Energetic Puzzle - thermodynamics, electrolysis on deep water "help"

[AngelLestat]

I'm always wondering what would be the most cost/efficient way to exploit and storage wind energy.

Now I want to know the benefics on deep water electrolysis, not sure if it would be cost/efficient (seems not) but I want to understand better the background physsics of this approach.

But I dont have practice with electrolysis calculations and Gibbs energy, also is not easy to identify the source of all energies involved (wind or enviroment).

So I will need your help to answer these questions:

1-What are the benefics (on efficiency or cost) doing deep water electrolysis vs surface?

2-what of the methods mentioned seems more cost/efficient? Energy sources involve?

3-Efficiency added if we exploit hydrothermal vents (black smokers) on electrolysis or other process.

Graphic details and extra info:

There are 3 methods, of course we can delivery the o2 and h2 using the same method, but this help to visualize in 1 image.

Something which is not very clear, is the pressure relation with the electrolysis efficiency, in theory taking into account the Gibbs Energy, it should cost more energy split water under pressure, but is the other way around. Also all best efficient methods work with high pressure electrolysis.

"high pressure electrolytes will consume less power in the process of electrolytic decomposition. The main reason was stated to be

the shrinking effect of pressure on the gas bubbles which cause the ohmic voltage drop and power dissipation to reduce. Moreover, high pressure electrolysis has less power demand for the phase of product compression", the max efficiency is at 700 bar with 250 c

http://www.electrochemsci.org/papers/vol7/7043314.pdf

Method A

This is the most simple case to visualize with Force * Distance = Work,

If we produce 1m3 of hydrogen with water at 700 bar, then in that volume we have 42kg of hydrogen.

This is equal to 5600 mjoules/m3 = 1583 kwh (energy of 1m3 h2 at 700 bar)

Lifting force and work:

9388 N * 7000 mts = 65.7 mjoules = 18 kwh

Then we need to add the oxygen lift force and work.

If we produce 1m3 of hydrogen, then we should (??) produce 1/2 m3 of oxygen, at 700bar weight 375 kg aprox, this provides 1225 N of force * 7000 mts = 8.5 mjoules = 2.3 kwh

18 + 2.3 = 20 kwh which seems negligible, but it does not need compression.

A1--> h2 load, displace water out

A2--> tank full of h2 rising

A3--> the tank release the h2 and let the water in, it remains open all the way down.

Method B

In this case we dont use a fixed tank, we use a bag, and we allow the bag expand meanwhile rise, this calculation is not simple, we should integrate, but we would need to include the pressure variable in some way. But is pointless, because the extra energy we would get, it will be the same (in theory) needed to compress the gasses back at 700 Bar.

Method C

Here we produce and exploit the flow that we get injecting h2 or o2 bubbles to a pipe, the bubbles transfer the kinetic energy to the flow, all bubbles over the 7000 mts help on this, depending how much water flow we allow under the tank it will determine the storage pressure (10 bar= huge flow same values than method B, 700 bar = small flow same values than method A)

In theory, we should have the same power output vs the other methods.

But with this we have extra benefics, we are injecting cold water full of nutrients in the surface, this increase the algae and plankton, which will absorb sun light (as solar panels) and absorb extra co2, it can be used as a perfect place to fish.

So? what methods seems better? we get some beneficts from the thermal gradient in the sea or is negligible?

Edited July 16, 2015 by AngelLestat

[PB666]

I'm always wondering what would be the most cost/efficient way to exploit and storage wind energy.?

Going by your photo, since chlorine only has to transfer one electron it preferentially undergoes oxidation and you produce CL2 gas not Oxygen. To make the system work you have to purify the water work. In addition the high level of salt will shorten the length of life of your expensive electrodes.

[NFUN]

Going by your photo, since chlorine only has to transfer one electron it preferentially undergoes oxidation and you produce CL2 gas not Oxygen. To make the system work you have to purify the water work. In addition the high level of salt will shorten the length of life of your expensive electrodes.

Chlorine would be reduced, not oxidized. It gains an electron.

And electrodes aren't expensive, they are preferably platinum.

[lajoswinkler]

Chlorine would be reduced, not oxidized. It gains an electron.

And electrodes aren't expensive, they are preferably platinum.

Nope.

Cathodes are electrodes where reduction occurs. (consonants)

Anodes are electrodes where oxidation occurs. (vowels)

Hydronium cations are reduced (given an electron), forming gaseous hydrogen.

Chloride anions would be oxidized (stripped from electron), forming gaseous chlorine.

Sodium cations are spectators, and they are joined by hydroxide anions forming at the cathode. It's a chloralkali process. You get hydrogen, chlorine and sodium hydroxide solution.

[AngelLestat]

Yeah, it seems that electrolysis in sea water is very complex, I read some time ago an article which explained how to extract all the compounds of seawater in a efficient way and then selling all those compounds by separate..

It was Na, Cl, h2, o2, some metals, etc. But I cant find the article.

But well that is not my question, but I understand that is still very difficult to answer because electrolysis seems to be more experimental than theory, and it has hundreds of different ways to do it.

There is no way to resume the problem to which is more efficient from the energy input perspective?

1 bar water electrolysis or 700 bar water electrolysis? Ignoring the fact that is seawater... But I guess this is one of those thing that can not be put in simple math. Or maybe the calculations will differ a lot from the real methods.

--------------------------------------------------------------------------------------------

It seems that I was not the only one with this idea.. here there is another method to do it:

https://scottishscientist.wordpress.com/2015/04/23/off-shore-electricity-from-wind-solar-and-hydrogen-power/

Edited July 17, 2015 by AngelLestat

[lajoswinkler]

Yeah, it seems that electrolysis in sea water is very complex, I read some time ago an article which explained how to extract all the compounds of seawater in a efficient way and then selling all those compounds by separate..

It was Na, Cl, h2, o2, some metals, etc. But I cant find the article.

Every chemical reaction is very complex and only by breaking it down to the simplest parts enables us to understand it when we fit the puzzle back again. As with all in science.

Electrolyzing sea water is different from electrolyzing aqueous solution of sodium chloride because sea water is a heterogeneous mixture. The solution itself contains alkali and earth alkali metal cations, as well as various anions, chloride being the dominant one, followed by bromide.

Before anything, sea water needs to be filtered and treated chemically to remove certain ions which would clog up the system when their precipitates form. For example, magnesium cation would react with hydroxide anions formed in the electrolysis to form poorly soluble magnesium hydroxide, eventually coating the electrodes and pipes.

But well that is not my question, but I understand that is still very difficult to answer because electrolysis seems to be more experimental than theory, and it has hundreds of different ways to do it.

There is no way to resume the problem to which is more efficient from the energy input perspective?

1 bar water electrolysis or 700 bar water electrolysis? Ignoring the fact that is seawater... But I guess this is one of those thing that can not be put in simple math. Or maybe the calculations will differ a lot from the real methods.

--------------------------------------------------------------------------------------------

It seems that I was not the only one with this idea.. here there is another method to do it:

https://scottishscientist.wordpress.com/2015/04/23/off-shore-electricity-from-wind-solar-and-hydrogen-power/

https://scottishscientist.files.wordpress.com/2015/04/deepseahydrogenstorage.jpg

I'm interested to see where's chlorine going. It's a decently valuable resource and if it's released into the environment, very poisonous, although short lived pollutant.

I know this is just a simple graphic, but the central thing here is high pressure electrolyzer which seems to be doing stuff very wrong.

[NFUN]

Nope.

Cathodes are electrodes where reduction occurs. (consonants)

Anodes are electrodes where oxidation occurs. (vowels)

Hydronium cations are reduced (given an electron), forming gaseous hydrogen.

Chloride anions would be oxidized (stripped from electron), forming gaseous chlorine.

Sodium cations are spectators, and they are joined by hydroxide anions forming at the cathode. It's a chloralkali process. You get hydrogen, chlorine and sodium hydroxide solution.

Oh, chloride anions. I misread, and thought he meant solitary chlorine in chemistry.

[peadar1987]

I saw a "free energy" concept based on this once. Basically you electrolyse water at high pressure, use the pressure differential or the buoyancy of the products to drive a turbine, then collect the hydrogen/oxygen mix at atmospheric pressure and pass it through a fuel cell.

Ignoring inefficiencies with the system (as these don't destroy energy, just create heat), a 100% efficient electrolysis process under pressure must take more energy than a similar process at atmospheric pressure, otherwise "free energy" would be possible. The electrolysis products have more energy when they are at high pressure, so they must take more energy to produce.

I think what the link is talking about is the efficiency of the electrolysis process itself, and the ease of storage. At higher pressure, the bubbles of hydrogen and oxygen produced will be smaller for the same mass. this means that less of the surface of the cathode and anode will be covered by non-conducting bubbles, meaning the overall resistance of the system (and therefore the amount of energy that gets wasted as heat) is reduced. You get a lower theoretical efficiency, but you can achieve a much higher proportion of it, similar to how the Otto cycle used for petrol engines has a higher theoretical efficiency than diesel, but diesel engines usually have higher real-world efficiencies, because they run at higher compression ratios, meaning they get a lot closer to their theoretical efficiency.

Also, gases need to be compressed for storage, so if they are produced at high pressure, you save yourself compression work later on.

[AngelLestat]

I'm interested to see where's chlorine going. It's a decently valuable resource and if it's released into the environment, very poisonous, although short lived pollutant.

I know this is just a simple graphic, but the central thing here is high pressure electrolyzer which seems to be doing stuff very wrong.

I still cant find that article that I am talking about,

I find some similar, but that one was very well explained, for example what % they left it as peroxide or clorine (I dont remember), and what they did with the others, all with their prices..

Something about molten salt and some addoms to release again the energy (in case they needed to produce electricity).

But well I will find it later.

About the last graphic, they said this:

So the technique will be to separate the custom more-concentrated electrolyte solution from the sea water by a semi-permeable membrane and allow pure water to pass through it by osmosis from the relatively dilute sea water.

But well, it does not seems like a serious study.

I saw a "free energy" concept based on this once. Basically you electrolyse water at high pressure, use the pressure differential or the buoyancy of the products to drive a turbine, then collect the hydrogen/oxygen mix at atmospheric pressure and pass it through a fuel cell.

Heh, there is not free energy there, even if we let the hydrogen expand in the bag meanwhile rise, to be extracted without compression, the energy is very far from the needed to split that amount of hydrogen back.

1 m3 of hydrogen has 1500 kwh, if you let it rise keeping the 700 bar, you get only 20kwh. That is close to 2% of the energy, and if we let it expand it will be 5% to 7% more, which is the energy needed to compress it.

Ignoring inefficiencies with the system (as these don't destroy energy, just create heat), a 100% efficient electrolysis process under pressure must take more energy than a similar process at atmospheric pressure, otherwise "free energy" would be possible. The electrolysis products have more energy when they are at high pressure, so they must take more energy to produce.

I think what the link is talking about is the efficiency of the electrolysis process itself, and the ease of storage. At higher pressure, the bubbles of hydrogen and oxygen produced will be smaller for the same mass. this means that less of the surface of the cathode and anode will be covered by non-conducting bubbles, meaning the overall resistance of the system (and therefore the amount of energy that gets wasted as heat) is reduced. You get a lower theoretical efficiency, but you can achieve a much higher proportion of it, similar to how the Otto cycle used for petrol engines has a higher theoretical efficiency than diesel, but diesel engines usually have higher real-world efficiencies, because they run at higher compression ratios, meaning they get a lot closer to their theoretical efficiency.

Also, gases need to be compressed for storage, so if they are produced at high pressure, you save yourself compression work later on.

Yeah, is like you said, it just make the method more efficient by other means, but not sure how much improve we are talking about...

Including compression vs other methods, maybe 3 or 5%? It would be great if we can get the heat from an hydrothermal vent, that would reduce the power needed by a lot, also all hydrothermal vent are pretty costant.

Not sure how the sea composition change with deep, this is on average: