An experimental wireless mesh network for monitoring forests for fire danger was recently demonstrated while being powered by the trees themselves during the Global Semiconductor Alliance Emerging Opportunities Expo & Conference, held earlier this month in Santa Clara, Calif. The demonstration used energy-harvesting hardware developed by a Massachusetts company called Voltree Power. The test took place at the National Interagency Fire Center in Boise, Idaho, under the auspices of the USDA Forest Service and Bureau of Land Management.
In the GSA talk, Darnell Group CEO Jeff Shepard characterized the harvested energy source as essentially a “potato battery.” As most EEs probably recall, the potato battery is a simple electrogalvanic cell in which the potato, or other medium, provides an electrolyte while a pair of dissimilar electrodes provides the potential difference. But the new technology may be more interesting since Voltree’s “potatoes” are still alive and growing.
In the forest, according to Voltree, its power sources provide an alternative to batteries for the Early Wildfire Alert Network (EWAN), which comprises a large number of humidity and temperature sensor nodes distributed over remote forestland (see the figure). Each node contains a ZigBee node that enables EWAN to instantly communicate the onset of wildfires as well as constantly monitor forest conditions for fire hazard prediction. Beyond the mesh, EWAN works with an existing network of remote automated weather stations (RAWSs) that transmit the collected data to Forest Service or Bureau of Land Management computers via satellite.
This is where it gets interesting. Voltree’s Web site is more Twittery than informative. But there is a lot of background (from Voltree CEO Stella Karavas) on the genesis and financing of Voltree in the entry on Wade Roush’s “Xconomy” blog titled, “The Roots of Power: How Voltree is Tapping Tree Energy to Save Forests”.
In addition to the company background, Roush describes experiments with aluminum and copper rods inserted in the tree and nearby earth, but reports that Voltree’s explanation for the voltage difference between them arises essentially from a difference in pH between the tree and the ground. So the tree in this case is not simply providing an ion-exchange mechanism, but is contributing to the reaction, and presumably must be alive to do so.
I’d be happy to field readers’ speculations on the mechanism. E-mail me at [email protected], with “tree” somewhere in the subject header. —Don Tuite