Stearic acid/expanded graphite as a composite phase change thermal energy storage material for tankless solar water heater

Tankless solar water heater (TSWH) integrated phase change materials (PCMs) is a promising field due to its low cost, low heat loss, and compact structure. To obtain a composite PCMs that suitable for tankless solar water heater application, the expanded graphite (EG) in different mass fraction (2 wt.%, 6 wt.%, and 10 wt.%) were added to stearic acid (SA) via melting impregnation. The thermophysical properties including phase change temperature, latent heat, and thermal conductivity of the SA/EG composites were characterized by DSC and thermal conductivity tests. The melting and freezing heats of SA/EG(6) composite were 163.5 J g(-1) and 167.3 J g(-1), respectively. The thermal conductivity of SA/EG(6) was as high as 9.6 times that of pure SA. Meanwhile, the infrared images suggested the addition of EG enhance the heat release capability of composites as well as improve the uniformity of heat transfer process of SA/EG(6). At last, a developed compact solar water heater was used to estimate the thermal performance of as-prepared composites. The experimental results shown that the melting time of SA/EG(6) was shorten 63.3% compared with that of the SA and the heat releasing rate of SA/EG(6) was the highest, suggesting the SA/EG(6) composites can be applied in TSWH system.