Modelling of radiative and convective heat transfer in an open cavity volumetric receiver for a 50-MWth beam-down integrated receiver-storage concentrating solar thermal system

This paper concerns solar-to-thermal energy conversion processes in an open cavity volumetric receiver for a 50-MWth integrated beam-down receiver-storage concentrating solar thermal system. A multiphysical model was developed in a COMSOL Multiphysics 6.1 environment. The model was validated against experimental and modelling data from the literature. The model incorporates specific solar irradiation profiles tailored to the beam-down optical system as the boundary condition, couples heat transfer with surface radiation and porous media radiation transport, and accounts for buoyancy effects on air convection within the cavity. The results reveal a significant reduction in the nonuniformity of net radiative heat flux distribution at cavity bottom, compared to the concentrated solar irradiation profile from the beam-down optics. The solar radiation is founded to absorb heat within the surface layer of the ceramic foam, with heat transfer in the porous media body dominated by volumetric convection. The buoyancy effects may cause air to escape from the cavity opening leading to non-negligible convective heat losses. The thermal performance is assessed by varying the concentration ratio, air flow rate, matrix thermal conductivity, and porosity of the ceramic foam. Under baseline conditions, the outlet air temperature could reach up to 1441 K, with solar-to-thermal and solar-to-exergy efficiencies of 39 % and 31 %, respectively. Heat losses are attributed to optical loss (33 %), cavity re-radiation (20 %), convection (6 %), and conduction (1 %). Performance could be further improved through increasing either the air flow rate or the concentration ratio, whereas the changes in porosity and thermal conductivity have a minor effect.