[extropy-chat] energy from osmosis

Mike Lorrey mlorrey at yahoo.com
Sat Oct 1 23:52:37 UTC 2005

--- spike <spike66 at comcast.net> wrote:

> The osmotic pressure of ocean water to fresh
> water is about 2.4 MPa. If we put an osmotic 
> membrane across the end of a pipe and shove 
> the empty pipe down into the ocean when the 
> pipe reaches a depth of about 240 meters 
> fresh water will start colleting on the 
> inside of the pipe. We can put a pump down 
> there and pump up fresh water. Nothing 
> tricky here. The energy to lift the water 
> 240 m is exactly the same energy that would 
> be required to push the sea water through 
> a membrane at ground level. 
> Now suppose we keep pushing the pipe down to 
> 290 m. The pressure across the membrane will 
> remain at about 2.4 MPa and this means that 
> the fresh water level inside the pipe will 
> rise about 50 m, so that the water surface 
> inside the pipe will remain at about 240 m. 
> We keep pushing the pipe further and further 
> down, the fresh water column gets longer and 
> longer, but the top of the fresh water column 
> remains at about 240 m below sea level. 
> No, that is too simple. Fresh water is slightly 
> less dense than sea water, so to keep the 2.4MPa 
> pressure differential across the bottom of the 
> pipe the fresh water column needs to rise faster 
> than the pipe sinks. Fresh water has a density 
> of 1000 kg/m^3 while ocean waters is about 
> 1025 kg/m^3. If the membrane end of the pipe 
> is down 1000m the freshwater column will be 785 m, 
> so the fresh water will rise to within 215 m of the 
> surface (25m higher than -240m). 
> If we keep pushing the pipe down to 9600 the 
> fresh water will come up to sea level. If we 
> push the pipe below 9600 me fresh water will 
> spew out of the top of the pipe above sea 
> level. We could then hook up a water wheel and 
> have a perpetual motion machine. Now we know 
> something is seriously wrong, but what?

What is the pressure limit of the osmotic membrane?

Okay, here goes:
Until you reach 240 m, you are trying to sink the pipe which is likely
positively bouyant. You can either force it down by some mechanism that
is anchored to the bottom, or else ballast it down with weights. The
walls of the pipe will need to get progressively thicker as you sink,
so this automatically adds weight as well (and of course energy to
refine the materials used in making the pipe). However, your perpetuum
might not work. While fresh water is less dense than salt water, a
fresh water column plus its surrounding piping might not be. If you can
come up with some unobtanium piping (nanotube wound compsites might do
it, esp considering pure carbon, and any hydro-carbons, are generally
less dense than water).

There is nothing wrong with what you propose, though, because the
energy differential between fresh and salt water is a real potential,
which you can use in the reverse in a riverine tidal zone at the
salinocline (provided you have a dam or dyke built there) to draw
electric power out of the ion influx, which is the more typical
application of osmotic filters for this purpose.

Mike Lorrey
Vice-Chair, 2nd District, Libertarian Party of NH
Founder, Constitution Park Foundation:
Personal/political blog: http://intlib.blogspot.com

Yahoo! Mail - PC Magazine Editors' Choice 2005 

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