Review of classical and nonlocal nanofluid models for solar collectors

Due to their significant improvement in thermal properties, nanofluids have found wide applications in engineering over the past three decades and have played a crucial role in solar collectors to enhance solar energy generation and transition to renewable energy sources. This study aims to investigate the application of nanofluids to increase the efficiency of solar collectors and to review their enhanced thermal performance as heat transfer fluids. Nearly all previously published review papers focus on the use of nanofluids in the solar energy field, highlighting the different engineering applications in addition to the important economic and environmental considerations. This work provides the first comprehensive review of the latest advances and the theoretical mathematical models used to simulate the nanofluid flow in solar collectors. The physical justification to use fractional calculus in the mathematical modeling of solar collectors is discussed from the viscoelastic mechanical viewpoint, which is a new research direction. A thorough novel classification of the nonlocal rheological and heat constitutive equations used to account for the anomalous mass diffusion and heat transfer arising in the nanofluid flow is presented. The existing nonlocal nanofluid models widely used are summarised and new insight into a unifying generalised theory for the underlying constitutive laws for heat and mass transfer is provided. These results provide valuable theoretical guidance for future research on harvesting solar energy to achieve Sustainable Development Goals (SDGs) 7.