Designing a Mechanically Robust Thermoelectric Module for High Temperature Application

We report a numerical study on the impacts of variations in the geometry, boundary conditions, and the coefficient of thermal expansion of the materials on the maximum shearing stress in thermoelectric power generator module (TEM) for high temperature applications. The maximum shearing stress in the TEM is evaluated for different designs focusing on their dependency on the fill factor. Although predictions by the previously developed analytical modeling are in partial agreement with numerical results, simplifying assumptions for the analytical model can limit the range of validity. Our numerical analysis shows that reduction of the fill factor alone under all the circumstances will not reduce the maximum shear stress. Imposing mechanical constraints at the boundaries, increasing the number of legs (6 × 6 in the analysis), and engineering the coefficient of thermal expansion are some of the key parameters controlling the maximum shearing stress and its changes with the fill factor

Mechanical Properties of Thermoelectric Materials for Practical Applications

Thermoelectric (TE) direct conversion of thermal energy into electricity is a novel renewable energy conversion method currently at a technological readiness level of 3–5 approaching laboratory prototypes. While approaching practical thermoelectric devices, an increase in the thermoelectric element’s efficiency is needed at the entire service temperature range. Yet, the main focus of research was concentrated on the electronic properties of the materials, while research on the mechanical properties was left behind. As it is shown in this chapter, knowing and controlling the mechanical properties of TE materials are paramount necessities for approaching practical TEGs. The material’s elastic constants, strength and fracture toughness are the most crucial parameters for designing of practical devices. The elastic constants provide understanding about the material’s stiffness, while strength provides the loading conditions in which the material will keep its original shape. Knowing the fracture toughness provides the stress envelope in which the material could operate and its susceptibility to inherent fabrication faults. The characterization methods of these properties are varied and may be physical or pure mechanical in nature. It is the authors opinion to prefer the mechanical methods, so the results obtained will describe more accurately the material’s response to mechanical loading

Minimizing thermally induced interfacial shearing stress in a thermoelectric module with low fractional area coverage

High temperature differences between the ceramic parts in thermo-electric modules (TEMs) intended for high temperature applications makes the TEMs vulnerable to the elevated thermal stress leading to possible structural (mechanical) failures. The problem of reducing the interfacial shearing stress in a TEM structure is addressed using analytical and finite-element-analysis (FEA) modeling. The maximum shearing stress occurring at the ends of the peripheral legs (and supposedly responsible for the structural robustness of the assembly) is calculated for different leg sizes. Good agreement between the analytical and FEA predictions has been found. It is concluded that the shearing stress can be effectively reduced by using thinner (smaller fractional area coverage) and longer (in the through thickness direction of the module) legs and compliant interfacial materials. (C) 2013 Elsevier Ltd. All rights reserved

Optimisation du rendement thermoélectrique de modules générateurs à pattes segmentées

L’amélioration de la performance thermoélectrique des modules générateurs thermoélectriques a fait l’objet de plusieurs études. Dans les dernières décennies, la plupart d’entre elles ont été faites à l’aide des éléments finis. Les simulations par éléments finis prennent à chaque jour une place plus grande dans la recherche. Des logiciels comme ANSYS Workbench fournissent des outils spécifiques pour chaque type d’analyses en plus d’un environnement convivial pour les nouveaux utilisateurs. Pour cette raison, dans ce mémoire où on s’est intéressé à l’effet de la segmentation des pattes dans la performance thermoélectrique du module, ainsi qu’aux propriétés mécaniques en générale et en particulier sur la résistance mécanique des modules, cette méthode d’analyse a été choisie. Le but du projet est de développer la simulation du comportement thermoélectrique et thermomécanique d’un module générateur thermoélectrique (MGT) à pattes segmentées afin d’établir et d’optimiser les paramètres permettant une performance supérieure dans la récupération de la chaleur perdue dans certains procédés industriels. De manière à bien atteindre cet objectif, on a construit trois géométries différentes de modules thermoélectriques afin de bien évaluer l’influence de la segmentation dans la performance des MGT ; deux modules standards,un module segmenté symétriquement et un modèle segmenté asymétriquement. Toutes les géométries ont été inspirées d’un MGT typique de 40 × 40 mm2 d’aire avec des pattes de 1.5 mm de longueur et 1.4 × 1.4 mm2 de section. Des alliages à base de tellurure de bismuth produits par extrusion à chaud à l’École Polytechnique ont été utilisés comme matériaux pour tous les modèles développés. Pour tous les modèles, plusieurs analyses ont été faites et comparées pour arriver à établir la viabilité du modèle proposé. Finalement, le modèle du MGT segmenté de façon asymétrique qui a été proposé a montré un gain en puissance de 9% par rapport au modèle simulé sans segmentation et a montré qu’il résiste aux contraintes mécaniques induites par le gradient de température. L’effet de la segmentation sur la performance des modules a été confirmé et l’effet de la déformation plastique de l’alliage de soudure sur la résistance mécanique des modules a été observé dans les résultats de la simulation.----------Abstract The performance improvement of thermoelectric generators (TEG) modules has been the subject of several studies, and in recent years most of them were made using a finite elements approach due to its reliability. For instance, software like ANSYS Workbench provides not only specific tools for each type of analysis, but also a pleasant environment for new users. For this reason in this work, where we are interested on how leg-segmentation affects the thermoelectric performance of the module as well as its general and particular mechanical properties and its strength, this method of analysis has been chosen. The project goal is to develop the simulation of the thermoelectric, and thermomechanical behaviour of a segmented-leg TEG module to establish, and to optimize the settings for enhanced performance in the recovery of waste-heat from industrial processes. In order to achieve this we built three different geometries of thermoelectric modules to assess the influence of the segmentation on module performance. A total of four modules were used for this purpose; two of them were standard modules, and the other modules were symmetrically and asymmetrically segmented. All the geometries have been created taking into account the dimensions of a typical TEG (40 × 40 mm2, legs of 1.5 mm length and 1.4 × 1.4 mm2 section.) Moreover, bismuth telluride alloys produced by hot extrusion, carried out at the École Polytechnique, were used as materials for all the models developed. Several analyses were carried out and models compared to determine the viability of the proposed model. Finally, the asymmetrically segmented TEG model has shown a power gain up to 9% with respect to the TEG without segmentation, and is capable to withstand the mechanical stresses induced by the temperature gradient. The effect of segmentation on the performance of the modules has been confirmed and the effect of plastic deformation of solder alloys on the strength of the modules has been observed in the results of the simulation. The model TEG asymmetrically segmented, proposed and developed in this work, is a versatile tool to introduce different variations in the model to verify the influence of other factors such as materials, temperature gradients, leg geometries and dimensions. This work has contributed a well-adapted tool to further study the thermomechanical and thermoelectric performance of TEG modules