Scheme to 'pull electricity from the air' sparks debate
Tiny charges gathered directly from humid air could be harnessed to generate electricity, researchers say.
Dr Fernando Galembeck told the American Chemical Society meeting in Boston that the technique exploited a little-known atmospheric effect.
Tests had shown that metals could be used to gather the charges, he said, opening up a potential energy source in humid climates.
However, experts disagree about the mechanism and the scale of the effect.
"The basic idea is that when you have any solid or liquid in a humid environment, you have adsorption of water at the surface," Dr Galembeck, from the University of Campinas in Brazil, told BBC News.
"The work I'm presenting here shows that metals placed under a wet environment actually become charged."
Dr Galembeck and his colleagues isolated various metals and pairs of metals separated by a non-conducting separator - a capacitor, in effect - and allowed nitrogen gas with varying amounts of water vapour to pass over them.
What the team found was that charge built up on the metals - in varying amounts, and either positive or negative. Such charge could be connected to a circuit periodically to create useful electricity.
The effect is incredibly small - gathering an amount of charge 100 million times smaller over a given area than a solar cell produces - but seems to represent a means of charge accumulation that has been overlooked until now.
Dr Galembeck suggests that with further development, the principle could be extended to become a renewable energy resource in humid parts of the world, such as the tropics.
However, while the prospect of free electricity from the air is tantalising, the prospect of harnessing enough of it to be widely useful is still a matter of some debate.
Hywel Morgan of the University of Southampton says that a similar effect has been known for some time; he points out that tribocharging - the generation of charge by rubbing wool over amber or water droplets over water droplets - is the origin of thunderstorms.
"What we think is happening is he's pumping the water vapour across his capacitor and during the pumping mechanism, tribocharging the water vapour."
That would result in a charge, but would not be the same as simply pulling the charge from still, wet air.
Marin Soljacic, the Massachusetts Institute of Technology physicist behind a wireless power transmission technology, known as Witricity, disagrees.
He calls the paper "very interesting" and "a good area of research".
He concurs, however, that the amount of charge gathered in the initial tests suggests the effect may be difficult to put to good use, saying that "at this point it is far-fetched to see how it could be used for everyday applications".
"It really warrants future research and understanding what all the limitations of this are, how far it can go," he told BBC News.
"[Prof Morgan] is right that a similar and closely-related effect is known to exist, but we're very pressed for finding new sources of renewable energy, [so] I think it's a bit early to discard this research."
Dr Galembeck is familiar with the controversy that this kind of work generates, saying that disagreement about the mechanism behind it forms "the motif for bitter discussions among scientists".
"There have been many attempts to harness electricity from the atmosphere and most had bad endings."