This spring, a company called Voltree Power plans to test a wireless sensor network that taps into electricity generated naturally by trees in order to provide an early warning system against wildfires. The humidity and temperature sensors will be distributed over a 100-acre plot of land provided by the U.S. Forest Service. Based on the readings relayed by each sensor, the network can continually monitor forest conditions to predict fire hazard.

The unique element about these sensors is their power source: the Voltree Power sensors harvest metabolic energy from the trees themselves and convert it into electrical energy. The use of ambient sources of energy eliminates the need to replace batteries, which makes the system substantially cheaper to maintain. A theory of how that metabolic energy arises was reported in the journal Public Library of Science ONE last month. The lead author on the study is Voltree’s vice president of research and development — and a senior at MIT.

According to the paper, the tiny amount of energy from the trees emerges from a difference in the pH of a tree and its soil. Think of the soil as a chemical solution. The tree, its roots, and the surrounding soil form a region with one pH level, while the surrounding soil has a different pH. The two regions will attempt to reach equilibrium, so that the pH is the same throughout the soil. In doing so, the gradient between the different pH regions produces a voltage, and that can be exploited to produce and store electrical energy.

Much like the rain power we wrote about last January, tree power would only provide a trickle charge, tiny amounts of energy slowly accumulated and stored in a battery or ultracapacitor. Most environmental sensors are currently battery-powered or use small solar panels, though a few varieties may use piezoelectric materials, which produce a voltage when placed under mechanical stress. Voltree has designed the sensors to work with the Forest Service’s 2,200 remote automated weather stations to simplify the exchange of data between the nodes and fire managers. These stations form the backbone of the U.S.’s wildfire monitoring network by monitoring the weather and air quality, which, when combined with satellite imagery, can be used to predict fire.

Voltree’s founders believe that their sensors will provide more specific and timely information about the spread and threat of wildfire, enabling the Forest Service to move more quickly to save their trees.

This article originally appeared in "Plenty" in September 2008.

Copyright Environ Press 2008