Investigation of a novel small-sized bifacial cavity PTC and comparison with conventional configurations

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In this study, a cavity and a full circular absorber small-sized parabolic trough collector (PTC) were compared regarding their optical and thermal behavior. First of all, the cavity receiver configuration was optimized regarding its optical performance by modifying the cavity aperture width and the distance between the focus of the parabola and the receiver’s center line. Moreover, the local concentration ratio and the local absorbed solar irradiation intensity at the receiver’s periphery were defined and calculated in each configuration. After that, both configurations were examined thermally under several different inlet operating temperatures (10–200 °C) considering water as the working medium. More specifically, the thermal efficiency and the overall heat losses were first calculated while the convective heat transfer coefficient was also determined and the Nusselt number as well as the friction factor were calculated and validated through theoretical models. In addition, an analytical method was developed in order to validate the simulation results and deviations lower than 1% have been found. The cavity receiver it was found to absorb 13% more solar irradiance and appeared 10.3% greater thermal efficiency on average compared to the full receiver one. Finally, the particular geometries were compared with an optimized conventional PTC configuration and it was found that the cavity PTC exceeds the last one in thermal performance by 10.35%. Also, the cavity PTC was finally optimized and compared to the already optimized conventional PTC and a mean enhancement of 12.16% was revealed. This enhancement should be taken into account in similar comparisons, since it is independent from the mass flow rate. Last but not least, a tracking error analysis was conducted and it was found that the tracking error does not affect significantly the comparison between the cavity and the conventional PTC. The design and the simulation of the examined models were conducted in Solidworks environment.