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Author Topic: How pure does water have to be to produce hydrogen? (and other questions)  (Read 4862 times)

Offline tvanover

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I was wondering how pure water has to be to be able to generate hydrogen for applications in fuel cells.  Would one have to use distilled water, tap water, or will gray water (runoff/ rain water passed through a basic sand and stone filter for the purposes of this question) work just fine?  What happens to the rest of the stuff in the water during electrolysis?  Does it evaporate, turning to a gas along with the hydrogen and oxygen?  Does it get left behind in some sort of crud?

Also, when converting the hydrogen back to water vapor to produce power what sort of temperature is it at?  And how is the power produced?  Does the hydrogen engine capture thrown electrons from the reaction or does the reaction produce force that drives a turbine or piston?

If you were using gray water to produce your hydrogen and oxygen for use in power generation would you have to somehow separate the impurities from the hydrogen and oxygen gas before use in the generator?  If so what would be the ways of going about that?  If not, since the water coming out is already a vapor, would it be possible to condense it and separate the pure water out from the impurities that were carried along.

If you could point me to resources or figures I would be very happy.  I am brainstorming a means of producing steady power and drinking water from an non-continuous power source.


 

Offline Evie

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Doesn't a fuel cell turn hydrogen and oxygen into water to create electricity?
 

Offline tvanover

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Doesn't a fuel cell turn hydrogen and oxygen into water to create electricity?

Yes, but it takes electricity, or a chemical reaction, to produce the hydrogen in the first place. 
 

Offline Pumblechook

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If it was distilled water there would be no current flow from the power source.  You need (a weak solution) sodium chloride or an acid to allow a flow. 

Problem is efficiency..  The energy available from the hydrogen produced is far less than the electrical energy needed to produce it.
 

Offline tvanover

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If it was distilled water there would be no current flow from the power source.  You need (a weak solution) sodium chloride or an acid to allow a flow. 

Ah, so if I am using sand filtered rainwater I just need to make sure that ph is low before I intake it.

Quote
Problem is efficiency..  The energy available from the hydrogen produced is far less than the electrical energy needed to produce it.

True, but there are a few mitigating factors to this that justify the inefficiency.
1st: Energy storage.  As far as I can tell hydrogen has the best storable energy to volume ratio over time for the cheapest material cost.  Batteries, and other energy storage techniques, while able store more energy initially have a greater storage capacity, lose capacity quickly.   Whereas hydrogen storage is cheap, and long lasting.  A large storage tank can last decades, and only needs to remain airtight.

2nd: Energy use rate.  Most energy storage techniques can only accept power at a limited rate.  If you push too much electricity into a storage medium the medium may fail catastrophically.  But with electrolysis, the amount of energy that can be converted to hydrogen is only limited by the water available, and the capabilities of the anode and cathode.  This is useful in situations where the power coming into the process varies greatly, as in the case of wind power.

3rd: Valuable output.  With a proper distillery at the end of the process we will be left with distilled water, fit for human consumption.  Comparing the energy lost from the input to output, with the energy it would take to distill an equal amount of water, we should come up close to equitable.

Thus this process would provide 2 valuable functions, store energy long term, and produce drinking water.  Inefficiencies of the one would be made up for by the nearly free supply of the other.
 

Offline Pumblechook

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I think you need to put some figures to that.

 

Offline tvanover

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Exact
I think you need to put some figures to that.

Exactly.  I am trying to find some resources that would have such figures.  Any Ideas where I can look?
 

Offline Bored chemist

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We use electrolytic hydrogen generators at work. They use high purity water and an ion exchange membrane as the electrolyte.
 You don't really want chloride in a system like this because you get some chlorine formed which is rather corrosive.
Rain water (collected on a clean surface) might be good enough but grey water is probaly a non starter.
Any impurities in the water are likely to reduce the efficiency of the system.
 

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