Extracting Hydrogen from water

Mike O'Dell mo at ccr.org
Sun Mar 21 14:25:14 CDT 2010

splitting water into hydrogen and oxygen is simply a way
of storing energy - the energy that went into splitting the
molecule of water.

the energy produced by the recombination of that hydrogen
and oxygen is limited by the laws of thermodynamics to
being *LESS* than the energy that went into splitting.

what is going on here is mis-named in the headline;
it is nothing like photosynthesis - it is simply electrolysis
with the electricity coming from silicon PV cells.

the contribution is a catalyst which improves the efficiency
of water electrolysis. in normal circumstances, the process
is not very efficient - it takes far more electrons to split
the water molecule than will be produced in recombination.

the catalyst acts by making it take less power to split
the water molecules, so the round-trip loss is less.
(round-trip meaning water to H2 and O2 and back to water)

the net result is that for the same solar flux, more hydrogen
is liberated than would be without the catalyst, but all of
it is still subject to the laws of thermodynamics:
	(1) you can't win
	(2) you can't break even
	(3) you can't get out of the game

all of this ignores energy required to store hydrogen.
a kilogram of hydrogen is *LARGE* at standard temp and pressure,
while a litre of hydrogen at STP is bupkis when it comes to energy
generation in a perfectly-efficient fuel cell (of which there
ain't none). so storing H2 requires further energy for compression,
further reducing the round-trip efficiency.

even rocket engines that burn H2 and O2 (eg, Saturn V second and third
stage engines and the reincarnation, the S1X for Ares) can't get
enough density with just liquid hydrogen. it's actually stored as
a "liquid hydrogen slushy". NASA discovered how to use nickel as a
catalyst to make LH2 start to freeze into slush without lower
temps or higher pressure. this increased the density of the LH2
sufficiently to make it a viable rocket fuel.

Hydrogen is just hard.


On 3/21/10 12:53 PM, Alex Fraser wrote:
> Could someone explain why this is against the laws of thermodynamics?
> Don't the numbers add up?
> Philip Miller Tate wrote:
>> On 18 Mar 2010, at 22:35, Andre Kesteloot wrote:
>>> http://www.popsci.com/technology/article/2010-03/video-artificial-photosynthesis-produces-enough-energy-power-house-one-bottle-water_______________________________________________Tacos
>>> mailing listTacos at amrad.org
>>> <mailto:Tacos at amrad.org>http://www.amrad.org/mailman/listinfo/tacos
>> It's depressing how far down the discussion list you have to go to
>> find the first person who understands the Laws of Thermodynamics. And
>> if ARPA don't, lord help you all. After all, even Homer told Lisa off
>> for making a perpetual motion machine: "In this house we OBEY the laws
>> of thermodynamics!"
>> Phil M1GWZ
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>> Tacos at amrad.org
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"Of course it's hard!

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