Trade-off between the near-field heat transfer and the space charge effect in graphene-anode thermionic energy converters

Abstract

The high work function of electrode materials inhibits the improvement of the performance of thermionic energy converters. Space charge effect and near-field heat transfer are also significant factors restricting the thermoelectric conversion capability. A micro-scale graphene-anode thermionic energy converter is proposed, in which a monolayer graphene sheet is placed on the substrate as the collector. The effects of both the space charge effect and the near-field heat transfer in the vacuum gap of the device are discussed based on Poisson's equation and the framework of fluctuation electrodynamics. Expressions for the power output density and efficiency of the system are derived. The systemic performance characteristics are comprehensively analyzed by optimizing main parameters. The results show that the electrode temperatures, current density, and energy fluxes in each portion are significantly regulated by the voltage output and the distance between the electrodes. There is a trade-off between the near-field heat transfer and the space charge effect so that the system can achieve optimum performance. The maximum values of the power output density and efficiency of the proposed system are, respectively, enhanced by 2.135% and 8.824% compared to those of the metal-thermionic energy converter. The optimal selection criteria of key parameters are determined, providing valuable guidance for actual operation.