Transient natural convection induced by the absorption of concentrated solar radiation in high temperature molten salts

Abstract

Solar-thermal energy systems that involve the deposition of radiation in absorbing high temperature molten salts to harness the entire solar spectrum and achieve high efficiencies and low Levelised Cost Of Energy (LCOE) are of considerable interest for power generation. From a design stand point, to achieve a competitive solar power generation devices, it is imperative to have an accurate knowledge of the inherent physical processes of such a fluid system. Thus under high temperature conditions, detailed understanding of the heat transfer and fluid flow characteristics in an irradiated fluid is considered. The work investigates the spectral dependent heat transfer and fluid dynamics in a thermal storage concept which uniquely combines a volumetric receiver and a single tank thermal store. The Thermal Energy Storage (TES) is protypical of a small scale concept concentrated solar plant. Advances in computing power, has seen Computational Fluid Dynamics(CFD) consolidated as a powerful tool employed by researchers and engineers to simulate real world behaviour and complex phenomena to a certain degree of accuracy with low effort in time, personnel and resources. This thesis is focused on the development of a realistic numerical model capable of predicting the local volumetric absorption of solar radiation in a fluid layer which provides an improved understanding of the hydrodynamic and thermal conditions in an enclosed fluid layer. Computational Fluid Dynamics is used to simulate the transient heat transfer and fluid flow determined by a combined influence of volumetric absorption and natural convection in a high temperature fluid filled enclosure. The enclosure is studied for the specific case in which a high temperature salt is first heated by direct volumetric absorption of the incident solar radiation and secondly by natural convection from a absorber plate located at the bottom of the enclosure whose sole purpose is to absorb all non-absorbed radiation reaching the lower surface. The current models considers the depth dependence absorption of solar radiation based on (i) a solar weighted absorption coe cient (assumed constant over all wavelengths) and (ii) spectral absorption coe cient characterised by wavelength band based on a standard solar spectrum reference. A commercially available CFD Package based on Finite Element Method (FEM), COMSOL Multiphysics is used to discretise and solve the Navier Stokes and energy equation under transients heating conditions for a non Boussinesq condition by accounting for the temperature variable properties of molten salts. A time-dependent and Backward Differentiation Formula (BDF) solver using implicit time-stepping methods is combined with refined mesh to solve the non-linear PDE. Validity of the numerical tool has been conducted, by comparing results from published results found in literature with corresponding numerical results. The mesh element optimum sizes and time steps used conform to those obtained in validation models. Simulations have been conducted for a daily charging period of three hours as used in conjunction with a solar system. The effects of bottom absorber plate, flux Rayleigh number, aspect ratio, variable Air Mass and inclination angle have also been investigated. Numerical results are presented in terms of surface plots, temperature contours, and velocity contours and streamlines which show the thermal field distribution and flow structures, for volumetric absorption of thermal radiation coupled with natural convection. Performance criteria are based on quantification of the level of thermal stratification using the MIX number, the dimensionless exergy and capture efficiency. Three dimensionality effects were studied by considering three dimensional simulation for the same problem. The results show that the present method and models are capable of capturing the main features of the flow and the overall performance of these turbulence models in terms of predicting time-averaged quantities. Results obtained indicate a nonlinear temperature profile consisting of two distinct layers: a surface layer and a bottom layer. The numerical results reveal natural convection in the cavity follows an initial stage, a transitional stage and a quasi-steady stage. Results indicate that volumetric absorption of solar radiation, when coupled to natural convection has a direct influence on the thermal field through the disparities in absorption and emission phenomena. The isotherms and streamlines show that the natural convective heat transfer and flow are quite different from those obtained in differentially heat enclosures. Thus the heat transfer mechanism destroys a symmetry of the system that relates clockwise and counter clockwise flows. Temperature and flow field are found to be greatly influenced by the aspect ratio (H/D) of the store and the flux Rayleigh number. It is found that the predicted heat transfer from the lower surface in the cavity is increased when the simulation is extended from two to three dimensional. Results obtained indicated that increasing the aspect ratio, Air Mass and inclination angle all result in increasing levels of thermal stratification. Natural convection from the lower absorber surface is found to increase with increasing

flux Rayleigh number.