Convection in volumetrically absorbing solar thermal receivers: A theoretical study

Abstract

While now considered a proven technology, molten salt concentrated solar power systems with tubular surface absorbers are still limited by low capture efficiencies, which contribute to higher levelized costs of electricity. Volumetric receivers have long been proposed as promising alternatives to surface absorbers. They consist of semi-transparent media that are directly irradiated and absorb solar radiation volumetrically. They can tolerate higher solar fluxes and eliminate temperature differences between the absorber and the heat transfer fluid, which in turn reduces heat losses. Until now, no studies have quantified the theoretical limits of the performance of volumetric absorbers due to the complex interaction between radiation, natural convection, and volumetric heating. We present a mechanistic model of liquid-based volumetric receivers for solar thermal energy conversion. The model is benchmarked against computational fluid dynamics simulations with good agreement (<1.7°C difference for bulk temperatures and <9°C for boundary temperatures). The model is applied to study the theoretical limits of the capture efficiency and temperature uniformity of a molten salt nanofluid receiver for a range of design parameters. Three characteristic regimes are identified: single-mixing-layer, triple-layer, and conduction-dominant. We propose a dimensionless volumetric receiver number, RV, that captures the regime transitions for a wide range of operating conditions. The results are summarized in the form of design diagrams, providing guidelines for optimal receiver design. The model’s fast (real-time) computation speed can enable digital twinning for volumetric receivers.