How new technologies could clean up air travel

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As a climate reporter, I sometimes hesitate to admit this, but I feel it's time that I came clean on something ... I love flying. It's not even just about traveling and seeing new places: I truly enjoy the process, from sitting in an airport terminal to sliding into a window seat. I even appreciate the elegance of a smooth trip through airport security. Unhinged, I know.

My love affair with flying and my work covering climate change feel at odds because aviation makes up about 3% of the world's greenhouse-gas emissions-almost a gigaton in 2019. Airline traffic could more than double from today's levels by 2050.

And we really don't know what we're going to do about it.

Aviation is one of those notorious hard-to-decarbonize" sectors, where the technical challenge of cutting emissions is especially steep. Fuels for planes need to be especially light and compact, so planes can make it into the sky and still have room for people or cargo.

The industry has some ideas for technologies that could cut emissions, and some are even starting to make it to test flights. Last week, a startup completed a test flight of the largest plane yet that was powered by hydrogen. So for the newsletter this week, let's take a look at the technologies that could transform aviation, how long they might take to make an impact, and where that test flight fits in.

There are a few potential technologies on the table that could help cut emissions from aviation (and help relieve my flying guilt).

Sustainable aviation fuels, or SAFs, are drop-in replacements to jet fuel made from non-fossil sources. There's a pretty wide range of SAFs out there, including those made with waste oils and fats, those derived from biomass, and fully synthetic e-fuels.

Each of these categories has its pros and cons. Waste oils are a relatively advanced technology and are even blended into commercial fuel today in small amounts, but the supply probably won't be big enough to power all of aviation. Synthetic e-fuels, on the other hand, aren't a proven technology and could be prohibitively expensive.

SAFs of one type or another are likely going to be a big piece of aviation's decarbonization strategy. They make up about 65% of the planned emissions reductions in the International Air Transport Association's 2050 decarbonization plan. But most SAFs don't cut carbon emissions to zero, and they could still lead to pollution when burned.

Batteries could power planes, at least for short distances. Some companies have been trying out test flights of electric planes powered this way, mostly small eVTOL (electric vertical take-off and landing) aircraft that can carry just a few people. Unlike combustion-powered aircraft, electric planes wouldn't produce pollution, and they could reach zero emissions if charged with renewable energy.

Batteries have the benefit of being a technology that's widely used today in electric vehicles, and they've gotten much better over their decades of development. But batteries will have to keep improving dramatically for electric planes to carry a significant number of people any significant distance. (Check out my story from last year on electric planes for more.)

Hydrogen could be a versatile fuel for aviation in the future. Planes might use hydrogen in two different ways. It could be burned in combustion engines, similar to how jet fuel is used today. Alternatively, hydrogen could be used in fuel cells, where chemical reactions generate electricity. We love to have options.

Hydrogen's environmental impact and feasibility will depend on how it's being used. Combustion will lead to some tailpipe emissions, though these would be mostly water. Hydrogen-electric planes, like aircraft powered by batteries, could be free from climate pollution depending on how the hydrogen is produced.

In either case, hydrogen has one key thing going for it: it contains a lot of energy without being too heavy (unlike batteries). When a vehicle has to lug its power source 30,000 feet into the air, it's better for that power source to be really light-and hydrogen, as the lightest element on the periodic table, fits this bill perfectly.

The problem is, while hydrogen is light, it also takes up a lot of space. In order to get it into a small enough volume to carry onboard a plane, hydrogen will likely need to be cooled to cryogenic temperatures (below -250 C). Designing these systems and getting them onto planes will be difficult. So will sourcing and distributing large amounts of hydrogen made with renewable energy. And there's the small fact that while there have been some experiments with flying hydrogen-powered planes over the years, the technology still needs work. It's hard to remake an industry, which is why SAFs, the drop-in solution, are probably the most likely to be adopted in the near future, while hydrogen will take decades to break through.

But there's been some exciting movement on using hydrogen for aviation over the past couple of years, with big players like Airbus getting into the game and announcing planned test flights.

And last week, startup ZeroAvia was in the news again, announcing it had completed a test flight of a 19-seat Dornier 228, the largest plane flown partly on hydrogen fuel cells. Before this test, the company had tested a smaller, nine-seat aircraft.

There are a few caveats with this announcement, chief among them that the plane was mostly powered by a combination of batteries and fossil fuels. Also, the test flight only lasted about 10 minutes. But the company says this is the first step to using its system in larger planes and breaking into commercial flight, a milestone it plans to reach by the end of the decade.

Check out my story on the announcement for all the details about the test flight and more info on what it would take for hydrogen planes to make an impact. And if you're curious to read more about aviation technology, here are a few stories from the MIT Technology Review vault from the past year.

• This $1.5 billion startup promised to deliver clean fuels as cheap as gas. Experts are deeply skeptical.
• How engineered microbes could cut aviation emissions
• This is what's keeping electric planes from taking off
• The aviation industry can hit its emissions goals, but it needs new fuels

Another Thing

It always seems there's a surplus of bad news on climate change. From weather disasters to new heights in greenhouse-gas emissions, there's plenty to be worried about.

But I'd argue there's some progress that we should appreciate, too. Emissions are dropping in many parts of the world, renewable energy is finding new footing globally, and other technologies that could cut emissions, like EVs, are hitting their stride.

In case you're looking for a little silver lining, I put together some data showing progress on climate change. One of my favorite bits was this chart, showing how some countries are starting to see economic growth that relies less on fossil fuels.

It's not all sunshine and rainbows: we're still moving too slowly to keep warming under international targets. But I think it's important to keep in mind the progress we are making, when we can.

Looking for more positive news? Or want to see how far we still need to go? Check out the full story.

Keeping Up with Climate

Mexico banned geoengineering experiments after a startup began dabbling in the technology. (MIT Technology Review)

My colleague James Temple broke news about the startup's work in December. (MIT Technology Review)

An alternative to batteries called thermal energy storage could help support industry and the grid. The technology may finally be ready to take off. (Canary Media)

EV batteries might actually help the grid, not harm it. If enough EV owners opt in, a distributed network of batteries could be used to smooth out supply and demand. (Wired)

Climate-change denial is making a comeback on social media. And misleading ads aren't helping. (The Verge)

Getting people to accept plant-based meat is a challenge with deep roots in human psychology. (Washington Post)

Bloomberg's take? Fake meat is just another fad." (Bloomberg)

Renewables could be 25% of the US electricity mix very soon. (Inside Climate News)

Still confused about gas stoves? I loved this walkthrough from ProPublica, in which a reporter discovers that her stove is leaking and grapples with the risks posed by the appliances. (ProPublica)