Simulation and experimental study of an air tube-cavity solar receiver

High temperature air is a potential candidate as a heat transfer fluid to transport energy from concentrated solar power to gas turbines. A 15-turn helically coiled tube cavity receiver with an optical splitter at the bottom is designed and fabricated. Its performance is investigated with a five 7-kW Xe-arc lamps array system as heat source. Eight K-type thermocouples are placed from top to bottom with an equal interval. The outlet temperature experimentally ranges from 593 degrees C to 546 degrees C when the air flow rate increases from 1 m(3)/h to 5 m(3)/h for up-flows, while it ranges from 662 degrees C to 570 degrees C for down-flows, when the average flux on aperture is around 120 kW/m(2). The Monte-Carlo ray-tracing method and the Lambert testing method with a charge-coupled device (CCD) camera are used to simulate and evaluate the concentrating radiation energy distribution on the cavity's internal walls, and then the actual flux distribution of each turn of the helically coiled tube is obtained. A comprehensive simulation model is proposed and validated by the experimental results, where the outlet temperature deviations are within 8.0% and 2.5% for down and up-flows, respectively. The model provides a detailed analysis of heat flows at different conditions, and indicates optimization ways to improve the efficiency and reduce heat losses. The simulation results show that the outlet temperature can increase up to around 800 degrees C at 5 m(3)/h under an average flux of 300 kW/m(2), and the thermal efficiency can be improved from around 56% to around 64% by decreasing the inner radius from 6 mm to 4 mm at the expense of increasing pressure drop of around 56 kPa. (C) 2015 Elsevier Ltd. All rights reserved.