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Comment by xt00 | Hacker News Reader

While air transport has dramatic challenges to switch to non fossil fuels, if we created vast high speed rail networks we could focus air traffic on the longer distance flights. As the human race approaches peak population (at least for a while) we should figure out what the long term infrastructure should be. Like next 500 years. Once we start branching out to the stars/galaxy, population will start increasing again.

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> to switch to non fossil fuels

There's no real reason to switch so long as we are using these fuels for anything else. Let air transport be the very last things changed over, we have a ton of much lower hanging fruit that we can work on first.

Heard that the cargo ship fleet is a big contributor to greenhouse gasses.

It's large overall but tiny per amount of cargo moved a distance.

I agree that there is much lower hanging fruit that we should put our primary focus on first, but that doesn't mean that we shouldn't be actively working on the problem. It is a much harder problem and likely going to be one of the last things switched regardless.

Is it really so challenging to remove fossil carbon from air travel, or is it merely expensive? With abundant renewable power you can make synthetic fuels that a turbine can burn. It might sound costly but it can be completely carbon neutral.

IIRC (though I'm not a chemist) a challenge with synthetic fuel generation is the continuous high power needed to drive those processes. Most renewables don't provide that yet (with the exception of hydro), at a sufficient level to make aviation fuel. There was a company mentioned here a while back that is trying to do that with small scale nuclear sited at airports, but that has other issues.

I'm not seeing the problem. Synthetic diesel is its own energy storage medium. If the plant was powered at night by burning its own daytime products, and powered in the day by solar, it would still be carbon neutral.

I think the problem is the need for uninterrupted high power input to the Fischer Tropsch process in order to maximize the utilization factor. You could get this in theory from a massive dedicated battery that kicked in when clouds rolled over and reduced solar output. It's doubtful that the economics of that work out.

Another problem is getting an uninterrupted supply of H2 and CO to feed the process. None of the convenient sources for those are carbon neutral, and obtaining either in a carbon neutral way (i.e. renewable electricity powered electrolysis) adds significant additional inefficiencies.

> Synthetic diesel is its own energy storage medium. If the plant was powered at night by burning its own daytime products, and powered in the day by solar, it would still be carbon neutral.

Yes, but why would you power it at night with the syn diesel you created during the day? You'd end up with at best 25% of the syn diesel you started the night with, because the Fischer Tropsch process is only 50% efficient, and running a diesel generator is at best 50%.

I'm not a process engineer of course, but my feeling is that abundant free energy changes these equations. It simply does not matter what the efficiency is when the energy input is free. Also you wouldn't burn the fuel for electricity, you'd burn it for direct heat. A lot of petroleum refinery equipment is heated by burning things that have just been made in the refinery itself. That doesn't seem very different.

Afaik the problem with air travel is not only the CO2 released from burning fuel, it's also the various byproducts getting deposited in the higher atmosphere. Synthetic fuels avoid the net CO2 Release but still have those impacts.

Yes, we approaching peak population, but local population density will change over time. Currently, the population of cities is increasing, but it can change if remote work will be more accepted(outside of IT).

I'm not sure I agree with the notion that the sole reason for continued urbanization is employment access.

I think we've reached a point where a large and growing portion of the population wants a level of amenities/services that can only be realistically provided in areas of sufficient population density.

Now, there could certainly be some shift that changes that, but I'm not sure what it would be.

> Now, there could certainly be some shift that changes that, but I'm not sure what it would be.

As I said: remote work.

You are right we need "sufficient population density". I wouldn't want to live in the country side for long term. I want to access things that only cities can provide, but I only moved to London because of job opportunities. I would happy to live in a small city if I have access to the same jobs.

If remote work would be more widespread the countries would be more decentralized.

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