The optical efficiency of three different geometries of a small scale cavity receiver for concentrated solar applications

The demand for energy is continually increasing day after day; but at the same time, investigations around the world into sustainable sources of power are growing in number. Concentrated Solar Power (CSP) can act as an efficient low cost energy conversion system to produce electricity which could lead to reducing the continuous demand on conventional fossil fuels. Most of the literature concerning CSP concentrates on the heat losses and their relationship to the receivers’ geometries; where these receivers are evaluated according to their thermal efficiency. The majority of the literature has often neglected heat gain enhancement by the receivers’ geometries, which helps to increase the heat transfer to the working fluid. This work concentrates on the optical efficiency as well as the heat flux distribution of three different geometries. The cylindrical, conical and spherical geometries of a cavity receiver are considered with the objective of analysing their optical and thermal behaviour optically and thermally, using the ray tracing method and a Computational Fluid Dynamic (CFD) model. The results showed that the conical shape of the receiver gathered, as well as absorbed, a higher amount of reflected flux energy than the other shapes, with about 91% and 82% for 75% and 85% absorption ratios respectively. The cavity receiver shapes and their absorption ratio are key parameters which affect the focal point location; thereby there is an optimum distance for each design depending on these two parameters. The results of the simulated work are validated using the experimental work found in the literature. Overall, in order to evaluate the heat balance, 3-D thermal analysis was employed using Fluent 15 and the amount of heat losses for the three shapes was determined. It was observed that the conical shape receiver experienced a lower heat loss. To ensure more confidence in the results, the thermal outcomes were validated against experimental works in the literature and they demonstrated good agreement.