA) at an airport, in particular near or at a runway of said airport;

[0018] - before takeoff-roll, predicting the actual obtainable temperature operating range (AT) of the fuel cell that is expected to be obtainable at the runway of the airport during takeoff by subtracting the ambient temperature (TA) from a predetermined desired maximum operating temperature (TFC.DES) of the fuel cell, i.e. AT = TFC.DES - TA;

[0019] - if AT is lower than a predetermined minimum threshold ATMIN for which a cooling system of the aircraft was designed, i.e. AT < ATMIN:

[0020] - limiting the power output PFC supplied by said fuel cell by a value PFC.LIM; and

[0021] - providing a supplementary power source for use during takeoff that has a maximjum power output PSUP.MAX S PFC.LIM to compensate for the set limititation of the power output PFC.LIM supplied by said fuel cell to thereby guarantee that, during takeoff, a total available power output PTOT of the fuel cell and the supplementary power source together, i.e. PTOT = PFC + PSUP, is equal to or higher than a maximum power of the fuel cell PFC .MAX-

[0022] The predetermined desired maximum operating temperature (TFC.DES) of the fuel cell is an operating temperature at which a predetermined desired maximum power output (PFC) , that can be supplied by said fuel cell at a predetermined ambient temperature (TA) at an airport, can be obtained. For example, an aircraft may be designed to be used at airports mostly having an ambient temperature TA of about 30 °C maximum. For such a use case and a fuel cell having a desired maximum operating temperature TFC.DES of about 80 °C, the AT = TFC.DES - TA = 80 °C - 30 °C = 50 °C. To meet this use case, the cooling system will be designed to be able to maintain the fuel cell at its desired maximum operating temperature TFC.DES of about 80 °C. However, in some situations the ambient temperature may be higher, e.g. during a heat wave, or the aircraft may be applied in a hotter climate. For example in the Middle East region, an ambient temperature TA of about 40 °C is no exception. This would however mean that the temperature range of the fuel cell, when it is operated at its desired maximum operating temperature TFC.DES is AT - TFC.DES — TA= 80 °C - 40 °C = 40 °C. If the cooling system is designed for a AT > 50 °C, this would mean that it would have (about 10 °C) insufficient cooling capacity to allow the aircraft to operate at the predetermined desired maximum operating temperature (TFC.DES) of the fuel cell and still provide a power output (PFC) that is sufficient for, or required to be available during, takeoff.

[0023] The supplementary power source is able to provide supplementary power PSUP to compensate for the lower power output of the fuel cell. It is however noted here that the power of the supplementary power source may only be readily available as back-up power in case of an emergency. For example, it may be preferable to use a longer distance on the runway during takeoff, if said longer distance is available. However, even if such a longer distance on the runway is available, it will still be necessary that sufficient power is always available, which is guaranteed by the supplementary power source.

[0024] Thus, by predicting the actual obtainable temperature operating range (AT) of the fuel cell that is expected to be obtainable at the runway of the airport during takeoff before takeoff-roll, i.e. the moment the aircraft starts rolling to initiate the takeoff, it is possible to optimize operation of the hydrogen fuel cell to provide a predetermined electrical power output (PFC) supplied by said fuel cell during takeoff, possibly supplemented by power output (PSUP) provided by a supplementary power source.

[0025] According to a preferred embodiment, the supplementary power source for use during takeoff is an electric power source. An electric power source does not require conventional fossil fuels, and may be easily charged with the hydrogen fuel cell, even during flight.

[0026] According to a further preferred embodiment, the supplementary power source for use during takeoff comprises an Auxiliary Power Unit (APU) of the aircraft, and said method comprising the step of configuring the APU to be used to provide power for propulsion during takeoff. An Auxiliary Power Unit (APU) of an aircraft is a power unit that is on-board the aircraft for providing power for on-board systems other than propulsion. By configuring said APU to be used to provide power for propulsion during takeoff, said APU will get additional functionality as supplementary power source. As the APU is already present on-board, using the APU as the supplementary power source does not add additional weight to the aircraft. If the power capacity of the APU would be insufficient to act as the supplementary power source on its own, using the APU as (part of) the supplementary power source still has the benefit of weight savings. Configuring the APU to be used to provide power for propulsion during takeoff may comprise the step of a power conversion to match the voltage and the electrical current (AC-AC, AC-DC (rectifier), DC-AC (inverter), DC-DC) of the APU with the propulsion system of the aircraft.

[0027] It is noted here that the APU could in theory be a kerosine-electrical power generator that uses kerosine to produce or store power for use while the main aircraft engine is “off’.

[0028] According to an even further preferred embodiment, the supplementary power source of the aircraft comprises a rechargeable battery. Thus, if the supplementary power source of the aircraft comprises an Auxiliary Power Unit (APU) of the aircraft, said APU is preferably an electric power source having a rechargeable battery.

[0029] According to an even further preferred embodiment, the supplementary power source of the aircraft comprises a supercapacitor. A supercapacitor uses electric fields to separate charges within a material, without relying on chemical reactions. As a result, a supercapacitor allows for faster charging and discharging than batteries, that rely on chemical reactions to generate electricity. The faster charging and discharging makes the supercapacitor ideal for applications requiring quick bursts of energy, such as acting as the supplementary power source for providing supplementary power during takeoff.

[0030] According to an even further preferred embodiment, the method comprises the step of adding one or more than one additional supplementary power source to the aircraft. As mentioned above, the supplementary power source may comprise the APU, that is already on-board the aircraft for other purposes. In many situations, the APU may be able to provide sufficient supplementary power. However, if this is not the case, such as in extreme hot conditions, the method may comprise the step of adding one or more than one additional supplementary power source to the aircraft. The one or more than one additional supplementary power source are thus only added when needed due to the actual circumstances, thereby preventing that unnecessary weight is added in circumstances where the supplementary power source alone is able to provide sufficient supplementary power. According to an even further preferred embodiment, the additional supplementary power source comprises an additional battery and / or an additional supercapacitor.

[0031] According to an even further preferred embodiment, the method comprises the step of recharging the supplementary power source during flight after takeoff. When the supplementary power source is used to provide supplementary power during takeoff, the supplementary power stored in said supplementary power source is at least partially depleted. However, if the supplementary power source is an electrical power source, it may be recharged during flight with electricity provided by the hydrogen fuel cell. Recharging may already start immediately after lift-off, as soon as the actual obtainable temperature operating range (AT) is larger than the predetermined minimum threshold ATMIN for which a cooling system of the aircraft was designed, i.e. AT > ATMIN. In practice, recharging may already start 10 to 30 seconds after lift-off. By recharging during flight, the power stored by the supplementary power source is replenished. In this way, it may be guaranteed that the supplementary power source may provide back-up power if a through start would be required during landing of the aircraft.

[0032] According to an even further preferred embodiment, said method further comprises optimizing the cooling system of the aircraft to compensate for AT < ATMIN by increasing a radiator capacity of the cooling system. If the radiator capacity is increased, the cooling capacity thereof increases. As a result, the predetermined minimum threshold ATMIN of the cooling system of the aircraft increases. Consequently the actual obtainable temperature operating range AT of the fuel cell also increases.

[0033] According to an even further preferred embodiment, the step of increasing the radiator capacity of the cooling system comprises: providing a water tank; and actively cooling a radiator with water from the water tank during takeoff. By actively cooling a one or more than one radiator of the radiators that are already installed in the aircraft, the cooling capacity thereof increases without requiring constructional changes, such as adding an additional radiator or replacing an installed radiator by a replacement radiator having a larger radiator capacity than the previously installed radiator. This allows for swift adaptations, such as filling a water tank with water. For conditions wherein actively cooling of the radiator is not required, the water tank is left empty, so it only minimally adds to the weight of the aircraft. Moreover, the water tank may be actively drained during flight. In this way, the weight of the aircraft may be reduced, for example after a takeoff at an airfield in hot conditions, and travelling towards cooler conditions where the additional cooling capacity with water from the water tank will not be required anymore.

[0034] According to an even further preferred embodiment, the step of increasing the radiator capacity of the cooling system comprises adding an additional radiator. The radiator capacity may be increased temporarily, e.g. by adding an additional radiator. Because the additional radiator is only added when needed, it is guaranteed that the additional weight of such an additional radiator is only added to the aircraft when needed.

[0035] According to an even further preferred embodiment, the method further comprises the step of optimizing an efficiency of the hydrogen fuel cell by the substeps of: providing an oxygen tank; and actively providing oxygen at an air intake of the fuel cell during takeoff. A higher oxygen concentration in the fuel cell increases the efficiency of the fuel cell.

[0036] According to an even further preferred embodiment, the method further comprises temporary operating the hydrogen fuel cell at an elevated operating temperature TFC.EL that is higher than the desired operating temperature (TFC.DES) , i.e. TFC.EL > TFC.DES. This measure would result in accelerated aging of the fuel cell, and is therefore preferably only relied on as a last resort, and more preferably always in combination with at least some of the above mentioned more preferred options. In this way, the more preferred options reduce the elevated operating temperature TFC.EL to a minimum.

[0037] According to an even further preferred embodiment, the step of operating the hydrogen fuel cell at the elevated operating temperature TFC.EL lasts less than 90 sec, preferably less than 60 sec, more preferably less than 45 sec, and most preferably about 30 sec. For example, the predetermined desired maximum operating temperature TFC.MAX of the fuel cell may be set at 80 °C. If the temperature operating range AT is too small, the hydrogen fuel cell will be operated at the elevated operating temperature TFC.EL that is higher than the predetermined desired maximum operating temperature TFC.DES, in other words: TFC.EL > TFC.DES. Once airborne, the aircraft will reach cooler air, and the temperature operating range AT increases. Consequently, the step of operating the hydrogen fuel cell at the elevated operating temperature TFC.EL only has to last for a short period of take, i.e. during takeoff.

[0038] According to an even further preferred embodiment, the step of operating the hydrogen fuel cell at the elevated operating temperature TFC.EL comprises a maximum temperature increase of the desired operating temperature TFC.DES that is less than 30 °C, preferably less than 20 °C, more preferably less than 15 °C, and most preferably about 10 °C. By applying an elevated operating temperature TFC.EL that only exceeds the desired operating temperature TFC.DES for a limited extend, the load on the fuel cell is maintained as low as possible and within an acceptable and safe range. Moreover, data on time period and extend of temperature elevation of such a temporary elevated operating temperature are stored, and may be taken into account for determining service intervals and / or replacement of the fuel cell.

[0039] According to an even further preferred embodiment, the method further comprises reducing an intake temperature (TINTAKE) of gaseous hydrogen at an intake of the fuel cell. Reducing an intake temperature (TINTAKE) of gaseous hydrogen at an intake of the fuel cell will result in a cooling effect. For example, reducing the intake temperature TINTAKE by 10 °C, e.g. from 70 °C to 60 °C may provide a cooling effect equivalent to a cooling power of for example 6.5 kW.

[0040] According to an even further preferred embodiment, the step of predicting or determining an ambient temperature (TA) at an airport comprises at least one of: measuring an air pressure, measuring a relative humidity, and measuring a wind speed and a wind direction. For example, an air strip at sea level (0 m altitude), will have a higher air pressure and higher relative humidity, resulting in a higher air cooling capacity, than an air strip at an altitude of 2500 m. After all, the air strip at altitude will have a lower air pressure, resulting in a lower air cooling capacity.

[0041] The invention is further related to an aircraft, comprising:

[0042] - a hydrogen fuel cell;

[0043] - a supplementary power source; and

[0044] - a controller configured to perform the method of operating the hydrogen fuel cell according to any of the foregoing claims.

[0045] The supplementary power source may be a power source that is already on board, such as an Auxiliary Power Unit (APU), or may be a power source that is actively added later. If an APU is used as supplementary power source, it is configured to provide power for propulsion during takeoff, in addition to its conventional functionality of providing power for on-board systems other than propulsion. The supplementary power source may also be a power source that is selectively added dependent on current conditions. In this way, the aircraft may be optimized for varying conditions, while also guaranteeing that no unnecessary weight is added.

[0046] The various aspects and features described and shown in the specification can be applied, individually, wherever possible. These individual aspects, and in particular the aspects and features described in the attached dependent claims, may be an invention in its own right that is related to a different problem relative to the prior art.

[0047] In the following description preferred embodiments of the present invention are further elucidated with reference to the drawing, in which:

[0048] Figure 1 is a schematic top view of an embodiment of an aircraft according to an aspect of the present invention.

[0049] Figure 2 is a schematic view of an embodiment of a method according to another aspect of the present invention; and

[0050] Figure 3 is a schematic view explaining the invention by comparing different scenarios.

[0051] Figure 1 shows a schematic top view of an embodiment of an aircraft 1 according to an aspect of the present invention. The aircraft 1 comprises a hydrogen fuel cell 2 and a supplementary power source 3, a cooling system 5, an oxygen tank 9, a Gaseous Hydrogen (GH2) line 10, onboard sensors 11 , a propeller 12, and a Hydrogen tank 13. Further sensors 12 may be place outside the aircraft 1 , e.g. on the runway 20.

[0052] In the shown embodiment, the supplementary power source 3 comprises one or more than one of: an auxiliary power source (APU) 3B, a Kerosine-electrical power generator 3A, a rechargeable battery 3C, a supercapacitor 3D, an additional battery 3E, and an additional supercapacitor 3F.

[0053] In Figure 1 , the cooling system 5 comprises a radiator 5A, an additional radiator 5B and a water tank 5C.

[0054] The onboard sensors 11 and / or sensors 12 outside the aircraft 1 , may comprise temperature sensors 11 A, 12A, air pressure sensors 11 B, 12B, humidity sensors 11C, 12C, wind speed sensors 11 D, 12D, and wind direction sensors 11 E, 12E.

[0055] The aircraft 1 furthermore comprises a controller 4, configured to perform the steps of a method 100 of operating the hydrogen fuel cell 2 of the aircraft 1. The method 100, according to another aspect of the present invention, is schematically shown in figure 2 and is arranged to provide a predetermined electrical power output PFC, that is supplied by the fuel cell 2 during takeoff.

[0056] In a first step 101 , an ambient temperature TA near or at a runway 20 of an airport is predicted or determined before takeoff-roll. Ambient weather conditions can for example be measured by the onboard sensors 11 and / or the sensors 12 arranged on or near the runway 20 of the airport, wherein the ambient temperature TA is predicted or determined by measuring the temperature using the temperature sensors 11A, 12A, by measuring 101A the air pressure using the air pressure sensors 11 B, 12B, by measuring 101 B a relative humidity using the humidity sensors 11 C, 12C and / or by measuring 101C a wind speed by the wind speed sensors 11 D,12D and the wind direction by the wind direction sensors 11 E, 12E.

[0057] In another step 102, the actual obtainable temperature operating range AT of the fuel cell 2, that is expected to be obtainable at the runway 20 of the airport during takeoff, is predicted before takeoff-roll. The actual obtainable temperature operating range AT is predicted by subtracting the ambient temperature TA from a predetermined desired maximum operating temperature TFC.DES of the fuel cell 2, such that AT = TFC.DES - TA.

[0058] When during step 102 it is predicted that AT is lower than a predetermined minimum threshold ATMIN for which a cooling system 5 of the aircraft was designed, that is when. AT < ATMIN, the controller 4 of the aircraft 1 is arranged to perform the step of:

[0059] - starting to execute the step of limiting 103 the power output PFC supplied by the fuel cell 2 by a value PFC.LIM; and

[0060] - one or more than one of the following steps:

[0061] - 1) start to execute the step of optimizing 108 the cooling system 5 of the aircraft 1 to compensate for AT < ATMIN by increasing a radiator 5A capacity of the cooling system 5; - II) execute the step of optimizing 112 an efficiency of the hydrogen fuel cell 2 by providing 112A the oxygen tank 9 and actively providing 112B oxygen at an air intake of the fuel cell 2 during takeoff;

[0062] - Ill) execute the step of temporary operating 113 the hydrogen fuel cell 2 at an elevated operating temperature TFC.EL that is higher than the desired maximum operating temperature TFC.DES, that is TFC.EL > TFC.DES, in such a way that the elevated operating temperature TFC.EL lasts less than 90 sec, preferably less than 60 sec, more preferably less than 45 sec, and most preferably about 30 sec and wherein a maximum temperature increase of the desired operating temperature T FC.DES is less than 30 °C, preferably less than 20 °C, more preferably less than 15 °C, and most preferably about 10 °C; and

[0063] - IV) execute the step of reducing 114 an intake temperature TINTAKE of gaseous hydrogen at an intake of the fuel cell 2.

[0064] After starting to execute the step of limiting 103 the power output PFC supplied by the fuel cell 2 by a value PFC.LIM, some further steps are preferably followed, that will be discussed in more detail below.

[0065] A first preferred step of the method is the step of providing 104 a supplementary power source 3 for use during takeoff that has a maximum power output PSUP.MAX PFC.LIM to compensate for the set limitation of the power output PFC.LIM supplied by said fuel cell 2. In this way, it is guaranteed that, during takeoff, a total available power output PTOT of the fuel cell 2 and the supplementary power source 3 together, i.e. PTOT = PFC + PSUP, is equal to or higher than a maximum power of the fuel cell 2 PFC, MAX minus the set limitation of the power output PFC.LIM, i.e. PTOT S PFC, MAX - PFC.LIM.

[0066] The supplementary power source 3 for use during takeoff is preferably an electric power source 3A. More in particular, the supplementary power source 3 for use during takeoff comprises an Auxiliary Power Unit (APU) 3B of the aircraft 1. In that case, the method 100 comprises the step of configuring 105 the APU 3B to be used to provide power for propulsion during takeoff. More preferably, the supplementary power source 3 of the aircraft 1 comprises a rechargeable battery 3C. Alternatively, or additionally, the supplementary power source 3 of the aircraft 1 may comprise a supercapacitor 3D. The method preferably further comprises the step of adding 106 one or more than one additional supplementary power source 3 to the aircraft 1. The additional supplementary power source 3 preferably comprises one or more than one of: an additional battery 3E or an additional supercapacitor 3F.

[0067] A further preferred step of the method comprises the step of recharging 107 the supplementary power source 3 during flight after takeoff.

[0068] After the step I) of starting to execute the step of optimizing 108 the cooling system 5 of the aircraft 1 to compensate for AT < ATMIN by increasing a radiator 5A capacity of the cooling system 5 , some further steps are preferably followed. First of all, increasing the radiator 5A capacity of the cooling system 5 preferably comprises:

[0069] - providing 109 a water tank 5B; and

[0070] - actively cooling 110 a radiator 5A with water from the water tank 5B during takeoff.

[0071] Alternatively, or additionally, the step of increasing the radiator 5A capacity of the cooling system 5 comprises adding 111 an additional radiator 5C.

[0072] In the schematic view of Figure 3 different scenarios are shown as an example to further elucidate the invention.

[0073] Scenario 1 indicates optimal operating conditions for the fuel cell 2 powered aircraft 1 , wherein the ambient temperature TA is 20 °C. Assuming a predetermined desired maximum operating temperature TFC.DES of the fuel cell 2 of 80 °C, the AT = TFC.DES - TA = 80 - 20 = 60 °C. Assuming the cooling system 5 is designed for a maximum AT = 50 °C, the cooling system 5 does not even have to run at maximum cooling capacity to maintain the fuel cell 2 at its predetermined desired maximum operating temperature TFC.DES of 80 °C.

[0074] In Scenario 2, the ambient temperature TA has increased to 30 °C. Assuming the cooling system 5 is again designed for a maximum AT = 50 °C, the cooling system 5 now needs to run at its maximum cooling capacity to maintain the fuel cell 2 at its predetermined desired maximum operating temperature TFC.DES of 80 °C. These conditions are still perfectly fine, yet at the upper limit of the cooling capacity of the cooling system 5.

[0075] In Scenario 3, the ambient temperature TA has increased even further, and now reaches 40 °C. The colling system 5 of Scenarios 1 and 2, that was designed for a maximum AT = 50 °C is not able to maintain the fuel cell 2 at its predetermined desired maximum operating temperature TFC.DES of 80 °C if the fuel cell 2 would be required to provide an electrical power output PFC that covers the total power required for takeoff. However, instead of allowing the temperature of the fuel cell 2 to increase above 80 °C, the invention proposes to limit the power output PFC supplied by said fuel cell by a value PFC.LIM. This is represented by a shorter vertical bar B in Scenario 3. However, to compensate for this power reduction of the fuel cell 2, the invention proposes to provide a supplementary power source 3 for use during takeoff that has a maximum power output PSUP.MAX ^ PFC.LIM . In this way, the supplementary power source 3 is able to compensate for the set limitation of the power output PFC.LIM supplied by said fuel cell 2 and thereby guarantee that, during takeoff, a total available power output PTOT of the fuel cell 2 and the supplementary power source 3 together, i.e. PTOT = pFC+ PSUP, is equal to or higher than a maximum power of the fuel cell 2, PFC, MAX minus the set limitation of the power output PFC.LIM, i.e. PTOT S PFC, MAX - PFC.LIM.

[0076] In Scenario 4, an even further increased ambient temperature TA of 50 °C may result in the situation that the supplementary power of the supplementary power source 3 is insufficient. This may be the case even taking into account other optional measures such as I) optimizing the cooling system; II) optimizing the efficiency of the fuel cell 2 (e.g. by actively providing oxygen at the air intake of the fuel cell); and III) reducing the intake temperature of gaseous hydrogen at the intake of the fuel cell. In such cases, it may be an option to temporary operate the hydrogen fuel cell 2 at an elevated operating temperature TFC.EL that is higher than the desired maximum operating temperature TFC.DES, i.e. TFC.EL > TFC.DES. If this is done only for a very short time during takeoff, and only for a limited temperature increase above TFC.DES, this may be considered an alternative for some extreme situations.

[0077] The above described embodiment is intended only to illustrate the invention and not to limit in any way the scope of the invention. Accordingly, it should be understood that where features mentioned in the appended claims are followed by reference signs, such signs are included solely for the purpose of enhancing the intelligibility of the claims and are in no way limiting on the scope of the claims. The scope of protection is defined solely by the following claims. List of abbreviations and notations

[0078] APU: Auxiliary Power Unit

[0079] PFC: power output supplied by fuel cell

[0080] PFC, MAX: maximum power output of fuel cell at its predetermined maximum operating temperature TFC.DES

[0081] PFC.LIM: value for limiting the power output supplied by fuel cell

[0082] PSUP: supplementary power output supplied by supplementary power source

[0083] PSUP, MAX: maximum power output supplied by supplementary power source

[0084] PTOT: total available power output PTOT of the fuel cell and the supplementary power source together, i.e. PTOT = PFC + PSUP

[0085] AT: temperature operating range

[0086] ATMIN: minimum threshold

[0087] TA: ambient air temperature

[0088] TFC.DES: predetermined desired maximum operating temperature of the fuel cell

[0089] TFC.ELV: elevated operating temperature of the fuel cell that is higher than the desired maximum operating temperature, i.e. TFC.EL > TFC.DES

[0090] TINTAKE: intake temperature of gaseous hydrogen

Claims

CLAIMS1. Method (100) of operating a hydrogen fuel cell (2) of an aircraft (1) to provide a predetermined electrical power output (PFC) supplied by said fuel cell (2) during takeoff, wherein said method (100) is characterized in that it comprises the steps of:- before takeoff-roll, predicting or determining (101) an ambient temperature (TA) at an airport, in particular near or at a runway (20) of said airport;- before takeoff-roll, predicting (102) the actual obtainable temperature operating range (AT) of the fuel cell (2) that is expected to be obtainable at the runway (20) of the airport during takeoff by subtracting the ambient temperature (TA) from a predetermined desired maximum operating temperature (TFC.DES) of the fuel cell (2),1.e. AT = TFC.DES - TA;- if AT is lower than a predetermined minimum threshold ATMIN for which a cooling system (5) of the aircraft was designed, i.e. AT < ATMIN:- limiting (103) the power output PFC supplied by said fuel cell (2) by a value PFC, LIM; and- providing (104) a supplementary power source (3) for use during takeoff that has a maximum power output PSUP.MAX ^ PFC, LIM to compensate for the set limitation of the power output PFC, LIM supplied by said fuel cell (2) to thereby guarantee that, during takeoff, a total available power output PTOT of the fuel cell (2) and the supplementary power source (3) together, i.e. PTOT = PFC + PSUP, is equal to or higher than a maximum power of the fuel cell (2) PFC, MAX.

2. Method (100) according to claim 1 , wherein the supplementary power source (3) for use during takeoff is an electric power source (3A).

3. Method (100) according to claim 1 or 2, wherein the supplementary power source (3) for use during takeoff comprises an Auxiliary Power Unit (APU) (3B) of the aircraft (1), said method (100) comprising the step of configuring (105) the APU (3B) to be used to provide power for propulsion during takeoff.

4. Method (100) according to claim 2 or 3, wherein the supplementary power source (3) of the aircraft comprises a rechargeable battery (3C).

5. Method (100) according to any of claims 2-4, wherein the supplementary power source (3) of the aircraft comprises a supercapacitor (3D).

6. Method (100) according to any of the foregoing claims, comprising the step of adding (106) one or more than one additional supplementary power source (3) to the aircraft.

7. Method (100) according to claim 6, wherein the additional supplementary power source (3) comprises an additional battery (3E).

8. Method (100) according to claim 7, wherein the additional supplementary power source (3) comprises an additional supercapacitor (3F).

9. Method (100) according to any of the foregoing claims, comprising the step of recharging (107) the supplementary power source (3) during flight after takeoff.

10. Method (100) according to any of the foregoing claims, wherein said method (100) further comprises optimizing (108) the cooling system (5) of the aircraft to compensate for AT < ATMIN by increasing a radiator (5A) capacity of the cooling system (5).

11. Method (100) according to any of the foregoing claims, wherein increasing the radiator (5A) capacity of the cooling system (5) comprises:- providing (109)_a water tank (5B); and- actively cooling (110) a radiator (5A) with water from the water tank (5B) during takeoff.

12. Method (100) according to any of the foregoing claims, wherein increasing the radiator (5A) capacity of the cooling system (5) comprises adding (111) an additional radiator (5C).

13. Method (100) according to any of the foregoing claims, wherein said method (100) further comprises the step of optimizing (112) an efficiency of the hydrogen fuel cell (2) by the substeps of:- providing (112A) an oxygen tank (9); and- actively providing (112B) oxygen at an air intake of the fuel cell (2) during takeoff.

14. Method (100) according to any of the foregoing claims, wherein said method (100) further comprises temporary operating (113) the hydrogen fuel cell (2) at an elevated operating temperature TFC.EL that is higher than the desired maximum operating temperature (TFC.DES) , i.e. TFC.EL > TFC.DES.

15. Method (100) according to claim 14, wherein the step of operating (113) the hydrogen fuel cell (2) at the elevated operating temperature TFC.EL lasts less than 90 sec, preferably less than 60 sec, more preferably less than 45 sec, and most preferably about 30 sec.

16. Method (100) according to claim 14 or 15, wherein the step of operating (113) the hydrogen fuel cell (2) at the elevated operating temperature TFC.EL comprises a maximum temperature increase of the desired operating temperature TFC.DES that is less than 30 °C, preferably less than 20 °C, more preferably less than 15 °C, and most preferably about 10 °C.

17. Method (100) according to any of the foregoing claims, wherein said method (100) further comprises reducing (114) an intake temperature (TINTAKE) of gaseous hydrogen at an intake of the fuel cell (2).

18. Method (100) according to any of the foregoing claims, wherein the step of predicting or determining (101) an ambient temperature (TA) at an airport comprises measuring (101A) an air pressure.

19. Method (100) according to any of the foregoing claims, wherein the step of predicting or determining (101) an ambient temperature (TA) at an airport comprises measuring (101 B) a relative humidity.

20. Method (100) according to any of the foregoing claims, wherein the step of predicting or determining (101) an ambient temperature (TA) at an airport comprises measuring (101C) a wind speed and a wind direction.

21. Aircraft (1), comprising:- a hydrogen fuel cell (2);- a supplementary power source (3); and- a controller (4) configured to perform the method (100) of operating the hydrogen fuel cell (2) according to any of the foregoing claims.

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