Effect of Boundary Condition on Heat Losses From Solar Cavity Receiver

Abstract

The solar cavity receivers used for high temperature applications were generally modeled as plain walls with isothermal boundary conditions, where radiation and conduction was observed to be constant and independent of cavity inclination. The recent experimental investigations on model receiver have studied the heat loss from a cylindrical cavity receiver, subjected to a range of constant heat flux boundary conditions. In constant heat flux boundary condition, the temperature distribution along the cavity walls is seen to vary, and is a function of cavity inclination. This paper presents the effect of boundary condition on heat loss through solar cavity receiver. Paper presents mathematical analysis of heat losses from model cylindrical cavity receiver subjected to isothermal wall condition and compare the heat loss from wall condition subjected to a range of constant heat flux boundary conditions. The convection heat loss is maximum at zero inclination and decreases greatly with the cavity inclination in the order of second degree of polynomial in constant heat flux. The conduction heat transfer increases linearly with the inclination. Whereas the radiation heat loss increases with cavity inclination, in the order of second degree of polynomial and the rate of radiation heat loss decreases with increase of cavity inclination. The convection heat loss varies exponentially with temperature in case of isothermal boundary condition. The radiation and conduction losses are not constant as initially estimated for isothermal boundary; they increase with increase in cavity inclination. It led us to conclude that it may not be sufficiently accurate to predict convection heat loss using previously developed correlations based on the isothermal boundary wall condition. Secondly, even though the variation in radiation and conduction heat losses with cavity inclination is small, it needs to be considered for accurate design of solar parabolic dish receiver system.