A Study on the Performances of Solar Air Collectors Having a Hemispherical Dimple on the Absorber Plate

Abstract

In order to increase the efficiency of solar air collectors, a new variant with a protrusion is proposed in this study, and its performances are analyzed from two points of view, namely, in terms of optics and thermodynamics aspects. By comparing and analyzing the light paths of the protrusion and the dimple, it can be concluded that when sunlight shines on the dimple, it is reflected and absorbed multiple times, whereas for the sunlight shining on the protrusion, there is no secondary reflection or absorption of light. When the lighting area and the properties of the surfaces are the same, the absorption rate of the dimple is 10.3 percentage points higher than that of the protrusion. In the range of Reynolds number from 3000 to 11000, numerical simulations about the effects of the relative height (e/Dh = 0.033-0.1) and relative spacing (p/e = 4.5-8.5) of protrusions on air heat transfer and flow resistance show that, in terms of comprehensive evaluation coefficient (PF), the best relative height is 0.085, when the relative spacing is 5. A correlation of Nu and f with Re, e/Dh and p/e is obtained by linear regression of the results, in order to provide a useful reference for the design and optimization of this kind of solar air collector.