Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Claude Giddings on 27/09/2015 11:46:49
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Claude Giddings asked the Naked Scientists:
Let's say you have a very long strong tube uncapped at both ends, and laying at sea level. It would have one atmosphere pressure inside.
Put a cap on one end and stick that end down to near the bottom of the Mariana Trench. Now pop the cap off.
The water would rush in to fill the one atmosphere pressure inside the tube.
Wouldn't this create a perpetual geyser from the end at the surface?
It seems to me that it would come out with enough pressure to run a generator. To me it seems that the pressure of the water coming out would be very close to the same as the pressure at the intake. Would it be considered perpetual motion or just hydraulics?
Why wouldn't this set-up solve the world's clean energy needs? Please pop my bubble. What am I not accounting for? I need to know WHY this won't work.
What do you think?
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Think about it this way.
The pressure on the water at the bottom of the trench is due to the weight of the column of water above it. The tube now becomes a similar column of water so the pressure at the bottom of the tube is the same as that of the water around it. So no net pressure difference.
Hope that helps
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The best way to do this is to build a square or rectangular structure that sits in the sea and is anchored to the bottom. Have inlets that allow sea water in to drive turbines. However you will need a good evaporation strategy to prevent the system from filling with sea water. This could be by solar power or some other environmental means.
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a very long strong tube
The longest capped tube lowered into the Marianas Trench was the vessel Trieste (http://en.wikipedia.org/wiki/Bathyscaphe_Trieste), which was less than 20m long. The part that withstood the pressure at this depth was a sphere only 2m in diameter. So, in practice, noone has created a tube long enough and strong enough. http://en.wikipedia.org/wiki/Bathyscaphe_Trieste
Deploying a long, thin, hollow tube to this depth would be very difficult - it would have to be assembled in sections, like drilling for oil in the deep sea.
Wouldn't this create a perpetual geyser from the end at the surface?
If you could deploy it, the water would initially shoot up the tube, and come out the top.
But the reason the bottom water is at the bottom is because it is denser than the tropical surface water. It is colder (around 4C, the temperature where water is densest), and it is saltier than waters near the surface. Once the initial rush subsided, the water would actually tend to flow down slightly, as the tube is now filled with cold, salty, bottom water.
Why wouldn't this set-up solve the world's clean energy needs?
There have been discussion of generating electricity from the temperature differences (http://en.wikipedia.org/wiki/Ocean_thermal_energy_conversion) between the surface and deep ocean. But this would be constructed by lowering a hollow tube (uncorked), so there is no great pressure difference between inside & outside.
a perpetual geyser from the end at the surface?
There is a self-sustaining fountain (http://www.youtube.com/watch?v=I24ftPo-_5w) in volcanic lake Nyos (http://en.wikipedia.org/wiki/Lake_Nyos#Degassing), driven by dissolved CO2 like a soda fountain.