Parametric Optimum Design of a Graphene-Based Thermionic Energy Converter

A new model of the thermionic energy converter (TEC) configured with the graphene-based cathode is proposed, which includes the thermal radiation between the cathode and the anode electrodes and the heat losses from the anode to the environment. Analytic expressions for the power output density and efficiency of the TEC are derived. The performance characteristics of the TEC are analyzed by numerical calculations. It is found that the maximum efficiency and power density can, respectively, reach 30% and 0.575 Wcm^-2 when the TEC is operated between the two heat reservoirs at temperatures 1500 and 300 K. In addition, the optimum regions of the efficiency, power density, voltage output, electric current, and anode temperature are determined. The maximum efficiency and power density of the graphene-based TEC operated at different temperatures are calculated and compared with those of the metal-based TEC. It shows that the graphene-based TEC operated at 1200-1800K displays the better performance than the metal-based TEC. The results obtained here may provide guidance for the appropriate selection of electrode materials and optimum design of practical TEC devices.

I. Introduction

Thermionic energy converters (TECs) [1], [2] are a class of devices that can conveniently convert the heat energy at relatively high temperatures [3]–[5] into electricity and have attracted considerable attention [6]–[9]. For instance, Wang et al. [10] proposed the model of the vacuum thermionic generator including internal and external irreversible heat losses and gave the optimum design criteria of the device. Gil et al. [11] presented an analytical expression for the current density across an isotype heterojunction valid for arbitrary doping concentration ratios and generalized the standard expression. Choi et al. [12] achieved a micrometer-sized thermionic electron emission source with the maximum current density of 1.2 A/cm² with full crystallization of the LaB₆. Datas [13] established a hybrid thermionic-photovoltaic converter through the combination of the thermionic and photovoltaic energy conversion and offered an alternative approach for thermal energy harvesting. Contrasted with other types of energy conversion devices, TECs have some significant advantages such as the relative simplicity of design and operation [1], high-power density [2], and without mechanically moving parts [10]. The energy conversion efficiency of a TEC is closely dependent on the materials of the cathode working at higher temperatures and the space charge effect [14]–[16]. Besides conventional metals or semiconductors as the cathode, which new materials can be used as the cathode material to more effectively implement thermionic emission? This is a new worthy issue to be investigated [17]–[20].