Analytical and experimental studies of thermoelectric devices and materials

Abstract

Interest in thermoelectric devices (TEDs) for waste-heat recovery applications has recently increased due to a growing global environmental consciousness and the potential economic benefits of increasing cycle efficiency. Unlike conventional waste-heat recovery systems like the organic Rankine cycle, TEDs are steady-state, scalable apparatus that directly convert a temperature difference into electricity using the Seebeck effect. The benefits of TEDS, namely steady-state operation and scalability, are often outweighed by their low performance in terms of thermal conversion efficiency and power output. To address the issue of poor device performance, this dissertation takes a multi-faceted approach focusing on device modeling, analysis and design and material processing.

First, a complete one-dimensional thermal resistance network is developed to analytically model a TED, including heat exchangers, support structures and thermal and electrical contact resistances. The purpose of analytical modeling is twofold: to introduce an optimization algorithm of the thermoelectric material geometry based upon the realized temperature difference to maximize thermal conversion efficiency and power output; and to identify areas within the conventional TED that can be restructured to allow for a greater temperature difference across the junction and hence increased performance. Additionally, this model incorporates a component on the numerical resolution of radiation view factors within a TED cavity to properly model radiation heat transfer. Results indicate that geometric optimization increases performance upwards of 30% and the hot-side ceramic diminishes realized temperature difference. The resulting analytical model is validated with published numerical and comparable analytical models, and serves as a basis for experimental studies.

Second, an integrated thermoelectric device is presented. The integrated TED is a restructured TED that eliminates the hot-side ceramic and directly incorporates the hot-side heat exchanger into the hot-side interconnector, reducing the thermal resistance between source and hot-side junction. A single-state and multi-stage pin-fin integrated TED are developed and tested experimentally, and the performance characteristics are shown for a wide range of operating fluid temperatures and flow rates. Due to the eliminated to thermal restriction, the integrated TED shows unique performance characteristics in comparison to conventional TED, indicating increased performance.

Finally, a grain-boundary engineering approach to material processing of bulk bismuth telluride (Bi₂Te₃) is presented. Using uniaxial compaction and sintering techniques, the preferred crystallographic orientation (PCO) and coherency of grains, respectively, are controlled. The effect of sintering temperature on thermoelectric properties, specifically Seebeck coefficient, thermal conductivity and electrical resistivity, are determined for samples which exhibited the highest PCO. It is shown the performance of bulk Bi₂Te₃ produced by the presented method is comparable to that of nano-structured materials, with a maximum figure of merit of 0.40 attained at 383 K.