Harvesting energy from the environment to monitor remote sensors enables more ambitious applications, such as monitoring the weather under the canope of trees in the forest, rather than just in clearings where weather stations can communicate with satellites. University of Washington, MIT and Voltree are advancing the state of the art. Look for tree-powered remote sensors to be deployed by the U.S. Forest Service as early as 2010. R.C.J.
Trees can be used to power circuits, but their voltage is too small to charge conventional batteries. University of Washington (Seattle) researchers recently demonstrated a nanoscale "boost converter" that integrates the ultra-low-voltage potentials generated by trees. A tree's output voltage can be as low as 20 millivolts, according to the researchers. They designed a circuit to nevertheless accumulate enough power over time to produce a 1.1-volt output--enough to power wireless sensors. Last year, the theory behind "tree power" was demonstrated by MIT which spun-off the company Voltree Power (Canton, Mass.) which recently won a contract from the U.S. Forest Service to supply a wireless mesh sensor network powered by trees. The Forest Service operates 28,000 U.S. weather stations, each transmiting conditions on the ground to a satellite. Voltree's energy harvester gathers information from the surrounding forest and transmits it wirelessly to existing weather stations. The architecture extends the stations' reach under the forest canopy for the first time. Pilot sites will be tested this fall, with the first weather station upgrades scheduled for the first quarter of 2010.