Effect of electrostatic stabilization on thermal radiation transfer in nanosuspensions: Photo-thermal energy conversion applications

Solar thermal collectors are among the most important photo-thermal energy conversion systems. Effectiveness of these systems is measured by the ability of working fluid to absorb incident radiative energy. Although nanosuspensions are considered very promising for this purpose, there is a concern about their stability and their long-term use. Electrostatic and steric stabilization methods are among the two approaches used for colloidal suspensions. In thermal applications, electrostatic stabilization is usually preferred; especially in high temperature applications. The aim of this study is to investigate, both experimentally and numerically, the effect of electrostatic stabilization on the thermal radiation transfer mechanisms in TiO2 and Al2O3 nanosuspensions. The experimental section covers nanosuspensions preparation and characterization, where the effects of electrostatic stabilization (pH and zeta potential values) on the increasing effective particle size due to agglomeration behaviour are explored. The numerical part covers the estimation of radiative properties and thermal radiation transfer based on the average particle agglomerate size obtained from the particle size distributions in the experimental part. The radiative properties are assessed using the single scattering approximation technique based on the Lorenz-Mie theory. The thermal radiation transfer is obtained by solving the radiative transfer equation by the discrete ordinate method. The results show remarkable stability behaviour under the effect of the pH value for the two nanosuspensions types. The effect of the different particle agglomerate size shows a considerable enhancement in the radiative properties specifically in the UV/Vis spectrum, which has a significant impact on the thermal radiative transfer phenomena, due to the solar spectrum. It is also shown that nanosuspensions with different particle agglomerate sizes have a significant effect on the volumetric radiative heat flux, where the radiative energy losses decrease in comparison to those of pure water.