Thermoelectric Energy Conversion: First-principles Simulations, Energy Harvesting and Deep Cooling Systems

Abstract

Thermoelectric devices can directly convert heat into electricity and electrical power into cooling or heating without moving parts. The energy conversion efficiency is dominated by the materials’ figure of merit zT, which is linearly proportional to the electrical conductivity, the square of the Seebeck coefficient, and inversely proportional to the thermal conductivity. Researchers worldwide have made great progress finding materials with higher zT through trial-and-error experiments during the past decades. This thesis contributes to understanding thermoelectric transport via first-principles simulations and the expansion of thermoelectric technologies through developing power generators using human metabolic heat and low-temperature freezers for vaccine storage.

First, we present density functional theory simulations of all thermoelectric transport properties of SiGe alloys over wide ranges of compositions, doping levels, and temperatures. This is the first time we are able to simulate realistic thermoelectric materials from first principles without fitting parameters. Surprisingly, the phonon drag effect that is supposed to be only significant at low temperatures still contributes to 10-20% of zT at 1100 K in SiGe alloys. The favorable comparison between our calculations and reported experiments brings us closer to predicting the transport properties of practical thermoelectric materials. Second, we present a flexible thermoelectric generator (f-TEG) design based on bulk thermoelectric materials, featuring multifunctional copper electrodes that enable flexibility, efficient heat concentration, and dissipation. The f-TEG reaches a record-high power density of 48 µW/cm² and can power an LED for reading in a completely dark room at 17.5°C when worn on the wearer’s forehead. This high-performance, low-cost f-TEG offers a broad prospect for low-power monitors or control systems in healthcare, agriculture, transportation, and industrial automation. Lastly, we develop a small affordable compressor-thermoelectric- hybrid freezer that can achieve an ultra-low temperature (ULT) of -70°C. Our ULT freezers can be readily commercialized into products with features of lightweight, portable size, low energy consumption, and competitive prices, which would facilitate transporting and storing clinical products around the world and make vaccines much more accessible to people in remote areas.