Rheological and thermophysical properties of hybrid nanofluids and their application in flat-plate solar collectors: a comprehensive review

Conventional working fluids such as water, glycol, and synthetic oils are commonly employed in solar thermal applications, but they have relatively low thermal conductivity, which leads to minimized heat transfer and lower thermal performance. As an alternative, a novel category of fluids known as hybrid nanofluids has proven to be highly beneficial for solar thermal applications due to their enhanced thermophysical properties. Recently, numerous studies have explored hybrid nanofluids as a continuation of nanofluids research for the improvement of the thermophysical properties of mono nanofluids. In this review paper, publications of hybrid nanofluids are in depth analyzed. The review is focusing on hybrid nanofluid preparation methods and characterization procedures, as well as techniques for assessing stability and improving it. The impact of hybrid nanoparticles on thermophysical properties such as thermal conductivity, viscosity, specific heat, and density is extensively reported. Furthermore, the current review provides a comprehensive overview of various experimental, numerical, and theoretical investigations where the hybrid nanofluids in flat-plate solar collectors were studied with a focus on thermal efficiency improvement, which is regarded as the primary parameter for using hybrid nanofluids. Lastly, recent challenges and limitations are outlined, and future recommendations are presented for further investigations.