Osmotic power
was first theorized in the 1950s. Osmotic power plants harness osmotic
pressure. One method of recovering pressure from osmosis is known as
pressure-retarded osmosis (PRO). It was first developed in the 1970s by
Professor Sidney Loeb, co-inventor of reverse osmosis desalination. It is a
cheaper method than the other method, which is known as reversed
electrodialysis (RED). Osmotic power, or salinity gradient power, also known as
‘blue energy,’ is generated by harnessing the difference in salt concentration
between seawater and river water or freshwater. The osmotic pressure is created
at the membrane that separates water of different salinities. The energy
recovery process is analogous to that of refrigeration, where a cycle of
compression and expansion of gases creates a temperature gradient that moves
energy. Since it is based on pressure differential, or gradient, it also
reminds me of natural gas letdown power generation.
The figures below, via
Wikipedia, depict pressure-retarded osmosis.
Norway, Denmark, and Japan
have been developing PRO, and the Netherlands has been developing RED for
possible commercialization. Denmark and, most recently, Japan have the only two
functioning osmotic power plants in the world.
Osmosis involves the movement
of water from a lower concentration solution through a semipermeable membrane
to a higher concentration solution. This process generates pressure that can be
converted into power. The thermodynamic variable known as Gibbs Free Energy
explains how the power generation works through the energy given off by a
chemical reaction that occurs when seawater and river water mix. The formula
for Gibbs Free Energy is as follows:
where R is the gas constant, T is the absolute temperature,
Kf is the final equilibrium constant, and Ki is the initial equilibrium
constant.
Osmotic pressure is created on the seaward side of the membrane and is determined by the van’t Hoff equation as follows:
More information is given
below about power production methods, thermodynamics, challenges, and
efficiency.
According to
Thermal-Engineering.org, from which the above figures are derived, heat
transfer and its optimization play a very important role in osmotic power
generation. Therefore, optimized heat management is a key to successful osmotic
power production, as detailed below. Optimized heat management allows higher
pressure to be maintained, which increases energy generation. This is explained below.
One problem with osmotic
power plants is that there is a lot of energy lost in pumping and from the
frictional loss across the membranes. Thus, making the process as efficient as
possible is the key to scaling up the technology. There is ongoing osmotic
power R&D in several areas of the world where desalination plants are
present.
Japan’s New Osmotic Power Plant
Japan recently turned on the
world’s second osmotic power plant, the first being in Denmark, which came
online in 2023. This project, in the Fukuoka District, is expected to power 220
households as well as the desalination plant. Osmotic power operates
continuously. It is not variable like wind and solar, but reliable.
“The Fukuoka District Waterworks Agency said the plant
began operations on August 5 and is expected to produce 880,000 kilowatt-hours
a year, power that will be fed to a desalination facility serving the city and
neighboring areas.”
I noted previously that
the higher the pressure differential is, the higher the power generation
potential is. The higher the salinity differential is, the higher the pressure
is. Thus, the higher the salinity differential is, the higher the power
generation potential is. The Japanese plant takes advantage of this by
combining waters of two very different salinities. The seaward side water is
the water expelled from the desalination plant after the freshwater is extracted.
This is very saline water that can be harmful to local marine life. The other
water stream is treated effluent from a local municipal sewage treatment
plant.
An article in Interesting
Engineering by Kaif Shaikh explains some of the challenges of generating
osmotic power and what is being done about it by the Japanese researchers:
“One of the major barriers has been membrane cost and
efficiency, because large surface areas and high pressures are required, and
pressure-related and frictional losses erode net gains.”
“Recent advances aim to tackle those constraints.
Hollow-fiber forward-osmosis membranes developed by Toyobo are designed to
allow water molecules to pass while rejecting salts and impurities, improving
overall efficiency in modern setups. These were used in the world’s first fully
functioning osmotic power plant in Denmark.”
“In parallel, emerging approaches such as Ionic Nano
Osmotic Diffusion (INOD) from French startup Sweetch Energy use bio-sourced raw
materials and nano-osmotic diffusion principles to enhance ionic selectivity
and reduce losses, pointing toward more scalable blue-energy capture.”
According to 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," said Akihiko Tanioka, an expert in osmotic power and professor
emeritus at the Institute of Science Tokyo.
References:
Inside
Asia’s first osmotic power plant: How Japan turns saltwater into electricity: Japan
joins Denmark in proving osmotic power can work at scale. Kaif Shaikh.
Interesting Engineering. August 25, 2025. How
Japan’s first osmotic power plant turns saltwater into energy
Japan's
1st osmotic power plant begins operating in Fukuoka. Kyodo News. August 16,
2025. Japan's
1st osmotic power plant begins operating in Fukuoka
Osmotic
power. Wikipedia. Osmotic
power - Wikipedia
Thermodynamics
of osmotic power generation. Thermal Engineering. Thermodynamics
of osmotic power generation
Heat
transfer in osmotic power generation. Thermal Engineering. Heat
transfer in osmotic power generation
Reversed
electrodialysis. Wikipedia. Reversed
electrodialysis - Wikipedia
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