Colin Barras, New Scientist 10 Sep 09;
Shoving electrodes into tree trunks to harvest electricity may sound like the stuff of dreams, but the idea is increasingly attracting interest. If we can make it work, forests could power their own sensor networks to monitor the health of the ecosystem or provide early warning of forest fires.
Children the world over who have tried the potato battery experiment know that plant material can be a source of electricity. In this case, the energy comes from reduction and oxidation reactions eating into the electrodes, which are made of two different metals – usually copper and zinc.
The same effect was thought to lie behind claims that connecting electrodes driven into a tree trunk and the ground nearby can provide a current. But last year Andreas Mershin's team at MIT showed that using electrodes made of the same metal also gives a current, meaning another effect must be at work. Mershin thinks the electricity derives from a difference in pH between the tree and the soil, a chemical imbalance maintained by the tree's metabolic processes.
Practical power
While proving that trees can provide a source of power is a significant step, a key question remains: can the tiny voltage produced by a tree be harnessed for anything useful?
Trees seem capable of providing a constant voltage of anywhere between 20 and a few hundred millivolts – way below the 1.5 volts from a standard AA battery and close to the level of background electrical noise in circuits, says Babak Parviz, an electrical engineer at the University of Washington in Seattle. "Normal circuits don't run from very small voltages, so we need ways to convert the small voltages to something that is usable," he says.
His team has managed to obtain a usable voltage from big-leaf maple trees by adding a device called a voltage boost converter. The converter spends most of its time in a kind of stand-by mode as it stores electrical energy from the tree, periodically releasing it at 1.1 volts.
To provide that periodic wake-up call, Parviz's team developed a clock, also powered by the tree, which keeps time by tracking the quantum tunnelling of electrons through thin layers of insulating material. It operates at 350 millivolts and uses just a nanowatt of power.
Parviz thinks trees could power gadgets to monitor their own physiology or their immediate surroundings, for ecological research. And, he adds, as electronic components continue to shrink and require less power, it is possible tree electricity could one day have a wide range of uses.
Green power race
Parviz's team isn't the only one trying to harness the tiny voltages trees can provide. Voltree Power, a company based in Canton, Massachusetts, patented a tree-powered circuit in 2005, says the company's CEO, Stella Karavaz.
Her firm is using energy harvested from trees to power sensors that monitor temperature and humidity inside forests. Earlier this year the company trialed a wireless sensor network to detect forest fires.
Devices that lose water the way trees transpire through their leaves could also be used to supply power, according to Michel Maharbiz at the University of California, Berkeley. His team recently showed that evaporation from simulated leaves can act like a mechanical pump, and that the effect can be harnessed to provide power.
Journal references: Parviz's paper: IEEE Transactions on Nanotechnology, in press, Mershin's paperPLoS One, DOI: 10.1371/journal.pone.0002963, Maharbiz's paper: Applied Physics Letters, DOI: 10.1063/1.3157144
