A Review of Convective Loss Data From Solar Central Receivers

Abstract

A summary of previous experimental evaluations to determine convective losses from solar central receivers is given. In this review studies performed directly on actual central receivers or simulations that closely approximate central receiver geometries are considered. Because of the diversity of the previous studies, there are a number of classifications of the studies that can be made. From one perspective, two types of evaluations have been carried out: either those using performance data (often called efficiency tests), or those configuring the receiver so that losses can be more directly inferred (loss tests). In addition to classifying the experimental configurations according to geometry type (either external or cavity forms), it is also beneficial to distinguish between evaluations that take place when normal solar flux levels are incident upon the receiver (flux-on tests) and those made with no flux present (flux-off evaluations). Previous tests are categorized according to the method of testing used. Special attention is given to the range of the physical parameters represented by the experiments, particularly the Grashof number. Too often sufficient information to characterize forced convection contributions is not available, but it is shown that the total convective loss sometimes is lower than the predictions for natural convection alone. It is shown that performance-based data does not correspond well with loss-based data. The effects of directional effects of forced convection and general aspects of mixed (natural and forced) convection are not well established in the existing data. There have been some flux-on techniques recommended recently that may overcome some of the previously established concerns about the accuracy of flux-on data.