🚀 go-pugleaf

Peter Connor wrote:
> 
> > So the liquid pressure of lesser density and at lesser depth fresh water
> > will be lower than the water at higher density and further depth sea
> > water on the other side of the membrane. Then the difference in
> > pressures at the depth of the membrane is proportional to the difference
> > in water densities. But the power needed to get a flow going in a full
> > pipe is proportional not only to the pressure differential, but also the
> > force of gravity over the distance of the flow (not to mention other
> > factors such as average velocity). So even though there is an upward
> > pressure due to the density difference, wouldn't there still need to be
> > a differential greater than 9800 Newtons per meter^3 x the depth the
> > fresh water would have to travel to reach the exit of the tube, in order
> > to force the water out? (Density of fresh water is 1000 kg/m^3.)
> >
> > -lozang trinlae
> 
> The pressure on the water at the bottom of the tube owing to the volume
> of water above it would be equal to that on the water outside the tube
> at the same depth, (imagine the water without the tube being there), the
> only difference to this equilibrium is the additional pressure placed on
> the pure water due to osmosis, this extra pressure forces the water up
> out of the tube.
> To answer another query on this thread the energy is actually coming
> from a reduction in the potential energy of the ocean as a whole as
> pushing the water out will of course reduce the overall sealevel.
> 
> Peter Connor

If this was true, you could build a machine that would create energy.
Imagine hooking a generator to the top of the tube, where the fresh
water is exiting. You could extract energy from the water movement.
Since the fresh water flows back into the sea, there is no sea level
change. The fresh water disolves back into the salt water
exothermically, so more energy can be removed there. At no point in this
cycle is there any energy input, just energy released! The system,
therfore, would violate the first law of thermodynamics unless the fresh
water level is at (or below) sea level. The entire pressure drop has to
be across the membrane.

John.