A few weeks ago I wrote an entry about generation of electricity from the temperature differences that exist between seawater at the surface and at depth (OTEC) and it is common knowledge that it is feasible to generate power from waves, tidal currents or tidal water level changes. However, it is less well known – by which I mean that I had never ever heard of the idea – that it is possible to generate power from the difference in salt content between freshwater and typical seawater. An article in New Scientist, Issue 2697 (28 February 2009) introduces this idea, the basis of which is some kind of cell in which freshwater and seawater are separated by a special membrane. There are two ways that this arrangement could then lead to the generation of electricity depending on the set-up and the membrane used. First, the process of osmosis (in which water moves from a weak solution to a strong one across a semi-permeable membrane) can lead to water molecules from the freshwater side crossing the membrane into the seawater and thereby causing an increase in its pressure that can drive the water through a turbine. Alternatively, a more complex arrangements of membranes can be created that allows the salt ions to move in different directions (e.g. positively-charged sodium ions one way, negatively-charged chloride ions another way) so that the positive and negatively charged ions move towards a cathode and an anode respectively producing a voltage across the cell (basically a big battery). There are plans for a prototype power plant to be up and running soon but it does seem that this technology would only ever be a minor/local player in global power generation (despite figures that suggest it could provide 40% of the world’s electricity demands), especially as any such power plants could only ever be cited in regions where this is ample supply of both seawater and freshwater – namely large estuarine systems that are almost always both environmentally sensitive and quite highly developed already.
Tim O'Hare 