Effects of Aluminum Fin Thickness Coated with a Solar Paint on the Thermal Performance of Evacuated Tube Collector

In commercial evacuated tube collector (ETC) with double layers evacuated tube, a solar absorber is coated on the outer surface of the inner glass vacuum tube. In order to reduce cost of production and maintenance, our previous research work has been successfully initiated to place an aluminum fin inside the ETC to be a solar receiver together with clear double layers evacuated tube. Therefore, the objective of this present research is focused on the effect of aluminum (Al) fin thickness coated with solar paint on the thermal performance of ETC. Commercial Al fins with three different thicknesses (11, 13, and 24 µm) were considered in this study. An available solar paint (Thurmolax 250 selective black solar collector coating) was coated on the Al fin solar absorber using an air spray deposition technique with three different coating thicknesses. Spectral reflectance and thickness of the coatings were characterized using Ultraviolet-visible-near infrared (UV-Vis-NIR) spectrophotometer at the wavelengths 300-2500 nm and a Mini Test 730 equipment respectively. The solar absorptance (α) was calculated based on the relationship of observed reflectivity (R), and the solar spectral irradiance of AM 1.5 in the whole wavelength range of the solar spectrum. The collector thermal efficiency (η) of ETC with different thicknesses of Al fin coated with solar paint as solar receiver was evaluated, following a standard ISO 9806-1. As a result, it is found that the α of all the thicknesses of solar paints considered was identical (α=0.94), indicating that the thickness of solar paint was an insignificant effect on the α. The η, heat removal factor ( FR), and overall heat loss coefficient (UL ) were calculated for different Al fin thicknesses with 1x solar paint coating thickness and were found to be increasing with decreased Al fin solar absorber thickness. The results revealed that the FR was predominant for the increase in the η with the reduction of Al fin solar absorber thickness. It can be concluded that the thinner Al fin (11µm) with single layer solar paint coating is suggested to be used in ETC due to its relatively higher η and FR, including light weight and low cost.