Thermal-fluid characterization and performance enhancement of direct absorption molten salt solar receivers

Abstract

This thesis presents an in-depth thermal-fluid analysis of direct absorption molten salt solar receivers. In this receiver concept, an open tank of semi-transparent liquid is directly irradiated with concentrated sunlight, where it is absorbed volumetrically and produces internal heat generation. The intensity distribution of the internal heating depends on the optical properties of the absorber liquid and the dimensions of the receiver. This heating results in a combination of thermal stratification and radiation-induced natural convection in the receiver, which govern the general thermal-fluid behavior and performance of the system. Direct absorption requires molten salts to be contained in open tanks directly exposed to the environment; consequently, the liquid absorber experiences thermal losses to the environment which reduces absorption efficiency and produces large temperature gradients immediately below the exposed liquid surface. The thesis presents an apparatus that allows for the precise measurement of light attenuation in high temperature, nearly transparent liquids. The apparatus is used to measure and characterize the absorption properties of the 40 wt. % KNO₃:60 wt. % NaNO₃ binary nitrate and the 50 wt. % KCl:50 wt. % NaCl binary chloride molten salt mixtures. The analytical model of the thermal stratification, radiation-induced convection, and radiative cooling effects highlights the key parameters and conditions for optimizing the thermal-fluid performance of the receiver. Computational fluid dynamics and heat transfer modeling of the CSPonD Demonstration prototype of a direct absorption molten salt solar receiver provide further insight into its performance. The findings from the analytical and computational analyses give motivation to create a new cover design for open tanks of molten salts consisting of floating hollow fused silica spheres. The cover concept is demonstrated experimentally and the analysis shows the cover's ability to reduce thermal losses by 50%.