Evaluation of efficiency factors and internal resistance of thermoelectric materials

Abstract

It is well known that the figure of merit (ZT) is unreliable in calculating the efficiency (Ƞ) of micro thermoelectric generators system level and unrealistic when comparing the performance of thermoelectric (TE) materials in the same metric units. To solve this problem, we have used COMSOL multiphysics to design a single leg of micro thermoelectric generators model for computing efficiency factors (ϕ) and internal resistance using TE materials' constants, such as electrical conductivity (σ), TE conductivity (K), and Seebeck coefficient (α). The TE materials were placed between two copper electrodes, and the first data analyzed were the voltages per meter and electric currents per meter. The internal resistances were calculated by taking the ration of voltages to electric currents, and at the same time, the electric powers were calculated from the products of electric currents and voltages yielding power per unit area in μW cm−2. The ϕ were calculated using changes in power (ΔP), temperature gradient (ΔT), and the surface area (A). The obtained results showed that the TE materials with highest ϕ when the temperatures are between 375 and 550 K are n-type SiGe and p-type SiGe. When the temperatures are between 550 and 780 K, the TE materials with the highest ϕ are PbTe-Pbl2, PbTe-CdTe, and PbTe-SrTe-Na. We noted that the ϕ obtained from eight TE materials in this work are within the range as those reported in the literature between 0.001 and 0.091 μW cm−2 K−2. The TE materials with high internal resistances such as PbS, PbTe, and PbSe have ϕ that is <0.0001 μW cm−2 K−2, and those with low internal resistances have ϕ in the range between 0.002 and 0.0091 μW cm−2 K−2. This work has shown that COMSOL multiphysics is a powerful computational tool that can be used to analyze internal resistances and ϕ of TE materials in the same temperature ranges. Copyright © 2016 John Wiley & Sons, Ltd.