Design consideration of a filled layer U-pipe evacuated tubular solar collector with flat diffuse reflector: energy and exergy analysis

Abstract

A lack of research about critical design factors, notably the center-to-center tube spacing-to-tube diameter ratio (C/D) and the relationship between length and diameter, is conspicuously evident in the context of U-pipe evacuated tubular solar collectors (ETSCs) employing the filled-layer approach. Furthermore, using a U-pipe configuration in ETSCs coupled with a flat diffuse reflector (FDR) remains conspicuously absent from the scientific literature as a potential enhancement strategy. This study evaluates the influence of collector tube length, diameter and the C/D on output parameters, including T out , η En and η Ex . The findings reveal that while η En experiences a decrease with increasing C/D, the η Ex does not reach its minimum value when C/D was three due to the corresponding increase in outlet temperature. Furthermore, introducing the FDR into the filled U-pipe evacuated tubular solar collector enhances daily η En and η Ex by 12.91% and 0.65%, respectively. Moreover, a suggestion of the length and diameters of a filled type boosted with FDR is L = 2000 mm for Dg/Dp = 47/37 mm and L = 1800 mm when Dg/Dp = 58/47 mm, which has the maximum η Ex per day of 2.87% and 1.87%, respectively. However, if the objective is to maximise η En per day, lengths of L = 1450 mm for Dg/Dp = 47/37 mm or L = 1200 mm for Dg/Dp = 58/47 mm would be preferable. Abbreviations: ESC: evacuated solar collector; ETC: evacuated tubular collector; DUFET: filled-type evacuated tube with double U-tubes; ETSC: evacuated tubular solar collector; FDR: flat diffuse reflector; C/D: center-to-center tube spacing to the collector's tube diameter (cover tube); Dg/Dp : diameter of cover tube and diameter of absorber tube