An updated model of the hybrid system consisting of a vacuum TIG (thermionic generator) and a multi-couple TEG (thermoelectric generator) is proposed, in which the main internal and external irreversible losses of the system are considered and the temperatures of the electrode plates of the TIG and the cold side of the TEG are determined by energy balance equations rather than some specified parameters. Analytical expressions for the power output and efficiency of the TIG, TEG, and hybrid system are derived. The effects of the voltage output and work functions of the TIG and the electric current of the TEG on the power output and efficiency are discussed. The maximum power output and efficiency of the hybrid system are numerically calculated. The optimally operating regions of main parameters are determined. (C) 2015 Elsevier Ltd. All rights reserved.