The long haul to Jet Zero
With over a hundred thousand commercial flights now operating each day, air travel has opened up our world. So how do we safeguard this opportunity for future generations?
The opportunities brought by commercial air travel since the 20th century come at a cost to our climate. The industry is responsible for 2.1 percent of global CO2 emissions and accounts for seven percent of greenhouse gas emissions in the UK.
With demand only set to grow, the UK government has committed to making air travel sustainable by reducing its CO2 emissions to net zero by 2050 and delivering net zero transatlantic flight within a generation. Its Jet Zero strategy identifies a need for new technologies, fuels, airplane designs, and carbon removal techniques.
Imperial is supporting Jet Zero through interdisciplinary centres such as the Brahmal Vasudevan Institute for Sustainable Aviation and the Institute for Molecular Science and Engineering (IMSE), which recently published a briefing on sustainable aviation fuels.
In the face of considerable uncertainty about technologies and timescales, researchers from across the university are supporting efforts to chart a path to sustainable aviation.
The shape of things to come
Some of the most ambitious changes that could be made in aviation are new airplane designs.
Professor Rafael Palacios, Director of the Brahmal Institute, investigates a range of aviation technologies and has turned his attention in recent years to the overall design of aircraft.
He explains: “The airplane is an excellent piece of engineering, but it’s looked the same for many years. Airplane designs from 50 years ago, which were based on the technology available at the time, shaped the infrastructure such as airports and runways that grew up around them. New airplane designs are now constrained by the need to work with that established infrastructure. We can tinker with the design, but we rarely redesign the whole system from scratch.”
But fundamental design changes could make flying considerably more fuel-efficient. Professor Palacios has been looking at how to develop aircraft with extraordinary aerodynamic efficiency and believes the best way to do it is to develop planes with very long, thin wings.
Airplane designs from 50 years ago shaped the infrastructure such as airports and runways that grew up around them. New airplane designs are now constrained by the need to work with that established infrastructure.
“Wings with a very high aspect ratio are coming within 10 to 15 years,” he says. “They will need to be made of lightweight material such as advanced fibre-reinforced plastics. The only question is exactly how long we will manage to make them.”
One challenge will be that these super long wings won’t physically fit in an airport, so we will also start to see wings that fold up in order to fit at the gate, he says. “It is possibly the most striking feature of aircraft to come."
Less dramatic changes could also improve airplane efficiency in particular when it comes to engine performance. The new Airbus A320neo, for example, delivers a 20 percent fuel saving over previous generations by increasing the relative size of an engine’s fans compared to its core, where combustion takes place.
Professor Rafael Palacios is Director of the Brahmal Vasudevan Institute for Sustainable Aviation, launched in 2022 to work towards environmentally friendly aviation. Photo: Fergus Burnett
Professor Rafael Palacios is Director of the Brahmal Vasudevan Institute for Sustainable Aviation, launched in 2022 to work towards environmentally friendly aviation. Photo: Fergus Burnett
Professor Palacios says: “As a rule of thumb, an airline will not launch a new plane unless it delivers a minimum of 10 to 15 percent improvement in fuel efficiency. The challenge is that even these significant improvements in efficiency are not nearly enough to achieve net zero carbon emissions. This is where sustainable fuels come into play.”
The Institute for Molecular Science and Engineering, which brings together researchers from across disciplines to find molecular-based solutions, recently launched a briefing on alternative aviation fuels. Pictured here (left to right) are Dr Mai Bui, Dr Nadine Moustafa, Dr Michael High, Dr Andrea Fantuzzi, Prof Bill Rutherford, Dr Isabella von Holstein, Dr Paola Saenz Cavazos. Photo: Jo Mieszkowski
The Institute for Molecular Science and Engineering, which brings together researchers from across disciplines to find molecular-based solutions, recently launched a briefing on alternative aviation fuels. Pictured here (left to right) are Dr Mai Bui, Dr Nadine Moustafa, Dr Michael High, Dr Andrea Fantuzzi, Prof Bill Rutherford, Dr Isabella von Holstein, Dr Paola Saenz Cavazos. Photo: Jo Mieszkowski
Fuelling the future
A new generation of alternative fuels could reduce carbon emissions in flight and in some cases their production could remove CO2 from the atmosphere.
Alternative fuels in development, which could complement improved aircraft and infrastructure design, include biofuels, synthetic fuels, and hydrogen. They each come with a different level of availability, technological readiness and cost.
Biofuels
Biofuels are fuels created from biomass such as manure, agricultural crop waste, municipal waste, algae, and animal fats, which is processed biologically or thermochemically to produce a fuel almost identical to fossil fuel petroleum.
The advantage of biofuels is that they can make use of a range of crop sources and burn more cleanly than traditional fuels. Growing biomass for fuel production can also remove CO2 from the atmosphere, and biodiesel production is also already established at scale for road transport. But biofuels are not a magic solution.
Dr Nadine Moustafa in Imperial’s Centre for Environmental Policy explains: “Biofuels are easy to produce and reasonably cheap, but their production policy varies internationally."
“In the UK, they can only be made from waste products, which limits how much fuel can be produced but safeguards their low environmental impact. In the US, crops can be grown specifically to make biofuel, but there still isn’t sufficient landmass to generate sufficient quantities for a wholesale shift in fuel choice,” she says.
“One other issue with biofuels is that they present several environmental concerns. If you are dedicating land to growing crops for biofuel production, we need to ask if that land could be better used for food production. This is one of the reasons the UK and EU limit biofuel production to waste products.”
Biofuels can be used in conventional aircraft like this Airbus, which flew from Toulouse to Paris. Photo: Eric Cabanis/AFP via Getty Images
Biofuels can be used in conventional aircraft like this Airbus, which flew from Toulouse to Paris. Photo: Eric Cabanis/AFP via Getty Images
Synthetic fuels
Another option is synthetic or e-fuels, which are produced by mixing CO2 and hydrogen. This chemical reaction produces a range of different synthetic fuels (kerosenes, gasolines and diesels), which can be transported over long distances. Producing them can involve removing and utilising CO2 from the atmosphere. But making a truly sustainable e-fuel requires getting your CO2 from the right place.
Dr Paola Saenz Cavazos in Chemical Engineering and the Hitachi-Imperial Centre for Decarbonisation and Natural Solutions researches techniques for carbon capture, a key part of the production process of synthetic fuels. She believes it is crucial to understand the whole lifecycle of emissions throughout the fuel supply chain. “You could use dirty hydrogen and fossil fuel CO2 to make your synthetic fuel,” she explains. “But ultimately that is going to be more harmful than just continuing to use fossil fuel kerosene.”
To produce a genuinely carbon neutral synthetic fuel, the CO2 needs to be captured directly from the atmosphere rather than from burning fossil fuels. The hydrogen needs to be produced by electrolysis powered by renewable energy (which is currently only feasible on a small scale), and you need additional renewable electricity to power the whole reaction.
So despite the potential advantages of synthetic fuels, analyses by scientists like Dr Saenz Cavazos reveal that yielding enough synthetic fuel to reduce the carbon footprint of aviation requires committing to what are currently extremely expensive production methods on a large scale. The technology exists and works, but is very energy-intensive and costly.
“It is hard to put a timeline on it,” explains Dr Saenz Cavazos. “With the right policy incentives and the right carbon price you could deploy the technology more widely in the next three to five years. But the research needed to drive the cost down could take five to ten years.”
While synthetic and biofuels are certified, their other limitation is that they can currently only be used in commercial aviation in a blend with fossil fuel kerosene up to 50 percent, which limits how much they actually reduce CO2 emmissions.
The main advantage of both biofuels and synthetic fuels is that they are ‘drop in’ fuels – you don’t have to change the design of an aircraft to use them. While they must pass tests to secure certification, those tests mainly revolve around what kind of composition they have and how flammable they are.
Hydrogen
Another alternative fuel is hydrogen, which can be produced using renewables and – unlike biofuel and synthetic fuels – does not release CO2 when used. However it requires a much larger scale change to the design of the aircraft .
Dr Mai Bui in the Centre for Environmental Policy believes that hydrogen-powered aircraft will have a role to play in short haul and small-scale aircraft in the future, leaving sustainable aviation fuels to carry the long-haul flights.
Hydrogen can only be used in aircraft specially designed for it, like this plane from APUS Group on display at the Hanover technology fair. Photo: AXEL HEIMKEN/AFP via Getty Images.
Hydrogen can only be used in aircraft specially designed for it, like this plane from APUS Group on display at the Hanover technology fair. Photo: AXEL HEIMKEN/AFP via Getty Images.
“Liquid hydrogen is much lighter than conventional jet fuel, but takes up a lot more space, which means changing the position of the fuel tanks and the structure of the aircraft,” she explains. “It would be difficult to design a plane that can carry sufficient fuel for a long-haul flight.”
Hydrogen is also not necessarily a viable short-term solution. Dr Bui adds: “Airlines have invested a lot of money into their current fleet and profit margins on short haul flights can be tight, so airlines would need to maximise their value by waiting to adopt this new technology until the fleet have reached the end of their lifetime.
“If you consider the conservative nature of the aviation sector and that regulation changes take time, a realistic timeline to deploy zero-emission aircraft (such as hydrogen or electric) would likely be after 2050,” she adds.
The right blend
So none of these fuels presents a complete solution to the challenge of sustainable aviation. Professor Palacios says that there has been a lot of soul searching over the last ten years about which are the ones to focus on.
He believes that while it is hard to find a fuel that fits the bill, hydrocarbons are the most viable solution; because these fuels behave in the same way as existing kerosene, they make life as an aircraft designer much easier.
The main advantage of both biofuels and synthetic fuels is that they are ‘drop in’ fuels – you don’t have to change the design of an aircraft to use them.
But they also come at a cost, which restricts how much fuel an engineer can use. Fuel costs account for around one third of a flight’s total cost, so any change in fuel use is likely to see costs passed on to the consumer as a 10 to 15 percent increase in ticket prices.
When IMSE produced its briefing paper on sustainable fuels, it found that all these fuels are potential alternatives, but will be needed at different times, in different places or for different purposes.
Dr Isabella von Holstein, IMSE’s Policy Manager adds: “It is hard to find a fuel that ticks all the boxes. Our work showed that there is a complex a balance between them and we need to have a pragmatic outlook; this technology is coming forwards but it’s not a magic solution and will need a blend of thinking at both the molecular and engineering scale.”
Imperial's Dr Marc Stettler (fourth from right) was one of 100 experts on board Virgin Atlantic's Flight 100, which in 2023 was the world's first transatlantic flight by a commercial airline to use 100% Sustainable Aviation Fuel. Photo: Virgin Atlantic
Imperial's Dr Marc Stettler (fourth from right) was one of 100 experts on board Virgin Atlantic's Flight 100, which in 2023 was the world's first transatlantic flight by a commercial airline to use 100% Sustainable Aviation Fuel. Photo: Virgin Atlantic
Reducing contrails
While contrails look elegant, they trap heat that would otherwise escape into space. The net warming effect is thought in some cases to be more than the effect of the carbon dioxide.
Condensation trails, or contrails, are besides burning fuel one of the major ways flying contributes to climate change. These line-shaped clouds that follow aircraft in the sky are caused by particles emitted from an aircraft’s engines.
Dr Marc Stettler in the Department of Civil and Environmental Engineering explains: “If the plume of an aircraft engine is very humid – or as we say, super-saturated – then the water vapour it contains will want to condense onto a surface as it cools. The particles in the plume provide that surface, allowing water vapour to form droplets, which quickly freeze and form an ice-cloud.”
This layer of insulation means that the net warming effect of contrails is thought in some cases to be more significant than the effect of the carbon dioxide produced by flying.
“The climate impact of contrails can be highly variable, but if we look at the effect that flights today have from now into the future, contrails are responsible for around one third of the total effect, the other two-thirds coming from CO2,” Dr Stettler adds.
Virgin Atlantic Flight 100, the first transatlantic flight by a commercial airline to use 100% sustainable aviation fuel, had 100 experts on board including Dr Marc Stettler.
Biofuels and synthetic fuels can play an important role in reducing contrail production. They typically burn more cleanly and reduce the particle emissions of aircraft engines; this in turn reduces the number of particles on which contrail ice particles can form, and shortens the lifespan of the contrail.
“As we only have a limited resource of these alternative fuels, we need to consider both the CO2 and non-CO2 benefits. And you only really see the impact of these fuels if you use them at higher blends – say 50 percent or higher," Dr Stettler explains.
Fuel is only one consideration, he adds: “While fuels can reduce particle emissions by about half, advances in combustor technology could have an even greater impact. We want to provide the evidence that the industry needs to set targets for future engine technologies."
“Reducing contrails through minor changes to flight paths is probably the fastest and cheapest way to reduce the climate impact of aviation. Several airlines are conducting trials and we’re looking to scale this up in the near term.”
In addition, contrail production depends on where and what time of day the aircraft are flying, and the atmospheric conditions that prevail. This presents an opportunity.
Dr Stettler says: “Once a contrail has formed, for it to persist, the environment around it also needs to be supersaturated. Although these supersaturated regions can extend across a considerable distance horizontally, they are actually quite thin. By changing the airplane’s flight level by as little as 2,000 feet it is technically possible to avoid contrail formation altogether.”
Dr Stettler was on the world’s first transatlantic flight by a commercial airline to use 100 percent Sustainable Aviation Fuel – in that case a blend of biofuel and synthetic fuel – saving 95 tonnes of carbon dioxide. Virgin Atlantic’s Flight 100 travelled from London to New York in November 2023 with 100 industry and academic experts on board, including Dr Stettler.
“By forecasting the regions of supersaturation on our journey," he explains, "we predicted that no contrails would be formed along the planned flight path and that no deviation was needed; this was confirmed with satellite imagery."
“It also demonstrated to both Virgin and other airlines that there is real potential to incorporate contrail data into existing flight planning tools.”
He adds: “Reducing contrails through minor changes to flight paths is probably the fastest and cheapest way to reduce the climate impact of aviation. Several airlines are conducting trials and we’re looking to scale this up in the near term.”
Flight’s digital frontier
Planes usually last 25 years, and it takes years to get a new model into flight. But digital technologies could improve the performance of existing aircraft.
The long time it takes to bring a new airplane into production makes it important not only to create more efficient planes but also to consider how to improve the performance of existing aircraft.
Professor Laura Mainini, Associate Director of the Brahmal Institute for Sustainable Aviation, works at the intersection of engineering and computational science. Her research emphasises the design and integration of onboard systems, using applied maths to assess the structural health of a plane and to provide system diagnostics and prognostics.
She says: “To accelerate airplane development, and to really enable the transition to net zero, we need to have different technologies onboard. Retrofitting an aircraft is not straightforward – you can’t just change an element and expect to ‘plug and play’. You have to consider how any new component interacts with other systems, in a snowball effect."
“For instance, the challenge of bringing hydrogen-based solutions on board isn’t just about finding space for storage tanks. We need to reimagine power architectures and thermal management systems – and reconfigure the whole aircraft and airport infrastructure.”
Currently, aircraft need to be grounded to collect and analyse data on those systems. But advances in sensors means that more refined diagnostics and prognostics could be carried out while the plane is in flight.
“The adoption and integration of new technologies could be accelerated without compromising safety,” explains Professor Mainini, “because computational reasoning would be able to tell us in the very early stages if something is going wrong.”
The information gathered from these diagnostics can inform maintenance strategies, which in turn impact the sustainability of the plane over its whole lifecycle, and can inform future generations of aircraft as engineers learn from the history of the fleet.
“Simulating different configurations of aircraft can be expensive, computationally demanding, and time-consuming,” she adds. “Using maths to accelerate this process can also support engineers in their decision-making process when it comes to designing new planes.”
Synergy for the skies
The challenge of making aviation sustainable cannot be solved just by one discipline. It needs people from a range of fields to make systematic and cumulative improvements.
For those working on this challenge, there is a common thread of optimism and collaboration.
Dr Stettler explains: “Net zero aviation is feasible. It’s very easy to assume it’s a single issue, but it’s much more complex than that – we need to come together, and quickly.”
IMSE’s Dr von Holstein agrees: “It’s a complex and varied state of play. We need to play a catalytic role and bring together academics, policy makers and industry to find solutions, in the way that Imperial does so well through centres such as IMSE.”
Dr Saenz Cavazos says: “It’s not something that’s going to happen overnight. It needs a lot of work and will be a combination of technologies, aircraft redesign, and understanding patterns of supply.”
Professor Mainini is optimistic: “The goals set up at international level are very ambitious and require very integrated and synergistic action, and this won’t come from technology alone. But we all work every day to make things that seem impossible, possible!”
Dr Stettler adds: “Admittedly some of the pathways are emerging and not yet proven at scale, but we have already seen rapid change in other industries where there’s been strong and consistent policy (for example biofuels in road transport), and I believe with similar policies the same shift can occur in aviation.”
“If we want to scale up, I think we can,” agrees Dr Moustafa. “If you look at how much we have progressed in other areas in last ten years – from greenhouse gas removal to renewables and EVs, even COVID-19 – if we need to do something, we can. It will depend a lot on academia, policy, and industry deciding together what the next things are we need to focus on.”
The goals are very ambitious and require very integrated and synergistic action, and this won’t come from technology alone. But we all work every day to make things that seem impossible, possible!
“I don’t think a specific fuel is the silver bullet,” she adds. “And in general, further gains will come from infrastructure and operation efficiencies within the airport, new technologies and greenhouse gas removal.”
For Dr Bui the route ahead will need all the pathways, to ensure that if something doesn’t work, it doesn’t leave us stranded. “There’s going to need to be a balance for sustainable aviation to work. There are synergies between direct air capture and fuel production – supporting both pathways will pay dividends,” she says.
Dr Moustafa adds: “Inevitably it might become more expensive to fly, but aviation has increased globalisation, contributing jobs, socioeconomic opportunities, cultural awareness, and innovation in general to the world, we cannot just shut it down.”
Professor Palacios agrees: “Imagine life without airplanes; there is no alternative. We need to make it work, to fix it and find solutions to help us live the life we want and keep the earth running alongside us. That includes strategies and choices we can all make, such as flying less, replacing domestic flights with trains and using video calls more. But we all need to push for it, because there’s no plan B for the planet.”
This feature was produced by Imperial Enterprise and the Institute for Molecular Science and Engineering (IMSE) with support from the Brahmal Vasudevan Institute for Sustainable Aviation.
- Imperial Enterprise helps businesses access the university's expertise, talent, resources and technology. Find out how it can support your organisation.
- IMSE brings together world-leading researchers from across the university's four faculties to find molecular-based solutions to major societal challenges.
- The Brahmal Institute is a collaborative research centre for blue-sky thinking towards environmentally friendly aviation.
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