Solar tower power (STP) plants integrated with thermal energy storage (TES) subsystems are expected to be a potential solution to meet the world's future energy needs. Air rock bed TES subsystem emphasizes to be efficient and cheap storage for STP plants. In the present study, the discrete element method (DEM) coupled with the computational fluid dynamics (CFD) model are adopted to study the fluid flow and heat transfer process of an air rock thermocline TES tank. The study is carried out to investigate the thermal behaviour of the TES thermocline tank by changing the Reynolds number. The results show that the temperature difference between both the heat transfer fluid (HTF) and the spheres for charging cycle decreases with increasing of the Reynolds number, but also the total rate of heat transfer increases. The results illustrated that the difference in temperature between the HTF and the spheres for discharging cycle decreases with increase in the Reynolds number and this difference in temperature is less during the discharging cycle than the charging cycle. In addition, the Re = 1,200 case exhibit the optimized behaviour as it possesses the higher matching in temperature distributions of spheres with the temperature profiles of HTF. The Re = 400 shows reduced charging/discharging performance as the low rate of heat transfer between spheres and HTF.