Abstract
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Solar energy is assumed to provide an everlasting solution to the modern energy crisis since it is the most abundant source of renewable energy and solar energy harvesting systems do not generate any harm to the environmental stability as well. Current research trends on solar energy are mainly focused on solar thermal energy harvesting systems because of their huge potential for domestic as well as industrial applications. The efficacy of a solar thermal energy harvesting system is dependent on its design aspects (i.e., geometry and material) and the fluid used for photothermal/photovoltaic-thermal conversion and subsequent thermal energy transportation. Considering the crucial role of the working fluid, ample research is underway on the development of new fluids having superior optical and thermophysical characteristics. One of the recently developed thermal fluids called hybrid nanofluids possesses high thermal potential which makes them a suitable candidate for application in solar energy systems. This review article contains a brief discussion on important characteristics of hybrid nanofluids and a summary of the major findings of recent research studies that have tested hybrid nanofluids in/for solar thermal energy harvesting systems. Considering the findings of reviewed articles, the most important aspects of hybrid nanofluids influencing the performance of hybrid nanofluid based solar thermal energy harvesting systems are; optical characteristics, thermal conductivity, viscosity, pressure drop, nanoparticle size, nanoparticle type, nanoparticle shape, nanoparticle concentration, base fluid, nanoparticle suspension uniformity, working temperature and flowrate, compatibility of individual nanoparticles, preparation method, period and power of magnetic stirring and ultrasonication, surfactant, nanoparticle agglomeration/clustering, and sedimentation. Extensive review of literature reveals that the inclusion of hybrid nanofluids can improve the performance of solar energy harvesting systems by two times at optimal operational conditions. The reported results showed that metal oxide and nano carbon-based hybrid nanofluids exhibit higher enhancement in energy efficiencies as compared to other nanofluids under the same conditions. Despite of obvious advantages of using nanofluids in spite of conventional fluids, there are some limitations as well such as clogging in the pipes, and thermal hammering in pipes due to extreme cycles of temperature variation. Moreover, the use nanoparticles can cause some environmental problems such as suspension of nanosized metallic particles in the air that would eventually cause air quality degradation and toxicity in the air. The article eventually presents some important recommendations for future research.