The focus of this study is the development of a dimensionless model that allows a better understanding of the thermocline storage system behavior. The proposed model is based on a dual phase approach. We had identified four remarkable dimensionless groups of parameters which control the thermocline system performances. The influence of each dimensionless group of parameters has been analyzed separately to understand its influence on the overall thermocline storage system performance. During this investigation, three concepts are deeply analyzed: the thermocline zone thickness, dimensionless discharge time and discharge efficiency. Using this model, we have demonstrated that, for each combination HTF/TESM, a universal optimum can be highlighted, which makes it possible to find, according to a given specification, the optimal configuration of the storage tank. Based on the obtained results, we had proposed a new reduced model based on dual-phase approach for rapid calculation of optimal sizing of a thermocline storage tank. The proposed method requires no discretization method and provides accuracy similar to those of the dual phase model but with a drastically reduced computation effort. (C) 2020 Elsevier Ltd. All rights reserved.