A pinch of salt | Nature Materials

In the end, life comes down to electrochemical gradients. But we haven't yet found very effective ways of making what works for the cell work for society: harnessing the energy released by

the equalization of solute concentrations. The power that could be tapped from the entropy of mixing of saline and fresh water, as rivers reach the sea, might seem tiny locally, but like

everything else associated with the oceans, becomes vast on the global scale: of the order of a few terawatts, which would provide a significant fraction of current world energy demand.

Highlighting this overlooked energy source in 1954, the engineer R. E. Pattle estimated that the energy released in a river estuary is equivalent to that available if the river were plunging

over a 680-foot waterfall1.

Of course, energy production from salinity gradients suffers the curse of all renewables: global sums sound appealing, but the energy is so dispersed that capturing even a small part of it

is immensely challenging, technologically and economically. That hasn't prevented attempts since Pattle first proposed his 'hydroelectric pile'. Most notably, in the 1970s Sidney Loeb

imagined harnessing the pressure generated as fresh water is drawn through a salt-obstructing membrane — a method called pressure-retarded osmosis, now being developed by the Norwegian-based

renewable energy company Statkraft, which aims to have a commercial generator by 2015.

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