Heat Transfer Analysis in Encapsulated Phase Change Material Capsule for Thermal Energy Storage

摘要

Transient two dimensional heat transfer simulations of an encapsulated phase change material (EPCM) capsule as applicable to high temperature energy storage (＞300℃) for concentrated solar power (CSP) and other applications are conducted. Sodium nitrate, NaNO_3, is used here as a potential phase change material (PCM) with stainless steel as the encapsulation but the results have broader implications. 76.2 mm (3 inch) diameter horizontally placed cylinder is recommended for the applications according to the simulations for both horizontally placed cylinder and vertically placed cylinder. For vertically placed cylinders, some PCM at the bottom area could melt first with unduly large pressures in the capsule under certain circumstances. Such effects are less likely in horizontal EPCM cylinders. Therefore, horizontally placed cylinder is proposed for the simulations. Different types of heat transfer fluids (HTFs) are used for the analysis - air and liquid VP-1. According to the simulations, the liquid heat transfer fluid could significantly shorten the energy storage times for EPCM compared to air as HTF. Thus, larger size of EPCM capsule could be used for energy storage when using liquid as HTF. The current work describes effects of and predictions related to buoyancy-driven convection in the molten phase change material and void space inside the EPCM capsule that are essential for EPCM used for thermal energy storage. The effects of natural convection in the liquid PCM can decrease the time for phase change that could also be relevant for larger EPCMs. The volumetric expansion of molten NaNO_3 during phase change may require about 20% of void space inside the capsule. The effects of density variation in liquid PCM during phase change and the gravity effect of solid PCM are considered. During the charging (energy input) process, the volume of the void would decrease while the pressure in the void space would increase but with proper design the resulting pressure can be kept under acceptable values of stress in the capsule. The void space inside the capsule would slightly increase the energy storage times because the air in the void would act as an insulator. Understanding the heat transfer process inside the EPCM capsule, enables the optimization the size of EPCM, choice the type of PCMs, as well as help with the design of EPCM capsule and EPCM based thermocline.