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Comment by scythe | Hacker News Reader

You can't simply ignore that liquid hydrogen is stored at 700 bar. That increases the system weight substantially. Direct ammonia fuel cells have competitive system weights even though NH3 is only 17% hydrogen:

https://www.hydrogen.energy.gov/pdfs/review19/arpae11_abbasi...

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That document refers to compressed, not liquified hydrogen. I don't know of any reason to keep LH2 under extreme pressure, other than to increase its boiling point to a more comfortable temperature, or to stiffen tank walls.

The last would probably be a consideration mainly for rockets.

The critical point of hydrogen is 33.2 K, that is Kelvin, or -240 degrees celsius. Any higher temperature than that, hydrogen is not a liquid, and exerts pressure on the walls of its container. ... so, you have the choice of making a very strong container, or a very well-isolated one, preferably both. But probably not a lightweight container, which is what you'd prefer.

I thought that writing "liquified hydrogen" would adequately suggest that I was talking about hydrogen in liquid phase; maybe I needed to be clearer, and say that liquified hydrogen really is liquid, and not gas, but I still don't see why.

Aerogel is both very, very light in weight, and also extremely effective at isolating substances of very different temperatures such as LH2 and aircraft parts. Thus, practical LH2 tanks on aircraft would be insulated with aerogel.

Given a tank of LH2 insulated with aerogel, there would be no need for the tank to be "very strong", as it would not need to hold back any more pressure than the (very light) weight of the column of LH2 contained. Liquified-gas tanks are routinely equipped with fart valves to relieve any pressure that arises from heat leaking in and boiling the contained fluid.

I think terrorism fears will keep ammonia far away from use as a fuel. Isn't ammonia / ammonium nitrate used in some home-made IED designs?

Ammonium nitrate is a common, cheap explosive, but the key to explosives is nitrate, not ammonia. Almost every practical high explosive is a nitrate or an organic nitro compound.

Major obstacles to ammonia fuel include the smell and its corrosiveness. Only a small amount (~12%) of energy is lost converting hydrogen to ammonia, and you save energy by avoiding compression/refrigeration.

Ammonia is used industrially, today, by the millions of tons, mostly in pure liquid form. Management of large quantities of liquid ammonia is very mature, and has been for most of a century.

Tens of thousands of warehouse-scale refrigeration systems use ammonia as refrigerant. It is the preferred refrigerant in most places besides homes, small offices, and road vehicles.

Liquified ammonia is the odds-on favorite to replace bunker oil in intercontinental shipping, as engines already in place could easily be converted to burn ammonia just by replacing tankage and plumbing, which would not require any structural changes. You need a larger space for the ammonia than for bunker oil, but ships have plenty of room.

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