Comprehensive theoretical framework for the analysis of microgap thermionic energy conversion under constant heat inputs

We present a comprehensive model to fully characterize the behavior of thermionic energy conversion devices subject to constant heat flux inputs. Thermionic energy converters (TECs) have recently gained enormous attention mainly due to technological advances enabling the fabrication of converters with micro/nanometer interelectrode gaps. The reduced gap appeared to solve the long-lasting issue of negative space-charge build-up within TECs. However, the decrease in gap distance leads to the appearance of new physics in the energy interactions between the two electrodes, such as the image-charge effect and near-field thermal radiation. The intertwined energy exchange mechanisms require a deeper look and a systematic approach for accurately modeling the performance of TECs in the micro/nanoscales, especially in practical applications where the device is exposed to external energy sources such as concentrated solar power. We conducted a steady-state energy balance analysis to calculate the operating temperatures of the electrodes by considering constant heat flux inputs (such as concentrated solar energy) to the emitter and various cooling rates of the collector. The potential barrier profile within the interelectrode gap, the net current density, and the power output are subsequently calculated for a TEC with electrodes made of tungsten with and without electron reflection. The optimal values of the gap distance are also reported for various input scenarios revealing considerable deviation from the previously reported ones. An overall efficiency of 38.3% with a power output of 19.2 kW/m2 can be reached for a TEC with perfectly absorbing electrodes at the gap distance of 6μm operating under a heat flux input of 50 kW/m2 and convective heat transfer coefficient of 500 W/m2K. We believe that the present work provides an unabridged framework for the rigorous analysis of thermionic power generation with (sub) micrometer interelectrode distances and offers substantial insight into the design and characterization of TECs.