Prandtl number effect on heatline and entropy generation through direct absorption solar collector

Abstract

Solar energy is often used in applications such as electricity generation, thermal heating and chemical processing due to its renewable and nonpolluting nature. "Flat-plate" type solar heaters are the most cost effective, but these suffer from relatively low efficiency and outlet temperatures. The feasibility of using a direct absorption solar collector (DASC) is analyzed numerically and compared its performance with that of a typical flat-plate collector in the present study. Water and copper nanoparticles-is used as the heat transfer medium. A two-dimensional heat transfer analysis is developed in which direct sunlight is incident on a thin flowing film of nanofluid. The effects of absorption and scattering within the nanofluid are accounted for. The governing partial differential equations are solved using finite element method with Galerkin's weighted residual technique. It is observed that the Prandtl number variation increases the rate of heat loss, collector efficiency and entropy generation for nanofluid more than that of pure water. According to the results obtained from this study, under similar operating conditions, the efficiency of a DASC is found to be higher than that of a flat-plate collector. Generally a DASC performs better than a flat-plate collector, however, much better designed flat-plate collectors might be able to match or outperform a nanofluid based DASC under certain conditions.