Sylvia:

My initial thought was "no way" but there may be some merit in this concept as a starting point.

A couple of queries -
1. if you have a spring overcoming the piston then it would need to be VERY light given that the osmotic effect would have very little inertia. Ever thought of having a double acting process.
2. I would imagine that the added salt could be achieved by having 'ordinary' water (that is the water going through a conventional RO) on the outside of the membrane and a waste stream from a conventional RO on the piston side.
3. There is no provision for the waste stream - any thoughts.

I can just picture a bank of these things chuffing quietly away producing reasonably low salt water.

Forrest:

I am not ignoring - just digesting. Been looking at a number of sites and need to consolidate in a way that my boss still thinks I am working...

Gonzo,

I don't know that the concentration gradient between RO input and RO brine output would typically be high enough to drive the design as it stands. The design could be modified so that the piston is split into two with each half having a different area, and then it could work, albeit at considerable extra complexity (high pressure piston seals, for example).

However, all that really amounts to is making the desalinator more energy efficient at the expense of additional hardware, which means more cost. There are already cost versus energy efficiency trade-offs in desalinator designs. If one wants a more energy efficient desalinator, then I imagine that there are cheaper ways of achieving it than by bolting on a variant of my non-patent salt-powered desalinator.

As with so many ideas, it's the economics that kills it, not the physics or the engineering.

Sylvia

Sylvia,

You haven't been reading my post on GrahamY's topic "James Pollack is driving a case, but does he drive a car?" have you? I confess to instantaneously thinking the worse of you that you were pulling my leg, because your invention does look horribly like a variant of a perpetual motion machine. I'm still not sure (mine is a pedestrian intellect-I will have to check the physics of it), but you just may be onto something. Should it be that there is a flaw in the theory (its not instantly obvious), then good one, you got me in!

Should it prove to work, would it detract too much from your right to have the device identified with you to suggest that you share the naming rights with James Watt of immortal memory? (You know, he of the steam engine and industrial revolution.) The device to become known as the Watt-Else salt powered desalinator.

And in the event of it being a vicious, wicked hoax, being named the "Else-what? green-baiting perpetual osmosis oscillator".

Right or wrong, this will bring out the latent Luddite in every control freak. You will be put under a fatwah for daring to dream of such a thing! And as for doing it for your own amusement, well, what contempt for the suffering of parched, dehydrating humanity you display. What an insalt!

Forrest Gumpp, I promise you it's no hoax. That energy can be extracted from a combination of solutions at different concentrations is well established physics.

See http://exergy.se/goran/cng/alten/proj/97/o/ for a discussion about using salt to generate power.

My salt powered desalinator is simply using that energy in a particular way, to drive a RO desalinator. In the process it reduces the difference in the concentrations of the two input solutions by transfering water from the lower concentration to the higher one, which is why the input solutions have to be replaced after each cycle. The combination of input solutions is really just a kind of fuel, albeit an unusual one.

Now, someone might be tempted to try to run the thing off its outputs. Doing that would be as naive as trying to run an electric motor off the output from a generator that the motor drives, and would be equally unsuccessful. There are energy losses in the system, so it cannot be expected to work that way. Consequently, it does not even approach being a perpetual motion machine, any more than the motor-generator set up does.

Providing the high concentration input solution would involve either digging up salt lakes and transporting the salt to the coast where the desalinator was, or establishing huge evaporation ponds. Either way, the economics don't stack up. Perhaps it could be made to run off the brine output from a conventional RO desalinator as Gonzo suggested, thus increasing the net energy efficiency, but as I've said, there are surely cheaper ways of achieving that result.

Sylvia.

Sylvia, Sylvia,

You are right! Have just re-read (after over 40 years) the chapter on osmotic pressure in Booth & Nicol; Physics: Fundamental Laws and Principles, pp 151-157. "How the brain doth ossify, and the flex of thougt embrittle with the years! And now, to horse! Ere this day pass we shall have seized Es Salt. England shall yet outshine fair Aquitaine!" (Shakespeare, Richard I, Act III - perhaps more well known from the film "The Lion in Summer")

You may be a little hasty in suggesting the economics of it do not add up. I can think of one place where a shortage of fresh water, highly concentrated brine, high evaporation and the availability of seawater are in close proximity: the Dead Sea; Israel, and Jordan. Construction of a tunnel from the Mediterranean to the Dead Sea has, I think, been considered for hydro-electric generation alone (there is a drop of around 1,200 feet, if you know what they were): perhaps the prospect of salt powered desalination may move this project into viability, and the Med-Dead scheme come to life. Loch Haim!

This invention has started out a bit like the Light Horse charge at Beersheba in 1917. An eyewitness account has it that "at the start of the charge we were all laughing, the Bedouin were all laughing, even the Turks were laughing." At the end of the day the Light Horse held the wells, the Turks were gone, and the horses all got to have a drink. (Incidentally, the Light Horse took the town of Es Salt, located in either present day Jordan or Syria, during the two-pronged advance of 1918 under Viscount Allenby of Armageddon.) Sylvia, you may yet have a tree planted in your name in Jerusalem.

Gonzo,

Boy, are you in deep dudu! Here you were, surfing the net in the boss's time, and a world-shaking invention bursts on the scene right before your eyes. What are you going to do now? You need a good spin doctor, mate.