Energy harvesting with single-layer and stacked piezoelectret films

Vibration-based energy harvesting with single- or multiple-layer (stacked) piezoelectret films is discussed. First, a mechanical and an electrical model for this kind of energy harvesting is established and equations for the generated charge, current, voltage, and power are derived. In particular, the case where the electrical corner frequency equals the mechanical resonance frequency of the film stack combined with the seismic mass is of importance since harvesting at this frequency results in maximal generated power. Then, experimental results for harvesters with single layers and with stacks of piezoelectret films are presented. The data confirm the predicted dependence of the generated power on terminating resistance, frequency, seismic mass, acceleration, and number of film layers in the stack. If the stress on the piezoelectret layers, generated by the accelerated seismic mass, is larger than 3 kPa, a nonlinear increase of the generated power with stress is observed. The maximum power harvested with a single-layer piezoelectret is 18 μW for an acceleration of 9.81 m/s2 and for a seismic mass of 40 g. A three-layer harvester yields an increase of the maximum power close to a factor of √3.