Japan’s New Osmotic Power Plant Turns Saltwater Into Electricity

We generally think of the mosaic of renewable energy technologies as being defined by the big three: solar, wind, and hydroelectric power. There are also-rans like geothermal and oceanic power, and more controversial green technologies like nuclear. Still, for the most part, we pretty much know how future renewable energy will be made.

That’s why it’s always interesting to see a truly different idea get some traction. Japan has now joined Denmark in making real investments in osmotic power, which uses differences in salt concentration to generate electricity. With plenty of access to seawater, the island nation created an osmotic power plant capable of generating about 880,000 kilowatt hours of electricity per year.

At first glance, that doesn’t sound like a lot—and it’s not. There are close to 8,800 hours in a year; if the plant is operated continuously, that would give it a capacity of around 100 kilowatts. A 100-kilowatt diesel generator (very much not green power) is about the size of a dumpster—much smaller than this new Japanese facility, which likely nears 100,000 square feet. Still, osmotic power has the advantage of not only being zero emissions, but of being totally safe and usable in many populated parts of the world.

The Three Gorges Dam in China is the largest power station on the planet, generating well over 100,000 times more electricity per year than this new osmotic plant. Credit: Dan Kamminga

The technology works by keeping an enormous amount of fresh water next to an equally huge amount of seawater, with a semi-permeable separation between them. Osmosis draws salt into the fresh tank, which causes this tank to expand. This outward pressure is then harnessed to generate electricity.

The fresh water used is generally captured on its way out to sea anyway, so as long as the resulting brackish wastewater is dumped into the ocean, all the same water and salt end up in the same place they were headed, anyway.

The plant will power an existing desalination plant that serves Fukuoka, making it a neat little seawater-based package. If used more conventionally, the energy could provide for about 290 average homes.

Again, that’s not a terribly large number of homes. Given that this technology is so intrinsically safe to have near people, the obvious question is whether it could be scaled down; using a very large building to power 290 homes isn’t outstanding, but what about using 10,000 square feet to power 29 homes? For communities relatively close to the sea—and with access to a river for fresh water—we could imagine a small complex of perhaps 30 homes powered by a centralized, 100-foot by 100-foot seawater generator.

For now, osmotic power is a proven renewable energy technology that needs improvements in efficiency, but it definitely has its virtues. It creates zero heat and poses functionally no risk to public safety, which means it could be integrated more closely than any technology other than solar photovoltaic. Unlike solar panels, however, it’s unaffected by the weather.