Water droplet impact energy harvesting with lead-free piezoelectric structures

Abstract

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. 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, representing an exciting opportunity for further investigation regarding kinetic to electrical energy harvesting. Piezoelectric materials are known to generate electrical energy through applied mechanical strain, and as such are ideally suited for such an application due to their inherently strong electromechanical coupling effect. The key research aim of this work is to analyse the parameters influencing the energy transfer efficiency of droplet impact harvesting, using this knowledge to produce piezoelectric structures that encourage efficient energy transfer from the mechanical energy of water droplet impact into electrical energy. It was found that careful consideration of both transducer bending stiffness and resonant frequency is required. Fabricated P(VDF-TrFE) on stainless steel foil cantilever beams produced a peak energy output of 28 nJ, from the impact of a 5.5 mm diameter droplet at the beam end, when the transducer bending stiffness was within the range of 0.067 to 0.134 N/m. Energy output was further increased when the beam resonant frequency was close to/at the droplet impact frequency. Whilst this result is intuitive, it has been reliably demonstrated that droplet-surface interactions are not trivial, being proposed elsewhere that energy transfer efficiency is more dependent on the relation between the beam resonant frequency and the natural vibration frequency of the impacting droplet. Furthermore, an expansive finite element analysis of ideal geometries for droplet impact energy harvesting highlighted the desirable mechanical characteristics of spiral shapes, with double-armed Archimedean spiral transducers composed of P(VDF-TrFE) deposited onto copper foil investigated. In order to drive the spiral transducers efficiently, a tiered tank system is presented which passively controls the diameter and impact frequency of dispensed droplets from a stored water volume. A total peak output power of 58.9 µW is achieved by a single spiral transducer arm driven by 1 litre of water dispensed as droplets, relating to a power density of 16 mW/cm3. This power output demonstrates how an array of stacked harvesters could produce a theoretical output power of 0.33 mW for every litre of water which descends through the guttering of a two storey building (estimated 5.7 m vertical height). With a suitable energy accumulation and management system, it is feasible to use this for powering applications such as low-power sensor systems.

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ORCID for Suan-Hui Pu: orcid.org/0000-0002-3335-8880

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