Heat transfer analysis of encapsulated phase change material for thermal energy storage

Transient two dimensional heat transfer analysis is conducted to investigate a high temperature energy storage using encapsulated phase change materials (EPCMs) for concentrated solar power applications. The phase change material (PCM) considered is the NaNO3 encapsulated by a stainless steel in a cylindrical shaped capsule (or tube). Energy storage/retrieval into/from various sizes of EPCM capsules is simulated for both laminar and turbulent flow conditions of the heat transfer fluid (HTF) by an accurate modeling of the propagating liquid/solid interface in a PCM. Numerical simulations are conducted by employing a front-tracking method and an enthalpy–porosity approach. The results predicted by the two methods agree well. A two-dimensional cylindrical shaped EPCM capsule or tube is considered in simulations using gas (air) and liquid (Therminol/VP-1) as heat transfer fluids in a cross flow and an axial flow arrangement. The energy storage/retrieval times into/out of the EPCM capsule is dictated by the surface heat transfer of the EPCM for the capsule sizes considered in this study. A single horizontally placed rod in a channel with different blockage ratios for laminar and turbulence flows of HTF is studied in the present paper. It is illustrated by the present work that enthalpy-porosity method can be applied to simulate heat transfer at the capsule level and the system level. System level storage module is a thermocline that includes an arrangement of several EPCMs for several megawatts of thermal energy storage (TES) for several hours used in concentrated solar power applications and other industrial thermal systems.