Piezoelectric energy harvesting from water droplet impact

Abstract

In recent years there has been an increased interest in scavenging energy from ambient sources such as precipitation which is a freely available source occurring mainly in the form of raindrops. This work investigates the design analysis of a piezoelectric system, capable of converting the mechanical energy generated by the impact of water droplets into electrical energy. The system is not designed for any specific rain type and responds to the whole range of raindrops to provide energy to low-powered devices or applications. An off-the-shelf piezoelectric transducer with Polyvinylidene Difluoride (PVDF) polymer film was used as a water droplet impact harvester for this research. Detailed analysis of water droplet impact harvester shows two distinct stages in the voltage output, first a logarithmic growth, then an exponential decay during a single droplet impact event. The energy of the growth stage was found to be much higher than that of the decay stage. Thus, growth stage of the impact process has a significant contribution to the overall output of the harvester. The experimental results show a power output of 2.5 µW with a 4 mm diameter droplet at a maximum height attained of 47 cm, which resulted in a low energy conversion efficiency of 0.12 %. A Harvester Array Model (HAM) then developed to characterise the output power of a single unit harvester, which is then applied to an array of rain impact harvesters. An energy harvesting module is then developed consisting of multiple units where only one unit was excited. The module was tested during a single droplet impact event which generated a power of 3.6 µW and giving the energy conversion efficiency of 0.67 %. A technique was also presented which was able to identify the efficiency of the impact conversion mechanism as the droplet interacts with the device and the efficiency of the mechano-electric conversion mechanism due to internal losses. Furthermore, a prototype is developed with multiple units and multiple impact events which resulted in an average power of 0.05 µW, over a light rain shower of 2.5 seconds with an array area of 0.003 m2 . A mathematical model of the prototype was presented to predict design parameters for different rainstorms. Harvested power in this research was found to be in the µW region. Nevertheless, with suitable energy technology it can provide power to specialist application for low-powered devices.