Hydrogen in the future of air transport

Aunque quedan muchos desafíos por superar en relación con el uso del hidrógeno como combustible de aviación, no cabe duda de que será una opción a tener en cuenta también en el camino hacia la descarbonización.

Hydrogen is going to be the fuel of the future in aviation. We are all aware of this and that it will have a major impact on the entire aeronautical sector, which will have to prepare for the appropriate infrastructures and procedures for its manufacture, distribution and storage. In return, it will be a major player in the journey towards decarbonisation.

Since the end of the last century, the aeronautics industry has been embarked on a race towards the reduction of their impact on the environment and, in particular, as regards the decarbonisation of air transport. Many lines of work are underway to this end, and among them, the use of hydrogen as a fuel is one of the most attractive and promising.

Hydrogen is the lightest, most basic and common element in the universe and its molecular form H2O, an energy source with a bright future because it does not pollute. However, hydrogen is not a primary energy sourceThe energy produced by other sources, both renewable (giving rise to so-called green hydrogen) and non-renewable (blue and grey hydrogen), can be conserved.

The theory is simple. Aircraft can fly through the use of hydrogen fuel cells for electricity generationwhich can then be used to power aircraft engines. Fuel cells convert hydrogen and oxygen into electricity, with water and heat as the only by-products. It is certainly a more environmentally friendly alternative to traditional jet fuels (mostly fossil-based hydrocarbons), which release greenhouse gases and contribute to climate change. However, it still there are challenges that need to be addressedincluding the limited availability of hydrogen fuel infrastructure (mainly manufacturing and storage).

A basic summary of how the process works is as follows:

1. Hydrogen is stored on board the aircraft in tanks.
2. Hydrogen feeds a fuel cellwhere it reacts with oxygen to produce electricity.
3. Electricity produced by the fuel cell powers an electric motorwhich drives the propellers or turbines of the aircraft.
4. The only by-products of this reaction are water and heatwhich are ejected from the aircraft.

This process is therefore based on a clean and efficient source of energy for the aircraftwithout the emissions of traditional aviation fuels. However, the use of hydrogen in aviation is still in the development phase and there are some barriers to overcome.

One of these, storage on the aircraft itself or in airport facilities, poses the following challenges:

1. Low densityUnder normal conditions, hydrogen is a light gas that occupies a large volume relative to its mass. This makes it difficult to store compactly, as it requires large containers to hold a significant amount of fuel. The use of cryogenic (liquid) hydrogen significantly reduces this problem by reducing the volume and pressure required.
2. High pressureAs a consequence, to store hydrogen in compact form, it must be compressed to high pressures, which requires heavy, specialised tanks. This is also minimised by using it in liquid form.
3. Cold temperatureCryogenic hydrogen means that it must be stored at extremely low temperatures to maintain a smaller volume and to avoid pressure build-up.
4. High reactivityHydrogen is highly reactive, which means that it can easily combine with other elements and can cause unwanted reactions (even explosions) if not stored properly.
5. CostHydrogen: Currently, storing hydrogen safely and efficiently is costly due to the specialised equipment and infrastructure required.
6. Fuel tank designThe requirements for hydrogen storage do not allow, at least for the time being, the traditional use of the wings as a storage tank for this fuel, but would require the use of tanks in the fuselage of the aircraft itself.

The solution that reduces some of these disadvantages is to use cryogenic hydrogeni.e. liquefied to a temperature of around -253°C. In this case, the main advantage is that the cryogenic tank will be subjected to lower pressure, as liquids are less compressible than gases. This is a storage solution that is common in the space sector and is now on the table with the possibility of incorporating hydrogen as an energy option in aeronautics. Of course, the use of cryogenic hydrogen is, a priori, a more attractive and logical alternative to the pressurised tank for space and safety reasons.

The above challenges mean that hydrogen storage is a complex and costly processemissions, which has so far limited its widespread use as a fuel source. However, research and development efforts are underway to improve hydrogen storage technology so that it can become more accessible and affordable.

But before reaching this point, the difficulties of production must also be assessed.

Currently, the most common method of producing hydrogen is through steam methane reforming, which involves converting natural gas into hydrogen. This method is relatively cheap, but has the major drawback of producing 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 provided by fossil fuels with the consequent emission of CO2 into the atmosphere, it is called "grey hydrogen".

A more environmentally friendly alternative is the so called "green hydrogen. The method by which it is produced uses electric current to separate the hydrogen from the oxygen in the water, so if the electricity needed is obtained from renewable sourcesIf all the hydrogen generated in the world were "green", we would finally produce energy without emitting carbon dioxide into the atmosphere. If all the hydrogen generated in the world were "green", the 830 million tonnes of CO2 emitted into the atmosphere per year would be saved, compared to when it is produced using fossil fuels. The underlying problem is that replacing all the world's grey hydrogen with green hydrogen would mean an additional 3,000 TWh of renewable energy demand per year, about the same as Europe's total demand today. As technology and infrastructure improve, the cost can be expected to fall, making hydrogen a more affordable alternative to traditional fuels.

Between grey and green hydrogen is blue hydrogen, which is produced using fossil fuels as an energy source, but without emitting greenhouse gases, especially carbon dioxide, into the atmosphere.

The use of hydrogen as a fuel source in aviation is expected to evolve in the following ways:

1. Increased adoptionAs more hydrogen fuel infrastructure becomes available, the use of hydrogen in aviation is expected to become more widespread, leading to a reduction in greenhouse gas emissions and an improvement in air quality.
2. Improved technologyHydrogen fuel cells: The development of hydrogen fuel cells is expected to make them better and more efficient. Also, advances in hydrogen storage and transport will make it easier and more cost-effective to use hydrogen as an aviation fuel source.
3. New aircraft designsThe use of hydrogen as a fuel source may lead to the development of new aircraft designs that are optimised for hydrogen fuel cell systems. It will also evolve into lighter and more fuel-efficient aircraft.
4. Reduced costsAs the technology and infrastructure for hydrogen fuel in aviation improves, the cost of hydrogen use can be expected to fall, making it a more affordable alternative for aviation use.
5. New air transport configurations and systems. In recent years, a new type of mobility has emerged that will also contribute to the development of the use of hydrogen in air transport, bringing new concepts beyond traditional aviation. This is Advanced Air Mobility AAM. Energy generated by hydrogen cells can be a game changer for eVTOLs (electric vertical take-off and landing aircraft). Distributed electric propulsion enables novel aircraft configurations that should be more efficient, safer and quieter.

As we have seen, hydrogen has great potential in aviation in its use in fuel cell technologies, but it is not the only alternative. Companies such as Airbus are also working on a zero-emission aircraft that will be equipped with new engines that will use hydrogen as fuel directly, not hydrogen cells that produce electricity. They plan to have this technology ready by 2035.

Finally, it is worth mentioning what the use of hydrogen will mean for airports. Managing its manufacture, storage and distribution at an airport will also be a challenge in the coming years. It will certainly not be as straightforward as it is today with fossil fuel, which is at room temperature.

It is foreseeable that airports will have hydrogen manufacturing facilities and will be able to supply hydrogen directly to aircraft. The current trend towards increasing renewable energy installations at airports around the world will lead to a large amount of green hydrogen. However, other airports will need to purchase it and will require distribution procedures and adequate storage facilities.

Overall, the development of hydrogen aviation has the potential to reducing industry's impact on the environment and creating 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 as the only by-product. Tempting, isn't it?

 

For more details on the future of hydrogen in aviation, the May 2020 report by Clean Sky 2 (now Clean Aviation) on this topic is particularly recommended, which you can download and read here.