Scientists from CEA/Leti-Minatec in France have created a way to harness the vibrations caused by raindrops. Raindrops landing on a flexible surface set of the vibrations for producing power using the piezoelectric principle, and the system works with raindrops ranging in diameter from 1 to 5 mm. It is possible to recover up to 12 milliwatts from larger drops but the energy produced depends on the size of the raindrop, their frequency and the size of the piezoelectric membrane on which they land.
To capture energy from the raindrops the scientists used a 25 micrometer thick PVDF (polyvinylidene fluoride) polymer, a piezoelectric material that converts mechanical energy into electrical energy. When a raindrop hits this surface it vibrates and the electrical charges created are recovered by embedded electrodes. The scientists used a testing apparatus which controlled the drop size, frequency and height from which they fell.
The Testing Apparatus
Source: Romain Guigon, et al
Romain Guigon, who conducted the research with fellow engineers Jean-Jacques Chaillout, Thomas Jager, and Ghislain Despesse, said "Our calculations show that even in the most unfavourable conditions, the mechanical energy of the raindrops...is high enough to power low-consumption devices."
Although harvesting power from the sun will produce more energy, the energy produced by raindrops could be used in combination with solar energy for example in wireless sensors used to monitor environmental conditions.
Top image source: Soil Erosion.net
For more attend: Energy Harvesting & Storage USA 2009