Radiation-induced natural convection in volumetrically absorbing solar thermal receivers: An experimental study

Abstract

Volumetric solar thermal receivers are composed of semi-transparent media that are irradiated from the top and absorb solar radiation directly. Previous theoretical studies have suggested that these receivers can capture a high percentage of the incoming energy by eliminating temperature differences between the absorber and the heat transfer fluid. However, the complex interaction between radiation-induced natural convection and volumetric heating which governs the receiver thermofluid behavior has never been experimentally investigated. We present a comprehensive experimental study in which these interactions are investigated from a design perspective. A 6.5 kW solar simulator is installed in a beam-down configuration, and molten nitrate salts are used as base fluids to replicate real-life conditions. Key receiver parameters, namely the fluid absorption coefficient, receiver height, surface emissivity, input flux and heating time are varied experimentally to investigate their influence on operating regime transitions and capture efficiency. Receivers dominated by natural convection are shown to achieve capture efficiencies up to 20 times higher than those dominated by conduction. The ${\chi }^2$ goodness-of-fit test is employed to demonstrate that theoretical predictions regarding the receiver physics are reasonably accurate and existing models can be effectively used for design optimization. Finally, a proof of concept for chloride salts-based high-temperature volumetric receivers is presented. The design insights obtained from the experiments are summarized as design guidelines for future commercial scale-up.