Abstract

Energy harvesting is probably one the most sought after solutions that is being given attention to and has become of great importance for last few years. Due to advances in microelectronics and growing demand of autonomous devices, researchers have been working on harvesting energy from ambient sources such as solar, thermal, wind and kinetic energy. Also, growth of rechargeable battery technology has resulted in research in ambient energy harvesting for charging purposes. In this field, piezoelectric effect has been identified as a viable solution to address both low power applications and battery charging applications. Piezoelectric effect is described as the phenomenon of generating a voltage from a mechanical stress and vice-versa. Piezoelectric elements have been seen to offer outstanding performance in scavenging energy because of their high power density, which make them suitable for integrated micro-generators. Many vibration-based harvesting technology use piezoelectric transducers as AC power source. This work emphasizes on vibration based piezoelectric energy harvesting from a very low input voltage source. The main objective of the thesis is to design a power converter that can successfully rectify and boost piezoelectric AC voltages from a few hundred millivolts to a stable usable DC voltage without the use of a bridge diode rectifier circuit.
The thesis begins with the introduction to the concept of energy harvesting and piezoelectricity, followed by investigation of a 13x25 mm, 28µm thick, laminated piezoelectric thin film made of Polyvinylidene Fluoride (PVDF) acting as the transducer. The transducer was subjected to a repeated vibration impulse and its resultant voltage response was determined. The thesis then moves towards presenting an integrated AC-DC rectifier converter which eliminates the use of full bridge diode rectifiers that have been known for being inefficient for low power energy harvesting. The stages of operation of the power converter is presented along with the simulation results. The work has also been extended to show the charging application of a Lithium-Ion thin film cell under constant voltage charging scheme using a MATLAB/SIMULINK battery model. A prototype of the converter was also built in the laboratory and presented to show the performance of the integrated AC-DC rectifier converter. A dSPACE controller board was employed to implement the open loop control and the converter switching scheme. Experimental results were presented and assessed before finally moving onto the conclusion and suggested future works.