INFLUENCE OF THERMAL CONTACT RESISTANCES ON MICRO-GAP HEAT LOSSES FOR MICROTHERMIONIC POWER GENERATORS AT LOW OPERATION TEMPERATURE WITH A SIC EMITTER

摘要

Thermionic power generation is a safe and clean energy source that allows for converting heat into electrical energy using thermionic electrons. The miniaturization is an advantage of this technology that led to the recent development of micro-gap thermionic power generators. In this work, a SiC emitter and a Pt collector are assembled with 10 μm gap, made of square SiO_2 columns with a width 600 μm. The emitter is heated at 774°C using an IR lamp. As a result, a thermionic power density of 130 mW/cm~2 is obtained. The heat transfers from the emitter electrode are evaluated and thermal conduction to the collector is by far the predominant thermal transfer (86% of the global emitter heat losses). In order to take into account the contact resistances, the thermal resistance of the micro-gap is measured to be 18.3 K/W, which allows for estimating a more realistic value for the heat loss by thermal conduction from the emitter to the collector via the gap (33% of the global emitter heat losses). The experimental efficiency is then calculated to be 6.8×10~(-3). A reduction of the insulators to 10 μm of width and the use of a Mo emitter would minimize the emitter heat losses, making it possible to reach an experimental efficiency of 8.7%. On a complementary study, thermal contact resistances between the micro-gap insulators and the emitter as well as between the micro-gap insulators and the collector are measured. A thermal resistance of 48.6 K/W is obtained by downsizing the insulators until 200 μm of width, demonstrating a high impact for decreasing the micro-gap conduction heat loss density from the emitter to the collector from 28 W/cm~2 (theoretical value obtained without considering contact resistances) to 5.6 W/cm~2. By downsizing the width of the insulators from 700 μm to 200 μm, an increase of the power conversion efficiency from 9.1×10~(-5) until 1.5×10~(-3) is observed.