Are Artificial Trees the Future of Renewable Energy?
In 2007, Eric Henderson watched the heart-shaped leaves of a redbud rustle in the wind outside of his home in Iowa. A gust came through, whipping around the tree’s branches, causing the leaves to oscillate in the turbulent stream of air.
“And that got me thinking,” he says.
Henderson, a molecular biologist at Iowa State University, started toying with the idea of harvesting these random gusts. “It’s not wind that will ever see a turbine because it’s low to the ground and it’s going through little eddies and swirls,” he says. But there is still energy there.
This started him on an obsession with leaves—studying their shapes, aerodynamics, oscillations at the slightest provocation. He recruited two other researchers from the university, Curtis Mosher and Michael McCloskey, to help him, and together, the concept of the faux forest blossomed. The idea was that by creating leaves out of certain materials, they could harvest the energy from the bending leafstalks.
Everything hinged on a method known as piezoelectrics, which has been around for over a century. Discovered by Jacques and Pierre Curie in 1880, they have been used in a variety of gadgets—from early phonographs (where piezoelectrics turned the vibrations from the needle into electric current) to spark lighters.
The concept is based on manipulation of materials that have a regular array of covalent bonds, a chemical connection in which two atoms share electrons. “In a crystal, all those [bonds] are in a very ordered state,” says Henderson. “If you squeeze it, or push it, or tweak it, it shifts.” And if manipulated properly, this shuttling back and forth of electrons can generate electricity.
The basics of the researchers’ idea was simple: build a tree-shaped electricity generator with plastic leaves that have stalks made out of polyvinylidene fluoride (PVDF), a type of piezoelectric plastic. Plunk the tree outside in any region with a breeze and harvest the energy as the fake leaves sway to and fro.
But, as they recently published in the Journal PLOS ONE, the situation is much more complicated. “It all sounds great until you try to do the physics,” Henderson says.