Parametric analysis of empirical parameters and solar radiation uniformity on the thermal performance of porous volumetric solar receivers

Volume-averaging method constitutes an efficacious approach for analyzing and optimizing the overall efficiency of the porous receivers. However, the accuracy in the prediction of heat transfer process is significantly contingent upon the precise assessment of empirical parameters. The comprehensive parametric study of empirical parameters and solar radiation uniformity is conducted by applying volume-averaging method in conjunction with local thermal non-equilibrium model. The typical ranges of the effective thermal conductivity, the convective heat transfer coefficient, and the extinction coefficient for the porous receivers are considered. The incident solar irradiation with uniform to extremely non-uniform profiles is considered. The findings indicate that the empirical parameters influence the solar energy propagation and coupled energy transfer process which causes large deviation in the prediction of receiver thermal performance. The discrepancy between upper and lower limits of outlet air temperatures, along with the maximum error in predicting the receiver's conversion efficiency, amounts to 62.0 K and 14.3 %, respectively, due to variations in the selection of empirical parameters. The enhanced heat conduction and convection are beneficial of improving the thermal efficiency by limiting the thermal radiative loss. However, the optimal extinction coefficient is determined by balancing solar radiation volumetric absorption against transmission loss, particularly in receivers with smaller thicknesses. The non-homogeneous incident solar radiation results in elevated maximum receiver temperature, evident temperature gradient and thermal efficiency deterioration by 5.9 %, which damages the long-term and safe operation of the receiver.