An array of coiled absorber tubes for solar trough concentrators operating with air at 600 °C and above

Abstract

An entirely novel solar receiver design for solar trough concentrators is proposed using air as heat transfer fluid at operating temperatures exceeding 600°C. It consists of an array of helically coiled absorber tubes contained side-by-side within an insulated groove having a rectangular windowed opening. Secondary concentrating optics are incorporated to boost the geometric concentration ratio to 97×. The multiple absorber tubes are connected via two axial pipes serving as feeding and collecting manifolds. The steady-state energy conservation equation coupling radiation, convection, and conduction is formulated and solved numerically using the finite volume technique. The solar flux distribution incident at each absorber tube is determined by Monte Carlo ray-tracing using spectrally and directionally dependent optical properties. Thermal radiative heat exchange is analyzed using the gray-band approximated radiosity method for an enclosure with a selective window. Model validation is accomplished by comparison to on-sun experiments with a 1m-long solar receiver prototype composed of 7 absorber tubes, mounted on a 4.85m-aperture solar trough concentrator. Feeding rates in the range of 5–20ln/min to each absorber tube led to air outlet temperatures of 621–449°C and a peak receiver efficiency of 64%.