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Solar energy remains the most ubiquitous and inexhaustible source of energy. This energy can be utilized by several approaches including the use of solar collectors. Several studies have illustrated that the efficiency of solar collectors can be significantly improved by the introduction of nanofluids which have shown improved thermal conductivity up to 160% with a subsequent reduction in greenhouse gases such as CO2. To produce nanofluids, nanoparticles such as Al2O3, hybrid ZnO + Al2O3, and metals (Al, Cu) particles are dispersed into the based fluids such as water, glycerol, and bio-fluids. The added nanoparticles enhance the viscosity, absorption rate, convective heat transfer coefficient and heat losses of the fluid. The performance of details the applications and effectiveness of different nanofluids in four types of solar collectors - parabolic trough, flat plate, direct absorption, and evacuated tube. In addition, the work sheds light on the future trend and challenges of nanofluids (including toxicity) in solar collectors. Regardless of its toxicity, researchers have shown more interest in nanofluids use in solar collectors because of its strong sustainability to a safe environment and the exploration of hybrid nanofluids to better enhance solar collectors. Solar collectors can also be modulated by using different nanofluids at varying concentrations.

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