Thermal Modeling of High Temperature Energy Storage Using Encapsulated Phase Change Materials

Transient heat transfer analysis is conducted to investigate high temperature energy storage using encapsulated phase change materials (EPCMs) for concentrated solar power applications. The phase change material considered is the eutectic mixture of NaCl-MgCl₂ (57 mole% NaCl and 43 mole% MgCl₂) encapsulated by stainless steel in a cylindrical shaped capsule (or tube). Energy storage into EPCM and energy retrieval from EPCM is simulated for various flow conditions of the heat transfer fluid. Heat storage/retrieval times are determined from numerical simulations for various sizes of capsules and flow conditions by an accurate modeling of propagating solid/liquid interface. Numerical simulations are conducted by employing a front tracking method and the enthalpy–porosity approach. A two-dimensional horizontally placed cylindrical shaped EPCM capsule is considered in simulations using gas (air) and liquid (VP1-Therminol) as heat transfer fluids. The results predicted by the front tracking method agree well with those predicted by the enthalpy–porosity method. It is illustrated by the present work that enthalpy–porosity method can be employed to simulate the modeling at the single capsule level and system level. System level storage unit is a thermocline that includes an arrangement of several EPCMs.