Osmosis is the process where a solvent, generally water, within something dissolved in it, generally a salt, passes though a semi-permeable membrane (or OsmoticMembrane) from a less concentrated solution to the a more concentrated solution.
Osmosis has a major impact on pretty much all life. If you life in the sea, you have to keep your salt balance right or you loose water. In plants, osmosis is used to generate pressure, which is how plants move, in response to the sun or even more rapidly in carnivourous plants.
More recently, reverse osmosis is being used for large scale desalination plants. Basically, if you apply pressue to the high concentration solution, the water passes backward through the semi-permeable membrane, generating fresh water. You need fairly highly pressures, which takes a lot of Energy, but it's still a lot cheaper than distillation.
Okay, so this is an Energy wiki, not a desalination tutorial, so how does this relate? It means that large scale osmotic plants are a reality and reasonably well developed. Instead of consuming Energy, you should be able to reverse the system to generate it. If you have brine and fresh water in one place, you should be able to pressurise the brine, by letting it become more dilute. If you have a pressure gradient you can use this to generate Energy through a WaterTurbine. This is shown diagrammatically below. This system works in batch; that is some of the time it is pressurising and sometime it's generating Electricity. You can run this continuously as well. In this case, you the brine section of the system permenately sealed and pump fresh brine into it. The rotation of the turbine could be directly used to drive this pump.
|Diagram of an Osmotic Generator|
Of course, there are lots of places in the world where brine and fresh water co-exist; the mouth of any river is an obvious example. Probably, you would need to pipe water from a couple of miles up stream, or it would be brackish, which would reduce the efficiency of the whole system. Alternatively, you could take the water from a WaterSolarTower as in a SolarValley, or a ThermalSaltSedimentator.