Heat Transfer and Fluid Flow Behavior of Nanoparticle Dispersions in Volumetrically Heated Solar Thermal Systems

Abstract

This thesis studies the role that volumetric absorption-based systems can play in efficiency enhancement of incumbent solar thermal technologies. Solar energy is considered as the most abundant renewable energy resource, but it accounts for only ~0.04 % of total energy demand globally due to low efficiencies and high costs compared to fossil-fuel based technologies. Currently, solar thermal systems employ surface absorption-based systems in which the surface gets heated and in turn heats up the working fluid flowing inside it. However, these systems suffer from low efficiencies due to high radiative and convective losses. Volumetric systems in which solar radiation is captured by allowing radiation to transmit through a transparent glass such that it interacts directly with the working fluid promise higher efficiencies as losses from the surface of the receiver are considerably reduced. However, studies catering to the employment of such systems in solar thermal technologies are relatively few. The phenomenon of direct volumetric absorption of radiation by the nanofluid and redistribution of the absorbed energy within the nanofluid has been critically analyzed. In particular, transport phenomena in two different flow situations have been considered: firstly, transport phenomena in channel flow wherein fluid is made to flow through a rectangular channel and at the same time it interacts directly and volumetrically with the incident solar irradiation. Secondly, transport phenomena in cavity flow wherein direct volumetric interaction of the solar irradiation induces fluid flow in the closed cavity. A comprehensive theoretical modeling framework was devised to study these systems and investigate the role of pertinent parameters for both cases (forced and natural convection) strictly in the laminar regime. The study of volumetrically heated channels (forced convection) considers the effect of the Reynolds number of flow, the solar concentration ratio, inlet temperature of the fluid, optical properties of the fluid