Why can't we use the hydrostatic pressure gradient to do work?

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Is there any way to harness the hydrostatic pressure gradient to generate energy?
The pressure at the surface of an ocean is atmospheric pressure.As we descend down the ocean, the pressure increases .After a point, the pressure will be very high.Why cant we use this pressure difference to do work?
 

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  • #2
anorlunda
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That's a fair question, but you can answer it yourself. If you put a soda straw in a glass, the pressure at the bottom of the glass is bigger. How fast does water flow through the straw? It ought to make a fountain, right?
 
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That's a fair question, but you can answer it yourself. If you put a soda straw in a glass, the pressure at the bottom of the glass is bigger. How fast does water flow through the straw? It ought to make a fountain, right?
I havent done it before, but I think the water will stop at the top of the glass.It will not rush out like a fountain.I think its because, the weight of the column is the reason for the pressure, so it cant lift itself.But initialy the water rushes to fill the straw, can this be used to do work?.Something like water rushing through a hole in a ship.Can this rush be used to do work?
 
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anorlunda
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But initialy the water rushes to fill the straw
No it does not. The water doesn't move at all. The end of the straw goes from top to bottom. The water at the top of the column is the same water that was on top before the straw.

Edit: but if you plugged the end of the straw first, then push it into the glass, then open up the end, water would rush in and you could extract energy, but first you had to put in the same energy (or more) to push the air-filled straw down.
 
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Merlin3189
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(I see Anorlunda has made my main point while I was writing.)
... initialy the water rushes to fill the straw, can this be used to do work?.Something like water rushing through a hole in a ship.Can this rush be used to do work?
Yes they could.
But, thinking about the straw, you would have to do work to push an empty straw (sealed) to the bottom of the glass against the buoyant force. When you open the straw, the inrush of water is just returning (some of) this energy.
Rather than making holes in the ships (and sinking them), you could seal them up, then pull them to the bottom with strong strings (hawsers.) When you want your energy back (some of it) you can allow them to float to the surface doing work by pulling on the strings. This is easy to repeat.

It is more or less (hedging his bets!) equivalent to lifting some water up and later letting it fall again.
In UK the electricity supply companies do something similar. They pump water up from a low pond to a high pond when they have spare energy, then let it run down again through turbines, when they need the energy. I suppose the advantage is that they can have a big difference in the heights of two lakes, rather than having to have one very deep lake and lots of large ships on hawsers.
 
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Thank you guys.And, the idea of storing excess energy as water potential energy is really nice.I have learnt that A.C. power cannot be stored.So this is a good way to store it.
 
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Thank you guys.And, the idea of storing excess energy as water potential energy is really nice.I have learnt that A.C. power cannot be stored.So this is a good way to store it.
We already do that with hydroelectric dams.
 
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Thinking laterally, would an air-lift suction system qualify, as used by underwater archaeologists and artisanal gold dredgers ??
 
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anorlunda
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Thinking laterally, would an air-lift suction system qualify, as used by underwater archaeologists and artisanal gold dredgers ??
It depends on your definitions. You could extract energy from the air lifted column, but you need to provide more energy to compress the air and get it to the bottom. So the net is negative. How would you count that?
 
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Yes, you are using air-power, but it leverages a previously static system to do useful work by creating a powerful suction which entrains otherwise intractable stuff.

Slightly further off-topic, I'm reminded of the 'permanent soda fountains' now warily de-gassing those African lakes with volcanic CO2 etc saturated bottom layers, potentially lethal should they 'overturn' again....
 
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anorlunda
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Yes, you are using air-power, but it leverages a previously static system to do useful work by creating a powerful suction which entrains otherwise intractable stuff.
You have to be careful how you say that. It may be very useful, very valuable, but in terms of energy you can't get out more than you put in. Using the word leverage implies over unity gain which is impossible, and a forbidden topic on PF. But that's not what you meant, right?
 
  • #12
OmCheeto
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You have to be careful how you say that. It may be very useful, very valuable, but in terms of energy you can't get out more than you put in. Using the word leverage implies over unity gain which is impossible, and a forbidden topic on PF. But that's not what you meant, right?
After a few hours of thinking about it, I think it would make an interesting homework problem:

2018.09.08.pf.perpet.motion.seawater.thingy.png


Of course, the friction is always the booger in the perpetual motion scheme of things.

BTW, I just spent a couple of hours doing experiments on my very ancient pressure measuring device, to determine how accurate it would be if I were to do such an experiment.

2018.09.08.todays.physics.problem.png


ps. I decided that the maths was quite a bit too difficult, and I therefore cheated, as usual.

Answer: 3.6 cc
 

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No over-unity gain, that's a given.
Sorry, it is such bedrock Science, I sometimes forget about woo-fringe ijits' sad tenacity...
 

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