Graphene nanoplatelets and copper foams for improving passive cooling performance of PCMs in Singapore's tropical climate

Abstract

In Singapore's hot tropical climate, phase change materials (PCMs) hold significant promise for year-round passive cooling solutions in buildings. However, the narrow diurnal temperature range of 24–32 °C poses a great challenge for pure PCMs with poor thermal conductivity to undergo complete melting-freezing cycles. Herein, we tested two composite PCMs in a self-designed thermal box: (1) paraffin RT28HC/graphene nanoplatelets (GNP-PCMs) and (2) paraffin RT28HC/copper foam (CF-PCMs). The passive cooling performance of the composite PCMs was evaluated based on their charging power at 32 °C and discharging power at 24 °C. The focus is on convection heat transfer between the composite PCMs and ambient air, with considering various factors, including GNPs content (0.1–0.9 wt%), CF pore density (40 pores per inch (PPI) and 70 PPI), airflow rate (natural convection and forced convection), and orientation (horizontal and vertical). The results indicate that a minimum temperature range of 26.6–30.3 °C is necessary for complete melting-freezing cycles. The effect of GNPs content is more sensitive to orientation, whereas the effect of CF pore density is more affected by airflow rate. In all cases, the CF-PCMs exhibit superior temperature uniformity than the GNP-PCMs. Under forced convection, the charging power and discharging power of pure PCM reached a maximum of 110.8 and 163.5 W/m2, respectively. The addition of GNPs enhanced the charging power and discharging power of pure PCM by up to 14.2 % and 10.2 %, respectively. Moreover, the incorporation of CF boosted the charging power and discharging power of pure PCM by up to 75.1 % and 64.3 %, respectively.