Study on optimization of rectifying, smoothing and charging circuits for multi-array piezoelectric energy harvesting system

Abstract

Piezoelectric energy harvesting (PEH) methods are promising technology owing to their typically high mechanical-to-electrical energy conversion efficiencies. This paper suggests the design in array configuration and circuit part from impact-based PEH system to investigate the charging amount. The piezoelectric modules were struck by sticks attached to a motor shaft. In structural design, the array was designed into two conditions: impacted simultaneously and impacted sequentially with a phase difference (30°, 60°, 90°, 120°, 150° and 180°). We show that the impacted sequentially with a phase difference allows faster charging rates of energy harvesting. Additionally, distribution of the torque required to transform the piezoelectric modules was found to be another advantage from this condition. In design of the rectifier, it is suggested that rectifying the signal from each piezoelectric module separately generates more electrical energy and charges a capacitor faster than using a single rectifier for all modules connected in series. With individual rectifiers for each piezoelectric module, the rms voltage of the capacitor after 400 s was 7.82 V and 8.40 V with no phase difference and 90° phase difference respectively. On the other hand, with just single rectifier, 4.79 V and 5.44 V were measured with no phase difference and 90° phase difference respectively. With no phase difference, ripple factors of 3.58%, 2.37%, and 2.18% were measured at input filter capacitances of 500, 1000, and 2000 ㎌, respectively, while adding phase reduced the corresponding ripple factors to less than half (1.58%, 1.07%, and 0.88%). This not only reduced ripple factor but also increased charging efficiency—which improved by a factor of 1.62 at an impact frequency of 4 Hz—although the total charge was significantly reduced. The charging efficiency increases gradually with impact phase difference, and in all cases where phase difference is present it is higher than 95%. For example, the charging efficiency increases from 59.68% (no phase difference) to 96.58% (with phase difference). Through experimental analysis, the optimal phase difference was found to occur at a phase of π/4. At this point, the amount of generated power is higher than at the π/2 efficiency maximum, while at the same time both the total charge and the charging efficiency at π/4 are higher than in the no-phase case. These results point to the benefits of using impact phase differences as well optimizing the phase angle.