Layout of phase change materials in a thermal energy storage system

Abstract

The objective of this work is to investigate the performance of a thermal energy storage system using multiple phase change materials (PCMs). This study is based on latent heat thermal energy storage. Three phase change materials namely, Potassium Hydroxide (KOH), Potassium Nitrate (KNO3), and Sodium Nitrate (NaNO3) have been selected for this study. These PCMs have been chosen because of their inherent thermal stability, high melting point, high latent heat of fusion per unit mass, relatively high thermal conductivity, high specific heat, non-flammable properties, and availability. In this work, the performance of the thermal energy storage system is analyzed by evaluating key parameters such as liquid fraction and the amount of energy stored and extracted during charging and discharging respectively. Two types of PCM layouts, uniform and cascaded, have been employed. In case of uniform PCM layout, only one type of PCM is used at a time throughout the bed. In case of cascaded PCM layout, multiple PCMs are used at a time throughout the bed. The cascaded layout further has two types of arrangement. The first type of arrangement is the slope down arrangement where the PCMs are placed in the descending order of their melting temperatures. The second type of arrangement is the slope up arrangement where the PCMs are placed in the ascending order of their melting temperatures. Overall, the cascaded layout excels in performance when compared to the uniform layout in terms of PCM melting and solidification time and in terms of energy stored and extracted. Keeping these factors in mind, we recommend using a cascaded layout in a Thermal Energy Storage System (TESS) as opposed to a uniform layout.