What's Wrong With Hydrogen
I'm not going to make the whole laundry list of technology and engineering problems with the use of hydrogen as fuel (some of those are, admittely, not that big), but I'll concentrate on the basics.
The first and foremost problem is pure thermodynamics:
Which means, in order to crack liquid water into gaseous hydrogen and oxygen (at 298 K and 1 bar), 237.13 kJ of energy are required for each mole of water. And that's quite a lot of energy. Steam reforming of methane to CO and hydrogen has ΔH0 around +200 kJ/mol as well. One kilo (roughly, one liter) of water contains 55.5 mol, so the energy required to crack water is 13.161 MJ/kg.
Moreover, no conceivable process will have 100% effieciency - a "clean" process starting with photovoltaic cells will have more likely 10% efficiency if that: electrolysis, especially when hydrogen is involved, is an inherently inefficient process. Solar furnaces and all that are probably better, but 50% efficiency would be a huge success, in my opinion.
If the task is to reduce global energy consumption, hydrogen as a fuel is not the solution.
On the other hand, if the task is to reduce air pollution in urban centers, hydrogen as a fuel is a possible solution.
The first and foremost problem is pure thermodynamics:
H2O(l) --> H2(g) + O2(g)
ΔG0 = 237.13 kJ/mol
ΔG0 = 237.13 kJ/mol
Which means, in order to crack liquid water into gaseous hydrogen and oxygen (at 298 K and 1 bar), 237.13 kJ of energy are required for each mole of water. And that's quite a lot of energy. Steam reforming of methane to CO and hydrogen has ΔH0 around +200 kJ/mol as well. One kilo (roughly, one liter) of water contains 55.5 mol, so the energy required to crack water is 13.161 MJ/kg.
Moreover, no conceivable process will have 100% effieciency - a "clean" process starting with photovoltaic cells will have more likely 10% efficiency if that: electrolysis, especially when hydrogen is involved, is an inherently inefficient process. Solar furnaces and all that are probably better, but 50% efficiency would be a huge success, in my opinion.
If the task is to reduce global energy consumption, hydrogen as a fuel is not the solution.
On the other hand, if the task is to reduce air pollution in urban centers, hydrogen as a fuel is a possible solution.
postato da Fabio alle
14:07
5 Commenti:
I once had a wild thought about hydrogen and nanotechnologies. It's probably just a purely science fiction scenario that my non-scientifical mind cannot recognize as such, but I'm gonna write it down here anyway, so you can roll your eyes at my ignorance.
I've once read about the possibility of building "nano-filters" that could disassemble materials atom by atom. Like the water passes through a kind of "nano-net" with holes big enough to let hydrogen atoms but not oxygen atoms through. Or instead of a "net" or "filter" a barrier of nano-arms that grab one kind of atom but not the other. If something like that could be built, do you think hydrogen could be a more viable energy vector?
On a less "science-fiction" scenario, I've read japanese scientists are trying to separate oxygen and hydrogen using genetically modified bacterias.
Di
Anonimo, Alle
2/4/07 23:51
The thermodynamics of the reaction will not change appreciabily whatever method is used - electrolysis, thermal cracking, photolysis, enzymes/bacteria - but the use of nanotechnology can achieve more efficient processes.
However, modern steam reforming plants can already achieve 94% efficiency from methane...
Di
Fabio, Alle
3/4/07 08:12
So basically, if I understand correctly, if bacterias had to produce hydrogen you'd had to feed them enough to produce 13.161 MJ per kilogram of hydrogen produced.
Di
Anonimo, Alle
3/4/07 18:30
Yes indeed; there is no way around thermodynamics.
The actual energy required may be slightly different because the reaction does not occur at standard conditions; but the order of magnitude is definitely right.
If you could point me to a paper with more details on those Japanese studies, I'd be grateful.
Photosynthetic bacteria will use solar energy to crack water, and that is definitely a feature. I don't think an hydrogen revolution is coming, however.
Di
Fabio, Alle
4/4/07 21:39
Actually about the japanese experiment so far I just read a couple of lines long piece of news somewhere on one of the technology sites I occasionally surf (you can find them all under "Tecnologia" at my link deposit blog http://wellington-lantieroe.blogspot.com/. perhaps the MIT one, I can't recall specifically), but there where no details whatsoever. They just said Japanese scientists where experimenting with these GM bacterias.
Anyway, after a brief search I found this:
http://www.fao.org/docrep/w7241e/w7241e0g.htm
and this
http://en.wikipedia.org/wiki/Biological_hydrogen_production_(Algae)
If you google using "biological hydrogen production" as keywords you should find more stuff.
There's also this site here but it is so obviously inoculated with ideology that I wouldn't swear on its reliability.
http://www.hydrogencommerce.com/index15.htm
Di
Anonimo, Alle
4/4/07 23:48
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