Thermophysical and chemical characterization of used cooking oils converting into eco-friendly phase change materials for energy storage applications

This study explores the conversion of waste cooking oils (WCOs) into phase change materials (PCMs) for storing thermal energy, offering a sustainable pathway for waste cooking oil valorization. Globally, over 190 million metric tons of WCO are produced annually, and per capita cooking oil consumption rose from 10.33 L in 2015 to 11.58 L in 2021. This study examines the effect of blending waste cooking sunflower oil (WCSO) and waste cooking coconut oil (WCCO) with soybean oil (SO), at 90/10 and 70/30 ratios, targeting low- (-20 to 5 °C) and medium- (5 to 40 °C) temperature-range applications, respectively. Fatty acid composition (GC), functional groups (FTIR), and phase-change behaviour (DSC, thermal conductivity) were characterized. The SO addition increased latent heat capacity by 1–6%. WCCO blends showed both melting and freezing temperature decreasing (25.00 to 16.75/14.20 °C) with increasing SO content, while WCSO blends showed freezing decreasing slightly (-10.00 to -10.30/-10.10 °C) and melting increasing slightly (-7.50 to -6.75 °C), narrowing the melt–freeze range. The WCSO blends stored 180–183 J/g with conductivity of 0.19–0.25 W/m·K and WCCO blends stored 160–169 J/g with conductivity of 0.21–0.29 W/m·K. These changes are consistent with SO-driven fatty acid compositional shifts, indicating WCO’s potential as low-cost PCM feedstocks, pending further validation.