[extropy-chat] energy from osmosis

spike spike66 at comcast.net
Wed Oct 5 03:49:32 UTC 2005

> bounces at lists.extropy.org] On Behalf Of "Hal Finney"
> Subject: Re: [extropy-chat] energy from osmosis
> There are two different questions raised by this thought experiment.
> ...  Imagine that the entire pipe
> is made of a semipermeable osmotic membrane, instead of just the cover
> at the bottom.  The whole pipe allows water molecules to pass through
> freely, while blocking salt.
> If we again assume an ocean at equilbrium, without surface heating, wind
> or wave action, it should be clear that once the pipe fills with fresh
> water to the initial depth, nothing more will happen.  At each depth,
> fresh water moves between the pipe and the ocean, in equal amounts.
> There is no net inflow at some depths which is balanced by a net outflow
> at other depths.  That would involve a constant circulation of water
> even though we are at equilibrium, which would violate the second law...
> Hal

Hal I disagree.  In your thought experiment, the
pressure difference between the seawater outside
the pipe and the fresh water inside is maximum at the
bottom end and decreases linearly to zero at the
surface.  Water flows into the pipe below 9600 meters
and out of the pipe everywhere above 9600.  Fresh
water flows upward thru the pipe.  The process is
driven by the energy released by falling salt: the
depths are getting more salty and the upper layers
less so.  Assuming no mixing of currents and tides,
the process continues until such time as equilibrium
is established, with the surface of the fresh water
240 meters below the surface and the salinity
increasing all the way down such that there is no 
flow in or out.



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