Pore-scale evaluation on a volumetric solar receiver with different optical property control strategies

Pore-scale flow and heat transfer model is built for a volumetric solar receiver subjected to highly concentrated irradiation, in order to provide insight into the detailed thermal and hydrodynamic performance. Direct three-dimensional simulations on the coupled radiation-convection-conduction are performed based on the Weaire-Phelan structure which is used to represent the open-cell ceramic foam absorber. The energy conversion characteristics with two ceramic materials (SiC and Al2O3) are firstly predicted and compared. And then several cases focused on the control approaches for the optical property (absorption/reflection/emission) of the solar absorber are introduced and investigated, including gradient surface absorptivity, spectral selectivity, and combination design of semi-transparent and opaque configuration. The results show that a moderate volumetric effect can be achieved at very low inlet fluid velocity for both ceramic materials, however presenting low conversion efficiency. Increasing the inlet velocity could enlarge the thermal non-equilibrium between solid and fluid phases and lower the outlet temperature. SiC absorber shows improved performance compared to Al2O3 absorber, and the efficiency changes obviously with the inlet velocity, while a variation of 24% is found. The optical property of the absorber front region significantly affects the overall performance. Decreasing absorptivity design and spectral selective design both show positive impact, especially for the Al2O3 absorber with an increment by 33% in efficiency relative to the basic one. Among the different strategies, spectral selective improvement shows the best effectiveness. Besides, adding porous fused silica as the front absorber layer can effectively move the high-temperature area inward at a cost of small decrement in efficiency, herein, honeycomb structure shows preponderance compared to the foam silica.