To investigate the behavior of the round-trip efficiency of transcritical-CO2-cycle-based TEES (thermoelectric energy storage) according to the changes in the temperature of the low-temperature hot storage tank, the charging and discharging processes were optimized at various temperature conditions of the tank. In contrast to previous studies, to achieve a more realistic simulation, heat exchangers of identical shape and size were shared by charging and discharging processes, and the characteristics of the working fluid's heat transfer and pressure drop were considered. Based on this study, it was confirmed that an optimal temperature exists in the low-temperature hot storage tank for maximizing the round-trip efficiency. The reason for this existence was discussed. Furthermore, it was found that a common design approach-using all the stored heat as a heat source during discharging-does not always yield optimal results from the viewpoint of round-trip efficiency. (C) 2014 Elsevier Ltd. All rights reserved.