The aim of this work is to show the current computational possibilities of three-dimensional computational fluid dynamics (CFD) simulations, using loosely coupled parallel computers (Beowulf clusters) in the virtual prototyping of thermal storage tanks. The transient thermal behaviour of a storage tank forming part of a thermosyphon solar heating system is simulated numerically. The influence of the inlet mass flow rate on the degree of thermal stratification during an unloading process is analysed. Special attention is given to the validation of the assumed mathematical model, the verification of the numerical solutions, and the post-processing tasks carried out in order to quantify the level of thermal stratification. The computational possibilities and limitations of this kind of detailed numerical experiments are pointed out. (C) 2004 Elsevier Ltd. All rights reserved.