# Pressure Harvesting - from ocean depths

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

1. ### DaveC426913Valued Senior Member

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I am not being silly.
You are simply seeing the conundrum in a more obvious way and realizing it obviously can't be right. You're almost there.

How is your way simpler?
A rigid tube requires no moving parts and no energy input.

If your ideas about energy from pressure were sound, why would the tube not work?

That is a question left for you.

Once you solve it, you will see why no system can extract ocean-depth water pressure to do work.
And that, if compressed air is the goal, it can be made at less energy cost with resources we already have.

Ocean water pressure is no more a source of harvestable energy than rocks at the bottom of a cliff. They are as low as they can get. The only way to extract any energy from them is to first put energy in to raise them.

Last edited: Mar 8, 2020

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

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no it isn't
try this:
Take a manual hand bike tire pump. You know the sort people use to pump up their bicycles tires.

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Pull the plunger out to its maximum.
Block off the output port...keep the input ports open.

Throw it in to 1000 meters of water.

What happens to the pump? ( other than getting wet lol)
The laws of physics will state that the plunger will compress the pump air chamber due to ambient water pressure as the pressure forces the pump plunger to pressurize the air in the air chamber.
you good so far?
The pump is sitting at 1000 meters with compressed air trapped inside it.
Lock down the plunger so the compressed air chamber can not increase in size and retrieve the pump to the surface.
Bingo... you now have a pump with compressed air in it.

as I said it ain't rocket science...
and I have just now harvested pressure from the ocean depths...using a bike tire pump.

nothing perpetual about it...

As Write4u posted in another thread
"The oceans can be used as a natural compressor" or something to that effect...

Last edited: Mar 8, 2020

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5. ### Quantum QuackLife's a tease...Valued Senior Member

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The intriguing thing about this is that we can take advantage of the ambient pressure at ocean depth with out reducing that ambient pressure.

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7. ### originHeading towards oblivionValued Senior Member

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Great, now what do you do?

8. ### billvonValued Senior Member

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Since it would be buoyant, it would take energy to winch it down there. Since the ascending chamber will have less volume, it will be less buoyant, so you can't recover that energy. You might as well just use the energy you spent to compress the gas to begin with.

There's no free lunch.
Yep. But you have to put a lot of energy in to compress it that far. Again, do the math.

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9. ### billvonValued Senior Member

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It, being full of air, will float.

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10. ### exchemistValued Senior Member

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That is exactly what it is.

When you lower your chamber into the water, a volume of water equal to its volume is displaced, reducing its apparent weight by the amount of buoyancy due to the volume of air inside. If we assume the chamber is still heavier than the buoyancy force it will sink, doing work on your pulley system as it does so.

When it gets to the bottom you open the valve, allowing seawater to compress the volume of air inside and greatly reducing the volume it occupies.

You then have to pull the chamber up to the surface again with the reduced volume of air inside, i.e. with a much reduced buoyancy force. So you do more work pulling it up than the work it does on your pulley system in descending.

It turns out this extra work you have to do is exactly equal to the work you can extract from the stored energy in the compressed air, once you have recovered the chamber at the surface.

So you have no net energy available to do any work. In other words, your idea will not work.

11. ### exchemistValued Senior Member

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SNAP!

Ninja'ed.

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12. ### originHeading towards oblivionValued Senior Member

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This is more accurate statement, The oceans can be used as an inefficient and expensive natural compressor.
It is much more efficient to use a mechanical compressor.

13. ### exchemistValued Senior Member

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Your error is in neglecting the energy you have to expend getting the pump down and then back up again. It will differ by the amount of stored energy in the compressed air that you trap in the pump. No free lunch.

If Write4U's quote was not ballocks, it will have referred to wave energy. This you can extract and use to compress air and drive a turbine, because it is extra kinetic energy of the water, imparted by the wind, which you can convert to something else.

What you can't do is magically get energy continuously out of a static body with no energy input. That defies the 1st law of TD. And that is what you are trying to dowith your device.

Last edited: Mar 8, 2020
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14. ### Quantum QuackLife's a tease...Valued Senior Member

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Fact is I can sink my bike tire pump into deep water lock it down and retrieve compressed air with out depleting the ambient pressure.
Your error is in projecting your fears onto a simple idea..
I do not think it conflicts with any laws but perhaps you can explain how you think it does...
You are yet another member fixated on perpetual energy devices...this idea of pressure harvesting has nothing to do with it.
Why do you think it is referring to a perpetual energy device?

Gosh when they discovered nuclear energy did they have the same hysterical problem?

Write4U was simply stating that the ocean depths can be used as a natural compressor... is this not true?

15. ### exchemistValued Senior Member

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. You can certainly achieve compression by taking something to the bottom of the sea. But the sea cannot be run as a compressor, no. As I have already said, you cannot continuously extract energy from a static object with no net energy inputs.

You are ignoring what billvon and I have both independently pointed out: that work is done when the container is made to sink to the bottom of the sea and work is done when it is recovered. And the two will not be the same. The difference will be the amount of stored energy in the container due to the compression of the air in it.

There. This has now been explained to you 3 (three) times. It is about time you started to address this explanation, rather than demanding that we explain it to you a fourth time.

By the way, these references you are now making to "fear" and "hysteria" are most peculiar. All three times the explanation has been given (once by billvon and twice by me) this has been done quite calmly and dispassionately.

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

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well at least you agree that it isn't a perpetual energy device... sigh!

The thing to do first is to agree in principle that the ocean depths provide a naturally occurring resource called "Pressure".
That this pressure is a resource whether expensive or not, efficient or not is irrelevant to the principle.
You could build a water tank tower that is 100 meters deep/high and you would have a similar pressure resource.

As to cost effectiveness the current cost of providing medium scale mechanically compressed air for industry is one of the most expensive forms of energy delivery.

A few cubic meters of 36000 kPa (5221 psi) compressed air can provide enormous supply when reduced to the pressures commonly utilized.
(A quick google: The average industrial grade diesel compressor runs at about 116psi at 125cfm)
At 5221 psi compressed air could replace the need for coal to fuel power stations.
You could decentralize the power stations and lower the pressure required for example.

Cost effectiveness after clever design and engineering would be vastly superior, is my guess if anything because it is entirely renewable, clean and incredibly versatile.

Last edited: Mar 8, 2020
17. ### Quantum QuackLife's a tease...Valued Senior Member

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so you still think we are discussing a perpetual energy device.... why?
Of course in simple terms there is a pressure retrieval cost. A cost that could be mitigated but never removed entirely by clever engineering and clever design.

18. ### exchemistValued Senior Member

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This will not work.

Please address the points made in my post 32 and the previous ones by myself and by billvon.

19. ### exchemistValued Senior Member

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I have told you why, twice.

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

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Do you think I should work out how it could become a perpetual energy device?
Doomed to fail but perhaps it could be fun!

21. ### Quantum QuackLife's a tease...Valued Senior Member

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when you actually address the thread topic I might do so... but at the moment you are answering the wrong questions.
1. Sink your bike pump (plunger extended)
2. lock of the plunger ( at the depth required)
3. retrieve it
and you have harvested the pressure at depth.

Yes or no?
Agree or disagree?

Last edited: Mar 8, 2020
22. ### Quantum QuackLife's a tease...Valued Senior Member

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23,326
• we are extracting potential energy in the form of pressure using air as a medium from an ocean that is providing the energy inputs.
• sinking a container of air IS compressing the air in the container.
• the oceans do provide a natural form of compression as the container sinks.
Primary school science..

Last edited: Mar 8, 2020
23. ### Quantum QuackLife's a tease...Valued Senior Member

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If you could solve (zero) the retrieval energy cost problem then and only then would have a close approximation of a perpetual energy device...

Have a go at it ... you might learn something about displacement and buoyancy while you are at it..