Heat transfer simulations and predictions of the thermal energy storage capability using encapsulated phase change materials (EPCM) at high temperatures are conducted. NaNO_3 is considered as a phase change material (PCM). The PCM is encapsulated by a stainless steel shell. Two dimensional simulations of a cylindrical capsule are considered. The effects of the buoyancy driven convection in the molten PCM as well as the thermal and volume expansions due to phase change are included in the thermal analysis. An initial void level of 20% is considered in the simulations of the EPCM capsules. EPCM capsules store energy not only by sensible heat but also by the latent heat of fusion as heat is stored or extracted from the capsule. The solid/liquid interface inside the PCM propagates radially inward during the melting process. The effect of a void on the thermal energy storage and on the evolution of the solid/liquid interface is characterized. Two cases are presented, that of a local void initially at the top of the EPCM capsule and an initially random void distribution. The initial location of the void within the capsule has a profound effect on the shape of the solid/liquid interface and the isotherms within the capsule. The results of these simulations can be the basis for the design of an EPCM based thermocline for thermal energy storage (TES) at a concentrated solar power plant and other applications.
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