Improving Thermionic Power Generation via the Reduction of Emission Barriers

This thesis investigates methods to improve the performance of thermionic energy converters (TECs) by reducing the emission barriers. A TEC generates electricity from thermal energy when electrons are emitted from a hot electrode (the emitter) through a vacuum gap and absorbed by a cooler electrode (the collector). Due to their lack of working fluids and inherently compact nature, TECs have garnered attention over the years for their potential as a means of power generation in space or high-grade waste heat recovery. Historically, however, the practical implementation of TECs has been hindered by significant technical challenges. Recent advancements in microfabrication techniques and the development of advanced materials have renewed interest in TECs. Notably, it has been shown that decreasing the interelectrode vacuum gap to a distance of a few micrometers can significantly improve performance by mitigating the build-up of negative space charge (NSC), thereby decreasing the potential barrier formed between the electrodes. By taking advantage of this, as well as other microscale phenomena that reduce the potential barrier faced by electrons (i.e., the emission barrier), the TEC can be realized as a powerful means of energy production. The first part of this work investigates how microscale structures can be utilized on the emitter’s surface to instigate Schottky barrier-lowering and increase the emitting surface area. An analytical framework is used to show that the increase in emitting surface area has a greater influence over Schottky barrier lowering, and that while output power density can be increased substantially, the improvement in efficiency is limited. The second part of this work focuses on a near-field enhanced solid-state TEC, where the vacuum gap is replaced by a thin semiconducting material, and the emitter is a p-type semiconductor exposed to near-field thermal radiation. Replacing the vacuum gap with a semiconducting interelectrode layer facilitates better electron emission through the Schottky barrier formed at the electrode interfaces. Exposing the p-type emitter to nearfield thermal radiation raises the average energy of electrons in the emitter, effectively lowering the emission barrier. Together, these effects enable a powerful, novel device capable of harvesting high-grade waste heat. Advisor: Mohammad Ghashami