Optimization on the photo-thermal conversion performance of graphite nanoplatelets decorated phase change material emulsions

Carbon nanomaterials decorated phase change material emulsions (PCMEs) are promising working fluids for direct absorption solar collectors (DASCs). Aiming at realizing an optimization, eleven PCMEs with different mass fractions of graphite nanoplatelets (GNPs) (0.04%, 0.055%, 0.07%, 0.085% and 0.10%) and paraffin (15%, 20% and 25%) were prepared, and their thermophysical characteristics, optical absorption properties, and photo-thermal conversion performance were investigated systematically. It is shown that the thermal conductivity and optical absorption of the GNPs decorated PCMEs increase with the mass fraction of the GNPs, but show a reverse trend as the mass fraction of the paraffin is increased. The viscosity increases with the increase of the paraffin mass fraction at the same temperature, and gradually decreases along with the temperature increases. The photo-thermal conversion performance of the 0.04% GNPs decorated PCMEs is inferior to that of the ones at higher loadings of the GNPs; while, at high loadings of the GNPs, an interaction between the effect of the GNPs and that of the paraffin should be considered. It is revealed that thermal conductivity is an important factor influencing the temperature distributions within the PCMEs. And to avoid the evaporation of the water included in a PCME to weaken the incident light consequently, its optical absorption property should match with its thermal conductivity. The 0.07% GNPs decorated 20% PCME possesses the best photo-thermal conversion performance, and the relative heat storage capacity is 1.64 times as higher as that of distilled water. The 100 heating-cooling cycles test indicates that the 0.07% GNPs decorated 20% PCME exhibits good thermal reliability, making it show great potentials for use in DASCs.