3O3- t1_iycl90b wrote on November 30, 2022 at 11:59 AM

Reply to comment by UnCommonSense99 in Rolls-Royce successfully tests hydrogen-powered jet engine by je97

Very cold take, H2 as approximate 3x the energy density of kerosene (120 vs 40 MJ/kg)

The problem is the size required to store it, not the mass, which is a completely separate technological issue (compression)

QVCatullus t1_iycntuk wrote on November 30, 2022 at 12:27 PM

It still goes exactly to what the post you're responding to said. They didn't claim that hydrogen had a low energy density, they said that a hydrogen tank that fit on a plane would either not have enough fuel (like, not even remotely enough -- the volumetric "energy density" of hydrogen is on the order of 1/3000 that of kerosene per atmosphere the hydrogen is stored at, so even with a high-pressure 700 bar tank you need 4ish times the fuel storage), or would have to be rated to such high pressures/low temperatures (for cryogenic storage, which is maybe not a good choice for air travel in any kind of near future) that it would be impractically heavy.

In other words, it seems like your comment is calling out a mistake in the post you're responding to, when really you're just restating it, since precisely the problem at hand the PP was referencing was the compression issue.

3O3- t1_iyezs2w wrote on November 30, 2022 at 10:11 PM

It does not at all, since the mass, not space (implied by the original post), is the limiting factor for flight. Imagine a standard commercial plane but 1/2 the seating space is now a fuel tank. There, you already have the 4x space, with no innovation in design, and with 1/3 of the fuel mass. As the original post pointed out however, what is currently limiting the application of H2, is in the mass (and general impracticality) of the current fuel compression technology.

There is definitely room for improvement in storage technology, and there are certainly no “fundamental constraints of physics and engineering” that limit to the mass of containers which store hydrogen to precisely what is currently available.

Simply, planes can be made much bigger (to accommodate the space needed to store the hydrogen even in the absence of significant advances in fuel compression technology) without being unviable (demonstrated categorically by the presence of huge commercial jets), which is already partly offset due to the huge mass savings thanks to very high MJ/kg of hydrogen.

If only The original commenter were working for Rolls Royce, they could have warned them it was useless due to the fundamental laws of physics and engineering, theyd have saved millions, and we would all have been spared this “greenwashing”

danielv123 t1_iycnhqx wrote on November 30, 2022 at 12:24 PM

Generally, by energy density we mean volumetric energy density. Specific energy is the common term for gravimetric energy density.