Trees energize wireless networks

Via EE Times Asia:
1 December 2008

Researchers say that by harnessing the voltage difference between a tree and the ground, new ultralow-power sensors can transmit sensor data from almost any tree. The approach would also eliminate the need to clear a forest floor for solar power.

"We believe that by installing wireless sensors on just four trees per acre, we can provide better fire prediction modeling, earlier alerts and much better local climate data than is available in any forest today," said Andreas Mershin, a professor at Massachusetts Institute of Technology.

Potential power
MIT scientists tried to determine why trees have a different voltage potential than the ground. After eliminating several other possibilities, such as electromagnetic radiation, they unearthed a principle from the 19th century: pH differences can create a voltage potential. "It's the imbalance in pH between the inside of the tree and the soil in which it was potted that generates a voltage," said Mershin. "You get about 59mV for every step in pH mismatch."

The pH imbalance between the inside of the tree and the soil generates a voltage.

What is significant about the voltage potential between a tree and its soil, according to Mershin, is that the metabolism of the tree itself works to maintain it; no matter whether it's day or night, fall or spring, summer or winter, rain or sun. "Because the tree has to have a certain concentration of these ions inside of it to be happy about its metabolic and other biological functions, the tree itself actively regulates its own pH," said Mershin. "So, no matter what pH soil you put a tree in, it will work hard using its metabolic energy to keep its own pH constant.

Mesh network
The voltage potential could be reliably used to trickle-charge a battery or capacitor. This, in turn, could be used to power a wireless sensor node four times a day to transmit data. By implementing a mesh network among the nodes installed on trees, the researchers plan to pass along sensor data from node to node until it reaches a regional weather station.

- R. Colin Johnson