Why Coastal Cities Are Betting on Osmotic Power

What if you could power cities just by mixing river water with seawater? Around 15,000 TWh of energy pours into the oceans each year as freshwater blends with saltwater at river mouths. That amounts to roughly half the energy the world consumes in electricity every year.¹² And we may have finally found a way to tap it. It’s called osmotic power, and it could provide clean energy around the clock.

As freshwater becomes scarcer, more cities are turning to desalination plants to meet their water needs. But desalination is energy-intensive, which drives up both water and electricity costs. Osmotic power could flip that equation by using the same salty wastewater from desalination to generate electricity instead of just dumping it back in the ocean.

Pilot projects are proving that there’s more than one way to shake up this hydrological cocktail. One approach creates voltage like a battery; another builds pressure like a mini hydropower dam. But how does this tech work? Could a combination of fresh- and saltwater be the “solution” to the world’s ever-increasing energy demands?

We already get energy from sunlight, wind, and dams. But what about from estuaries? When freshwater spills into seawater, energy is released as salt levels equalize. What if we could draw off some river water while it’s still fresh, and some seawater while it’s still salty, and combine them in a way that lets us put that free energy to work? It turns out, we now can. This is “osmotic power,” and Ocean Energy Systems, the blue energy wing of the International Energy Agency, predicts it could compete with other low-emissions, renewable energy sources like wave, tidal, and offshore wind at 50-100€/MWh, or about $60-$120/MWh.³

How much electricity are we talking about? Estimates range from about 17% of today’s worldwide electricity demand, according to the Dubai Future Foundation,⁴ down to just 2% in a study that modeled using only ideal river mouths and diverting just 20% of their flow.⁵ Still… that’s the amount of energy you’d get from covering all of Puerto Rico with solar panels.⁶

A 2016 study of coastal resources found that at least 448 river mouths around the world are good candidates, but osmotic power isn’t limited to just estuaries.¹ If we tap saltwater outflows from desalination plants and other industries and pair them with freshwater from municipal wastewater streams, the amount of energy we could generate through osmosis jumps even higher.

Currently, a French start-up is using a membrane made from wood pulp to separate the salts in seawater fast enough to create voltage. The company says it could generate enough power for 1.5 million people with seawater from the Mediterranean and just a fraction of the Rhône River’s flow.⁷⁸

In Japan, a very different approach is already online; the country’s first osmotic power plant uses turbines to generate electricity from a desalination plant’s salty wastewater and freshwater from the local treatment center.

How can mixing water possibly generate electricity?

The trick used in Japan is to separate the sweet water from the saltwater with a membrane that keeps salts on the saltwater side, but lets water molecules from freshwater come through (like a one-way valve).⁹¹⁰ Water then rushes from the freshwater side to the saltier side, balancing the salt in a natural process called osmosis.⁹

It’s kind of like pickles. Hang with me for a second. It’s the same force that pickles cucumbers. It drives water out of the plump veg and into a salty brine until the pickle is both shriveled…and delicious. I really need to not film these when I’m hungry.

Anyway, all that freshwater passing into saltwater builds up pressure on the saltwater side. The pressurized water is then released through a turbine, spinning a generator and producing electricity.⁹¹⁰¹¹

This process is called PRO, or pressure retarded osmosis, because freshwater flows into the saltwater even though the saltwater is moderately pressurized. Osmosis is that powerful.

Calling in the PRO

It’s no surprise, then, that PRO is making such a global splash. Groups in Australia, South Korea, Spain, and Qatar have launched prototypes and pilot projects hoping to launch it as a major league renewable.¹⁰

Norwegian company Statkraft built a 4 kW PRO demonstration site in 2009 with dreams of reaching 25 MW.⁹¹²¹³ It produced power… but not at a price that could compete, and in 2013, Statkraft shut the plant down. The salinity difference between the fjord’s seawater and the nearby river just wasn’t high enough to drive cheap power production with the membranes that existed back then.¹⁴

So when Danish company SaltPower built out the world’s first full-scale osmotic power plant in 2023, it used the saltiest water possible: brine from the Nobians Saltworks. The water is so salty, it can’t hold any more salt.¹⁵ At the saltworks, freshwater is pumped deep underground to dissolve salt deposits, then pumped back up as a concentrated brine. SaltPower added its osmotic generator into that loop.¹¹¹⁶

The system generates about 100 kW of power, enough to offset some of the energy needed to run the pumps. Thanks to modern, higher-efficiency membranes, the cost of that power is finally approaching solar and wind with one big difference: SaltPower’s osmotic generator runs 24/7.¹¹

The next step in bringing PRO to its full potential is to use water somewhere in the middle between seawater and saturation. The water is similar to the double-concentrate of seawater that’s produced when freshwater is extracted in desalination plants.¹⁷

In August 2025, the world welcomed a PRO plant in Fukuoka, Japan, linked to the Uminonakamichi Nata Seawater Desalination Center.¹⁸¹⁶⁹ It’s pulling salty brine from the desalination plant and treated sewage from the nearby Washiro Water Treatment Center. And it’s expected to generate 880 MWh of electricity per year or roughly 110 kW of net power output. That electricity helps offset part of the desalination plant’s own energy use, but would otherwise be enough to supply around 220 Japanese homes.¹⁸

The plant just opened, so real-world performance data isn’t available yet. But we do have results from a two-year pilot study in South Korea. That desalination facility used PRO not to generate electricity, but as an energy recovery device to help pressurize water for desalination.¹⁹ The researchers found that adding PRO cut the plant’s overall energy use by around 20%. That’s huge.

The world is already in a freshwater crisis, with more desalination plants constructed each year to meet freshwater demands.¹⁷ As of 2019, desalination consumed an estimated 205 to 361 TWh of electricity each year.²⁰ That’s about the same as the annual electricity use of Spain or Mexico.²¹ A 20% discount on that level of energy consumption could help keep freshwater prices accessible, even as rivers and lakes dry up.

For PRO to make economic sense at a desalination plant, it only has to beat the price the plant already pays for electricity: the market rate.

That’s the pressure approach to osmotic power. But watt if there’s another way to harvest osmotic power by capturing voltage like a battery instead of spinning turbines? What if it’s efficient enough to produce electricity cheap enough to sell to the grid … electricity that could feed your home, your phone, and your EV?

Sweetching It Up

Sweetch Energy in France is scaling up an osmotic power technology it says will clear the renewable energy limbo bar at about $110 per MWh within the next three years.²²²³ At that price, an around-the-clock energy source like osmotic power can compete with intermittent renewables like solar and wind with battery storage added in.²⁴ And above that bar? Well, we know how that goes.

Started in 2015, Sweetch created an “osmotic generator” utilizing a technology they call Ionic Nano Osmotic Diffusion or INOD. The membranes in Sweetch’s design aren’t designed to block salt’s sodium and chloride ions like desalination and PRO membranes. Instead, they’re covered in fairly large holes around 10 nanometers wide, which is much larger than the ions themselves.²⁵ The salt is meant to move through into the freshwater channels that flank each saltwater channel.

How you do get electricity if you’re just letting the salt rush through? By cleverly separating salt’s positively-charged sodium ions from its negatively-charged chloride ions. On one side of each saltwater channel, the membrane’s pores carry a positive charge; on the other side, the pores are negatively charged. That means when salts are drawn into the freshwater, the sodium ions are pulled through one side, while the chloride ions head out the other.²⁵

Think of the salt ions like horror enthusiasts and musical buffs queueing up at the box office. Even if both theater doors open at the same time, the movie-goers sort themselves. That separation creates a voltage difference between the two ends of the stack, driving electrons through an external circuit to generate electricity.

The water keeps flowing and so does the electricity. This is baseload power for a grid produced from just the mixing of saltwater and freshwater.²⁶

Of course, a real river is very different from a lab. To see how well the membranes perform with actual river and sea water, Sweetch built a pilot facility where France’s Rhône river meets the Mediterranean sea.²⁷ In field tests, power density has climbed from 1.2 to 1.6 W/m², reaching 40% efficiency in turning osmotic power into electricity. That may seem like a big drop compared to 4.3 W/m², but that’s typical when switching from pure salt solutions in the lab to the variety pack of salts in real river- and seawater.²⁸²⁹ With continued testing, the company says it can very likely hit 60 to 65% efficiency.⁷²³

The plan is to gradually scale the site from a several kilowatt pilot into a full-on demo facility, selling energy to the grid within three years.⁸²²²³ Sweetch says that if it ran just 10% of the Rhône river through its stacks, it could obtain 400MW of electricity. That means Rhône’s osmotic power could cover the electricity needs of the Marseille metropolitan area and its 1.5 million people.⁷⁸ Sweetch has its sights set on delivering grid power; but along the way, it’s partnering with local industries like desalination plants to turn their salty wastewater into energy, too.²³⁷

Sweetch’s first stop? Opening its big new production facility in France to scale up membrane production, optimizing the manufacturing process just as it’s optimized its unusual membrane.²⁶⁷

In desalination and PRO, membranes are typically petroleum-derived polyamide films.³⁰ But Sweetch makes its membranes with an inexpensive natural polymer: cellulose from wood, the same stuff we turn into paper, cellophane wrap, and even rayon fabric.²²³¹ The company manufactures the membranes itself in a roll-to-roll process, tuning the membrane’s physical and chemical properties to its needs.³²⁷

According to a 2024 report by French energy company Engie, Sweetch’s tech stands at technology readiness level 6 ³³, though reliable power export to a grid would bring them to a 7.

Although the membrane was designed with river- and seawater in mind, all the INOD membrane stack needs to generate electricity is a reason for ions to flow. That reason can be osmosis or, the company says, just plain old heat… the kind produced by run-of-the-mill industrial processes like, well, running a textile mill, a chemical plant, or a data center.³⁴

Sweetch’s idea is to convert a temperature gradient into a salinity gradient, using specialized salt solutions that change behavior at different temperatures.³⁵ Think of a liquid that acts like freshwater at 10 C or 50 F, but acts like saltwater at 50 C or 120 F.²³

The European Union has awarded Sweetch a $2.8 million dollar grant to turn its theory into a reality. The project wraps up in October 2026, and hopefully then we’ll get the full scoop on the tech. When I spoke to Sweetch about this approach, the team sounded very optimistic.

However, the technology isn’t the only part of osmotic power systems that needs deep study. Diverting an estuary’s river water could have downstream effects by changing the way water and sediments flow.²⁰¹⁵ That could impact oyster bars, sea grass beds, and mangroves. Estuaries are known as the “nurseries of the sea” because so many species of fish and other critters grow up there before heading offshore.³⁶

All that life and sediment is no good for membranes: it slimes them up. Desalination plants rely on heavy-duty filtration and even chemical conditioning to prevent that. Sweetch has a system in place for pre-filtration of sediment, but biofouling is still an issue. Because it’s returning the water directly to the estuary, it has to beat back biofouling without chemical treatment.

Still… with global desalination capacity expected to double by 2050, the potential for osmotic power keeps rising.¹⁷ Meeting clean-energy targets will take a mix of technologies. This means tried-and-true solar and wind plus newcomers like osmotic power and wave energy, which I covered recently if you want to check that out.³⁷

Just like how hydropower dominates the rainy Pacific Northwest and solar fields cover sunny Texas, each renewable has its place.³⁸³⁹ And osmotic power’s place might be exactly where we need it most: at coastal desalination plants that are already dealing with salty wastewater and rising energy costs.

I’ll be watching to see if osmotic power scales up at desalination plants and whether Sweetch starts exporting power to the grid in France within the next few years.