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Dumb Question

  1. May 19, 2007 #1
    Hey, im a bit confused about this...

    sorry if im being stupid, im only 16...

    I was thinking about hydrogen and the fact that it is less dense than air, and water and the fact that it is denser than air... Could the gpe gained by hydrogen, and the gpe converted into kinetic energy by water be used?

    By that i mean, water is allowed to fall, some sort of dynamo is turned by the falling water (as in power stations) generating electricity. This electricity is used to electrolyse water, producing hydrogen (and oxygen). Hydrogen is allowed to rise, then when it reaches the top again, it is burnt, producing water again and energy. The water is then allowed to fall again. Surely there is an excess energy here or am i missing something blindingly obvious? Surely the larger the distance travelled down by the water and up by the hyrodgen, the greater the excess energy...


  2. jcsd
  3. May 19, 2007 #2


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    That's the catch - there is no excess energy. One would have to evaluate the mass transfer and energy balance in order to evaluate the system.

    One is transferring/transforming energy, not creating energy. The water from the combustion of hydrogen has to cooled and condensed to liquid, which then flows down the gravity field to a collection vessel, which then flows back to the electrolysis vessel.

    Along the way, there are losses due to friction and heat conduction from the system.
  4. May 19, 2007 #3
    HMM i dont quite understand you... surely the heat is yet more excess energy...

    I dont know if this is right, but if you just elctrolyse the water, then combust the hydrogen back into the water (and use the energy of the heat in the water - returning the water to its original temperature). In a perfect world there would be no energy loss (due to donds broken/formed). But then if you include the factor of the water falling, generating electricity, being elctrolysed in the vessel it fall into, and then the hydrogen rising, it seems there is an excess...

  5. May 19, 2007 #4


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    One has to put energy IN to break the H-O bonds in water. Then in combusion the formation of H-O bond results in a release of energy, but it is the same energy as that put in - on the bond level. There are inherent dissipative effects that prevent 100% conversion of energy.

    Don't forget, water is falling, but H is rising. Where does one get the O to recombine with the H? That too will have to be transferred. If one uses a closed system, then one is constrained with the convervation of mass - mass transfer up = mass transfer down.
  6. May 19, 2007 #5
    yes, i knew that 100% energy is not possible, some energy loss will happen in the electrolysis and combustion process.

    it need not be a closed system, surrounding air could be used...

    the logical part of me i saying this cant work, but i just cant work out how it cant work... the oxygen is already present in the air at the top, so could that be where the excess energy is coming from (the oxygen at the top from the surrounding air that has gpe)
  7. May 19, 2007 #6


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    What happens with the oxygen from the dissociation of H2O. That has to be transported somewhere - and if across the system boundary, then it is not a closed system. Air is ~21% oxygen, so the combustion of H2 in air will dissipate some energy to N2. Certainly, one can take advantage of convection in the atmosphere.

    BTW - your original question is not stupid. Asking such a question is the first step in understanding the physics of what is involved in this process. :smile:
  8. May 19, 2007 #7
    Thanks :biggrin: , ive always liked physics (my best subject) and im also concerned about the environment. so its always in my mind. This idea first came when i thought about the sea and if you could have a large vessel that allows water to fall in. if the water could fall far enough to generate enough energy to be electrolysed, you would have a hydrogen source (as the major problem in hydrogen cars etc is the fact you have to input energy to get the hydrogen!). (btw how far must a molecule of water fall (assuming ALL ke is converted to electrical energy) to generate enough energy to be electrolysed, does it depend on what it is dissolved in? i was thinking acidulated water...) But then i realised sea water would be too impure and so i though of this idea - using the "same" water over and over...

    yes after posting my last post i realised that this could be a use of the natural mixing of gasses (through brownian motion or convection). The oxygen at the bottom would naturally over time mix back into the atmosphere... Its not a problem if it isnt a closed environment is it?

    dont quite understand the importance of this? do u mean nitrogen dioxide will be produced (as in car engines)? Any car can be run on hydrogen so the actual combustion would be no problem, the combustion could turn a dynamo etc

    Im starting to want this to work lol... But now that the source of energy has been identified as convection, it seems a bit more pheasable...
    Last edited: May 19, 2007
  9. May 19, 2007 #8
    so how pheasable do you think this is?
  10. May 19, 2007 #9


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    As pheasable as a phrog phrolicking in a phorest. :tongue:

    Sorry, I couldn't help it. The word is spelled 'feasible'.
    It seems as if there are two ways to approach this, each of which boils down to a pre-existing concept.
    The first is a closed system, which amounts to the old idea of using electricity generated from falling water to pump the water back up to the top. Clearly, that is a perpetual motion machine and therefore impossible.
    The second is the collection and use of naturally occurring rain to generate electricity. Possible, but not particularly practical.
    Your idea seems to be a combination of the two. Part of the problem, I suspect, is that you are underestimating the amount of electricity needed to crack the water apart.
  11. May 19, 2007 #10
    hence i asked how far a water molecule would have to far to generate enough energy to be electrolysed. whats is the energy require to electrolyse 1 molecule of water in acidulated water?

    oops @ feasable, i have an english exam soon :S

    Really, the falling part would only needto generate enough energy to counter any energy loss and any output energy. the main amount of energy to electrolyse the water would come from the combustion of hydrogen.

    Hmm im not sure if its a combination of those two, as they only cover half of the system, the other half would be the electrlysing the water, allowing the hydrogen to rise, combusting it, cooling it. However, i have seen a large water power station in the past. During peak times water from the top lake is allowed to fall and is used to generate electricity. Then it is pumped back up to the top when there is less demand on the elctricity grid...

    so how FEASABLE (:D) is this then?
  12. May 19, 2007 #11


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    Right then... that's the first time that you mentioned extracting energy from the combustion process. Just how do you plan to do that? Do you think that your method would come close to providing enough electrolysis energy?
    As for the dam reversal, it is far from being an over-unity process. There are losses at every stage. Economically, however, it makes sense for the power company to take that route. As you noted, it's done only during slow periods when excess electricity is being generated. Since the turbines are spinning anyhow, it's logical to put them to some use rather than just leaving unbought electricity lying around.
  13. May 19, 2007 #12
    sorry i thought thast was implied.

    as stated earlier, in a perfect world, the energy produced from combustion would equal the energy needed to electrolyse. However this is not a perfect world. But this would go some way towards providing enough energy for electrolysis. Therefore the falling water would make up the rest and anyexcess is your product...

    Combustion, maybe fuel cells, or just convention combustion (ie engine) with a dynamo. obviously the exhaust gasses would need cooling but the heat can be used to make more electricity, reducing the amount of electricity the falling water has to make up...

    Yea dam reversal struck me as being pretty dumb as it is a large waste of energy. but i guess it helpy cope with peaks in electricity usage...

    so you agree that the energy that is the product in this idea is basically coming from convection, raising the oxygen up over time? Does this all sound within reason?
    Last edited: May 19, 2007
  14. May 19, 2007 #13


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    Uh... it's your plan; you tell us where it's coming from. First you said from the falling water (via turbine, I assume), then from hydrogen combustion, and now convection?
    I'm sorry, but no matter how you slice it, it ain't cake. I'm going to defer to Astronuc and other professionals to get into the details with you.
  15. May 19, 2007 #14
    nonono im saying the source of the energy that i got confused about is coming from convection... IE the oxygen u r using at the top has gpe. The oxygen at the bottom has less. This is why it can be a closed unit... I was just kinda confirming what i had worked out from my discussion with astronuc earlier.

    Basically there is a an excess energy (ie the output) and that is created because of convection distibuting oxygen evenly...
  16. May 19, 2007 #15


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    How do you figure that oxygen anywhere has gpe? If you had a completely massless balloon and filled it with oxygen, would it fall in our atmosphere? If so, how fast? How much energy could be extracted from it? How much would be expended to get it back up?
    As mentioned, this is getting out of my area. The others will have to handle it from here out. It's not that I don't want to converse with you further; I'm just at the end of my ability here.
  17. May 19, 2007 #16
    no its cool, i think im just describing it badly. Basically the problem that astronuc posed was: "Don't forget, water is falling, but H is rising. Where does one get the O to recombine with the H? That too will have to be transferred."

    The answer to that was the O to recombine with the H comes from the atmosphere. Oxygen is slightly denser than air itself. So if you think about it, i am taking oxygen from higher up, where it has gpe and outputting it lower down. BUT because of convection and brownian motion (that gasses will naturally become uniformly conecntrated) the oxygen gains gpe in the atmosphere. I am not however saying that i will use the same oxygen. The oxygen is just released into the atmosphere to do its funky thing ovewr time. So, the actual source of the energy (because the HAS to be one due to the law of conservation of energy) is natural convection/brownian motion...

    Well at least, thats what i think...

    So should i give up my day job (school)? Or should i stop thinking about this and get on with my english literature revision lol!
  18. May 19, 2007 #17


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    :surprised :surprised :surprised
    Don't ever give up thinking about anything! :grumpy:
    Bloody hell, man... you could be the next Buckminster Fuller. Think as much as you can about as much as you can.
    I did get the part about not recycling the oxygen, but I don't think that it makes that much difference. As for oxygen's gpe, as far as I know Brownian motion far overwhelms it. After all, we wouldn't have a greenhouse effect (or animal life) if carbon dioxide settled to the bottom of the atmosphere.
  19. May 19, 2007 #18
    yea i thought so, therefore the oxygen at the bottom will, over time make its way up again... this seems to perfect to be true... but its all so simple, i only have a basic knowledge of science, but i cant see what should complicate this. there seems to be nothing i am rellying on to be true that has exceptions of that i am misinterptetting?

    As to thinking about this as much as i can, thats gonna be difficult with gcses at the moment, but aslong as this doesnt get dissproved by the end of them, i might have to look into it further...

    i still wouldnt mind astronuc's final opinion on this just to confirm lol...
  20. May 19, 2007 #19


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    We're now at the point where I can be of no help, because it comes down to number-crunching. You would have to plot efficiency curves for all of the process involved, energy gains vs. losses at every point, etc.. I can't balance my checkbook, so count me out of this stage.
    I don't know what gcses are, but I assume some sort of exam. If it's that English thing, you need some warm-up time. While it doesn't matter much here, an English teacher probably won't be impressed with your current spelling, grammar, and punctuation. And since you're in England, they're probably pickier about it than someone over here would be. In any event, good hunting.
    And one of the great things about Astronuc and several other of the more valuable posters is that they seldom have a 'final opinion' as in dropping a hammer on you. They encourage you to understand why things are the way they are.
  21. May 19, 2007 #20
    yea. im not that bad at english... dont make errors in basic spelling like that often and when online im not always thinking about it... yes gcses are english examinations... you take them at 16. universities normally look at them because when you apply to a university you havent completed your a-levels yet. English is more about analysis etc, pretty boring. I just need to refresh my memory on to kill a mockingbird, educating rita and some poetry. But unfortunately i also have a latin test the same day, so that requires some revision.

    I thought it owuld come down to some calculations... most vitally, the energy produced by the hydrogen combustion in comparison to the energy used in the elctrolysis. I would imagine that there would be quite a lot of energy lost in conventional engine combustion because of the hot exhaust gasses. Hopefully if some of the heat energy can be utilised, less energy would need to be generated in the falling water. I could only do basic calculations myself, eg gpe=mgh ke=1/2mv^2 and i know some basic things about electrolysis and electricity - v=ir vi=w 96500 electrons/coulomb and q=it but beyond that i'm not aware of how to even go about calculating the rest. I imagine that the electrolysis calculation will be quite complicated too, i think the number of ions present and the time is important. water as is is has very few ions, meaning slow electrolysis so im not sure how you would calculate the number of ions in diff solutions etc.
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