[extropy-chat] energy from osmosis

Mike Lorrey mlorrey at yahoo.com
Mon Oct 3 00:34:39 UTC 2005

One other factor one should calculate in: capillary action should
reduce the 9800 m depth required, particularly if your pipe is a whole
lot of tiny pipes so as to maximize the amount of water contact with
wall surface (which is normally a problem with hydropower systems, you
want to minimize wall surface area, but not in this instance as we are
trying to create a potential difference, not exploit it, yet)

--- The Avantguardian <avantguardian2020 at yahoo.com> wrote:

> --- 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?
> This is an interesting idea, Spike. I at first thought
> it couldn't possibly be right but when I did the
> calculation myself, it turned out to be essentially
> correct. Using your figure for the osmotic pressure, I
> get a result of the break even depth for the pipe to
> be 9800 meters. Anything below that is a fresh water
> fountain that should go above the pipe. I think the
> error is caused by you getting the sign of the osmotic
> pressure wrong since that will give a constant term of
> -245 meters for the fresh water but at increasing
> depths it gets overcome by the factor 1.025. This is
> evident in your header as well since what you are
> doing is technically REVERSE osmosis as the osmotic
> pressure is actually being overcome by the difference
> in hydrostatic pressure between salt and fresh water
> caused by the higher density of salt water.
> This should theoretically work as Mike is correct in
> that there is a real potential energy difference being
> exploited here. There are some ways that reality might
> throw a monkey wrench into the plans however. There is
> probably a salinity gradient in the ocean with the
> saltiest water being deeper down. I ignored this in my
> calculation, but it could make the osmotic constant
> larger. There is certainly a temperature gradient as
> Mike pointed out but since the temperature gets lower
> the further down one goes, this might actually
> counteract the salinity gradient if there is one.
> Any ways, well done, Spike. That is a GREAT idea. Even
> if it is just a dribble instead of a gushing fountain
> you may have figured out a way to desalinate ocean
> water, essentially for free. Considering that nothing
> says the pipe has to be straight once it reaches the
> surface, you might have invented the giant bendy straw
> that could cause the Sahara to bloom. This would be a
> blessing for desert countries like Libya and might
> make wars over freshwater (like Israel versus its
> neighbors over the Golan Heights) obsolete.
> I am going to figure out if there is a way to test
> this at a scale that will fit on a lab bench or at
> least in swimming pool.
> Kudos, man.
> The Avantguardian 
> is 
> Stuart LaForge
> alt email: stuart"AT"ucla.edu
> "I don't want to achieve immortality through my work. I want to
> achieve it through not dying." - Woody Allen
> "Our hope of immortality does not come from any religions, but nearly
> all religions come from that hope" - Robert G. Ingersoll
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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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