Optimizing the thermal performance and heat losses of self-stored U-tube based ETC for residential water heating applications: A combined experimental and theoretical approach

Using evacuated tubes as solar collectors has immense potential for residential and industrial water heating. The primary goal of this research is to develop an annular copper fin integrated U-tube based evacuated tube collector (ETC), which is integrated with phase change material (PCM) as thermal battery backup. The annulus space of the evacuated tubes and U-tube heat exchangers was filled with stearic acid (PCM) and combined into a single unit. In contrast, another U-tube based ETC without PCM is treated as a reference system for performance comparison. Both collectors were operated under simultaneous charging mode (SCM) and midday charging mode (MCM) at five flow rates for complex environmental conditions of J&K, India. The outcomes of the study revealed that the thermal output of the collector with storage ranged between 73.85 and 87.84 % for SCM and 71.23-85.44 % for MCM for all flow rates. In contrast, the thermal performance of the reference system was obtained in the range of 60.37-74.87 % for the same flow rates. The performance enhancement ratio of the storage-based collector was found in the range of 9-21 % for SCM and 9-16 % for MCM compared to a reference system. The total heat loss coefficient (UL) was minimal for the designed system compared to the reference system. Also, the study investigates the effect of flow rate, input temperature of water and solar radiation on the outlet water temperature of a designed system employing fuzzy logic algorithm. The innovative system was concluded to act as a thermal enhancer, utilizing a PCM to store heat. This approach extends the duration of hot water production while mitigating temperature fluctuations in the water.