Scientists at the CEA/Leti-Minatec in Grenoble, France are looking at ways to produce electricity from the vibrations caused by falling raindrops.
It's the latest step toward exploiting piezoelectric principles. In piezoelectrics, bending or otherwise deforming an object can produce power. If you take a tiny wire and bend it, for example, a negative charge gets produced on the stretched side while a positive charge gets created on the compressed side. When the pressure on the wire is relieved, an electrical current can be detected.
Using the CEA's concept, raindrops hitting a flexible surface set off the vibrations for producing power. The original paper is here. Thomas Jager, one of the authors of the paper, told Physorg.com that the system works with raindrops ranging in diameter from 1 to 5 millimeters. Large drops can generate enough vibrations for 12 milliwatts. Building a big system that could capture energy from a lot of raindrops, however, remains to be seen.
A number of other organizations are working on piezoelectric devices as well. Zhong Lin Wang at the Georgia Institute of Technology has devised a sensor that can harvest mechanical energy by bending zinc oxide nanowires and convert it into electricity. He wants to put it in a boot so you can walk and generate power. Other experiments have involved harvesting energy from a flexible floor.
The CEA is also working on devices that can produce energy via the Seebeck effect, or temperature differentials. Last year, it presented a paper on devices that could derive power by being exposed to ambient temperatures. A group at Princeton is conducting similar experiments.