Numerical investigation of the optimization of phase change thermal storage units with air as heat transfer fluid

Abstract Phase change thermal storage units (TSUs) that use air as heat transfer fluid (HTF) are in huge demand in the fields of building heating, solar energy utilization, and flue gas heat recovery. Researchers have developed various air-based TSUs and used different methods to optimize their structure. In the current study, the structural parameters of a novel TSU with air as the HTF are optimized using a comprehensive method that combines overall thermal resistance analysis and numerical simulation. The influence of different HTF injection methods on the natural convection of a phase change material (PCM) and the effects of different structural parameters on the TSU's charge performance were investigated. Results indicated that the heat transfer thermal resistance of the developed TSU is mostly concentrated in the PCM side. When the depth direction of the flat tube was perpendicular to the direction of gravity and the HTF adopted side injection, the phase change completion time was shortened by 20.59% compared with that in the original TSU. Thermal resistance balance was achieved in the air and PCM sides when the rib height, pitch, and thickness of the flat tube were 0.03, 0.0232, and 0.00025 m, respectively. At this moment, the compact factor and effectiveness of the novel TSU were 1.63 times and 10.52% higher than those of the original TSU, respectively.