Thought experiment - water at the bottom of the sea

[sophiecentaur]

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.

 

[russ_watters]

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?

 

[russ_watters]

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.

 

[sophiecentaur]

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.

 

[russ_watters]

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).

 

[Ilythiiri]

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 (:

 

[vaidya14]

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.

 

[sophiecentaur]

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.

 

[Ilythiiri]

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" (:

 

[sophiecentaur]

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.