Hydrogen will be the fuel of the future in aviation. We are all aware of this and of the fact that it will have a major impact on the entire industry, which will have to equip itself with the appropriate infrastructures and procedures for its manufacture, distribution and storage. In return, it will be a major stakeholder in the journey towards decarbonisation.
Since the end of the last century, the aeronautical industry has been immersed in a race to curb its impact on the environment, in particular with regard to the decarbonisation of air transport. Many lines of work are being pursued to this end and, among them, the use of hydrogen as a fuel is one of the most attractive and promising.
Although there are still challenges to overcome in relation to the use of hydrogen as an aviation fuel, it is clear that it will have a key role to play on the road to decarbonisation.
Hydrogen is the lightest, most basic and common element found in the universe, and its molecular form H2O, a future-proof source of energy because it does not pollute. However, hydrogen is not a primary energy source, although it is used to conserve energy produced by other sources, both renewable (green hydrogen) and non-renewable (blue and grey hydrogen).
The theory is simple. Aircraft can fly by using hydrogen fuel cells to generate electricity, which can then be used to power the aircraft’s engines. Fuel cells convert hydrogen and oxygen into electricity, with only water and heat as by-products. It is arguably a more environmentally friendly alternative to traditional jet fuels (mostly fossil-based hydrocarbons), which release greenhouse gases and contribute to climate change. However, challenges remain to be addressed, including the limited availability of hydrogen fuel infrastructures (mainly manufacturing and storage).
A basic summary of how the process works is given below:
- Hydrogen is stored on board the aircraft in tanks.
- The hydrogen is fed into a fuel cell, where it reacts with oxygen to generate electricity.
- The electricity produced by the fuel cell powers an electric motor, which drives the aircraft’s propellers or turbines.
- The sole by-products from this reaction are water and heat, which are expelled from the aircraft.
This process is based on a clean and efficient energy source for the aircraft, without the emissions of traditional aviation fuels. However, hydrogen use in aviation is still in its infancy and there are some hurdles to overcome.
One of these, storage on the aircraft itself or at airport facilities, raises the following challenges:
- Low density: hydrogen is a light gas that occupies a large volume relative to its mass. This makes it difficult to store in a compact form, because large containers are required to hold a significant amount of fuel. The use of cryogenic (liquid) hydrogen dramatically reduces this problem by reducing the volume and pressure required.
- High pressure: as a result of the above, to store hydrogen in compact form, it must be compressed to high pressures, requiring heavy, specialised tanks. This is also minimised by using it in liquid form.
- Cold temperature: cryogenic hydrogen means that it must be stored at exceedingly low temperatures, typically around -253 °C, to maintain a smaller volume and to avoid a build-up of pressure.
- High reactivity: hydrogen is highly reactive, meaning that it can easily combine with other elements and can cause unwanted reactions (even explosions) if not stored correctly.
- Cost: at present, storing hydrogen safely and efficiently is expensive on account of the specialised equipment and infrastructure required.
- Fuel tank design: the requirements for storing hydrogen do not enable, at least for the time being, using the wings as a fuel tank as per tradition, but would require the use of tanks that would be located in the fuselage of the aircraft itself.
The solution that alleviates some of these drawbacks is to use cryogenic hydrogen, i.e. hydrogen liquefied at a temperature of around -253 °C. In such a case, the major advantage is that the cryogenic tank will be subjected to a lower pressure, given that liquids are less compressible than gases. This storage solution is common in the space industry and is now on the table with the possibility of incorporating hydrogen as an energy option in aeronautics. Naturally, using cryogenic hydrogen is, in principle, a more attractive alternative to a pressurised tank for reasons of space and safety.
The aforementioned challenges make hydrogen storage a complex and costly process, which has thus far curtailed its widescale use as a fuel source. However, research and development efforts are ongoing to enhance hydrogen storage technology to make it more accessible and affordable.
But before reaching that point, it is also necessary to consider the difficulties of producing it. Currently, the most common method of producing hydrogen is by steam methane reforming, which consists of converting natural gas into hydrogen. This method is relatively cheap, but a major drawback is that it produces carbon dioxide as a by-product, which contributes to climate change and takes us back to square one. When the energy needed for the electrolysis process is obtained using fossil fuels with the consequent emission of CO2 into the atmosphere, it is called “grey hydrogen”.
A more environmentally friendly alternative is “green hydrogen”. The method used to produce it involves separating the hydrogen from the oxygen in the water using an electric current, so if the electricity needed is obtained from renewable sources, we will ultimately produce energy without emitting carbon dioxide into the atmosphere. Making all the hydrogen generated in the world “green” would save the 830 million tonnes per year of CO2 emitted into the atmosphere when it is produced by fossil fuels. The underlying challenge is that replacing all the world’s grey hydrogen with green hydrogen would require an additional 3,000 TWh of renewable energy per year, about the same as the total demand in Europe at present. By improving technology and infrastructure, we can expect the cost to fall, making hydrogen a more affordable alternative to traditional fuels.
Hydrogen used as a fuel source in aviation is expected to evolve in the following ways:
- Increased adoption: as more hydrogen fuel infrastructure becomes available, hydrogen use in aviation is expected to become more widespread, leading to a reduction in greenhouse gas emissions and an improvement in air quality.
- Enhanced technology: development of hydrogen fuel cells is expected to make them better and more efficient. Furthermore, advances in hydrogen storage and transport will make using hydrogen as an aviation fuel source easier and more cost-effective.
- New aircraft designs: using hydrogen as a fuel source may lead to the development of new aircraft designs optimised for hydrogen fuel cell systems. It will also evolve into lighter and more fuel-efficient aircraft.
- Reduced costs: as technology and infrastructure for hydrogen fuel in aviation improves, the cost of hydrogen use is expected to drop, making it a more affordable alternative for aviation use.
- New air transport configurations and systems. A new type of mobility has emerged in recent years that will also contribute to developing the use of hydrogen in air transport by bringing new concepts beyond traditional aviation. This is Advanced Air Mobility (AAM). Energy generated by hydrogen cells can change the rules of the game for eVTOLs (electric vertical take-off and landing aircraft). Distributed electric propulsion will enable novel aircraft configurations that should be more efficient, safer and quieter. In fact, it is expected that HFC-powered eVTOL designs will have much higher capacities than those powered by conventional electric batteries.
Lastly, it is worth mentioning what using hydrogen will mean for airports. Managing the manufacture, storage and distribution of hydrogen at an airport will also be a challenge in the forthcoming years. It will clearly not be as straightforward as it is today with fossil fuel, which is at room temperature.
Airports are likely to have pure hydrogen manufacturing facilities and will be able to supply hydrogen directly to aircraft. The current trend towards expanding renewable energy installations at airports worldwide will lead to a large amount of green hydrogen. However, some other airports will need to purchase it and will require distribution procedures and suitable storage facilities.
Generally, the evolution of hydrogen aviation has the potential to greatly ease the industry’s impact on the environment and create new opportunities for innovation in aircraft and fuel system design. There are many challenges to overcome, but hydrogen promises abundant, clean and sustainable energy, with water vapour being its only by-product. Tempting, isn’t it?
For more details on the future of hydrogen in aviation, the May 2020 Clean Sky 2 (now Clean Aviation) report on this topic is highly recommended, which can be downloaded and read here.