Preliminary characterization and thermal evaluation of a direct contact cascaded immiscible inorganic salt/high-density polyethylene as moderate temperature heat storage material

Abstract

The present work assesses the direct contact cascaded immiscible phase change material (PCM) for solar heat storage systems (SHSS) in water heating applications. The direct contact cascaded immiscible PCM uses ternary salt mixture (TSM) and high-density polyethylene (HDPE). Improvement on phase behavior and thermal properties are obtained for the direct contact cascaded immiscible TSM/HDPE. The plasticity effect of HDPE leads to a stable phase transition of the mixture with the lowest supercooling degree by 0.9 °C. The initial thermal decomposition temperature for TSM and HDPE is above 400 °C, which is suitable for moderate temperature operation of SHSS. The immiscibility between TSM and HDPE leads to a multistep phase transition between 110.2 and 139.6 °C. The phase behavior during the liquid-solid transition demonstrates the lowest temperature gradient of 13.9 °C, implying a steady phase transition. It allows the stored heat can be released effectively without causing significant volume expansion of the TSM. Moreover, the enthalpy of fusion for TSM/HDPE is increased, making it favorable as solar thermal energy for water heating applications in SHSS.