Okay — but there’s no harm to the environment. You’re just not improving it.
The grandparent claimed this was a “fig leaf”: instead, you made an ideological post about how we need to not do even environmentally neutral things — that doesn’t make carbon neutral air travel a “fig leaf”.
Like oil, natural gas and coal which is top 3 of our energy sources worldwide. Together in 2021 they amounted to 130 000 out of 160 000 TWh of energy produced.
Wasting energy is bad. Even if we actually used synthetic fuel. Even if we actually moved to 100% renewable energy sources - we'd still have limited amount of energy. And before we do - it's just PR.
What's missing from all this is how stunningly impractical the route from atmospheric CO2 to aviation fuel actually is. It's multiple GJ/tonne just to reverse atmospheric entropy and get the CO2 where you want it, and that's before you have to climb backwards up combustion's enthalpy cliff to turn it back into usable fuel. But the capture cost is a rounding error, even at 1GJ/tonne.
People often have the wrong mental model of how energy dense aviation fuel actually is. A 747 burns 4 litres of fuel per second. Do the maths on aviation fuel energy content, and that comes out in the order of 120MW. Megawatts. With an M. 10^6. Those small modular reactors everyone's excited about? One of those. That's the thermodynamic range anything proposed has to operate in.
Ok, so divide that by the number of passengers (say 400 to make the maths easy) and it's 300KW per passenger. Multiply by 9 for our fancy new supersonic plane: 2.7MW per passenger. Concorde had 128 seats; let's say ours has 100. 270MW for the whole thing.
Now, a transatlantic flight might be 10 hours. Assume a spherical cow in a vacuum and 270MW x 10h = 2.7GWh. Say two flights per day doubles it, but they're only 5 hours each supersonic, so halve back down and that cancels out. Round to 3GWh to make the maths easy. In joules: 11TJ. With a T. 10^12.
Now, we need to fuel this thing. Which means we need to convert enough energy into enough fuel. If we completely ignore the energy cost of actually capturing the carbon, 11TJ is the absolute lower bound. It will take more than that, but in the interests of generosity let's put that to one side. 86400 seconds per day gives us 127MW.
So the good news is that the total energy requirements for this one aircraft to run on fuel made of already captured carbon are within the bounds of what we know how to build. You can, if you are a government, choose to buy these things. The bad news is that you need one per aircraft, for the eventual ability to fling 100 meatsacks across the Atlantic a bit faster.
I would suspect, looking at the costs (in terms of energy, money, space) that this is not a choice a rational actor would make.
Now, leaving aside the hypothetical supersonic jet planes, we still have this problem with the existing airplanes. If we want to decarbonize aviation, then how do we go about this?
On one hand, maybe 747 is an old plane and not that fuel optimized. I checked for 787-9, and it burns only 1.5 liters per second, which translates into 55 MW. It can only carry about 300 passengers vs 520 for a 747, so overall there is an improvement by about 33%. It's not nothing, but it's not that impressive either.
Anyway, with whatever fleet of commercial airplanes the world has now, some old, some new, we consume about 100 billion gallons of fuel per year. That's about 300 million tons.
Let's say we switch to hydrogen somehow. Which has more than double the energy density of jet fuel. We would need about 150 million tons H2 per year. According to the DOE [1], a 1GW nuclear power plant can produce about 150 kT of H2 per year, so we'd need 1000 of them. The whole world produces about 400 GW from nuclear reactors, we'd need to multiply the current fleet by a factor of 2.5, just to decarbonize air travel.
Yep. It's better than nothing, but we'd need to find orders of magnitude from somewhere. I don't think they're there to be found, personally: keeping big lumps of metal off the ground with aerodynamic lift runs into fundamental physical constraints relatively quickly (you can do the maths on glide slope and gravitational potential energy - just by rough order of magnitude it's mindboggling that it works at all).
And you're right, we do have this problem right now. Anything that wants to keep our travel patterns as they are today needs to confront the reality that its energy requirements make any other consideration a rounding error. From my point of view the big picture is fusion or bust, but that's, uh, a bit of a long shot.
The good news is that air transport emissions are only about 2% of the total, so there are much more productive lower-hanging fruit to tackle.
Thinking a bit more, there's nothing special about airplanes. The world consumes about 100 million barrels of oil per day. I guess most is transportation, but in the end it does not matter. There are about 7.5 barrels to a ton, so about 13 million tons daily, which is very nearly 5 GT per year. Each ton of oil produces about pi (3.14) tons of CO2 (of course pi is just a coincidence, but it's easy to remember). So about 15 GT of our worldwide 50 GT greenhouse gas emissions come from burning oil.
How are we going to replace that?
5 billion tons per year means about 150 tons per second. At about 40 megajoules/kg, that's 6000 GW. Six thousand large (1 GW) power plants.
You say it's fusion or bust. What if we had fusion today? Commercial, economical fusion. As easy to build as natural gas power plants. How are we going to build 6000 large such power plants?
The grandparent claimed this was a “fig leaf”: instead, you made an ideological post about how we need to not do even environmentally neutral things — that doesn’t make carbon neutral air travel a “fig leaf”.