Finite element formulation to study thermal stresses in nanoencapsulated phase change materials for energy storage Josep Forner-Escrig a , Roberto Palma b,* , Rosa Mondrag´on a a Department of Mechanical Engineering and Construction, Universitat Jaume I, Av. de Vicent Sos Baynat, s/n 12071 Castell´ on de la Plana, Spain; b Department of Structural Mechanics and Hydraulic Engineering, University of Granada, Spain ARTICLE HISTORY Compiled 17th February 2020 Abstract Nanoencapsulated phase change materials (nePCMs) –which are composed of a core with a phase change material and of a shell that envelopes the core– are currently under research for heat storage applications. Mechanically, one problem encountered in the synthesis of nePCMs is the failure of the shell due to thermal stresses during heating/cooling cycles. Thus, a compromise between shell and core volumes must be found to guarantee both mechanical reliability and heat storage capacity. At present, this compromise is commonly achieved by trial and error experiments or by using simple analytical solutions. On this ground, the current work presents a thermodynamically consistent and three-dimensional finite element (FE) formula- tion considering both solid and liquid phases to study thermal stresses in nePCMs. Despite the fact that there are several phase change FE formulations in the literat- ure, the main novelty of the present work is its monolithic coupling –no staggered approaches are required– between thermal and mechanical fields. Then, the FE formulation is implemented in a computational code and it is validated against one- dimensional analytical solutions. Finally, the FE model is used to perform a thermal stress analysis for different nePCM geometries and materials to predict their mech- anical failure by using the Rankine’s criterion. KEYWORDS Finite Element Method; Thermoelasticity; Phase Change; Nanoparticles; Heat storage 1. Introduction 1 One of the major concerns that society faces currently for its development is producing 2 and supplying energy. In fact, evolution of mankind has been closely related to a 3 progressive increase in energy consumption through history [1]. Therefore, research 4 in energy production appears to be crucial for society. Concerning the production 5 of energy, two different paths seem to arise: searching and exploiting new sources of 6 energy or optimizing the existing facilities of energy production processes to gain in 7 efficiency. In connection with this last alternative, a considerable amount of research 8 in thermal energy storage is being carried out [2–5]. More precisely, in this field, heat 9 storage systems based on phase change materials are continuously attracting attention, 10 *Corresponding author: rpalgue@ugr.es