Combined heat loss analysis of solar parabolic dish - modified cavity receiver for superheated steam generation

In this article, a 3-D numerical modeling is carried out to determine combined convection and surface radiation heat losses from a modified cavity receiver of parabolic dish collector used as mono-tube boiler for sub-cooled, saturated and superheated steam generation conditions. The forced convection heat loss from the modified cavity receiver is estimated using Nusselt number correlation developed for the modified cavity receiver. The effect of receiver inclination (β), operating temperature (Tw), emissivity of the cavity cover (∊), thickness of insulation (tins) on the combined heat losses from the modified cavity receiver is investigated. The boundary conditions for wall temperature and insulation thicknesses are chosen to match the three steam generation conditions. It is found that the natural convection heat losses are higher at β=0° (receiver facing sideward) and lower at β=90° (receiver facing down) whereas the forced convection heat loss is higher at β=90° and lower at β=0°. The variation of radiation heat losses is marginal for all values of β and vary with Tw. The effect of various parameters such as receiver inclination, wind direction (φ), wind speed and diameter ratios on forced convection heat loss from the receiver has also been studied. The forced convection heat loss at lower wind speeds (<2.5 m/s) is lower than the natural convection heat losses. The maximum heat losses occur at side-on wind direction (φ=0) followed by head-on wind directions (φ = 30-90°) and back-on wind directions (φ=-90° to -30°). The heat losses vary with diameter ratios for different configurations of the receiver. The forced convection heat loss is 1.2-9 times higher than natural convection heat loss for diameter ratio (ratio of cavity diameter to aperture diameter, d/D) = 0.4 at 5 m/s and receiver inclinations varying from 0° to 90°. Nusselt number correlations have been proposed based on the numerical analysis to estimate the combined convective and radiative heat loss. The present study attempts to estimate natural convection, forced convection and surface radiation heat losses from the modified cavity receiver under various conditions.