The study's lead author, Prof Jun Yao told the Guardian that it was all an accident and his team was only interested in making a simple sensor to measure humidity. 

"For whatever reason, the student who was working on that forgot to plug in the power" and the UMass Amherst team 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 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," says Yao. "You have a positive pull and a negative pull, and when you connect them the charge is going to flow."

"Even though a thin sheet of the device gives out a very tiny amount of electricity or power, in principle, we can stack multiple layers in vertical space to increase the power."

Another team, Prof Svitlana Lyubchyk and her twin sons, Profs Andriy and Sergiy Lyubchyk, are trying to increase the power. Svitlana Lyubchyk and Andriy are part of the Lisbon-based Catcher project, whose aim is "changing atmospheric humidity into renewable power", and along with Sergiy they have founded CascataChuva, a startup intended to commercialise the research.

So far they have created a thin grey disc measuring 4cm (1.5in) across which can generate a relatively modest 1.5 volts and 10 milliamps.

However, 20,000 of them stacked into a washing machine-sized cube, they say, could generate 10 kilowatt hours of energy a day — roughly the consumption of an average UK household. Even more impressive: they plan to have a prototype ready for demonstration in 2024.

The average net present cost of electricity generation for a generator over its lifetime — from these devices will indeed be high at first, but by moving into mass production, they hope to lower it significantly, ultimately making this hygroelectric power competitive with solar and wind.

The team accept that it may take years to optimise a prototype and scale up production, but if they're successful, the benefits are clear. Unlike solar or wind, hygroelectric generators could work day and night, indoors and out, and in many places.