Heat transfer and thermal stress of a parabolic trough solar collector with secondary reflector using microencapsulated phase change material slurries

The traditional parabolic trough solar collectors (PTSC) often face the problems of high thermal stress and the deformation of the tubular receiver. In this paper, a secondary reflector (SR) is put into the traditional PTSC, and the microencapsulated phase change material (MEPCM) slurries with 7vol%, 12vol%, and 15vol% are used as the operating fluids. By associating the Monte Carlo ray-tracing (MCRT), finite volume (FVM), and finite element (FEM) methods, the optical-thermal-mechanical coupling problem of the absorber tube is settled, the flow and heat transmission of MEPCM slurries in the collector tube are numerically investigated by applying the Eulerian-Eulerian multiphase flow model. The results show that the solar flux and wall temperature of the absorber tube with the SR are more uniform along the circumferential direction. The use of SR enhances the local heat transfer of the MEPCM slurries at the phase change section in the absorber tube, while when the phase changes finish, the local heat transfers of the slurries in the PTSC with SR have no difference with the traditional PTSC. Compared with the water, the MEPCM slurries obviously enhance the heat transfer of the absorber tube due to the phase change and the interactive motion of the microencapsulated particle, and the higher the slurry concentration, the larger the mean Nusselt number, the smaller the thermal stress. The addition of SR significantly decreases the thermal stress of the absorber tube, the maximal equivalent stress is reduced by 81.8%