Salinity gradient power, osmotic power, or blue energy, is the energy available from the difference in salt concentration between fresh river water and seawater.
Salinity gradient energy is based on using the osmotic pressure difference between fresh water and sea water and relies on evaporation to separate water from salt. Osmotic pressure is the chemical potential of concentrated and dilute solutions of salt.
Pressure Retarded Osmosis (PRO)
Differing salinity gradient power generations exist but one of the most commonly discussed is pressure-retarded osmosis (PRO). Within PRO, seawater is pumped into a pressure chamber where the pressure is lower than the difference between fresh and salt water pressure. Fresh water moves in a semipermeable membrane and increases its volume in the chamber. As the pressure in the chamber is compensated a turbine spins to generate electricity.
Two solutions, A, salty seawater and B, fresh water, are separated by a membrane and only water molecules can pass the semipermeable membrane (see Fig. 1). As a result of the osmotic pressure difference between both solutions, the water from solution B will diffuse through the membrane in order to dilute solution A. The pressure drives the turbines and powers the generator that produces the electrical energy.