Using CFD and ray tracing to estimate the heat losses of a tubular cavity dish receiver for different inclination angles

Abstract

The process of obtaining an accurate estimate of the heat losses of a tubular cavity receiver absorbing concentrated solar energy from a parabolic dish at various inclination angles and wind speeds is described. Computational fluid dynamics (CFD) was used to simulate the conjugate heat transfer of the absorbed solar radiation to the heat transfer fluid while considering thermal radiation as well as forced and natural convective heat losses. Validation is performed against an experimental heating test at full-scale using heated air and measured wind conditions. On-sun conditions were modelled using the ray-tracing software, SolTrace, adapted for complex geometry receivers using ANSYS mesher and user coding. A 200 million ray result was found to be ray and mesh independent for a meshed receiver surface containing 30 000 elements. The SolTrace heat flux distribution was implemented as a volumetric source in ANSYS Fluent employing user-defined functions. The losses due to thermal radiation out of the cavity, and due to natural convection (using the buoyancy-driven mechanism afforded by gravity and the ideal gas formulation) and forced convection (due to the atmospheric wind) are presented. For the dish considered, 40–50% of the absorbed solar power was transferred to the heat transfer fluid for dish orientations from −45° to 45°, and wind speeds between 0.5 m/s and 4 m/s. This variation was mainly due to a variation in convective heat losses, with thermal radiative heat losses remaining constant at about 30%. The Nusselt numbers from the CFD simulations are compared against correlations from literature.