Preparation and thermal properties of low melting point alloy/expanded graphite composite phase change materials used in solar water storage system

Abstract

The focused solar water heating system can automatically adjust its focus according to the solar angle for higher heating rate and source temperature. One of the major challenges for such solar heat collector is stabilizing its daily heat outputs. This study intends to utilize a bulk low melting point alloy (140 °C)/expanded graphite (mass ratio of 9:1) composite phase change energy storage material to assure the stability and availability of the hot water output. To investigate the thermal properties of the proposed new material with different apparent densities, this study conducted scanning electron microscopy (SEM), X-ray powder diffractometry (XRD) and differential scanning calorimetry (DSC) tests to examine its thermal conductivity, storage density, and stability. The results of SEM show that the porous structure of expanded graphite (EG) can effectively adsorb the alloy and prevent material leakage. The XRD test shows that the formation of the low melting point alloy and the expanded graphite is a physical process without new substance creation. The DSC test shows the phase change temperature of the composite phase change material (PCM) is about 140 °C with an enthalpy of 45.05 kJ kg−1 (0.46% different from its theoretical value). The thermal conductivity of the bulk alloy/expanded graphite composite PCM with density of 2083 kg m−3, 2500 kg m−3, 3125 kg m−3, 4167 kg m−3 and 5000 kg m−3 are 12.65 W m−1 K−1, 14.64 W m−1 K−1, 20.80 W m−1 K−1, 28.16 W m−1 K−1 and 32.64 W m−1 K−1, respectively, which has an improved thermal conductivity compared to pure alloys (12.44 W m−1 K−1), especially the bulk composite PCM with a density of 5000 kg m−3 has the best thermal conductivity and the volumetric energy storage density reaches 2.25 × 105 kJ m−3, higher than the phase change heat storage density of most PCMs for thermal management, such as paraffin (volume heat density is 1.8 × 105 kJ m−3）, and repeated heating in an environment of high temperature of 180 °C does not leak. The results suggested that the developed composite phase change energy storage material with an apparent density of 5000 kg m−3 has high heat storage capacity, thermal conductivity, energy density and is able to effectively improve the heat exchange rate of the focused solar water heating system.