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njl
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[*] posted on 24-5-2021 at 06:42
Green hydrogen production


What are your thoughts on promising green hydrogen technologies? More specifically, which reactions/chemical technologies are of interest moving forward? My understanding of current industrial obstacles is that we still don't have satisfactory technologies for making, storing, or using hydrogen. IMO the most promising technologies right now are photocatalytic water splitting, chemical processes like the Bunsen reaction, and polymer hydrogen storage.

What about lab-scale preparations? The only ways I know of are stoichiometric reactions like reducing agent plus acid/base, or electrolysis. Stoichiometric reactions are much faster and easier to carry out, but require large reaction volumes for larger quantities of gas. Electrolysis is much cleaner but is quite slow and requires expensive electrodes to hold up over time. Anybody know how to make a medium pressure hydrogen bottle?




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[*] posted on 24-5-2021 at 06:58


Volume-energy ratio is one of the biggest factors against utilizing hydrogen as an energy storage medium. 70kg of liquefied H2 will occupy 1m3 of space, and contain 10 000 MJ of energy, while ordinary hydrocarbons will weight about 750kg and contain about 33 000 MJ of energy, and require no pressurization of any sort (700bar for hydrogen? Shotgun generate only about 600bar peak) and generally do not leak through many ordinary structural materials and make them brittle so they will eventually spontaneously fail/explode anyway.

Just the practical side. Theory, and wherever hydrogen is needed in chemical feedstock at 1g per mol, is a different matter. If it is consumed in the very place it is generated and bound to some more stable molecules with high energy density, it would likely render ground oil obsolete if a breakthrough in manufacture were developed.

Someone, somewhere a long time ago proposed of storing hydrogen in fullerene carbon balls. Is this fiction, and would it allow for greater energy densities, and what kind of composition this sort of fuel would have? Be an extremely airborne black soot? If it were gelatinized with something, it could possibly be viable as a pumpable, heavy fuel oil type gunk, but still exceedingly messy if any sort of leak were to occur.
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clearly_not_atara
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[*] posted on 24-5-2021 at 12:20


Pretty much the only thing that is relevant is photocatalytic water splitting; everything else has a big penalty vs. batteries. As to how relevant, it's not clear; some things look promising at lab-scale, but the general consensus is that improvements in both efficiency and cost will be necessary for a viable system.



[Edited on 04-20-1969 by clearly_not_atara]
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S.C. Wack
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[*] posted on 24-5-2021 at 13:54


Quote: Originally posted by clearly_not_atara  
Pretty much the only thing that is relevant is photocatalytic water splitting


...except for the wind systems now operating or in the pipeline; which are definitely the future, and long overdue. No need for salt domes in areas with existing gas pipelines, just inject some H in...not really much need for huge storage capacity; use it directly and supplement with something else instead of the other way around.




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clearly_not_atara
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[*] posted on 24-5-2021 at 17:37


^ Okay, I'm curious. We're turning mechanical energy into chemical energy? If you're talking about wind systems coupled to compressed air energy storage, I agree that it's a good idea, but I feel like if you mean making hydrogen, you've skipped a few highly novel steps that would be of great interest to a chemistry trivia hound like me.

For the same (kinda) reason, I'm not huge on thermal hydrogen when molten salt thermal storage already works so well. In a lot of applications, hydrogen is like the competition, but with extra steps.




[Edited on 04-20-1969 by clearly_not_atara]
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[*] posted on 24-5-2021 at 18:41


As a fuel, hydrogen has a couple of really attractive aspects. Energy density on a per mass basis is the best possible. There are no pollutants whatsoever. It is possible to combust or to use a fuel cell to produce electricity. In that sense it is versitile.

And that is where the fairytale ends.
  • Energy density on a volume basis is very low.
  • Transportation of the fuel is hazardous. As is transfer of the fuel / refuelling.
  • You end up dealing with many additional steps such as pressurisation, liquification, or, in the case of BMW's experiments with H2 cars, absorption of H2 into aluminium blocks.
  • Gaseous hydrogen leaks out of everything.
  • Hydrogen embrittlement of steel is a problem in any component where steel is used structurally. (Including pipes.)
  • Production of hydrogen by most processes holds no advantage over more conventional fuels. Typically, it is produced from methane at some energy cost with no advantage over fossil fuels.
  • Production of H2 via electrolysis is very energy inefficient.


    Photocatalytic water splitting sounds like it is interesting. On the face of it, it seems like it would be direct competition with photovoltaic cells. With additional processing required. and still the need to work with an unfriendly fuel.

    If we are using it as an energy storage medium then there are a lot of competitive options and you would need to show that it is advantageous over the alternatives. Which include
  • Pumped hydro
  • Compressed gas
  • Molten salt
  • Liquified air (Being developed in the UK IIRC)
  • Flow batteries
  • Other battery storage (of which there are dozens in development)
  • Flywheel technology

    All of these are dependent on factors specific to the conditions you are developing for. But I am not seeing a case for replacing solar farms with hydrogen equivalents or for the use of hydrogen as a storage medium for moderating electrical grids.
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    S.C. Wack
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    [*] posted on 24-5-2021 at 20:30


    Quote: Originally posted by clearly_not_atara  
    ^ Okay, I'm curious. We're turning mechanical energy into chemical energy?


    I'm talking about using wind-powered hydrolysis to produce H to replace methane, in the most low-hanging-fruit way, e.g. existing and potential users of H, and natural gas in particular.

    IIRC the Germans say 10% H in gas lines has no bad effects.

    The cost need not be competitive with the subsidized fossil and biofuels, especially not right now.

    There is however much rejection these days of all things coming from "the educated liberal elite".




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    [*] posted on 24-5-2021 at 22:59


    I would love to see cost-benefit analysis for wind powered electrolysis to combine with piped natural gas as an energy supply. I sense some pitfalls.
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    [*] posted on 25-5-2021 at 01:15


    Well, it ain't ready for primetime. I've read lots of papers on using Metal Hydrides as a storage vehicle for hydrogen. The idea being release of Hydrogen, followed by Metal Hydride regeneration, followed by release of Hydrogen, etc. etc. ad infinitum.

    Nice idea, and we can't do it yet.

    And then of course, there is no free lunch. The energy to split H2O, has to come from somewhere.

    Now, if we could strip H2 from hydrocarbons, leaving behind elemental carbon, and then sequester the carbon... We could stop global warming.

    Though at some point, we might start depleting the atmosphere of O2. Via the 2 H2+O2--->2 H2O reaction!

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    [*] posted on 25-5-2021 at 05:32


    Green hydrogen from.... wind power ?
    Have you compared the CO2 emission for nuclear and wind power ?!

    For green hydrogen you need nuclear power. Lots of it if you believe the current trends.
    Fusion would be good but for the meantime fission will do.




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    clearly_not_atara
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    [*] posted on 25-5-2021 at 12:11


    Quote: Originally posted by S.C. Wack  

    I'm talking about using wind-powered hydrolysis

    What is "wind-powered hydrolysis"? Do you just mean electrolysis?




    [Edited on 04-20-1969 by clearly_not_atara]
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    [*] posted on 25-5-2021 at 13:38


    It's what you get after spending the evening inhaling low concentrations of toluene.

    Was electricity from anything, and gas heat, as cheap as wood, kerosene, oil, coal, and coal gas? Were there safety complaints? (an aunt of mine died in a gas explosion btw) Was there a ready made infrastructure?




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    [*] posted on 25-5-2021 at 13:42


    Quote: Originally posted by S.C. Wack  

    IIRC the Germans say 10% H in gas lines has no bad effects.

    I remember to have read that back then gas lines were restricted to just 5% hydrogen because of the explosion hazard.
    Talking about gas lines for labs explicitly though.
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