Design analysis and fabrication of a mobile energy harvesting device to scavenge bio-kinetic energy.

The increasing prevalence of low power consumption electronics brings greater potential to mobile energy harvesting devices as a possible power source. The main contribution of this thesis is the study of a new piezoelectric energy harvesting device, called the piezoelectric flex transducer (PFT), which is capable of working at non- resonant and low frequencies to harvest bio-kinetic energy of a human walking. The PFT consists of a piezoelectric element sandwiched between substrate layers and metal endcaps, the endcaps are specifically designed to amplify the axial force load on the piezoelectric element, instead of conventional designs of piezoelectric energy harvesters that focus on utilising resonant frequency in order to increase power harvested. This thesis presents the analyses, design, prototyping and characterisation of the PFT using a coupled piezoelectric-circuit finite element model (CPC-FEM) to show the energy harvesting capability of the proposed and developed novel device to harvest bio-kinetic energy. Prior to the study of the new PFT, an initial focus was given to a traditional Cymbal device to investigate its potential as a bio-kinetic energy harvesting device. To gain an understanding, effects of geometrical parameters and material properties of the device on its energy harvesting capability were studied and in doing so issues and problems were identified with the traditional Cymbal device for use as a bio-kinetic energy harvesting device. Its structural materials were not able to withstand higher than a 50N applied load and it was proposed that a small adhesion area connection in a fundamental part of the structure may have been at high risk of delamination. In order to study these, the CPC-FEM model was developed using the commercial software of ANSYS and validated by experimental methods. Later, based on a modelling and experimental study, a novel PFT was proposed and implemented to overcome the issues and problems of the traditional Cymbal device. For this initial study, the Cymbal was analysed by studying how key dimensional parameters affect the energy harvesting performance of the Cymbal. In addition to this, how piezoelectric material properties affect the energy harvesting performance were studied using the developed CPC-FEM model through comparisons of different piezoelectric materials and their electrical performances to aid with selecting high power producing materials for the final PFT design. It was found that (1) d₃₁ is a more dominant material property over other material properties for higher power output, (2) Figure of Merit (FOM) was more linear related to the power output than either the k₃₁ or the d₃₁, and (3) εᵀ r₃₃ had some role when the materials have an identical d₃₁; a lower ε ᵀ₃₃ was preferred. A combined FOM with d₃₁ parameters is recommended for selection of piezoelectric material for a higher power outputs. The design of the new PFT is partly based on the traditional Cymbal however, the new PFT has more potential for withstanding higher forces due to an addition of substrate layers that reduced delamination risks. Using a similar approach to designing the traditional Cymbal, the new PFT was designed and tested with force frequencies of less than 5Hz and forces of up to 1kN. In the design process, the validated CPC-FEM was used 1) to analyse then utilise correlations between geometric parameters and power outputs, and 2) to ensure structural integrity by monitoring mechanical stress in the PFT. The PFT was retrofitted into a shoe and the harvested power was used to power an in-house developed wireless sensor module whilst the subject with a body weight of 760N was wearing the shoe and ran at 3.1mph (equivalent to 1.4Hz on the shoe), the PFT produced an average maximum power of 2.5mW over 2MΩ load and the power produced is able to power the wireless module approximately every 10 seconds.