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The use of air as heat transfer fluid (HTF) in solar parabolic trough collectors at operating temperatures up to 600 °C is investigated for electricity generation and high-temperature process heat. A novel solar receiver design that addresses the low density and thermal conductivity of gaseous HTFs is developed. 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 proof of concept is delivered by on-sun experiments with a 1 m-long solar receiver prototype composed of 7 absorber tubes, mounted on a 4.85 m-aperture solar trough concentrator. Feeding rates in the range of 5 – 20 ln/min to each absorber tube led to air outlet temperatures of 621 – 449 °C and a peak receiver efficiency of 64%. 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. Model validation is accomplished by comparison to experimental results. The validated heat transfer model is further applied to a parameter study of the array of coiled absorber tubes. Numerical simulations are performed for variable solar direct normal irradiance (DNI) and incidence angle, and the annual mean solar collector efficiencies are calculated. For a typical yearly DNI of 2400 kWh/m2 and air outlet temperatures in the range 420 – 660 °C, the maximum attainable annual mean DNI-to-thermal efficiency varies from 0.49 to 0.37. Industrial scale demonstration is realized with a 212 m-long parabolic trough concentrator and associated solar receiver designed for air operating

Concentrating solar trough collectors based on air as heat transfer fluid for electricity generation and high-temperature process heat