Thermal performance of galactitol/mannitol eutectic mixture/expanded graphite composite as phase change material for thermal energy harvesting

Abstract

The eutectic mixture of galactitol and mannitol with the melting point of 157.6 °C and high latent heat of fusion (ΔHm=271.07 kJ/kg) is regarded as an ideal phase change material used in medium temperatures. However, the shortcomings of low thermal conductivity and severe supercooling restrict its application for thermal energy storage. To address these challenges, expanded graphite (EG) was introduced to obtain eutectic mixture/EG composites by vacuum impregnation method. SEM images showed that the pores of EG were filled by sugar alcohols and no chemical reaction happened during this process as confirmed by FTIR spectroscopy and XRD. The thermophysical properties of phase change material (PCM) composites were measured. The result revealed that the supercooling degree (Tsc) of PCM composites decreased more than 26.2% compared to that of the eutectic mixture. Although the ΔHm of PCM composites decreased with an increase in content of EG, most of them are higher than 200 kJ/kg. Unlikely the ΔHm of PCM composites, the latent heat of crystallization (ΔHc) first rose by 15.3% and then dropped slightly. Their thermal conductivities were greatly improved by EG, especially with a 14 wt.% EG loading producing a more than 30-fold increase. Also, the PCM composite with 14 wt.% EG loading shows great form stability and thermal reliability after 20 charging-discharging cycles. These results prove that galactitol/mannitol eutectic mixture/EG composite has a promising prospect in potential for thermal energy harvesting and thermal management systems application.