A Novel Bio-Oil–Based Binary Nanofluid with Enhanced Thermal Conductivity for Heat-Transfer Applications

The conversion of solar radiation into thermal energy depends significantly on the performance of heat-transfer fluids (HTFs), which are responsible for both heat transport and thermal energy storage. In recent years, bio-oil-based nanofluids have attracted considerable attention as sustainable, efficient, and environmentally friendly alternatives to conventional HTFs due to their renewable origin and enhanced thermophysical properties. However, challenges associated with thermal stability, long-term dispersion stability, and the development of binary nanocomposite formulations capable of further improving the thermal conductivity of bio-oil-based HTFs remain largely unresolved. This study presents the development of a novel palm kernel oil (PKO)-based binary nanofluid produced through transesterification of PKO followed by the two-step nanoparticle dispersion method. Binary nanoparticles consisting of Al₂O₃ and TiO₂ were dispersed in the bio-oil to enhance its heat-transfer performance. The synthesized nanoparticles and the resulting nanofluid were comprehensively characterized using Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDX), Fourier Transform Infrared Spectroscopy (FTIR), Transmission Electron Microscopy (TEM), and Zeta Potential analysis to evaluate their morphology, composition, chemical interactions, and dispersion stability. The experimental results revealed a maximum thermal conductivity enhancement of 61.1% at a nanoparticle concentration of 0.3 wt% and a temperature of 80 °C, demonstrating a substantial improvement over the base fluid. These findings confirm that the developed PKO-based Al₂O₃–TiO₂ binary nanofluid possesses excellent thermophysical properties and stability, making it a promising heat-transfer fluid for low- and medium-temperature thermal energy systems, particularly in solar thermal applications. Furthermore, this work provides an innovative and sustainable approach to the development of renewable, high-performance, and environmentally friendly heat-transfer technologies.