Piezoelectric energy harvesting utilizing human locomotion.

Abstract

Previous studies have shown that not only are piezoelectric materials feasible for energy harvesting, they are feasible as an energy harnessing medium in shoes during walking. Continuing in that vein, this thesis provides new designs to better apply mechanical stress and achieve higher power output. Two points of stress during walking were used for energy harvesting. 1.) The heel of the shoe, for when a person’s foot first lands on the ground during the initial stage of the step. 2.) The ball of the shoe, for the curling motioning of the foot as the person propels forward finishing a step. A flexible, multilayered insole was developed for the ball of the shoe operation and integration into the sole of a specially selected “street shoes”. The insole consists of six layers of PVDF sheets, three sheets per side, adhered to a thick but flexible Nylon core. The PVDF absorbs the mechanical compression or tension stress, depending on the side they are on, thereby creating a charge differential across the surface of each sheet. A rigid, reversed clamshell piezoceramic transducer was developed and integrated into the heel of the same shoe. The insert consists of two Thunder PZT unimorph connected in parallel and mounted inside a steel housing to facilitate optimal force transference. The inherent capacitive property of the piezoelectric materials and its very low frequency of operation (~ 1Hz or 1 step per second), allows for very little current to be extracted through conventional full-wave rectifier harvesting circuit. Due to previous research success with resonating an inductor in series with the piezoelectric source, an energy harvesting circuit coined “Synchronized Switch Harvesting on Inductor” SSHI was utilized to increase power output. However, due to the inability to correctly synchronize the switching circuit and lack of proper piezoelectric source modeling, SSHI circuit only provided marginal improvement in power output ~10-20% as oppose to previous study demonstrating 250%+ output. Nevertheless, by using only full-wave rectifier harvesting circuits, the new PVDF insole and PZT insert designs have propelled harvestable energy to 11-13mW from one shoe, with a combined generation of 22-26mW for both shoes.