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If you're assuming hydrogen is green, you may as well assume you can simply make jet-fuel equivalent biochemicals. Right now most of the hydrogen comes from nat

If you're assuming hydrogen is green, you may as well assume you can simply make jet-fuel equivalent biochemicals. Right now most of the hydrogen comes from natural gas, so that's not so helpful. But fertilizers that might make biochemicals come from hydrogen that comes from natural gas. And like hydrogen, with lots of infrastructure costs, shipping biological feedstocks and fertilizers around also causes emissions and lowers energy returns. I would not bet on hydrogen for air travel, unless you mean lighter than air craft then maybe with careful design. For planes there's enough existing bio-based options that are simply liquids and much easier to handle. Batteries probably make more sense than liquid fuel in a car if you're assuming the liquid fuel comes from renewables. If that's the case you might as well use the starting energy (electricity) and use it directly rather than converting to liquids and then back again.

_hypx 4y ago
Neither biofuels nor batteries will ever make sense for airplanes. In the former, it is because it is basically an inefficient way of turning carbon dioxide into fuel. If we need liquids for airplanes, it will almost certainly be some kind of e-fuel or synfuel. In the latter, it is because the energy is well over an order of magnitude too low. It's pretty much impossible for something that resembles a conventional battery to work. Not even the theoretical limits of conventional batteries will make for a worthwhile plane. So battery planes will pretty much always been short-ranged aircraft that can't replace jetliners. As a result, the future of air travel is guaranteed to either be some kind of e-fuels/synfuels, or hydrogen itself. There are no credible alternatives unless you're imagine nuclear powered planes.
- pfdietz 4y ago
  > There are no credible alternatives unless you're imagine nuclear powered planes. Do you consider power beamed by laser from space to not be credible, even into the unlimited future?
  - _hypx 4y ago
    And if the beam fails, everyone is expected to die? Nevermind the feasibility of such a beam. These are highly implausible ideas. On the order of aerial battery swapping. You can imagine such things, but they won’t happen in any sort of practical way. Far less than just figuring out a greener fuel.
    - pfdietz 4y ago
      You could select a wavelength that doesn't penetrate the troposphere. Why is this implausible? Mere assertion of incredulity is not sufficient, especially at long time horizons.
      - _hypx 4y ago
        A wavelength that doesn’t penetrate the troposphere will make it impossible. You don’t seem to take your own idea very seriously… You are confusion implausible with impossible. Sure, it is not “impossible.” It is just absurdly unlikely. I mean seriously, you will need lasers on the order of hundreds of MWs in power, beaming to tens of thousands of planes flying at any given time. It doesn’t take much effort to realize the sheer cost and unreliability of such an idea. And your appeal to “long time horizon” is just an excuse to not think through the issue. At long time horizons, other ideas become viable, and ones that are much more practical.
        pfdietz 4y ago
        The aircraft could get to altitude on batteries, then switch over to beamed power. You need to expand your imagination when you're trying to rule out a technology: you are arguing all possible ways to do it don't work. This is difficult task and one you are not taking seriously. Facile dismissals of the kind you are spouting here are not sufficient. You are the one who is writing checks his thinking can't cash.
        _hypx 4y ago
        Now it becomes doubly complex. The is both an self propelled vehicle while also able absorb many MWs of power from lasers or whatever. Sorry, but you are one fantasizing here. Everyone can imagine what this is. You're the one who actually thinks this is possible for a real aircraft. Again, the alternative is simply figuring out a greener fuel. So no matter how much you insist that over a long enough time horizon that this is possible, it will be quickly displaced by far more plausible ideas first. As a result, it's highly unlikely to happen on any time frame.
- Schroedingersat 4y ago
  Next gen biofuels are ~1-2% efficient rather than <0.1%. At that point land isn't as much of a constraint. My money is on a biofuel/synfuel hybrid in the short term. Pyrolised/digested bagasse or some form of grass with electrolysed hydrogen used to modify hydrocarbons and increase energy yield.
pfdietz 4y ago
If one is making biofuel based jet fuel, one can roughly double the amount of fuel per unit of biomass by adding extra hydrogen. Cellulose is basically (CH2O)n; the overall reaction is (CH2O)n + nH2 --> (CH2)n + nH2O. The carbon in existing waste streams in the US would be more than enough to satisfy existing US jet fuel demand.
narrator 4y ago
Zero Avia[1] is already working on hydrogen retrofits for existing planes. You can produce and store it via electrolysis directly at the airports. You won't have to pipe it around. Unlike a car charging network, you'll only need production and storage at a small number of airports instead of thousands of charging station, so it's far more practical for that application. Biofuels use a lot of fossil fuel derived fertilizer and require a lot of energy to distill into jet fuel. It might be a stop gap solution, but not a long-term one especially since people could eat the crops grown where biofuel crops are grown. [1] http://zeroavia.com http://zeroavia.com

_hypx 4y ago

Neither biofuels nor batteries will ever make sense for airplanes. In the former, it is because it is basically an inefficient way of turning carbon dioxide into fuel. If we need liquids for airplanes, it will almost certainly be some kind of e-fuel or synfuel. In the latter, it is because the energy is well over an order of magnitude too low. It's pretty much impossible for something that resembles a conventional battery to work. Not even the theoretical limits of conventional batteries will make for a worthwhile plane. So battery planes will pretty much always been short-ranged aircraft that can't replace jetliners. As a result, the future of air travel is guaranteed to either be some kind of e-fuels/synfuels, or hydrogen itself. There are no credible alternatives unless you're imagine nuclear powered planes.

pfdietz 4y ago
> There are no credible alternatives unless you're imagine nuclear powered planes. Do you consider power beamed by laser from space to not be credible, even into the unlimited future?
- _hypx 4y ago
  And if the beam fails, everyone is expected to die? Nevermind the feasibility of such a beam. These are highly implausible ideas. On the order of aerial battery swapping. You can imagine such things, but they won’t happen in any sort of practical way. Far less than just figuring out a greener fuel.
  - pfdietz 4y ago
    You could select a wavelength that doesn't penetrate the troposphere. Why is this implausible? Mere assertion of incredulity is not sufficient, especially at long time horizons.
    - _hypx 4y ago
      A wavelength that doesn’t penetrate the troposphere will make it impossible. You don’t seem to take your own idea very seriously… You are confusion implausible with impossible. Sure, it is not “impossible.” It is just absurdly unlikely. I mean seriously, you will need lasers on the order of hundreds of MWs in power, beaming to tens of thousands of planes flying at any given time. It doesn’t take much effort to realize the sheer cost and unreliability of such an idea. And your appeal to “long time horizon” is just an excuse to not think through the issue. At long time horizons, other ideas become viable, and ones that are much more practical.
      - pfdietz 4y ago
        The aircraft could get to altitude on batteries, then switch over to beamed power. You need to expand your imagination when you're trying to rule out a technology: you are arguing all possible ways to do it don't work. This is difficult task and one you are not taking seriously. Facile dismissals of the kind you are spouting here are not sufficient. You are the one who is writing checks his thinking can't cash.
        _hypx 4y ago
        Now it becomes doubly complex. The is both an self propelled vehicle while also able absorb many MWs of power from lasers or whatever. Sorry, but you are one fantasizing here. Everyone can imagine what this is. You're the one who actually thinks this is possible for a real aircraft. Again, the alternative is simply figuring out a greener fuel. So no matter how much you insist that over a long enough time horizon that this is possible, it will be quickly displaced by far more plausible ideas first. As a result, it's highly unlikely to happen on any time frame.
Schroedingersat 4y ago
Next gen biofuels are ~1-2% efficient rather than <0.1%. At that point land isn't as much of a constraint. My money is on a biofuel/synfuel hybrid in the short term. Pyrolised/digested bagasse or some form of grass with electrolysed hydrogen used to modify hydrocarbons and increase energy yield.

pfdietz 4y ago

If one is making biofuel based jet fuel, one can roughly double the amount of fuel per unit of biomass by adding extra hydrogen. Cellulose is basically (CH2O)n; the overall reaction is (CH2O)n + nH2 --> (CH2)n + nH2O. The carbon in existing waste streams in the US would be more than enough to satisfy existing US jet fuel demand.

narrator 4y ago

Zero Avia[1] is already working on hydrogen retrofits for existing planes. You can produce and store it via electrolysis directly at the airports. You won't have to pipe it around. Unlike a car charging network, you'll only need production and storage at a small number of airports instead of thousands of charging station, so it's far more practical for that application. Biofuels use a lot of fossil fuel derived fertilizer and require a lot of energy to distill into jet fuel. It might be a stop gap solution, but not a long-term one especially since people could eat the crops grown where biofuel crops are grown. [1] http://zeroavia.com http://zeroavia.com