Transient numerical study of thermo energetic performance of solar air heating collectors with metallic porous matrix

Abstract

This paper presents the numerical modeling of the thermo-energetic behavior of solar air heaters with a porous matrix to enhance the heat transfer from the absorber plate to the circulating air. The porous matrix was modeled by defining an average strand at which the porosity, the effective cross-sectional area for conduction heat transfer, and the effective lateral area for convection and radiation heat transfer were determined. The pressure drop along the collector was small because the porosity of the matrix was high (97 %). For this collector, the thermohydraulic efficiency reaches a maximum value of 63 % for an air mass flow of 0.06 Kg/s and thereafter decreases as the power consumed by the fan increases. Using data measured during the winter of 2015 in a prototype solar collector, the numerical model was validated. The fit relative error between measured and modeling values was 3 % for the air output temperature and 5 %, on average, for the useful energy gain of the collector. The dependence of the collector heat removal factor, FR, on the porosity was studied through numerical simulation. The results of this study revealed that, as the porosity of the matrix decreases, the FR factor increases. Therefore, in order to maximize the thermal efficiency of the collector, it is recommended to use matrices with a porosity close to 90 %. The thermal efficiency of solar air heaters of double-pass counterflow with a metallic porous matrix with a porosity of 90 % is 20 % higher than that of the same collector without a porous matrix for the same operating parameters. The results of this study are of technological importance for the efficient design of solar air heaters of double-pass with a porous matrix.