Piezoelectric Vibration Energy Harvesters for Low Frequency Applications

UoM administered thesis: Master of Philosophy

  • Authors:
  • Jie Wang

Abstract

Piezoelectric energy harvesters are a promising power generation solution for widespread adoption of wireless sensors in remote locations. A well-recognized class of these devices employs highly flexible PVDF (Poly-vinylidene fluoride) polymers known for low resonant frequencies and are thus adequate for harvesting mechanical energy within low frequency applications. This thesis contributes to the state-of-art in the design of PVDF-based vibration energy harvesters through the provision of three novel studies into the effects of tip mass, planform geometry, and prolonged operation on the dynamics and power generation performance for this class of harvesters. The first study proposes the use of solar panels as active tip masses to understand the effect of their inclusion on the dynamics and power generation performance of cantilevered PVDF-based energy harvesters. Four different harvester planforms with and without solar panels are tested using off-the-shelf components. The experimental results show that the flexible solar panels considered are capable of reducing resonance frequency by up to 14% and increasing the PVDF power generated by up to 54%. Two analytical models are developed to further investigate this concept; employing both an equivalent concentrated tip mass model to represent the case of flexible solar panels and a distributed tip mass model to represent rigid panels. For the flexible solar panel model, it is found that the highest PVDF power output is produced when the length of solar panels is two thirds of the total length. On the other hand, results from the rigid solar panel model show that the optimum length of solar panels increases with the relative tip mass ratio, approaching an asymptotic value of half of the total length as the relative tip mass ratio increases significantly. Meanwhile, there is currently very limited understanding on how to size this class of harvesting devices to operate efficiently in response to a given excitation level. As such, the second study provides a thorough experimental investigation into the effect of planform geometries and excitation levels on the dynamics and power generation performance of PVDF-based cantilevered vibration energy harvesters. Three main findings are obtained: First, there is an optimal width for the harvester where the output power is maximized. This optimal width value depends on the excitation amplitude in such a way that narrower harvesters are more suited for small excitations whereas wider harvesters perform better upon experiencing large excitations. Second, it is shown that the selection of length is critical in that it should be decided to ensure a linear device response to the operation excitation as if non-linear effects are triggered, they will drastically deteriorate the power density performance. Finally, the second moment of area for the harvester cantilever is particularly effective at capturing the geometric effect on the power density. Finally, the third study in this thesis considers investigating how piezoelectric vibration energy harvesters typically employed within low-frequency applications degrade during long-term operation in realistic harvesting conditions. Here, not only PVDF-based harvesters were investigated but other piezoelectric material options were considered for cross-comparison. The harvesters tested are unimorph cantilevers based on three of the most commonly used piezoelectric options: polyvinylidene fluoride (PVDF), macro fiber composite (MFC), and Quick Pack (QP). Testing was carried out under single-frequency excitation (1040 Hz) of 1g amplitude for three million vibration cycles. The results show that larger cumulative variation in natural frequency and optimum load resistance yields a larger variation in power output, thereby linking the power performance degradation to the degradation of the mechanical and/or electrical properties of the harvesters. The study also indicates that increasing the tip mass does not necessarily improve the average power output, suggesting that a larger tip mass may exacerbate the degradation of the mechanical and/or electrical properties of the harvester. It was also shown that stiffer harvesters show the highest signs of power degradation. Nevertheless, QP harvesters managed to demonstrate the highest power density values. When cost is considered within the assessment, PVDF harvesters managed to demonstrate the highest power density to cost ratio.

Details

Original languageEnglish
Awarding Institution
Supervisors/Advisors
Award date1 Aug 2021