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 in the cavity and the nature of enveloping surfaces such as glass, heat mirror etc. and compares their performance to channels where energy is supplied through surface heating. Performance characteristics reveal that particularly at high solar concentration ratios, volumetric absorption-based receivers could have 45%–51% higher thermal efficiencies compared to their surface absorption-based counterparts. Further, this could translate into a significant increase (by 15%–18%) in overall energy conversion efficiency of concentrated solar power plants. The parametric study of volumetrically heated cavities (natural convection) considers two orientations of incident flux: (a). cavities heated from side and (b). cavities heated from bottom, and investigates the effect of Rayleigh, Prandtl and Biot numbers, the optical thickness of the fluid, the aspect ratio of the cavity and linearly varying flux. The concept of heatlines has been extended to this study and the following were the key observations of the study: (1). the existence of regimes (conduction and convection-dominated) is identified for both orientations (side and bottom-heating); in convection-dominated regimes the pattern of heatlines begins to resemble that of the streamlines; (2). nature of the flux distribution (uniformly distributed vs linearly varying) at the transmitting wall can significantly impact the temperature distribution and (3). there exist optimum values of optical thickness (for various Rayleigh numbers), which ensure minimum heat loss and maximum useful energy gain in these systems” Overall, through the present work key design and operational parameters have been deciphered while quantifying their impact on the performance of volumetrically heated systems especially pertaining to their role in solar thermal technologies. However, the applications of this work are not limited to solar thermal systems alone. The study of fluid filled enclosures leads us to a better understanding of the environment around us – the natural circulation of the atmosphere enveloping our planet, the circulation of ocean currents, and the movement of the molten core beneath the surface of Earth’s crust, can all be studied by choosing an appropriate system.