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It's pretty clear that hydrogen-powered automobiles are not going to be a thing -- electric batteries work very well for that use case. There are some cases wh

by curtis 7y ago

It's pretty clear that hydrogen-powered automobiles are not going to be a thing -- electric batteries work very well for that use case. There are some cases where I think hydrogen might still work better than the alternatives. The most obvious one is aviation. There are a few battery-electric aircraft flying today but they have very short range. I don't think there's any conceivable battery chemistry that will allow anywhere near the range of hydrocarbon-powered aircraft. However, liquid hydrogen has much greater energy density than hydrocarbon fuels. It's also quite difficult to work with. But when you're talking about a vehicle the size of a 737 (much less an A-380), the economics might ultimately be favorable. It would probably require aircraft models specifically designed for liquid hydrogen -- the tanks are so large that the aircraft will have to be designed around them. That's probably a major impediment to adoption. It's possible that biofuels or synthetic fuels made from CO2 will instead replace fossil fuels in aviation, instead.

nradov 7y ago
The additional costs related to cryonics, aerodynamic drag, safety issues, and material embrittlement mean that liquid hydrogen will never be a viable commercial aviation fuel. It seems attractive in theory but ends up being totally impractical when you get into real world details. The way forward is absolutely going to be battery power for short flights and synthetic liquid hydrocarbons for longer distances.
- kortex 7y ago
  Totally agree. Airliners have a fuel fraction (fuel wt / total wt at takeoff) around 50%. Half the gross weight of the entire system is fuel. And that's with liquid, room-temp, atmospheric pressure fuel that's easy to transfer and can fit into every liquid-tight nook and cranny. Furthermore, you can pump it around for weight trim. I don't see hydrogen in any form, including hydrides, meeting these criteria. Not to mention, the entire distribution infrastructure is tooled out for liquid hydrocarbons. Synfuel is expensive compared to fossil fuel, but not dramatically so. You're currently looking at ~$0.64/L for FF jet fuel, ~$0.95/L for biodiesels, and $1.15/L for fully synthetic fuels eg Fischer-Tropsch. https://s3.wp.wsu.edu/uploads/sites/192/2018/01/Bann-Seamus-Endres-2016-17.pdf https://s3.wp.wsu.edu/uploads/sites/192/2018/01/Bann-Seamus-...
  - curtis 7y ago
    > Half the gross weight of the entire system is fuel. That's true for jet propellant, but it's not true for hydrogen. A hydrogen powered airplane would have to have a much greater volume than an equivalent airplane fueled with regular jet fuel, but it would have substantially less weight. I'm not sure how this would work out for overall performance, though -- you'd have substantially more direct drag to overcome, but on the other hand you'd have a lot less lift-related drag simply because you'd need a lot less lift. As a thought-experiment, you could imagine an aircraft roughly the size and shape of a 757 but with the weight and payload capacity of a 737. This might be an acceptable design for a cargo carrying aircraft, but realistically, a passenger-carrying aircraft would need large cylindrical tanks on the wings for safety reasons, but also with increased drag. > Not to mention, the entire distribution infrastructure is tooled out for liquid hydrocarbons. In the specific case of hydrogen-powered commercial aviation, this doesn't matter at all, because there would not be a distribution infrastructure, at least as we generally think of it. Instead, we'd need to distribute water and electricity to airports, and the hydrogen would be produced and stored on-site. The equipment to make and store liquid hydrogen wouldn't necessarily be cheap, but over any significant period of time it would likely be dominated by the cost of electricity necessary to produce the hydrogen through electrolysis. The economics of synthetic fuels for aviation are probably still better, but I think that it might be a lot closer than people imagine. If you could cheaply retro-fit existing airframes for hydrogen they'd be a lot closer still, but I don't think that's likely. It's an interesting possibility to think about though.
    - kortex 7y ago
      > realistically, a passenger-carrying aircraft would need large cylindrical tanks on the wings for safety reasons, but also with increased drag. Precisely. Any weight savings of energy density per wt of hydrogen are offset by the poor energy/volume, plus the need for cylindrical tanks for stress reasons. Wing fuel tanks are roughly wing-shaped, meaning no increase to cross section. Hydrogen would require sets of cylinders, which add walls which add weight yet don't directly aid the engineering constraints, or exist outside the current flight envelope, adding drag. Maybe some big advancement in construction methods could allow synergy of hydrogen tanks walls with aircraft structure, but you are still stuck with 1/3rd volumetric density of energy, meaning larger cross section, more drag, and more fuel usage. https://en.m.wikipedia.org/wiki/Energy_density#/media/File%3AEnergy_density.svg https://en.m.wikipedia.org/wiki/Energy_density#/media/File%3...

by curtis 7y ago

nradov 7y ago

The additional costs related to cryonics, aerodynamic drag, safety issues, and material embrittlement mean that liquid hydrogen will never be a viable commercial aviation fuel. It seems attractive in theory but ends up being totally impractical when you get into real world details. The way forward is absolutely going to be battery power for short flights and synthetic liquid hydrocarbons for longer distances.

kortex 7y ago
Totally agree. Airliners have a fuel fraction (fuel wt / total wt at takeoff) around 50%. Half the gross weight of the entire system is fuel. And that's with liquid, room-temp, atmospheric pressure fuel that's easy to transfer and can fit into every liquid-tight nook and cranny. Furthermore, you can pump it around for weight trim. I don't see hydrogen in any form, including hydrides, meeting these criteria. Not to mention, the entire distribution infrastructure is tooled out for liquid hydrocarbons. Synfuel is expensive compared to fossil fuel, but not dramatically so. You're currently looking at ~$0.64/L for FF jet fuel, ~$0.95/L for biodiesels, and $1.15/L for fully synthetic fuels eg Fischer-Tropsch. https://s3.wp.wsu.edu/uploads/sites/192/2018/01/Bann-Seamus-Endres-2016-17.pdf https://s3.wp.wsu.edu/uploads/sites/192/2018/01/Bann-Seamus-...
- curtis 7y ago
  > Half the gross weight of the entire system is fuel. That's true for jet propellant, but it's not true for hydrogen. A hydrogen powered airplane would have to have a much greater volume than an equivalent airplane fueled with regular jet fuel, but it would have substantially less weight. I'm not sure how this would work out for overall performance, though -- you'd have substantially more direct drag to overcome, but on the other hand you'd have a lot less lift-related drag simply because you'd need a lot less lift. As a thought-experiment, you could imagine an aircraft roughly the size and shape of a 757 but with the weight and payload capacity of a 737. This might be an acceptable design for a cargo carrying aircraft, but realistically, a passenger-carrying aircraft would need large cylindrical tanks on the wings for safety reasons, but also with increased drag. > Not to mention, the entire distribution infrastructure is tooled out for liquid hydrocarbons. In the specific case of hydrogen-powered commercial aviation, this doesn't matter at all, because there would not be a distribution infrastructure, at least as we generally think of it. Instead, we'd need to distribute water and electricity to airports, and the hydrogen would be produced and stored on-site. The equipment to make and store liquid hydrogen wouldn't necessarily be cheap, but over any significant period of time it would likely be dominated by the cost of electricity necessary to produce the hydrogen through electrolysis. The economics of synthetic fuels for aviation are probably still better, but I think that it might be a lot closer than people imagine. If you could cheaply retro-fit existing airframes for hydrogen they'd be a lot closer still, but I don't think that's likely. It's an interesting possibility to think about though.
  - kortex 7y ago
    > realistically, a passenger-carrying aircraft would need large cylindrical tanks on the wings for safety reasons, but also with increased drag. Precisely. Any weight savings of energy density per wt of hydrogen are offset by the poor energy/volume, plus the need for cylindrical tanks for stress reasons. Wing fuel tanks are roughly wing-shaped, meaning no increase to cross section. Hydrogen would require sets of cylinders, which add walls which add weight yet don't directly aid the engineering constraints, or exist outside the current flight envelope, adding drag. Maybe some big advancement in construction methods could allow synergy of hydrogen tanks walls with aircraft structure, but you are still stuck with 1/3rd volumetric density of energy, meaning larger cross section, more drag, and more fuel usage. https://en.m.wikipedia.org/wiki/Energy_density#/media/File%3AEnergy_density.svg https://en.m.wikipedia.org/wiki/Energy_density#/media/File%3...