A new technique could make the salinity difference between fresh water and salt water a viable source of renewable energy.
Power yields from existing techniques are not high enough to make them viable a team led by physicists at the Institut Lumière Matière in Lyon, in collaboration with the Institut Néel, has discovered a new means of harnessing this energy.
The team has discovered that osmotic flow through boron nitride nanotubes generates huge electric currents, with 1,000 times the efficiency of any previous system, and their findings are published in today’s issue of Nature.
To achieve the result, the researchers developed a novel experimental device that enabled them, for the first time, to study osmotic fluid transport through a single nanotube.
When a reservoir of salt water is brought into contact with a reservoir of fresh water through a special kind of semipermeable membrane, the resulting osmotic phenomena make it possible to produce electricity from the salinity gradients.
This can be done in two different ways: either the osmotic pressure differential between the two reservoirs can drive a turbine, or a membrane that only passes ions can be used to produce an electric current.
Concentrated at the mouths of rivers, the Earth's osmotic energy potential has a theoretical capacity of at least 1 terawatt – the equivalent of 1,000 nuclear reactors – but, the technologies available for harnessing this energy are relatively inefficient, producing only about 3 watts per square meter of membrane.
Extrapolating the teams results to a larger scale, a 1m2 boron nitride nanotube membrane should have a capacity of about 4kW and be capable of generating up to 30 megawatt-hours per year – three orders of magnitude greater than that of the prototype osmotic power plants currently in operation.
The next step for the researchers in the project will be to study the production of membranes made of boron nitride nanotubes and test the performances of nanotubes made from other materials.