Design, modelling and testing of a novel energy harvesting device

Abstract

This work is a feasibility study to develop a novel energy harvesting device. Energy harvesting devices capture energy in various forms from the surrounding and transform it into usable electrical energy. These devices do not require any refuelling or recharging and are virtually a never ending source of energy. The energy harvesting devices rely on di erent mechanisms of energy conversion, depending on the energy source. This work focuses on conversion of mechanical energy from vibrations into electric energy using piezoelectric materials. Most of the existing devices are shaped like a cantilever beam, thus limiting the tunability to a single resonance frequency. It is believed that by modifying the geometry of the energy harvesting device and applying a pre-load to the active material (piezoelectric), a variable tunability can be achieved. Also, the application of an axial compressive pre-load helps to further increase the power output of the device. Therefore, in this present work, the performance of a simply supported beam shaped energy harvesting device is investigated both numerically and experimentally. For the numerical analyses nite element simulations are carried out using ANSYS. An electro-mechanical model of the simply supported beam has been developed through a series of approaching models with increasing complexity, starting from an analytical solution. The nal three-dimensional model was used as a base to create a model of the beam that has been used during the experimental tests. Shape optimization studies were carried out on this nite element model to analyse the power output of the device. It has been observed, through pre-stressed modal analyses, that the axial pre-load decreases the resonance frequency of the beam, thereby giving the beam the ability to be tuned. Also,it has been observed that an optimisation of the beam footprint shape can increase the power output by almost 40%.The experimental work focussed on the investigation of the harmonic behaviour of the simply supported beam under di erent pre-load conditions. It was observed that the experimental results were in disagreement with the nite element simulations and also with the reference literature. The disagreement was identi ed to be due to the hinge design that does not ensure the alignment of the two tips of the beam and therefore the application of a perfectly axial pre-load. From the work presented here it emerges that the possibility to develop a simply supported beam shaped energy harvesting device that rely on the application of an axial pre-load to obtain tunability and an higher power output is promising. The nite element simulations gave good results on the beam behaviour and on the possibility to further increase its output by optimising the shape of its footprint. The experimental work allowed to identify the hinge design as a problem area to design a pro table device.