NUMERICAL INVESTIGATION OF ENTROPY GENERATION DURING THE DISCHARGE OF ENCAPSULATED PHASE CHANGE MATERIAL-BASED THERMAL ENERGY STORAGE

Abstract

A latent heat thermal energy storage system (LHTES) has a potential to improve the load stability by mitigating the fluctuations encountered in concentrated solar thermal power plants. In this paper, heat transfer analysis of LHTES during the discharging period is studied by estimating pressure losses, first-law efficiency, and temporal variation of entropy generation. The LHTES considered in this study is of packed bed type where a phase change material (PCM) is encapsulated in a spherical shell, which forms the solid portion, and heat transfer fluid (HTF) flows through a porous zone in a packed bed. The numerical model considered is two-dimensional axisymmetric, which takes into consideration the mass, momentum, and energy conservation equations in a porous medium. Heat transfer between HTF and PCM is modeled using two-temperature equations coupled with an enthalpy-porosity technique to analyze the isothermal phase change behavior during solidification of PCM. The numerical model is first validated with the published experimental results. The effects of several parameters, such as porosity, inner encapsulation diameter, encapsulation shell thickness, and encapsulation shell material are further studied. It is found that the LHTES produces more entropy due to the internal diffusion process, which is prominent in the system with a large coefficient of overall volumetric heat transfer between HTF and PCM. The first-law efficiency of discharging is affected significantly by porosity rather than by any other parameter considered in the study.