Hacker News Reader

Comment by mattashii | Hacker News Reader

> It's not any inherently greener then synthetic hydrocarbon production, which can be produced from atmospheric carbon for zero net carbon impact

As I understand it, the production of complex hydrocarbons that can be used as a replacement for jet fuel is even more inefficient when compared to the production of hydrogen, as many of the known processes that start from CO2 actually use hydrogen in the process. So, although you could get energy-denser fuels, that would be at the cost of even more end-to-end efficiency.

Finally, hydrogen has a great energy / kg ratio, making it a very interesting fuel for aviation if the material science problems of hydrogen can be overcome, as it allows for longer distance flights / flights with more/heavier cargo.

View on HN

That makes sense. I guess the questions would be whether the material science can really allow us to make use of that, and how the end-to-end efficiency weights against the infrastructure/engineering greenfield costs that synthetic fuels avoid. Being able to just keep using all existing engines/storage/distribution knowledge we already have is a major economic advantage. Electric will cover most land based transportation, and we need negative carbon technology anyway, so de novo hydrogen R&D has less to spread out across. If solar/nuclear becomes cheap enough, less efficient for more convenient might still be a worthwhile trade off. But I guess that's something for experts to crunch out. Thanks for the reply.

Don't forget that wind turbines still produce at night when there is less demand. So either you use that power to pump water upstream (if you have dams) or you can use it to produce Hydrogen.

There is also work underway on solar cells which directly produce hydrogen instead of electricity.

Or you can put it into a battery.

> As I understand it, the production of complex hydrocarbons that can be used as a replacement for jet fuel is even more inefficient when compared to the production of hydrogen, as many of the known processes that start from CO2 actually use hydrogen in the process.

Sure.

Here's the thing though: we can efficiently store those complex hydrocarbons. We can't efficiently store hydrogen. It destroys containers, and can escape any container we have devised(unless in liquid form?)

Many of the proponents of electrolysis claim that renewables will make energy a non-factor(or they'll propose nuclear power, etc). If that's the case, then add another step to capture CO2 as well.

> Finally, hydrogen has a great energy / kg ratio

Yes but energy per unit of volume is terrible.

The idea here is to do basically a cryoplane. Hydrogen can be generated on demand for the planes to refuel and can be stored highly compressed for that particular use case. We're not talking about using it for GA or cars.

As for volume - there's a reason why they call them cryoplanes ;) From Airbus - Liquid hydrogen is used as fuel for combustion with oxygen.

Liquified. So, low pressure. Aerogel insulation makes the tanks practical.

Volume on an aircraft is not expensive. Weight is expensive. Aerogel is very, very light. As is LH2.

Airports can make the LH2 directly on the spot from solar and wind power, so no need to transport or refine fuels.

Cryogenic fuels also give possible efficiency boost by precooling inlet air, but that's bigger gain for Sabre-style engines (HOTOL, Skylon, etc. SSTOs) or for efficient supersonic airliners (proposed derivation of Tu-144 used liquid H2 to gain low operating costs for it's typical Ma 2 cruise speed.)

> start from CO2

Why would that route even be taken though? Biofuel for jet engines is proven technology in limited use for many years now. Let plants deal with the CO2, since they're good at it, then process the resultant vegetable oils to produce a suitable diesel or jet fuel.

https://en.wikipedia.org/wiki/Aviation_biofuel

> Let plants deal with the CO2, since they're good at it

They aren't good with it at all. Convert CO2 slowly, take space to do that, take other resources like fertilizers, require some shielding from pests and herbs, can produce massive fires... They have advantages, but it's not a silver bullet, especially when you're in a hurry.

I believe popular proposals involve the use of algae. Regardless, farming in general is something modern civilization already has a strong competency in. Synthesizing jet fuel from scratch might make sense for the USN since they have nuclear reactors and all the water they want, but I think it's fair to say our civilization in the general case struggles with generating huge amounts of clean electricity. Plants do it with solar energy on a scale we should be jealous of. And so what of they take a few months to do it? They take that long to grow the food you eat too and it's not like you starve for months waiting for them.

Society uses absolutely mind-gobbling amounts of energy, mostly from fossil fuels. There's just not enough land available to produce even a significant fraction of that from biofuels. Never mind the ecological devastation that would be caused by converting the little wilderness left to intensive biofuel cultivation.

That is not to say biofuel don't have a role to play. Agriculture and forestry wastes, municipal biowaste, etc. can all be used for energy. And yeah, it makes sense to use the available biofuel for sectors that are otherwise very hard to decarbonise, such as aviation.

Algae, well, that has been researched for decades with very little to show for it. It has great potential, but it seems very difficult to make it actually work.

But this is my point; biofuel makes a ton of sense for aviation. Biofuels would be used in cases where there is no realistic alternative, particularly with airplanes. It's more practical and likely to succeed in this domain than hydrogen, and isn't a total pipe dream like battery powered planes. In other domains, like grid power or automobiles, solar/wind/etc are the way to go (I never suggested we run power plants off biofuel!), but there is still a fuckton of coal and natural gas plants that need to be replaced. Trying to also replace jet fuel with synth fuel at the same time will only set back efforts to close coal and natural gas plants. If we already had a surplus of clean electrical energy then things would be different, but we don't. Every solar farm dedicated to the production of synth fuel is a solar farm that isn't replacing a coal plant.

I just don't see any way that synthesis of jet fuel from atmospheric carbon makes any sense for anybody but a nuclear Navy.

The problem is twofold: The first is that biofuel doesn't scale. Maybe if we shifted all biofuel production to aviation we could do it. But the market is growing over the long term. Which means biofuels will eventually stop being a solution.

The other problem is that biofuel isn't actually all that green. It still needs lots of fertilizer, pesticides, farm equipment, etc., that ultimately gives it a relatively high GHG output.

At some point, we need to bite the bullet and go with hydrogen or one of its derivative fuels (e.g. ammonia, methanol, etc.). People are constantly trying to imagine a solution that avoids this outcome, but no one has come up with anything that looks like a viable alternative.

Maximum efficiency of 2%.

Even the worst electrolysis driven by cheap solar does better.

You've forgotten that we have a limited capacity for solar cell production and those solar cells are put to better use replacing coal plants than synthesizing something plants can synthesize.

Biofuel is just very, very expensive. The only reason it is used at all is heavy, heavy subsidies, often connected with corruption. (Cf. Archer-Daniels-Midland.)

And hydrogen or synth fuel aren't expensive? If these alternatives weren't more expensive then the problem would be moot because the industry would be voluntarily using them already.

They have always been more expensive than petroleum, and anyway would be made with power generated from petroleum or coal, or (in the case of hydrogen) from natural gas.

But that was before solar and wind became substantially cheaper than petroleum.

> Let plants deal with the CO2, since they're good at it,

This is never going to happen at the necessary scale. Where do you want to put all those plants? The planet's land area that is suitable for agriculture is already being used for food production. Well, apart from a few areas with natural reserves but you really don't want to touch those if you care about the environment. And if you don't then the whole clean energy topic is not for you anyway.

Sure some areas can be made a bit more efficient, but that's never going to be enough.

Biomass fuel production averages 1 watt per square meter compared to the 100 watts per square meter that solar power gives. Before fossil fuels civilization was in many ways energy constrained, forced to make harsh tradeoffs in how much land was devoted to forests for heating/iron production and how much to farmland for food.

Speaking generally about biofuel, it's a terrible idea - it's taking farming output and turning it into fuel, which is a bad idea because 1) it's unreliable and climate-dependent, and 2) we're basically just turning oil into plants and then turning the plants back into oil.

algae biofuels is what I'm hoping for.

Reply on news.ycombinator.com