Comprehensive Review of Bio-Oil-Based Nanofluids as Cutting-Edge Heat Transfer Fluids with a Focus on Solar Thermal Systems

The increasing demand for efficient renewable heat transfer fluids in solar system applications has intensified interest in exploring bio-oil as a renewable and sustainable alternative to conventional thermal fluids. Bio-oils, derived from biomass, offer a promising alternative for industrial applications. However, their relatively low efficiency has prompted the incorporation of nanoparticles to create bio-oil-based nanofluids. Despite the notable improvement in heat transfer effectiveness, the full-scale implementation of bio-oi-based nanofluids is constrained by several critical challenges. These include the tendency of the nanoparticles to aggregate over time, compromising long-term stability, and inherently high viscosity of the bio-oil fluid, which hinders the fluid flow and energy efficiency. Additionally, thermal stability under varying operating conditions and bio-oil scalability for industrial applications remains a significant concern. The present review comprehensively examines the potential of bio-oil-based nanofluids as an improved heat transfer fluid, focusing on their environmental and sustainable benefits over conventional fluids. It also presents recent advanced studies on bio-oil-based nanofluids, addressing critical issues related to long-term stability, thermal stability, and high viscosity through different preparation and characterization techniques to improve the thermophysical properties to optimize their performance in solar thermal systems. The review identifies key challenges and prospects for full-scale implementation of bio-oil-based nanofluids in solar thermal technologies. It aims to provide researchers in this field with valuable insight into advancing an improved heat transfer fluid that is environmentally friendly and suitable for solar thermal applications that contribute to the global transition toward a greener future.