Energy storage is one of the most important components of renewable energy systems. Among different methods, thermal energy storage (TES) in forms of sensible or latent has been the subject of many studies in the past decades. The main difficulty in optimal design of storage tanks is associated with low thermal conductivity of the storing (solid or phase change) material. In fact, the distribution of thermal energy from a source to the body of storing material poses a volume to point problem which is the subject of constructal theory. Therefore, the objective of the present paper is to investigate the transient behavior of a rectangular thermal energy storage tank equipped with fin configurations optimized for heat conduction based on constructal theory. Results of numerical simulations reveal that the more complex configurations perform better in sensible TES systems; almost as well as what is expected based on analytical steady state solutions. However, because of the convection currents in the melting process of a PCM tank, the final full charging time of the latent systems are approximately the same. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.