[extropy-chat] energy from osmosis

The Avantguardian avantguardian2020 at yahoo.com
Sun Oct 2 23:11:09 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?

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 
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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