Pressure Harvesting - from ocean depths

Discussion in 'General Science & Technology' started by Quantum Quack, Mar 8, 2020.

1. Quantum QuackLife's a tease...Valued Senior Member

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If i install a bike pump inside the Titanic i can assure you with out doing any math what so ever that it will sink and the air will compress.

Last edited: Mar 9, 2020

3. Quantum QuackLife's a tease...Valued Senior Member

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I suppose you think a bike pump will float on the surface if you threw it in the ocean?
Ever done it?
It only has to be heavier than the water it displaces and it will sink.

5. exchemistValued Senior Member

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...from someone at the bottom of the class, yes.

Last edited: Mar 9, 2020

7. exchemistValued Senior Member

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What we are telling you is you cannot "solve the energy retrieval cost problem". It is intrinsic.

The change in buoyancy results from the difference in volume of air. The resulting net work you have to do in lowering and raising the cylinder is exactly equal to the stored energy in the compressed air once you have it back at the surface. There is no getting away from this. No technology can magic it away.

And this is why nobody has thought of it before. It is, to any decent engineer or physical scientist, a useless idea. Sorry, but there it is.

8. exchemistValued Senior Member

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But if it sinks, you then have to pull it back up. Which takes energy.

9. exchemistValued Senior Member

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You seem determined not to see the point here. You can call it "harvesting" pressure, as you put it, but it will cost you at least the same energy to "harvest" it as the energy that you "harvest". So your "harvesting" exercise is a waste of time and provides no energy.

10. Quantum QuackLife's a tease...Valued Senior Member

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Just as a matter of interest and relevance have a read of the following quoted from wiki..
https://en.wikipedia.org/wiki/Compressed-air_energy_storage

Constant-pressure storage
In this case the storage vessel is kept at a constant pressure, while the gas is contained in a variable-volume vessel. Many types of storage vessels have been proposed, but the operating conditions follow the same principle: The storage vessel is positioned hundreds of meters underwater, and the hydrostatic pressure of the water column above the storage vessel allows maintaining the pressure at the desired level.

This configuration allows:

• Improve the energy density of the storage system, because all the air contained can be used (the pressure is constant in all charge conditions, full or empty, the pressure is the same, so the turbine has no problem exploiting it, while with constant-volume systems after a while the pressure goes below a safety limit, and the system needs to stop).
• Improve the efficiency of the turbomachinery, which will work under constant inlet conditions.
• Opens to the use of different geographic locations for the positioning of the CAES plant (coastal lines, floating platforms, etc.).[16]
On the other hand, the cost of this storage system is higher, due to the need of positioning the storage vessel on the bottom of the chosen water reservoir (often the sea or the ocean) and due to the cost of the vessel itself.[16]

Plants operate on a daily cycle, charging at night and discharging during the day. Heating of the compressed air using natural gas or geothermal heat to increase the amount of energy being extracted has been studied by the Pacific Northwest National Laboratory.[15]

Compressed-air energy storage can also be employed on a smaller scale such as exploited by air cars and air-driven locomotives, and can use high-strength carbon-fiber air-storage tanks. In order to retain the energy stored in compressed air, this tank should be thermally isolated from the environment; else, the energy stored will escape under the form of heat, since compressing air raises its temperature.

so it is not a new idea and is currently employed ( in part )

Key concepts:
Variable volume vessel that reduces volume as pressure is exploited yet maintains a constant pressure in the process.

Still researching...

Last edited: Mar 9, 2020
11. exchemistValued Senior Member

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This is energy storage. That is fine. You put energy in, and later you take the same energy back (apart from process losses). Of course you can do that.

It is not at all the same as trying to use something as an energy source, which is what you were claiming could be done.

12. Quantum QuackLife's a tease...Valued Senior Member

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23,328
in way it is... but not quite...
It is a variable volume storage system (VVSS) that used ambient pressure to maintain constant pressure in the vessel as it is being exploited.

The ambient water pressure is being used to maintain that pressure as the volume inside the tank reduces due to exporting the air pressure to the surface..
But this is not the full story IMO... there is something else that could improve this system. upon.
From what I understand the VVSS is fed pressure from the surface using mechanical pumps when they recharge it.
It must be economically viable to do so, other wise they wouldn't be doing it.

To improve on it's economics I would be looking at delivering pressure harvested from the ambient pressure instead of pumping it down from the surface.
====
Say you establish a large Bulk VVSS down about 1000 meters. ( 1472 psi)

You sink down pressure harvesting VVSS submersibles to top it up by docking with the Bulk VVSS.
The energy cost to go down by sinking is negligible.
No need for mechanical pumping at the surface.
All pressure to the bulk storage VVSS is harvested from ambient pressure.
====
If designed properly the submersible VVSS could be recycled and used over and over again...to top up the bulk storage.
It would have to sink lower than the Bulk VVSS so that it has the pressure stored that is higher when rising to dock with the bulk VVSS and transfer harvested pressure.

How much compressed air is needed to float the submersible to the surface?
This could be provided by releasing a small quantity from the stored air pressure. Just like submarines do.

End result:
A constant air pressure of 1472 psi delivered to the surface....from the bulk VVSS via tubing or pipes. All of which is supported and provided by harvested ambient water pressure.

Last edited: Mar 9, 2020
13. exchemistValued Senior Member

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This system is "harvesting" nothing. It's a storage system.

You pump air down. This takes energy, because you have to compress it to the pressure of the water at the chosen depth. When you want to run a turbine or something from the stored compressed air, you get the same energy back that you previously put in. I say again, nothing is "harvested".

The advantage of the system, as described, is that because it is open to the water, the volume changes as you charge or discharge it, rather than the pressure. So however much air remains in the storage vessel, it is always at the same pressure. This is unlike a closed compressed air cylinder, in which it is the volune that is constant and so the pressure obviously drops as you uses it. Having a constant working pressure allows you to optimise both the compressor and the turbine or engine (in fact the two may be the same machine).

14. Quantum QuackLife's a tease...Valued Senior Member

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To maintain pressure in the system described in the wiki they make use of ambient pressure ... yes?
Call it what you like but with out the ambient pressure being exploited the system would not be a VVSS.

btw I use the term harvested because you are reaping pressure and then taking it somewhere...
perhaps exploited would be better....

Last edited: Mar 9, 2020
15. exchemistValued Senior Member

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Oh I read it. But I was resisting calling it what it is. But my patience is exhausted: you force my hand. What it is is, well, idiotic.

The phrase in which the stubborn stupidity sets in is this one:-

"To improve on it's [sic] economics I would be looking at delivering pressure harvested from the ambient pressure instead of pumping it down from the surface."

In other words, to "improve its economics" you would - airily, with a snap of the fingers - change it from an energy storage device (which we all agree is feasible) to an energy source. This is exactly what I have been pointing out, at length, is impossible to do.

But I'm losing interest now. I begin to think you are either trolling for your own amusement or else you are an irredeemably stupid person. Whichever it is, I am close to having had enough of it.

16. Quantum QuackLife's a tease...Valued Senior Member

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nope...
I would be supplying the storage device with out using mechanical pumping...and snap both fingers doing it...

17. billvonValued Senior Member

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Yes. And if you have unlimited Titanics to sink (and thereby convert gravitational potential energy to work) then you could convert their potential energy to actual energy that way. However, that would be converting a lot of energy into less energy. It would be, in effect, an energy dissipator.
Yes.
Yes.

exchemist likes this.
18. DaveC426913Valued Senior Member

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No matter how many tweaks you add to your system, it will take energy to move it somewhere.

If you try to take uncompressed air down to depth, it takes energy to do so (that's why deep sea divers of yore needed surface compressors).
If you try to lift compressed air up from depth, it takes energy to do so (just like picking a rock up off the ground) because it is not buoyant.

You can't escape this.

If you use some contraption (such as solar power or whatever) to assist your system, then again you might as well bypass the whole ocean pressure aspect and use the energy source directly ...

... because ocean pressure is not a source of energy - any more than rocks at the bottom of a cliff.

19. geordiefValued Senior Member

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2,143
Could wave power be used to lower and raise objects under water?

The change in pressure within the objects could be harvested although there would be no net gain as the extra energy (if it could be harvested) would come from the waves in the first place.

It might be a way of transferring energy from the surface to below the surface
I feel quite baffled by QQ's arguments but still ,can I ask whether the whole introduction of the ocean /sea is a red herring since the atmosphere itself is just a rarefied form of the ocean ,with its own body of gases rather than H2O and it's own accumulation of pressure at ground or sea level.

What would QQ's argument look like if transposed to the atmosphere rather than the sea, I wonder?

20. Quantum QuackLife's a tease...Valued Senior Member

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This puzzles me...
A submarine with uncompressed air for the crew only has to fill it's ballast with water and it sinks no problemo...
So I am not sure what you are talking about...
It takes no added energy ( except opening a valve ) to sink a sub.
By the time it gets to 1000 meters it's crew are living in approx. 1472psi of compressed air. If it were not the sub would implode.

So please explain why you think it is other wise other than just claiming it to be...

Last edited: Mar 9, 2020
21. DaveC426913Valued Senior Member

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I was actually thinking about offering such an analogy, hoping it might help QQ see the flaw in his arguments, but I don't know if it will help.

I also thought it might be better to go with the scaled-down "swimming pool" setup QQ himself suggested. It might help to show the flaws.
To remove the apparent mystery of what water pressure does, instead of water molecules, fill the pool up with baseballs (we'll even make them frictionless) thhee feet dep.

It's pretty easy to see now that trying to push any object (say, a soup can-sized container of air) down to the bottom of a pool of baseballs requires some effort. Not because of the friction - we've eliminated that. It take effort because, to place such an object at the bottom of a full swimming pool of baseballs requires making room for it at the bottom - by effectively lifting up a number of baseballs equivalent to the volume of the soup can. That's lifting a dozen baseballs about 6 inches.

What you're doing here is exactly equivalent to this:
put the soup can on the ground,
pick up a stack of a dozen baseballs
lift the stack six inches
set the stack on the soup can.

The effort it took to lift the stack of baseballs 6 inches and place them on top of the soup can is exactly equivalent to the effort requires to ignore the baseballs, and simply push down on the top of the soup can with your hand.

And this works in reverse - if the soup can is compressed at the bottom of the pool and you want to lift it up.

22. Quantum QuackLife's a tease...Valued Senior Member

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Sorry if I have confused you...it is an evolving idea...

It is all about pressure differentials
Sea level air pressure is about 14.69 psi
maximum low pressure is zero. Gain (-)14.69psi when returned to surface.

Average ocean depth allows over 4000 psi differential
High density resource of potential energy.

23. exchemistValued Senior Member

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I was having a look at the numbers on this
Because you are wrong about the air in a sub. It does not get compressed. A sub is a pressure vessel that withstands the pressure due to depth. If the air became compressed the submariners would get the bends if they surfaced too quickly.

The sub sinks or rises due to its weight minus the buoyancy of its hull. When the tanks are flooded the buoyancy is reduced to close to neutral (ie almost matching the weight), allowing the sub to go up or down in response to the angle of the hydroplanes. When it surfaces, air is pumped into the tanks, displacing water out and increasing buoyancy again, so that it rises.