Volume 4, Issue 2 -

A tremendous amount of research has been performed on the design and analysis of vibration energy harvester architectures with the goal of optimizing power output. Often, little attention is given to the actual characteristics of common vibrations from which energy is harvested. In order to shed light on the characteristics of common ambient vibration, data representing 333 vibration signals were downloaded from the NiPS Laboratory “Real Vibration” database, processed, and categorized according to the source of the signal (e. g. vehicle, machine, etc.), the number of dominant frequencies, the nature of the dominant frequencies (e. g. stationary, band-limited noise, etc.), and other metrics. By categorizing signals in this way, the set of idealized vibration inputs (i. e. single stationary frequency, Gaussian white noise, etc.) commonly assumed for harvester input can be corroborated and refined. Furthermore, some heretofore overlooked vibration input types are given motivation for investigation. The classification determined that, of the set of signals used in the study, 64 % of the animal source signals are best described with nonstationary dominant frequencies, 58 % of machine source signals are best described with stationary frequencies, and vehicle source signals are poorly described by any one signal type used in the classification. Nonlinear harvesters with a cubic stiffness term have received extensive attention in the scholarly literature; a numerical simulation and optimization procedure were performed using several representative signals as vibration inputs to determine the prevalence with which such a nonlinear harvester architecture might provide improvement to power output. The analysis indicated that a nonlinear harvester architecture may prove beneficial in increasing power output over a linear counterpart if the signal contains a single, dominant frequency that is not stationary in time, as evidenced by a 14 % increase in harvester power output when employing an architecture with a nonlinear cubic stiffness function. Other studies have indicated that nonlinear architectures may be beneficial for signals with nonstationary frequencies or filtered noise. 53 % of the all characterized signals fall into categories that could potentially benefit from a nonlinear oscillator architecture.

This study presents the characterisation of a prototype thermoelectric generator including ceramic Ca3Co4O9${\rm{C}}{{\rm{a}}_{\rm{3}}}{\rm{C}}{{\rm{o}}_{\rm{4}}}{{\rm{O}}_{\rm{9}}}$ p-type and metallic Cu0.57Ni0.42Mn0.01${\rm{C}}{{\rm{u}}_{{\rm{0}}{\rm{. 57}}}}{\rm{N}}{{\rm{i}}_{{\rm{0}}{\rm{. 42}}}}{\rm{M}}{{\rm{n}}_{{\rm{0}}{\rm{. 01}}}}$ n-type legs. The generator was constructed applying the conventional rigid chessboard design, which provides an electrical series connection of the thermoelectrically active materials in between of two electrical isolating cover plates made of alumina. Ag-epoxy resin was used for electric connectors which were directly bond to the thermoelectrically active materials. The generator was rebuild in the framework of a finite-element model to simulate the generator’s transport characteristics using the physical data of the single materials as input parameters for the simulation tool. The simulation results give a view to the distributions of temperature and electric potential as well as vector plots showing the generated current density of charge inside the regarded materials. The finite-element simulation tool was used to consider the electrical contact resistivities between the thermoelectric legs and the connector material. The input parameter for the specific isothermal electric resistivity of the Ag-epoxy connectors were varied by including the electrical contact resistances to match the simulated to the measured data in order to receive the correct electric current densities in the modeled device.

Thermoelectric generator (TEG) elements typically made of Bismuth Telluride (Bi 2 Te 3 ) have good thermoelectric properties but are very brittle. In practice, however, TEG elements often are subject to both mechanical and thermal loading. Although clamping is the main source for mechanical loading in TEGs, other loadings such as from vibrations can occur and inducing stresses which can lead to failure. If the allowable stress is exceeded, then device failure will result. Axial stress is predominantly found in vertically oriented elements. Elements oriented in other positions experience both axial and bending stresses. However, when shear and bending occur, failure is far more likely. Therefore, TEG shape and orientation relative to the thermal and structural loading are critical. In this context, a topology optimization approach is posed to develop a compliant TEG, capable of maintaining thermoelectric functioning and sustaining mechanical loadings. This approach builds on previous research on topology optimization for multifunctional materials, but uniquely deals with multifunctional design of a composite TEG. First a tool is developed and validated to study the unique compliant structure and second a composite 3-D unit cell comprised of structural and thermoelectric materials is created. The volume fractions and orientation of the two materials are optimized to support applied structural shear, bending, and axial structural loads and thermal loads. A optimal structural model was shown to have equal shear and adjoint loads that resulted to a an increase of 9.61 % displacement while using 8.5 % less material. The integrated model (structural and thermal) used 8.5 % less material and had a 9.64 % increase in displacement. The implication of this research is that it could help to inform 3-D printing of more compliant TEGs optimized for a particular application.