Investment and advances drive sustainable aircraft propulsion.
By Owen Davies
By 2050, that could rise to 25%, thanks to emission reductions in other industries and to the continuing growth of air travel. That idea is not welcome. Environmentalists are already targeting the aviation industry. Last October, IATA passed a resolution supporting a goal of making aviation carbon-neutral by 2050.
And companies worldwide are working to make it happen. Four technologies bid to replace fossil-derived jet fuel with cleaner ways to store energy. At least 1 – probably 2 – could propel short-range commercial aircraft before 2030, with any laggards arriving soon after. IATA’s estimates are about 5 years later than ours.
Sustainable aviation fuel
Sustainable aviation fuel (SAF) is almost indistinguishable from ordinary jet fuel. Most of it is made from biomass, recycling carbon that otherwise would be released. SAF delivers clear benefits over Jet A/A1 without requiring special infrastructure or equipment changes.
Over its life cycle, SAF should release only 20% as much carbon as fossil-based jet fuel. Much of the particulates and sulfur and nitrogen oxides also disappear. Understandably, the industry has welcomed this synthetic aviation fuel eagerly.
SAF has powered more than 370,000 flights since 2016, according to IATA, and more than 45 airlines have experience with SAF. 2030 will be a critical year for SAF. By then, Boeing says all its planes will run on pure SAF. AIG, parent company of British Airways, plans to power its flights with SAF.
So do the 60 companies of the “Clean Skies for Tomorrow” coalition, whose members include Airbus, Delta Air Lines, United, KLM, United Airlines, fuel providers BP and TotalEnergies, and bizjet operators McKinsey and Deloitte Willie Walsh, IATA’s director general, envisions a bright future for SAF.
“A potential scenario is that 65% of [aviation-related] carbon emissions will be abated through sustainable aviation fuels,” he estimates. However, that potential scenario faces significant obstacles. One is manufacturing capacity.
Some 100,000 tons were scheduled for 2021. This rounds conveniently to 0% of the 300 million tons of jet fuel airlines need in an average year. Some 14 billion tons are in forward purchase agreements, but that still doesn’t make a dent in the fuel market.
Worse, flying with SAF will hit passengers in the pocket. It costs 3 to 4 times as much as fossil jet fuel. The cost should come down as production expands, but it will be a long time before SAF costs as little as naturally liquefied dinosaur food.
An unlikely alternative
During World War II, when the military had first call on petroleum, some ground vehicles burned another liquid fuel – ammonia. They spewed nitrogen oxides, but at least CO2 was not a problem. Unfortunately, making ammonia is as dirty as burning it.
The process in use today combines hydrogen, from natural gas or coal, with nitrogen in high-pressure reactors that consume energy from fossil fuel. In all, making ammonia uses 2% of the world’s energy and releases 1% of its CO2.
However, this is beginning to change. Australia’s Fortescue Future Industries is producing green hydrogen and ammonia using wind and solar energy, with hydro and geothermal in planning. Several groups are developing reverse fuel cells that create ammonia by packing energy into a mix of hydrogen and nitrogen.
Others have demonstrated fuel cells and other efficient cracking processes to release hydrogen as engines need it. Ammonia, however, is not an ideal avfuel, as it contains less than half the energy by weight of Jet-A, and 15.5% as much as hydrogen when both are in liquid form.
Yet, it has one big advantage over hydrogen – it becomes liquid when cooled to -33º C, hydrogen only at -253º C, just 20º C above absolute zero. Ammonia saves the weight and cost of cryogenics.
The Advanced Research Projects Agency–Energy is interested enough to contract with Raytheon Technologies to evaluate ammonia’s use in aviation. At midpoint in the study, researchers had identified some engineering issues to be solved in designing ammonia-fueled powertrains, but none were considered critical.
In 2020, Airbus abandoned work on electric aircraft in favor of hydrogen power. The company now plans to have hydrogen aircraft on the market by 2035. The case for hydrogen is obvious. It contains 3 times as much energy by weight as conventional jet fuel. Burn it, and the exhaust is water.
Put it through a fuel cell to generate electricity, and you get water. No particulates. No unpleasant oxides. No one worries about emissions they can drink.
Its negatives seem to be either livable or repairable. Disadvantages include:
• Hydrogen is 4 times as expensive as fossil fuel, so it will be a while before the cost reaches parity.
• Measured by volume instead of weight, hydrogen contains only about 25% as much energy as jet fuel. Therefore, aircraft require large and sturdy pressure vessels to contain the hydrogen – a need hydrogen enthusiasts clearly accept.
• Converting to hydrogen power will require vastly more supply, and probably a new distribution infrastructure, to meet even a fraction of aviation’s needs.
• And very little hydrogen today is really “green.” It comes from cracking petroleum, with all the carbon that entails.
Fortunately, the supply of carbon-free hydrogen from electrolysis powered by solar or wind energy is set to grow quickly in the years ahead. The Australian Renewable Energy Agency even breaks water into hydrogen and oxygen by focusing sunlight on a catalyst, avoiding electricity completely.
Two companies aim to market hydrogen airplanes by mid-decade. Universal Hydrogen, in Hawthorne CA, is developing a fuel-cell power train for the ATR 72 and de Havilland Canada Dash-8 twin-turboprop regional aircraft. Key to Universal Hydrogen’s plan is a modular tank system that can be carried by existing freight networks and loaded directly onto the aircraft, eliminating infrastructure problems.
Universal Hydrogen is backed by an impressive list of investors, including Airbus and JetBlue. ZeroAvia, in the UK, is converting a 19-seat Dornier 228 twin-turboprop to hydrogen-electric power. Plans call for first flight in mid-2022 and certification and sales of the powertrain for the Dornier and competitors by 2024.
Aircraft with 50 to 80 seats are planned for 2026, and with 100 to 200 seats and 1000-nm range in 2028. For 2040, they are looking at aircraft with more than 200 seats and a range of 5000 nm. The company has also demonstrated a self-contained refueling system for airports that can produce its own green hydrogen, store it, and deliver it to fuel-cell aircraft.
Customers are buying in to ZeroAvia’s vision. Bangalore-based Hindustan Aeronautics will retrofit Dorniers and build its own hydrogen-electric equivalent. Ireland’s ASL Aviation plans to convert 10 ATR 72 freighters to ZeroAvia powertrains.
RTM (Rotterdam, Netherlands) says it will introduce the world’s first hydrogen-electric international passenger route from London to Rotterdam using ZeroAvia-modified aircraft. And Rose Cay, a Delaware investment firm, plans to buy up to 250 preowned aircraft, convert them to ZeroAvia powertrains, and lease them to operators beginning in 2024.
On December 10, 2020, a de Havilland Canada Beaver floatplane rose from the Fraser River in Vancouver BC. The 10-minute flight was a first for a scheduled airline because the plane, operated by Harbour Air Seaplanes, was powered by a 750-hp electric aircraft motor from magniX. Harbour Air founder, CEO & Test Pilot Greg McDougall plans to convert the company’s 50-odd floatplanes to electric power as soon as he can get STCs for them.
Harbour Air is on track to become the world’s first all-electric scheduled airline. Second could be Loganair, which serves the Scottish Highlands and Islands, including Orkney. The company plans to convert its 8-passenger Britten-Norman Islanders to electric power.
That 2 little short-range airlines are leading the transition to electric power says something. So far, electric motors are best suited to small planes and brief flights. Harbour Air’s flight was powered by lithium-ion batteries, as Loganair’s planes probably will be.
The best now available hold less than 300 Wh/kg, compared with 12 kWh/kg of kerosene. Carry batteries for a long flight, and your payload shrinks to nothing. Batteries also deteriorate rapidly. After 1000 charging cycles, most lithium-ion batteries hold only half as much energy as when they were new.
Replacing them would burn capital fast. Electrics offer more than emission-free flight. The magniX motors, with few moving parts and little vibration to fatigue them, are expected to go 10,000 hours before needing significant work.
Greg McDougall expects them to save $1 million each over their operating life. It’s a big incentive to go electric. Better batteries are on the way. Magnesium, sulfur, vanadium, iron, and many other elements have found their way into new batteries.
So have new electrolytes and refined electrode structures that store more energy per battery weight, and prolong the unit’s operating life. Lithium-ion batteries now in the lab hold up to 400 Wh/kg. Other chemistries are said to store even more.
It seems clear that tomorrow’s batteries will be cheaper, more durable, and more energy-dense than any now available. As a result, tomorrow’s electric airplanes will fly further and faster than current examples. Yet, tomorrow’s game-changer may not be battery chemistry, but battery structure.
Several research groups are working on batteries strong enough and stiff enough to replace some of the metal that makes up an airplane. Even if they lack the storage capacity of more conventional batteries, displacing some of the airplane’s metal will reduce the weight penalty of battery-electric power.
Probably the best so far comes from MIT. The 2 structural outer layers of carbon fiber, sintered into a kind of ceramic, serve as the electrodes. The positive side is doped with lithium-iron phosphate. Sodium trisilicate waterglass, an inorganic adhesive, binds the structure together.
It is strong, rigid, and heat stable, and it easily passes lithium ions moving from one electrode to the other. The resulting battery is sturdy enough to replace aluminum in lightly stressed parts, can be molded as needed, and holds 93.9 Wh/kg – less than other batteries but about 4 times as much as the present second-place contender.
Even without advanced batteries, the race to market battery-electric aircraft is well under way. Ignoring urban air taxis, a market they dominate, here are a few leading contenders:
• Sweden’s Heart Aerospace is developing a 19-seat aircraft with 4 electric motors of 536 hp each. It will be powered by lithium-ion batteries. The company says it will begin deliveries in 2026. The aircraft’s planned range is only 215 nm, but that is enough for many markets. Heart Aerospace has signed a letter of intent with Finnair, and has lesser commitments from 8 other carriers including Air Greenland and SAS.
• Los Angeles-based Wright Electric is working with British carrier EasyJet on the M-Star, a 180-passenger V-tailed aircraft with a range of 300 miles. Power is by fans mounted inside full-chord ducts atop the wing and between the tails. They report that a high-net-worth individual wants one as his 5th private jet.
• Eviation’s 9-passenger Alice has evolved since its prototype was announced in 2017. The production version is wider than the original, and wing-mounted engines have been replaced by 2 tail-mounted magni650 motors from magniX. Max cruise is 250 ktas.
Range is 440 nm. First flight was scheduled for 2021, but seems likely to slip until 2022. DHL Express has ordered 12 cargo models for service in 2024.
A study by Roland Berger found a rough consensus that an electric aircraft with 50-plus seats would make its first commercial flight between London and Paris around 2043.
A fully-electric aircraft that could replace an executive jet was expected after 2050. Those due dates now seem a bit too cautious. Given the progress visible all over the world, low-emission, high-efficiency aircraft using any of these new technologies should make the London–Paris run before 2035.
Once high-performance batteries reach production, more capable zero-emission airplanes will be no more than 5 years away.