Power Enhancement of Piezoelectric Energy Harvester by Interface Circuit and Structural Design

Abstract

Energy harvesting technologies have received considerable attention during the last decade and is continuing to grow at a rapid pace. Most of the external energy sources such as solar, tidal, thermal, and vibration can be transformed into electrical energy by using various energy conversion technologies. Among these technologies, piezoelectric materials which harvests mechanical energy have been widely used because piezoelectric materials have relatively high power density and large feasibility compared to other energy harvesting materials. Also the external mechanical energy is more transferable than other external energies from energy harvesting. There are a wide range of applications for piezoelectric materials in the field of energy harvesting, including low-powered wireless sensor nodes for structural and medical field, alternatives of batteries in systems, and power electronic devices. Although the use of piezoelectric materials has been increased in industrial areas, it still has disadvantages. Piezoelectric materials are composed by ceramic structures which is prone to cracking under overstress. Also they have a tendency to possess high internal impedance, hence they require to be connected to an auxiliary circuit as well as an AC-DC conversion circuit. Recently, multidisciplinary approach comprised of mechanical, electrical and material part was studied to improve efficiency on electrical energy harvesting and diverse adaptation. In this dissertation, first of all, the circuit interfaces for piezoelectric energy harvesting were compared to the output power. Most of previous interfaces use standard interface which bases on AC-DC rectifier circuit and external capacitor Crect. However, the standard interface is inefficient for energy harvesting because piezoelectric materials have high internal capacitance Cp. A synchronized switch harvesting on inductor(SSHI) interface can be more effective since it uses switching and inductor. Therefore, a self-powered SSHI circuit was designed as well as the output power difference between standard and SSHI interfaces was investigated in this work. Secondly, the strain-stress effects on piezoelectric materials and the crack formation were investigated. The electrical output energy is proportional to the applied strain. However, electrical output energy reduces abruptly beyond a certain point since cracks develop on the surface of piezoelectric materials. In this work, the substrate layer was exposed to impacts at different distances from the free end of the cantilever beam to exert various mechanical strains until crack formation appeared, and then silver electrode paste was coated on top of the cracks to connect damaged electrodes. The results showed that damaged piezoelectric material could be successfully restored by connecting the damaged electrodes. Thirdly, the reinforcement method of piezoelectric materials was also investigated. In order to prevent the cracks and to reinforce the piezoelectric materials, ultraviolet(UV) curable resin was laminated on the piezoelectric ceramic. The results showed that UV coating method can extend the yield point in the strain-stress curve and thus this allowed longer life time cycle of piezoelectric materials. Fourthly, the characteristic of stiffness of piezoelectric energy harvesting system was investigated in impulsive conditions. In a system having a single degree of freedom (SDOF), piezoelectric beam which comprised substrate beam and piezoelectric material was simplified with the equivalent mechanical system diagram and investigated. Based on this model, additional springs were attached to the free end of the piezoelectric cantilever beam to change the total stiffness. The results showed that a higher total stiffness caused a higher natural frequency of the cantilever beam. Thus, the external matching impedance was decreased while the output power was increased. Finally, application of piezoelectric energy harvesting on the infrastructure such as trains and under pavements was investigated. The amount of power generated is completely dependent on the nature of the ambient mechanical energy. Thus the mechanical energy from vibration was analyzed first then subsequently a suitable design of an energy harvester was made and applied for each case. Experimental results showed that the piezoelectric energy harvesting could be used for harvesting micro scale to macro scale electrical energy from ambient source. All approaches focus on the optimizing methods for energy harvesting for piezoelectric materials. In order to improve the output power and reliability, the electrical and mechanical properties were designed and investigated. The simulation and experiments result are presented to verify the proposed methods.