[extropy-chat] energy from osmosis

[Hal Finney]

That's a good puzzle, it stumped me.  Looking at the discussion from
the second link BillK provided,
<http://www.halfbakery.com/idea/Supersimple_20reverse_20osmosis>,
I now think I understand it.

If we think of a column of salt solution in equilibrium, there are
two opposing tendencies.  One is for the salt molecules to sink to
the bottom, because they are heavier than water.  The other is for
all the molecules to have uniform concentrations, due to diffusion.
The interaction of these two tendencies produces a concentration gradient.
The solution becomes more concentrated as we go deeper.  In equilibrium,
this concentration gradient is stable and there is no tendency for it
to change.

Apparently this does happen in the ocean.  Although the top few hundred
meters are reasonably well mixed due to wind and wave action (ultimately
the effects of solar energy) beyond that depth we find a salinity
gradient, with deep waters being more salty than near the surface.

This is a problem for Spike's invention, because the necessary
pressure differential for reverse osmosis will increase as the water
becomes saltier.  Therefore as he sinks his pipe lower, the increased
saltiness of the ocean will cause a greater pressure difference across
the membrane, which will reduce the pressure needed on the fresh water
side of the membrane.  As a result, his fresh water column will not grow
as high as hoped.

If the oceans are close to salinity equilibrium at depth, then the
fountain will not work.  However, if solar energy does provide enough
mixing in the deep ocean to reduce salinity levels significantly below
equilibrium, then the fountain could work, and by raising fresh water
to the surface would increase the salinity gradient and move it closer
to equilibrium.  In that case, as Spike notes, it acts indirectly as a
heat engine based on solar energy.  However the comments in the link
above suggest that the deep ocean is near to salinity equilibrium,
so my guess is that it will not work.

Hal