Finite Element Simulations on Scaling Effects of 3D SiGe Thermoelectric Generators
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Finite Element Simulations on Scaling Effects of 3D SiGe Thermoelectric Generators Nicholas Williams1, Ali Gokirmak1, and Helena Silva1 1
Electrical and Computer Engineering, University of Connecticut, 371 Fairfield Way, Storrs, CT 06269, USA ABSTRACT We report 3D finite element simulations analyzing scaling effects on the performance of single Silicon Germanium thermoelectric generator with 170 µm tall metal contacts. Temperature dependent material parameters are included to accurately model device performance. Power density was extracted for a range of widths, heights, and operating temperature. Depending upon cross sectional area of the SiGe leg and operating temperature, height can be optimized for maximum power density. INTRODUCTION Thermoelectric generators (TEGs) are solid state devices that are used for direct conversion of thermal energy to electrical energy (Seebeck effect) or for active heating/cooling (Peltier effect) if electric power is supplied. ZT is a common dimensionless figure of merit for TE materials which is a measure of the thermodynamic efficiency of a material and is defined as:[1] (1) Where S is the Seebeck coefficient, σ is the electrical conductivity, k is the thermal conductivity and T is the temperature. The possibility of using TEGs for thermal-electrical energy exchange has recently regained interest as larger ZT has been observed for a range of materials[2, 3]. An ideal thermoelectric material would have low thermal conductivity (allowing for large temperature differences) with a large power factor (S2σ). Efforts to increase ZT have focused on decreasing thermal conductivity (nanostructuring[4], superlattices[5], alloying[6], milling[7]) or increasing Seebeck coefficient via optimized doping concentration[1] and modified density of states in confined materials[3]. The recent development of improved thermoelectric materials has spurred significant interest into incorporating TEGs into waste heat recovery systems on automobiles, power plants, and factories. Several car companies (GM[8], BMW[9] and Volkswagen[10]) are developing TEG equipped cars which show a 5% net increase in fuel efficiency. If a 10% improvement in full efficiency can be achieved, the Department of Energy will retrofit existing large trucks with TEG modules[11]. Besides the ZT of the material, TEG efficiency and performance depends on the design of the TEG structure. In this study, 3D finite element simulations in Synopsys Sentaurus TCAD[12] are used investigate the effect of scaling the device dimensions and changing the operating temperature on the performance of a single TEG composed of Silicon Germanium (SixGe1-x). The TEG leg height (HLEG) ranges from 100 nm to 10 µm while the width and depth range from 100 nm to 10 mm. SixGe1-x is a commonly used high temperature thermoelectric material with a peak ZT value of ~ 1 for n-type and ~ 0.6 for p-type at 900° C[13]. ZT values of 1 are achieved as a result of a considerable decrease in thermal conductivity for SiGe as compared to pure Si or Ge.
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