Thermal efficiency enhancement of mono and hybrid nanofluids in solar thermal applications – A review

Everywhere throughout the world, primarily in all industrial sectors, there is a tremendous need for energy. The only option to meet the energy demand is via fossil fuels. Global warming and environmental pollution are caused by the usage of fossil fuels and the fast expansion of industry. In addition, relying on renewable energy sources is vital due to the finite availability of fossil fuels. The most promising renewable energy source in the world is solar energy, which is generally accessible on the surface of the globe. In the solar flat plate collector, solar energy can be transformed into thermal energy. The working fluid employed in the collector is the only factor that influences its thermal efficiency.The majority of research found that increasing the collector's thermal efficiency can be accomplished by substituting high thermal conductivity fluids called nanofluids and hybrid nanofluids for the working fluid. A few decades ago, studies involving nanofluids in solar collectors were carried out. In order to significantly increase the efficiency of solar collectors employing hybrid nanofluids, researchers are currently working on these devices. By replacing one fluid in solar thermal systems with high thermal conductivity fluid, larger effectiveness has been achieved. Nanofluids offer larger thermal conductivity values over their base fluid. By using these nanofluids in solar thermal systems can provide the augmented heat transfer coefficient, effectiveness and thermal performance.Hybrid nanofluids are high thermal conductivity fluids compared to mono-dispersed nanofluids. Usage of hybrid nanofluids enhances the fluid thermal properties in solar thermal systems including the thermal conductivity, density, viscosity and specific heat. The synthesis of hybrid nanoparticles, hybrid nanofluids, characterization, thermophysical characteristics, stability analysis and application of mono and hybrid nanofluids in solar thermal systemsspecifically to flat plate collectoris critically reviewed and presented in this paper.