ALBERT

Description: 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.

Description: During the summer and fall of 1987 in Almeria, Spain, a wire-pack receiver was tested by the International Energy Agency/Small Solar Power Systems (IEA/SSPS). The basic operation of the receiver is that: air is drawn through several layers of stainless steel wire screen; concentrated solar flux is directed on the face of the screen pack; the oxidized wires absorb the solar energy; and heat is transferred to the air flowing through the screen. Although the experiment goal was strictly proof-of-concept and was not receiver characterization, modeling efforts were initiated to help understand the experimental results. The steady-state performance of the receiver is modeled using the fact that the net solar and infrared radiative energy absorbed by each screen layer must be transferred to the air by convection. Basic performance trends and typical calculations of receiver efficiency are given. Model predictions and experimentally measured temperatures and flow rates are compared. Model predictions of receiver power and efficiency are generally higher than the test results (operational modifications of the receiver absorber as tested are believed to have produced nonideal conditions), but trends are consistent with experimental data.