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.^[1] 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.

This method of generating power was invented by Prof. Sidney Loeb in 1973 at the Ben-Gurion University of the Negev, Beersheba, Israel.^{[2] [3]} The concept was also published independently by Richard Norman in 1974.^[4][5]

The ideal power production formula, which applies to an idealized situation, predicts that the optimal hydraulic pressure difference, ${\displaystyle \Delta P}$ is one-half the osmotic pressure difference between the saline and pure water streams ${\displaystyle \Delta \Pi /2}$.^[4][6] For a seawater to fresh water PRO system, the ideal case corresponds to an optimal power pressure of 26 bars. This pressure is equivalent to a column of water (hydraulic head) 270 meters high.^[7]

In a real-world system, both the hydraulic pressure and the osmotic pressure will vary through the PRO system as a result of friction, water removal, and salt build up near the membranes. These factors reduce the achievable power below the ideal limit. The amount of membrane area that can be used is limited by cost and other practical considerations, and this factor limits achievable power production.^[8] A significant portion of the electrical power generated by PRO must be used by the pumps that circulate water through the plant.^[9] Appropriate membranes are also necessary. All these factors have limited the economic viability of PRO.^[10]

PRO has the potential to extract osmotic power from waste streams, such as desalination plant brine discharge or treated wastewater effluent. The potential power output is proportional to the salinity difference between the fresh and saline water streams. Desalination yields very salty brine, while treated municipal wastewater has relatively little salt. Combining those streams could produce energy to power both facilities. However, powering an existing wastewater treatment plant by mixing treated wastewater with seawater in a mid-size city could require a membrane area of 2.5 million square meters.^[11]

PRO uses a water–permeable membrane with an osmotic pressure difference to drive water flux from a low–concentration "diluate" stream, into a slightly pressurized higher–concentration. An energy recovery device on this stream provides the energy output, and must exceed the pumping pressure input for net power production.

The world's first osmotic plant with capacity of 10 kW was opened by Statkraft, a state-owned hydropower company, on 24 November 2009 in Tofte, Norway.^[13] It had been estimated that PRO could generate 12 TWh annually in Norway, sufficient to meet 10% of Norway's total demand for electricity.^[14]

In January 2014, Statkraft terminated their pressure-retarded osmosis pilot project ^[15] due to economic feasibility concerns.

Starting in 2021, SaltPower is building another commercial osmotic power plant in Denmark using very high salinity brine from a geothermal power plant.^[16]

• Electrodialysis reversal (EDR)
• Forward osmosis
• Green energy
• Osmotic power
• Osmotic pressure
• Renewable energy
• Reverse electrodialysis (RED)
• Reverse osmosis
• Semipermeable membrane
• Van 't Hoff factor