Engineers at Rutgers University in the USA have now paved the way for so-called blue energy that can be produced where rivers flow into the sea.
The process consists in the fact that positive sodium ions in salt water escape through a membrane into fresh water, while the membrane, on the other hand, stops negatively charged chlorine ions.
This creates a voltage difference that can be used with electrodes connected to two water tanks.
This is how blue energy is formed:
Previously, the scientists had made membranes with boron nitride nanotubes through which ions could pass, but the capacity was not enough.
The problem is to arrange the nanotubes tightly enough and completely perpendicular to the membrane to allow enough ions to pass through.
The American engineers have now succeeded in this. Their idea was based on using a magnetic field to control the nanotubes so that they fit correctly in the polymer film that forms the membrane.
First, the nanotubes were coated with a positive charge and then bathed in a solution with iron oxide atoms.
After this, it was possible to control the orientation of the nanotubes with the utmost precision in the polymer mass, and when it had solidified, the scientists had in their hands a 6.5 micrometer thick membrane with 10 million nanotubes per square centimeter.
In the tests that followed, it was found that the membrane produced four times more electricity than older membranes, and the performance can probably be improved even more, since only 2% of the nanotubes were open at both ends.
Now the researchers plan to find ways to etch both sides of the membrane so that more tubes open up.
The potential of blue energy is enormous. If it were possible to extract energy from the entire mixing of river water and the sea, the output would be 2.6 terawatts of blue energy, which corresponds to the electricity production of about 2,000 nuclear power plants.
