Valorisation of Plastic Waste: Recycled Polyethylene Terephthalate as a Scalable Spectrally Selective Coating for Dual-Mode Solar Thermal and Radiative Cooling Applications in Tropical Buildings

Spectrally selective surfaces are pivotal for advanced thermal systems but are often characterized by complex, energy-intensive, and costly manufacturing processes, limiting their scalability and accessibility. This work presents a sustainable and economically viable alternative by developing, fabricating, and characterisation of a low-cost spectrally selective coating derived entirely from recycled Polyethylene Terephthalate (PET) plastic waste. The coating was engineered for dual-mode operation in a hybrid solar thermal and nocturnal radiative cooling system, requiring a balance of high solar absorptance (α) and high thermal emittance (ε) in the atmospheric window (8–13 μm). The PET film was fabricated through a process of pulverizing post-consumer bottles and applying the resulting powder onto a mild steel substrate. When experimentally characterized within a full-scale hybrid system under tropical conditions, the coating demonstrated remarkable dual-functional properties: a solar absorptance of α ≈ 0.86-0.94 and a long-wave infrared emissivity of ε ≈ 0.42- 0.48. The integrated system, leveraging this coating, achieved a solar thermal efficiency of 47.11% and a nocturnal radiative cooling power of 196.86 W/m2. These results are competitive with systems using more complex commercial or engineered surfaces. This study conclusively establishes recycled PET as a viable, abundant, and effective material for spectrally selective surfaces, bridging the gap between circular waste management strategies and the pressing need for accessible renewable energy technology. The simple fabrication process offers a disruptive pathway for the scalable production of low- carbon thermal management systems, particularly in developing economies.