There's Energy in Swaying Trees

Imagine a forest of trees, swaying in the breeze. Now imagine you could capture that energy and use it for good — say, to prevent the deadly collapse of a bridge.

That’s what Ryan Harne, a researcher at Ohio State University, is trying to do. Well, sort of. He’s built little machines that mimic the structural properties of trees, which allows energy to be captured more efficiently and converted into electricity.

What is a tiny tree generator good for? Imagine this: All over the world, buildings, bridges, and airplanes are equipped with sensors to measure vibrations, which signal if the structure is deteriorating and at risk of collapse.

“These have been proliferating after a number of large structural catastrophes, like bridges collapsing,” Harne tells Inverse.

Individually, these sensors don’t use a lot of energy, but taken together, their power consumption is quite large. Today, they rely mainly disposable batteries, which are wasteful and harmful to the environment.

“The lightbulb was that, hey, let’s try and power the sensors using the vibrations that they’re actually monitoring,” says Harne. It’s a question that researchers have been working on for about 25 years.

And that’s where trees come in. Trees have really interesting structural properties that regulate how they react to windy conditions.

It turns out that they exhibit this strange phenomenon called internal resonance: Smaller branches at the outside of the tree oscillate quickly in the wind, and this energy is transferred to longer period oscillations in larger branches and in the trunk. In this way, the energy is dissipated and the tree can withstand more violent gusts.

“There’s just some really good evidence from botanists that trees use some really remarkable distribution of their frequencies that they oscillate at, and that helps to consolidate the energy for dampening purposes,” says Harne.

He wondered if he could harness the same principle towards another end — capturing energy. He built a simple tree-like structure from steel: a six-inch “trunk” with a single three-inch “branch” attached at a 90 degree angle. The structure was equipped with a strip of electromagnetic material, which serves to convert the vibrational energy into electricity.

Then he attached his little tree to a device that shook it back and forth at a high frequency. At first, it didn’t appear to do much, although some electrical activity was registered.

But when he injected some “noise” into the system — a randomization of the vibration pattern that might mimic a gust of wind or an earthquake — the system reached a tipping point, and the higher frequency oscillations of the branch were transferred to a longer period, and larger amplitude oscillations in the trunk of the tree. What’s neat about that is how much more efficiently those vibrations are converted to electricity — the system more than doubled its voltage output after the internal resonance was triggered.

The experiment proves the potential of vibrational sensors that power themselves. But Harne says it’s tough to drum up interest from industry in a world that is so attached to disposable technology.

“When do I see it coming online? We would probably require a greater demand for self-sustainability. I’m not sure that that is as richly demanded in our society,” he says.

Harne isn’t the only one trying to commercialize the power potential of vibrations. One company has proposed wind power generators that oscillate instead of spinning, as a way of reducing noise and harm to wildlife. Ocean wave power, which could potentially generate an enormous amount of electricity, operates on the same principles.