Water droplet impact energy harvesting with P(VDF-TrFE) piezoelectric cantilevers on stainless steel substrates

Harvesting energy from ambient environmental sources using piezoelectric transducers has seen a tremendous amount of interest from the scientific community in recent times. The practicality of energy scavenging technology looks set to see continued relevance, with decreasing power demands of electrical systems, such as Wireless Sensor Networks (WSN), allowing such technology to progressively act as an energy source to drive and sustain them independently. In light of this, there is a growing opportunity for piezoelectric materials to prolong, or even replace, battery powered sensor systems positioned in remote or difficult to reach areas. It has been demonstrated that falling water droplets of millimetric-scale diameter can impart forces of over a thousand times their resting weight upon surface impact. As such, the potential for utilising piezoelectric transducers to drive sensor systems, by converting the kinetic impact energy of falling water droplets into useful electrical energy, is investigated. The key parameters that affect the efficiency of energy transfer between incident water droplets and piezoelectric cantilever structures made of stainless steel foil coated with the lead-free piezoelectric material P(VDF-TrFE) are investigated. A peak power output of 28 nJ achieved from the impact of a 5.5 mm diameter droplet upon a single energy harvesting transducer illustrated that, for droplets of diameter 3.1 mm to 5.5 mm impacting from heights between 0.5 to 2.0 m, it is desirable to utilise piezoelectric transducer beams of bending stiffness in the range of 0.067 to 0.134 N/m in order to achieve good energy transfer efficiency. Although the active electrode area was constrained in order to maintain consistency between samples, reducing the peak energy output, the achieved results correspond to a 15.9 J/m^3 energy density, representing the significant energy transfer efficiency achievable through appropriate transducer mechanical tailoring to the excitation source.