This study is to systematically analyze the effect of various kinds of design factors on the stratification performance of a rectangular storage tank. Special interest is focused on the diffuser configuration, which crucially impacts the performance of a storage tank. Herein, a new diffuser shape is proposed, which exemplifies improved performance. Three-dimensional unsteady numerical experiments are conducted for four design parameters of a stratified thermal storage tank: Three design parameters with three levels (i.e., the Reynolds number = 400, 800, and 1200; the Froude number = 0.5, 1.0, and 2.0; and the area ratio of the diffuser to tank cross-section = 0.0327, 0.0582, and 0.131) and one design parameter having two levels (i.e., diffuser type = radial plate type and radial adjusted plate type). Orthogonal array L-18(2 x 3(7)) is adopted for the analysis of variance. The result gives quantitative estimation of the various design parameters affecting the performance and helps to determine the main factors for the optimum design of a stratified thermal storage tank. In the range of parameters considered, the Reynolds number is found to be the most dominant parameter. Moreover, the diffuser shape plays a significant role on the performance of a stratified thermal storage tank. (c) 2008 Elsevier Ltd. All rights reserved.