Reverse ElectroDialysis (RED)

Reverse Electrodialysis (RED) is based on the principle that ionic compounds such as salts dissolve in water into charged ions: equal parts positively and negatively charged ions. In sea water, the predominant salt is sodium chloride (NaCl), which dissolves into positively charged sodium ions (Na+) and negatively charged chloride ions (Cl-). A RED system uses ion-exchange membranes to separate fresh and salt water and control the mixing of the ions between the two solutions. One type of ion-exchange membrane only allows positively charged ions to pass (also known as a cation-exchange membrane, CEM), and the other only allows negatively charged ions to pass (also known as an anion-exchange membrane, AEM).


When salt water is feed in between a pair of these ion-exchange membranes (with fresh water on the other side of either membrane) then the ions will passively diffuse from the salt water into the fresh water; however, the diffusive transport of the ions will be preferential due to the ion-exchange membranes. All the positively charged ions will only be able to diffuse through the CEM, likewise all the negatively charged ions will only be able to diffuse through the AEM. In this way, the bulk transport of positively charged ions will be in one direction and the negatively charged ions in the opposite direction. The opposing transport of the positively and negatively charged ions creates positively and negatively charged poles, similar to a battery, and more accurately defined as an electrochemical cell.


The amount of electrical energy available is, in principle, determined by the concentrations of the two solutions, the composition of the dissolved salts, and the temperature of the water used in the electrochemical cell. The actual generated electricity is dependent upon a number of technical factors, such as the membrane selectivity and the internal resistance of the cell. The membrane selectivity is the degree to which the membrane is able to permit only one ion species to pass, while blocking all other ions. The resistance of a cell is a measure of the disruption ions encounter en route from the salt water to the fresh water.

Click here for a movie.

This movie was made by Wetsus (Centre of Excellence for Sustainable Water Technology) and REDStack

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