[brilee]

Desalinated water is also less dense than normal seawater, so the water column inside the output pipe would create a pressure imbalance with the water column outside the pipe, assisting in the outflow? I'm having trouble figuring out how to resolve this seeming perpetual motion machine

[VonTum]

The minimum pressure differential needed to perform reverse osmosis is bigger than the pressure differential between a collumn of fresh and a collumn of salt water of equal height.

[josefx]

I think it would stop in an isolated setup once most of the water is desalinated.

[shiandow]

That still makes no sense, water can't desalinate itself in the same way it cannot spontaneously cool itself.

[XorNot]

Not in a static system, but the ocean isn't static - there are currents.

Until the membrane fouled, if you sank a system like this to the bottom, fresh water would naturally spill out at the surface while brine built up around it.

If the brine doesn't flow away (brine is weird like this) then eventually the system hits equilibrium and stops. But if ocean currents (powered by the sun, tectonics etc.) keep removing brine at the bottom...then it can in fact run indefinitely because there is an energy input.

[shiandow]

A steady supply of salty water doesn't help enough in the same way a steady supply of warm air cannot cool a house.

The problem with your system is that it you can power an engine with the flow of salt ions and that really isn't the kind of thing you are supposed to be able to do to something that happens spontaneously.

And really water spontaneously desalinating is about as clear a violation of the second law of thermodynamics as you can get. With the scale of latent energies involved it would be like water flowing up a 70 meter wall.

Look maybe I am missing something somewhere that secretly compensates for the apparent decrease in entropy but I am not seeing it. Brine will flow away e entually, the water returns to the ocean and in the meantime you can power your power plant by salinating the water, indefinitely.

[XorNot]

The system isn't closed.

You're functionally drawing solar energy off the system very inefficiently (if you wanted kinetic motion).

A different way to look at the problem is that you can't have water spontaneously move up hill, but if you dam a river you can absolutely extract useful energy from it.

A turbine underneath the ocean could extract energy from ocean currents and this is the same problem.

[shiandow]

So according to you you could simply lower a membrane and a pipe into a still body of salt water and it would spontaneously separate into sweet water and brine until the build up of brine prevented this from continuing?

Yeah I am still not seeing it. If that were the thermal equilibrium I don't see how it wouldn't separate spontaneously, or why you can mix salt and water with no input of energy whatsoever.

It goes against anything I know about entropy and osmotic pressure.