How Japan is turning saltwater into clean electricity

Japan has officially entered the global race to generate energy from the meeting of saltwater and freshwater, unveiling its first osmotic power plant in Fukuoka earlier this month.

The facility, operated by the Fukuoka District Waterworks Agency, is only the second in the world after Denmark’s pioneering project in 2023.

Experts see it as a crucial development in the search for renewable energy sources that can function continuously without weather limitations or carbon emissions.

What we know about Japan’s first osmotic power plant

The newly launched installation in Fukuoka uses the difference in salinity between seawater and freshwater to produce electricity.

According to the waterworks agency, the plant is expected to deliver about 880,000 kilowatt-hours per year, a volume of energy equivalent to the consumption of approximately 220 Japanese households.

Instead of supplying homes directly, the output will be channelled into a desalination facility that provides clean water to residents of Fukuoka and surrounding areas.

The agency highlighted osmotic generation as “a next-generation renewable energy source that is not affected by weather or time of day and emits no carbon dioxide.”

By relying on the natural flow of water molecules across a semipermeable barrier, the technology avoids the intermittency problems faced by solar and wind power, which depend on sunlight or favourable weather conditions.

Akihiko Tanioka, professor emeritus at the Institute of Science Tokyo and a veteran researcher in the field, expressed optimism about the breakthrough, telling Kyodo News, “I feel overwhelmed that we have been able to put this into practical use. I hope it spreads not just in Japan, but across the world.”

The science behind osmotic power

The principle of osmosis, known to science for centuries, is at the heart of the system. When freshwater and saltwater are placed on either side of a semipermeable membrane, water molecules naturally flow toward the saltier side to balance concentrations.

Osmotic power plants exploit this process by placing two water streams with different salinities opposite each other, separated by specialised membranes that allow only water molecules to pass.

In the case of the Fukuoka facility, the freshwater side comes from treated wastewater sourced from a sewage plant, while the saltwater side is composed of concentrated seawater created as a byproduct of desalination.

As the freshwater crosses the barrier, pressure builds on the saltwater side. This pressure is then used to drive a turbine connected to a generator, producing electricity.

The concept is straightforward but technically demanding. Large amounts of water must be pumped into the system, and membranes must withstand high pressure while maintaining selectivity to avoid impurities passing through.

The Fukuoka plant’s design reflects recent advances in both areas, including improvements in hollow-fibre forward-osmosis membranes that enhance efficiency.

Why this was a long-time coming

Although osmotic energy has only recently reached commercial-scale implementation, the idea itself is not new. The theoretical foundations were laid in the mid-20th century.

In 1954, researcher RE Pattle first suggested that the mixing of freshwater and seawater could be used to generate power.

In the 1970s, Professor Sidney Loeb, best known as co-inventor of reverse osmosis desalination, developed the method known as pressure-retarded osmosis (PRO) after observing the natural mixing of the Jordan River and the Dead Sea.

Since then, multiple attempts have been made to move the technology from laboratory prototypes to viable energy plants.

Pilot projects have been tested in Norway, South Korea, Spain, Qatar, and Australia, each exploring variations of osmotic processes. However, most initiatives struggled with technical limitations and costs, preventing widespread adoption.

From Denmark to Japan

The breakthrough came in 2023 when the Danish company SaltPower launched the world’s first commercial osmotic power plant in Mariager.

The project used Toyobo’s hollow-fiber forward-osmosis membranes to capture blue energy at scale. The Japanese installation in Fukuoka, though similar in generating capacity, is larger in design and is the first in Asia.

Experts view the Japanese plant as an important step forward. Dr Ali Altaee, a specialist at the University of Technology Sydney, explained that while the plant’s annual electricity production matches Denmark’s, its larger size demonstrates Japan’s commitment to scaling up.

He told The Guardian that the electricity generated is “the equivalent of powering about 220 Japanese households.”

Altaee has worked on osmotic prototypes in Australia, Spain, and Qatar, though some projects were disrupted during the Covid-19 pandemic.

He suggested that facilities in places such as New South Wales could benefit from similar systems if adequate government support were made available, pointing to Australia’s salt lakes as untapped resources.

What hurdles does the method face

Despite the enthusiasm, experts acknowledge that significant challenges remain. Energy must be expended to pump both saltwater and freshwater into the facility, and frictional resistance inside the membranes causes additional losses.

As University of Melbourne professor Sandra Kentish told The Guardian, “While energy is released when the salt water is mixed with fresh water, a lot of energy is lost in pumping the two streams into the power plant and from the frictional loss across the membranes. This means that the net energy that can be gained is small.”

Kentish added that Japan’s decision to use concentrated seawater brine, the residual from desalination plants, was a smart approach because it increases the difference in salt concentration between the two streams, thereby enhancing the amount of energy available.

She also pointed out that advances in membrane and pump technology are gradually reducing inefficiencies.

Why this method is sustainable

Unlike solar and wind energy, osmotic power plants can operate continuously, as long as there is access to freshwater and seawater. This quality makes them particularly attractive for regions seeking stable renewable energy sources.

For Japan, which has limited domestic fossil fuel reserves and relies heavily on imports, the development of such technologies is strategically significant.

Globally, researchers are continuing to refine osmotic systems. In France, startups like Sweetch Energy are experimenting with Ionic Nano Osmotic Diffusion (INOD), a process that uses bio-based raw materials and nano-scale principles to boost efficiency.

These efforts, alongside Japan’s achievement in Fukuoka, indicate that osmotic power could soon move from experimental curiosity to a practical component of the renewable energy mix.

The hope is for Japan’s success to encourage further adoption.

With inputs from agencies