Can we make flying "Green"?

[This is a transcript of the video embedded below. Some of the explanations may not make sense without the animations in the video.]



“Flight shaming” is a social movement that originated in Sweden a few years ago. Its aim is to discourage people from flying because it’s bad for the environment. But let’s be honest, we’re not going to give up flying just because some Swedes think we should. I mean, we already shop at IKEA, isn’t that Swedish enough?

But seriously, maybe the flight shamers have a point. If aliens come to visit us one day, how are we supposed to explain this mess? Maybe we should indeed try to do something about airplane emissions. What are airlines doing anyway, isn’t it their job? What are the technological options, and will any of them give you a plausible excuse if flight shamers come for you? That’s what we’ll talk about today.

Flying may be good for watching four movies in a row, but it really isn’t good for the planet. It’s the third biggest contribution to carbon emissions from individuals, after having children and driving a car. Altogether, flying currently accounts for around 2 point 5 percent of global carbon dioxide emissions, that’s about a billion tons each year. 81 percent of this comes from passenger flights, and another 60 percent of that, so about half of the total, are international flights.

Most of the flights, not so surprisingly, come from high-income countries. If flying was as a country itself, it would rank sixth in carbon dioxide emissions, and it would congratulate the new British Prime Minister by reminding her that “The closest emergency exit may be behind you.”

The total carbon dioxide emissions from flying have been increasing steeply in the past decades, but the relative contribution has remained stable at 2 point 5 percent. This is partly because everybody is emitting more with everything, but also because planes have become way more fuel efficient. Planes consume today about half as much fuel as they did in the mid 1960’s.

Carbon dioxide emissions are not the only way that flying contributes to climate change. It also adds some other greenhouse gasses, and it creates clouds at high altitude that trap heat. But in this video, we’ll focus on the carbon dioxide emissions because that’s the biggest issue, right after the length of this video.

There are four ways that airlines are currently trying to reduce their carbon emissions, that’s electric planes, hydrogen, biofuel, and synthetic fuel. We’ll talk about each of those starting with electric planes.

1. Electric planes

The idea of electric planes is pretty obvious, charge a battery with a carbon neutral source of energy, use the battery to drive a propeller, try to not fall off the sky. Then you can partly recharge the battery when you’re landing.

And at first sight it does sound like a good idea. In 2016, the Swiss aircraft Solar Impulse 2 completed its first trip around the world. Its wings are covered with solar cells with a wingspan that is comparable to that of a Boeing 747.

A Boeing 747 typically flies at about one thousand kilometers per hour and carries 500 or so people. The Solar Impulse carries two and it flies about 70 kilometers per hour. At that speed it would take about 4 days to get from Frankfurt to New York which requires more in-flight entertainment than the CDC recommends.

You might think the issue is the solar panels, but the bigger problem is that electric batteries are heavy, and you don’t need to be Albert Einstein to understand that something that’s supposed to fly better not be heavy.

One way to see the problem is to compare batteries to kerosene, which is illustrated nicely in this figure. On the vertical axis you have the energy per mass and on the horizontal axis the energy per volume. Watch out, both axes are logarithmic.

You want energy sources that are as much in the top right corner as possible. Kerosene is up here, And lithium-ion batteries down here. You can see that kerosene has 18 times more energy in the same volume as a typical lithium-ion battery and sixty times more energy in the same mass. This means it’s difficult to pack power onto an aircraft in form of electric batteries. Consequently, electric planes are slow and don’t get far.

For example, in 2020, the Slovenian aircraft company Pipistrel brought the first electric aircraft onto the market. It’s powered by lithium-ion batteries, can carry up to 200 kilogram, and flies up to 50 minutes with a speed of about 100 kilometers per hour. It’s called Velis Electro which sounds like a good name for a superhero. And indeed, carrying 200 kilograms at 100 kilometers per hour is great if you want to rescue the new British Prime Minister from an oncoming truck, I mean, lorry. Though there isn’t much risk of that happening because the lorries are stuck at the French border. Which, incidentally, is farther away from London than this plane can even fly.

Nevertheless, some other companies are working on electric planes too. The Swedish start-up Heart Aerospace plans to build the first electric commercial aircraft by 2026. They ambitiously want to reach 400 kilometers of range and hope it’ll carry up to 19 passengers. Presumably that’s 19 average Swedes, which is about the same weight as 2 average Germans.

So, unless there’s a really big breakthrough in battery technology, electric planes aren’t going to replace kerosene powered ones for any sizeable share of the market, though they might be used, for example, to train pilots. Train them to fly, that is, not to rescue prime ministers.

A plus point of electric planes however is that they are more energy-efficient than kerosene powered ones. An electric system has an efficiency of up to ninety percent, but kerosene engines only reach about fifty percent efficiency. To this you must add other inefficiencies in the gearbox and the mechanics of the propeller or fan and so on. The total efficiency is then around 70-75 percent for electric engines and between thirty and forty percent for kerosene engines.

The technological developments that are going to have the biggest impact on electric planes are new types of batteries, that are either lighter or more efficient or, ideally, both. Lithium-sulfur and lithium-oxygen batteries are two examples that are currently attracting attention. They pack three to ten times more energy into the same mass as than Lithium-ion batteries.

2. Hydrogen

Let’s then talk about hydrogen. No, we don’t want to bring the Zeppelin back. We are talking about planes powered by liquid hydrogen. If we look back at this handy figure, you can see that liquid hydrogen really packs a lot of energy into a small amount of mass. It has, however, still a fairly large volume compared to kerosene. And volume on an airplane means you must make the plane bigger which makes it heavier, so the weight-issue creeps back in.

Also, hydrogen usually isn’t liquid, so you have to either cool it or keep it under pressure. Cooling requires a lot of energy, which is bad for efficiency. But keeping hydrogen under pressure requires thick tanks which are heavy. It’s like there’s a reason fossil fuels became popular.

The downside of hydrogen is its low efficiency, which is only around 45 percent. Still, together with the high energy density, it’s not bad, and given that burning hydrogen doesn’t create carbon dioxide it’s worth a try.

Hydrogen powered airplanes aren’t new. Hybrid airplanes that used hydrogen were being tested already in the 1950’s. Airbus is now among the companies who are developing this technology further. Just a few months ago, they presented what they call the ZEROe demonstrator. It’s a hydrogen powered engine, that will be tested both on the ground and in flight, though in the test phase the plane will still be carried by standard engines.

They recently did a 4-hour test flight for the hydrogen engine. The plane they used was an A380, that’s a two-deck plane that can transport up to eight hundred passengers or so. They used this large plane because it has plenty of room for the hydrogen tanks plus the measurement equipment plus a group of engineers plus their emotional support turkeys. However, the intended use of the hydrogen engine is a somewhat smaller plane, the A350. Airbus wants to build the world’s first zero-emission commercial aircraft by 2035.

The Airbus competitor Boeing is not quite so enthusiastic about hydrogen. Their website explains that hydrogen “introduces certification, infrastructure, and operational challenges”. And just to clarify the technical terms, “challenge” is American English for “problem”. Because of those challenges, Boeing focusses on sustainable fuels. According to its CEO, sustainable fuels are “the only answer between now and 2050”. So let’s look at those next.

3. Biofuel

Bio-fuels are usually made from plants. And when I say plants, I mean plants that have recently deceased and not been underground for a hundred million years. Bio-fuels still create carbon dioxide when burned, but the idea is that it’s carbon dioxide you took out of the air when you grew the plants, so the carbon just goes around in a cycle. This means unlike regular jet fuel, which releases carbon dioxide that was long stored underground in oil, bio-fuels don’t increase the net amount of carbon dioxide in the atmosphere.

The most common bio-fuel is ethanol, which can be made for example from corn. It can and is being used for cars. But ethanol isn’t a good choice for airplanes because it’s not energy dense enough, basically, it’s too heavy. In the figure we already looked at earlier, it’s up here.

Another issue with bio-fuel is that, to be used for aircraft, it must fulfil a lot of requirements, in particular it must continue to flow well at low temperatures. I’m not much of an engineer but even I can see that if the fuel freezes midflight that might be a challenge.

A bio jet fuel which fits the bill is synthetic paraffinic kerosene, which can be made from vegetable

oils or animal fats, but also from sugar or corn. Paraffinic kerosene is in some sense better than fossil kerosene. For example, it generates less carbon dioxide and less sulfur.

The International Airport Transport Association considers bio-jet fuel a key element to get off fossil fuels. Indeed, some airlines are already using biofuels. The Brazilian company Azul Airlines has been using biofuel from sugarcane on some of their flights for a decade. British Airways has partnered with the fuel company LanzaJet to develop biofuels that are suitable for aircraft. The American Airline United is also investing into biofuels. And the Scandinavian airline SAS has the goal to use 17 percent biofuel by 2025.

The problem, I mean challenge, is that bio jet fuels still cost three to six times more than conventional jet fuel. Moreover, researchers from the UK and Netherlands have estimated that the start-up cost for a commercial bio jet fuel plant is upwards of 100 million dollar which is a barrier to get things going.

But making the production of bio jet fuel easier and more affordable is a presently a very active research area. An approach that’s attracting a lot of attention is using microalgea. They produce a lot of biomass, and they do so quickly. Microalgae reach about three to eight percent efficiency in transforming solar energy to chemical energy, while conventional biofuel crops stay below 1 percent. Take that, conventional biofuel crops!

Algae also generate more oil than most plants, and genetic engineering can further improve the yield. A few years ago, ExxonMobil partnered with Craig Venter’s Synthetic Genomics and they developed a new strain of algae using the gen editing tool CRISPR. The gene engineered algea had a fat content of 40-55 percent compared to only 20 percent in the naturally occurring strain.

But bio fuels from algae also have a downside. They have a high nitrogen content so the fuel produced from them will release nitrogen oxides. If you remember, we talked about this earlier in our video on pollution from diesel engines. Then again, you can try to filter this out, at least to some extent.

Another way to make biofuels more affordable is to let the customer pay for it. SAS for example says if you pay more for the ticket, they’ll put more biofuel into the jet. So, it’s either more legroom or saving the planet, though times for tall people.

4.Synthetic jet fuel

Finally, you can go for entirely synthetic jet fuel. For this, you take the carbon from the atmosphere using carbon capture, so you get rid of the plants in the production process. Instead, you use a renewable energy source to produce a chemical that’s similar to kerosene from the carbon dioxide and water.

The resulting fuels are not completely carbon neutral because of the production process but compared to fossil fuels they have small carbon footprint. According to some sources, it’s about 70 to 80 percent less than fossil fuels though those number are at present not very reliable.

Synthetic kerosene is already in use. Since 2009, it can be blended with conventional jet fuel. The maximum blending ratio depends on the properties of the synthetic component, but it can be up to fifty percent. This restriction is just a precautionary requirement and it’s likely to be relaxed in the future. The problem, I mean challenge, is that at the moment synthetic kerosene is about 4 to 5 times more expensive than fossil kerosene.

Nevertheless, a lot of airlines have expressed interest in synthetic kerosene. For example, last October, Lufthansa agreed on of annual purchase of at least 25 thousand liters for at least five years. That isn’t a terrible lot. Just for comparison, a single A380 holds up to three hundred twenty thousand liters. But it’s a first step to test if the synthetic stuff works. Quantas announced a few months ago that they’ll invest 35 million dollars in research and development for synthetic jet fuel. They hope to start using it in the early 2030s.

But let me give you some numbers to illustrate the… challenge. In 2020 the market for commercial jet fuel was about 106 billion gallons globally. Twenty-one billion-gallon in the US alone. According to the US Energy Information Administration, it is expected to grow to 230 billion gallons globally by 2050.

At current, the global production of synthetic kerosene is about 33 *million gallons per year. That’s less than a tenth of a percent of the total jet fuel. Still, the International Air Transport Association is tentatively hopeful. They recently issued a report, according to which current investments will expand the annual production of synthetic kerosene to 1 point three billion gallons by 2025. They say that production could reach eight billion gallons by 2030 with effective government incentives, by which they probably mean subsidies.

So, even if we’re widely optimistic and pour a lot of money into it, we might be able to replace 5 percent of jet fuel with synthetic fuel by 2030. It isn’t going to save the planet. But maybe it’s enough to push transatlantic flight down on the sin-list below eating meat, so the Swedes can move on from flight-shaming to meat-shaming.