Energy from the difference in salinity of two water bodies (also known as Salinity Gradient Power) can be extracted by placing membranes between a fresh and a salt water stream. There are two promising techniques for generating electricity from fresh-saltwater differences: Reverse ElectroDialysis (RED) and Pressure Retarded Osmosis (PRO) (Witteveen+Bos & CE Delft, 2019).
In reverse electrodialysis (RED), two types of ion-selective membranes allow positive and negative sodium and chloride ions to pass through, creating a small current. By placing several membranes in series, a voltage difference is created, which is converted into electricity (Witteveen+Bos & CE Delft, 2019). Reverse electrodialysis is the opposite of the process used to desalinate salt water using membranes (electrodialysis or ED).
With Pressure Retarded Osmosis the membrane allows water to pass through, but not dissolved salt. Because water naturally wants to flow from the fresh to the salt side, a pressure difference arises with which electricity can be generated via a turbine (Witteveen+Bos & CE Delft, 2019).
The development of techniques that extract energy from fresh-salt water is highly dependent on the development of membranes. Learning effects of the development of comparable membranes for other applications, such as desalination of salt water or storage of electricity in fresh and salt water (see AquaBattery, 2020), are therefore also relevant.