In this paper, numerical analyses have been carried out to describe the velocity and temperature fields inside a storage tank to be used in a solar system under various boundary conditions with upper and lower inlet flows. The aim of the study was to evaluate the effect of different plate sizes situated opposite the inlet in order to increase the thermal stratification. A numerical model was developed, and validated using experimental results. Two different initial temperature assumptions were taken into account along with a two-layer configuration. Two cases were analysed, the thermocline in the vicinity of the plate and the thermocline in the middle of the tank. In the latter case, the plate diameter had little impact, but moving the thermocline closer to the plates resulted in the diameter having a greater influence. It was also found that larger plates made it possible to preserve stratification with at larger inlet flow rates than the flow rates of the conventional low flow systems. Cold water inflow into the top of the tank was also studied. The influence of the plate diameter for the colder inflow was examined along with two temperature differences between the inlet and the tank. It was found that the diameter of the plate and the distance between the plate and the top of the tank have a significant effect on the temperature stratification within the tank when cold water enters at the top of the tank. (C) 2003 Elsevier Ltd. All rights reserved.