Thought experiment - water at the bottom of the sea

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  • #26
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(unless i'm wrong my assumption that dissolved gas in seawater is saturated).
You're wrong.
 
  • #27
russ_watters
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You're hinting that i'm a "Omahgad, is it really free energy?!!!1" guy? No. But harvesting - yes.
In this case energy would come from ocean mixing currents(ultimately from Sun).

New thought experiment:
I pump water from 1km depth to surface using 10m diameter pipe.
At some point inside pipe, pressure change due to pumping and water column reduction creates dissolved gasses(mostly N2 i assume) oversaturation and outgassing in bubble form.
Resulting density change does work by moving unsaturated water upwards and sustains the process once the flow is laminar.
Energy lost due to water friction not a total waste - heat reduces water density/is useful.

Please criticize this one, i don't see any obvious holes(unless i'm wrong my assumption that dissolved gas in seawater is saturated). [emphasis added]
Well, the bolded part is basically gibberish. Yes, the density changes as the water rises up the pipe (for several reasons; release of dissolved gases being the least of them if it even happens) but that doesn't make it self-sustaining nor does it have anything at all to do with laminar flow.

You could, however, use the expansion of the water from it warming up and de-pressurizing as it rises to drive a turbine, generating an absolutely miniscule amount of power that comes nowhere close to countering the loss from pumping the water out of the ocean.

Look, there are a host of ways that something dug or pumped out of the ground will absorb or release energy spontaneously when reaching the surface. Almost anything you pump or dig out of the ground will. But none of them come anywhere close to the energy value of the resource itself -- with the exception of geothermal energy in which the resource is heat which will be spontaneously absorbed or rejected it if not recovered.
 
  • #28
sophiecentaur
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generating an absolutely miniscule amount of power
Why only a tiny amount? Wouldn't the Potential Energy be reasonably significant? In general, compressed gases are a good way of storing mechanical energy (the energy put into compressing air into a scuba tank is enough to cause an explosion when a tank bursts.. The energy could be greater than the GPE involved in raising the gas (or even gas+water) by 2km which would be supplied by hydrostatic pressure from the sea. Nitrogen takes time to dissolve and evolve so the bubbles could form after the water was near or at the surface. (Diving tables for relatively shallow air breathing dives involve many minutes of decompression time.)
 
  • #29
russ_watters
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Why only a tiny amount? Wouldn't the Potential Energy be reasonably significant?
This thread is severely lacking in numbers, so here's a back-of-the-envelope calc (please feel free to check my math):

Here's a study of the solubility of CO2 in water, with data up to 1,000 ATM:
https://www.researchgate.net/public...f_literature_data_and_thermodynamic_modelling

It seems to drop off below 10C, but we can ignore that and use the 10C value (assuming it is saturated, which is a big assumption).

At 10C and 1000 ATM (equivalent depth 10,000m), the solubility is about 4% by moles. One cubic meter of water contains 55,555 moles of water and therefore 2222 moles or 286 cubic meters of CO2 at room temp and atmospheric pressure. Since solubility varies with pressure you only average half the pressure delta in the expansion, so the energy in the expansion is 14.3 megajoules. That's more than I would have expected, but by comparison:

-The thermal energy capacity of the water is 67 megajoules (with a 16C temperature rise).
-A cell phone battery is on the order of 32 megajoules.
-A cubic meter of diesel has an energy density of 35,800 megajoules.

So, miniscule.
In general, compressed gases are a good way of storing mechanical energy (the energy put into compressing air into a scuba tank is enough to cause an explosion when a tank bursts..
Really? I'm having trouble thinking of a practical example where compressed air is used for energy storage. It is proposed for things like cars, but has yet to be practically implemented as far as I know. The best I can do is CO2 cartriges for specialty applications like paintball guns.

The fact that a scuba tank can explode doesn't really mean anything since it isn't being stored for its energy. Knocking over a building releases a lot of energy too...
The energy could be greater than the GPE involved in raising the gas (or even gas+water) by 2km which would be supplied by hydrostatic pressure from the sea.
I didn't include it as an example because as you say you can get it back if you have a piping loop, but no, it's nowhere close. For my example at 10 km depth, the GPE of 1 cubic meter of water is 98 MJ.
 
  • #30
olivermsun
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I'm having trouble thinking of a practical example where compressed air is used for energy storage. It is proposed for things like cars, but has yet to be practically implemented as far as I know. The best I can do is CO2 cartriges for specialty applications like paintball guns.
Air tools running off a compressed air tank are a good practical example. Air brakes with reservoirs for failsafe operation are another example.
 
  • #31
sophiecentaur
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Really? I'm having trouble thinking of a practical example where compressed air is used for energy storag
It isn't a front runner but there is work going on, based on the sort of numbers that would be relevant here. This link, as well as the Wiki stuff is a bit lightweight but shows that someone, somewhere, finds it worth while thinking about.
For my example at 10 km depth, the GPE of 1 cubic meter of water is 98 MJ
Yes, as you say, that energy is not relevant because it comes from the ocean water falling by an equivalent amount - for free.
The fact that a scuba tank can explode doesn't really mean anything since it isn't being stored for its energy.
I didn't make the right point about that. To produce a scuba tank full at 300Ats involves a few kWh. That's the sort of energy that would be available (about 25% of it, actually). 300Ats corresponds to around 3km depth so it's a representative figure.
But the question would be how much constant supply could be obtgained and what would a pipe that deep cost to instal (no idea of diameter that would be practical but we'd be talking the costs of an oil well, I suppose.

It clearly wouldn't be a staggering money maker but it would be continuous and reliable.
 
  • #32
russ_watters
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Air tools running off a compressed air tank are a good practical example. Air brakes with reservoirs for failsafe operation are another example.
I like the air brakes example, but it is single use, so not much energy. But are machine shops ever really run with stored compressed air (besides the surge tank?)? Are there portable air tools?
 
  • #33
russ_watters
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I didn't make the right point about that. To produce a scuba tank full at 300Ats involves a few kWh. That's the sort of energy that would be available (about 25% of it, actually). 300Ats corresponds to around 3km depth so it's a representative figure.
But the question would be how much constant supply could be obtgained and what would a pipe that deep cost to instal (no idea of diameter that would be practical but we'd be talking the costs of an oil well, I suppose.
You aren't suggesting this could be done passively (with a single, CO2 filled pipe?), are you? That's the PMM fallacy that started the thread: the pipe just fills with water and after that nothing else happens. To do this at all requires many km of piping, both up and down, and a pump to circulate the water.

Note: I previously mentioned the thermal energy capacity of the water: Water is harvested in this manner for its thermal energy capacity, but only at depths of about 200m and at very high initial cost.
 
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  • #34
sophiecentaur
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I'm talking (and I thought the thread was) about using the existing dissolved gases (N2 etc?) and encouraging a kind of convection which uses the PE of the dissolved gases and which would be self sustaining. I guess that the main argument against this would be that it doesn't occur naturally.
 
  • #35
russ_watters
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I'm talking (and I thought the thread was) about using the existing dissolved gases (N2 etc?) and encouraging a kind of convection which uses the PE of the dissolved gases and which would be self sustaining. I guess that the main argument against this would be that it doesn't occur naturally.
Ok, well, with no process to analyze, there isn't much that can be said about that speculation other than that per the OP's example, this type of speculation tends to lead to or be based on perpetual motion fallacies. Generally, as you imply, you need an already existing natural process to harness because extracting energy from spontaneously happening processes in the environment generally means you are interrupting a process that is already occurring (such as interrupting a river by building a dam).
 
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  • #36
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I like the air brakes example, but it is single use, so not much energy. But are machine shops ever really run with stored compressed air (besides the surge tank?)? Are there portable air tools?

Yes there are. For safety in explosive environments though, not for energy storage aspects.

Laminar flow - i meant when process/flow is stable, constant across the length of pipe.

In essence, i hoped dissolved gas phase transition would give additional usable energy(idea from decompression sickness).
Anyway, apparently this doesn't happen spontaneously(for example with underwater volcanos), so no cake here.

Back to clathrates i guess (:
 
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  • #37
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The work that you would have to put into forcing that empty pipe to a depth of 2km would be the same as the work you would get out. As compressed water is admitted into the empty pipe, it will expand to its normal density (a bit of an incidental, actually) pressure at the bottom will force more and more water into the pipe and the water will accelerate. By the time it reaches the surface, it will still have a vertical velocity so you can expect it, indeed, to shoot out at the surface if the pipe is wide enough to cause low drag. If the pipe end is significantly above the surface then the movement will carry the level high than the sea surface but then the level will fall back until there is equilibrium. The level will oscillate and will reach equilibrium as the kinetic energy is dissipated. This is the same with water in a U tube.
To my understanding if the pipe is open on both is its end the water will not go beyond the sea level(as you depict oscillate). This is because..let it see in the form of energy....the total energy, the upward flowing water will be less than or equals to the energy of the water body exerting this energy.(here the sea water). I would like to know your thoughts about it..
Thanks.
 
  • #38
sophiecentaur
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To my understanding if the pipe is open on both is its end the water will not go beyond the sea level(as you depict oscillate).
If the pipe starts full of just air, the water entering at the bottom will be under pressure all the time the pipe is filling up. It will gain speed unless the pipe is extremely narrow. That Kinetic Energy will be in addition to the GPE it will have gained on the way up. It will 'keep going' until the KE is used up in giving the column extra height above the top of the pipe. This process can be seen in any U tube in which the levels start off unequal and will be an oscillation that dies down as losses take the mechanical energy away.
 
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  • #39
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I've thought about saturation part.
Water capacity for dissolved gasses considerably increases with pressure, but below sea level there's no source of nitrogen to saturate that increased capacity, only diffusion from surface layers.
Any vertical mixing will decrease dissolved atmospheric gas concentration in deeper water layers, counteracting diffusion.
Resulting equilibrium is far from saturation.

Special case would be gasses created underwater.

Pick lake or sea with little mixing, resulting anoxic bottom environment and subsequent H2S saturation by bacterial activity - for example Baltic sea. Stick aforementioned pipe to the bottom. Expend energy to create initial flow. Oversaturation and bubbles happen, sustaining process. Harvest excess energy from this process.
Since releasing H2S into atmosphere will create abundant sulphuric acid rains anyway, separate and burn H2S. Energy and profits!

Obviously this would require adopting common corporate practice of ignoring hidden costs - "pollution is not my problem as long as there's no fines, then some lobbying expenses" (:
 
  • #40
sophiecentaur
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Water capacity for dissolved gasses considerably increases with pressure, but below sea level there's no source of nitrogen to saturate that increased capacity, only diffusion from surface layers.
Assuming a large area of ocean plus some currents, diffusion in the local area would not necessarily be the limiting factor to N2 concentration at depth. But I see what you're saying.
Special case would be gasses created underwater.
I thought that had been dealt with higher up the thread.
 

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