3 ms·

It can make sort of sense for applications for which the energy density of the fuel (like for airplanes or rockets) matters a lot. The energy density of current

It can make sort of sense for applications for which the energy density of the fuel (like for airplanes or rockets) matters a lot. The energy density of current generation lithium ion batteries is insufficient to allow for long-range flights. Synthetic fuels could allow planes to still travel around the world while being effectively net zero emitters because the CO_2 was harvested from the atmosphere (or another, unavoidable CO_2 source). The production of the fuel might be inefficient but at least it would work.

yholio 5y ago
There is already a good solution for planes: ammonia, manufactured efficiently from green hydrogen. You will never be able to use gaseous or liquid hydrogen on planes due to the high volume, weight and risk associated with known storage methods. Ammonia seems to strike the right balance: https://www.ammoniaenergy.org/articles/zero-emission-aircraft-ammonia-for-aviation/ https://www.ammoniaenergy.org/articles/zero-emission-aircraf...
- _Microft 5y ago
  How are they planning to work around ammonia's toxicity problem? It's fatal if breathed in at concentrations like we currently have CO_2 in the atmosphere (> ~400ppm). If a tank with conventional jetfuel or methane springs a leak or has to be dumped, you've got a huge problem. If that happens with an ammonia tank, you are most likely toast either way.
  - ncmncm 5y ago
    Millions of tons of ammonia are produced every year. It is the favored refrigerant in industrial sites, such as warehouse-sized freezers. We have most of a century's experience handling ammonia in such quantities. But ammonia is probably a better choice for fueling trains and ships than for aircraft.
ncmncm 5y ago
Liquid hydrogen LH2 electrically synthesized on-site at airports from excess peak production will be important in aviation after maybe 2035 (provided civilization doesn't collapse first). Aside from LH2's usefully extreme energy density, pumping less CO2 directly into the stratosphere would be good. We may reasonably expect production of the needed aerogel-insulated LH2 tankage to be mature by then.
- MauranKilom 5y ago
  Asking as someone only remotely familiar with hydrogen as a fuel: Don't you have the problem of tanks and pipes becoming brittle from hydrogen diffusing into the material? Are there existing/upcoming solutions for this? I was under the impression that this is a pretty fundamental problem with hydrogen being just a proton essentially..
  - ncmncm 5y ago
    Aluminum is supposed to be pretty tolerant of extra protons in its matrix. You might passivate the surface to be electropositive, to repel the ends of the H2 molecules as they try to weasel into the gaps. Liquified hydrogen is anyway much less mobile than gaseous hydrogen, and when used at atmospheric pressure, nothing is trying to force the molecules in. Gaseous H2 at the extreme pressure people try to use is a much greater nuisance.
- _Microft 5y ago
  You need to consider the volumetric energy density, not the specific energy (which is energy per mass). LH2 has a high specific energy but pretty low volumetric energy density which means that much larger tanks are needed to store the same amount of energy than for other fuels. The conditions for storage of LH2 and demands on materials in contact with LH2 are much harder than for e.g. methane.
  - ncmncm 5y ago
    Low mass density means you can't keep the LH2 in wing tanks. Inboard tankage might be unwise. Slinging tanks under the wings, alongside the engines, minimizes plumbing. It might seem like that would add a lot of drag, but aerodynamics is a very unintuitive science.

yholio 5y ago

There is already a good solution for planes: ammonia, manufactured efficiently from green hydrogen. You will never be able to use gaseous or liquid hydrogen on planes due to the high volume, weight and risk associated with known storage methods. Ammonia seems to strike the right balance: https://www.ammoniaenergy.org/articles/zero-emission-aircraft-ammonia-for-aviation/ https://www.ammoniaenergy.org/articles/zero-emission-aircraf...

_Microft 5y ago
How are they planning to work around ammonia's toxicity problem? It's fatal if breathed in at concentrations like we currently have CO_2 in the atmosphere (> ~400ppm). If a tank with conventional jetfuel or methane springs a leak or has to be dumped, you've got a huge problem. If that happens with an ammonia tank, you are most likely toast either way.
- ncmncm 5y ago
  Millions of tons of ammonia are produced every year. It is the favored refrigerant in industrial sites, such as warehouse-sized freezers. We have most of a century's experience handling ammonia in such quantities. But ammonia is probably a better choice for fueling trains and ships than for aircraft.

ncmncm 5y ago

Liquid hydrogen LH2 electrically synthesized on-site at airports from excess peak production will be important in aviation after maybe 2035 (provided civilization doesn't collapse first). Aside from LH2's usefully extreme energy density, pumping less CO2 directly into the stratosphere would be good. We may reasonably expect production of the needed aerogel-insulated LH2 tankage to be mature by then.

MauranKilom 5y ago
Asking as someone only remotely familiar with hydrogen as a fuel: Don't you have the problem of tanks and pipes becoming brittle from hydrogen diffusing into the material? Are there existing/upcoming solutions for this? I was under the impression that this is a pretty fundamental problem with hydrogen being just a proton essentially..
- ncmncm 5y ago
  Aluminum is supposed to be pretty tolerant of extra protons in its matrix. You might passivate the surface to be electropositive, to repel the ends of the H2 molecules as they try to weasel into the gaps. Liquified hydrogen is anyway much less mobile than gaseous hydrogen, and when used at atmospheric pressure, nothing is trying to force the molecules in. Gaseous H2 at the extreme pressure people try to use is a much greater nuisance.
_Microft 5y ago
You need to consider the volumetric energy density, not the specific energy (which is energy per mass). LH2 has a high specific energy but pretty low volumetric energy density which means that much larger tanks are needed to store the same amount of energy than for other fuels. The conditions for storage of LH2 and demands on materials in contact with LH2 are much harder than for e.g. methane.
- ncmncm 5y ago
  Low mass density means you can't keep the LH2 in wing tanks. Inboard tankage might be unwise. Slinging tanks under the wings, alongside the engines, minimizes plumbing. It might seem like that would add a lot of drag, but aerodynamics is a very unintuitive science.