Thermal Storage Characteristics of Honeycomb Phase Change Unit with Different Fluid Parameters

Phase change thermal storage tanks are important components of clean energy heating systems, and their charging performance depends strongly on storage-unit geometry and heat transfer fluid (HTF) conditions. This study develops a two-dimensional numerical model of a hexagonal honeycomb phase change storage unit and investigates the effects of cell number, HTF inlet temperature, inlet velocity, and fluid type under an approximately constant PCM inventory. Compared with the 4-cell configuration, the 9-, 16-, and 25-cell units reduce the times required to reach an average liquid fraction of 0.8 by 58.3%, 73.9%, and 83.5%, respectively. At 9000 s, the total stored heat of the 25-cell unit is 106.0% higher than that of the 4-cell unit. Increasing the inlet temperature from 338 K to 343 K and 348 K reduces the complete melting time by 27.4% and 42.5% and increases the total stored heat by 14.6% and 26.9%, respectively. An inlet velocity of 0.01 m/s provides the best thermal charging performance, while further increases cause slight deterioration. Water performs best, although differences among the three HTFs remain small. These results provide a quantitative basis for honeycomb PCM storage-unit design.