Thermal performance of modified melamine foam/graphene/paraffin wax composite phase change materials for solar-thermal energy conversion and storage

The development of phase change materials (PCMs) is hampered by issues like leakage, poor thermal conductivity, and poor light absorption. In this study, we innovatively combined modified melamine foam (MF) and graphene nanoparticle (GNP) to address these defects of PCMs in the solar-thermal energy system. The MF was modified to endow the solar-thermal energy conversion ability and superhydrophobic interface characteristics due to the polypyrrole (PPy) by the in-situ polymerization and the polymerized octadecylsiloxane (PODS) layer coating on the sponge skeleton. The composite PCMs consisting of the modified MF, paraffin wax (PW), and GNP were fabricated and exhibited good leakproof ability, high thermal conductivity, and excellent solar-thermal energy conversion capability. Results showed that due to the effects of the PPy and the PODS on the sponge skeleton structure, MF@PPy-PODS/PW composite PCMs showed a decrease in the mass loss ratio to less than 0.34% and an increase in the thermal conductivity of 0.33 W/m·K and the solar-thermal storage efficiency of 75.68%. Besides, adding GNP with higher concentrations impacted positively on the thermal conductivity and the solar-thermal energy conversion ability but negatively on the phase change enthalpies. MF@PPy-PODS/GNP3/PW composite PCMs increased the thermal conductivity to 0.59 W/m·K and the solar-thermal storage efficiency to 79.36% while decreasing the phase change enthalpy to 130.61 J/g. Furthermore, a thermoelectric conversion system driven by solar energy was developed to show the potential of composite PCMs for cleaner energy production. The open-circuit voltages of the MF@PPy-PODS/GNP3/PW composite PCMs achieved 0.78 and 0.91 V at 2 and 4 Sun, respectively. The open-circuit voltages remained for a period of time and slowly dropped without the simulated solar light irradiation. This study provided a potential strategy for the optimal performance of composite PCMs and their application in solar thermal systems.