6 ms·

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

> 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.

xoa 6y ago
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.
- dcustodio 6y ago
  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.
  - dathinab 6y ago
    There is also work underway on solar cells which directly produce hydrogen instead of electricity.
  - nickik 6y ago
    Or you can put it into a battery.
outworlder 6y ago
> 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.
- JAlexoid 6y ago
  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.
  - ncmncm 6y ago
    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.
    - p_l 6y ago
      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.)
liability 6y ago
> 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 https://en.wikipedia.org/wiki/Aviation_biofuel
- avmich 6y ago
  > 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.
  - liability 6y ago
    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.
    - jabl 6y ago
      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.
AtlasBarfed 6y ago
algae biofuels is what I'm hoping for.

xoa 6y ago

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.

dcustodio 6y ago
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.
- dathinab 6y ago
  There is also work underway on solar cells which directly produce hydrogen instead of electricity.
- nickik 6y ago
  Or you can put it into a battery.

outworlder 6y ago

> 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.

JAlexoid 6y ago
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.
- ncmncm 6y ago
  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.
  - p_l 6y ago
    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.)

liability 6y ago

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

avmich 6y ago
> 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.
- liability 6y ago
  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.
  - jabl 6y ago
    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.

AtlasBarfed 6y ago

algae biofuels is what I'm hoping for.