First analysis of a novel design of a solar chimney with absorber elements distributed in the air channel

The study of solar chimneys arouses great interest because of the wide variety of its applications, above all passive heating, cooling and ventilating of buildings. Mathematical models to predict the obtainable air flow rates and the conversion efficiency have been developed. Some studies focused on the possibility of increasing the amount of heat transferred to the air, and thus the efficiency, through an increase of the absorbing surface area inside the channel. The present work arises in this research line, and in this study a never investigated novel solar chimney configuration is proposed, with four arrays of cylindrical absorber elements in the air channel. Aim of this system is not only to increase the absorbing surface, but also the convection exchange coefficient between absorber surface and air, thus increasing thermal efficiency compared to a traditional system. A numerical model has been developed to investigate the proposed system, with an accurate approach to the radiative heat transfer exchanges, that have usually been rather neglected in previous studies. Convection heat transfer coefficient have been calculated and simulations have been conducted both in the hypothesis of natural and mixed convection. Head losses have also been taken into account. Thermal efficiency and air flow rate have been calculated for different values of the solar height. Results show that in the investigated configuration (a direct power equal to 669.1 W has been considered) the efficiency is always higher than in the traditional system, reaching values up to 61%. Moreover, efficiency value does not decrease as solar height increases, as it happens in the traditional system, where efficiency maximum value is 38% for a 10 degrees solar altitude. Compared to the traditional system efficiency values, the proposed system values result increased, between a minimum of 47% at a solar altitude equal to 3 3 degrees and a maximum of 128% at a solar altitude equal to 71 degrees. Also airflow rates are higher, from a minimum of 7% to a maximum of 21% respectively for the solar altitude values of 33 degrees and 71 degrees. With reference to the average of the efficiency values (in the ten solar altitude values examined in the range 10 degrees -71 degrees), it has been verified that is 68% higher for the configuration with cylinders than for the traditional one. The average air flow rate is 11% higher.Also the power lost by the glass surface do not depend on solar height, and results much lower in the proposed system than in the traditional; the power loss due to the direct power on the inlet opening in the proposed system is lower than in the traditional system, especially for high solar altitudes, having a value up to 135 W, while in the traditional system it reaches 280 W. Moreover, the proposed system efficiency values in several cases results also higher than those found with solutions presented in other studies to increase the absorbing surface.Results therefore show considerable advantages in adopting a novel solar chimney system such as the one proposed in the present work, and its performance appears to be worthy of further investigation, focusing on convection mode and on the optimization of the arrangement of the cylinders (position, number of arrays,..) with respect to the solar height.