CMDR_kamikazze OP t1_iyc18jk wrote
Reply to comment by Helios4242 in People who are promoting hydrogen as a green fuel always forget what water vapor is one of the most powerful greenhouse gases. by CMDR_kamikazze
I'm an engineer, I'm not forgetting anything.
- It's minuscule in comparison with what hydrogen engines will do. You will burn 1 kilogram of hydrogen and you will get 9 (yes, NINE) liters of water. It's chemistry.
- Yes, but by using it in jet aviation we will introduce the all-new very effective way to move that water high in the stratosphere, thus creating a lot of new clouds.
Helios4242 t1_iyc4hhn wrote
>kilogram
yeah let's talk moles.
1 mole H2O per 1 mole H2
N+1 mole H2O per 1 mole N-Carbon hydrocarbon
Hydrogen fuel has about 2.8x more specific energy (120MJ/kg) than a traditional jet fuel (43MJ/kg) as well (source) So that 1kg H2 is worth the energy in 2.8 kg jet fuel. It's never exact to get a molecular weight off of jet fuel, but a JP-8 similar to the commercial A-1 (with the reported energy density I could easily find on wikipedia) had 180 MW reported (source). OK so 15.56 mol jet fuel (mostly between 9 and 16 Carbon, let's say an average of 12.5 Carbon--which checks out this would give an estimated avg MW of 177). This means one mole of jet fuel is producing 13.5 moles water for 210 moles water in 2.8kg jet fuel.
Compare that to the 500 moles produced by 1kg H2.
42% is not "miniscule in comparison". Yes, H2 combustion is producing more per Joule (a little over twice as much), but again all that water came from the water cycle rather than from buried hydrocarbons that were never part of the water cycle.
Moreover, the amount produced is relatively small compared to the amount already in the atmosphere, unlike with CO2.
CMDR_kamikazze OP t1_iyc5606 wrote
Indeed, but don't forget more water emissions in the sky means more clouds. If your calculations are correct (which seems like it from my perspective) this means we will effectively double the water vapor emissions from aircraft only. And clouds have a greenhouse effect too, but besides that, they capture the infrared (thermal) energy in the atmosphere in one place and release it in another one, thus moving energy around the globe through weather systems. Additional clouds would mean more energy would be trapped and moved around. That's not as harmless as it may seem, these effects should definitely be researched ahead.
Helios4242 t1_iyc6bfk wrote
>water vapor emissions from aircraft only.
Which is miniscule compared to the amount already evaporating from oceans. Double of miniscule is still miniscule.
By and large, the relative impact of using hydrocarbons on greenhouse gases is not the water vapor but the carbon dioxide. Eliminating the impact on carbon dioxide while doubling its miniscule impact on water vapor is considered to be a good tradeoff by experts in the field (if of course we could reach low-carbon hydrogen production). There's also less nitrogen oxides and particilates. The water vapor is taken into account. It's just not much to account for.
Secondly, as I mentioned, since water splitting would be done with water that is already part of the water cycle, it's just moving it through the cycle. The same amount of water remains on the Earth's surface. Combustion of a fossil fuel adds water to the Earth's surface from where it was stored as a hydrocarbon.
CMDR_kamikazze OP t1_iyc6pk1 wrote
>if of course we could reach low-carbon hydrogen production
The biggest culprit of all issues with using hydrogen. The easiest way of producing it isn't much better for the environment than directly burning hydrocarbons. And it's exactly the way big fuel corporations want it.
Helios4242 t1_iyc71wl wrote
of course, but it's good to have the mechanisms in place to use it if the attempts to produce enough clean electricity bear fruit, alongside improvements to hydrogen production efficiency and usage.
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