Modelling analysis and optimisation of cantilever piezoelectric energy harvesters

Patel, Rupesh (2013) Modelling analysis and optimisation of cantilever piezoelectric energy harvesters. PhD thesis, University of Nottingham.

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Over the last decade there has been a growing increase in research in the field of vibrational energy harvesting - devices which convert ambient vibrational energy into electrical energy. The major application area for such devices is as power sources for wireless sensors, thereby replacing currently used batteries which suffer from a finite lifespan and pose environmental issues during disposal. The vast majority of designs are cantilever beams comprising of piezoelectric layers having coverage identical to the substrate layer. It is evident from the literature that rudimentary work has been performed on design optimisation, with reliable and extensive parametric studies on geometry, especially piezoelectric layer coverage, being overlooked. As a result of this, outcomes from previous research are yet to be seen in designs for practical applications.

In this work a versatile linear model is developed which can accurately predict the performance of cantilever piezoelectric energy harvesters. An integral part of the model uses a transfer matrix method to accommodate the difference in structural dynamics of both uniform and non-uniform structures with model validation provided through extensive experimental work. The linear model developed is used to carry out parametric studies on the geometry of three distinct energy harvester cases thereby providing comprehensive knowledge on key variables and geometrical changes which can improve performance. In one of the cases examined, an improvement in performance of over 100% is predicted by solely altering piezoelectric layer coverage. However, the load resistance, i.e. electrical condition, has a significant effect on the trends in generated power which led to work directed toward harvester optimisation in a more realistic electrical scenario.

Investigation on harvester geometry whilst utilising an electrical scenario comprising of an energy storage medium is undertaken in this work. The developed model ensures the effects of electro-mechanical coupling remain and provides a solid basis from which users can readily apply model extensions through inclusion of further electrical components to resemble practical circuitry. Theoretically, for all examined case studies, improvements in performance were realised through alterations to piezoelectric layer dimensions with the most notable result indicating an improvement of over 200% during optimisation of piezoelectric layer length. In conjunction to theoretical findings, outcomes of extensive experimental work are provided in order to highlight the accuracy and reliability of the presented theoretical models in both electrical scenarios. Variation in mechanical damping magnitude plays a pivotal role throughout experimental testing and is one key factor in explaining why devices comprising of shorter piezoelectric layers have high performance.

A methodology behind unbiased design comparisons is also provided in this work, and involves comparing devices with identical fundamental frequencies. The reasoning behind this approach is to allow for each device to perform as efficiently as possible in the same excitation scenario. Systematic alterations to multiple geometric parameters are used to achieve this. Geometric parameters such as the substrate thickness are observed to provide adequate frequency control. Using this approach, performance improvements from adjustments to piezoelectric coverage still remain.

The occurrence of non-linearity in piezoelectric materials is a widely known phenomena and so lastly, a more robust model is provided which incorporates material and geometric non-linearity. This model is useful in determining dynamical responses of uniform and non-uniform piezoelectric energy harvesters when subjected to moderate-to-high acceleration levels. A thorough validation of the theoretical model is achieved using extensive experimental data obtained from a range of samples. For the harvester composition tested in this work, the occurrence of mild non-linearity at base acceleration levels as low as 1 meter per second squared is witnessed with softening behaviour causing the resonant frequency to decrease with base acceleration. In order to avoid reduced efficiency in the final application, the prediction of possible frequency shifts is vital during the design process.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: McWilliam, S.
Popov, A.A.
Keywords: Piezoelectric energy harvesting, Non-linearity, Geometric optimisation
Subjects: T Technology > TK Electrical engineering. Electronics Nuclear engineering > TK7800 Electronics
Faculties/Schools: UK Campuses > Faculty of Engineering > Department of Mechanical, Materials and Manufacturing Engineering
UK Campuses > Faculty of Engineering
Item ID: 13246
Depositing User: EP, Services
Date Deposited: 24 Oct 2013 11:42
Last Modified: 13 Sep 2016 12:24

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