Fuel Cell systems in hybrid-electric aircraft

EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476245 A1409 Page 1/9 A1409 Fuel Cell systems in hybrid-electric aircraft Markus Meindl* (1,2), Fabian Forster (1), Bernd Eckardt (1), Martin März (2) (1) Fraunhofer IISB, Erlangen/Germany; (2) Institute of Power Electronics, Friedrich Alexander Universität, Nuremberg/Germany; *Contact corresponding authors: www.EFCF.com/ContactRequest Abstract With their actual power densities, fuel cell systems are too heavy for aviation. The power density of a fuel cell system can be increased by further developing the subcomponents. Intelligent integration of individual components in the aircraft could be a solution. For this purpose, the aircraft dimensions and simulations of 200 nmi and 600 nmi range from an EU project are used. A more granularly divided series connection of fuel cell systems is considered to integrate these systems in the aircraft more intelligently. A smart way is to equip each fuel cell system with a 640 V (323 kW) system voltage and specially integrated DC-DC boost converters. This choice of system voltage and the use of DC-DC boost converters allow for thinner cross-section cables, easier integration of DC-DC converters into the FC systems housing, and increased system redundancy. The overall control of the fuel cell systems can be carried out via low-weight CAN bus low-voltage signal cables, which hardly contribute to the aircraft's weight. EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476245 A1409 Page 2/9 Introduction The growing awareness of the environmental impact of technological progress is leading to new challenges that science must address. In the aviation industry, the efficient use of existing technologies and the introduction of new ones are crucial elements of corporate strategy. Hybrid-electric propulsion architectures are among the emerging technological innovations and promise a greener and more fuel-efficient future for aviation. Nevertheless, the economic attractiveness for potential operators cannot be neglected. With an expected average aircraft load factor of 82.5% in 2024, air transport is proving to be one of the world's most efficient, safest, and most reliable modes of transport. However, aviation emissions must be reduced to achieve the European Union's (EU) Flightpath 2050 targets. Innovative technologies and concepts are currently competing with environmental expectations and growth. The aviation sector is experiencing rapid growth. According to recent estimates, demand for air transport will increase by an average of 3.7% to 4.3% per year over the next 20 years [1–3]. To reduce emissions in the aviation sector, several measures are available, including new strategies for aviation routing [4], different types of aviation fuels [5], new aircraft designs [6, 7], and new propulsion systems based on, e.g., electrification [8] and hydrogen. In the latest press release at the 2025 Airbus Summit, the aircraft manufacturer presented its new concept of a hydrogen aircraft powered by four, two megawatt electric propulsion motors, each driven by a fuel cell system that converts hydrogen and oxygen into electrical energy [9]. The starting point of the analysis presented here is a scenario analysis of the regional aircraft segment from the Clean Sky 2 project, GENESIS [10], which has raised awareness of the need for more environmentally friendly aircraft. The author of this paper was the technical leader of this project. The Top Level Aircraft Requirements (TLAR) were defined through a detailed study of the most critical regional aircraft with turboprop (TP) and jet engines. The key factor in determining the TLAR is the economic attractiveness of the new 50-passenger regional aircraft, considering current and future routes. The developed aircraft concept in the project was based on an ATR 42. Studies have shown that this aircraft is currently the most efficient, commercially operating regional aircraft. Furthermore, essential technology streams for energy storage with batteries and fuel cell (FC) systems were explored. After defining the TLAR, the main goal was to examine three major technology streams for energy storage. Batteries and fuel cell system strategies, including possible combinations of those technologies, are elaborated through the execution of various designs of experiments (DOE) to identify the most promising solutions in terms of aircraft configurations. The design chain has been described as well as the MATLAB® based software tool used during the entire process. In the end, the results for the three above-mentioned time horizons have been presented in [6]. These aircraft were compared over flight distances of 200 nmi and 600 nmi. It was shown that the basic structure of the plane would need to be slightly lengthened to accommodate all systems. However, the simulations confirm over the flight phases that a hybrid-electric 50 PAX aircraft with fuel cells and battery systems can maintain an MTOW of just under 26 tons with liquid hydrogen tanks [6, 11]. 1. Aircraft requirements and design To define the TLARs, the simulation tool in [6, 11] was modelled with a commercially available ATR 42-600 with 48 PAX, a cruising altitude of 24,000 ft, and a cruising speed of Mach 0.48 (298 kn (535 km/h)), with a maximum deviation of 0.25% between the design weights and the aircraft's actual values. This verified that the tool was sufficiently accurate, and the components' specific fuel consumption and specific energy can be considered. Thus, flight missions with 600 nmi ranges are an interesting and competitive alternative, even for short-haul flights within Europe. The cruising speed is 295 - 300 kn (~556 km/h). EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476245 A1409 Page 3/9 Short runway lengths of less than 1,200 m are defined as TLARs. Figure 1 shows the hybridelectric combination of fuel cell and HV battery (BZ-Bat-Configuration). This aircraft is electric and, with a parallel hybrid powertrain, is expected to have a maximum takeoff weight of less than 26 tons in 2045. As techniques, proton exchange membrane fuel cells (PEMFCs), advanced PMSMs, and lithium-air (Li-O2) batteries will be used in 2045. This period also coincides with Airbus's introduction plans [9]. The TLARs and key performance indicators of the hybrid-electric aircraft are listed in Table 1. Figure 1: 50 PAX hybrid-electric aircraft, according to [6, 11] Table 1: TLARs and aircraft parameters, according to [6, 11] TLARs Aircraft parameters Flight distance 600 nmi PEMFC 2 x 1273 kWel. Time to Climb 13 min PMSM 10 x 600 kWel. Cruise speed 295-300 kn Li-O2 Batterie 2233 kWh Length runway < 1200 m Specific fuel consumption 0.0627 kg/(kWh) Length landing strip < 1200 m Li-O2 Battery mass 2363 kg Max. payload 4750 kg Li-O2 Bat specific energy 945 Wh/kg MTOW < 27 ton MTOW 26.1 ton 2. Fuel Cell Systems in hybrid-electric aircraft Hydrogen as a "fuel" for a fuel cell system (FCS) theoretically convinces with its high energy density as a promising fuel of the future. The potential energy densities for liquid hydrogen are well known. However, further challenges must be overcome to effectively utilize hydrogen in the aviation industry. The primary focus of this chapter is to explain the functioning of a fuel cell system (FCS) and the potential integration into a hybrid-electric regional aircraft based on concrete results. One of the main issues is the scaling up of the technology from, for example, an automotive application (as already present in series production in the Toyota Mirai) [12, 13] and a knowledge transfer from the laboratory to LH2 Batteries Galleys Toilets Exits Emergency Exits LH2 Fuel Cells Liquid hydrogen tanks Attendants seats Luggage Electric machines LH2 EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476245 A1409 Page 4/9 aviation applications. A fundamental understanding of fuel cell vehicles and hybrid-electric drive systems from fuel cell and battery technologies is necessary for this knowledge transfer. Under laboratory conditions, electrode potentials are determined as the voltage of a half-cell against a hydrogen electrode as a reference point. This allows the reversible cell voltage U0 to be calculated from the electrode potential difference between the cathode U01 and the anode U02. Each stack in the FC-Bat configuration consists of 400 metallic, planar cells. An individual cell active area of 250 cm² is assumed in 2040 and 300 cm² by 2050. These are plotted in Figure 2 along their current-voltage characteristics and show the behavior of the cells as a function of current density. A current density that is too low leads to low power and causes an inefficient use of the cells. A current density that is too high causes high losses and leads to high thermal stresses, which leads to poorer cell efficiency. On the left side of Figure 2, all voltages are plotted on the ordinate [a] and the cell efficiencies on the ordinate [b]. The thermoneutral voltage (black) indicates the theoretically maximum achievable voltage under standard conditions. The entropy component T ∆S decreases this voltage, which cannot be utilized due to the respective ambient conditions. This results in the standard potential (violet, dashed), which describes the cell's open-circuit voltage under standard conditions. The open-cell voltage (OCV; green) is the measurable cell voltage without load, often called the open-terminal voltage. Figure 2: Current-voltage characteristic of the fuel cells for the aircraft The OCV is generated by hydrogen diffusion processes from the anode to the cathode, electron flow despite open terminals (since the electrolyte is not a perfect insulator), and side reactions. Therefore, the resulting cell voltages are below the OCV minus the overvoltage, which can describe the irreversible activation, resistance, and diffusion losses. As the current density increases, the cell voltage and thus the cell efficiency decrease. The cell power is plotted on the right side of Figure 2. After the activation energy is applied for the electrochemical reaction, the optimal operating point of the fuel cell lies in the transition region between the activation and ohmic ranges, as the cell voltage here is sufficiently high. 2,5 1,0 0,5 0 Luft-Massenstrom [kg/s] 2,0 1,5 3,0 00,2 0,4 0,6 0,8 1,0 1,2 Elektrische Leistung [MW] Maximale BZ-Anlagenleistung für 2050 im Betrieb Maximale BZ-Anlagenleistung für 2040 im Betrieb 1,4 1,6 0,10 0,05 0 Wasserstoff-Massenstrom [kg/s] 00,2 0,4 0,6 0,8 1,0 1,2 Elektrische Leistung [MW] 1,4 1,6 0,20 0,15 0,30 0,25 0,40 0,35 0,45 MPP: Maximum Power Point Max. cell operating performance 2040 Thermoneutral voltage [V] Cell-Voltage 2040 [V] Standard potential [V] Open cell voltage (OCV) [V] Cell-Voltage 2050 [V] Cell-Efficiency 2040 Cell-Efficiency 2050 Cell-Power 2040 [W] Cell-Power 2050 [W] Ordinate left [a]: Voltage Ordinate left [b]: Efficiency Ordinate right: Power 1,2 1,0 1,4 0,6 0,4 0,8 0,2 000,5 1,0 1,5 2,0 2,5 3,0 0 Current density [A/cm2] Ordinate left [a, b] Ordinate right Preferred operating range MPP 1,50 1,25 1,75 0,75 0,50 1,00 2,00 2,25 0,25 Activation losses dominating Ohmic losses dominating Mass transport losses dominating 1,6 2,50 Max. cell operating performance 2050 EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476245 A1409 Page 5/9 However, cell losses are still low enough to achieve good cell efficiency. At higher current densities, concentration losses increase, leading to further voltage drops up to the maximum power output (MPP). This operating point is not ideal for fuel cells, as it entails high losses and low efficiencies, shortens cell lifetimes, and exposes them to high thermal and mechanical stress. Therefore, operating the fuel cell at moderate current densities is more sensible, as shown in Figure 2 in the favorable operating range. The maximum power of the individual cell is 1.4 W in 2040 and 1.64 W in 2050. Therefore, the optimal cell voltage for the cells in this study is 0.8 V, and for the fuel cell stack, 240-320 V. In green, Figure 3-a) shows schematically a fuel cell system. The fuel cell stacks shown in blue must be expanded with the following components for operation as a fuel cell system: • Fuel cells as a stack • Cooling subsystems (pumps, pipes, connectors, ...) • H2 subsystems (H2 recirculation pump, pressure reducer, injectors, ...) • Air subsystems (air compressor, air filter, connectors, etc) • FC control (monitoring temperature, power, humidity, ...) • FC safety systems that allow for controlled shutdown of the FC. Figure 3: Fuel cell system (green) with the necessary components, a) and a potential fuel cell system for a hybrid-electric aircraft, b) In the scientific literature and many EU projects, a 2 kW/kg power density is required for fuel cell systems [10, 14, 15]. When calculating power density, the weight components of the diagram highlighted in green (Figure 3-a) must always be considered. Additional components of the fuel cell system are shown in brown. One example is the power electronics connecting the FC system to the High-Voltage-DC-Bus (HV-DC-Bus). Figure 3-b) shows a schematic of how the fuel cell stacks and the fuel cell system (green) could be connected for a hybrid-electric aircraft. The fuel cell system can also be merged with a fuel cell plant (FC plant 1, FC plant 2) to meet the power requirements of an aircraft. For example, FC plant (1) could be installed in the right wing and FC plant (2) in the left wing of the aircraft, as shown in Figure 1. Every FC plant needs its own unidirectional DCDC converter to connect the plant to the (mostly 800 VDC chosen) HV-DC-Bus. Due to the System 1.1 System 1.3 System 1.2 System 1.4 Stack 1.1.2 Stack 1.1.1 Stack 1.2.1 Stack 1.2.2 Stack 1.3.1 Stack 1.3.2 Stack 1.4.1 Stack 1.4.2 System 2.1 System 2.3 System 2.2 System 2.4 Stack 2.1.2 Stack 2.1.1 Stack 2.2.1 Stack 2.2.2 Stack 2.3.1 Stack 2.3.2 Stack 2.4.1 Stack 2.4.2 Cooling subsystem H2-subsystem Air-subsystem FC control FC safety systems Fuel Cell stack Cooling Hydrogen supply Air supply Power electronics Auxiliary power supply Higher-level control/shutdown HV-DC-Bus FC plant 1 FC plant 2 HV-DC-BUS DC DC a) b) DC DC EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476245 A1409 Page 6/9 expected high weight of the FC plant of almost two tons in the FC-Bat configuration (see Chapter 3), other integration options as a compact system in the aircraft are also possible. This decision is up to the respective aircraft manufacturers. 3. Results and discussion Finally, the conservative results of the fuel cell experts from the GENESIS project are taken into account and presented in Table 2. These can be read in detail in Deliverable D2.4 [16] of the project and provides an overview of the used technology, power, number of stacks, weight, current, voltage, and other results that should give an insight into the FC-Bat configuration regarding the FCS. The results in [16] for 2040 show that with a power density of 1.39 kW/kg of the FCS, the initially demanded 2 kW/kg cannot be achieved. Due to the ongoing research and development work regarding the subcomponents of the FCS, it is expected that [16] the power density of 2.2 kW/kg can be achieved by the time horizon 2045. Table 2: Results of the PEMFC fuel cell plant, according to [16] Parameter Unit 2040 PEMFC 2050 PEMFC Fuel Cell Stack Current (max) A 750 750 Voltage (max.) V 320 320 Weight kg 40,7 33,7 Electrical power (Brutto) kW 180 210 Thermal power kW 147 140 Electric efficiency (min) % 55 60 Number of cells 400 400 Grav. power density kW/kg 4,42 6,23 Fuel Cell System (FCS) Number of FC stacks for FCS 2 2 Current (max) A 750 750 Voltage (max.) V 640 640 Weight kg 228,5 169,3 Auxiliary energy demand kW 37 42 Netto Nominal power kW 323 378 FCS-Efficiency % 45-48 45-48 H2-consumprion rate1 kg/h 20 21 Generated water kg/h 202 202 Grav. power density kW/kg 1,39 2,2 Fuel Cell plant (FCP) Number of FCP for aircraft 2 2 Number of FCS for FCP 4 4 Peak power requirement kW 1.293 1.512 FCUEfficiency % 45-48 45-48 Weight kg 982 581 Current (max) A 1.500 1.500 Voltage (max.) V 1.280 1.280 1: Including purge losses at nominal power However, the results also show that, based on initial technology forecast analyses and the expected technological advances, achieving the required power densities of fuel cell systems will still require considerable research and development. Therefore, a crucial lever will be the intelligent integration and further development of subcomponents. This topic is being addressed and advanced in current research programs by the author in cooperation with fuel cell manufacturers. Accordingly, this would be an essential optimization approach to achieve a lower system weight of the fuel cell system and, consequently, higher power EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476245 A1409 Page 7/9 densities. To integrate intelligent fuel cells as a system in the aircraft, Figure 4 considers a more granular series connection of fuel cell systems than was already presented in Figure 3. The ATR design requires that the powertrain components be supplied with electrical energy and hydrogen from the rear fuselage to the wings via cables and supply lines above the passenger cabin. The wings in this type of regional aircraft are mounted above the cabin. Figure 4: Redundant integration of fuel cell system ideas in the wing of a hybrid-electric aircraft It might be more sensible to equip each fuel cell system with a 640 V (323 kW) system voltage and its own integrated DC-DC boost converter, rather than a fuel cell system with a 1,280 V (1,293 kW) voltage and an external DC-DC buck converter. This allows for the use of cables with a thinner cross-section, enables an easier integration of DC-DC converters into the fuel cell system housing, and increases system redundancy. The overall control of the fuel cell systems can be achieved via low-voltage signals on buses over lightweight CAN bus cables, which barely contribute to the aircraft's weight. This fuel cell system arrangement in the wing is sketched in Figure 4-b) on the right side of the wing. This would make it conceptually feasible to integrate the fuel cell systems with a shared cooling, hydrogen, and oxygen supply into a single wing, as shown in Figure 4-a). The air compressor and the fuel cell cooling system would have to be housed in the aircraft's fuselage. The fuel cell system voltage would be 640 V or higher. Assuming a voltage of 640 V, this would only need to be increased by 140 V from the individual DC-DC boost converters to the 800 V DC bus voltage. Further, this provides greater redundancy and reliability in the system and increases the complexity of controlling and regulating the system. However, the weakest cell limits the performance in the respective series connection and considers a lower risk for multiple Wingspan 26.76 m ~2.5 m ~12.22 m AC DC AC DC AC DC AC DC AC DC LH2 ~1,5 m Luggage HV-DC-Bus incl. control and safety H2-Tank-subcomponents LH2-Tank H2-supply O2-supply Cooling ~1.37 m HV-Battery Unidirektional FC DC-DC converter DC DC AC AC DC DC-AC inverter DC DC Bidirektional Battery DC-DC converter Negative voltage Positive DC-Bus voltage Airspeed as vWind a) b) System 1.1 System 1.3 System 1.2 System 1.4 Plant 1 HV-DC-BUS Stack 1.1.2 Stack 1.1.1 Stack 1.2.1 Stack 1.2.2 Stack 1.3.1 Stack 1.3.2 Stack 1.4.1 Stack 1.4.2 DC DC DC DC DC DC DCDC EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476245 A1409 Page 8/9 parallel 640 V systems (with integrated 800 V DC-DC converters). Furthermore, an intelligent balance between series and parallel connection of the fuel cell systems achieves higher currents. If the cables are not too heavy for the high current load, this solution is more attractive for (future cryogenic) powertrains than the high voltage of the fuel cell system in Figure 3. If a series connection fails, the rest of the system can still operate, and the defective fuel cell system can be replaced. Contactors as bypass switches between the fuel cell systems are also conceivable. Through the intelligent integration of the gravimetrically lightweight power electronics, subcomponents within the system could be further developed and installed optimally. This results in enormous weight savings and redundancy in the system, which has positive aspects regarding the certification of the components for aviation. However, control and monitoring become more complex and require a comprehensive system understanding. Operating at higher system power limits comes at the expense of the system's service life. This would be highlighted as a negative sustainability point. Therefore, a trade-off should always be considered, as not all systems should be pushed to physical limits in the drive train. 4. Conclusion In summary, regarding the integration of fuel cell technology in hybrid-electric aircraft, the first PEMFC systems must achieve a power density of 1.39 kW/kg to be considered for regional aircraft with reasonable ranges. Integrating the balance of plant components and power electronics is key to achieving this. The choice of the system voltage significantly influences the design. An intelligent granular connection of fuel cell systems increases reliability advantages on the construction and system sides. A power density of 2.2 kW/kg is projected for the fuel cell system by 2050. This requires further development of the FC stack and its subcomponents. References [1] F. 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