Here's the explanation of the physics they gave:

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,” says Yao. “You have a positive pull and a negative pull, and when you connect them the charge is going to flow.”

Wow. Presumably coastal northern communities could be big beneficiaries of this in the future. Lets hope it's not literal vapourware

American southeast could single handedly power the country

One thing that hasn't been mentioned here is that at a large scale this probably has significant impacts on weather patterns.

The device they have come up with is the size of a thumbnail, one-fifth the width of a human hair, and capable of generating roughly one microwatt – enough to light a single pixel on a large LED screen.

So probably only going to be usable for low power devices for a while