[FoRK] Magnesium / Water cycle hydrogen production

Ken Meltsner meltsner
Wed Oct 26 16:29:28 PDT 2005


Thinking about this sort of problem thermodynamically, which is always
dangerous, it occurs to me that the energy produced by aluminum or
magnesium + water reacting together to make hydrogen + metal oxide,
and then burning hydrogen to make water, is the same as burning metal
without the business of making hydrogen.

I realize that it's important to make hydrogen so that the (internal
combustion) engine can run -- burning metal is inconvenient at best in
an IC engine -- but it also means that you have to worry about
condensing the water vapor produced (if you want to get every last bit
of energy out of the metal-oxygen reaction).  If you don't, there's
another chunk of energy gone.

In detail:

M (Aluminum or magnesium)

M + H2O liquid -> MO + H2 (produces heat and H2)
H2 + 1/2 O2 -> H2O vapor  (produces heat and water vapor)
H20 vapor -> H2O liquid

Since you're starting with liquid water (except in the midwest in a
few months...), you end up losing the heat of condensation (energy
released when vapor -> liquid) as well.  This is not a trivial amount
of energy -- it's the reason that high efficiency gas furnaces require
a condenser to return the maximum amount of energy from combustion. 
Numbers escape me -- it may be about 5%.

If you used external combustion -- just burned the metal and used the
energy to power a Stirling engine, I suppose -- you could avoid
producing H2 entirely, and you'd pick up a few more points of
efficiency.  Which is beside the point, but one more sign that this
isn't the best idea overall.

Ken Meltsner



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