Most of the world's electricity still comes from burning 100-million-year-old dead life-forms, but new alternatives keep appearing. The latest uses protein nanowires to draw electricity from the atmosphere, although since moisture from humidity is required, the air the power comes from isn't entirely thin.

The new device, named the Air-gen, is the brainchild of microbiologist Dr Derek Lovley and electrical engineer Dr Jun Yao, both from the University of Massachusetts, Amherst. They claim their creation avoids the key outstanding problem of other sorts of renewable energy. "The Air-gen generates clean energy 24/7," Yao said in a statement. 

Some attempts to produce electricity from novel sources rely on exceptionally expensive raw materials. The film of electrically conductive nanowires connecting the Air-gen's two electrodes, however, is produced by Geobacter bacteria fed on a cheap diet. Gold leaf electrodes were used, but substitution with inert carbon also worked. The film is less than a hundredth of a millimeter thick and only partially covered by the top electrode, exposing it to the air. Pores in the film absorb atmospheric water vapor and a moisture gradient induces ionization, with positive charges diffusing one way and electrons the other.

On a laboratory scale, the Air-gen's energy density is high compared to most novel renewable energy devices. The test model Yao and Lovley describe in Nature produced 4mWatts/cm² at best – about a fifth of a solar panel in full sunlight. That's almost as high as another recent renewable novelty, a panel that turns the kinetic energy of raindrops into electricity. However, existing modules are tiny – a quarter of a square centimeter, and scaling up may be a challenge.

On the other hand, Air-gen's capacity is uniquely resilient to changing weather conditions. Production dropped when humidity was outside an ideal range, but even Saharan conditions didn't stop it entirely. Electricity has been generated from humidity before, but with much lower power densities and only in bursts that last less than a minute. Air-gen appears able to keep going indefinitely, although even under constant humidity electricity production fell slowly over a day.

Initially, the authors hope to produce patches that power wearable electronic devices, and then paint the walls of off-grid houses with the nanowires. “Once we get to an industrial scale for wire production, I fully expect that we can make large systems that will make a major contribution to sustainable energy production,” Yao said. 

Lovley identified the Geobacter in Potomac River mud 30 years ago, and later discovered it eats electrons off rocks and produces electrically conductive protein nanowires. The idea of using these wires for electricity production came about by accident when Yao's PhD student Xiaomeng Liu was trying to use them to make sensors and noticed under certain circumstances current was produced.