[extropy-chat] energy from osmosis

[spike]

> bounces at lists.extropy.org] On Behalf Of Mike Lorrey
> Subject: Re: [extropy-chat] energy from osmosis
> 
> ... 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?

I don't know, but it is high.  Seawater is currently
desalinated by pumping it thru osmotic membranes.

> 
> 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...

No issue, fill it with fresh water.

> 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)...

OK.  Ordinary modern steel alloys could carry the 
weight if we weld in thicker wall steel at the top
of the pipe.  Do the calcs Mike, you know how.  {8-]  The
density of steel is close enough to 8g/cc for
this calc.  Ordinary steel can go 400 megaPascals
tensile, and you know the maximum compressive pressure 
on the pipe is 2.4 mPa, which is nothing to small
diameter pipe.  Submarines can down that far.

...
> 
> 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


OK I figured out the solution this morning; Mike's
post kinda hinted me there.  I went crazy last night
trying to figure it out.  I will post what I think
is the right answer in about a little later.  The hint that
Mike posted is in the comment "There is nothing wrong
with what you propose..."

spike