Parameters influencing the performance of nanoparticles-laden fluid-based solar thermal collectors: A review on optical properties

The continuous and increasing usage of fossil fuels is adversely impacting the environment and also presents economical and geo-political challenges for policy makers around the world. Solar energy is one of the forms of renewable energy resources which is freely and abundantly available on the earths' surface and hence could be a very good potential alternative to fossil fuels. Apart from directly generating electric power (using solar photovoltaic cells), more and more attention is being paid to the use of solar thermal collectors for heating various fluids for applications ranging from space heating/cooling, process heating, steam generation etc. Such solar thermal collectors are generally are of conventional type, having a 'black' surface-based absorption of solar irradiation. In order to improve the overall efficiency of such collectors, a new type of direct absorption solar collectors (DASC) have been proposed recently. In DASC, a nano-sized particle are added to the heat transfer fluid and due to these particles the optical and thermo-physical properties of the fluid improves, and it is able to directly absorb the solar irradiation, eliminating the need of a conventional 'black' surface. This review paper presents a state-of-art of such nanoparticles-laden fluid-based solar thermal collectors, and focuses on factors like volume fraction, material and shape of the nanoparticle and mass flow rate of the fluid which affects the performance of the nanoparticles-laden fluid-based-solar thermal collector. In particular, special attention has been given to the studies involving the effect of using nanoparticles of various classes of materials – metallic, core/shell, graphite – on the overall optical properties of the working fluid (nanoparticles-laden fluids).