Applicability of synchronized charge extraction technique for piezoelectric energy harvesting

In the past few years, various circuit techniques have been proposed to improve the efficiency of piezoelectric energy harvesting, among which the synchronized charge extraction (SCE) circuit has been enthusiastically pursued. In the literature, the SCE technique is claimed to increase the power output of a piezoelectric energy harvester (PEH) by four times based on the assumption that the vibration of the harvester is not affected by the energy harvesting process. Under such assumption, the circuit model of a PEH is usually over-simplified as an ideal current or voltage source with the piezoelectric internal capacitance placed in parallel or in series. In this paper, the applicability of the SCE technique is investigated by electrical simulation. First, a more accurate circuit model of a cantilevered PEH is derived, taking into account the backward electromechanical coupling effect on vibration. Subsequently, the designed SCE circuit is connected with the simplified and the accurate circuit models of the PEH for simulation. The applicability of the SCE circuit for different cases are investigated, including the PEH excited at resonance and off-resonance frequencies as well as the PEH with various degree of electromechanical coupling. The results show that when the coupling of PEH is not negligible, the SCE technique cannot improve or even reduces the efficiency of energy harvesting for the PEH vibrating at resonance. The SCE technique is found applicable for efficiency improvement only for the PEH vibrating at off-resonance or with a weak coupling coefficient because of the very minimum of electrical damping effect from the energy harvesting process, in which cases, the simplified and accurate circuit models are approximately equivalent.