The Long Haul to Zero emmissions in aviation
Aviation remains one of the hardest sectors to decarbonise. While new technologies are under development, today few economic alternatives exist to propel a commercial aircraft as efficiently as fossil fuel. And therein lies the heart of the problem.
Before the pandemic grounded the world’s passenger fleet, aviation accounted for about 2.4 per cent of global emissions. According to Manchester Metropolitan University’s Centre for Aviation, Transport and the Environment, including noncarbon effects such as nitrogen oxide and contrails, aviation’s environmental impact rises to about 3.5 per cent. As other sectors move more quickly to cut their carbon impact, aviation’s share will only continue to increase. Although CO2 emissions per passenger flight have fallen 54 per cent since 1990 thanks to better engines and improved operations, the total volume has jumped 34 per cent over the past five years because of rising air traffic.
Few economic alternatives exist to propel a commercial aircraft as efficiently as fossil fuel and therein lies the heart of the problem.
By 2050, airlines are expecting 10 billion passengers to fly 20 trillion kilometres and this will introduce significant amounts of additional carbon to add to the atmosphere, even as the need to reduce emissions grows ever more acute. There are alternatives being developed. Electric cars have caught lots of people’s imagination, but the use of batteries for aviation is far more complicated, mainly due to their power and energy density limitation.
Long haul hybrid electrification concepts are being considered by Airbus and RollsRoyce’s E-fan X1, replacing one of the four jet engines with a 2 MW electric motor powered by a gas turbine and a battery pack. The other aviation giant, Boeing and NASA are developing ‘SUGAR Volt’2, where the engines were designed to burn fuel when power requirement is high (such as during take-off) and to use electricity while cruising. Both concepts rely on hydrocarbon liquid as energy source and hence not zero emissions, but they could lower emissions.
Electric cars have caught lots of people’s imagination, but the use of batteries for aviation is a lot more complicated, mainly due to its power and energy density limitation. Full electrification, where battery is the only energy source will be more useful in the short to medium haul segment. There are more than 100 projects worldwide looking into Urban Air Mobility3. Uber’s Elevate, Google’s Kitty Hawk, Lilium Jet all have VTOL (vertical takeoff and landing) offerings at various stage of development, suitable for air-taxi application. For more ‘conventional looking’ planes, EasyJet and Wright Electric are developing a 186 seat, 300 nautical miles (nmi) plane for commercial service in 2030; Eviation Alice has a 440nmi plane, but only as a nine-seater. ZeroAvia has a slightly larger plane for 20 passengers at 500nmi but requires hydrogen storage and fuel cell powertrain to spin the propeller, which has better power and energy density. Solid-state batteries boast much higher energy density but will require decades of technological advancement before appearing on commercial aircraft. Where today’s planes shed weight as they burn fuel, a battery-powered plane would land weighing as much as it did at take-off. So, while short-haul electric flights are on the horizon, batteries are just too big and heavy to fly hundreds of people thousands of miles without a long-term paradigm shift in the technology.
Another potential option is biofuel, made from feedstocks ranging from plants, to used cooking oil and municipal and household waste. However, like all sustainable fuels, it is expensive; two to four times the cost of standard jet fuel. Hit by the collapse in global air travel during the COVID-19 pandemic, no carrier wants to pay more for fuel, which has at times accounted for up to 30 per cent of airline operating costs, depending on oil prices.
Environmental groups are also worried by differing definitions of what can be classified as a sustainable feedstock. Ultimately, some that are currently acceptable may prove unsustainable. This might be because there is not enough to meet competing demand from different sectors, as with used cooking oil. Similarly, if the growing of biomass for use in biofuels displaces other forms of agriculture or, worse, forested areas, these indirect consequences mean such biofuels may do more harm than good.
Another solution may lie in synthetic fuel, artificially created to replicate kerosene, but this is not straightforward either.The creation of so-called power-to-liquid or e-fuels requires huge amounts of green electricity, which makes them very expensive, whilst massive investment is needed in both renewable energy and fuel production to cut the cost. Synthetic fuels also emit carbon, although only what has been taken from the atmosphere. “The cost of these e-fuels in the 2030s could be as low as today’s low-cost biofuels,” said Daniel Riefer, aviation partner at consultants McKinsey. “But you cannot scale up right away.”
E-fuels have one big advantage. Like clean biofuel, they can be dropped into the tanks of today’s aircraft and use existing fuel infrastructure. “The benefit of sustainable aviation fuel is that we don’t have to change very much,” said Russ Dunn, chief technology officer at GKN Aerospace. Other technologies on the radar appear promising but would involve more significant re-designs.
Hydrogen is the only potential true zero fuel option we know about now. It is not a new concept and was once at the heart of a top-secret US cold war project codenamed Project Suntan. In the late 1950s, a fertiliser factory outside West Palm Beach, Florida was a front for the world’s largest liquefied hydrogen plant, part of a clandestine programme to develop a hydrogen-powered spy plane. Two years after Project Suntan started, it shut down. The challenges of delivering a hydrogen-fuelled aircraft of the right size and range were too great. GKN, for instance, is also working on hydrogen-powered propulsion as part of its sustainable fuel programme.
More than 60 years later, hydrogen is back on the aerospace agenda, even if many of the challenges faced by Project Suntan remain. “Hydrogen is one of the technologies to take us there,” said Grazia Vittadini, chief technology officer at Airbus, which is planning to have a zero-emission, hydrogen-powered aircraft ready for service by 2035. The project is a flagship of the EU’s multibillion-euro COVID-19 stimulus package, aimed at greening the bloc’s economy.
Yet not everyone shares Airbus’s confidence that the obstacles encountered by Project Suntan can be overcome by 2050. These obstacles include concerns over hydrogen’s stability as an aviation fuel, as well as its transportation and storage.
Rival Boeing hence takes a more cautious view to the prospects of hydrogen in aviation.“Our belief is that it will take a while for all the technology and elements of hydrogen propulsion to be worked out before we can get tocommercial use,” said Sean Newsum, director of environmental strategy at Boeing Commercial. “Our belief is that sustainable aviation fuels are a higher near-term priority.”
But possibly the biggest obstacle is that it would require trillions in investment, investment in new aircraft, in fuel storage systems, in fuel distribution systems and in production itself. Some of the transition to net-zero may come from technological change. Some of it may come from consumers simply opting to fly less and vacation locally. Getting to net-zero requires a broad rethink across all these strategies. For large wide body aircraft, there is no obvious solution other than liquid fuel. The industry admits it will be a very tough challenge to get to net-zero by 2050.
In the end, it is debatable whether aviation as we know it can ever truly be emissions free. Changing consumer patterns, as already observed as a result of the COVID-19 pandemic and the impact of new consumer taxes on short-haul flights may lead to behavioural shifts. Some of the transition to net-zero may come from technological change. Some of it may come from consumers simply opting to fly less, and vacation locally. Getting to net-zero requires a broad rethink across all of these strategies.