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Electrolysis at pressure.

Posted on 2002-06-27
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Does it take more energy to electrolyse an amount of water into hydrogen and oxygen at 100 atmospheres of pressure than at one atmosphere and if so how much more? In whatever units you want.
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Question by:andyalder
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by:ozo
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Yes.  The same amount of energy that it would take to compress hydrogen and oxygen from one atmosphere to 100 atmospheres.
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by:andyalder
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That's what I think as well but can you find a proof or equation for this preferably available on the Internet?

Just want to prove why http://my.voyager.net/~jrrandall/BublGen.htm doesn't work.
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by:ozo
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I suspect that http://my.voyager.net/~jrrandall/BublGen.htm
will actually work despite the electrolysis, not because of it,
just from the temperature difference between the cold deep ocean water and warm surface water.                                                                                                                                                                                                                                                                                                                                                                          

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by:BigRat
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No. The electrolysis of a substance causes the binding to be broken and requires the binding energy to do it. (Farady's Law) The elements are freed at the electrodes and often react with the solvent. Electrolysis of saline solution creates hydrogen not sodium, for example.

The gasification of the elements occurs only if the critical tempurature is exceeded. And of course if the element is not soluable in the solute.

In your example a certain amount of oxygen will be lost. One would also need to look at the conductivity of sea water at that location and at that depth, in order to determine efficiency. A certain amount of energy will be lost in simply heating up the water - electrolysis is not without thermic losses you know!

One minor point. The article suggests that the energy comes from the warmth of the water and gravity. The latter is not true, well at least not as suggested. The electrolysed water will be replaced by that generated from the fuel cell which is then dumped back into the sea. The latter is lighter since it contains no minerals and it is warmer! This water will eventually get recycled into the depths by the tides - gravitational actions due to the earth and the moon. If you extract too much energy the moon will recede from the earth, making my cheese gathering trips more difficult.
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by:rid
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As you say, BigRat, electrolysing sea water would yield Cl2 and H2 rather than O2 and H2. The solution reaching the surface at the "positive" riser would be a rather complicated cocktail and what would be the use of all the Chlorine, anyway? Surrounding sea should become fairly sterile after a while, of course, and you'll probably get dioxines and a nice assortment of other lethal substances to contend with. Ugh.

Regards
/RID
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by:kaller2
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I cant get the whole page to load, but enough to get an idea. There are two issues, the author's description of what he thinks will happen, and the author's explanation of that.  Sadly there are errors on both counts, and the whole scheme is just bad science.

>>Here is a way to generate energy from warm sea water. This may look complicated but it uses simple technology. To start, electricity is sent down deep into a warm lake or ocean.

OK, so there is a cost already.

>Oxygen and hydrogen are produced using electrolysis.

They probably would be.  Along with chlorine and caustic soda.  The conductivity of seawater is not ideal, and that would waste some of your energy for a start.  The higher pressures would make it more difficult for gases to form. And as mentioned, you would get a build up of hydroxide and chlorine from the salt, probably producing corrosive hypochlorite if the chlorine and caustic crap produced were allowed to mix.  I agree, yeccchhhh.  Not to mention the hazards of explosive gases.  And you are investing large amounts of chemical energy for a hoped-for small mechanical effect.

>The gases come up thru seperate pipes causing a large volume of water to rise up thru the pipes. This is referred to as a bubble pump and can produce very large flows of water.

Major error.  Bubbles don't ascend by lifting water, apart from small volume of water lifted during the bubble creation.  It is the re-lowering of this small volume that drives the bubble to rise.  He has got it all wrong.

>This flow of water is caused by the high water pressure deep under the sea which is produced by gravity.

Funny I though that gravity acted downwards.  Anyway, a straight-forward pressure pump driven by electricity would be more efficient at lifting water than a costly electrolytic process, regardless of how it operated.  The energy that has gone into the electrolysis needs to be recovered (by fuel cells etc) and that brings in even more inefficiencies.

>As the gases rise they decompress and cool, chilling the water.  

Well since the deep water stayed down there, only the surface water would be cooled, but not by much though. The rest of the article is irrelevant.  And anyway, why try to lift cold water to exploit it?  Much better to leave it down there and run a heat pump between the hot and cold layers.  

Hope this answers the question.
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by:andyalder
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I only posted the bubble pump URL since it mentioned electrolysis at pressure, I'm not interested in that article per se but cannot find an authoratitive document, I was thinking more of a belt with umberellas on it driven by gas at the lower electrodes and it provided perpetual motion which can't be.

If no extra energy was required to turn water into gas at pressure then you'd get free energy by electrolysing at depth and recombining at the surface. However big the inefficiencies become you can compensate by making the column longer if there was no extra energy needed.
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kaller2 earned 100 total points
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The laws involved are the conservation of energy and the laws of thermodynamics.  Umbrellas or rubber balloons it makes no difference, the water flows around them, there is no free lunch.
It comes down to this - suppose you create a bubble by any means inside a liquid.  It can be biological, electrochemical, chemical.  For example you could mix two gases or liquids or solids etc. to make a gas.  What lifting power does the bubble have?  The lifting power will be less than the energy required to create the bubble in the first place, by displacing the water.  At greater pressure this energy can simply be stored in a smaller volume of gas.  The work done is the weight of water displaced - the weight of the gas produced, ie the buoyancy.  

Since you are creating the bubble this time by electro-chemical means, there is also the vastly more energy needed to make the gas as well.  

The concept you need to uderstand this better is called "free energy". G=H-T*S In this case there is a strong drive to produce a gas from a non-gas, caused by the entropy contribution S.  You very slightly inhibit this by operating at pressure, but the change would be insignificant.  It would affect the equilibrium processes at the electrodes slightly.  By how much?  The gas would be produced at a higher pressure, and the entropy contribution by compressing a gas can be accurately calculated.  Most likely the voltage required to electrolyse the water at depth would go up by a small fraction, exactly the amount that you were (sadly) looking to gain.

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by:andyalder
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>Most likely the voltage required to electrolyse the water at depth would go up by a small fraction, exactly the amount that you were (sadly) looking to gain

I think that must be the answer, the only time you can get owt for nowt is when you use simplified rules and theories.
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by:kaller2
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I need to look this up really to check my reasoning, but thankyou for the points.  I neglected to mention the solubility factor which might also play a role - the lower temperature and high pressures would encourage solubility of the gases.
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by:brm123
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I disagree with BigRat regarding temperature. Electrolysis of seawater is not dependant on temperature.
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by:andyalder
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The end products are dependant on temperature because of the post-electrolysis chemical reaction: see brine electrolysis under http://voltaicpower.com/Dictionary/Dictionary-B.htm
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by:brm123
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Remember: "Gibbs Free Energy" and the "outside" temperature rises as the bubble ascends into warmer water. The temperature differential is not a significant factor. Look at the potential for work by the bubble and the amount of energy required to create it.

Do you agree with the following statements?

1- The potential for work by the bouyancy of the bubble, as depth increases, is a linear relationship.
2- Energy required to sustain an electrolytic reaction at increased pressure IS NOT linear.

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by:brm123
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Reply to andyalder:

The link you referenced is not talking about the affect of temperature on the electolytic reaction. It is talking about the affect of temperatore on the products of the reaction only.
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by:brm123
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Reply to andyalder:

The link you referenced is not talking about the affect of temperature on the electolytic reaction. It is talking about the affect of temperatore on the products of the reaction only.
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by:andyalder
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brm, I don't remember enough physics to work it out, that's why I asked in the first place. I don't think either of us is going to break the second law of thermodynamics however hard we try though, not even Flanders and Swann managed that.
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by:brm123
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Well let's keep trying. I posted the idea in 98 but did not get any  serious responses. So when I saw this site ...... anyway I can learn the math just time consuming.
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