Can Hydrogen and Water Create a Sustainable Energy Cycle?

[samski]

Hey, I am a bit confused about this...

sorry if I am being stupid, I am 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 traveled down by the water and up by the hyrodgen, the greater the excess energy...

Thanks

sam

 

[Astronuc]

Surely there is an excess energy

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.

 

[samski]

HMM i don't quite understand you... surely the heat is yet more excess energy...

I don't 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...

:s

 

[Astronuc]

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.

 

[samski]

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 can't work, but i just can't work out how it can't 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)

 

[Astronuc]

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.

 

[samski]

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.

Thanks , I've always liked physics (my best subject) and I am 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 realized sea water would be too impure and so i though of this idea - using the "same" water over and over...

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.
Certainly, one can take advantage of convection in the atmosphere.

yes after posting my last post i realized 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 isn't a closed environment is it?

so the combustion of H2 in air will dissipate some energy to N2.

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...

 

[samski]

so how pheasable do you think this is?

 

[Danger]

As pheasable as a phrog phrolicking in a phorest.

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.

 

[samski]

hence i asked how far a water molecule would have to far to generate enough energy to be electrolysed. what's 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 I am 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?

 

[Danger]

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.

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.

 

[samski]

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?

 

[Danger]

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?

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.

 

[samski]

nonono I am 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...

 

[Danger]

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.

 

[samski]

no its cool, i think I am 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, that's 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!

 

[Danger]

should i stop thinking about this and get on with my english literature revision lol!

Don't ever give up thinking about anything!
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.

 

[samski]

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 can't 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, that's going to be difficult with gcses at the moment, but aslong as this doesn't get dissproved by the end of them, i might have to look into it further...

i still wouldn't mind astronuc's final opinion on this just to confirm lol...

 

[Danger]

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.

 

[samski]

yea. I am not that bad at english... don't make errors in basic spelling like that often and when online I am 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 haven't 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 I am not sure how you would calculate the number of ions in diff solutions etc.

 

[Danger]

Hot exhaust gasses aren't by any means the only loss in an IC engine, and some of that can be recovered with a turbocharger. A lot, if not most, of the thermal losses are through the block and heads into both the air and the cooling jacket which then subsequently dumps it into the air through the radiator. There are also enormous frictional losses through piston ring contact, wrist pin rotation, rod bearings, crank bearings and seals, cam bearings and seals, rocker arm pivots... and on and on.
Electric motors are far more efficient; the problem is matching the energy storage density of gasoline or diesel with a battery pack.
As for the electolysis, the energy required to split the molecular bond is constant and has no relationship to the electrolyte content of the water. That only affects how easily current can be transferred throught the medium.

 

[samski]

aha! i had an inkling that it would simply speed up the process, after all you are only attracting the ions, not splitting the water molecule...

would some kind of fuel cell be more efficient? Also can these work without hydrogen being under pressure?

 

[Danger]

I really don't know anything at all about fuel cells. If my understanding of their function is correct, my semi-educated guess would be that both gasses should be at equal pressure. Don't they just diffuse through a semi-permeable membrane of some sort? I could be quite wrong about this, since I haven't studied up on them at all.

 

[samski]

yea, I am pretty sure that in any car it is pressurised, but I am not sure if that is just to save space...

a fuel cell would be perfect - its output is electricity so no need for dynamos as with an engine

 

[Danger]

But yet again, there is no way in the world you can extract as much energy from a fuel cell as is required to split the hydrogen and oxygen apart in the first place to provide the fuel for it.

 

[samski]

but the vast majority can be provided. the falling water genertaing electricity would provide the rest.

Electrolysis doesn't use up lots of energy... it can be done with 6v

 

[Danger]

Electrolysis doesn't use up lots of energy... it can be done with 6v

Voltage isn't the only factor. You could discharge a Van de Graf generator at a couple of million volts into the tank and get nothing. A 12V car battery at 1,000 amps, however, could produce enough to create a serious explosion hazard.
And I dispute your assertion that 'the vast majority' of the electrolysis energy could be created by a fuel cell powered from that same water. This is once more, though, in the realm of math.

 

[samski]

in a perfect world it would create all of the energy. bonds broken and bonds formed. When elctrolysing it, you have effectively broken 2 hydrogen water bonds and formed 1 hydrogen hydrogen bond. in combustion the opposite happens. obvioudly its not a perfect world however I am sure the energy produced would go most of the way to electrolysing it.

 

[Danger]

In a perfect world, I'd be married to Sandra Bullock, have Angie Harmon for a mistress, and never have to work for a living...
Again, it comes down to the numbers. You have to work it out and post actual figures in order to make your point.

 

[samski]

from howstuffworks: "If the fuel cell is powered with pure hydrogen, it has the potential to be up to 80-percent efficient. That is, it converts 80 percent of the energy content of the hydrogen into electrical energy. "

however I am not sure if this is pressurised hydrogen...

edit: here is another source from wikipedia: "A cell running at 0.6V has an efficiency of about 50%, meaning that 50% of the available energy content of the hydrogen is converted into electrical energy; the remaining 50% will be converted into heat."

However I am pretty sure many methods exist to make use of the heat energy (steam turbines etc). Just as in many power plants, the steam produced can passed close to water pipes, heating it up and producing high pressure steam which drives turbines.


edit2: this is pretty interesting: http://en.wikipedia.org/wiki/Reversible_fuel_cell

 

[Astronuc]

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

NOx are consequences of combustion in air, but that can be addressed. The issue is that any energy released from combustion will be somewhat dissipated, in this case through N2 which is relatively inert. At the other end, one has to put more energy into the electrolysis process than is required to break each bond. So in either step, there is a loss of useful energy.

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...

One has to look at the energy transfer in various processes to see how feasible.

Basically, hydropower utilizes some of the steps you've mentioned, but without electrolysis and separation of H from O in H2O. The sun evaporates moisture, which is transported from a lower level (e.g. sea level) to higher level (mountains) - there is a change in the gpe of that water. The atmosphere (wind, water vapor and clouds) does the mass transport. Condensation causes vapor to liquid, which then runs down into streams, rivers, into lakes/reservoirs. The water is allowed to fall through hydroturbines which drives generators, which generate electricity. The mass transport system is huge - and the solar energy is one the order of 1 kW/m².

Then there is wind power, which extacts energy from wind, which is driven by large mass flows (convection) in the atmosphere.

 

[Danger]

Pressurization is irrelevant; it has to be de-pressurized for use. 80% return sounds like a reasonable figure, but that's a far cry from 'vast majority'.

edit: Oops! A bunch of bloody customers came in while I was composing, so I've been gone for a while. Missed Astro's post until now. In fact, I've still missed it. Have to go back and read it now.

edit#2: And now that I've read it, I'll add only one comment. Wind power is still solar-driven, since the heat energy that causes convection comes from the sun.
Thanks for re-joining, Astro. I'm at the end of my knowledge here.

 

[Astronuc]

Don't confuse energy conversion (and it's attendant efficiencies) with energy creation.

Bullock and Harmon, eh? Hmmmm. lol

 

[Danger]

Bullock and Harmon, eh? Hmmmm. lol

Yeah, well... you know...
Jeez, I'm glad that W doesn't read this stuff.

 

[Astronuc]

Yeah, well... you know...
Jeez, I'm glad that W doesn't read this stuff.

I had the same thought. Could be detrimental to one's longevity.

 

[samski]

ok, so is there anyway of getting some estimates on this? not too sure where to go for estimates of energy loss etc

good to know that it doesn't need to be pressurised, that could have used quite a lot of energy.

so, we have ~20% energy loss to make up for the combustion process (dont know if this inlcludes getting energy back from the exhaust gas heat) and how much in the electrolysis process would you say is it in thr order of about 20% again? As well as other things, such as waste heat (if that wasnt included in the 20%above). When making this up, any turbines turned by the water also have frictional loss... do you have anyywhere i could get numbers for this? i imagine it depends of turbine size.

Yes i can see the similarity between this and hydropower... however its disadvantages are that you can't just turn rain on and off. And the same for wind power... This seems to be much more reliable (although more complicated than just a large turbine/impellor (i think that's the name for it)...

anyway, its getting late here in merry england so i'll call it a night!

thanks very much both of you...

sam

 

[samski]

Hey, I've started a few calculations. I got the overall energy input to to break 2 mol of water as 469.6kJ. There are 4 moles of O-H bonds to break @ 458.9kJ/mol each (source: http://en.wikipedia.org/wiki/Bond_energy)

You form 2 H-H bonds @ 436kJ/mol each (same source). And you form one O=O bond @ 494kJ/mol.

458.9 * 4 - 436 * 2 - 494 = 1835.6 - 872 - 494 = 469.6

at the combustion side I am assuming you would get this energy out (in a perfect world)

At the combustion side, assuming 80% efficiency, the energy output is
0.8*469.6 = 375.68kJ/2mol = 187.84kJ/mol

at the electrolysis side, if i have 80% efficiency (this not a real number), is it 1.2 *469.6 = 563.52kJ/2mol = 281.76kJ/mol

therefore, total energy to make up/mol = 281.76-187.84=93.92kJ/mol

OK, i found this about water on (http://en.wikipedia.org/wiki/Pumped-storage_hydroelectricity): "For example, 1000 kilograms of water (1 cubic meter) at the top of a 100 meter tower has a potential energy of about 0.272 kW·h."

That's 979.2kJ so that's 979.2J per 1000grams, 1mol water = 18g. 0.9792 * 18 = 17.6256J

This is getting rediculous! per meter that's 0.176256J. we want 93.92kJ. so total meters to fall is 93920/0.176256 = 532861.29m = 533km

LMAO.

I think this is over...

sam

 

[samski]

no wait i think we're ok:
"However, the exosphere can extend from 500 up to 10,000 km above the surface,"

lmao :P

 

[Danger]

Okay, so we're agreed that it's over. But isn't it far more satisfying and eductional to have come to that conclusion yourself through logical analysis rather than if you'd just been told to pack it in?

 

[samski]

yea its definitely much better.

further musings:

if the drop was 500m, the total generated would be 1.36kwh = 4896kJ per 1000 kg of water = 4.896j/g. Times 18 is 88.128J/mol lost. Tot energy input = 88.128+469600 = 469688.128J Tot energy out = 469600j

469600/469688.128 * 100 = 99.98123% efficiency

hmmm... interesting

 

[Danger]

I'll have to wait for Astro to verify those numbers; it's way over my head. Good on you for going through all of that.

 

[samski]

hehe.. i was surprised i could actually work it out with my knowledge of chemistry and physics...

Thanks both of you for your help... it was nice to get to the bottom of it