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Abstract: The thermocline energy storage tank (TEST) serves as a crucial component in thermal energy storage systems, utilizing the working fluid that enters through a diffuser to store and harness energy. However, the conventional double-plate radial diffuser is ill-suited for a single medium TEST's full tank storage due to its unidirectional fluid inflow. There has been a notable lack of optimization analysis of diffusers. Two innovative tubular diffuser designs with reduced cross-sectional areas have been introduced: the annular arranged diffuser (AAD) and cross arranged diffuser (CAD). To elucidate the impact of diffuser designs on energy storage efficiency, a three-dimensional transient computational fluid dynamics (CFD) model was established to simulate a thermocline formation under two diffuser types. The model was validated against experimental data. Results showed that the thermocline of AAD was 11.39% thinner than that of a traditional double plate diffuser. In the process of charging and discharging, the time-varying thermocline and factors influencing thermocline thickness were analyzed. Results indicate that in the mixed dominant region, increased turbulent kinetic energy correlates with reduced thermocline thickness. Notably, the AAD's stable thermocline was 4.23% and 5.41% thinner than the CAD's during charging and discharging, respectively, making the AAD preferable for engineering applications. The effects of tube diameter and orifice opening angle on temperature stratification performance were also examined. The findings suggest that an inclined impact jet and large diameter tubes are more conducive to temperature stratification. Moreover, an orifice diameter optimization method was developed, which can decrease the thermocline by 6.78%.