Decarbonising the aviation industry with hydrogen
The aviation industry currently accounts for almost 10% of UK’s carbon emissions, which amounts to a sizeable 35 million tonnes of carbon dioxide . Alongside this, the aircrafts release soot and leave a water vapour trail in the upper atmosphere. This roughly doubles the net global warming impact of any aircraft journey . Figure 1 showcases the impact of global warming on our planet:
Figure 1: A visual representation of the scale of the challenge posed by global warming .
To tackle these problems, the sector has set out an ambitious Net Zero target for 2050. Moreover, the UK government has recently set out its ten-point plan for a green industrial revolution  and has committed to several investments  which heavily allude to a wider hydrogen-centred economy. In light of this, there has been widespread discussion about the role of hydrogen in decarbonising aircrafts. But why has this not already been done?
The challenges of decarbonising the aerospace industry
Aircrafts carry heavy loads across long distances, which requires a reliable source of energy to power the engine. The energy source must also fuel the entire journey, as aircrafts cannot refuel mid-air economically or sustainably. These factors put enormous constraints on the types of fuels that can be used – as well as providing energy security, they must also be efficient enough to transport heavy loads. Striking the right balance in these technical challenges with the current green energy options has proved to be considerably difficult so far. In conjunction with this, airlines need to retain their passengers’ confidence. This means that they need to be in a position to guarantee that the clean aircraft fuels do not compromise on flight safety and thus acquire societal acceptance for any novel technologies.
Hydrogen in aviation – the reasons and the challenges
As the UK actively seeks clean fuel sources, hydrogen stands out as an obvious candidate – available abundantly through sea water, it could prove to be a reliable alternative for our current power-trains. It can be produced through a variety of methods such as acidogenesis, electrolysis, etc. Acidogenesis works by recycling food and animal waste, whereas electrolysis works by breaking down water into oxygen and hydrogen. As demonstrated in Figure 2, electrolysis requires some energy input in order to break the molecules apart, which makes it inefficient. Engineers are still researching how to do this at scale in a carbon neutral way.
Figure 2: How does electrolysis work? 
For airlines to truly adapt to hydrogen aircrafts, they must have the guarantee that none of their fleet will be stranded at an airport due to a lack of hydrogen to refuel. This means that hydrogen must be available at all airports. To achieve this, hydrogen production must be global as shorter supply chains produce less carbon dioxide. Implementing this requires international policy changes, which brings about a multitude of political challenges.
However, the appeal of hydrogen for the aviation sector remains undeniable. Hydrogen has a high energy density as compared to jet fuels, i.e., the amount of energy stored per unit weight of pure hydrogen is found to be almost three times more than the traditional jet fuel . This means that the weight of the fuel required for any aircraft journey will decrease considerably. Unfortunately, hydrogen also has a lower volumetric ratio, i.e., it takes up more space than the conventional jet fuel. This means that it might need to be compressed before storage. When considering hydrogen as a fuel, engineers need to strike the right balance between exploiting its energy storage properties and keeping the size of the aircraft at an acceptable level.
Hydrogen’s explosive nature combined with its tendency to leak in gaseous form pose a set of challenges for transportation and storage. This has also adversely affected its public perception – in a survey conducted recently, only 49.5% of respondents believed that hydrogen is gener- ally safe. These fears stem from historic accidents, such as the 1937 Hindenburg disaster. The passenger airship exploded due to the ignition of leaked hydrogen , as illustrated in Figure 3. Engineers need to collate evidence to reassure the public that hydrogen is safe.
Figure 3: The Hindenburg disaster 
In aviation, hydrogen will be mainly used through fuel cells. Fuel cells, as demonstrated in Figure 4, work simply and can be thought of as being the opposite of electrolysis – they combine the oxygen from the air and the stored hydrogen to produce energy. The main challenge for fuel cells is the cost of production. Alongside this, engineers also have to conduct further research to optimise the life cycle of fuel cells in order to reduce waste.
Figure 4: How does a fuel cell work? 
Hydrogen-powered aircrafts – how close are we?
Airbus has launched hydrogen-powered concept aircrafts, one of which is illustrated in Figure 5.
Figure 5: An innovative blended wing-body design .
This aircraft attempts to address numerous problems – including carbon emissions and noise pollution. The blended body reduces turbulent flow of air, i.e. it allows the air to pass more smoothly over the aircraft body and hence reduces the noise pollution. Additionally, it also creates the much-needed space to store hydrogen.
Airbus cannot, however, implement any of these ambitious ideas unless there is change within the entire aviation ecosystem. This calls for policy changes at a government-level, innovative solutions for hydrogen distribution at a technical level and most important of all – societal acceptance at a consumer level. Although the appetite for change is unprecedented, zero-carbon alternatives remain fraught with technical and social challenges that are yet to be solved.
For an industry heavily burdened with profit cuts due to the COVID-19 pandemic, a green recovery is vital for its long-term security. The carbon emissions from aviation dropped by over 60% during the peak lockdown due to a reduction in the number of passengers . However, with ever-increasing globalisation, it seems unlikely that behavioural changes will happen soon enough or be widespread enough to truly counteract the adverse environmental impact. This means that the industry’s primary focus should be on decarbonising its fleet of aircrafts.
Hydrogen could potentially provide a way out. Without compromising the security of the aircrafts, it could lead the way to a more sustainable and greener aviation industry. Airbus has proposed numerous solutions as part of their ZeroE initiative, as illustrated in Figure 6. However, the road to getting there is littered with challenges that the engineers must work hard to overcome. The production, storage and transportation of hydrogen raise technical challenges that require focused research. Similarly, the public perception of hydrogen and the politics of the implementation of a hydrogen economy bring about social challenges which must be solved through diplomacy. The combination of these factors has hindered our path to a hydrogen economy so far, but we must now pull together to cut down our emissions and save our planet.
Figure 6: Airbus ZeroE initiative concept aircrafts .