The Shocking Truth: Trees are Electric

Two engineers have designed innovative circuits that draw their power directly and solely from the weak electric currents generated by maple trees. The new research advances ongoing efforts to construct networks of tree-powered sensors that collect ecological data and can even detect forest fires.

Working with nanoelectronics, University of Washington electrical engineers Babak Parviz and Brian Otis built a boost converter circuit that needs only 20 millivolts of tree power to output a steady 1.1 volts, which is not much less than the 1.5 volts a AA battery generates. The two researchers hooked up their converter circuit to bigleaf maple trees around the university campus in a successful test, the results of which are now available online via the journal IEEE Transactions on Nanotechnology. This is the first peer-reviewed study of any tree-powered circuits.

“I have a lot of confidence that these circuits can support low-power sensors,” said Otis. Systems of wireless sensors powered by trees could be used in ecological research to monitor the changing conditions of forests and other natural environments, he said.

A Massachusetts-based company called Voltree Power has already successfully tested a tree-powered network of environmental sensors in Idaho’s Boise National Forest. Their network is designed to detect forest fires by monitoring humidity and temperature and wirelessly transmitting the data to a central station. Although the United States Forest Service and Voltree recently signed their first contract, there’s a bit of a problem: Voltree’s devices still require batteries. The trees provide enough natural voltage to extend the life of those batteries, but they must be replaced eventually — a costly effort requiring substantial labor, especially for those trees in the most remote locations.

This is where the new boost converter device designed by the University of Washington engineers could prove especially useful. The circuits Parviz and Otis constructed run solely on tree power, theoretically for as long as the tree lives. Adapting these circuits to support the kinds of sensors Voltree uses would eliminate the problem of battery replacement.

“In the next five to ten years we will be able to run these sensors right from the tree,” predicted Andreas Mershin, a biophysicist at the Massachusetts Institute of Technology and scientific advisor to Voltree. Once batteries are out of the picture, the Forest Service could furnish trees with circuits, sensors and wireless transmitters — none of which are harmful to the tree, Mershin emphasizes — and leave them to function as autonomous sentinels of national parks and remote forests.

Scientists have known that trees produce a tiny voltage upwards of a few hundred millivolts since at least the 1970s, but when Voltree began working with tree-powered circuits back in 2005, people still weren’t sure how to explain the phenomenon, according to Voltree’s chief executive, Stella Karavas.

Mershin and colleagues at MIT solved the puzzle in a 2008 study published in the journal PLoS ONE. They identified the pH imbalance between the inside of a tree and surrounding soil as the responsible mechanism. Trees continually pump water, nutrients and charged particles through their xylem and phloem — specialized tissues inside the trunk — actively maintaining this pH imbalance and creating the potential for an electric current.

Tree-powered technology has inspired other ideas for using living things as energy sources. “Many organisms lose a tremendous amount of energy in the form of heat,” said Otis, who sees this heat as another potential power source for circuits similar to those he built for bigleaf maples. He is currently working towards a chip powered by the heat a flying insect generates — a chip that could give biologists access to data they’ve never had before.

The work of Parviz and Otis confirms tree-generated voltage as a feasible source of energy for low-power electronics — a class of devices, like Voltree’s sensors, deliberately designed to consume very little power. Their new study further exemplifies some recent trends in electronics research: first, the challenge to make circuits run on less and less power; secondly, the need to completely rethink what makes a given power source viable or not.

Unfortunately, tree power isn’t going to solve the energy crisis or revolutionize bio-powered technology. The voltages involved are just too low to make such green dreams a reality. As Voltree’s Karavas was quick to point out, “You’re not going to be plugging your cell phone into a tree any time soon.”