Modeling of a heat capacity peak and an enthalpy jump for a paraffin-based phase-change material

TL;DR

This paper develops a microscopic model to accurately fit the heat capacity peak and enthalpy jump of a paraffin-based phase-change material, enabling better understanding of its thermal properties and latent heat.

Contribution

A novel microscopic development that models heat capacity and enthalpy changes in PCMs, applicable to other materials with polycrystalline structures.

Findings

The model accurately fits experimental heat capacity and enthalpy data.

Latent heat accounts for about 84% of total enthalpy change.

Most of the phase change occurs within 5°C of the peak.

Abstract

Rubitherm RT 27 is a paraffin-based phase-change material (PCM) in which a change between a solid and liquid phase is used to store/release thermal energy. Its enthalpy and heat capacity, as measured in a quasistatic regime by adiabatic scanning calorimetry, has a single distinct jump and peak, respectively, at about 27.3 C. We present a microscopic development from which the jump and peak can be accurately fitted and that could be analogously applied even to other PCMs. It enables us to determine the baseline and excess part of the heat capacity and thus the latent heat associated with the phase change. It is shown to be about 84 % of the total enthalpy change that occurs within 5 C from the peak maximum position. The development is based on the observation that PCMs often have polycrystalline structure, being composed of many single-crystalline grains. The enthalpy and heat capacity measured in experiments are therefore interpreted as superpositions of many contributions that come from the individual grains.