Osmotic power generates electricity from the natural tendency of water to move through a semipermeable membrane from a less concentrated solution into a more concentrated one—a process known as osmosis. The volume of salty water increases as it is diluted by fresh water passing through the membrane, and some of the resulting outflow is tapped to drive a turbine.
Water has been a source of renewable power for more than 2,000 years. The Chinese used water wheels to grind grain and pound ore. In the 1700s water wheels kickstarted the Industrial Revolution in Britain. In the 1870s William Armstrong installed the world's first hydroelectric system to light his family home, Cragside, in north-east Britain.
Two forms of osmotic power generation exist. The first uses membranes that allow only water molecules to pass from one solution to another; the volume increase on the salty side is then used to drive a turbine. The second uses membranes that let through charged particles (ions) instead; the movement of ions creates an electric potential difference—the basis of a battery—and a current is produced directly.
Recent advances in precisely tailored membranes have made osmotic power more viable. These membranes have improved water permeability and are less liable to clogging by impurities.
Osmotic power could provide base-load energy to coastal communities with abundant salty water, in areas such as Australia and the Middle East. It could also recover energy from desalination plants, which produce a waste stream of extremely salty brine that, when pumped back into the sea, can harm the local environment. Because osmotic power relies on the flow of rivers into the sea, it can generate electricity day and night, whatever the weather—unlike solar or wind, which require batteries for sustained output.
In August 2025 a ¥700m ($4.5m) osmotic power plant opened in Fukuoka, a city on the northern shore of Japan's Kyushu Island. With a generating capacity of 110 kilowatts—enough for 200 typical homes—the energy is used to run an adjacent desalination plant. The Fukuoka plant uses waste brine from the nearby desalination plant and treated water from a sewage works, rather than fresh water.
Sweetch Energy is building an osmotic power plant on the river delta where the Rhone meets the Mediterranean Sea in southern France. The warm, sunny climate causes a high rate of evaporation, making the Mediterranean's waters particularly salty. If a trial plant proves the technology, the company aims to construct a larger installation over the next decade with a generating capacity of 500 megawatts—enough to supply the 1.9m people of Marseille and its surrounding districts.
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