A skutterudite thermoelectric module with high aspect ratio applied to milliwatt radioisotope thermoelectric generator

Abstract

This study focuses on the design and optimization of a high aspect ratio skutterudite (SKD) thermoelectric module (TEM) for a milliwatt Radioisotope Thermoelectric Generator (RTG). The RTG is a solid-state energy conversion device that utilizes the heat generated by radioactive isotope decay to generate electrical energy through the Seebeck effect. The aim of this work is to enhance the output performance of the RTG by investigating the influence of TEM structure on temperature distribution and overall performance using thermal resistance network and finite element analysis (FEA) methods. In this work, the RTG's Radioisotope Heater Unit (RHU) and diameter are kept constant as 4 W and 70 mm, while the effects of leg length (L), number of legs (n), and cross-sectional area (A) on voltage, output power, and conversion efficiency are studied. It is observed that increasing L, selecting an appropriate A, and reducing n result in a larger temperature gradient within the TEM, ultimately improving the output power and conversion efficiency of the milliwatt RTG. For A = 0.64 mm2, L = 25 mm, and n = 36, the milliwatt RTG generates maximum output power (Pmax) of 86.95 mW and maximum conversion efficiency (ηrmax) of 2.17%. To optimize the output power and conversion efficiency of the milliwatt RTG while simultaneously assuring the large voltage and manufacturability, a SKD TEM with 64 legs, each with dimensions of 0.8 × 0.8 × 25 mm3, is chosen. The milliwatt RTG installed with this SKD TEM will provide a voltage output (Vout) of 1.13 V, maximum output power (Pmax) of 63.30 mW, and maximum conversion efficiency (ηrmax) of 1.58%. Furthermore, it exhibits excellent environmental adaptability, capable of operating in ambient temperatures (T∞) up to 538 K. The thermal stress distribution in the high aspect ratio SKD TEM is also analyzed. Due to the varying coefficients of thermal expansion (CTE) between the SKD material and the adhesive, the maximum stress within the SKD TEM occurs inside and around the p-SKD legs. Finally, a SKD TEM of 9.9 × 9.9 × 25 mm3 containing 64 legs with each leg's dimension of 0.8 × 0.8 × 25 mm3 is fabricated. It shows an open circuit voltage (Voc) of 3.51 V, a maximum output power (Pmax) of 194.45 mW, and a maximum conversion efficiency (ηtmax) of 3.4% at the hot side temperature (Th) of 723 K and cold side temperature (Tc) of 296 K.