Thermal performance and design analysis of U-tube based vacuum tube solar collectors with and without phase change material for constant hot water generation

Solar water heaters are the most promising technology, and they can be effectively used for hot water generation in cold climatic conditions. The moto of this research is the design and development of two compact vacuum tube solar collectors (VTSCs): (i) modified copper finned U-tube based VTSC filled with PEG6000 as a phase change material (PCM). However, it integrates heat transmission and storage capabilities into a unit (ii) conventional U-tube inserted VTSC without PCM. The developed VTSC with PCM was operated under dual (simultaneous and mid-day charging) modes, and their performance was compared with the conventional system at various flow rates. This study explored the design analysis, working principle, and the outcome of fixed mass flow rates on the system's thermal output. The results display that the daily energy output of the developed collector with PCM is 81.54–89.74 % for simultaneous mode and 78.21–86.52 % for mid-day charging mode. Whereas the daily energy output of conventional solar collector without PCM was 69.31–75.54 % for set mass flow rates. The highest thermal efficiency was around 89.74 % for U-tube inserted VTSC with PCM (simultaneous mode) at a high flow rate (30 LPH). The global heat transfer coefficient (UL) was maximum at a low mass flow rate (10 LPH) for conventional system without PCM, followed by mid-day charging and simultaneously operated VTSC with PCM. After comparing both systems, it is found that using PCM in VTSC improves the daily energy efficiency and increases the operating time for 3–4 more hours than conventional system. This analysis revealed that the developed U-tube inserted VTSC with PCM can substantially supply the hot water demand for domestic applications constantly at night/off sunshine hours.