Designing microscale gas discharges to enhance thermionic energy conversion

Abstract

Thermionic energy converters (TECs) are devices that convert heat directly to electrical work via thermionic emission. In a TEC, a hot cathode emits electrons that are then collected by a cold anode. However, these emitted electrons buildup in the gap between the electrodes, retarding the flow of current. There are many strategies for combatting the build-up of negative space charge, all with benefits and challenges. One popular strategy is to shrink the interelectrode gap; this reduces, but does not eliminate, the build-up of negative space charge. Another strategy is to use a plasma with positive ions to cancel the effect of the build-up of negative space charge, typically using a cesium plasma; however, cesium is extremely corrosive, which limits the materials that can be used. Operating using an inert gas plasma can bring the benefits of plasma-enhanced TECs, without the drawbacks of cesium plasmas. In previous work, we showed that microscale inert gas plasmas could also enhance thermionic emission under steady state conditions, by enhancing the local electric field at the cathode. However, energy must be injected into the system to ignite the microplasma, potentially negating any benefit from thermal energy conversion. To overcome this, microplasma-enhanced TEC devices must be operated transiently. A typical cycle is broken into an “on” sub-cycle where a voltage near, at, or above the gas's breakdown voltage is applied, causing a large number of ions to be produced due to electron impact ionization. Then, during the “off” sub-cycle these ions slowly drift towards the cathode. Due to the long residence time of the ions compared to electrons, there is net positive space charge in the interelectrode gap, causing a strong electric field that enhances the emission of electrons as well as mitigating negative space charge. In this work, we use the plasma modeling software Zapdos to model an inert gas microplasma-enhanced TEC. We explore the effect of various system parameters on the net power produced by microplasma TECs, and look to optimize these parameters to maximize power output.