DESIGN OF ENHANCED EFFICIENCY ARTIFICIALLY ROUGHENED SOLAR COLLECTOR BASED ON THERMO-HYDRAULIC . CONSIDERATIONS

Abstract

coefficient from absorber plate to the air. This low heat transfer coefficient results in relatively higher absorber plate temperature leading to higher thermal losses to the environment and hence lower thermal efficiency. Artificial roughness in the form of repeated ribs can be employed for the enhancement of heat transfer coefficient between the absorber plate and air and thereby improving the thermal performance of solar air heaters. However, use of artificial roughness also results in an increase in friction factor and hence increased pumping power, thereby, offsetting some of the gain heat transfer. The effective efficiency and cost per unit energy delivered by the system are the useful parameters which takes into account the effect of enhancement in useful energy gain due to higher heat transfer coefficient between absorber plate and air and the increase in pumping power due to higher frictional resistance. The roughness parameters can be optimised on the basis of maximum effective efficiency or minimum cost criterion. In the present work, solar air heater having artificial roughness has been optimised on the basis of maximum effective efficiency and minimum cost with respect to roughness parameters. Two types of roughness geometries have been considered for the analysis, namely, transverse rib roughness and V-shaped rib roughness. In the case of transverse rib roughness the optimisation has been carried out with respect to relative roughness height (e/D), while in the case of V-shaped rib roughness, the relative roughness height (e/D) and angle at attack (a) have been considered. The result of optimisation have been presented in the form of tables. The design tables can be used for selecting optimum roughness parameters for a particular temperature rise parameter and insolation and the corresponding thermal efficiency can be predicted from the plots. Eli