Conjugate radiation-convection-conduction simulation of cubic lattice solar receiver with high porosity for high-temperature heat absorption

Direct simulation was performed for the solar porous receiver having cubic lattice as elementary structure with high porosity (0.80 - 0.98). The cell size was changed in four steps from 0.49 mm to 2.94 mm at three levels of the power over air mass (POM). Silicon carbide (SiC) was assumed as receiver material, and the absorptivity of the wall surface was assigned 0.9. The numerical data for the porous receiver was compared with the honeycomb receiver with low porosity. Such comparison proved the superiority of the highly porous receiver: the receiver-exit temperature and the receiver efficiency surpass those of honeycomb receiver at the same level of POM. Regarding the size effects, the performance of the porous receiver improves as the cell size reduces. When the cell size is 0.98 mm at POM =1000 kJ/kg, the receiver exit temperature increases beyond 1000 K and the receiver efficiency 0.8. When the cell size is 0.49 mm at POM = 2000 kJ/kg, the receiver exit temperature exceeds 1700 K and the efficiency 0.8. The numerical results thus demonstrated that the smaller cell size maintains the efficiency at the higher levels of POM. The mechanism for superiority of the small cell size was investigated through analysis of the heat transfer loss. This investigation revealed that the convective heat transfer enhancement in the case of small cell size decreases the temperature in the inlet region and attenuates the thermal radiation loss. Therefore, the high performance of the small-cell size receiver is attributed to the cooling effect of the air stream with high heat transfer coefficient.