Flat-Plate Solar Collectors for Water Heating with Improved Heat Transfer for Application in Climatic Conditions of the Mediterranean Region

Abstract

The aim of this research project is to improve the thermal performance of passive flat plate solar collectors using a novel cost effective enhanced heat transfer technique. The project work focuses on the process of energy conversion from the collector to the working fluid. This is accomplished by employing an aluminium grid placed in the channels of a collector to induce a gradient of heat capacitance. This novel technique is tested both theoretically by means of simplistic designs using Computational Fluid Dynamics (CFD) and experimentally using two unglazed collectors. One collector has the aluminium net inserted in its channels and it is tested against an identical conventional collector in order to have a direct comparison at the same time. The obtained CFD data and the experimental findings are coupled and show a good agreement. All the obtained results are validated with the literature. The results both theoretical and experimental demonstrate an enhancement in the heat transfer coefficient by 9 % resulting to an increase in the output temperature of the working fluid in the collector with the metallic insertion. Other parameters such as the Nusselt and Raleigh numbers supported these findings. Three novel expressions that correlate the Nusselt and the Rayleigh number, for different heat fluxes, were developed using data from CFD and experimental results. These correlations can be applied on any flat plate collector with an aluminium grid in its pipes, in order to predict its performance. Furthermore an existing lumped parameters model that predicts the output temperature of a collector was simplified and improved.