Experimental testing of a tubular cavity receiver for a small scale solar thermal brayton cycle

Abstract

Paper presented to the 3rd Southern African Solar Energy Conference, South Africa, 11-13 May, 2015.The open solar thermal Brayton cycle uses air as working fluid, which makes this cycle very attractive for use in waterscarce countries, such as South Africa. An open-cavity tubular solar receiver may be used in a dish-mounted solar thermal Brayton cycle. This solar receiver has been optimised in recent work using the method of entropy generation minimisation. The performance of the optimised open-cavity tubular receiver can be modeled analytically, but this model should be validated. The purpose of the study was to test the receiver in an experimental setup to validate a heat loss model which considers the errors associated with the collector. A 4.8 m diameter aluminium parabolic dish with 45° rim angle was constructed on a two-axis solar tracking system. An optical error of 5 mrad was measured. The receiver had an aperture area of 0.25 m x 0.25 m and was constructed from 88.9 mm diameter stainless steel 316 tube sections. The solar tracking was done manually to provide a tracking error of less than 1°. A blower was used to blow air through the receiver tube to determine the net heat transfer rate to the air. A hightemperature test was also done wherein the receiver was heated up to steady state to determine the convection heat transfer coefficient. The temperatures on the receiver surface were measured with K-type thermocouples. The maximum temperature reached was around 650 K. It is shown that the insulation arrangement around the receiver tube is important.