Newsgroups: sci.space.policy
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Breaking CO2 down to release oxygen.
Date: Sun, 13 Feb 2000 20:58:16 GMT

In article <38A6D6CF.EB4409EA@peterlynnkites.co.nz>,
Robert Lynn  <robert@peterlynnkites.co.nz> wrote:
>I'm well aware of photosynthesis as a process for converting CO2 and
>water into sugar and oxygen but are there any other methods for
>extracting oxygen from CO2 that are feasible?

Well, it depends on what you mean by feasible.  There are lots of things
that can be done, like CO2 + 4H2 -> CH4 + 2H2O followed by electrolysis of
the water and pyrolysis of the methane, but generally it is difficult and
awkward.  Photosynthesis is preferred.

In fact, unless you are recycling food, there is a better approach.  The
overall reaction of human metabolism is roughly CH2O + O2 -> CO2 + H2O.
That is, not only do you get CO2 out, you also get water.  Dumping the CO2
and electrolyzing the water is a much simpler way to recover oxygen.  For
example, that's where Mir's normal oxygen supply comes from.
--
The space program reminds me        |  Henry Spencer   henry@spsystems.net
of a government agency.  -Jim Baen  |      (aka henry@zoo.toronto.edu)

Newsgroups: sci.space.policy
From: henry@spsystems.net (Henry Spencer)
Subject: Re: How does a space colony deal with moisture fom human breath?
Date: Tue, 28 Nov 2000 19:32:29 GMT

In article <k3FFBDKf08I6EwXo@merlyn.demon.co.uk>,
Dr John Stockton  <jrs@merlyn.demon.co.uk> wrote:
>>More precisely, they convert food to water.  Human metabolism, to a very
>>crude first approximation, is food+O2 -> CO2+H2O.  Oxygen could be
>>recovered from either the CO2 or the H2O, but getting it out of the H2O
>>is currently rather simpler.
>
>But you have an O2 on the left, and an H2O on the right.  Half the
>oxygen breathed does not go into the water.  And, as carbohydrates are
>approximately (CH2O)n, the proportions in your equation are about right.

Yes, although I wasn't paying careful attention to proportions.  There is
much more waste water than just what comes out of the food, unless you are
recycling water (which currently-flying systems don't).  Even discounting
wash water and such, only a few desert animals can live entirely on the
water from food metabolism; everything else, including humans, runs water
through as well.

For non-recycling life-support systems, it turns out that there is
generally enough waste water for it to serve as the oxygen source.

If you start recycling water, then yes, you have to think about recycling
CO2 as well, because although there will be a water excess (unless you're
recycling food as well), it won't be large enough to supply oxygen.
--
When failure is not an option, success  |  Henry Spencer   henry@spsystems.net
can get expensive.   -- Peter Stibrany  |      (aka henry@zoo.toronto.edu)

Newsgroups: sci.space.policy
From: henry@spsystems.net (Henry Spencer)
Subject: Re: How does a space colony deal with moisture fom human breath?
Date: Wed, 29 Nov 2000 15:26:24 GMT

In article <3A24856F.9696C889@biogate.com>,
Julian Bordas  <jbordas@biogate.com> wrote:
>> ...only a few desert animals can live entirely on the
>> water from food metabolism;
>
>And the koala bear !!  Not a desert animal

Not much fresh water available up in those treetops -- it may not normally
be considered a desert, but in practice that's what it is...
--
When failure is not an option, success  |  Henry Spencer   henry@spsystems.net
can get expensive.   -- Peter Stibrany  |      (aka henry@zoo.toronto.edu)