Mixing fresh water with salt water releases a substantial amount of energy, but scientists have struggled to find a way to harvest this inexpensively. Doriano Brogioli at the University of Milan-Bicocca, based in Monza, Italy, has proposed a new technique.
He charged two porous carbon electrodes in salty water, then flushed the device with fresh water. Salt ions diffused away from the electrodes, increasing the voltage between them from 300 to 333 millivolts. Discharging the electrodes allowed energy to be extracted. (Apparatus and technique are detailed in graphic, below.)
Brogioli estimates that the device could yield about 1.6 kilojoules per litre of fresh water, which is on a par with the power produced by costly membrane-based methods.