Morphological comparative assessment of a rooftop solar chimney through numerical modeling

Abstract

An assessment on the performance of three different configurations at the outlet sections of a rooftop solar chimney is conducted in this work. The starting configuration corresponds to a representative dwelling-solar chimney system. On the basis of the knowledge reached in the technical bibliography, a numerical modeling is developed, considering both the wind-driving and the buoyancy-driving forces. The turbulent nature of the airflow is simulated through an enough validated two-transport equations model, including a low-Reynolds treatment at walls. Special care is taken in the numerical simulation of the wind, implementing a logarithmic speed profile to properly introduce the atmospheric boundary layer. At walls, a heat-flux heating condition (q uniform) is imposed in order to realistically simulate the heating from irradiation. Focus is posed on a comparative and systematic study of the behavior of airflow through the three considered morphologies, free outlet, covered outlet and side outlet. The more relevant differences found in the performance of each constructive shape are highlighted, both for near-calm as well as for wind-dominant conditions. A wide range of heating conditions are considered, 0.5≤q≤1000 W/m2 (corresponding to Rayleigh numbers from 1.085×1010 to 2.170×1013), as well as reference wind velocities from windless condition up to 10 m/s. Several correlations for the air ventilation rate and the dimensionless heat transfer coefficient are proposed, oriented to engineering practical applications. Wind reveals as dominant from reference velocities in the range 2-3 m/s. The conditions under which a certain better performance wall-to-wall spacing between walls forming the solar tower can be encountered, are analyzed and discussed. Illustratively, in some cases, the maximum value of the average Nusselt number reaches twice the value corresponding to aspect ratio equal to unity.