Relationship between current and impedance in piezoelectric energy harvesting system for water waves

Piezoelectric energy harvesting system can convert mechanical energy to electrical energy across a wide range of areas by using the water waves. However, piezoelectric energy harvesting has high impedance and low current relative to its high voltage, which limits the scope of its application. In this study, a piezoelectric energy harvesting system designed for use with water waves was replicated to investigate whether the low current could be improved and the high impedance reduced. The free end of the cantilever structure of a piezoelectric device was connected to a vibration exciter; thus, the frequency and strain could be treated as independent variables. Although the strain of the piezoelectric ceramic changed, the internal impedance was nearly constant. The output current increased in proportion to the strain, whereas the output power increased by a factor of the squared current. When multiple piezoelectric devices were connected in parallel, the internal impedance decreased and the output current increased. The absolute change in impedance with frequency decreased by 82.2 % and the output power increased to 18.76 mW. Therefore, the impedance is readily matched without any additional impedance-converting circuit owing to the low and constant internal impedance regardless of the frequency and strain of the piezoelectric ceramic.