Investigation of thermal performance and charging/discharging characteristics of finned coaxial evacuated tube collector filled with phase change material

Abstract

Present numerical model simulates the charging (melting), discharging (solidification) and heat transfer phenomenon of a phase change material (PCM) filled in the finned type coaxial evacuated tube solar collector. Thermal performance of the proposed evacuated tube has been predicted with Paraffin wax as the PCM. Solar radiation data measured on March 13, 2022 at Srinagar (Garhwal), Uttarakhand, India (latitude 30.2247° N and longitude 78.7986° E) has been applied as heat flux (450–965 W/m2) on wall of the evacuated tube. Heat transfer fluid (HTF) at the varying inlet temperature flows through the tube and absorbs heat, consequently its temperature increases. Results predict that during the charging process, 90 % of the PCM was melted by 11:30 AM for 0.0063 kg/s mass flow of the HTF having inlet temperature of 303 K. For the same operating conditions, maximum temperature rise achieved by HTF is ≈ 30 K at 14:00 PM. In the absence of solar radiation, discharging process (during 18:00 to 23:00 PM) of the PCM occurs that liberates the heat which is carried by HTF. Hence, heating of the HTF is continued during this period, however, outlet temperature continuously drops as night progresses. For the considered inlet temperature of the HTF = 303 K, maximum and minimum outlet temperatures were found to be 316.87 K and 307.55 K respectively. Furthermore, mass flow rate of the HTF significantly affects the melting rate of the PCM. At the mass flow rate of 0.0063 kg/s, 99.36 % of the PCM was melted by 12:30 PM. Whereas, till 14:00 PM, only 91.68 % and 70.54 % of the PCM melts when mass flow rate increases to 0.016 kg/s and 0.026 kg/s respectively. Findings also confirm that by increasing the HTF inlet temperature from 288 K to 308 K, melting period of the PCM is reduced by 40 %. Hence, it may be concluded that varying intensity of the solar radiation has significant effect on the performance of the evacuated solar tube. Moreover, the performance also depends on various other operating parameters.