Harvesting electrical energy from water drops falling on a vibrating cantilever

In this work a novel thin-film device combining piezoelectric and contact electrification energy harvesting is created with the aim of investigating how it responds to water droplet impact during vibrations. The two energy harvesting principles utilize the same ground electrode, but the electrical signal outputs are independent and show entirely different electrical signal characteristics in presence of external forcing. While piezoelectricity gives rise to a nearly quadratic increase in harvested energy as a function of vibration velocity, the energy due to contact electrification reaches saturation for larger water drop velocities. On the other hand, when the water stream transitions from discrete droplets to a continuous stream the energy gathered from the piezoelectric mechanism exhibits saturation, whereas the energy due to contact electrification decreases. The proposed device may have applications as a self-powered environmental sensor that allow one to distinguish between forced oscillations and water droplet impacts.