The promise and challenge of solar volumetric absorbers

This study investigates the potential performance of volumetric absorbers as a function of geometric and material properties, aiming to identify the best absorber design parameters and the highest efficiency that may be expected. A simplified one-dimensional model was used to represent a planar slab of ceramic foam absorber, with local thermal non-equilibrium and effective volumetric properties. Three approaches for modeling the radiative transfer were considered, and the S₄ discrete ordinates model was selected based on validation against a detailed Monte-Carlo simulation. The boundary conditions were investigated in detail. This model is simple enough for fast computation and parametric study, yet reasonably realistic to represent real absorbers. The results reveal several guidelines to improve the absorber performance. Optimization of geometry (porosity and characteristic pore diameter) is insufficient to reach high efficiency. A significant increase in convection heat transfer is required, beyond the normal behavior of ceramic foams. A reduction in the thermal conductivity of the absorber material is also needed to maintain the desired temperature distribution. Finally, spectral selectivity of the absorber material can also help to further increase the absorber efficiency, in contrast to the common opinion that it is effective only at low temperatures. With a combination of these measures, absorber efficiencies may be increased for example from around 70% to 90% for air heating to 1000°C under incident flux of 800kW/m².