Comments
CMDR_kamikazze OP t1_iyc0219 wrote
Well yes, but imagine all of the jet aviation to switch to hydrogen engines. I doubt the sheer amounts of water vapor introduced into the stratosphere as a result of that will do any better than current amounts of CO2 do. The greenhouse effect could even be worse. The effects of water vapor emissions should really be researched and compared to effects of CO2 emissions before we will put too much time and effort in hydrogen tech.
notmynose t1_iycah8m wrote
Actually you make fair points here. Cloud formation in the stratosphere has an negative impact on both the ozone layer and on surface temperatures. I retract my objections.
Helios4242 t1_iyc0t18 wrote
And what YOU are forgetting is that
- Combustion engines are also producing water upon combustion of fuel
- Hydrogen production typically involves water splitting, consuming an equal amount of water as would be produced upon combustion.
CMDR_kamikazze OP t1_iyc18jk wrote
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.
Caro_lada t1_iyc1c4e wrote
You should also consider while vapour can act as an greenhouse gas, clouds (which are very small water droplets) are created from vapour as it condenses in the atmosphere and reflect the sun rays. They are thus doing the opposite of vapour. It is too cold in the upper atmosphere for the water to stay vapour for a longer time.
CMDR_kamikazze OP t1_iyc1u83 wrote
That's a common misconception. Clouds have a greenhouse effect too. They're just capturing the infrared (thermal) energy in the atmosphere in one place, and releasing it in another one, thus trapping and then moving energy around the globe through weather systems. Additional clouds would mean more energy would be trapped and moved around.
Caro_lada t1_iycbw2j wrote
That is also partially correct. Clouds reflect energy, they don't capture and release it. Both going outwards and coming inwards. As more energy is coming from the sun to earth than vice versa they're not bad per se.
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[deleted] t1_iybzkb5 wrote
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[deleted] t1_iyc006q wrote
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mayorqueyo3 t1_iydjl11 wrote
I thought about that too but i guess i undertood that it was only suposed to be used by power companys, not vehicles at a largo scale
Due-Department-8666 t1_iybzf5c wrote
Condense the water vapor into water and pipe it to places suffering from water scarcity
CMDR_kamikazze OP t1_iyc082f wrote
It won't work this way with combustion engines of moving vehicles, you know.
Due-Department-8666 t1_iyc0ngg wrote
Because we can capture/filter other emissions with catalytic converters but water just escapes our abilities.
Snaps fingers* Shoot, we were so close too.
CMDR_kamikazze OP t1_iyc1foa wrote
No, it's ain't a joke really. When you will burn 1 kilogram of hydrogen you will get 9 (yes, NINE) liters of water. Good luck capturing that. Imagine all this water in the winter on the highway. Or up in the stratosphere as clouds.
Due-Department-8666 t1_iyc2bdi wrote
Got a link for the numbers?
CMDR_kamikazze OP t1_iyc39o7 wrote
Do you need a link to a chemistry textbook?
Assuming we're burning 1000 grams of hydrogen and have complete combustion and a reaction that is 100% ideal (it won't be like that, but pretty close):
2H2+O2->2H20
With that, amount=mass in grams/Mr or Ar. Hence the amount of H2 would be 500 mol. Using a stoichiometric ratio amount of water is 500 mol.
Now mass = amount×Mr or Ar, hence 500×(2+16)=9000g of water from burning 1000g of hydrogen.
Helios4242 t1_iyc4q7c wrote
Great now tell me how many moles H2O produced in a combustion reaction from jet fuel providing an equal amount of energy :)
Providing one and not the other is deceptive even if accurate.
CMDR_kamikazze OP t1_iyc5fwc wrote
BTW while with jet fuel it's about twice as much water produced, how about common car fuels, like gasoline and diesel (OMG I hate diesel, ban this thing for good already), could you calculate it? I'm not very good with hydrocarbons.
Helios4242 t1_iyc6vdb wrote
for simplicity, I'm assuming octane (C8H18). Energy density is 46MJ/kg, so we need 2.6 kg to match the energy in 1kg H2. MW of 114 means we are using 22.8 moles, producing 9 moles of H2O per mole octane. That's 206 moles, sopretry similar. Diesel isn't gonna be much different.
notmynose t1_iybzf8x wrote
Crazy thing about water. It condenses. Methane and co2 don't.
E: although stratospheric emission of water vapour is a different story. Yes, this does need more attention.