Enhancing phase change material performance through the incorporation of thermally conductive objects at the solid-liquid interface

Abstract

Latent heat storage using phase change materials (PCMs) is a promising technology for storing and recovering waste heat. PCMs offer high energy density and can be tailored for specific melting temperatures, making them suitable for various applications, including temperature stabilization in buildings and thermal management of electronics and batteries. However, a significant disadvantage of PCMs is their low thermal conductivity, which slows the process of charging and discharging thermal energy. This work explores a novel approach to enhance the melting rate of PCMs by incorporating thermally conductive objects (TCOs) within the PCM. The TCO density lies between the solid and liquid PCM densities and is designed to follow the solid-liquid interface. First, an analytical model based on the Stefan problem formulation and a numerical model developed using Ansys Fluent, with locally modified thermal conductivity at the solid-liquid interface, were created to simulate the thermal behavior of the PCM with the addition of TCOs. An experimental setup, consisting of a rectangular enclosure with an organic paraffin as the PCM and hollow aluminum cylinders as the TCOs, was employed to validate these models under purely conductive melting conditions, with heating from the top and cooling from the bottom. Experimental results indicate that the addition of cylinders increased the melting rate by 19% under purely conductive conditions compared to the scenario without cylinders. However, the cylinders enhanced heat flux only within the first 12 mm, beyond which the thermal resistance of the liquid PCM became dominant, preventing further heat flux improvements. During solidification, the cylinders did not move with the solid front and were engulfed at the bottom. In the second part of this work, the developed Fluent model is used to study hypothetical scenarios where a TCO is added to the PCM, but heating comes from the side walls and melting is driven by convection. In this scenario, the inclusion of a TCO at the solid-liquid interface acts as a moving fin, increasing the melting rate by 62% and enhancing thermal power dissipation from 26.5 W to 75.4 W. Future research should focus on experimentally validating this scenario under convective melting, exploring methods to mechanically return the TCO to its starting position for repeatability, and conducting an energy analysis to determine whether the benefits of an enhanced melting rate outweigh the energy required to move the object and the increased manufacturing costs of the latent heat storage system.