Reverse Electrodialysis

Reverse electrodialysis, or RED, is a technology that retrieves salinity gradient energy from the difference in salt concentration between seawater and river water. A method of harnessing the energy this process produces by means of a heat engine was invented by Sidney Loeb in 1977 at Ben-Gurion University of the Negev. In reverse electrodialysis, a salt solution and fresh water are passed through a stack of alternating cation and anion exchange membranes; the chemical potential difference between the salt and fresh water generates a voltage across each membrane, and the total potential of the system is the sum of the potential differences over every membrane in the stack. The process works through a difference in ion concentration rather than an applied electric field, and, as in a fuel cell, its cells are stacked; a module with a capacity of 250 kilowatts is about the size of a shipping container.