On-demand operation of a compact solar water heater based on U-pipe evacuated tube solar collector combined with phase change material

Abstract

Abstract Space and weight requirements coupled with time delay between energy production and consumption represent great barriers toward further deployment of typical solar water heating systems in existing buildings with limited space. Therefore, a new compact U-pipe evacuated tube solar collector (ETC) integrated with paraffin wax (ALEX WAX 600) for energy storage is presented in the current study. The ALEX WAX 600 is an organic chemical-based phase change material (PCM) having an average melting temperature of 60 °C and a thermal conductivity of 0.21 W/m K. The key issue of the developed system is the elimination of entire systems components by storing the energy in the evacuated tube itself through the usage of paraffin wax. Due to the low thermal conductivity of paraffin wax, heat transfer plate (fin) with an area of 0.1251 m2 is integrated in the proposed system. The present study investigates the developed system under two configurations; un-finned and finned U-pipe evacuated tube solar collector side by side with a typical forced recirculation solar water heating system (FSWHS) under the same operation and weather conditions. The operation of the solar water heating systems is studied during the on-demand operation under a simultaneous operation and a real water consumption profile. The results clarify the favorable performance of the developed compact solar water over the typical FSWHS during different operation scenarios and weather conditions due to their low thermal inertia. Furthermore, the utilization of fin in the developed system has a substantial effect on improving the heat transfer characteristics of the PCM and enhances the overall system stability. During simultaneous operation tests, the total effective energy discharged for the un-finned system is higher than FSWHS by 35.8% under clear day weather conditions. However, the finned system is higher than FSWHS by 47.7%. The simultaneous long-term predictions based on regression modeling show that the average annual efficiency is 71.8%, 85.7% and 40.5% for the un-finned, finned and FSWHS systems, respectively. During real water consumption profile tests, the daily system efficiency is, 33%, 26% and 20% for the un-finned, finned and FSWHS systems.