From thin air The race to draw energy from the atmosphere

Scientists have generated electricity from the humidity in the air-but can we ever make enough to power our homes?

- Ned Carter Miles

In the early 20th century, Serbian inventor Nikola Tesla dreamed of pulling limitless free electricity from the air. He was thinking on a vast scale, effectively looking at the Earth and upper atmosphere as two ends of a giant battery. His dreams were never realised, but the promise of air derived electricity - hygroelectricity - is capturing researchers' imaginations again. The difference: they're not thinking big, but very, very small.

In May, a team at the University of Massachusetts (UMass) Amherst published a paper declaring it had successfully generated a small but continuous electric current from humidity in the air. It's a claim that will probably raise a few eyebrows, and when the team made the discovery that inspired this new research in 2018, it did.

"To be frank, it was an accident," said the study's lead author, Prof Jun Yao. "We were actually interested in making a simple sensor for humidity in the air. But the student who was working on that forgot to plug in the power."

The UMass Amherst researchers were surprised to find that the device, which comprised an array of microscopic tubes, or nanowires, was producing an electrical signal regardless.

Each nanowire was less than one-thousandth the diameter of a human hair, wide enough that an airborne water molecule could enter, but so narrow it would bump around inside the tube. Each bump, the team realised, lent the material a small charge, and as the frequency of bumps increased, one end of the tube became differently charged from the other.

"So it's really like a battery," said Yao. "You have a positive pull and a negative pull, and when you connect them the charge is going to flow."