Could mechanical trees be the next big green technology?

Scientists find that small vibrations already happening in the hustle-bustle of the modern world could trigger electricity generation in tree-like structures.

Felipe Dana/AP
Palm trees are silhouetted against an early evening sky, at Ipanema beach in Rio de Janeiro, Brazil, Jan. 23.

Tiny leafless mechanical "trees" may be the next frontier for harvesting green energy. 

New research reveals that tree-like units can generate electricity by being shaken slightly. This power generation could be triggered by the wind, a tall building swaying, traffic on a bridge, or even the quaking of the Earth itself.

The key is in the vibrations, a team of scientists reports in the Journal of Sound and Vibration.

These "trees" – really just a trunk with a handful of branches – would be made of sensitive electromechanical materials. When the random vibrations innate in the hubbub of every day life shake the mechanical trees, the structures would convert those forces into strong structural vibrations that can in turn be used to generate electricity.

"Buildings sway ever so slightly in the wind, bridges oscillate when we drive on them and car suspensions absorb bumps in the road," project leader Ryan Harne said in a news release. "In fact, there’s a massive amount of kinetic energy associated with those motions that is otherwise lost. We want to recover and recycle some of that energy."

Scientists who proposed similar technology before dismissed the random vibrations as too inconsistent for electricity generation. But Dr. Harne used mathematical modeling and revealed that tree-like structures could in fact maintain consistent vibrations even from the widely varying inputs.

The researchers tested their findings by building a mock mechanical tree. They started by shaking their "tree" slightly with one device. The vibrations were at a high frequency and produced a small amount of power.

But then the team decided to add more inputs into the system. This added noise created what Harne called "saturation phenomena." The extra pushes and shakes pushed the "tree" past a tipping point where the high frequency energy became low frequency, which was more conducive to electricity generation.

The researchers don't necessarily envision fields of this new kind of crop, but suggest mechanical trees could prove useful for electricity generation where other green options don't fit. 

Windmills, for example need to be in wide open space where the wind will spin it sufficiently. But these mechanical trees would do well in busy metropolitan areas, stuck to the bottom of a bridge or even built into a skyscraper.

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