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In recent years, solar thermal energy (STE) has attracted energy researchers because of its higher efficacy compared to the photovoltaic solar cell. STE is one of the forms of solar energy whereby heat is transferred via a secondary medium called heat transfer fluids (HTFs). Therefore, the overall performance of STE depends on the thermophysical properties and thermal performance of the HTFs. Traditional HTFs suffer from low decomposition temperature, high melting point, and higher vapor pressure. To overcome these limitations, researchers have recently begun working on new HTFs for STE. Ionic liquids (ILs) are considered as a potential candidate for the next generation of HTFs because of their enhanced thermophysical properties, such as thermal stability at high temperature, insignificant vapor pressure, and high ionic conductivity. In addition, thermophysical properties and thermal performance of ILs can be further enhanced by dispersing nanoparticles, which is one of the emerging research interests to improve the efficiency of the solar thermal system. This paper summarizes the recent study of ILs-based nanofluids as HTFs. These summaries are divided into two sections (i) thermophysical properties studies, such as density, viscosity, thermal conductivity, and heat capacity, and (ii) thermal performance studies such as natural convection and forced convection. Synthesis of ILs-based nanofluids and thermophysical properties measurement techniques are also discussed. Based on these state-of-the-art summaries, we offer recommendations for potential future research direction for ILs-based nanofluids.

Ionic liquids (ILs) are considered a potential candidate for a heat transfer fluid (HTF) in concentrated solar power (CSP) applications. There are already many CSP sites in operation throughout the world. These complex energy systems use various subsystems such as mirrors and lenses to concentrate solar energy onto a central collector. These CSP sites rely on having a stable HTF in order to maintain high energy storage capacity and to reduce costs. This research seeks to develop a robust set of workable data that can be used to better understand the nanoparticles shape effect on viscosity and thermal conductivity of ionic liquids (ILs) based nanofluids. ILs based nanofluids were prepared by pouring 1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([C4mim][NTf2]) base IL and Al2O3 nanoparticles. Three different particle shapes (platelets, blades, and spherical) were used to prepare the 1 wt% ILs based nanofluids. Experimental results shows that the needleshaped nanoparticle provided the greatest effective thermal conductivity compared to the base ILs.

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