Vibration Energy Harvesting from Raindrops Impacts: Experimental Tests and Interpretative Models

The kinetic energy of raindrops is a large and renewable source of energy that nowadays can be exploited by means of piezoelectric harvesters. This study focuses on a new cantilever harvester that uses the impact of a drop on a liquid surface created on the harvester in order to improve the conversion from kinetic energy to electric energy. Experimental tests, carried out both outdoors and indoors, were performed to assess the validity of the proposed design. The voltage obtained with the impact on the liquid surface was about four times larger than the one obtained with the impact on a dry surface. The phenomena that lead to the increased performance of the harvester were analyzed both experimentally, by means of a high-speed camera, and analytically, by means of a mathematical model. The camera footage showed a clear relationship between the waveform of the generated voltage and the various phases of the impact (crown formation, crown collapse, and sloshing). The mathematical model developed herein, which was based on the oscillation of the liquid mass caused by the impact and on the linear momentum equation, is simple and can be used to estimate the measured voltage within a good approximation.