This paper reports a numerical and analytical study of time dependent storage of energy by melting a phase change material in a cylindrical tank. In the first part of the study the heater was a vertical spiral tube positioned coaxially with the tank. In the second part, the heater consisted of two coaxial spirals positioned on the same axis as the cylinder. In accord with the philosophy of constructal design, the heat and fluid flow configuration was endowed with degrees of freedom in order to morph toward better overall performance for a fixed amount of heating material. The paper documents the effect of the geometrical features such as helix diameter and pitch on the overall performance. (C) 2014 Elsevier Ltd. All rights reserved.