Experimental justification of poor thermal and flow performance of solar chimney by an innovative indoor experimental setup

Abstract

Power generation of solar chimneys effectively depends on their thermal performance. The manufactured samples, available in the literature, have many functional weaknesses and their obtained output power is much lower than expected. Due to the variation of operating conditions (i.e. solar radiation, wind velocity, ambient temperature etc.) of outdoor experimental setups, the thermal performance and heat transfer coefficient cannot be properly evaluated for those systems. Herein, a new indoor test setup is employed to study the steady-state thermal performance of the solar chimney collector while the effects of environmental changes on the tests are minimized. The flexibility of the custom-designed experimental setup allows the investigation of effects of angle of collector, height of chimney, and the area of collector on the convection heat transfer coefficient, velocity and temperature distribution at various points, and the ratio of energy transferred to that introduced to the collector (RET). Our analyses show that the poor performance of common solar chimneys can be justified by existence of secondary flows beneath the collector. The results are important in designing the next generation of solar chimneys with optimized geometry and enhanced thermal efficiency. Results show that, high thermal and frictional losses, due to formation of secondary flows, cause less than 9% of the input energy to enter the chimney and generate power. The collector angle is known the most responsible parameter for controlling formation of the secondary flows. For the divergent collector, increasing the collector angle from 0 to 14 reduces the thermal losses by 1.72% and enhances the heat transfer coefficient by 41.6%. On the other hand, higher amounts of secondary flows in convergent collectors, increases thermal losses by 98.8%. Based on the obtained results, increasing the collector area, as commonly suggested by many researches, while increasing the suction velocity, significantly reduces thermal efficiency of the chimney; This issue along with increasing the manufacturing costs, indicate that increasing the area acts marginally and cannot be a good idea to improve chimney performance most of the times.