The aviation industry, like many others, is under increasing pressure to reduce its environmental impact. Amidst various innovative solutions, hydrogen has been hailed as a potential game-changer for sustainable air travel. However, despite its promise, the practical challenges associated with hydrogen, particularly its volumetric energy density compared to traditional aviation fuel, make its widespread adoption in commercial air travel unlikely in the near future. Let’s delve into the mathematics behind this assessment.
Understanding Energy Density
The crux of the issue lies in the fundamental differences in energy density between hydrogen and conventional aviation fuel (jet fuel).
- Jet Fuel: Aviation fuel, such as Jet-A, has an energy density of approximately 43 MJ/kg (megajoules per kilogram) and about 35 MJ/litre (megajoules per litre).
- Hydrogen: Hydrogen, on the other hand, boasts a higher energy density by weight, at around 120 MJ/kg. However, its volumetric energy density is significantly lower. In its liquid form, hydrogen has an energy density of only about 8.5 MJ/litre. In gaseous form at standard temperature and pressure, this drops even further to approximately 0.0108 MJ/litre.
Volume and Weight Calculations
To understand the implications of these differences, let’s compare the amount of each fuel required to achieve a similar energy output.
- Weight Comparison:
- For a given energy requirement, say 1000 MJ, the weight of jet fuel needed would be:
- The weight of hydrogen needed to achieve the same energy output would be:
- Volume Comparison:
- For the same 1000 MJ energy requirement, the volume of jet fuel needed would be:
- The volume of liquid hydrogen needed would be:
Implications for Aircraft Design
These volumetric differences have profound implications for aircraft design and operations:
- Fuel Tank Size: Aircraft would need to be significantly redesigned to accommodate the larger volume of hydrogen required. This could mean either much larger fuel tanks, which would impact aerodynamics and increase drag, or the sacrifice of passenger and cargo space, reducing the aircraft’s commercial viability.
- Structural Changes: Liquid hydrogen needs to be stored at cryogenic temperatures (around -253°C). This requires special, heavily insulated tanks that are more complex and heavier than those used for jet fuel, further compounding design challenges.
- Weight Distribution: The lower density of hydrogen means that even though the weight of the fuel might be lower, the distribution of this weight across larger volumes can affect the balance and stability of the aircraft.
Operational Challenges
Beyond the physical design constraints, operational issues also pose significant barriers:
- Refuelling Infrastructure: Airports would need to invest heavily in new infrastructure to handle hydrogen storage and refuelling, including safety measures for handling cryogenic liquids.
- Energy Efficiency: The process of producing, liquefying, and transporting hydrogen is energy-intensive, potentially offsetting some of the environmental benefits if the hydrogen is not produced from renewable sources.
- Economic Viability: The higher costs associated with hydrogen production, storage, and aircraft redesign might make hydrogen-powered flights economically unfeasible compared to traditional or other emerging sustainable fuels.
Conclusion
While hydrogen holds potential as a clean energy source, the current technological and logistical challenges, particularly related to its low volumetric energy density, present substantial hurdles for its adoption in commercial aviation. The significant redesign of aircraft, extensive infrastructure changes, and economic considerations make it unlikely that hydrogen will be realised as a mainstream fuel for commercial air travel in the near future. As the industry continues to innovate, other solutions, such as sustainable aviation fuels (SAFs) or electric propulsion for short-haul flights, may offer more practical pathways to greener skies.