Jet fuel smells like itself: oil, chemicals, potential. In a glass, it resembles vodka, or sometimes a pale tequila. It is less heavy than water, cheaper by weight than milk, and harder to set alight with a flaming match than ethanol. Where kerosene is more refined than diesel, so jet fuel is more refined than kerosene. For chemists, it is a jumble of Cs and Hs, and the occasional sulfurous S; for mechanics, it is something to avoid splashing on your skin. For the aviation industry, it is a puzzle.
That’s because this unglamorous substance is the best thing we’ve yet found to power the planes that allow us to cross the earth. It’s not the only thing—planes will run on many different kinds of fuel—but it is, for now, the best balance between cost and energy density.
For the planet, jet fuel is not the best option. The process of first extracting it from the earth and then refining it is exceptionally bad for the environment—burning it, meanwhile, pumps thousands of tons of carbon dioxide into the air every year.
Short on alternative immediate solutions, the aviation industry has mostly focused on improving fuel efficiency or avoiding switching on the engine wherever possible. It’s not unusual, for instance, for aircraft to taxi over to the runway using a self-driving device or to plug into electrical ground power at the gate. It helps a little, but doesn’t get us away from the fact that 1 kilogram (2.2 pounds) of conventional aviation fuel will generate 3.16 kilograms of carbon dioxide. (When burnt, the carbon in the fuel reacts with oxygen in the air to produce much heavier carbon dioxide.)
While we probably won’t run out of oil any time soon, recent UN regulations capping aviation emissions at 2020 levels will kick in in 2021, even as the number of passengers worldwide is likely to continue to rise. A decade ago, IATA, the aviation industry association, committed to reducing its 2005 level of emissions by half by 2050. Raising ticket prices will help, by reducing demand. The sector’s purchase of carbon offsets will also rise, and eventually become routine. But in the long run, if we want to continue to fly, we’ll need better solutions.
Thankfully, there are some. We may eventually see two types of planes altogether. Hybrid or fully-electric planes could be deployed for short-haul journeys, but for longer trips, we’ll need better fuels, ideally at comparable prices to jet fuel. Even as these solutions are not currently in our grasp, they may be only a few decades, or even years, away.
We already have electric cars, boats, buses and trains—why shouldn’t we have electric planes, too?
At the end of the day, it’s literally more of a lift: hauling thousands of tons of machinery into the air is much, much more energy intensive than simply pushing it along the ground. Add a need to travel farther and faster than ground-vehicles, and you’re left with a much greater engineering puzzle.
The solution is in batteries. Since the early 1990s, the lithium-ion battery has become the gold standard for batteries. They’re light, reasonably inexpensive, and one of the best ways we’ve found so far to store energy. But, as things currently stand, they aren’t really powerful enough for all but the very tiniest of electric planes. An 180-seater plane making the 1000-kilometer (621 mile) trek between New York and Indianapolis, or Beijing and Shanghai, would need a battery with approximately four times the energy density of even the very best battery on the market.
That’s why, in the immediate future, developers are setting their sights on more achievable targets: journeys of a couple of hundred miles, tops.
Happily, they’re making progress. Some prototypes are already certified for test flights while others are not far off. With the possible exception of the fossil fuel industry, it’s in everyone’s interests to find a way to make these electric planes work. They’re better for the planet, sustainable (so long as they’re powered by renewable energy), and much quieter, potentially making moot current noise-related laws limiting the hours in which planes can take off and land.
Given the difficulties involved in powering flight with electricity, manufacturers have a few different options. Some are focusing exclusively on smaller, air taxi-style planes with just a few seats. A $4 million electric nine-seater unveiled by Israeli startup Eviation Aircraft, for instance, took a “double-digit” number of orders at this year’s Paris Air Show.
Other manufacturers are betting on hybrid planes, where jet fuel gets the thing into the sky, then batteries kick in to power it along. Then there’s the riskiest approach of all: trying to revolutionize battery technology altogether. (Some manufacturers are exploring a combination of all of the above.)
There’s scope to put electric planes to use in areas outside of commercial aviation. The all-electric, two-seater plane currently being tested by Colorado-based Bye Aerospace is primarily intended for use in training pilots. High training costs are one significant contributor to the US pilot shortage CEO George Bye explains: Small electric trainers would help bring down the expense of getting trainee pilots their 1,200 hours’ experience. “The electricity cost that’s stored up in the aircraft batteries—the transfer of that energy into the batteries amounts to $4 or $5 of electricity per flight hour,” says Bye. Comparable legacy crafts may use $50 to $70 of fuel in an hour. The developmental prototype is already flying; now, the next step is seeking FAA certification. A four-seater plane is also in development.
The bigger these planes become, Bye says, the harder the task of all-electric flight. Happily, he says, “battery technology is getting better.” He cites Bye Aerospace’s current research project with OXIS Energy into a lithium-sulfur battery that could potentially double the energy density of traditional lithium-ion batteries. (These are the sort of batteries used by Airbus in its record-breaking, autonomous solar-powered craft.)
“The pathway to the future, for a faster airplane that can carry more people,” says Bye, “is enabled as these new generations of battery technologies are ready to go.”
Competitor ZeroAvia, based in Hollister, California, thinks hydrogen may be the answer. The crafts wouldn’t actually be fueled by famously explosive hydrogen, of course. Instead, CEO and physicist Val Miftakhov explains, you should think of hydrogen as a way to store energy. The company uses renewable electricity to generate hydrogen through electrolysis. The hydrogen is then loaded onto the planes and once in flight, it reacts with the air to produce electricity.
He describes the hydrogen as a mid-point: electricity goes in, electricity comes out. The cell would be as much as four times as energy dense as traditional batteries.
ZeroAvia flight testing began in February, with a six-seater prototype weighing around two tons. (The eventual commercial target is 19 seats.) The company believes it is the largest zero-emission aircraft in the world, with the potential to be used for short hops of about 300 miles between San Francisco and Los Angeles or Edinburgh and the Orkney Islands.
Not everyone believes a zero-emission model is the most realistic. Wright Electric, also based in California and headed by CEO Jeff Engler, is exploring a hybrid model, where it pairs lithium-ion battery technology with jet fuel to produce 100-seater planes by 2030. “Think of it like a Toyota Prius,” Engler says, “where it still uses jet fuels, but because it’s a more efficient design, it uses less fuel per mile than a typical airplane.” The company is currently working with easyJet.
The challenges are daunting, Engler says, but he’s hopeful about a future in which multiple technology companies come together to try to push the industry forward. “We’re also seeing incentives from the government,” he says, citing Heathrow’s promise to waive landing fees for electric or hybrid-electric planes for the first year of flight. “And Norway announced that they want all their short flights to be zero emissions by 2040. Then, of course, there’s tons of government support financially, around the world.”
Ultimately, the winners in electric fight are likely to be the established giants, who are buying up startups as well innovating on their own. Boeing, for instance, acquired Virginia-based Aurora Flight Sciences in 2017: earlier this year, its autonomous electric “air taxi” completed a test flight, in which it took off vertically, hovered above the ground, and then returned to land. Airbus, meanwhile, hopes to launch its hybrid E-Fan X for the first time in 2021, in what the company calls “a giant leap towards achieving zero-emission flight over the next 20 years.”
Better than biofuels
Electric planes may be great for pilot training, short flights, or air taxis. But even the greatest EV enthusiast is unlikely to pretend they’re the solution to low-emission long-haul flight. Here, biofuels are often presented as the best option—the utopian picture of planes powered by sunflowers, or soy, or algae. Already, commercial entities such as California-based AltAiror or Fulcrum BioEnergy are supplying US airlines such as United Airlines with biofuel made from agricultural waste, to be mixed with jet fuel.
But biofuels have problems of their own. They are shockingly land-intensive, and growing enough crops to produce enough fuel for the world’s planes would use an area roughly equivalent to the entire European Union, according to a recent IPCC report. If the global population continues to rise, that same land will be needed for the far more important task of growing crops to feed people.
Synthetic fuels are a much less discussed, but arguably more promising, option. These use a series of highly sophisticated chemical techniques to effectively conjure jet fuel from thin air.
It’s not quite that simple, of course. First, carbon dioxide must be sucked out of the ambient air, using technology like that developed by Zurich-based Climeworks. “It involves using a fan to pass air over a surface containing a chemical agent that only reacts with carbon dioxide, then exposing the newly formed compound to heat,” writes my colleague Akshat Rathi, who has covered its development extensively. “That energy breaks the bond, reversing the reaction and releasing carbon dioxide, which can be stored or put to some use.”
The resulting carbon dioxide might be turned to stone, or converted to methane and used to power trucks, or injected into oil fields to produce carbon-negative oil. (It has other applications in food and agricultural science.) It can also be used to produce synthetic jet fuel.
Through the Kopernikus project, funded by the German government, researchers are exploring the best way to store renewable energy: at the moment, wind or solar power goes to waste if there’s no immediate need for it. Liquid fuel is one possible solution. Here, carbon dioxide captured by Climeworks’ technology undergoes a series of reactions, eventually creating fuels that can be directly used: gasoline, kerosene, diesel.
For now, operations are fairly limited in scope. At the moment, Kopernicus produces just 22 pounds (10 kg) a day of this synthetic fuel. Once the researchers are confident the process works, the next step, says Michael Klumpp, a project researcher from the Karlsruhe Institute of Technology, is increasing the scale of production over a period of six or so years, until they can produce as much as 44,000 pounds (2000 kg) a day. (It’s still not enough to dent the commercial demand: a single transatlantic flight might use more than twice that much fuel.)
The process is also highly energy intensive—which means it’s only “green” if the power used is entirely renewable. “This is basically the most important factor for determining the cost of the final product,” says Klumpp. “As long as the energy, the electricity, is expensive, the product will be expensive, too.”
For the time being, the technology is expensive, slow, and resolutely early stage. But researchers hope that, as with solar or wind technology, mass production will lead to a drastic drop in price.
A beam of light through the clouds
There are plenty of reasons not to be optimistic about the solutions to carbon-powered flight: They’re expensive, not scalable, relatively untested. But the commitment is there, and the incentives will grow over time.
In the earliest years of flight, many small operators used one another’s experiments to push higher, harder, faster against the limits. For an optimist, any one of these companies has the potential to be tomorrow’s Boeing. (Just 103 years ago, the monolith was a humble seaplane manufacturer, based out of a shed near Seattle.)
Jet fuel, as it currently stands, is a compromise: the best solution available, given the constraints of weight and affordability. Whatever replaces it probably won’t be perfect, either. Now, manufacturers, airlines, and regulators must work out where they can make concessions, and where they must hold firm.