Pressure retarded osmosis (
PRO) is a technique to separate a solvent (for example,
fresh water) from a solution that is more concentrated (e.g.
sea water) and also pressurized. A
semipermeable membrane allows the solvent to pass to the concentrated solution side by
osmosis. The technique can be used to generate power from the
salinity gradient energy resulting from the difference in the salt concentration between sea and river water. In PRO, the
water potential between
fresh water and sea water corresponds to a pressure of 26
bars. This pressure is equivalent to a column of water (
hydraulic head) 270 meters high. However, the optimal working pressure is only half of this, 11 to 15 bar.