Piezoelectric is a versatile material and it is used in many different applications like.
The piezoelectric materials are practically used since the start of 1900, and their phenomena, working, resonance behaviour, equivalent circuit and circuit response was well understood at that time.
The "Piezoelectric Energy Harvesting" technology originated from the "Mechanical Structure Damping" work.
The mechanical structure damping techniques are used to suppress mechanical vibration and acoustic noises in the system. Hagood [Paper-1, 1991] [paper-2, 1989/1991] first time used the piezoelectric material for mechanical structural damping. When a piezoelectric material is bonded to the mechanical structure, the vibrations in the mechanical structure will generate energy, which will be converted into electrical energy by the piezoelectric material. By dissipating this energy in the external passive load resistor (joule heating), will result in suppression of mechanical structure vibrations. Hagood et. al papers discuss resistive (R) and series resistive+inductive (series R+L) shunting. The L was made using a toroidal inductor. [ Hamid PhD Thesis, Page, 15-16]
The series R+L technique presented by Hagood can work well for small values of R, but will require iterative adjustment for large R values. In series R+L circuit, R has to be chosen first, then L is designed. For any new value of R, the new value of L is required. To solve this problem, the parallel R+L circuit for passive structural damping was introduced by Shu-yau Wu [paper, 1996]. The L in the circuit was implemented using "Riordan gyrator" circuit also known as (synthetic inductor) [paper, 1967] also used in by Edberg [paper, 1992].
The transition of the piezoelectric and then use of piezoelectric materials for structural damping to piezoelectric energy harvesting started with a famous conference paper from the MIT [paper, 1998]. Later a piezoelectric micro-generator was presented by authors from the university of Southampton, UK [paper, 2001]