Heat Transfer Analysis for Thermal Energy Storage Using NaNO₃ as Encapsulated Phase Change Material

Storage of thermal energy is of practical interest for concentrated solar power (CSP) applications. Thermal energy storage (TES) for CSP systems has been accomplished with the use of liquid eutectic mixtures of KNO₃ and NaNO₃ that are exchanged between a cold and hot tank with appropriate heat exchangers in between. While such two tank systems primarily rely on the sensible heat of the fluid to store thermal energy the storage system that utilizes latent heat can significantly be smaller, more effective and less expensive. The current work is focused on TES at higher temperatures using encapsulated phase change material (EPCM) as NaNO₃ encapsulated by stainless steel that is applicable to Rankine and other power generation cycles. We present here two dimensional transient heat transfer analysis for the NaNO₃ encapsulated in a cylindrical shaped capsule for charging (storing thermal energy) and discharging (retrieving thermal energy) process. Energy stored and retrieved are in both sensible and latent heat form. Simulations are conducted for both vertically and horizontally placed cylindrical rods to investigate the effect of gravity on the charging and discharging process for various diameters of rods. It has been found that heat transfer into/from EPCM rods is not posing any problem for 6–8 hours storage/retrieval of thermal energy.