Optical properties of nanofluids considering particle size distribution: Experimental and theoretical investigations

Solar-thermal conversion is an effective means to collect solar energy. Nanofluids as thermal absorption media or optical filters have shown great potential for application in direct absorption solar collector or hybrid photovoltaic/thermal (PV/T) collectors. Many researches have demonstrated that nanofluids, by adding certain nanoparticles, can dramatically enhance the radiation absorption property of base liquid. The optical properties of nanofluids could also be significantly affected by the size and distribution of nanoparticles. What's more, due to the high surface energy of nanoparticles, they have a great tendency to collide and aggregate into larger clusters. Obviously there is a close relationship between the aggregations of nanoparticles in nanofluids and their on optical properties. However, a systematic summary and review of those reports is lacked. In this review, firstly, the models describing aggregation processes of nanoparticles, i.e., population balance equations, Monte Carlo simulation and Brownian dynamic simulation, have been systematically introduced. Then, the basic optical models and optical models considering aggregation are summarized. Two possible problems and their solutions in the experimental investigation of optical properties of nanofluids are also discussed. Based on the discussions, the challenges that are expected to limit the predicting accuracy of optical properties of nanofluid are summarized which are supposed to be worth much in-depth study in the future.