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Phononic crystals (PnCs) have been utilized to amplify the amount of input energy transferred to a piezoelectric energy harvesting (PEH) device by manipulating elastic wave propagation. When introducing a defect that has material properties and/or geometry different from a unit cell, mechanical resonance of the defect leads to localizing elastic waves inside the defect. This is called a defect mode. Several prior studies have explored a defect mode for PEH purpose; however, they have focused only on a single defect. When introducing an additional defect into a PnC, the coupling between two defects leads to splitting the defect band. Incorporating such split defect band phenomena into PEH can potentially widen frequency bandwidth and realize broadband energy harvesting. Thus, this study newly proposes a PnC-based PEH system that uses double defect modes under elastic waves. In particular, this study examines how an electrical circuit connection (i.e., Independent, Series, or Parallel) between two PEH devices attached on each defect affects PEH performances. Key findings from this study include (1) the shift in split defect bands as well as harvesting performance varies with the type of electrical circuit connection; (2) split defect band phenomena amplify the output electric power at double resonance peaks in an Independent connection; and (3) despite displacement amplification at certain resonance frequencies, no peak output voltage occurs due to the current source direction (for Series connections) and voltage cancellation (for Parallel connections). These key findings can provide design and selection guidelines for electrical circuit configurations between double defects for enhanced PEH.

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