Experimental analysis of thermal performance according to heat pipe working fluids for evacuated tube solar collector

The purpose of this study was to experimentally investigate the thermal performance and operating characteristics of various heat pipe working fluids in evacuated tube solar collectors. Heat pipe efficiency was evaluated by calculating the solar collector efficiency value when using four types of working fluids: water, ethanol, flutec-pp9 and methyl acetate under the same operating conditions on an indoor experiment apparatus. The experimental conditions were as follows: The inclination angle of the evacuated tube solar collectors were 20°, 40° and 60° from the normal basis. The mass flow rate into the manifold of the evacuated tubular solar collector was 0.3 kg/min. The heat flux on the collector surface was 870 W/m2. The results showed that, at an angle of incidence of 40°, the values of FRτα\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{F}}_{\text{R}} \left( {\tau \alpha } \right)$$\end{document} and FRUL\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{F}}_{\text{R}} {\text{U}}_{\text{L}}$$\end{document} with methyl acetate as the working fluid were 0.6572 and −2.0086, respectively, with water they were 0.6636 and −1.8457, respectively, ethanol they were 0.6147 and −0.6353, respectively, and with flutec-pp9 they were 0.525, and −3.2313, respectively.