Aviation’s kerosene conundrum
Image credit: Pipistrel Alpha
When it comes to flight, getting to zero carbon is going to be more of a struggle than for most sectors.
The way the world generates energy could look very different by 2050. By that point, most of our road vehicles could well be running on batteries. Industries will have converted to use renewables more or less completely. TSMC, which makes chips for the many companies who do not own their fabs, committed last summer to having its entire operations run on green energy by the middle of the century even though each fab can easily consume 100MW.
On the surface, flight looks to be in the same situation. The UK aviation sector has committed to net-zero carbon emissions by 2050. But flight is one of the hardest sectors to decarbonise fully. When it comes to large, long-haul aircraft, no one has zero-emission aircraft in their sight before 2050. For many flights, net-zero has to come about through carbon accounting even though electric flight is already a reality. You just have to be on a plane that is not going very far.
Analysts such as IDTechEx chairman Peter Harrop see the commercial prospects for electric flight as compelling for short-range services, likely beating the nearest competitor, hydrogen, not just in the short term but the long term. Though batteries suffer from the problem of their energy density being much lower than a full tank of kerosene and an engine, electric flight can have significant advantages in terms of performance. Electric aircraft can deliver high instantaneous power levels, which translates into rapid climb rates. This, in turn, points to possible applications in scaled-up vertical-take-off (VTOL) drones that can act as air taxis, though possibly computer guided. “With fixed-wing, you’re not desperate to get rid of the pilot’s weight,” Harrop notes.
Slovenian company Pipistrel obtained European Union Aviation Safety Agency flight certification for its first electric fixed-wing two-seater just over a year ago. By the start of 2021, it had delivered more than a hundred, kicking off with Swiss flying school AlpinAirPlanes, which installed solar panels at its ten locations across the country to recharge them. Other companies such as Bye Aerospace expect to join Pipistrel soon. Though Bye is waiting for its eFlyer 2 and eFlyer 4 to be certified, the company believes it will be the first to obtain US Federal Aviation Administration approval, and by March had notched up more than 700 orders.
For these aircraft, electricity brings advantages on top of being able to claim to be low-carbon, Harrop says: “The cost of running it per hour as you train a pilot or use as a taxi service is one-fifth of the cost per hour of today’s equivalent aircraft. It has a very disruptive influence on the industry.”
Though the certification process is still being thrashed out, the cost of development for these fixed-wing projects is relatively affordable. Pipistrel based its electric aircraft on the airframe of its existing combustion-engine Virus. Harrop says the costs for Bye are also low, noting that the company “is an example of all the fixed-wing [electric] companies: bringing to market at one-tenth the investment the electric VTOL people say they need or have raised”.
Instead of developing completely new aircraft, Falcon, a company set up by a masters student and colleagues at Eindhoven University of Technology, is planning to retrofit electric traction and batteries to the existing aircraft used by flying schools. Its founder Brandon van Schaik said in a seminar organised by the International Civil Aviation Organisation (ICAO) on low-carbon flight: “As they generally fly only short distances, the optimisations you could carry out for new aircraft are not necessary.”
Optimisations will be needed to go beyond pilot training and recreational flying. According to Ajay Misra, deputy director of the Nasa Glenn Research Center, battery packs with an energy density of up to 200Wh/kg, which is achievable today, can readily support two-seater designs. The aim is to get to 400Wh/kg, which Misra and others regard as the sweet spot for electric VTOLs as well as for small passenger or commuter planes.
Norwegian operator Widerøe is the lead customer for a joint project between Tecnam and Rolls-Royce. Three-quarters of its flights are under 275km and it serves more than 40 airports scattered across the Nordic country. The aim of the project is to deliver by 2026 a nine-seater that caters for Widerøe’s needs, though it will probably have a lower range than planes in the existing fleet. According to Matheu Parr, customer business director at Rolls-Royce Electrical, small propeller aircraft will help prepare the industry for electric products at scale, and the engine maker expects batteries to power aircraft with up to 20 passengers.
Harrop points to the 400 islands of Japan as an important market for electric aircraft. For remote airports everywhere, the ability to recharge the aircraft using renewables instead of flying in fuel at great expense will help promote the use of electrics.
Getting a long way beyond 200Wh/kg involves novel battery chemistries, such as lithium metal and sulphur combinations. Though battery manufacturers have avoided using metallic lithium in their designs up to now because of its flammability, by replacing the liquid oxygen-rich electrolytes with solid sulphur, the future batteries may prove to be a lot safer. That, in turn, may clear a path to easier flight certification, as a major roadblock at the moment lies in concerns about battery safety.
“I think the long-term future will be solid-state batteries. They have a low flammability risk as there are no liquids,” Misra says, but it is not clear how long their service life will be, as publicly available data is hard to find.
Service lifetime is important. Though the move away from combustion may cut maintenance and service costs for aircraft, battery-powered flight does incur significant ongoing costs that will also increase its environmental footprint. Using recent estimates, making a battery can generate between 60 and 110kgCO2/kWh, depending on where they are made and where the raw materials are mined.
Batteries will need to be replaced not because they lose too much capacity but because they can no longer deliver the high power levels needed for take-off, which could be a bigger headache for the urban VTOLs that are collecting venture capital investment right now. By the point of retirement, the battery will likely be able to charge up to 80 per cent of its rated capacity. “Batteries can and must be used in a second life,” says Tine Tomazic, CTO of Pipistrel. They may be transferred to fixed systems at airports or sold on for grid-stabilising arrays.
For larger aircraft, liquid or liquefied fuels remain the only options. This is where fuel cells and hydrogen-powered flight enter the picture, but it is nowhere near as quick a transition as that to smaller battery-powered aircraft. In addition to its work on electric powertrains, Rolls-Royce has a programme to develop hydrogen-fuelled engines but sees introduction as unlikely before 2035, though test vehicles could be in the air by the middle of this decade.
Glenn Llewellyn, vice president for zero-emission aircraft at Airbus, says: “In the medium term, from the 2030s, we believe there is huge potential in hydrogen-powered aircraft both in terms of their ability to reduce environmental impact and also in terms of cost.” He notes that growing government policy support for green hydrogen will help make the cost case. “A significant ecosystem is developing and it is one we want to think about when positioning for hydrogen in aerospace.”
Last year, Rolls-Royce unveiled three concept designs for hydrogen fuel: a turboprop designed to carry 100 people; a narrow-body turbofan for 200 people; and a more futuristic blended-wing body.
As with electric aircraft, hydrogen does not escape the obstacle of low range. “Before 2050, it is unlikely there will be hydrogen aircraft available for long-haul,” says Geert Decock, manager of electricity and energy at campaigning group Transport & Environment. “We want to see this, we want zero-emission aircraft to be part of fuel mandates,” he adds, but emphasises that hydrogen in flight faces major issues.
A big question for hydrogen is whether to combust it or direct it into fuel cells. These might overcome some of the problems faced by batteries in smaller aircraft, though they currently suffer from poor power delivery. If start-ups such as HyPoint can improve instantaneous power output that will help move fuel cells into VTOLs and possibly larger aircraft.
The technologically safest option, and the one that will probably cover most passenger-kilometres flown in the coming four decades, is to accept that carbon-based fuels are here to stay. The net-zero part comes from mitigating its environmental footprint through carbon accounting. The first step was ICAO’s CORSIA regime, which though it has drawn criticism for potential loopholes, standardises the way airlines account for their carbon offsets. The next step is to move to what the industry calls sustainable aviation fuel (SAF), which is kerosene-like fuel that does not come from fossil sources.
“At the end of the day, for long-haul flight, it’s SAF,” says Christoph Wolff, global head of mobility at the World Economic Forum.
As with the automotive industry in the 2000s, much of the early work on SAF focused on biofuels: blending them with conventional kerosene to deliver enough power to the jet engines. By the middle of 2019, Virgin had flown a total of a million kilometres on a fuel that included butanol fermented by re-engineered yeasts.
Waste provides another option that the ICAO has recognised in its list of approved SAF feedstocks. The Altalto plant being built on Humberside by start-up Velocys with government funding and help from British Airways is a prime example: it will be able to take in 500,000 tonnes of household waste a year and convert it into up to 60 million litres of aviation fuel a year.
Biofuel though cannot provide the answer to large-scale SAF. Aviation accounts for very little of the world’s total fossil-fuel supply but even its thirst would easily outstrip capacity. The US Department of Energy concluded in a report on SAF published last autumn that just US flights would need the country’s entire available biomass.
The answer seems to lie in renewables. Solar and wind energy provide the option to massively expand SAF in another direction and one that would also support an eventual migration to hydrogen-fuelled aircraft. To make SAF, you first make hydrogen through electrolysis and then combine that with carbon monoxide ‘syngas’ that is in turn derived from carbon dioxide captured from industrial plants or from the atmosphere. Converting carbon dioxide in this way is significantly less energy-intensive than the electrolysing step. Though industrial carbon dioxide looks far more economically attractive than direct air capture, which is difficult because of the thankfully low concentration of the gas in air, that has a similar challenge to biomass: there simply will not be enough to go round. So, any cost projections need to factor in the expense of direct air capture.
SAF is growing fast but it is beginning from a tiny base. According to Wolff, the capacity for SAF is likely to double by 2025. By even at that point, the majority will still be based on biofuel because green hydrogen production is only just getting off the ground.
This balance may shift in the second half of the decade. In principle, assuming producers in solar- and wind-rich Chile hit their targets, Wolff argues SAF made using green hydrogen could be cost-competitive with the biofuel options, with a cost of around $1,000 (£705) per tonne. However, Wolff’s projections do not see a rapid reduction in cost after that, with production based on green hydrogen and carbon dioxide captured from the air likely to be only slightly lower than biofuel even in 2040. The advantage that electrically derived fuel will have is perhaps a better image for sustainability: crops for biofuel will often compete with those for food.
There are also issues with pushing aviation into SAF too quickly: it may divert attention from activities that do more to limit greenhouse-gas emissions. Keith Whiriskey, deputy director of the Bellona Foundation, estimates Denmark alone would need 25TWh of energy to synthesise enough aviation fuel to satisfy just its own demand. That is more than two-thirds of its current generating capacity.
Theoretically, if the country were to set about focusing on aviation emission reductions, the saving would be about three million tonnes per year. If all of Denmark’s road vehicles ran on batteries, that would need just 8TWh per year and save 6.5 million tonnes of CO2, a recognition of the round-trip cost of converting electricity to chemical energy and back again.
“The ramp-up is mind-boggling if you look at the figures. This is a huge task,” says Wolff.
It gets worse when you factor in projected growth. Although the Covid pandemic has put a large kink in the curve, air travel is still expected to grow over the coming decades. “If we want to achieve a 50 per cent reduction compared to 2005, we require 80 per cent of our fuel to be renewable by 2050,” adds Karl Hauptmeier, managing director of Norsk e-Fuel, a start-up specialising in synthetic fuel.
Unless there is a huge increase in the construction of renewables capacity, which is something that with enough cash many countries close to and south of the equator could take on profitably, the only answer may be to discourage further growth in air travel.
Without that construction, the mitigation measures might be restricted to more efficient ways to make fossil-derived kerosene. Hydrogen probably has a future in lowering the carbon footprint of refining oil.
The focus in all the strategies should be on making sure the environmental calculations makes sense, but how decarbonisation really plays out will depend as much on economic incentives and government policies as on the underlying calculations.
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