# Using the pressure of the water in the sea

## Main Question or Discussion Point

I have a very basic question:

since the pressure of water in the sea is high when we go deep down, why not puting a tube, closed on the side below, deep down, and then opening it when it is very deep down?

The pressure should push the water up, that could be used to move machines.

Since it is very obvious and it has not been done, there should be a trivial reason for which it does not work, but I cannot find which one it is.

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pushing an empty tube down takes energy as well. Once you open the tube the water will only rise to sea level and you can't get more energy out of that than it took to push the tube down.

If you keep the tube 2 years, you will get more energy that keeping it 1 year. At some point in time, the energy delivered will be higher than the energy used to take down the tube.

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I can imagine a way how to extract energy from this tube, making some assumptions:

I have a very heavy, cilindrical piece of material that fits perfectly in the interior of the tube. I place this cylinder in the bottom of the tube. I connect it with the surface of the sea with a very long rod. On top of the rod, I have a recipient to put something in it (eg, water). Before opening the bottom of the tube, the top of the rod is at sea level. I fill the recipient with water. Now I open the tube, and the cylinder goes up a few meters, until its weight compensates water pressure. The recipient has gone up a few meters. Now, if at this height over the sea I can take the water that has gone up, I can let the water go down until sea level, and use the potential energy via hydromechanical energy.

Now, my assumption: I have lots of weights, and I put one of them to push the cylinder until the original height. We repeat the process again and again.

Of course, this process is not efficient, but I think it shows it is possible to extract energy from the pressure down the sea level.

If you keep the tube 2 years, you will get more energy that keeping it 1 year. At some point in time, the energy delivered will be higher than the energy used to take down the tube.
the only way to extract energy from the tube is letting it fill. how will you extract energy from the tube after it is full?

the only way to extract energy from the tube is letting it fill. how will you extract energy from the tube after it is full?
See my answer just before yours. Probably you have not read it before I posted it.

I see that other people has suggested a more efficient process: pump the cold water up (that should be almost energy-less) and use it as A/C.

You could also dig lots of holes through the earth and drop random stuff in there and use the friction as energy source.

Inefficient.

Redbelly98
Staff Emeritus
Homework Helper
If you keep the tube 2 years, you will get more energy that keeping it 1 year. At some point in time, the energy delivered will be higher than the energy used to take down the tube.
No, you'll get the same energy back in the end. You'll just have stored that energy for a year longer before using it.

You're idea is inherently the same as rolling a heavy ball up a hill, and then using the energy you get by letting it roll back down.

Doesn't matter if the ball sits on the top of the hill for 10 minutes or for 10 years. You'll get the same energy in the end, and no more than the energy it took to put it up there in the first place.

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mgb_phys
Homework Helper
I see that other people has suggested a more efficient process: pump the cold water up (that should be almost energy-less) and use it as A/C.
Pumping tons of water up from depth is not exactly energy less - but similair schemes have been used for office cooling. I think Toronto is one example.

russ_watters
Mentor
If you keep the tube 2 years, you will get more energy that keeping it 1 year. At some point in time, the energy delivered will be higher than the energy used to take down the tube.
Please reread post number 2. There is no energy gained by this process, so leaving it two years gives you 2x0=0 energy.

One useful way to look at pressure is to say that the pressure at the bottom of the ocean (or at any point below the surface, for that matter) is equal to the weight of a column of water over that point. That means the pressure doesn't change if you enclose that column of water in a tube.

Try this: insert a straw into a glass of water. Does water start shooting out the straw?

I'd like to make this more interesting....

considering that there is higher pressure at the bottom with respect to the top, why wont a syphon work from the high pressure below to the low pressure on top....with the tube never breaking the surface of the water at the top....

so basically, the curved peak of the straw is below the waterline...if you see what i mean....

DaveC426913
Gold Member
considering that there is higher pressure at the bottom with respect to the top, why wont a syphon work from the high pressure below to the low pressure on top
Ask this question: why is there a high pressure at the bottom? The answer is "because of the weight of the water on top of it".

It is the same outside the tube as it is inside the tube. The weight of the water inside the tube will balance exactly any pressure from the bottom.

Every square inch of ocean at one mile down is under the pressure of a column of water exactly one inch on a side and exactly one mile tall. Enclosing that square inch in a tube changes nothing.

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rcgldr
Homework Helper
Forget the tube, the real idea here is to drop matter into the ocean and harness the gravitational potential energy of the objects dropped into the ocean. You could simply strap a line to each chunk of mass and use the line to turn a generator while the chunk of mass descended to the bottom of the ocean, then either retrieve the line, or dump it as well and start with another chunk of mass and line. The issue is the cost of moving the mass to the ocean and producing the line in order to use this system.

A dam turbine or waterwheel would be more efficient, as it's power source is the Sun, which evaporates water, causing it to rise and accumulate, eventually turning into rain, falling down into rivers where a dam turbine or waterwheel can convert the energy.

Regarding sticking a very long vertical tube into the ocean, once an upwards flow is started, then differences in salinity result in difference in density of the water in the tube as it rises and heats up, resulting in a continuous upwards flow, but I don't know how much energy there is to be gained this way, and there are better means to utilize chemical energy from the ocean, although I'm not sure if there are harmful effects from doing this. If interested here's a good starting link:

http://www.energy.ca.gov/development/oceanenergy [Broken]

After checking out this web page some of the links are broken, but at least there's enough info to know what to search for (for example "osmotic energy").

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The OP doesn't understand mechanical energy in relation to fluid mechanics, water at the bottom of the tube will convert its pressure energy into elevational potential energy, in fact some will be wasted in the form of heat, so there will be no spare kinematic energy for the water to "move machines". What's more, once the tube has filled up, that's it, the whole system reaches hydrostatic equilibrium and you're left pondering how on earth you wasted all those millions of hard earned dollars fighting the buoyancy of an empty tube for such a mindless idea.

A more (potentially) useful application of the deep ocean's pressure is to use it to carry out experiments: experiments where you want to assess how mechanical objects respond to sustained large pressure over long time-scales. Because you can keep things at the bottom of the ocean for decades, maybe even longer. Those experiments would cost a lot of money to run in a great big cylindrical pressurizer in a lab in the basement of your local university.