The field of nanofluids has received interesting attention since the concept of dispersing nanoscaled particles into a fluid was first introduced in the later part of the twentieth century. This is evident from the increased number of studies related to nanofluids published annually. The increasing attention on nanofluids is primarily due to their enhanced thermophysical properties and their ability to be incorporated into a wide range of thermal applications ranging from enhancing the effectiveness of heat exchangers used in industries to solar energy harvesting for renewable energy production. Owing to the increasing number of studies relating to nanofluids, there is a need for a holistic review of the progress and steps taken in 2019 concerning their application in heat transfer devices. This review takes a retrospective look at the year 2019 by reviewing the progress made in the area of nanofluids preparation and the applications of nanofluids in various heat transfer devices such as solar collectors, heat exchangers, refrigeration systems, radiators, thermal storage systems and electronic cooling. This review aims to update readers on recent progress while also highlighting the challenges and future of nanofluids as the next-generation heat transfer fluids. Finally, a conclusion on the merits and demerits of nanofluids is presented along with recommendations for future studies that would mobilise the rapid commercialisation of nanofluids.

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The parabolic trough collector (PTC) is one of the most widely deployed concentrating solar power technology in the world. This study aims at improving the operational efficiency of the commercially available LS-2 solar collector by increasing the convective heat transfer coefficient inside the receiver tube. The two main factors affecting this parameter are the properties of the working fluid and the inner geometry of the receiver tube. An investigation was carried out on six different working fluids: pressurized water, supercritical CO2, Therminol VP-1, and the addition of CuO, Fe3O4, and Al2O3 nanoparticles to Therminol VP-1. Furthermore, the influence of a converging-diverging tube with sine geometry is investigated because this geometry increases the heat transfer surface and enhances turbulent flow within the receiver. The results showed that of all the fluids investigated, the Al2O3/Oil nanofluid provides the best improvement of 0.22% to thermal efficiency, while the modified geometry accounted for a 1.13% increase in efficiency. Other parameters investigated include the exergy efficiency, heat transfer coefficient, outlet temperatures, and pressure drop. The analysis and modeling of a parabolic trough receiver are implemented in engineering equation solver (EES).

Solar thermal collectors are systems that allow for the use of solar energy in thermal applications. These collectors utilize a heat transfer fluid to transport absorbed solar radiation to applications where they are needed. Scientists in a bid to improve the conversion efficiency of solar collectors have suggested different collector designs and improved collector materials. Over the last 25 years, the study of nanofluids and their applications have revolutionized material science, and nanotechnology has found applications in improving solar collector materials. This article reviews the impact of different nanomaterials on the efficiency of solar collectors. The study also outlines the limitations of applying nanofluids and discusses the long-term challenges of their application to solar collectors. Nanofluids have the potential to improve the overall efficiency of most solar collectors, however, the full potential of nanofluids in heat transfer applications cannot be completely achieved until some of the questions regarding hysteresis, stability, and the overall predictability of nanofluids are answered.