Thermal performance enhancement of flat-plate and evacuated tube solar collectors using nanofluid: A review

During the past decades, the technology to make particles in nanometer dimensions has been improved and a new kind of solid–liquid mixture, which is called a nanofluid, has appeared. Nanofluids are an advanced kind of fluid containing a small quantity of nanoparticles (usually less than 100 nm) that are uniformly and stably suspended in the liquid. The dispersion of a small amount of solid nanoparticles in conventional fluids such as water or ethylene glycol changes their thermal conductivity remarkably. Since then, nanofluids have been applied to enhance the thermal performance of many engineering systems. Recently, nanofluids have been used as heat transfer fluids to enhance the performance of solar collector devices. This paper reviews the recent progress and applications of nanofluids in flat-plate and evacuated tube solar collectors. Other than to review the efficiency of solar collectors with nanofluids, the paper also discusses the impact of nanofluids in solar collectors on economic and environmental viewpoints. Finally, the challenges and future trends in the application of nanofluids in thermal solar collectors are discussed.