Hydrogen Storage Vessel for a Proton-Exchange Membrane (PEM) Fuel Cell Auxiliary Power Unit for Commercial Aircraft

Abstract

Approximately 20% of emissions from air travel are attributed to the auxiliary power units (APUs) carried in commercial aircraft. This paper proposes to reduce greenhouse gas emissions in international air transport by adopting proton-exchange membrane (PEM) fuel cells to replace APUs in commercial aircraft: we consider the design of three compressed hydrogen storage vessels made of 304 stainless steel, 6061-T6 aluminium, and Grade 5 (Ti-6Al-4V) titanium and capable of delivering 440 kW—enough for a PEM fuel cell for a Boeing 777. Complete structural analyses for pressures from 35 MPa to 70 MPa and wall thicknesses of 25, 50, 100, and 150 mm are used to determine the optimal material for aviation applications. Key factors such as deformation, safety factors, and Von Mises equivalent stress are evaluated to ensure structural integrity under a range of operating conditions. In addition, CO₂ emissions from a conventional 440 kW gas turbine APU and an equivalent PEM fuel cell are compared. This study provides insights into optimal material selection for compressed hydrogen storage vessels, emphasising safety, reliability, cost, and weight reduction. Ultimately, this research aims to facilitate the adoption of fuel cell technology in aviation, contributing to greenhouse emissions reduction and hence sustainable air transport.

1. Introduction

The energy used in aviation accounts for between 2.5% and 5% of all global energy consumption [1]: increasing usage of fossil fuel will result in rising greenhouse gas emissions. The 781 Mt of CO₂ emitted from aeroplanes amounts to about 12% of global transport emissions. According to the International Air Transport Association (IATA), there will be 47,000 jets and about 8.2 billion passengers travelling around the world by 2036. The European Commission and IATA have developed a plan to reduce aviation emissions by 50% in 2040 [1]. Sustainable, eco-friendly fuels for aviation are necessary to reduce emissions and to ensure a safe and healthy environment [2]. Kerosene fuel is inexpensive but is the primary cause of the emission in aviation. Combusting kerosene emits the greenhouse gas CO₂ [3]. Fuel cells convert chemical energy from fuel (hydrogen) into electrical energy through an electrochemical reaction. A total of 20% of an aircraft’s emissions are from the auxiliary power unit (APU), and both Boeing and Airbus have deployed fuel cells to replace the gas turbine APUs used when the main engines are not engaged. This has provided a reduction in both emissions and fuel use [3,4]. Fuel cell-powered aircraft such as SUNTAN and CRYOPLANE use complete hydrogen propulsion systems [3]. However, combusting hydrogen forms small amounts of NOx [5]. Although hydrogen has high gravimetric (mass-based) energy density, it has low volumetric energy density at ambient conditions compared to conventional fuels, thus requiring larger storage volumes and posing weight challenges; also, safety is another concern [1].

There are different storage technologies for hydrogen, compressed hydrogen, liquified hydrogen, and cryogenic hydrogen [6]. Hydrogen is highly flammable; thus, storage vessels must have good structural integrity [7]. Although composite overwrapped pressure vessels (COPVs) are widely adopted in industry for hydrogen storage, this study investigates metallic tanks to establish a preliminary understanding of structural performance under high-pressure conditions [8].

The objective of this study is to conduct a preliminary structural analysis of compressed hydrogen storage vessels made of stainless steel 304, 6061-T6 aluminium, and Grade 5 (Ti-6Al-4V) titanium to evaluate their suitability for replacing conventional auxiliary power units with the PEM fuel cell system, and to identify the most promising material for safe and efficient hydrogen storage in commercial aircraft. The design of compressed hydrogen storage vessels suitable for commercial aircraft Boeing 777 producing 440 kW of power from a PEM fuel cell APU was demonstrated. The structure was analysed using ANSYS R19.2 to propose a material with high structural integrity. This will help the aviation industry choose suitable materials, optimising safety, reliability, weight, and cost, with reduced emissions.

Horde et al. [9] used PEM fuel cells to power aircraft at elevations of 200, 1200, and 2200 m to determine the viability of fuel cells at various altitudes. Li et al. [10] combined solid oxide fuel cells (SOFCs) with a piston engine to develop a hybrid system. The results of the analysis showed that SOFCs can be employed with a piston engine; and the system had a 52.29% energy efficiency, and fuel cell has an exergy efficiency of 92%. Massaro et al. [11] investigated hydrogen storage proton-exchange membrane fuel cell (PEMFC) electric aircraft and the size of the propulsion system using a real-world flight profile. Electric aeroplanes are generally 25% heavier than conventional aircraft, but weight can be reduced by enhancing energy storage methods for fuel cell technologies. Santin et al. [12] addressed the technical challenges of the performance of fuel cell hybrid systems with auxiliary power gas turbines, concluding that systems’ weight being greater than that of conventional APUs was the main concern.

Keim et al. [13] tested a PEMFC model to examine the impact of water and oxygen-depleted air (ODA) on the performance of the system. It was concluded that a PEMFC can theoretically produce enough water for flights reaching a distance of 16,000 km, avoiding the need for water fill-up prior to take-off and lowering the take-off weight. Ayar and Kavakoc [14] tested five different types of fuel cells, namely solid oxide fuel cells, proton exchange membrane fuel cells, alkaline fuel cells, photocatalytic fuel cells, and molten-carbonate fuel cells, taking into consideration different factors for each type of fuel cell using an analytical hierarchical process. The results showed that PEM fuel cells worked effectively in small aircraft when used as an APU to create a hybrid electric aircraft. Turan et al. [15] performed a design analysis for a lightweight aircraft in which the propulsion system operates with the PEMFC, and a Cri-Cri—the smallest twin-engine crewed aircraft—was chosen for system analysis and fuel cell-testing performance. The PEM fuel cell used 0.71 kg of hydrogen, and a total of 30.4 kWh of energy was consumed. Additionally, the Cri-Cri’s flying duration with the PEM fuel cell was 73 min at 150 km/h. In contrast, the Cri-Cri aircraft’s flying time without PEM fuel cell was only 30 min. Guida and Minutolo [16] designed a PEMFC power system to examine the feasibility of PEM fuel cells used for aircraft APUs; the fuel cell stack was sized to produce the developed design’s highest specific energy. It has been suggested that reducing the weight of the hydrogen vessel, which makes up 47% of the fuel system’s overall weight, will improve system overall efficiency. Kazula et al. [17] studied the potential limitations of using PEMFC in aircraft to increase the energy density of the fuel cell propulsion system, and for ice protection, heat exchangers and cryogenic hydrogen can be used as coolants for superconductors. Batteries are also suggested to prevent thrust loss brought by faulty fuel cell systems in emergency circumstances. Saleh et al. [18,19] developed and examined a simplified mathematical model for a 1 kW self-humidifying PEM fuel cell stack for high-altitude unmanned aerial vehicle (UAV) operation. The model incorporates major thermodynamic and electric parameters involved in the operation of fuel cells under different operational conditions. Schroder et al. [20] developed a PEMFC to optimise the operating parameters of the stack to maximise system effectiveness for hybrid aircraft. The results showed that the PEMFC operated effectively depending on the phase of flight. Hashimoto et al. [21] devised a plan to tackle the high weight issue of SOFCs for aeroplane applications by re-designing each material of the SOFC stack to achieve efficient operations at low temperatures (below 873 K). Saleh et al. [22] investigated the impact of using extracted air from high altitudes on the operation and performance of a PEM fuel cell whilst maintaining a certain level of delivered power, and they aimed to determine the most adequate trade-off choice between fast response and reactant consumption of PEM fuel cell used for unmanned aerial systems (UASs). Wang et al. [23] addressed major issues in using fuel cell systems in unmanned aerial vehicles (UAVs). Several hybrid fuel cell integrations with batteries, solar panels, and supercapacitors were investigated. It was found that combining fuel cells and solar cells considerably boosted the UAVs’ endurance and offered a lightweight design. Reid and Albayati [24] examined the design requirements for a UAS powered by a 1 kW PEM fuel cell for high-altitude operation and correlated them into a quantitative data model to produce a design-constraint diagram. Gradalla and Zafar [25] analysed UAV performance with a fuel cell and solar cells. The performance of the flight was improved, with an increase in endurance from 470 min to 970 min. Coutinho et al. [26] considered the issues of thermal management in fuel cell aviation systems and investigated different methods with possible solutions. Bradley et al. [27] conducted research on the performance of PEMFC propulsion in small UAVs. The cruise time of the tested UAV was 43 min, demonstrating the viability of PEMFC in UAV propulsion. Hyeyoung and Yu [28] reduced the weight and volume of the fuel cell system by removing external humidifiers and using water from the cathode to humidify the fuel cell. Coppola et al. [29] explored the best storage solutions for aircraft, considering the weight, cost increment, and emissions from hydrogen storage technologies. It was reported that the most effective way is to use materials with inherent porosity which limit emissions whilst using alternative storage technologies (compressed, liquid, and cryo-compressed hydrogen). The inherent porosity materials showed improved outcomes in weight and cost compared to metal hydrides. Dutczak [30] conducted a study on storage technologies for unmanned aerial vehicles based on performance. It was concluded that an Ion Tiger UAV employing liquid hydrogen storage technology was able to fly for 48 h due to the higher density of the liquid storage. Colozza and Kohout [31] conducted a study on aviation storage technologies such as physical and chemical storage. They found that compressed hydrogen storage technology, using a conformal vessel, rather than using a cylindrical one, would allow for 20% more hydrogen to be stored. Liu et al. [32] studied the performance of an optimised SOFC gas turbine hybrid system for power generation in aviation, reporting that hybrid systems are twice as efficient in producing power than conventional gas turbines. Elman et al. [33] reported that integration of a PEMFC and SOFC can perform better for aircraft operation.

2. Methodology

We examined various materials and the consequent design requirements for a 2D axisymmetric hydrogen vessel. The calculations required for the height, volume, and weight of the hydrogen storage vessels were performed using ANSYS R19.2 for three different materials—titanium, 304 stainless steel, and aluminium. Structural analyses were performed at various internal pressures (25, 50, 60, and 70 MPa) to confirm the structural integrity of these materials. Additionally, CO₂ emissions from an equivalent conventional gas turbine APU and the PEM fuel cell system were compared.

2.1. Hydrogen Vessel Design

We propose that a fuel cell system can be used as an alternative for an auxiliary power unit (APU) in the commercial aircraft Boeing 777, in which the APU provides power of 440 kW when the aircraft is on the ground and the main engines are not engaged for an hour [34]. A proton-exchange membrane fuel cell (PEMFC) with 60% efficiency will operate for one hour. The hydrogen required to produce 440 kW for one hour of continuous operation can be determined to design the storage vessel using Equation (1) [35,36].

$$Hydrogen amount required \left(\mathrm{K}\mathrm{g}\right)={\displaystyle \frac{P \times t}{ɳ \times LHV}}$$

(1)

where P is the power in kW, t is the time in hours, ɳ is the efficiency of the fuel cell system, and LHV is the lower heating value of hydrogen (33.33 kwh/kg or 120,000 kJ/kg).

$$Hydrogen\mathrm{ }required \left(\mathrm{K}\mathrm{g}\right)=\mathrm{ }{\displaystyle \frac{440 \times 1}{0.60 \times 33.33}}=22 \mathrm{k}\mathrm{g}$$

Compressing 1 kg of hydrogen to 35 MPa will give a volume of 45 litres and 26 litres for a pressure up to 70 MPa [37]. Therefore,

$$22 \mathrm{K}\mathrm{g} of hydrogen=22 \times 45=990 \mathrm{L} = 0.99 {\mathrm{m}}^3.$$

The volume of the vessel is equal to the volume of the required hydrogen, and for a cylindrical vessel with a diameter of 0.6 m, the volume of the vessel can be determined by Equation (2) [36]:

$$Volume of cylindrical vessel \left(V\right)= \pi r^2 h$$

(2)

where h is the height of the vessel (in m), V is the volume (in m³), and r is the radius (in m); thus, h will be equal to 3.5 m. This cylindrical hydrogen vessel is assumed to be designed with hemispherical ends. The volume of a hemisphere is half the volume of the sphere and can be calculated as given in Equation (3) [36]:

$$Volume of the Hemisphere = {\displaystyle \frac{1}{2}}\ast {\displaystyle \frac{4}{3}}\ast \pi \ast {\left(r\right)}^3$$

(3)

$$Volume of the hemisphere for one side= {\displaystyle \frac{1}{2}}\times \left(0.11\right)=0.057 {\mathrm{m}}^3$$

$$Volume of the middle part=overall volume - \left(2 \times one side volume of hemisphere\right)$$

$$Volume of the middle part=0.99 - \left(2 \times 0.057\right)=0.876 {\mathrm{m}}^3$$

$$Height of the middle part= {\displaystyle \frac{0.876}{\left(\pi \right){\left(0.3\right)}^2}}=3.09 \mathrm{m}$$

The overall height of the vessel is 3.5 m, and the height of the middle part is 3.09 m, with a 0.3 m radius. ANSYS was used to design 2D axisymmetric vessels with a capacity of 22 kg of hydrogen and with wall thicknesses of 25, 50, 100, and 150 mm. The 2D and 3D designs of the hydrogen vessel are presented in Figure 1.

Titanium, 304 stainless steel, and aluminium were used in the ANSYS design and analyses at internal pressures of 25, 50, 60, and 70 MPa, thus confirming the structural integrity of these materials; the properties of these materials are given in

Table 1. Material properties for compressed hydrogen vessel design.

Material Properties	Grade 5 (Ti-6Al-4V) Titanium	304 Stainless Steel	6061-T6 Aluminium
Density	4620 Kg m−3	7750 Kg m−3	2770 Kg m−3
Young’s modulus	9.6 × 1010 Pa	1.93 × 1011 Pa	7.1 × 1010 Pa
Poisson’s ratio	0.36	0.31	0.33
Tensile yield strength	930 MPa	207 MPa	280 MPa
Compressive yield strength	930 MPa	207 MPa	280 MPa
Tensile ultimate strength	1070 MPa	586 MPa	310 MPa

. Mesh generation in ANSYS is vital for breaking down the complex geometry into manageable elements to accurately analyse structural behaviour. The 2D body is meshed with quadrilateral elements for improved accuracy. Element quality was maintained close to 1.0, indicating good mesh quality. The total element after meshing is 224 and 817 nodes. The element size was set to 30 mm for optimal analysis. The mesh generation can be seen in Figure 2a. The boundary conditions are crucial in pre-processing. Frictionless support was applied at the vessel’s edge to prevent deformation. Uniform pressure loads of 35, 50, 60, and 70 MPa were applied inside the vessel. These loads simulate real-world scenarios and are crucial for evaluating structural performance. The boundary conditions applied can be seen in the Figure 2b.

2.2. CO₂ Emission Calculation

To produce 440 kW of power for one hour, the PEM fuel cell system requires 22 kg of hydrogen. While fuel cells do not directly produce CO₂ emissions, the method of hydrogen production does. If hydrogen is generated through the steam methane-reforming process, 8 kg of CO₂ is emitted per 1 kg of hydrogen produced [38]. Thus, the amount of CO₂ emitted from producing 22 kg of hydrogen is 176 kg of CO₂.

The amount of jet fuel required for a conventional gas turbine APU to produce 440 kW of power for one hour of operation and for a system with 30% efficiency can be calculated by using Equation (1) above, setting the heating value of jet fuel at 43,147 kJ/kg.

$$Jet fuel required \left(\mathrm{K}\mathrm{g}\right)={\displaystyle \frac{\left(440\right)\times \left(1\right)\times \left(3600\right)}{\left(0.30\right)\times \left(43147\right)}}=122.37 \mathrm{K}\mathrm{g}$$

It has been reported that a conventional gas turbine APU for Boing 777 produces 3.16 kg of CO₂ of per 1 kg of jet fuel combusted [39]. So, the emission of CO₂ for 1 h of operation is (122.37 × 3.16 = 386.68 kg) of CO₂. This approach might be applicable to other aircraft models such as the Airbus A350 or Boeing 787; however, further investigations are required in future work.

3. Results and Discussion

The structural analyses described in this section were based on deformation safety factors and equivalent Von Mises stresses for 304 stainless steel, aluminium, and the titanium vessels, and related to the cost and weight.

3.1. Analysis of 304 Stainless Steel Vessel

With 304 stainless steel at a pressure of 35 MPa and a thickness of 25 mm, the vessel exhibits a moderate deformation of 0.8371 mm. However, the equivalent stress is significantly high at 412.88 MPa, surpassing the material’s yield strength of 207 MPa. Consequently, the safety factor is relatively low at 0.5, suggesting a potential risk of failure at a pressure of 35 MPa. As the pressure increases to 50, 60, and 70 MPa, the deformation of the vessel increases to 1.1959, 1.435, and 1.6742 mm. Furthermore, as the pressure increases from 35 MPa to 70 MPa, the equivalent stress on the vessel increases significantly, from 412.88 MPa to 825.76 MPa. The safety factor decreases with increasing pressure, dropping to 0.35, 0.29, and 0.25 at 50, 60, and 70 MPa. When the thickness of the vessel is increased from 25 mm to 50 mm, there is an improvement in the deformation of the vessel, equivalent stress, and safety factor, but the safety factor remains below 1 for the 50 mm thickness. This indicates that 304 stainless steel cannot withstand pressures from 35 to 70 MPa with thicknesses of 25 mm and 50 mm. However, increasing thickness to 100 mm allows the vessel to perform well at pressures of 35 MPa and 50 MPa. At 35 MPa, the deformation is 0.2206 mm, and the stress is 138.93 MPa, which is below the yield strength of stainless steel of 207 MPa. Similarly, at 50 MPa, the equivalent stress is 198.47 MPa, and the safety factor exceeds 1, indicating that the vessel can withstand these pressures. However, the 100 mm thickness is insufficient to handle pressures of 60 MPa and 70 MPa, as the safety factor falls below 1. Increasing the thickness to 150 mm enables the stainless steel to withstand pressure of 60 MPa and 70 MPa, with safety factors of 1.1243 and 1, respectively. The deformation of a 304 stainless steel vessel can be seen in Figure 3. Figure 4 shows the relation of pressure and safety factors. The deformation for the stainless steel at different pressures and thicknesses is presented in

Table A1. Stainless steel (304) vessel’s deformation results.

	25 mm	50 mm	100 mm	150 mm
35 MPa
50 MPa
60 MPa
70 MPa

, and the equivalent stress is given in

Table A2. Stainless steel (304) vessel’s equivalent stress with 50 mm thickness.

Stainless Steel (304) Hydrogen Vessel (Equivalent Von Mises Stress)
35 MPa	50 MPa	60 MPa	70 MPa

.

3.2. Analysis of 6061-T6 Aluminium Vessel

Aluminium material with a yield strength of 280 MPa generally exhibits higher stress tolerance compared to 304 stainless steel. However, when analysed under pressures ranging from 35 MPa to 70 MPa, with a thickness of 25 mm, the aluminium demonstrated poor performance. The deformation-band equivalent stresses experienced are notably high, and thence they are associated with a low safety factor. Specifically, at 25 mm, the deformation increases with pressure: 2.1268 mm at 35 MPa, 3.0383 mm at 50 MPa, 3.6459 mm at 60 MPa, and 4.2536 mm at 70 MPa.

Table A3. Aluminium vessel’s deformation results.

	25 mm	50 mm	100 mm	150 mm
35 MPa
50 MPa
60 MPa
70 MPa

shows that that maximum deformation (highlighted in red) occurs in the middle of the vessel and inside the top of the hemisphere. Moreover, the safety factor falls below unity for pressures ranging from 35 MPa to 75 MPa, indicating expected failure of the 25 mm wall-thickness aluminium vessel. However, increasing the thickness from 25 mm to 50 mm improves the vessel’s performance, particularly under the lower pressure of 35 MPa. With a thickness of 50 mm and pressure of 35 MPa, the aluminium experiences moderate deformation of 1.08 mm, with a stress of 229.94 MPa—lower than the aluminium’s yield strength of 280 MPa. Additionally, the safety factor of 1.277 exceeds unity. However, the safety factor at a thickness of 50 mm under pressures of 50, 60, and 70 MPa falls below unity. To ensure the aluminium vessel can withstand higher pressures, such as 50, 60, and 70 MPa, the wall thickness needs to be increased. With an increase in thickness from 50 mm to 100 mm, the aluminium vessel becomes capable of withstanding pressures ranging from 50 MPa to 70 MPa. Although it experiences moderate deformation ranging from 0.8339 mm to 1.1675 mm at pressures of 50, 60, and 70 MPa, respectively, the stresses generated in the wall remain lower than 280 MPa. Additionally, the safety factor exceeds 1 for pressures of 50, 60, and 70 MPa when the thickness is increased to 100 mm, indicating that the aluminium vessel can withstand these pressures. Figure 5 and Figure 6 show the deformation and safety factors for the aluminium vessel under various pressures.

Table A4. Aluminium vessel’s equivalent stress for 50 mm thickness.

Aluminium Hydrogen Vessel (Equivalent Von Mises Stress)
35 MPa	50 MPa	60 MPa	70 MPa

presents the equivalent stress generated for the aluminium hydrogen vessel.

3.3. Analysis of Grade 5 (Ti-6Al-4V) Titanium Vessel

Grade 5 (Ti-6Al-4V) titanium with a yield strength of 930 MPa exhibited favourable results in terms the safety factor, particularly with a wall thickness of 25 mm: at this thickness, the safety factor remains above one when applying pressures ranging from 35 MPa to 70 MPa. Specifically, at 35 MPa, the deformation is 1.454 mm, which decreases to 1.0771 mm at 50 MPa and increases to 2.4926 mm at 60 MPa. The stresses generated start at approximately 412.85 MPa and increase with pressure up to 825 MPa. When increasing the wall thickness from 25 mm to 50 mm, the deformation appears slightly reduced, as indicated in Figure 7. With a thickness of 50 mm, the deformations at 35, 50, 60, and 70 MPa are 0.754, 1.0781, 1.29, and 1.50 mm. These deformations are relatively low, particularly considering the safety factor. Additionally, the stress levels generated range from 229 MPa to 459 MPa across pressures from 35 MPa to 70 MPa. Given that titanium has a yield strength of 930 MPa, these stress levels are manageable for the material. Consequently, a titanium hydrogen vessel is suitable for applications involving both lower and higher pressures. Moreover, the safety factor remains above 1 for all parameters across various pressure and thickness configurations, as illustrated in Figure 8. The deformation of the titanium vessel is presented in in

Table A5. Titanium vessel’s deformation results.

	25 mm	50 mm	100 mm	150 mm
35 MPa
50 MPa
60 MPa
70 MPa

. The equivalent stress for titanium vessel is presented in

Table A6. Titanium vessel’s equivalent stress with 50 mm thickness.

Titanium Hydrogen Vessel (Equivalent Von Mises stress)
35 MPa	50 MPa	60 MPa	70 MPa

.

Three different materials of hydrogen vessels were analysed. The titanium vessel showed impressive structural integrity across various pressure ranges. For a thickness of 25 mm, titanium showed good results, along with an acceptable safety factor, for applied pressures between 35 MPa and 70 MPa. The deformation is moderate whilst maintaining a greater-than-unity safety factor even at higher pressures. We conclude that titanium will be well suited for high- and low-pressure applications in aircraft.

At higher pressures of 50, 60, and 70 MPa, the aluminium vessel suffered from both unsatisfactory levels of deformation, reaching up to 4.25 mm and sub-unity safety factors: aluminium is therefore not suitable for higher pressure application for aviation. However, at lower pressures, aluminium showed reasonable performance especially at 35 MPa, the deformation is small 1.0859 mm and the safety factor is above unity making aluminium suitable for lower pressure aviation applications.

The 304 Stainless steel vessel showed low deformation with a below unity safety factor lower at both lower pressure and higher pressures, making it the least favourite choice for aviation.

3.4. Weight and Cost Estimation

The aluminium pressure vessel was lighter than the 304 stainless steel and the Grade 5 (Ti-6Al-4V) titanium pressure vessels, as seen in Figure 9. The 6061-T6 aluminium is the least dense material at a 25 mm wall thickness, weighing 512.24 kg, followed by the titanium vessel, at 854.34 kg, and the 304 stainless steel vessel at 1433.14 kg. However, when a 25 mm wall-thickness vessel is designed for 70 MPa, the weight of aluminium is reduced to 311.18 kg, the weight of 304 stainless to 870.64 kg, and titanium to 519.02 kg. As might be expected, as the wall thickness increases, the weight of the vessel increases. However, it has been found that the weight of the vessel decreases when the storage pressure increases; this is because when hydrogen is compressed to a higher pressure, the hydrogen volume required will be reduced, and this will reflect on the reduction in the size and weight of the vessel. The cost of the aluminium is low in contrast with the 304 stainless steel and titanium. An aluminium vessel costs around GBP 717 for a 35 MPa design, followed by stainless steel, which is GBP 1935, and titanium, which is around GBP 61,640. Meanwhile, for the 70 MPa design, the weight and cost of the vessel materials are presented in Figure 9 and Figure 10.

4. Conclusions

A comprehensive structural analysis was conducted on three different hydrogen vessel materials. It has been found that a Grade 5 (Ti-6Al-4V) titanium storage vessel can perform well with hydrogen compressed at different pressures (35 MPa–70 MPa) with minimal deformation and greater-than-unity safety factors for both lower- and higher-pressure conditions. Titanium with a minimal thickness of 25 mm has shown good structural performance for 35, 50, 60, and 70 MPa. The main drawback of using titanium is the cost, as titanium is expensive (GBP 61,640) and weighs more than the equivalent (854.34 kg) 6061-T6 aluminium vessel. Aluminium, on the other hand, showed good structural performance, along with having a safety factor above unity at 35 MPa; however, when the pressure increased, aluminium could not cope with higher pressures.

The aluminium vessel’s weight is lower (512.24 kg), and its cost is cheaper (GBP 717), than titanium and stainless steel. The 304 stainless steel failed to perform well at both lower and higher pressures. Moreover, instead of 35 MPa of compressed hydrogen, using 70 MPa pressure of compressed will require a lower volume of hydrogen to generate required power—this will positively impact the design of the vessel by reducing the size compared to the 35 MPa design. It is possible to gain a 39.2% reduction in weight with 70 MPa compressed hydrogen.

CO₂ emissions, using a fuel cell instead of conventional gas turbine for APU, are reduced by 45.6%, which makes the fuel cells in aviation for APU application very attractive: fuel cells act as a key player to reduce CO₂ emissions in the aviation industry.

Titanium’s exceptional properties make it an ideal choice for storing hydrogen when safety is the highest priority.

We recommend titanium vessels for higher-pressure (70 MPa) applications and aluminium for lower-pressure (35 MPa) applications. We suggest that this ultimately could lead to secure and safe electric aircraft, helping aviation and aerospace to promote an emission-free sector.

We show that aluminium exhibits less deformation in the hemispherical end caps. So, we recommend a further future investigation into a vessel design using two materials, aluminium for hemispherical end caps and titanium for the middle part of the vessel. We expect this approach to reduce capital and running costs.

In addition, we note that carbon composites have a yield strength of 1900 MPa and may be used for storing hydrogen with the benefits of low cost and weight. The cost of the carbon-reinforced composite is lower than that of titanium. Furthermore, to reduce the size of the hydrogen vessel, we recommend the use of a hybrid system of fuel cells and the conventional gas turbine APU, which will reduce the amount of hydrogen needed and, hence, the size and weight of the vessel. Finally, model limitations and operational factors such as electrochemical behaviour, thermal effects, fluid dynamics, vibration, thermal effects, and hydrogen embrittlement were not addressed in this study and are strongly recommended as future work.