An investigation on the thermal performance of volumetric solar absorbers with lateral wall power losses

This study investigates the impact of lateral wall heat losses on the performance of volumetric solar absorbers. The effect of absorber dimensions, absorber geometrical parameters and thermophysical properties, and receiver operating conditions on the promotion of lateral heat losses is analyzed. Additionally, the effect of external heat transfer conditions at the absorber lateral wall for a non-insulated receiver is studied. Finally, the combined effects of insulation material and thickness and external heat transfer conditions on the receiver thermal performance are thoroughly investigated. The results show that heat conduction from the absorber solid phase to the absorber lateral wall is the most relevant heat transfer route from the absorber internal region to the lateral wall. A considerable decrease in the receiver thermal efficiency is observed if no insulation were applied — the minimum efficiency decrease in relation to the adiabatic efficiency is about 5.6 percentage points (predicted under natural convection and with black walls). Inappropriate insulation materials may inadvertently promote lateral wall heat losses. An applied insulation layer can effectively transport heat from downstream to upstream absorber regions, potentially aiding in the uniformization of the solid-phase absorber temperature field. The one-dimensional approximation for heat diffusion in the insulation layer yields reasonable results.