Heat losses in a trapezoidal cavity receiver of a linear Fresnel collector: A CFD approach

This paper presents a thermal study of a cavity receiver designed for a Fresnel-type linear solar collector (LFC). The study utilizes a two-dimensional computational fluid dynamics (CFD) model implemented in ANSYS Fluent. The thermal behavior of air inside the cavity for a Fresnel collector is extensively examined. The receiver consists of a trapezoidal cavity with a set of six parallel absorber tubes, through which a thermal fluid circulates. The cavity has aluminum reflectors on the inner walls and glass window closing the aperture facing the primary reflectors of the solar collector. The two-dimensional numerical model represents a cross-section of the receiver, and aims to provide numerical results that allow to provide algebraic correlations for predicting heat losses in the receiver from the wall temperature of each of the six individual absorber tubes that compose it. The developed model is transient, utilizing the k-& epsilon; turbulent model. In addition, the study is completed with an analysis of the behavior of the air surrounding and inside the cavity, to evaluate its thermal performance. For this purpose, the velocity and temperature contours obtained with the two-dimensional model are discussed. Correlations are obtained to know the heat flux between the tubes and the heat loss through the window for any combination of temperatures for each pair of tubes, which has not been yet studied in the literature. The study reveals that radiative losses contribute to 81% of the total heat losses, with the outer tubes temperature being the main responsible for these losses. Furthermore, a dimensionless analysis examines the relationship between the Nusselt and Rayleigh numbers in comparison to reference problems based on canonical geometries dominated by buoyancy-driven flows. The performance is found to be similar to that of a downward hot flat plate.