One-tank molten salt thermochne heat storage system can reduce capital cost due to the use of cheap heat storage materials as compared with two-tank molten salt heat storage system. In order to develop a convenient method for the evaluation and design of the one-tank thermocline heat storage system, the idea of effectiveness-number of transfer unit method (epsilon-NTU) for heat exchanger design was employed in which three dimensionless design parameters are defined including the ratio between overall thermal conductance and the total thermal capacity of heat transfer fluid flowing through the system during discharging process (i.e., NTU), the ratio between thermal capacity of solid particles and total thermal capacity of heat transfer fluid through the system during the discharging process (i.e., C*(s)), the ratio between overall thermal conductance in the charging process and that in the discharging process (i.e., (hA)*). A transient one-dimensional dispersion-concentric model is then applied to study the effects of three dimensionless parameters on the effectiveness of one-tank system. It is shown that the effectiveness of the one-tank thermochne heat storage system is closely related to NTU, C*(s) and (hA)*. Based on the simulated results, general performance evaluation charts and the related effectiveness correlations are also proposed for evaluation and design of the one-tank system. Moreover, the proposed effectiveness correlations are finally applied in the design of a one-tank molten salt thermocline heat storage system in a 50 MWe CSP plant. (C) 2018 Elsevier Ltd. All rights reserved.