Nov 28 2022

The Challenge of Green Aviation

There is some good new when it comes to decarbonizing our civilization (reducing the amount of CO2 from previously sequestered carbon that our industries release into the atmosphere) – we already have the technology to accomplish most of what we need to do. Right now the world’s electricity generation is 63.3% from fossil fuels. We have the technology, through wind, solar, geothermal, hydroelectric, and nuclear power, to completely replace this if we wanted to.  We can debate the quickest and most cost-effective path, but there are many options that will work.

About 84.3% of total energy used by the world, however, is from fossil fuel. This includes not only electricity, but transportation, heating, and industrial use (other than through electricity). Of the transportation sector, 92% is ground vehicle (cars, trucks, and shipping). Battery electric vehicle technology is now more than capable of being the primary option for most users, with ranges >300 miles for passenger cars and 500 miles for shipping. Prices still need to come down, but they will as production ramps up.

Another way to look at this is that 73.2% of our carbon footprint comes from all energy, 18.4% from agriculture, 3.2% from waste, and 5.2% from direct industrial processes (like making cement and steel). Agricultural, waste, and industrial sources of carbon are complex, and these mostly require technological advances that we will hopefully chip away at over the next few decades. But we can rapidly eliminate that 73.2% from energy if we want to, with the exception of the 8% of transportation carbon from aviation. That remains a tough nut to crack.

The challenge of aviation is that jets and planes need to be light and have limits on size, so they require an energy source that has high energy density (energy per volume) and specific energy (energy per mass), more so than ground transportation. Right now the optimal fuel for those two features is hydrocarbons. This means that the best option for greener aviation is using biofuels (sustainable aviation fuel). Biofuels can be used with existing aircraft and have similar energy density and specific energy to existing fuels. The carbon footprint is usually not zero, but is much lower than fossil fuels. The carbon footprint of biofuels depends on the feedstock used and the methods of growing used. There are also land and water-use issues with mass-producing biofuels for aviation or other purposes. The best options are those that use waste feedstock.

Because of the scaling issue, biofuels are not a slam dunk. What other options for aviation are there? There is research into battery electric planes, but current battery technology suffers from poor energy density and specific energy, especially if you include all the supporting electronics. Battery powered electric jets are simply not on the table anytime soon. But a Swedish company is developing the Heart Aerospace ES-30, which is a four-propeller passenger plane that can carry 30 passengers and has a range of 200 km (124 miles). The range increases to 400 or even 800 km as a hybrid version. These can be useful for short-haul flights. This will only make a tiny dent in the carbon footprint of aviation, but still can fill a niche. As battery technology improves, that range will also improve, but even optimistic projections mean that the range of such planes will remain in the short-haul zone.

This leaves us with the only other real option being hydrogen. Hydrogen can fuel jet engines, which gets us out of the very short-haul range. Hydrogen also has its challenges, however. The primary problem is the energy density, which is about 25% that of hydrocarbon-based jet fuel. This assumes liquid (and therefore cooled) hydrogen. Compressed gaseous hydrogen is even less. Hydrogen, because it’s the lightest element, has a high specific energy, about three times that of jet fuel. That’s why it is perhaps the best rocket fuel, where specific energy is by far more important than energy density. For jet fuel, however, energy density (energy per volume) is critical. Those fuel tanks can only be so big, and increasing them by a factor of four would be challenging to say the least.  (As an aside, I see a lot of reporting that confuses energy density and specific energy, so be on the lookout for that.)

This means that commercial jets will likely have to be redesigned to accommodate larger fuel tanks, and tanks that can hold liquid hydrogen. That liquid hydrogen would then need to be fed to the jet engines, which themselves need to be redesigned to make optimal use of hydrogen as a fuel. Further, switching to hydrogen fueled jet engines is only helpful if we have a sufficient supply of green hydrogen. Right now we don’t. Most hydrogen is from fossil fuels (and is worse than just burning the fossil fuel), and some is so-called blue hydrogen, which is perhaps a little better because it incorporates carbon capture but some argue is still very bad.

All this means that hydrogen-fueled passenger jets are likely decades away. We should still develop the technology, but it’s not going to be a carbon solution anytime soon. What will an optimal low-carbon aviation industry look like eventually? It’s hard to say, because there is no perfect or optimal technology. Perhaps that means we will have a hybrid industry. We may be using battery electric planes for short-haul flights, starting at around 200km but then steadily but incrementally improving as battery technology advances. Perhaps organic solar cells will cover the tops of these planes as a modest range extender. For medium-haul flights we may eventually have hydrogen-powered jets, supported by a green hydrogen industry. For long-haul flights the only option in the foreseeable future is sustainable aviation fuel (biofuels).

Are there any disruptive aviation technologies on the horizon? Probably not. Nuclear-powered jet research was essentially abandoned in the 1950s. Nuclear batteries using short half-life isotopes may extend the range of electric planes, but we would need to develop better shielding, so not likely anytime soon. Fusion is obviously a distant hope for small aircraft like jets. There is research into using ground-based lasers to recharge electric planes in flight, and the proof of concept has already happened. Whether an infrastructure can be safely developed for commercial flight remains to be seen, as the tests have only been done on drones. Those are the only options I have seen. If any reader knows of other potential disruptive aviation technologies, let us know in the comments.

For now, there is essentially no clear solution. It will take decades to develop the hybrid aviation solutions I outlined above. Biofuels from waste feedstock is likely the best option for now.

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