Abstract
Heat absorbers critically affect efficiency in a wide range of applications, such as solar power generation and industrial heat utilization. The present study experimentally compares the thermal performance of two conceptually different flat-plate heat absorbers, based on: a) a plate with straight rectangular, parallel channels and b) a serpentine tube-on-plate configuration with a series of unconventional, composite U-bends. This is a novel study as the devices and operating conditions are primarily intended for use in the receiver of a linear-focus, concentrating solar collector with Fresnel reflectors.Experiments were performed on an electrically heated laboratory-scale setup with a thermal oil as the heat transfer fluid. Measurements were taken under a wide range of operating conditions, representative of a medium-temperature concentrating solar thermal system. Steady-state efficiencies, heat removal factors and heat transfer coefficients were determined.For the absorber with rectangular channels, the steady-state thermal efficiency was found to be up to 18% higher than the serpentine-tube configuration, except for the upper range of operating temperatures, where the latter configuration exhibited a higher overall efficiency by up to 9%. The thermal–hydraulic behavior of the two absorbers was analyzed in terms of several physical effects such as the flow regime (which differed between the two configurations), non-uniform heating and buoyancy. For the serpentine-tube configuration, a new correlation was proposed, accounting for the transition and buoyancy effects present in the operating range considered.Given that the majority of flat receivers for solar thermal systems investigated so far have been characterized by low geometric (length-to-width) aspect ratios, the study of the present elongated geometries, indeed including an unconventional composite U-bend concept, aims to provide insight into the thermal–hydraulic behavior and to contribute to the efficient designs of receivers for linear-focus, concentrating solar collectors.
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