What kind of energy density? There is energy density by volume (where it matches batteries, when you ignore all the stuff you need to keep hydrogen at 700bar safe), and energy density by weight (where it wins pretty handily against pretty much everything).

Am not historically familiar with this topic though the attached paper above shows a comparative figure of Specific Energy in terms of Wh/Kg at both 5k and 10k psi

Edit: interesting to do the conversion and realize 700bar is just over 10k psi

Edit2: further interesting to note how the 5k compression of the hydrogen has _higher_ Specific Energy density than 10k due to _decreased_ pressure vessel requirements

Just be careful with Hydrogen energy density as there’s a huge range of non obvious issues. Compressed you lose a great deal of energy converting it to kinetic energy and weight due to the containers + engines. Fuel cells also need electric motors and ideally a hybrid battery.

Cryogenic fuel looks great until you need to have it sit around unused. Embrittlement, volume, cost, and a rage of safety hazards make it unappealing for aircraft or ships.

So, my guess is the ideal long term solution barring “super batteries” is some sort of synthetic hydrocarbons that uses atmospheric CO2 but we are a long way from viability there.

Haven't we tried hydrogen for air travel before? ;-)

It's been used successfully for space travel, in liquid (cryogenic) form. The Space Shuttle's main engines ran on hydrogen fuel, for example.

However the latest generation of rockets seems to have moved away from hydrogen back to things like RP-1 (kerosene) and liquid methane. The high cost and difficulties of working with hydrogen is a factor in this.

Hindenburg

Anybody who needs to run 24/7 and/or can't count on an electric grid could use hydrogen power: A logistics network, an army, public transportation, utility companies, foresting or mining...

Or they could just use the well known https://en.m.wikipedia.org/wiki/Fischer–Tropsch_process to convert it the rest of the way to a hydrocarbon, not have the pesky raw hydrogen issues, and be able to use all their existing infrastructure to burn it? With the advantage of pulling carbon from the air in the process?

This is a technical requirement often overlooked by the Consumer Market compared to the Commercial Market - the heavy and long usage of equipment in remote places (generators that run for days or shipping across oceans).

Hydrogen may be a solution along with other hybrid approaches (WA state ferry electrification being relevant [1])

[1] https://wsdot.wa.gov/construction-planning/major-projects/fe...

Not really, because if you don't have access to electricity then you very likely don't have access to hydrogen either. Hydrogen is much more difficult to transport and store compared to natural gas, gasoline, diesel, etc.

Getting electricity out of hydrogen works the same way as getting electricity out of all the sources you mentioned, except your generator now produces zero emissions.

JCB are heavily in to hydrogen. They’ve shown how they can convert existing diesel equipment, and setup fuel bowsers on work sites.

Engineering Explained occasionally discusses hydrogen energy density: https://www.youtube.com/watch?v=vJjKwSF9gT8

They definitely fly better than a lead balloon (batteries).

Once you're already committed to green hydrogen (electrolysis-made hydrogen) you may as well go whole hog and do green methane (methane made through the Sabatier process, which consumes Hydrogen and CO2).

Making a methane-powered jetliner is far more practical than a hydrogen-powered one.

That's right, but there's an even better option: making synthetic kerosene from green H2 and CO2 so it can be used in existing jetliners.

I thought it was hard to contain since the particle is so small (leaks).

Liquid I suppose, temperature concern though.

For boats, fuel cells can make sense. But air travel not only requires a high energy density but also a high power density, and here the fuel cells are relatively weak even by car standards. The alternative is to burn hydrogen, which introduces an inefficiency that eats up most of your advantage. For a car, you can have a small auxiliary battery, but a jet engine will run at max power for several minutes continuously during take-off, and for existing jet engines the power-to-weight ratio is considered a crucial figure of merit.

For hydrogen storage, the DoE targets are truly sobering [1]. That whittles down hydrogen from roughly 100 times as dense as batteries to about 5. Throw a 50% thermodynamic efficiency (jet engines are not efficient!) on top of that and hydrogen still has an advantage, but it lags quite badly behind jet fuel. There have been four decades of intensive government-sponsored research effort into hydrogen storage materials, but all existing systems in practical use rely on fiber-composite tanks at 700 bar.

You get better results with an ammonia-burning jet engine. Here the tank weight is negligible and the hydrogen storage density is effectively 15% (after correcting for the enthalpy of formation). But ammonia still has just half the energy density of jet fuel, and it's rather unpleasant to work with. Plus, the existing production process of ammonia faces its own serious inefficiencies, and extensive investigation of more efficient ammonia production has been painstaking with only one Japanese startup [3] that is behind its timeline (probably a little COVID-influenced) and other contenders at lab stage.

With the direct ammonia fuel cell, the power density issue is even worse, but the energy density is very good. This technology competes well with fossil fuels for weight-sensitive applications that do not require high power (DARPA has been interested in a DAFC drone). But ammonia fuel cells are mostly at lab stage, partially because ammonia production remains disappointing (despite a theoretical energy cost of zero) despite considerable research effort.

1: https://www.energy.gov/eere/fuelcells/hydrogen-storage

2: https://www.google.com/search?client=firefox-b-1-lm&q=tsubam...