# Vibration Energy Harvesting from Raindrops Impacts: Experimental Tests and Interpretative Models
## 1 Introduction  
The possibility of exploiting the kinetic energy of raindrops by means of piezoelectric vibration energy harvesters has been analyzed since 2008 [1, 2, 3]. Some studies have focused on the possibility of feeding sensor nodes and small electronic equipment using the mild rainfall that occurs in countries with temperate climates [4, 5]. Other studies have focused on the possibility of harvesting large amounts of energy from tropical raindrops [6, 7]. Most of the proposed raindrop harvesters directly exploit the bending strain of beams or membranes hit by the raindrops in order to generate electric energy via the piezoelectric effect [5, 7]. However, there are some interesting examples of rotational piezoelectric harvesters [8, 9]. In this case, the raindrops cause the rotation of a small rain-mill that in turn bends a series of piezoelectric cantilevers that generate electrical energy. Regardless of the typology, these rain harvesters generate an AC power that has to be converted into DC power to be stored and exploited by electronic equipment. For this reason several researchers have focused on the development of conditioning circuits [10, 11, 12]. Moreover, rain generates a series of random impacts, causing transient peaks in AC power. To cope with this effect, specific electronic [5] and mechanical [13] solutions have been developed. The analysis of a raindrop harvester is a fully multi-physical problem. As in other piezoelectric harvesters, there is a coupling between the mechanical domain and the electrical domain, through the direct and inverse piezoelectric effects [14, 15], but the impact of the raindrop on the wet surface of the harvester also generates a coupling between the fluid-dynamic domain and the mechanical domain (the vibrating harvester). Some researchers have highlighted the effects of the accumulation of water on the harvester surface, with the formation of a water layer [16, 17, 18]. The presence of the water layer not onlymodifies the natural frequency and the damping ratio of the harvester [18] but also modifies impact dynamics. The impact of a raindrop on a dry solid surface generates spreading and splashing phenomena, which dissipate a significant part of the raindrops' kinetic energy. Conversely, when a raindrop impacts a water layer of sufficient thickness, different phenomena take place [19]: the formation of a crater, the rising of water surrounding the crater with the formation of a crown, the collapse of the crown, and the formation of water ripples. As a result of the above-mentioned phenomena, the presence of the water layer increases the amount of energy transferred to the harvester. This positive effect ends when the water layer reaches the borders of the harvester and a significant spill phenomenon takes place. A novel harvester design was proposed by a research group from Padova University in [20] in order to exploit these phenomena. In the new device, a square-based spoon was fixed to the free end of a cantilever harvester and exposed to rain. A small pool of water was generated in the spoon; hence, after the initial transient stage, the raindrops impacted on a liquid surface. A series of experimental tests carried out in the laboratory using simulated rain showed the following differences when the spoon was empty and when the spoon was filled with water:  
* The resonance frequency of the harvester strongly decreased due to the presence of the added mass of water in the spoon; this effect is rather intuitive and predictable. * The damping ratio increased due to the presence of water; this effect is predictable as well. * When the impact takes place on the liquid surface, the maximum voltage amplitude is not reached in the first period, but it is reached later. * The impact on the liquid surface leads to a large increase in the generated voltage, with a significant improvement in the performance of the harvester.