MIT researchers are developing a novel power scavenging systes for small wireless sensors that monitor for forest fires. The sensors are powered by the trees themselves. Each sensor's battery is trickle charged with the electricity generated by the imbalance in pH between the tree and the soil. From the MIT News Office:
A single tree doesn't generate a lot of power, but over time the "trickle charge" adds up, "just like a dripping faucet can fill a bucket over time," said Shuguang Zhang, one of the researchers on the project and the associate director of MIT's Center for Biomedical Engineering (CBE).
The system produces enough electricity to allow the temperature and humidity sensors to wirelessly transmit signals four times a day, or immediately if there's a fire. Each signal hops from one sensor to another, until it reaches an existing weather station that beams the data by satellite to a forestry command center in Boise, Idaho.