Date of Completion

2-1-2016

Embargo Period

2-1-2016

Keywords

thermoelectric generator minority carriers finite element simulations

Major Advisor

Helena Silva

Associate Advisor

Ali Gokirmak

Associate Advisor

Ranjeev Bansal

Field of Study

Electrical Engineering

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

Thermoelectric generators (TEGs) are solid state devices (no moving parts) that directly convert thermal energy into electrical energy utilizing thermoelectric phenomena known as the Seebeck and Peltier effect. TEGs have been studied over the past 100 years as a possible energy generation and cooling technology. Interest in TEGs has become considerably popular in the last 10-15 years due to the awareness of climate change, environmental issues, and advancement in material fabrication. TEGs can recover energy from waste heat, the byproduct of many industrial/commercial processes, an example of which is automobiles, where waste heat accounts for ~30% of energy losses. However, relatively low efficiencies have limited TEG application to niche areas. Significant increase in the efficiency is necessary before TEGs can be implemented in widespread applications.

In this work, the effects of scaling the dimensions of a TEG are analyzed using finite element modeling in Synopsys Sentaurus software. Temperature dependent material parameters and a thermodynamic model are utilized to determine the output power and efficiency of Silicon and Silicon Germanium TEGs for dimensions ranging from 1 mm to 5 nm and operating temperatures from 300 K to the melting temperature. The role of minority carriers is examined using TEG designs which utilize built-in electric fields to extract generated minority carriers and transport them to a corresponding majority carrier area.

Temperature dependent material parameters are critical for modeling TEG operation at high temperatures. TEGs can be tailored to achieve optimum efficiency and power generation depending upon operating temperature and dimensions. Large aspect ratios at small dimensions exhibit the greatest power density suggesting that nanowire TEGs are a possible option for waste heat recovery. Results show that minority carriers are one of the TEG performance limitations at higher temperatures. TEG geometries that use PIN junctions are able to extract and transport minority carriers to their corresponding majority carrier leg. Extraction decreases the minority carrier density improving efficiency at higher temperatures due to reduced recombination. If leg widths are scaled down, depletion of majority carriers occurs for lower operating temperatures. Depletion can be minimized by converting PIN junction into PN junctions.

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