Spin a turbine with clean and dirty water, make electricity
No fuel needed
Osmosis is a physical process which will raise the pressure of water in a tank in the right setting. It is possible to make electricity by bleeding off the pressure to spin a turbine connected to a generator. No fuel is needed.
This technology is getting press coverage as a new source of fuel-free electricity.
Working power plants are already in use. They are small at this point but the basic principle has been proven. There is a lot of development work ahead to make osmotic energy a cost-effective way to make electricity.
Reverse osmosis
Reverse osmosis is perhaps better known since it is used in municipal water plants to remove contaminants from raw water. Dirty creek water goes in, and clean drinking water comes out. (Extra dirty creek water comes out to and goes back to the creek.)
To accomplish this by reverse osmosis there is a special membrane placed down the middle of a tank of water. The sides could be called the contaminated water side and the clean water side. By pressurizing the contaminated water you can force its water molecules to go through the membrane to the clean water side and leave the contaminants behind.
Clean water is the desired result, and the creek water with extra contaminants is allowed to flow back to the creek.
What is osmosis?
Osmosis is the natural process that would occur if pressure was not applied to the contaminated side.
If there were a membrane between a contaminated water side and a clean water side and this system is left alone, water will go from the clean water side through the membrane to the contaminated water side and dilute the contaminants.
Water molecules move by osmosis through the membrane without being forced. The key to osmosis or reverse osmosis is the semi-permeable membrane.
Capturing the pressure
If the contaminated side is in a sealed tank, the pressure will increase as the osmosis process proceeds.
Energy can be captured from a pressurized tank by allowing some of the pressurized water to flow through a discharge pipe and control valve. The flow of water can spin a turbine as it leaves.
The two water streams must flow
The two steams must not stay stagnant. A flow of high-concentration water is needed to keep this process working. As clean water comes through the membrane into the contaminated water the high concentration of contaminants is diluted. The pressure produced is proportional to the concentration of contaminants in the high-concentration flow. If there was no replenishment of high-concentration water, its pressure will drop and eventually the turbine won’t spin.
Power plants in commercial service
There are three working power plants, one each in Denmark, France, and Japan. Several more projects are being pursued in other countries.
Denmark
SaltPower, a Danish company, has installed a plant producing about 100 kW, with the capability of expansion. The high-concentration stream is brine from the process of production of salt from the earth. The low-concentration stream is river water. The location is a salt production facility owned by Nobian, a chemicals company. The power plant was commissioned in 2023, following several years of testing smaller systems. SaltPower is active in research to improve the process.
Photo credit, SaltPower, https://saltpower.net/gallery/
France
Sweetch Energy has installed a pilot plant in the Rhone Delta. The two steams are river water and sea water. The two streams mix naturally. The osmotic plant captures energy that would otherwise be lost.
The plant was commissioned in 2023. Sweetch patented the membrane in use. The membrane has higher efficiency than its predecessors. The initial plant produces approximately 100 kW but the developers are making plans for expansion, eventually up to 500 MW.
The image shows an artist’s conception of a 25 MW plant.
Photo credit, https://meilleure-innovation.com/lenergie-osmotique-est-elle-une-solution-efficace-a-la-production-delectricite/
Japan
The most recent power plant is in Japan, owned by the Fukuoka Water Agency. It went into service at the agency’s desalination plant in August 2025. It is rated at 100 kW and provides electricity to the desalination plant itself.
The two streams are the high-concentration salty discharge stream from the desalination plant, and treated wastewater also on its way to being discharged. Treated wastewater has few contaminants and is often cleaner than creek or lake water.
The Fukuoka Water Agency also operates a small hydropower generator and has installed solar photovoltaic panels on one of its buildings.
References
https://www.polytechnique-insights.com/en/columns/energy/osmotic-energy-an-asset-for-the-planet/
https://saltpower.net/
https://www.city.fukuoka.lg.jp/mizu/somu/documents/eigoban.pdf
https://interestingengineering.com/energy/asias-first-osmotic-power-plant