Scientists in the Netherlands have previously succeeded in generating electricity at the point where salt and fresh water mix in an estuary using a membrane. Unfortunately, the membranes are expensive and delicate, making the technique costly.

Now Doriano Brogioli of the University of Milan Bicocca in Monza, Italy, has published a paper in the Physical Review Letters which takes a different approach that promises to be much cheaper.

Two carbon electrodes are placed in the salt water and given an initial electrostatic charge – one positive and the other negative. Positively charged sodium ions are attracted to the negative electrode and negatively charged chlorine ions are attracted to the positive electrode. When fresh water flows past the electrodes, the positive and negative ions are diffused away from the electrodes. This increases the electrostatic charge between the electrodes. In other words, some of the mechanical energy in the flowing water is converted into electrical energy.

Doriano Brogioli has demonstrated the technique, which is called an electrostatic double-layer (EDL) capacitor, on a laboratory scale. He calculates that it could eventually be cost competitive with wind power without the perceived environmental problems of wind turbines.