While the contribution that the aviation industry makes to human-induced CO2 emissions in the atmosphere is estimated to be relatively minimal, it is expected to grow over the next few years as other industries decarbonize and increasing affluence makes air travel more affordable in more of the world.
Many parts of the industry are moving quickly to embrace sustainable aviation fuel (SAF) as a way of decarbonizing aviation, but as we discussed in the second part of this set of articles, SAF only reduces rather than removes CO2 emissions from aviation activities.
There are currently two main contenders to completely remove CO2 from flying: hydrogen and battery. Either could require new aircraft designs and extensive changes to supporting airport infrastructure, which means that the industry needs to proceed carefully if it is going to maintain its financial viability.
Hydrogen is one of the most abundant elements on the planet, so the chemistry is well understood, and when it is mixed with oxygen it can offer enough energy to lift an aircraft and power it through the air.
Hydrogen reacts with oxygen to generate electricity and the only by-product is water vapor. As a result, hydrogen can potentially be used in a comparable way to current jet fuels (and was used on NASA’s space shuttle program between 1981 and 2011), importantly though, it can do it without the damaging CO2 emissions. However there are distinct downsides.
In its natural form at room temperature, hydrogen is a gas, but it needs to be condensed into liquid form to provide enough fuel for the reaction necessary to power an aircraft’s engines over long distances.
Unfortunately, liquid hydrogen is extremely flammable, and even in its liquid form, an aircraft would need to carry significantly more hydrogen fuel to fly the same distance using traditional jet fuel. To remain liquid, it also needs to be stored at -253°C (-423°F), it is odorless and burns with a transparent flame.
What this means in short is that hydrogen-powered aircraft with a compatible range to the current generation of hardware would need to have larger tanks, bigger cooling systems, and enhanced monitoring and warning systems. This additional hardware would potentially mean fewer passengers or less cargo on an aircraft, and it would also require extensive redevelopment of the infrastructure on the ground. This would have considerable implications for the economics of an aviation operation, and it also changes the risk profile.
That said, it is possible to retrofit existing aircraft, with studies such as the UK’s Project NAPKIN (New Aviation Propulsion Knowledge and Innovation Network), a cross-industry consortium, examining the viability of converting smaller aircraft (up to 100 seats) to liquid hydrogen on the UK’s regional aviation network as part of a wider evolution towards zero carbon emission flights. According to NAPKIN estimates, in the initial phases of the conversion process, London’s Heathrow airport would only need a single liquid hydrogen delivery truck each day for the first few years to support zero carbon emission flights by regional aircraft. 
While the weight of an aircraft might have to increase to accommodate enhanced fuel tanks and accompanying systems, it is worth pointing out that the successful commercial use of composite materials by global aviation manufacturers over the last couple of decades shows that while there may need to be a compromise on range, the industry is capable of reducing the weight of aircraft without compromising safety.
Like SAF, the source of the hydrogen used in aviation fuels would also be important. To create hydrogen in enough volume to meet aviation’s commercial needs, production would need to scale up significantly, which is where discussions around grey, blue and green hydrogen comes in. Keeping it short and not too scientific, the best solution would be to find a way to produce green hydrogen, which, as the name hints, uses energy from renewable sources to produce hydrogen. Grey and blue hydrogen are created through ostensibly the same processes, but they use fossil fuels to power the reaction that creates hydrogen. In the case of blue hydrogen, the greenhouse gasses produced during this process are captured and stored, but grey foregoes this part of the process. Unfortunately, as ever in this sort of situation, grey hydrogen is the cheapest and green is the most expensive to produce.
Meanwhile, because hydrogen is gaseous at room temperature, many of the airframe and infrastructure hurdles that face liquid hydrogen as a power source for the aviation industry could be overcome if a way can be found to remove the need to store hydrogen in its liquid form on an aircraft.
At this point however, we are a long way from being able to produce a gaseous hydrogen reaction with enough energy density to get something the size of a commercial aircraft into the air and move it over long distances efficiently and cost-effectively. Most commercial research and development is focused on using liquid hydrogen as part of the overall solution as a result.
There is a further complication that is worth pointing out with hydrogen engines. The fact that the reaction between hydrogen and oxygen creates water as a by-product means that contrails could still be a potential issue for the industry, as we discussed in the first part of these articles. If the aviation industry did move to hydrogen as its primary fuel source, there would still need to be a great deal of investigation into the contribution that hydrogen-induced contrails make to human-induced climate change.
There is one final point to make at this stage about the potential for hydrogen as a more environmentally considerate alternative to traditional jet fuel: While it would directly reduce an aircraft’s CO2 emissions, the process of producing, liquifying, storing and transporting it would be very energy intensive, particularly in the initial phases of conversion. In other words, it would bring down the airline sector’s emissions, but it could push up emissions for other sectors within the aviation industry.
Batteries are another way that the aviation sector could move. There is a great deal of interest across the sector about developing battery powered alternative aircraft, but there are a lot of steps that would need to be made before battery power becomes standard even in a limited capacity.
For a start, batteries are heavier than fuels, and from the perspective of an aircraft, they are more limited in the shape that they can be. Liquid fuel, whether it’s Jet A1 or hydrogen, can be poured into a tank of any shape, such as an aircraft wing, but a battery has to be a specific shape if it is going to fit in an aircraft. It is not an insurmountable challenge, but it does reduce the potential flexibility.
Equally, assuming there’s an appropriate runway, any aircraft can currently turn up at any airport, tank up with fuel and carry on with its journey. In a battery-powered future, a specific model of aircraft might only be able to turn up at a specific airport if it was going to swap out its battery. Not a problem if it wanted to recharge because a global standard for charging would be likely to emerge relatively quickly, but it would require careful management if an aircraft was looking for a replacement battery for a fast turnaround. It could potentially have implications for maintenance, repair and overhaul costs.
Batteries also don’t lose weight as the power is consumed, which would require a considerable change to the planning of long-haul flights because an aircraft’s range improves as it burns through fuel and becomes lighter. Again, this is not a major challenge, but it would require a certain level of rethinking around the logistics of the aviation industry.
The likelihood is that these alternative fuel sources will be phased in over the next couple of decades, and the industry seems to be evolving towards the next generation of aircraft deploying hybrid power systems that utilize the benefits of different power sources as far as possible while avoiding, or at least reducing, the downsides. This is why there is currently so much research going into innovative designs such as truss, delta and even canard wing airframes that completely rethink the current footprint of an aircraft to accommodate different requirements.
The scale of the change that the aviation industry needs to make if it is to meet its environmental commitments is significant, but from an insurance and risk management perspective, support will always be available so long as organizations are transparent in the way that they are developing their new approaches to the aviation industry. The need for change is becoming increasingly apparent, but much of the groundwork is already being done.
WTW is committed to living up to its ESG responsibilities and supporting clients as they look to enhance the way that they work and reduce any negative impacts of their business activities. We work with several global legal and academic organizations that help us respond to client concerns about their climate-related legal risk measurement and management. If you would like to hear more about our ESG activities, please visit our ESG webpage. As part of this, the WTW Research Network is an active participant in the Towards Zero Carbon Aviation (TOZCA) project, spearheaded by the Air Transportation Systems Laboratory at the University College London. TOZCA is evaluating several technologies, including fuels, to work out which have the potential to help aviation achieve net-zero by 2050 globally, and has support from the aerospace industry, governments and regulators. The three-year project’s scenario analysis report is due to be delivered in Q4, 2024.