Nanofluids in solar collectors: a comprehensive review focused on its sedimentation

The use of nanofluids as working fluids in energy systems, such as solar collectors, natural circulation loops, geothermal plants, and nuclear reactors, has the potential to enhance their efficiency. Nanofluids have improved optical and thermal properties, enabling better energetic performance than common working fluids. However, the stability of nanofluids depends on multiple factors that can cause nanoparticles to sediment in the base fluid, leading to the degradation of physical properties that affect system performance. Experimental, analytical, and computational-based approaches have been used to study the sedimentation process in nanofluids. This review presents a comprehensive comparison of these research methods, with a particular focus on the role of gravitational sedimentation when nanofluids are used in solar collectors. Sedimentation remains the primary limitation of the engineering application of nanofluids. Some key factors that affect this process, such as volumetric concentration, flow regime, and additives, have been addressed to solve the sedimentation problem. However, other factors that can influence sedimentation in solar collectors, such as thermal cycling, ultraviolet radiation, and rest periods, remain open problems that require extensive investigation in the future.