Power harvesting, energy harvesting, power scavenging, and energy scavenging are four terms commonly used to describe the process of extracting useful electrical energy from other ambient energy sources using special materials called transducers that have the ability to convert one form of energy into another. While the words power and energy have vastly different definitions, the terms "power harvesting" and "energy harvesting" are used interchangeably throughout much of the literature to describe the same process of extracting electrical energy from ambient sources. Even though most of the energy coupling materials currently available have been around for decades, their use for the specific purpose of power harvesting has not been thoroughly examined until recently, when the power requirements of many electronic devices has reduced drastically. The overall objective of this research is to typify the power source characteristics of various transducer devices in order to find some basic way to compare the relative energy densities of each type of device and, where possible, the comparative energy densities within subcategories of harvesting techniques. Included in this research is also a comparison of power storage techniques, which is often neglected in other literature sources. An initial analysis of power storage devices explores the background of secondary (rechargeable) batteries and supercapacitors, the advantages and disadvantages of each, as well as the promising characteristics of recent supercapacitor technology developments. Also explored is research into the effectiveness of piezoelectric energy harvesting for the purpose of battery charging, with particular focus on the current output of piezoelectric harvesters. The first objective involved presenting and verifying a model for a cantilever piezoelectric bimorph. Next, an investigation into new active fiber composite materials and macro fiber composite devices utilizing the d31 coefficient is performed in comparison to a monolithic piezoelectric bimorph. The information gathered here was used to design a two bimorph device termed the mobile energy harvester (MEH). Worn by a human being at the waste level, the MEH harvests energy from each footfall during walking or running. The next objective involved characterizing small temperature gradient (less than 200 oC) thermoelectric generators (TEGs). Four TEGs were linked in series and joined with a specially made aluminum base and fin heat sink. This device was then mounted to the exhaust system of an automobile and proved capable of recharging both an 80 and a 300 milliamp-hour battery. A switching circuit concept to step up the output voltage is also presented. However, the circuit proves somewhat difficult to implement, so an alternative DC/DC device is proposed as a possible solution. With the advent of highly efficient, low voltage DC to DC converters, it is shown that their high current, low voltage output can be converted to a higher voltage source that is suitable for many electronic and recharging applications. As extensive literature exists on the capabilities of photovoltaic and electromagnetic energy harvesting, no original experimentation is presented. Instead, only a brief overview of the pertinent technological advances is provided in this document for the purpose of comparison to piezoelectric and thermoelectric energy harvesting. The main research focus, as described above, is dedicated to designing and performing original experiments to characterize cutting edge piezoelectric and thermoelectric transducer materials. To conclude and unify the document, the final section compares the power harvesting techniques with one another and introduces methods of combining them to produce a hybrid, multiple energy domain harvesting device. A piezoelectric-electromagnetic harvesting combination device is presented and scrutinized, revealing that such a device could improve the amount of energy extracted from a single harvesting unit. The research presented here not only expands on the present understanding of these materials, but also proposes a new method of creating a hybrid power harvesting device utilizing two of the energy coupling domains, electromechanical and piezoelectric. The goal is to maximize the harvested energy by tapping into as many ambient sources as are available and practical. / Master of Science

O presente trabalho visa empregar uma metodologia numérica para determinar as propriedades macro mecânica de compósitos ativos (AFC - Active Fiber Composite ou MFC - Macro Fiber Composite), combinando o conceito de Volume Elementar Representativo (VER) com o Método dos Elementos Finitos (MEF). Inicialmente, apresenta-se a fundamentação teórica associada à abordagem numérica empregada. Posteriormente, os modelos numéricos desenvolvidos são aplicados na determinação dos coeficientes efetivos de materiais compósitos inteligentes transversalmente isotrópicos com fibras piezelétricas de seção com forma circular e quadrada, respectivamente. Finalmente, os resultados numéricos obtidos pela metodologia proposta são, então, comparados com resultados da literatura. Constata-se que os resultados obtidos são muito semelhantes aos resultados relatados pela literatura para arranjo quadrático e hexagonal com fibra de geometria circular, sendo que neste caso, compararam-se os resultados numéricos com analíticos obtidos através do Método de Homogeneização Assintótica. Em seguida, a metodologia é aplicada para determinação dos coeficientes efetivos para arranjo quadrático e hexagonal com fibra de geometria quadrada. Empregando diferentes frações volumétricas de fibras, os resultados via MEF foram comparados aos resultados analíticos obtidos através do Método dos Campos Uniformes (Uniform Field Method). Após a avaliação das limitações e potencialidades da metodologia, de forma direta, através de resultados analíticos, realizou-se a avaliação da mesma de forma indireta. Para tal, foram realizadas análises dinâmicas visando comparar as Funções de Resposta em Frequência (FRF) experimentais com as obtidas computacionalmente. Dessa forma, utilizou-se uma viga de alumínio estrutural engastada-livre, onde foram colados duas pastilhas piezelétricas, sendo uma para realizar a excitação da estrutura e, a outra para fazer a aquisição dos dados. Os modelos computacionais via MEF empregaram para o domínio das pastilhas, as propriedades efetivas determinadas através da metodologia desenvolvida. Os resultados obtidos demonstraram mais uma vez as potencialidades da metodologia proposta. Assim, conclui-se que a metodologia numérica não é somente uma boa alternativa para o cálculo de coeficientes efetivos de compósitos inteligentes, mas também uma ferramenta para o projeto de estruturas inteligentes monitoradas por materiais piezelétricos. / This work presents the development a numerical methodology to determine the mechanical properties of active macro composites (AFC - Active Fiber Composite, or MFC - Macro Fiber Composite), combining the concept of Representative Elementary Volume (REV) with the Finite Element Method (FEM). In the first instance, the theoretical framework associated with the numerical approach employed is presented. Later, numerical models based on unit cell are applied to predict the effective material coefficients of the transversely isotropic piezoelectric composite with circular cross section fibers. Finally, numerical results obtained by the proposed methodology are compared to other methods reported in the literature. It appears that the results are very similar to the literature results for square and hexagonal arrangement of fibers with circular geometry, in which case, it was compared numerical with analytical results calculated by Asymptotic Homogenization Method (AHM). After that, the methodology is applied to determine the effective coefficients for square and hexagonal array with square fiber geometry. Employing different fiber volume fractions, it follows that the results obtained by the proposed methodology were compared to analytical results calculated by the Uniform Field Method (UFM). After assessing the potential and limitations of the methodology, either directly, through analytical results, the evaluation took place in the indirect approach. Then, dynamic analyses were performed in order to compare the Frequency Response Functions (FRFs) determined by experimental tests with computational results. Thus, it was used a cantilever beam aluminum structure, which were bonded two piezoelectric patches, one to carry the excitement of the structure and the second to perform the data acquisition. The effective properties determined by the proposed methodology were applied for the dominium established by the piezoelectric patches. The results showed, again, the potential of the proposed methodology. Therefore, the numerical methodology is not only a good alternative for the calculation of effective coefficients of smart composite, but also a tool for the design of smart structures monitored by piezoelectric materials.

O presente trabalho visa empregar uma metodologia numérica para determinar as propriedades macro mecânica de compósitos ativos (AFC - Active Fiber Composite ou MFC - Macro Fiber Composite), combinando o conceito de Volume Elementar Representativo (VER) com o Método dos Elementos Finitos (MEF). Inicialmente, apresenta-se a fundamentação teórica associada à abordagem numérica empregada. Posteriormente, os modelos numéricos desenvolvidos são aplicados na determinação dos coeficientes efetivos de materiais compósitos inteligentes transversalmente isotrópicos com fibras piezelétricas de seção com forma circular e quadrada, respectivamente. Finalmente, os resultados numéricos obtidos pela metodologia proposta são, então, comparados com resultados da literatura. Constata-se que os resultados obtidos são muito semelhantes aos resultados relatados pela literatura para arranjo quadrático e hexagonal com fibra de geometria circular, sendo que neste caso, compararam-se os resultados numéricos com analíticos obtidos através do Método de Homogeneização Assintótica. Em seguida, a metodologia é aplicada para determinação dos coeficientes efetivos para arranjo quadrático e hexagonal com fibra de geometria quadrada. Empregando diferentes frações volumétricas de fibras, os resultados via MEF foram comparados aos resultados analíticos obtidos através do Método dos Campos Uniformes (Uniform Field Method). Após a avaliação das limitações e potencialidades da metodologia, de forma direta, através de resultados analíticos, realizou-se a avaliação da mesma de forma indireta. Para tal, foram realizadas análises dinâmicas visando comparar as Funções de Resposta em Frequência (FRF) experimentais com as obtidas computacionalmente. Dessa forma, utilizou-se uma viga de alumínio estrutural engastada-livre, onde foram colados duas pastilhas piezelétricas, sendo uma para realizar a excitação da estrutura e, a outra para fazer a aquisição dos dados. Os modelos computacionais via MEF empregaram para o domínio das pastilhas, as propriedades efetivas determinadas através da metodologia desenvolvida. Os resultados obtidos demonstraram mais uma vez as potencialidades da metodologia proposta. Assim, conclui-se que a metodologia numérica não é somente uma boa alternativa para o cálculo de coeficientes efetivos de compósitos inteligentes, mas também uma ferramenta para o projeto de estruturas inteligentes monitoradas por materiais piezelétricos. / This work presents the development a numerical methodology to determine the mechanical properties of active macro composites (AFC - Active Fiber Composite, or MFC - Macro Fiber Composite), combining the concept of Representative Elementary Volume (REV) with the Finite Element Method (FEM). In the first instance, the theoretical framework associated with the numerical approach employed is presented. Later, numerical models based on unit cell are applied to predict the effective material coefficients of the transversely isotropic piezoelectric composite with circular cross section fibers. Finally, numerical results obtained by the proposed methodology are compared to other methods reported in the literature. It appears that the results are very similar to the literature results for square and hexagonal arrangement of fibers with circular geometry, in which case, it was compared numerical with analytical results calculated by Asymptotic Homogenization Method (AHM). After that, the methodology is applied to determine the effective coefficients for square and hexagonal array with square fiber geometry. Employing different fiber volume fractions, it follows that the results obtained by the proposed methodology were compared to analytical results calculated by the Uniform Field Method (UFM). After assessing the potential and limitations of the methodology, either directly, through analytical results, the evaluation took place in the indirect approach. Then, dynamic analyses were performed in order to compare the Frequency Response Functions (FRFs) determined by experimental tests with computational results. Thus, it was used a cantilever beam aluminum structure, which were bonded two piezoelectric patches, one to carry the excitement of the structure and the second to perform the data acquisition. The effective properties determined by the proposed methodology were applied for the dominium established by the piezoelectric patches. The results showed, again, the potential of the proposed methodology. Therefore, the numerical methodology is not only a good alternative for the calculation of effective coefficients of smart composite, but also a tool for the design of smart structures monitored by piezoelectric materials.

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