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Comment by ncmncm | Hacker News Reader

Hydrogen's energy-density makes it by far the best possible fuel for aviation. Improvements in aerogel manufacturing are making liquid-hydrogen tanks a practical prospect. LH2 is not mass-dense enough to fit in the wing tanks of existing airframes, but at least the LH2 tanks won't need to be pressure vessels.

Production of LH2 from cheap solar- and wind-generated electric power and water, right at the airport, will make aviation the most practical place to start using hydrogen as a fuel. Maybe auxiliary LOX tankage could help reach cruise altitude more quickly, or enable flying higher, so as not to waste the oxygen by-product.

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Hydrogen only has a high energy density by weight, but by volume it's actually quite low compared to other liquid fuels. Then you have all the problems with handling liquid hydrogen, insulation needed, all sorts of additional functions are required of the storage system compared to jet fuel. For example, icing would probably be a problem if the wing tanks are full of liquid hydrogen!

Considering the freezing point of hydrogen is −259.14 °C, I doubt it would be a problem.

Wrong way around, they're saying the low temperature necessary to keep the hydrogen liquid will cause icing on the wings.

Ice on the outside isn't a problem for the hydrogen. It's the other way round. The weight calculations have to include enough insulation on the tanks to prevent rain from freezing to the wings.

Considering that the post you replied to specifically noted that:

(a) H2 is energy-dense but not mass-dense,

(b) wing tanks are not a practical place to carry LH2,

it makes me wonder what you think this comment adds to the discussion.

> Hydrogen's energy-density makes it by far the best possible fuel for aviation.

gasoline: 45.8 MJ/kg hydrogen: 120 MJ/kg

What about safety? I visited the Zeppelin museum in Friedrichshafen last week...

Wouldn't it be nice if turning CO2 into gasoline would enable us to re-use tall the existing infrastructure?

Zeppelins were essentially giant gaseous hydrogen balloons. That is directly flammable.

AFAIK liquid hydrogen tanks are orders of magnitude safer (as in “a totally different problem”).

Liquid hydrogen tanks are definitely a different problem, but I don't know if "orders of magnitude safer" is necessarily true. Many spacecraft have run into problems involving liquid hydrogen storage for a variety of reasons:

- Liquid hydrogen is so cold that many metals weaken

- Exposure to hydrogen itself weakens many metals (hydrogen embrittlement)

- Liquid hydrogen expands rapidly if exposed to heat, so venting is needed

- Hydrogen leaks through the smallest of seams

Now imagine trying to build a tank that prevents all that from happening, and it has to be lightweight for usage on spacecraft / aircraft.

I know nothing about it, but what about hydrogen cells ? It doesn't solve this issue ?

Hydrogen cells nothing to do with storage, it's for converting hydrogen to electricity

Oh, to me the name in french implied it stored energy. We say "Pile a Hydrogen".

In french we differenciate "batterie" and "pile", "pile" are one use, but I thought due to the name that the hydrogen cell would store energy.

The Hindenberg blew up because there was a bomb on board. No aircraft design is noted for resistance to onboard explosives.

Furthermore, the flames seen in the film were from kerosine fuel. All the hydrogen released went up, up, up. It is quite difficult to ignite hydrogen at atmospheric pressure; it requires oxygen mixed in a narrow range of partial pressures.

So, no, safety of H2 fuel is not a problem.

What is the best energy return of energy invested of hydrogen?

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