In November 2009, Norwegian state owned electricity company Statkraft opened the world’s first osmotic power plant prototype, which generates electricity from the difference in the salt concentration between river water and sea water. While osmotic power is a clean, renewable energy source, its commercial use has been limited due to the low generating capacities offered by current technology – the Statkraft plant, for example, has a capacity of about 4 kW. Now researchers have discovered a new way to harness osmotic power that they claim would enable a 1 m2 (10.7 sq. ft.) membrane to have the same 4 kW capacity as the entire Statkraft plant.
The global osmotic, or salinity gradient, power capacity, which is concentrated at the mouths of rivers, is estimated by Statkraft to be in the region of 1,600 to 1,700 TWh annually. Electricity can be generated through the osmotic phenomena that results when a reservoir of fresh water is brought into contact with a reservoir of salt water through the use of a special kind of semipermeable membrane in one of two ways –either by harnessing the osmotic pressure differential between the two reservoirs to drive a turbine, or by using a membrane that only allows the passage of ions to produce an electric current.
The Statkraft prototype plant (and a planned 2 MW pilot facility) relies on the first method, using a polymide membrane that is able to produce 1 W/m2 of membrane. A team led by physicists at the Institut Lumière Matière in Lyon (CNRS / Université Claude Bernard Lyon 1), in collaboration with the Institut Néel (CNRS), have developed an experimental device that they say is 1,000 times more efficient than any previous system, significantly enhancing the commercial viability of osmotic power as a power source.
The team’s experimental device uses the second method. It consists of an impermeable and electrically insulating membrane that was pierced by a single hole through which the researchers inserted a boron nitride nanotube with an external diameter of a few dozen nanometers. With this membrane separating a salt water reservoir and a fresh water reservoir, the team measured the electric current passing through the membrane using two electrodes immersed in the fluid either side of the nanotube.
The results showed the device was able to generate an electric current through the nanotube in the order of a nanoampere. The researchers claim this is 1,000 times the yield of the other known techniques for harvesting osmotic energy and makes boron nitride nanotubes an extremely efficient solution for harvesting the energy of salinity gradients and converting it immediately into usable electrical power.
Extrapolating their results to a larger scale, they claim a 1 m2 boron nanotube membrane should have a capacity of around 4 kW and be capable of generating up to 30 MWh per year, which is three orders of magnitude greater than that of current prototype osmotic power plants.
The researchers’ next step will be to study the production of membranes made of boron nitride nanotubes and test the performance of nanotubes made from other materials.
The research is detailed in a study published in the journal Nature.
Source: CNRS (Délégation Paris Michel-Ange)
Update: This article was updated on Mar. 14, 2013, to replace the per year capacity for a 1 m2 boron nanotube membrane originally stated as "30 mWh" to "30 MWh." Our apologies for error.
The reverse osmosis process will produce some high salt concentration waste water along with its fresh water and the Statkraft system can be used to generate the electricity to power the reverse osmosis pumps. Now that's what you call a win-win :)
If it works electricity will be renewable and cheap. Seafood will also be rare and expensive. Remember where the fresh water and the salt water mix is now call a marine estuary or a mangrove swamp. In the future it will be a power plant. Marine estuaries like mangroves are the nurseries of the oceans. No places on the planet are as important for the health of the oceans and we are now contemplating turning them into power plants. These power plants WILL destroy the mangroves and the estuaries.
Seafood is the last relatively cheap source of high quality, extremely tasty, high volume protein left on this crowded planet. All you need is a good boat, net and sonar to make a good living. You don't need to buy the land, pay land taxes, plant the crop, fertilize it, protect it from pests you just buy the combine and harvest.
We really need to think about what we are doing. When we do the cost benefit analysis on this technology through an environment assessment in whatever country has the money to build one of these plants please let us not assume that the loss to the marine ecosystem will be negligible and be made up for by the mangroves in Mexico or Bangladesh as these countries will be looking at this as a panacea too.
Up until I read this I thought the only really serious problem mankind faced was converting all our good cropland to asphalt.
You often get "estimates" of the total solar power of the planet which envision every square meter of land being a solar panel (i.e. abstract total power which means nothing as far as real potential goes but makes it sound really promising). I wouldn't be surprised if the global osmotic capacity was based on every ounce of water entering the sea being run through the process. I hope this technology does well but without subsidies from governments who are easily swayed by such "estimates".