Affordable BIO-PCM Composite Derived from Waste Cooking-Oil (WCO) for Outdoor Building Insulation—Experimental Study

The valorization of waste cooking oil (WCO) offers a sustainable pathway for improving building’s energy efficiency while supporting circular economy principles. This study developed a novel shape-stabilized bio-based phase change material (Bb-PCM) derived from WCO fatty acids for passive thermal regulation of building envelopes. Purified fatty acids were obtained through filtration, saponification, acidification, and solvent purification. The resulting Bb-PCM was then incorporated into a natural clay–cellulose supporting matrix containing four activated-carbon loading levels using a direct impregnation method. The composites were characterized using spectroscopic, thermal, and microstructural techniques. Differential scanning calorimetry under nitrogen at 2 °C min−1 showed melting temperatures of 34–35 °C and melting latent heats of 35.2–45.9 J g−1. Thermogravimetric analysis confirmed thermal stability below 100 °C, while microstructural characterization demonstrated differences in matrix densification and structural ordering among the composite formulations investigated. The composite containing 25 wt.% activated carbon exhibited the highest melting latent heat (45.9 J g−1) and favorable thermal conductivity (0.29 W m−1 K−1), representing the optimum composite formulation that balances thermal storage capacity, heat transfer, and structural stability. Outdoor evaluation demonstrated stable thermal performance, reducing indoor temperatures by approximately 2 °C, indicating strong potential for sustainable passive cooling applications in buildings.