Abstract
An experimental study was carried out to assess the thermal performance of a few evacuated tube solar collectors (ETSCs) for water heating. The thermal performance of two kinds of ETSC (heat pipe ETSC and direct-flow ETSC) was investigated using an indoor experimental apparatus in lab testing conditions with a solar simulator. Several experimental tests were carried out for the heat pipe ETSC system under different operating conditions, such as the solar intensity (300, 500, and 1000 W/m(2)) and the tilt angle (0 degrees, 30 degrees and 90 degrees) of the ETSC and the water flow rate (0.6, 1.2, and 2.4 LPM). Moreover, four configurations of direct-flow ETSC (U tube, double U tubes, coaxial tubes, and helical tube) were examined and compared to the conventional heat pipe ETSC. The results of the experiment proved that the ETSC system presents a great performance at higher solar irradiance and lower water flow rates, and the experiments indicated that with a 30 degrees tilt angle, the ETSC reaches the maximum thermal efficiency of 36%. Furthermore, compared to the conventional heat pipe ETSC and the other proposed configurations of direct-flow ETSCs, the helical tube-based ETSC has a better thermal efficiency, 69%, and can be considered a greater potential heat exchanger that can be integrated in ETSCs. To the best of our knowledge, it is the first time this helical tube type been integrated into the ETSC and tested under these conditions.