(345c) Mathematical Modelling of an Enhanced Volumetric Solar Receiver Based on Partially Reflective Surfaces with a Discussion on the Volumetric Effect Criteria

One of the remaining challenges of point focusing concentrating solar power systems is the realization of a true volumetric receiver, one whose entire volume is utilized for the absorption of irradiance. Current state-of-the-art receivers (e.g., HiTRec-II and SolAir-200) have not demonstrated the volumetric effect, because of low radiation penetration within the absorber. Earlier works have noted that radiation penetration can be improved by increasing porosity (void fraction), but at the cost of reducing the convective heat transfer area. More recent works have succeeded at improving radiation penetration in volumetric absorbers by axially grading the porosity of the structure, but those designs are complex and share the issue of manufacturability. Nevertheless, the improvements are notable and justify the pursuit of true volumetric receivers. This work discusses the conceptual design and numerical evaluation of a true volumetric receiver achieved by applying different reflectivity distributions to the irradiated surfaces to improve radiation penetration.

The square honeycomb receiver structure was reduced to a single channel to allow for detailed modelling of radiative phenomena. Monte Carlo ray tracing was used to model external irradiance and the conventional direct integration approach was used to model mutual irradiance. This radiative model was coupled with a 3-dimensional heat transfer model and a laminar flow model for a complete description of the problem. Furthermore, the relationship between the axial reflectivity distribution and relevant design parameters like porosity and residence time are explored via parametric sweeps, with solar-to-thermal efficiency exit gas temperature and the volumetric effect ratio as the monitored responses. This work was completed using COMSOL Multiphysics®.

The base case parametric study showed that the optimal parameters for a Silicon Carbide uniform reflectivity receiver are those of the HiTRec-II. Both varied reflectivity receiver cases considered exhibited an improvement in performance parameters for the same average emissivity of the base case. The best performance was achieved by a wall-varied reflectivity receiver, where every two walls were assigned a certain emissivity based on the amount of radiation they intersect. This receiver design is expected to achieve an increase of 5.2%, 6.1% and 8.2% in the exit gas temperature, thermal efficiency and volumetric effect, respectively, compared to the HiTRec-II.