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 prototype. The study is carried out to investigate the thermal behaviour of the TES thermocline tank by changing the Reynolds number. The results demonstrated that the higher void fraction near the wall results in a lower solid mass to absorb thermal energy and the temperature in the near-wall region is much higher than in core region at low Reynolds number but with increasing the Reynolds number this temperature difference decreasing. The results show that the temperature difference between both the 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 both the spheres and the HTF during the discharging cycle decreases with increase in the Reynolds number and this difference in temperature is less during the discharging cycle than the charging cycle.