Design of Piezoelectric Energy Harvesters with Different Electromechanical Couplings

The goal of the present thesis is to design the piezoelectric energy harvesters with different magnitudes of electromechanical coupling. In addition, it also proposes several methods for measuring the effective system parameters of an energy harvester. The methodology is based on the energy approach where the shape function is chosen either by the standard modal analysis or by the method of uniform load. Besides, the effective system parameters can also be determined based on the equivalent RLC circuit model. The proposed criterion for measuring the strength of electromechanical coupling of an energy harvester is defined by the ratio of electromechanical coupling to the mechanical damping ratio. The damping coefficient is assumed to the type of Rayleigh damping here. The first part of the present thesis is to make comparisons among the different proposed estimates of effective parameters. It is found the estimation of harvested power frequency response measured at the optimal load agrees quite well with experimental observations. However, the disagreement increases in the case of attaching larger electric loads. It is also found the estimation based on the equivalent circuit model shows the least error compared to other methods. The second part is to study the effect of different lengths of piezoelectric layers on the magnitudes of electromechanical coupling. It is found both analytic estimates and experimental observations exhibit the similar trends. Hence, our proposed estimates are capable of performance evaluations. The first observation from our analysis is that the behavior of the case of strong electromechanical coupling is similar to that of the weak electromechanical coupling. The second result is the strongest coupling is achieved when the ratio of the length of the piezoelectric layer to that of the substrate is about 0.5.The third observation is the harvested power per unit mass is not monotone increasing as the increase of piezoelectric layer. Instead, it is minimized when the ratio of the piezoelectric layer to the substrate is around 0.3. Interestingly, the harvested power per unit price is also minimized at around the similar range. Finally, the experimental observations confirm that the coefficient of Rayleigh damping is approximate to be a constant in spite of different lengths of piezoelectric layers. In addition, it is observed the coefficient of Rayleigh damping in the case of weak coupling is larger than that in the case of strong coupling.