Archimedes' water fountain; a water droplet energy harvesting system utilising piezoelectric spiral transducers

The arithmetic, or Archimedean spiral, was originally described by the Greek mathematician, Archimedes. In this research, the design, characterisation and fabrication of piezoelectric transducers inspired by double-armed Archimedean spirals for harvesting water droplet impact energy is presented. Such designs present a highly tuneable, self-supporting structure with multiple degrees of freedom, encouraging high sensitivity to droplet impact. It was found that spiral designs with droplet impact area bending stiffness within the range 7.9-9.9 N m(-1) produced the highest peak energy outputs. However, samples with lower bending stiffness produced a greater average energy output, highlighting the significant influence of geometry design on energy transfer efficiency. A tiered tank system is presented which collects an input volume of water, before dispensing this water as a series of frequency and diameter regulated droplets onto the impact energy harvesting spirals. Controlling such aspects significantly increases system efficiency-a total energy output of 58.9 mu J was generated by a single spiral transducer arm driven by 1 L of water dispensed as droplets of 6 mm diameter, 1 m release height and impact frequency 0-40 Hz, representing an energy density of 16 mJ cm(-2).