Most of the piezoelectric energy harvesters (PEHs) discussed in Chapter 5 are based on mechanical vibrations. They generate maximum power when the resonant frequencies of the PEHs are matched with the frequency of the available vibrational sources. A slight deviation results in a significant decrease in power-generation efficiency. The resonant frequency of a PEH is fixed after its fabrication, whereas the energy sources could differ, depending on the availability. A mismatch between the resonant frequency of a harvester and the vibrational frequency of an energy source is generally encountered. Various strategies and techniques have been developed to address this problem, among which two have been widely adopted: frequency tuning and bandwidth widening.
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