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To store enough hydrogen to be useful you probably need it in liquid form like rockets use. Hydrogen is energy dense by mass, but not by how much space it takes
by wlesieutre 7y ago
To store enough hydrogen to be useful you probably need it in liquid form like rockets use. Hydrogen is energy dense by mass, but not by how much space it takes up compared to liquid fuels.
Otherwise you'll have a really giant bag of gaseous hydrogen which does help your bouyancy, but then you've made an airship instead of a plane and a spark accidentally turns it into the Hindenburg.
- afiori 7y agoIs it so much less energy dense per liter to offset the gain in weight? In a plane energy per weight is rather important afterall
- wlesieutre 7y agoAs a gas at 1 atmosphere, wikipedia says 0.011 MJ/L, compared to jet fuel at 35 MJ/L. You'd save weight (it weighs less than air so this would make you float) but need 3000x as much space if it's not compressed. Compressed at 690 bar (about 680 atmospheres) it goes all the way up to 5 MJ/L, only 7x as much volume of fuel required compared to jet fuel. But the tanks to pressurize something that much are presumably very heavy and not a reasonable solution in an airplane. Liquifying it gets that up to 10 MJ/liter and I don't think you need as much pressure to store that (as long as you keep it very very cold). Per weight instead of volume you beat jet fuel (by 3-4x), so that aspect is a win. Having enough volume to store it and keeping the storage equipment light are the challenges.
- Tade0 7y agoCounterintuitively enough liquid ammonia packs more hydrogen than liquid hydrogen per unit volume - also it condenses at -33C (-27.4F), so it actually wouldn't require cooling once in the air. It can be burned in a gas turbine, but for now the NOx emissions are pretty horrible.
- jandrese 7y agoAlso, it's horribly corrosive and poisonous.
- usrusr 7y agoThe penalty for more volume is rising with higher speed, whereas the penalty for more weight is staying pretty much constant. In a fast plane, volume is just as important as weight, if not more.
- wlesieutre 7y agoAlso liquid fuel tanks in planes are usually out in the wings. They can be shaped pretty much like whatever as long as the fuel keeps flowing. Pressure vessels though, you're better off making them round.
- hfkajshfaks 7y agoIts so bad that only the largest space rockets use liquid H2. All the others use kerosene or the like. Either way, the Soviets tried gaseous fuels in the 80s - See Tu-155. They ran on liquid H2, CNG. If I recall, the weight increase didn't make it practical. On the other hand it was a turbine powered plane. It might work out with fuel cells (although you'd need a big and heavy compressor to run it)
- mannykannot 7y agoLiquified is the only option, I think. Compressing it into cylinders or storing it as a metal hydride is probably way too much weight. Until recently, making hydrogen by electrolysis was not a benefit, environmentally, as a lot of the electricity would have come from fossil fuels. I think decomposition of methane is an option that would allow the carbon to be sequestered.