A combined experimental and numerical study of heat losses in upward-facing helical cavity receivers

Cavity receivers improve the thermal efficiency of concentrated solar systems because the small aperture traps reflected sunlight and limits re-radiation and convective losses from the absorber. This study examines a small upward-facing helical cavity receiver, with an aperture diameter of 0.5 m and a cavity depth of 0.8 m, paired with a 60 m 2 Scheffler concentrator. In this fixed-focus arrangement the receiver is mounted separately from the tracking reflector, so the concentrator carries no receiver load, and the aperture faces upward. The coiled tubular absorber offers a large heat-transfer surface and a long fluid residence time, but the upward-facing aperture is exposed to buoyancy-driven convective loss that existing correlations do not describe. On-sun tests and three-dimensional conjugate computational fluid dynamics (CFD) simulations were used to quantify the effects of tilt angle ( 0 ∘ to − 90 ∘ ), inlet fluid temperature (320 to 550 K), and mass flow rate (0.05 to 2.0 kg/s). Thermal efficiency ranged from 22% to 84%, depending strongly on operating conditions. The CFD model reproduced the measured losses within the experimental uncertainty and resolved the recirculation and separation regions that govern convective loss. From the numerical dataset, a Nusselt number correlation was developed for natural convection in quiescent air, valid for Rayleigh numbers from 1 × 10 8 to 14 × 10 8 based on the aperture diameter, and was validated within ± 30 % against the experimental data. Wind skirts and 50 mm mineral wool insulation are expected to reduce losses, although their benefit was not quantified within the present test matrix. The results provide validated insight into cavity loss mechanisms and design guidance for Scheffler-based helical cavity receivers.