They recognized that the installation of heat insulation panels can effectively prevent the mixing of hot and cold fluids, thereby improving the heat storage performance of the heat storage tank. Furthermore, [PERSON] et al. [16] presented the experimental results obtained by ENEA in the Casaccia Research Centre (Rome, Italy), with a small-scale test section consisting of a 300 \(\mathrm{kW_{th}}\) steam generator inserted in an 8 \(\mathrm{m^{3}}\) storage tank with molten salt at high temperature. [PERSON] et al. [17] conducted an experimental analysis of a single media thermoline storage system equipped with a perforated, eccentrically mounted, vertical, and porous flow distributor, and analyzed thermocline formation and propagation during the charging cycle. It is observed that the thermocline layer is thinner for higher flow rates. [PERSON] et al. [18] carried out an experimental investigation of the sparsely studied'single medium thermocine' (SMT) based single tank sensible thermal energy storage and discussed the effect of Atwood number (density stratification) and mean temperature on TES effectiveness as well as providing the appropriate thermodynamic insights. [PERSON] et al. [19] proposed a kind of hybrid thermocline heat storage method in multi-scale structures and relevant experimental systems and experimentally investigated the fluid flow and heat storage performances of molten salt in a multi-scale structure. The results showed that the theoretical heat storage efficiencies among the three heat storage manners are less than 80% because of the existence of the thermocline. [PERSON] et al. [20] investigated the characteristics of thermal energy storage using the mixture of Xceltherm 600 synthetic oil and sand under various inlet temperatures and velocities. It is found that the TES efficiency of the sand-oil mixture increased by 18.5% compared to that of pure sand. [PERSON] et al. [21] selected the binary nitrates (KNO\({}_{3}\) + Ca(NO\({}_{3}\))\({}_{2}\)) as a heat storage medium, and comparatively studied three different inlet velocity conditions such as constant value, manual, and automatic adjustment. The results showed that the heat discharging power drops with the heat discharging time under constant inlet velocity. A lot of research on the single-tank thermal energy storage system adopted molten salt as a storage medium, and the studies based on water were rare. However, in some application scenarios, such as the combined heat and power (CHP) plants and solar heating for civil architecture and industrial application, the single-tank thermal energy storage system based on water is commonly employed because of its low cost and no solidification in a large range of temperature. Additionally, due to the low viscosity of water, it is easy to flow in disturbance to thin the thermocline and improve system efficiency. Therefore, the objective of this study is to attain a comprehensive investigation of the thermal performance of the single-tank thermal energy storage system as water is chosen as the storage medium. The performance of a single-tank TES system is closely related to the thermocline thickness. The thicker the thermocline is, the worse the performance is, so a novel efficiency evaluation model for the single-tank TES technology based on thermocline thickness is proposed in this paper. In order to highlight some crucial factors of thermocline on the thermal performance, a two-dimensional flow and heat transfer model of the single-tank thermal energy storage system is established, and the effects of time, flow velocity, and height-to-diameter ratio on thermocline thickness are considered.