Investigations on heat losses from a solar cavity receiver

Thermal as well as optical losses affect the performance of a solar parabolic dish-cavity receiver system. Convective and radiative heat losses form the major constituents of the thermal losses. In this paper, an experimental and numerical study of the steady state convective losses occurring from a downward facing cylindrical cavity receiver of length 0.5 m, internal diameter of 0.3 m and a wind skirt diameter of 0.5 m is carried out. The experiments are conducted for fluid inlet temperatures between 50 degrees C and 75 degrees C and for receiver inclination angles of 0 degrees (side ways facing cavity), 30 degrees, 45 degrees, 60 degrees and 90 degrees (vertically downward facing receiver). The numerical study is performed for fluid inlet temperatures between 50 degrees C and 300 degrees C and receiver inclinations of 0 degrees, 45 degrees and 90 degrees using the Fluent CFD software. The experimental and the numerical convective loss estimations agree reasonably well with a maximum deviation of about 14%. It is found that the convective loss increases with mean receiver temperature and decreases with increase in receiver inclination. Nusselt number correlations are proposed for two receiver fluid inlet temperature ranges, 50-75 degrees C and 100-300 degrees C, based on the experimental and predicted data respectively. Besides no-wind tests, investigations are also carried Out to Study the effects or external wind at two different velocities in two directions (head-on and side-on). The wind induced convective losses are generally higher than the no-wind convective loss (varying between 22% and 75% for 1 m/s wind speed and between 30% and 140% for the 3 m/s wind speed) at all receiver inclination angles, the only exception being the loss due to side-on wind at 0 degrees receiver inclination angle. This is because the wind acts as a barrier at the aperture preventing the hot air to flow out of the receiver. The head-on wind causes higher convective loss than the side-on wind. Nusselt number correlations proposed in this work are compared with the existing correlations in the literature. It is found that the correlations available in literature under-predict the convective losses at mean receiver temperatures between 100 degrees C and 300 degrees C. This is due to the fact that the correlations are developed for certain receiver geometries having the ratio of aperture diameter to receiver diameter equal to or lesser than one. (C) 2008 Elsevier Ltd. All rights reserved.