For the past years the scientists have been talking about energy sources that in contrast to wood, coil, oil, and gas can not be exhausted completely because they are constantly renewed. These sources are sun, wind, water, inner heat of the Earth, biomass. Solar energy industry is being developing the most successfully now. However, some research groups in the world including the Russian team from Vladivostok are working with such exotic source of energy as ocean water.
It has been well known for a long time that it is possible to get significant energy by mixing salt and fresh water and then transform easily this energy into electric power. The problem is to put this theory into practice. Experimental power stations that use the energy potential of fresh and salt water have been created and tested in the USA, Sweden, Israel. In Russia the installation that works with this source of energy have been first created in Vladivostok. The principal of its work is very easy and it is described in any textbook on Chemistry. The container is divided into two parts with porous partition, which only passes the molecules of water. One half of the container is filled with fresh water and the other one - with salt water. The two liquids start to mix trying to equalize the concentrations of the salts. How to use this property of liquids depends on your inventiveness. The simplest device can work according to the following scheme: the molecules of fresh water percolate through the partition and increase the pressure in the compartment with salt water. The salt water goes up under the pressure and performs useful work.
"We believe that electrodialysis is more perspective way to get energy from salt water", says the inventor of the installation Valerii Knyazhev. The scientists took the membranes that filter either cations or anions and fixed them alternatively. Thus they got many-layer 'sandwich' with alternate layers of fresh and salt water between the membranes. The gaps between the membranes were very small, just several millimeters, to reduce the electric resistance of water. The fresh and salt waters from adjacent compartments started to mix. As only ions of a certain charge pass through the partitions, the anions and cations from salt solution are accumulated in the compartments with fresh water making some total charge. In such a way voltage occurs, which provides electric current. Now the installation provides as much as 3 or 4 Volt (about 0.15 kilowatt hour per one cubic meter of fresh water). However, it is possible to get 0.7 kilowatt hour theoretically. This power is enough to iron a shirt or work with a notebook for an hour.
The experimental installation, which is half a meter in height, has worked for three years at Marine base of the Institute on Popov's Island near Vladivostok. "Such power stations have not been used in industry now: their economic success is not very high but we are going to improve the method because it is based on soft technology", Knyazhev explains. Power stations on salt water are expedient for seaside regions where it is possible to place them in the mouths of the rivers running into the ocean. Having passed the station the water remains clear and does not pollute the environment.
I have heard of this concept before. It somehow relies on a potential difference created by a silinity gradient across an osmotic membrane. Kind of analagous in some ways to how a fuel cell works. I remember way back when I was taking General Chemistry that we played around with some of the relevent equations--but I can't remember what the magnitude of potentially extracted energy that can be generated per liter per second of fresh water flow. But if the entire output of a river could be used, the power could be significant.
However, I really have to wonder how issues such as sedimentation (from muddy water) and of course fisheries could impact such a project. My feeling is that the potential harm--or cost of maintenance--may make such a system impractical. The sedimentation issue reminds of some written accounts of early paddle wheel steam boat operators running on the Mississippi River having to shovel steaming mud out of the clean out doors in their boilers because they regularly got clogged with silt! Big rivers like the Mississippi, Amazon, Nile, Volga, or the Danube will produce thousands of tons of silt per day--this would just play Merry Hell with osmotic membranes!
