[FoRK] Magnesium / Water cycle hydrogen production
Stephen D. Williams
Wed Oct 26 17:16:59 PDT 2005
Ken Meltsner wrote:
> 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%.
I have one of those and I don't think that it's the condensation that's
a key to efficiency, it's much more a matter of not throwing away a
temperature gradient. Condensation is probably part of that, but it
can't be a large part. The condensation happens and the induced exhaust
flow is needed because of the greatly lowered exhaust gas temperature.
Traditional gas appliances counted on a high temperature differential to
carry the gas (include CO) up the flue. When the unit reclaims 95% of
the heat, the resulting temperature differential means you can't use a
If you look at the internals of a high efficiency furnace, it has
combustion tubes that start large and then continue, winding back and
forth into the cold air stream, until they terminate in the inducer
plenum. The furthest reaches of the combustion tubes hit the coldest
air first. That air goes on to hit warmer and warmer parts of the
combustion tube as it travels. The inducer makes sure the exhaust gases
are pumped outside and the exhaust tube is slanted so that condensation
flows back to a trap at the inducer. It seems that most of the
condensation happens in the exhaust tube (a standard 4" thick wall PVC
pipe) where heat recovery is probably marginal and definitely not part
of the airflow.
> Ken Meltsner
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