Hydrogen Propulsion Technologies for Aviation: A Review of Fuel Cell and Direct Combustion Systems Towards Decarbonising Medium-Haul Aircraft

Abstract

Hydrogen propulsion technologies are emerging as a key enabler for decarbonizing the aviation sector, especially for regional commercial aircraft. The evolution of aircraft propulsion technologies in recent years raises the question of the feasibility of a hydrogen propulsion system for beyond regional aircraft. This paper presents a comprehensive review of hydrogen propulsion technologies, highlighting key advancements in component-level performance metrics. It further explores the technological transitions necessary to enable hydrogen-powered aircraft beyond the regional category. The feasibility assessment is based on key performance parameters, including power density, efficiency, emissions, and integration challenges, aligned with the targets set for 2035 and 2050. The adoption of hydrogen-electric powertrains for the efficient transition from KW to MW powertrains depends on transitions in fuel cell type, thermal management systems (TMS), lightweight electric machines and power electronics, and integrated cryogenic cooling architectures. While hydrogen combustion can leverage existing gas turbine architectures with relatively fewer integration challenges, it presents its technical hurdles, especially related to combustion dynamics, NOx emissions, and contrail formation. Advanced combustor designs, such as micromix, staged, and lean premixed systems, are being explored to mitigate these challenges. Finally, the integration of waste heat recovery technologies in the hydrogen propulsion system is discussed, demonstrating the potential to improve specific fuel consumption by up to 13%.

1. Introduction

Greenhouse gas emissions play a key role in climate change. The UN sustainability goal for climate change aims to keep the temperature increase by 1.5 degrees [1]. CO₂ emissions generated by current aircraft technology exceeded 918 million tonnes in 2019, accounting for 2–3% of worldwide emissions [2]. With air travel demand continuing to increase, studies by the International Council on Clean Transportation (ICCT) predict an annual passenger growth rate of 3% until 2050 [3]. Passenger traffic presents challenges for achieving emission reduction targets, emphasising the need for advancements in fuel efficiency, adopting sustainable aviation fuels, and developing low-carbon technologies to ensure the industry’s alignment with global climate goals.

Battery, hydrogen, SAF and hybrid propulsion systems are anticipated to replace traditional propulsion systems. Short-range flight routes offer the greatest potential for transition to electric propulsion, as they typically require minimal modifications to the existing airframe and configuration. Instead of redesigning the entire aircraft, electric systems can often be retrofitted in place of traditional propulsion, making the shift more practical and cost-effective. The energy density of today’s state-of-the-art lithium-ion batteries ranges between 180 and 300 Wh/kg at the pack level. However, a 1330 Wh/kg specific energy battery is needed to achieve a 1000 km+ range for a 150-passenger aircraft [4]. The optimistic projection of the battery specific energy density will reach nearly 1000 Wh/kg by 2050 (Figure 1) [5]. However, a specific energy density of at least 1000 Wh/Kg should be achieved to replace the propulsion of conventional single-aisle aircraft with batteries. This still falls ~25% short of the 1330 Wh/kg required for a 150-passenger, 1000+ single aisle aircraft. Another potential battery technology beyond Li-Ion is the Li-S battery and it is expected to reach an energy density level of 2654 Wh/Kg (theoretical specific energy density). However, this may provide only ~50% cell level efficiency output of the size of 1330 Wh/Kg. Moreover, currently, constraints in battery technology and operational requirements create an unfavorable situation for using batteries in conventional single-aisle aircraft or beyond. Yet, new battery electric aircraft concepts and designs could improve performance [6]. This gap in technology clearly suggests that battery electric propulsion alone is unlikely to be viable for single-aisle aircraft without significantly improving airframe efficiency or hybridization.

Universal Hydrogen and ZeroAvia have demonstrated the feasibility of using fuel cell powertrains in regional short-range aircraft [7]. Moreover, ZeroAvia is developing a megawatt-class powertrain for 1000 km+ and 90-passenger seat aircraft after successfully testing its 600 kW powertrain for the 400 km+ range and 19-passenger capacity. Figure 2 shows the successfully flown hydrogen electric aircraft using liquid hydrogen. Cranfield Aerospace Solutions and Airbus are developing hydrogen-electric powertrains capable of carrying up to 9 and 100 passengers, respectively.

H2Fly was the first passenger aircraft to successfully fly for several hours and demonstrate the feasibility of a hydrogen-electric powertrain capable of carrying four passengers. A multitude of organizations have invested in hydrogen aircraft. Major studies on hydrogen-powered aircraft such as Cryoplane, Tupolev 155, Lockheed, and FlyZero proved that hydrogen can be a viable solution for transitioning from hydrocarbon-based aircraft. However, significant technological improvements are required in the storage system, propulsion system, and aircraft configuration, as hydrogen does not contain similar properties to those of kerosene fuel [8,9].

Figure 2. Successfully flown powertrains (b,d) [10,11]; under development (a,c) [12,13].

Figure 2. Successfully flown powertrains (b,d) [10,11]; under development (a,c) [12,13].

According to the United Nations, it aimed to reduce 45% of emissions by 2030 [14]. IATA introduced a roadmap for aviation to achieve NetZero, and it requires speedy development in fuel cell systems to replace the regional market of aviation [15]. Also, direct combustion to medium-range aircraft should be parallelly improved to achieve the required technology readiness level. Currently, due to insufficient technology maturity, a quick transition from hydrocarbon fuel-based aviation to hydrogen may not be deemed viable [16]. However, recent developments in fuel cell systems with higher efficiencies and liquid hydrogen storage research show the potential and viability of converting existing configurations into hydrogen aircraft.

The literature review was conducted based mainly on four databases, and the source selection methodology is shown in Figure 3. This research reviews the current advancements in key technology metrics of the hydrogen-electric powertrains and hydrogen direct combustion engines. In particular, the technical feasibility of medium-range hydrogen-powered aircraft is reviewed in the light of their contribution to the aviation traffic. According to Rakhshani et al. [17], these kinds of aircraft would best satisfy the feasibility and validation criteria for hydrogen retrofit performance. And address the challenges in regulatory and infrastructure requirements.

2. Zero and Low-Emission Fuels Characteristics

Narrow-body and wide-body aircraft contribute to a larger portion of the total carbon dioxide emission as shown in Figure 4. In 2019, before COVID, the total estimated CO₂ emissions were nearly 905 million metric tons. Nearly half of the CO₂ emissions come from narrow-body and regional aircraft, i.e., 43% and 5% respectively. The CO₂ emission is almost twice as high for regional aircraft per passenger kilometre as medium-haul flights [18]. CO₂ emissions are one of the main contributors to global warming, which causes several uncertainties in the world, such as sea level rise, ocean warming, ice mass melting, etc. According to the International Energy Agency (IEA) and IATA, aviation contributes 2.5% of global CO₂ emissions. However, it accounted for 3.8% to 4% greenhouse gas emissions.

Alternative propulsion technology offers significant potential to mitigate aviation’s climate impact. A technically and economically viable solution will be needed to decarbonize the aviation sector to achieve NetZero. SAFs seem to be potential fuel for reducing aviation emissions, and they do not require any further modification in aircraft design or systems. However, they do not stop emitting CO₂ and other gases into the atmosphere. The life cycle emissions of SAFs may seem to be a viable option for now because it can reduce the emissions from aviation by 65% in 2050 and can reduce the CO₂ emission by 80% compared to jet fuel [19]. Currently, SAF is used in aircraft with a 50% kerosene blend but expect to run on 100% by 2030 and by this time, it would replace 30% of Airbus operations [20]. In 2023, Virgin Atlantic flew using 100% SAF from London to New York, proving that flying using 100% SAF (Power-to-liquid) is safe. A recent study by Markl et al. [21], shows that SAF, especially HEFA-SPK, reduce ice number concentrations and non-volatile particulate matter by 56% and 35%. SAF’s price is approximately 2.5 times higher than Jet A fuel, but it is expected to match the price after 2035. Moreover, IATA projections show that 449 billion liters of SAF are required to meet the 2050 target. It is not practical to achieve a true NetZero with SAFs. The AIA report by Cambridge University shows that SAFs are not the only solution to reduce emissions [22]. Battery-powered propulsion is currently not feasible for narrow-body aircraft due to the batteries’ significantly lower energy density than conventional aviation fuels. Jet fuel has an energy density of approximately 43 MJ/kg, whereas advanced lithium-ion batteries offer only about 1 MJ/kg [23]. This vast disparity means that the weight and volume of batteries required to power a narrow-body aircraft would be impractically considerable, severely limiting the payload capacity and range. Additionally, the weight of batteries remains constant during flight, unlike fuel, which is consumed, leading to further inefficiencies [24]. With sufficient technological advancements, alternative propulsion methods could replace jet fuel-powered aircraft. Also, AIA recommends improving the technologies to achieve a true zero flight, including further research and development on hydrogen-powered aircraft [22].

The key properties of hydrogen, such as zero carbon emission, high energy density, high ignition temperature, broad flammability range, and fast flame speed, make it a potential fuel for aviation. It only produces water as an emission. Moreover, the energy density of hydrogen is approximately three times higher than that of the current Jet A fuel. Recent research shows that fuel cell and direct hydrogen combustion engines can reduce the fuel mass by 50% and 80%, respectively, compared to Jet A [17]. However, the hydrogen volumetric density is four times less than kerosene, which requires more space to store hydrogen in the aircraft. The high specific heat of hydrogen increases the turbine inlet temperature and operating pressure ratio, which leads to a reduction in SFC [8]. Moreover, the lower density of hydrogen leads to a lower lift-to-drag ratio, lowering the wing loading at take-off. The major problem with hydrogen combustion is contrail formation and NOx emission. Contrails contributed to 70% of the change in temperature due to aviation in 2019 [25]. However, the temperature changes due to contrails and NOx emissions of hydrogen-powered aircraft are 50% and three times lower than those of kerosene-powered aircraft [26]. Also, this paper [26] evaluated and compared the environmental impact of hydrogen combustion due to emissions such as Nox, H₂O, and contrails with kerosene combustion. Another paper [27] reviewed the climate impact of hydrogen combustion, and further research on contrails is required, as there are still some uncertainties in correctly calculating the impact on the environment [28]. Table 1 shows the critical parameters required to determine the feasibility of the alternative propulsion systems.

Table 1. Technological parameters of alternative propulsion systems.

Parameter	Fuel Cells	Hydrogen Combustion	SAF	Battery Hybrid	References
Specific Energy	33–40 MJ/kg	33–40 MJ/kg	~43 MJ/kg	0.2–0.4 MJ/kg	[29,30]
Energy Efficiency	45–60%	35–45%	38–42%	80–95%	[31]
System Complexity	Very High: Stack, humidifier, compressor, cooling, and inverter	Moderate: Turbine and modified combustor	Low: Drop-in fuel in existing engines	High: Battery packs, inverters, electric motors, and BMS	[32]
Fuel/Storage Cost	EUR 6–10/kg LH2	EUR 6–10/kg LH2	EUR 1.5–3/kg	EUR 100–200/kWh	[33]
GHG Emissions	Zero	Water and NOx	~80% reduction	Zero	[32,33,34]
TRL	Medium	Medium	High	High	[32,33]
Constraints	Cooling, durability, and density	Flame stability, NOx control	Supply, feedstock, and LCA	Weight, cooling, and battery aging	[32]

Table 1. Technological parameters of alternative propulsion systems.

Parameter	Fuel Cells	Hydrogen Combustion	SAF	Battery Hybrid	References
Specific Energy	33–40 MJ/kg	33–40 MJ/kg	~43 MJ/kg	0.2–0.4 MJ/kg	[29,30]
Energy Efficiency	45–60%	35–45%	38–42%	80–95%	[31]
System Complexity	Very High: Stack, humidifier, compressor, cooling, and inverter	Moderate: Turbine and modified combustor	Low: Drop-in fuel in existing engines	High: Battery packs, inverters, electric motors, and BMS	[32]
Fuel/Storage Cost	EUR 6–10/kg LH2	EUR 6–10/kg LH2	EUR 1.5–3/kg	EUR 100–200/kWh	[33]
GHG Emissions	Zero	Water and NOx	~80% reduction	Zero	[32,33,34]
TRL	Medium	Medium	High	High	[32,33]
Constraints	Cooling, durability, and density	Flame stability, NOx control	Supply, feedstock, and LCA	Weight, cooling, and battery aging	[32]

Two potential fuels to replace fossil fuels in narrow-body and wide-body aircraft are hydrogen and SAFs. SAFs are not the solution for NetZero due to their life cycle emissions. However, they are likely to continue being used until alternative propulsion systems are developed for wide-body and narrow-body aircraft. Hydrogen can be utilized in two primary ways: to produce electricity through fuel cells or directly combusted in gas turbine engines. Hydrogen combustion produces 2.6 times more water vapor than kerosene fuel [35]. On the other hand, the overall efficiency of the hydrogen fuel cell system could be achieved by 40–60%, compared to about 30–40% for kerosene gas turbine engines. This implies that aircraft powered by fuel cells could consume less fuel for the same energy output [36]. The AIA model shows that hydrogen-electric aircraft will increase their range up to 4000 km by 2035, and this will replace the fleets of narrow-body aircraft such as the A320 and B737 with hydrogen-electric propulsion [22]. Additionally, the cost of green hydrogen production is expected to decrease after 2040, becoming more economical than crude oil, natural gas, and PtL SAFs [2,37]. Table 2 shows the potential hydrogen production pathways and their economic and environmental impact.

Table 2. Key parameters for gray, Blue, and Green Hydrogen Production Pathways.

Metric	Gray Hydrogen	Blue Hydrogen	Green Hydrogen	References
CO2e Intensity (kg CO2e/kg H2)	~9–11	~1.5–5	~0.05–1.5	[38,39]
TRL	9	8	9	[40]
Cost (USD/kg)	USD 1.00–USD 2.50	USD 1.50–USD 3.50	USD 2.50–USD 7.50
Feedstock	Natural gas	Natural gas + CCS	Renewable electricity + water	[41,42]
Water consumption (kg H2O/kg H2)	~4–7	~6–9	~9–12	[43]

Table 2. Key parameters for gray, Blue, and Green Hydrogen Production Pathways.

Metric	Gray Hydrogen	Blue Hydrogen	Green Hydrogen	References
CO2e Intensity (kg CO2e/kg H2)	~9–11	~1.5–5	~0.05–1.5	[38,39]
TRL	9	8	9	[40]
Cost (USD/kg)	USD 1.00–USD 2.50	USD 1.50–USD 3.50	USD 2.50–USD 7.50
Feedstock	Natural gas	Natural gas + CCS	Renewable electricity + water	[41,42]
Water consumption (kg H2O/kg H2)	~4–7	~6–9	~9–12	[43]

3. Hydrogen-Powered Aircraft

3.1. Fuel Cell Propulsion

Fuel cells convert fuel and air into electricity, producing only water as a by-product. Fuel cell technology, types, and systems are well characterized in many studies [44,45,46]. The application of fuel cells in aircraft propulsion is reported in the literature accordingly [47]. A fuel cell-based propulsion system is already integrated into road vehicles using compressed hydrogen gas as fuel [48]. However, adopting fuel cell powertrains in aircraft is challenging due to their lower specific power, specific volume, and efficiency. As shown in Figure 5, the fuel cell system consists of several systems and components. A fuel cell stack consists of several cells and it requires other subsystems to operate in an optimum condition.

The ATI provides the roadmap for fuel cells (Figure 6) with the technology requirements and projections on key performance metrics [49]. It shows that a shift in fuel cell type should be made, which could help achieve a power density of up to 6 kW/kg, similar to that of the conventional propulsion system.

3.1.1. Fuel Cell Types

There are two potential types of fuel cells for aviation applications: (a) the polymer electrolyte membrane fuel cell (PEMFC) and (b) the solid oxide fuel cell (SOFC). PEMFC has been classified into two types based on their operating temperature, LT-PEMFC and HT-PEMFC. LT-PEMFC has been used by major hydrogen powertrain developers (Table 1). This type of fuel cell has already achieved a significant milestone in retrofitting megawatt-class power-consuming aircraft (Dash 8-300). SOFCs and HT-PEMFCs can perform with a higher electrical efficiency compared to LT-PEMFC, but due to their low TRL, it is not yet suitable for now [51]. LT-PEMFC (Figure 7) polymer electrolyte membrane fuel cell systems become more complex and challenging as the output power increases [52]. One primary challenge is adequate heat and water management; higher power levels lead to increased heat generation and water production, which can cause membrane dehydration or electrode flooding, adversely affecting the performance and durability [53]. Additionally, at elevated power outputs, mass transport limitations become significant due to insufficient reactant supply and the accumulation of water in the catalyst layers, leading to voltage losses and decreased efficiency [54]. Platinum-based catalysts are poisoned by impurities like carbon monoxide, which reduces the effectiveness of the fuel cells [55]. Maintaining uniform temperature and reactant distribution across larger fuel cell stacks also becomes more challenging at higher power levels, negatively impacting the performance and durability [45]. Table 3 shows the successfully developed hydrogen electric powertrains using PEMFC.

Table 3. Various powertrains developed and tested successfully.

Industry	Fuel Cell Type	Aircraft Type	Capacity of the Aircraft	Powertrain Capacity	References
HY4	LT-PEMFC	Experimental aircraft	4-seater	80 kW	[56]
ZeroAvia	LT-PEMFC	Piper Malibu M350	6-seat aircraft	250 kW	[57]
ZeroAvia	LT-PEMFC	Dornier 228	19-seater	600 kW	[58]
Universal Hydrogen	LT-PEMFC	Dash 8-300	56-seater	1–2 MW	[59]

Table 3. Various powertrains developed and tested successfully.

Industry	Fuel Cell Type	Aircraft Type	Capacity of the Aircraft	Powertrain Capacity	References
HY4	LT-PEMFC	Experimental aircraft	4-seater	80 kW	[56]
ZeroAvia	LT-PEMFC	Piper Malibu M350	6-seat aircraft	250 kW	[57]
ZeroAvia	LT-PEMFC	Dornier 228	19-seater	600 kW	[58]
Universal Hydrogen	LT-PEMFC	Dash 8-300	56-seater	1–2 MW	[59]

On the other hand, HT-PEMFCs operate at 200 degrees and higher, leading to enhanced electrode kinetics, improved tolerance for fuel impurities like carbon monoxide, and simplified water management since water remains in the vapor phase, reducing flooding issues [60].

Recent advancements in high-temperature proton exchange membrane fuel cells (HT-PEMFCs) indicate promising potential for scaling hydrogen-electric propulsion systems to larger aircraft. In 2023, ZeroAvia developed an HT-PEMFC stack with a specific power of 2.5 kW/kg at cell level for a 20 kW module. It is expected to achieve the power of over 3 kW/kg at the system level in 2025 to support their ZA2000 powertrain, designed for a 40–80-seater aircraft. Feasibility studies of FlyZero show that single-aisle hydrogen-electric aircraft could become viable between 2035 and 2050. Their projections estimate that HT-PEMFCs, excluding balance of plant (BoP) components/cell levels, could reach a specific power density of approximately 16 kW/kg while maintaining a comparable system mass to LT-PEMFCs by 2035. The fuel cell system-level efficiency achieved for aviation applications is presented in Table 4. FlyZero further estimates that LT-PEMFCs, including the BoP/system level, could achieve 3–3.5 kW/kg and 75% efficiency by 2050, with additional improvements of up to 5–6 kW/kg possible through the adoption of high-temperature fuel cells and superconducting powertrains [49]. Comparatively, NASA has projected a specific power of 1.7 kW/kg for LT-PEMFCs by 2025. ZeroAvia’s 600 kW fuel cell powertrain, developed in the same timeframe, has reached a specific power of 1 kW/kg, closely aligning with NASA’s forecast. Additionally, HyPoint has projected a specific power of 3 kW/kg for HT-PEMFCs by 2025, a target that FlyZero estimates for 2035 [61]. ZeroAvia also anticipates that HT-PEMFC systems with a specific power of 4 kW/kg will be capable of powering 100+ seat single-aisle aircraft by the early 2030s, supporting the transition toward zero-emission, medium-range aviation [58]. Studies on wide-body Boeing 787-8 aircraft show that advancements in the well-to-wing energy efficiency and the power density of fuel cells can enable liquid hydrogen (LH2) fuel cell aircraft to carry 200 passengers over 6000 km and a 20% higher range than gas turbine aircraft [62].

Table 4 shows the efficiency, operation pressure, temperature, and power densities of potential fuel cell types in aviation. The SOFC seems unlikely to be feasible soon due to its specific power and complex thermal management, though it has a higher operating efficiency. NASA’s Glenn research center has developed an advanced solid oxide fuel cell (SOFC) that could achieve a specific power density five times higher than current SOFCs, i.e., up to 2.5 kW/kg. This highly efficient fuel cell can operate on a diverse range of fuels, including hydrogen and hydrocarbon-based options such as methane, diesel, and jet fuel, without the need for external reformers [63].

Table 4. Fuel cell for aircraft propulsion.

Type	Operating Pressure (Bar)	Operating Temperature (Celsius)	Power Density (kW/kg) (Cell Level)	System-Level Efficiency	References
LT-PEMFC	Up to 3	60–80	Up to 7	<60%	[50]
SOFC	1–10	600–1000	0.51	<65%	[64]
HT-PEMFC	Up to 3	120–200	2.5	<60%	[58]

Table 4. Fuel cell for aircraft propulsion.

Type	Operating Pressure (Bar)	Operating Temperature (Celsius)	Power Density (kW/kg) (Cell Level)	System-Level Efficiency	References
LT-PEMFC	Up to 3	60–80	Up to 7	<60%	[50]
SOFC	1–10	600–1000	0.51	<65%	[64]
HT-PEMFC	Up to 3	120–200	2.5	<60%	[58]

Several hydrogen-electric powertrains have been successfully developed and tested for commercial passenger aircraft of varying configurations and capacities (Table 3). These developments primarily focus on regional or short-range commuter aircraft, demonstrating significant progress in applying hydrogen fuel cell technology to regional aviation. Currently, no fuel cell powertrain has achieved the capacity to be installed beyond regional aircraft categories. However, using a sophisticated catalyst design and lightweight, oxygen-enriched air and better-performing materials in fuel cell stack components, it is possible to improve the power density by a factor of at least five, which helps to reduce the weight of the future aircraft [65].

3.1.2. Balance of Plant (BoP)

The BoP in a fuel cell system consists of auxiliary components and subsystems necessary to support the fuel cell stack operation. It manages the fuel supply, air delivery, thermal regulation, water management, and power conditioning to ensure the optimal performance and durability of the fuel cell system. Figure 8 shows the balance of plant modules and their interaction with other modules to support and improve the performance of the fuel cell system [66,67]. The BoP components such as humidifiers, compressors, and heat exchangers are required for high-power operations, resulting in higher parasitic power consumption, reducing the overall net power output of the fuel cell system [68]. Nearly half of the energy is converted into heat during electricity generation in the fuel cell. It must be removed from the stack to maintain optimal performance. High-efficiency fuel cells require less hydrogen for the same power output, reducing the overall fuel storage needs and minimizing the system weight and complexity. However, higher power output and efficient fuel cells may operate at lower temperatures (low-grade waste heat), impacting the thermal management approach. In such cases, systems with advanced heat exchangers or phase-change materials might be needed to regulate the temperature while keeping the weight low [69]. Microchannel heat exchangers have a high surface area-to-volume ratio that can effectively manage the high thermal loads generated by fuel cells and inverters without adding significant weight. It can reduce the system weight by up to 30% and reach the specific power of 0.65 kW/kg compared to other traditional cooling techniques [70].

A phase-change cooling system is capable of handling larger heat power, and it reduces the weight and complexity of the thermal management system. Phase-change materials (PCMs) can store excess heat temporarily and release it slowly, stabilizing temperatures and reducing the cooling system’s peak load. A study by Striednig et al. [70] shows that the phase-change heat pumps reduce engine and aerodynamic drag by 23% and 98.7% (94% less weight) compared to liquid cooling (LC) for MW-class LT-PEMFCs. A recent study on MW-class LT-PEMFC cooling systems shows that the mass of the two-phase cooling system (TPC) with an accumulator is 35% lower than the LC, and without an accumulator, the TPC weighs 2.4 times less than the LC [71]. Nanofluids can be applied in cooling systems as they have a higher thermal conductivity, which improves efficiency and reduces the size of the TMS, but it may increase the pumping power [72]. A study by Islam et al. [73] shows that the frontal area of the heat exchanger reduced by 21% compared to the baseline fluid for the same coolant flow rate at a 0.05% volume of nanoparticles, and the pumping power increased by around 1%. Another analytical study by Fly et al. [74] shows that the frontal area can even be reduced by 27% by changing from liquid to evaporative cooling for LT-PEMFC. On the other hand, HT-PEMFC minimizes the mass of the TMS by 65% compared to the LT-PEMFC system [75]. Figure 9 illustrates a typical schematic configuration of a cooling system for fuel cells that can be utilized in the BoP. Multitudes of similar configuration are reported in literatures such as [76,77,78].

In aviation, where ambient pressure and humidity vary significantly (due to ISA conditions), maintaining the optimal hydration of the fuel cell membrane without flooding becomes complex. Multiple strategies have been proposed to address this. Water management methods and techniques and materials such as changing the shape of the flow passage, material, and an efficient control system are improving the fuel cell efficiency, lifespan, and performance [79,80]. Nano-engineered GDLs improve water management and allow for effective excess water removal, leading to thinner and lighter electrodes and reduced weight in fuel cell systems, improving the power density by 14% with a gradient pore structure compared to a conventional structure [81]. Advanced polymer electrolyte membranes that incorporate hydrophilic additives and nanomaterials enhance water retention and enable thinner membranes, thereby reducing the overall weight of the membrane assembly [82]. Graphene-based coatings on membranes and electrodes have improved water transport and reduced membrane dehydration, further boosting fuel cell efficiency [83]. Furthermore, ML algorithms could improve the water flooding issue by monitoring and controlling the fuel cell system’s input and output parameters. Further quantitative studies are required on specific density variations and the mass of fuel cell systems by incorporating technologies and innovative approaches.

LT-PEMFCs face water management challenges because their operating temperature is below the boiling point of water, leading to potential water flooding issues. In contrast, HT-PEMFCs operate at significantly higher temperatures and do not experience serious water flooding issues. And it has a simple BoP design, higher impurity tolerance, and higher-grade waste heat [84]. A detailed review of the issues and gaps is identified on TMS in HT-PEMFC, and addressing these gaps provides a suitable performance for aerospace applications [85]. Future studies could enhance the system-level efficiency by focusing on the overall thermal system behavior and minimizing parasitic power losses through the integration of waste heat recovery systems [86].

3.1.3. Power Electronics Systems

Power management and distribution (PMAD) systems in hydrogen-electric aircraft propulsion encompass components such as inverters, cables, converters, buses, and switches. Traditional power transmission methods relying on metals and alloys contribute significantly to the overall weight of the PMAD. Recent advancements have focused on reducing weight and enhancing the efficiency of inverters used in megawatt-class powertrains. ZeroAvia inverters can produce a maximum of 450 kW power at 800 VDC, with a 20 kW/kg power density for the ZA600 powertrain and a maximum efficiency of 99% without using cryogenic cooling methods. The study conducted by Wang et al. [80] reviews the key projects and technologies that improved the specific power and efficiency of the inverter for mega-watt class powertrains used for aircraft propulsion applications, and this concludes that the inverters for mega-watt class powertrains are approaching 20 kW/kg and 99% of specific power and efficiency. NASA’s AATT project investigated several capacity inverters and other electrical components such as motors for turbo-electric and parallel hybrid electric powertrains suitable for tube wing narrow-body and hybrid wing-body aircraft. The NASA Glenn Research Center explored a cryogenically cooled inverter system for the MW-class propulsion, achieving a specific power of 18 kW/kg and 99% efficiency [87].

An experimental 1-MW inverter developed under NASA’s Advanced Air Transport Technology project achieved a specific power of 27 kW/kg and an efficiency of 99.47% at 500 kW. This inverter utilized advanced cooling systems, where liquid nitrogen cooled EMI filters and gaseous nitrogen regulated the junction temperature of SiC MOSFETs to maintain performance.

Additionally, researchers at MIT have designed an ultra-light inverter capable of producing 53.4 kW/kg specific power with over 98% efficiency for a 1-MW motor [88]. This inverter comprises ten 100 kW three-phase inverter modules, demonstrating the scalability of modular designs for electrified propulsion. Also, 1-MW integrated modular motor drive is developed at the NASA test facility (NEAT), with a specific power of 9 kW/kg [89]. The specific power density and efficiency of the potential inverters are shown in Figure 10.

Incorporating hyperconducting materials in the power transmission weighs up to 3–4 times less compared to conventional high voltage power distribution systems [90]. Hydrogen-electric PMAD systems rapidly advanced toward high efficiency (99%+) and high-power density (20–50 kW/kg). This research [91] shows a 60% mass reduction in aircraft power electronics due to smaller conductor cross sections. And the benefits of cryogenics in power electronic devices for aircraft propulsion are reviewed, also identified the components suitable for low temperatures [92]. Airbus is investigating cryogenics and superconducting technologies under the ASCEND program. The current limitation on the power density in power electronics could be overcome through weight reduction in the cable [93]. Cryogenic cooling and superconducting materials are key to reducing weight and improving performance.

3.1.4. Electric Motor

Researchers at the University of Illinois developed a 1 MW, 10 kW/kg, and 98% efficiency motor using the cryogenically cooled superconducting permanent magnet motor. The NASA Glenn Research Center is developing a 1.4 MW, 16 kW/kg, and 98% wound-field synchronous motor with a superconducting rotor. Ohio State University is investigating 2.7 MW, 13 kW/kg, and more than 96% induction motors using innovative variable cross section wet coil cooling on the stator. Moreover, it has completed a conceptual design of a 10 MW ring motor with approximately 1 m in diameter and 5000 RPM [94]. As part of the H2FlyGHT program, the University of Manchester is conducting advanced research on hyperconducting motor coil design for a cryogenically cooled electric motor, intended to power a 2 MW hydrogen-electric propulsion system [95]. A joint venture between Airbus UpNext and Toshiba aims to develop a superconducting motor for an MW-class powertrain [96]. ZeroAvia estimates that, for narrow-body aircraft such as the Airbus A320 or Boeing 737, electric motors must achieve a specific power density exceeding 15 kW/kg, as these aircraft consume over 12 MW of power at cruising, even with airframe improvements. Currently, ZeroAvia has developed a 660 kW electric motor operating at 2500 rpm, reaching a specific power density of 5 kW/kg [58].

FlyZero’s target for 2030 for the electric motor is 23 kW/kg and 25 kW/kg in 2050, enabling narrow-body aircraft (like A320 and 737) to be capable of flying with 100+ seat aircraft [49]. Figure 11 illustrates the projected specific power trends for electric motors from various studies. Furthermore, advancements in superconducting motors could boost power densities up to 20 kW/kg by 2035, significantly surpassing the current technology benchmark of approximately 5 kW/kg.

3.2. Hydrogen-Electric Powertrain

Based on the developments in powertrain components such as the converter, inverter, motor drive, and motor, it is possible to achieve a 70% efficiency without considering the fuel cell efficiency (Figure 12). A fuel cell consists of several components; its efficiency strongly depends on the BoP efficiency. So far, the fuel cell system has achieved nearly 50% efficiency at the system level.

The powertrain’s overall efficiency is around ~35% if the fuel cell is included. Power electronic components and motors have significantly improved the efficiency and specific power. FlyZero’s projections indicate a 75% fuel cell efficiency by 2050, resulting in a ~52% powertrain efficiency.

For a 1 MW powertrain, the weight can be estimated using the specific power values of the components in the hydrogen-electric powertrain across different timeframes. The component specific power values are shown in

Table 5. Specific power of fuel cell powertrain in different timeframes.

		Component Specific Power kW/kg
	Year	2025	2035	2050
Components
Fuel cell + BOP		2	2.5	3.5
PMAD		>20	>25	25–50
Motor		13	>15	>25

for different timeframes.

Hydrogen storage is not included in this calculation as it strongly depends on the technology and performance parameters of the aircraft. Moreover, the powertrain weight is expected to be nearly half in 2050, like FlyZero’s projection [50]. It is worth noting that the improvement in fuel cell systems strongly influences the weight of the powertrain. The total take-off mass of 9 to 20-seat aircraft increases by 10% to 13% for hydrogen-powered fuel cell aircraft [101]. Another study shows that converting Cessna 208 aircraft into fuel cell-powered increases its total-off weight by ~15% [102]. Both these studies are based on projected component performances for 2035. Figure 13 shows how the mass of the powertrain varies based on different timeframes.

Further improvements in the efficiency and specific power of the hydrogen-electric powertrain, which are considered as key parameters in assessing the scalability of a propulsion system, could be improved by adopting cryogenics and lightweight materials. According to the FlyZero study, there were no noticeable improvements in the efficiency or power density as the powertrain scaled from half a megawatt to 4 megawatts. However, this study [103] shows that the power density of the powertrain including the storage system could be achieved up to 1.5 kW/kg in 2035 with the integration of cryogenic cooling and superconducting technology. Additionally, TMS is identified as a significant sector for improvement as it accounts for 21% of the total weight of the hydrogen-electric propulsion system. However, as the future fuel cell system is HT-PEMFC, it significantly reduces the weight of the thermal management system.

4. Hydrogen Direct Combustion

Hydrogen’s high energy density by weight is another advantage for aviation applications, offering the potential for long-range flights with significantly lower carbon footprints than traditional jet fuel. A liquid hydrogen aircraft with a Pratt Whitney turbojet engine by Lockheed in 1956 was built and tested [8]. Another notable project in the 1980s was Russia’s Tupolev Tu-155, the first aircraft to fly partially powered by hydrogen combustion (Figure 14). This was a modified trijet Tu-154 airliner that successfully used cryogenically stored liquid hydrogen as fuel for one of its three engines, achieving multiple test flights. Even though hydrogen combustion engines are not yet commercialized, major conventional aircraft engine manufacturers such as GE, Safran, Roll Royce, and P&W are working on their roadmap to bring future hydrogen-powered turbine engines into the regional, medium, and long-range aviation market. GKN Aerospace aims to bring hydrogen gas turbines for single-aisle aircraft by 2035 with its multiple partners [104]. In addition, Airbus is working on three different projects including hydrogen combustion-based aircraft, and aims to the market by 2035; moreover, it successfully tested its ZEROe engine for about 4 h in Toulouse [105]. Rolls-Royce is developing its hydrogen turbofan engine to power narrow-body airlines by the middle of 2030, and it has tested using 100% hydrogen in the Pearl 700 engine in 2023 at DLR, Germany. BeautHyFuel conducted a successful test in January 2024 on the TP-R90 engine with gaseous hydrogen fuel [106]. And in January 2025, the first liquid hydrogen-fed gas turbine was successfully tested for light aircraft.

Feasibility studies on hydrogen-powered aircraft such as Lockheed, Cryoplane, and Flyzero show that hydrogen can be an alternative fuel for aircraft propulsion, including narrow-body aircraft with advancements in propulsion systems, storage, and aircraft configurations [9,49,107]. While the water produced by hydrogen combustion absorbs some of the heat and thus reduces NOx formation, water vapor itself can contribute to climate effects. In addition to this, the overall global warming impact of hydrogen is lower than that of jet fuel [108].

4.1. Combustion Techniques

Hydrogen combustion engines for aviation require significant modifications to the combustion chamber due to hydrogen’s high diffusivity and broad flammability range (4–75% by volume in air). Uneven fuel–air mixing, flame instability, and flashback are significant problems in hydrogen combustors, and hydrogen combustion produces a larger valve of temperature than kerosene at its stoichiometric ratio. Still, it can be burned at lean mixing ratios. As the Nox formation rate rises exponentially with the flame temperature, lowering the flame temperature is a crucial goal. Advanced injector designs (e.g., swirl, jet in cross-flow, and multi-point), premixing burners, and staged combustion mitigate these challenges by ensuring more uniform mixtures and stable operation. The primary purpose of the combustor design is to reduce or eliminate NOx emissions and overcome technical challenges associated with the development of a compatible combustor (Figure 15).

Several advanced combustion technologies are being explored to mitigate NOx emissions in hydrogen-fueled engines. This research [110] reviews the low emission combustion technologies for kerosene gas turbine engines at high and low TRL levels. Hydrogen’s flammability range is higher than kerosene, so it can be burned leaner than kerosene. One approach is lean-burn combustion, which involves a high air-to-fuel ratio to lower combustion temperatures, thus reducing NOx formation. Another technique is flameless or mild combustion, where fuel and air are premixed and combusted at lower temperatures, eliminating hot spots that produce lower NOx up to less than 90% of the baseline Jet A combustor [111]. Recent research on flameless hydrogen combustion based on lean azimuthal flame technology reduces NOx up to a single digit in hydrogen combustion engines [112]. Water injections and humidified air combustion are also being investigated as methods to lower NOx emissions in hydrogen combustion engines. Water injections directly cool the combustion chamber, reducing peak temperatures and offering a 50% NOx reduction for kerosene-powered combustion engines [113]. Steam injection and recovering water vapor from the engine could reduce NOx emissions by 80% in LH2 combustion gas turbine engines [114]. A recent numerical investigation on water injections in turbofan engines shows that a 40–50 K temperature drop across high-pressure compressors leads to a significant reduction in Nox [115].

Even though the premixing of air and hydrogen produces a higher performance and reduces NOx emissions, there is a flashback issue. On the A320 Auxiliary Power Unit (APU) GTCP 36-300, the use of the micromix diffusive combustion system resulted in a NOx reduction of approximately 60–80% compared to the original kerosene-based combustion system [116]. A numerical study shows that the micromix combustor has the potential to reduce the NOx emission by 80% and the possibility to scale up for real gas turbine applications [117]. Furthermore, an independent author suggests that micromix and staged combustion could be used in future hydrogen engines as it has lowered the NOx emission and improved the combustion efficiency [118]. This research [119] summarizes the numerical studies on micromix combustors for aircraft applications focused on reducing NOx emissions. A micromix burner is shown in Figure 15. In addition to the above-discussed combustors, there are other potential combustors at a higher TRL such as RQL, DAC, TAPS, and LDI and a lower TRL such as NASA multi-point LDI, LPP, ASC, and VGC that are reviewed at [110] for kerosene-based aero gas turbine engines to reduce NOx. Hence, a qualitative assessment of these engines concludes that the LPP combustor produces the lowest NOx emission but is currently at a lower TRL with combustion stability issues. The kerosene-powered P&W RQL combustor reduces Nox in the range of 55% to 72% of the CAEP/6 standard, and it is at a higher TRL-9. Rolls-Royce recently tested hydrogen fuel in the Pearl 15 combustor (RQL), showing the potential of using hydrogen as a fuel in advanced engines such as UltraFan and reducing Nox, which is in an acceptable level of CAEP [120].

The Twin Annular Premixing Swirler (TAPS) combustor concept has significant potential for NOx reduction in hydrogen-fueled engines by utilizing two premixing swirler zones that operate under lean conditions, thus limiting peak flame temperatures and mitigating thermal NOx formation. GE developed lean burn TAPS capable of powering wide-body aircraft and reducing NOx up to 52% below CAEP/6 and good combustion efficiency for kerosene-powered gas turbine engines [121]. The exact NOx reduction for a hypothetical 100% hydrogen TAPS combustor remains under active research; the lean premixed approach inherent to TAPS sets a solid foundation for low-NOx hydrogen combustion but demands careful control to prevent flashback and autoignition in premixing passages. NASA experimented without premixing the hydrogen and air using MLDI and quick mixing techniques to reduce NOx emissions and obtained a reduction of up to 66% relative to the CAEP/6 at higher pressures without any flashback and autoignition issues [122]. Combustors like RQL and TAPS, which are at a higher TRL, produce NOx emissions at an acceptable level; however, implementing these technologies in hydrogen-fueled engines poses challenges in fuel–air mixing and combustion stability, and further research is needed to refine these techniques for aviation applications. As the potential combustor design matures, it will be pivotal in transitioning to low-carbon or zero-carbon aviation.

4.2. Waste Heat Recovery Technologies

Modern aeroengines could reach a thermal efficiency of up to 50%, and the other half of the energy is wasted as heat. The key engine parameters like the turbine inlet temperature and overall pressure ratio could help to improve the thermal efficiency further [123]. However, due to emission regulations and material limitations, further improvements are difficult in this way. The waste heat recovery (WHR) in aero gas turbine engines is a method to improve energy efficiency, reduce fuel consumption, and minimize environmental impacts (Figure 16). Hydrogen burns at significantly higher flame temperatures (up to 2400 °C) than conventional aviation fuels like kerosene (~2000 °C), which can exceed the thermal limits of even advanced materials used in turbines. Without adequate cooling, critical engine components such as turbine blades, combustor liners, and nozzles are at risk of thermal fatigue, deformation, or failure. Several methods and techniques for recovering heat from the gas turbine engine include the Rankine cycle, thermoelectric generators, and heat exchangers. Waste heat recovery systems can improve fuel consumption through either modification to the thermodynamic cycle or power generation using a separate cycle or materials: the two ways are presented below [124].

For the WHRs, heat recoverability or exergy depends on the theoretical Carnot efficiency and available power. The hydrogen combustion engines have higher chances of being integrated with the WHRs as they produce high-grade heat, but hydrogen electric propulsion systems using low temperature fuel cells produce low-grade heat due to operating constraints. The Carnot efficiency ${\mathit{\eta }}_{carnot}$ is the maximum possible work extracted from the heat sources. It is defined based on temperature differences such as cold $T_c$ and hot $T_h$.

$${\mathit{\eta }}_{carnot}=1-{\displaystyle \frac{T_c}{T_h}}$$

(1)

Higher temperature at the exhaust produces a larger temperature difference in the gas turbine engine and increases the Carnot efficiency. For modern turbofan engines, the temperature reaches above 1700 K at the turbine inlet, and the Carnot efficiency becomes ~82%. In the case of hydrogen, it could reach further. Incorporating WHRs increases the overall efficiency of the engine, either producing power or reducing the SFC, as shown in Figure 15.

4.2.1. Organic Rankine Cycle

In recent years, several studies have investigated the integration of Organic Rankine Cycle (ORC) systems for waste heat recovery (WHR) in aeroengines. Saadon et al. [125], used an analytical approach to demonstrate that incorporating an ORC into a turbofan engine’s WHR system can significantly increase the net power output and energy efficiency. Thermodynamic analyses for turboprop engines have revealed that ORC integration can improve the thermal efficiency by approximately 10% [126]. A key metric for evaluating the effectiveness of ORC systems in such applications is fuel savings, which can be estimated using the following equation:

$$\mathrm{F}\mathrm{u}\mathrm{e}\mathrm{l} \mathrm{S}\mathrm{a}\mathrm{v}\mathrm{i}\mathrm{n}\mathrm{g}={\displaystyle \frac{m_f\left(h_{AR}-h_{AT}\right)}{\mathit{\eta }\times LHV}}$$

(2)

In this expression, $m_f$ is the mass flow rate of fuel, $h_{AR}$ and $h_{AT}$ represent the specific enthalpies of exhaust gases after and before the recovery process, respectively, η is the thermal-to-electric conversion efficiency of the ORC system, and LHV is the lower heating value of the fuel. Despite these promising benefits, ref. [127] have identified that the heat exchanger’s size and the associated pressure drop are the most critical limitations affecting the ORC WHR unit’s performance. These factors constrain the maximum achievable fuel savings. However, another optimization study by [128] indicates that with design improvements—specifically the implementation of louvered fins in the heat exchanger—it is possible to reduce the system weight by 10% and achieve a power density of 1.84 kW/kg.

4.2.2. Supercritical CO₂ (sCO₂)

The sCO₂ WHRs could achieve a 7–13% SFC reduction for gas turbine engines [129]. Another study on the turbofan engines using sCO₂ shows the potential to integrate WHRs with 33% efficiency and a 2.8% reduction in SFC. However, the weight of the sCO₂ system increases the overall SFC by 4% and requires further research on heat exchange weight reduction [130]. Integration of the sCO₂ WHRs with the fuel cell system in aircraft is still a gap to study further.

4.2.3. Intercooled Recuperated (ICR) Cycles

Modification in the thermodynamic cycle of the gas turbine engines has been explored in experimental and conceptual engines by major manufacturers such as Rolls-Royce, GE, Honeywell, MTU, and Europrop International. These engines demonstrate significant fuel consumption and emission reductions through efficient heat recovery and reduced compressor work. Lockheed and Cryoplane studies show that cooling compressor air and turbine exhaust air have higher SFC benefits and are summarized in review by [61]. The ICR cycle by means of engine’s schematics and T-S diagram is shown in Figure 17.

The minimum requirement for SFC reduction is more than 7% to justify using ICR [133]. Also, this research provides the minimum weight ratio/threshold for ICR and non-ICR engines. Numerical analysis on a two-pass cross-flow intercooler improves the SFC and BPR by 5.9% and 35%, respectively, than a non-intercooled engine, reducing the compressor exit temperature and NOx emissions [134]. A novel concept in the intercooled turbofan engine improves the heat recovery system benefit, reducing the SFC by 3% [135]. An optimization study by [131] shows innovative heat exchanger concepts that reduce the SFC by more than 9%, emissions, and weight compared to conventional heat exchanger design. Another optimization and design study on ICR by [132] shows the potential to improve thrust by 10% and reduce SFC by 10% compared with a traditional turbofan engine. A recent review by [136] concludes that the intercooler within the thermodynamic cycle reduces the compressor’s workload, overall efficiency, and output in the gas turbine.

4.2.4. Thermoelectric Generator (TEG)

TEG converts temperature differences into voltage. The heat is directly converted into electricity using thermoelectric generators on the engine. Thermoelectric materials like skutterudites and quantum-structured materials have the potential to produce power densities of around 927 W/kg but require large heat exchangers to produce a sufficient temperature difference across the thermoelectric material [137]. A recent study conducted as part of the German Aeronautical Research Program (LuFo-5) investigated the performance of a thermoelectric generator (TEG) integrated between the hot section of a propeller and the bypass flow of the cooler. The results demonstrated that the TEG achieved an efficiency ranging from 3% to 7%, with a power density between 1 kW/m² and 9 kW/m² [138]. The SFC could be improved between 0.052% to 0.1%, and in aircraft, it would be possible to improve the SFC further by increasing the nozzle surface and installing TE materials at places like de-icing of engine components [139]. The potential technologies for heat recovery in aviation are shown in Table 6.

Table 6. Comparison of waste heat recovery (WHR) technologies for aircraft engines.

Technology	SFC Reduction/Fuel Saving	Efficiency/Power Density	Key Benefits	Limitations	Research Gaps/Notes	References
Organic Rankine Cycle (ORC)	Up to 4%	Up to ~18%; power density: 1.5 kW/kg	Thermodynamic improvement; utilizes low- to medium-grade waste heat	Heat exchanger size and pressure drop limit performance	Lightweight heat exchangers Optimization of fin design for compact systems	[127,128]
Supercritical CO2 (sCO2)	7–13%	33% WHR efficiency (system weight increases SFC by 4% due to heat exchanger weight)	High thermodynamic efficiency, potential for compact cycles	High system weight; challenging heat exchanger design	Integration with fuel cell systems remains unexplored Lightweight heat exchangers needed	[130]
Intercooled Recuperated (ICR) Cycles	Up to 10%	Increased thermal efficiency by up to 8%	Reduces compressor work and emissions	Increased weight and volume due to complex design	Optimization needed for compact and efficient intercoolers Novel designs showing >9% SFC reduction	[131,140,141]
Thermoelectric Generator (TEG)	0.052–0.1% SFC	Efficiency 3–7%; power density: 1–9 kW/m2	Compact and low maintenance	Requires large heat exchangers and low overall efficiency	Further work on placement (e.g., de-icing zones), materials, and heat exchanger integration Introduction of low-cost TEG material	[139,142]

Table 6. Comparison of waste heat recovery (WHR) technologies for aircraft engines.

Technology	SFC Reduction/Fuel Saving	Efficiency/Power Density	Key Benefits	Limitations	Research Gaps/Notes	References
Organic Rankine Cycle (ORC)	Up to 4%	Up to ~18%; power density: 1.5 kW/kg	Thermodynamic improvement; utilizes low- to medium-grade waste heat	Heat exchanger size and pressure drop limit performance	Lightweight heat exchangers Optimization of fin design for compact systems	[127,128]
Supercritical CO2 (sCO2)	7–13%	33% WHR efficiency (system weight increases SFC by 4% due to heat exchanger weight)	High thermodynamic efficiency, potential for compact cycles	High system weight; challenging heat exchanger design	Integration with fuel cell systems remains unexplored Lightweight heat exchangers needed	[130]
Intercooled Recuperated (ICR) Cycles	Up to 10%	Increased thermal efficiency by up to 8%	Reduces compressor work and emissions	Increased weight and volume due to complex design	Optimization needed for compact and efficient intercoolers Novel designs showing >9% SFC reduction	[131,140,141]
Thermoelectric Generator (TEG)	0.052–0.1% SFC	Efficiency 3–7%; power density: 1–9 kW/m2	Compact and low maintenance	Requires large heat exchangers and low overall efficiency	Further work on placement (e.g., de-icing zones), materials, and heat exchanger integration Introduction of low-cost TEG material	[139,142]

5. Infrastructure and Regulatory Framework Requirements for the Implication of Hydrogen Technology in Aviation

Hydrogen adoption in aviation demands new regulatory requirements that refer to rules, standards, and certifications to ensure safety, environmental compliance, and operational reliability. These requirements are set by aviation authorities like the FAA, EASA, and ICAO. Hydrogen in aviation requires standards in various sectors such as fuel production, airport infrastructure, propulsion systems, emissions, testing, and aircraft systems. Several existing bodies/organizations are developing standards for various activities related to the above-mentioned sectors (e.g., SAE, EUROCAE, ASTM, NFPA, etc.). For example, the SAE airport task group is developing standards for hydrogen refueling, transportation, and storage for aviation applications. Hydrogen properties have both advantages and disadvantages in safety. There are a few existing studies and research on hydrogen fire hazards that are discussed in these review articles [27,61]. However, there are still regulatory gaps in the hydrogen fire and explosion protection, fuel cell and high-voltage systems, materials/structures, safety assessment methodologies, and cabin safety, which are identified as critical areas for certification [143,144]. The FAA provides a roadmap to identify technological challenges, safety concerns, and policy gaps to be addressed to adopt hydrogen as a fuel in aircraft [145]. The EASA organized a workshop on certification roadmaps for hydrogen, and it identified priority safety domains such as leak detection, cryogenic fuel storage and venting, high voltage and fuel cell systems, and the mitigation of fire and explosion hazards [146]. Both regulators identified similar areas and actions to be taken to address the regulatory gap in the hydrogen aircraft. Currently, there are no certified hydrogen-powered aircraft. However, ZeroAvia obtained G1 issue paper (contain certification basis) for the electric propulsion system (inverter and motor) from the FAA. Additionally, it has signed with the FAA to obtain P1-issue paper (contain means of compliance). Finally, it is aiming to certify its 600 kW hydrogen electric powertrain with UK CAA [147].

Hydrogen aircraft adoption requires scalable infrastructure across production, storage, transportation, and refueling and regulatory frameworks to ensure safe, secure, efficient, and environmentally friendly operation across the hydrogen infrastructure. The hydrogen-ready airport is crucial for the adoption of hydrogen aircraft, which requires collaboration and integration across the aviation stakeholders as well as on the cost of the hydrogen at the user end [148]. Research by Gu et al. [149] identified the challenges in adopting hydrogen in the airport and provided a roadmap to speed up the airport planning suitable for hydrogen. Another research identified that the most economical way to produce hydrogen at the airport can be achieved by incorporating a liquefaction plant on the airport, and the gaseous hydrogen could be transported via pipeline to the liquefaction plant at the airport [8]. The truck transportation cost of LH2 is identified as competitive to other modes of transportation [150]. Liquefaction is about ~25% of the total cost of LH2 at the end user and the production cost is around 69% [27]. The development of high energy efficiency liquefaction should be achieved, i.e., about 5 to 6 kWh/kg-H2, to reduce the cost at the end user [151]. SAE developed standards for refueling liquid hydrogen safely into the aircraft. It contains safety protocols, hydrogen type, operational procedures, and equipment specifications. Similarly, ISO and ASTM are parallelly working on standards for fuel quality, materials and tanks to ensure safe operation across the hydrogen infrastructure.

6. Conclusions

This review comprehensively assesses hydrogen-electric and hydrogen combustion propulsion systems, emphasizing component-level performance and integration challenges relevant to decarbonizing medium-range aviation. Notable advancements in electric motors, power electronics (PMAD), and fuel cell technologies show promising potential, particularly when coupled with cryogenics, which enhances the power density and thermal management. Despite these gains, scaling from kilowatt- to megawatt-class powertrains remains a major hurdle. Most existing technologies operate at the kW level, and further investigation is required to assess the impact of scaling on mass, efficiency, and system integration.

Fuel cell systems have matured considerably, with development trends aligning with 2035 and 2050 aviation targets. However, their current integration capacity is still focused on smaller-scale applications. Liquid hydrogen offers additional benefits through its potential as a heat sink in thermal recovery systems such as heat exchangers, thermoelectric generators, and Rankine cycles. Future work should focus on designing and optimizing cryogenic cooling systems for fuel cell systems and integrating waste heat recovery systems with a hydrogen powertrain.

The review can be summarized in the following points.

• SAFs, hydrogen, and batteries are going to be the future energy mix in aviation.
• Hydrogen-electric powertrain development has gained momentum, as it has the potential to achieve zero CO₂ emission flight.
• Hydrogen electric powertrain efficiency is expected to be nearly 55% and half of the weight by 2050, requiring a technology breakthrough to achieve this.
• Hydrogen combustors are expected to reduce NOx and require careful control of the combustion process and properties.
• Gaps in regulatory requirements may delay the entry of hydrogen commercial flights.
• Incorporating a heat recovery system into the powertrain could improve the SFC by up to about 13%.