Natural convection heat loss inevitably occurs in cavity-type receivers in high concentrating solar dishes, downward focusing systems and solar towers. In most applications, it can contribute a significant fraction of total energy loss, and hence it is an important determining factor in system performance. To investigate natural convection losses from cavity type receivers, an electrically heated model receiver, was tested at inclinations varying from −90 deg (cavity facing up) to 90 deg (cavity facing straight down), with test temperatures ranging from 450 to 650 deg C. Ratios of the aperture diameter to cavity diameter of 0.5, 0.6, 0.75, 0.85 and 1.0, were used. In addition to measurements of overall heat loss, the Synthetic Schlieren technique was used to visualize the flow pattern out of the cavity. Numerical modeling of the convection losses from the cavity was carried out for positive angles with the commercial computational fluid dynamics software package, Fluent 6.0. Good agreement was found between the numerical flow patterns at the aperture region with the schlieren images and between measured and predicted values for heat loss. Of the previously published work that has been reviewed, a model proposed by Clausing, A. M., 1981, “An Analysis of Convective Losses from Cavity Solar Central Receivers,” Sol. Energy 27 (4) pp. 295–300 shows the closest prediction to both numerical and experimental results for downward facing cavities despite its original use for bigger-scale central receivers.

Natural convection inevitably occurs in solar thermal systems, e.g., open cavity receivers in high concentrating solar dishes and solar towers. In most applications, it can contribute a significant fraction of total energy loss, and hence it is an important determining factor in system performance. The present study investigates both experimentally and numerically the natural convection loss from the aperture of open cavity receivers used in paraboloidal dish concentrators. The aim of this study is to quantitatively determine the relationship between convection loss to various pertinent parameters such as ambient temperature, operating temperature, cavity inclination and cavity geometry. The experimental investigation was based on an isothermal electrically heated model receiver, with a test temperature of 450°C. The model receiver is tested at inclinations varying from 0° (cavity facing the side) to 90° (cavity facing straight down). With the numerical work the commercial CFD software package, Fluent 6.0 was used to model cavity convection losses. The range of input parameters used is similar to those of the experimental model. In addition, two cases of full-scaled receivers currently used in ANU dishes are considered. The accuracy concerning grid structure and grid resolution of the numerical model is also discussed. The numerical results obtained are qualitatively in good agreement with those predicted by various previously proposed correlations. The Clausing model [1981] shows the closest prediction to both numerical and experimental results despite its original use for bigger-scale central receivers.