Experimental analysis of heat loss of a non – evacuated parabolic trough collector receiver subjected to uniform and non-uniform wall heat flux condition

Abstract

ABSTRACT The parameters that largely influence the design and performance of a parabolic trough collector system for delivering process heat are optical efficiency, incidence angle modifier, solar irradiation data, and receiver heat loss. The heat loss and optical properties of a receiver greatly influence the overall efficiency of the solar thermal system. This paper focuses the attention on the experimental investigation of heat loss associated with a non-evacuated receiver subjected to i). Uniform wall heat flux (UWHF) and ii). Non – uniform wall heat flux (NUWHF) boundary condition. A series of heat-loss measurements are carried out in the novel test stand capable of emulating both the boundary conditions, on the indigenous non – evacuated receiver developed for moderate temperature process heat applications. A comparative analysis of natural and forced convection heat loss associated with the bare and glass-covered receiver is performed. The results show that the non – uniform wall heat flux boundary condition has a decisive influence over the forced convection heat loss, especially at higher wind velocities. The work concludes that i) the surface temperature of the receiver associated with natural convection heat loss and subjected to NUWHF boundary condition is1.5– 5% higher than the receiver subjected to UWHF boundary condition. ii) In forced convection mode of heat loss the surface temperature of the receiver is 4–13% higher than the receiver subjected to UWHF. Thus, a more realistic characterization of the thermal behaviour of a non-evacuated receiver for moderate temperature process heat application is possible only by investigating the receiver subjected to non – uniform heat flux distribution. Such a characterization of the receiver will considerably influence the aperture area and thus the cost of the collector to deliver process heat.