CFD modeling of an evacuated U-tube solar collector integrated with a novel heat transfer fluid

Solar water heating (SWH) system is an eco-friendly way to generate hot water for household/commercial applications. Recent studies have shown that utilizing phase change material (PCM) as an energy storage medium in SWH systems to alleviate the requirement of a booster unit when solar radiation is inconsistent. This study proposes the combination of heat transfer and storage, both in a single unit by applying a U-tube containing the novel PCM-based heat transfer fluid (HTF) inside the evacuated U-tube collector (EUTC). The selected HTF is erythritol due to its unique sub-cooling behavior. Additionally, a solid-liquid PCM, namely Tritriacontane paraffin, was integrated to provide direct heat storage and delayed release of heat. A 3D computational fluid dynamics (CFD) modeling of the PCM-based HTF EUTC is performed in stagnation (on-demand) operation. A performance comparison of heat pipe ETC (HPETC) and PCM-based HTF EUTC, as well as the effect of different HTFs towards the system's functionality were studied. The CFD results show a fin temperature enhancement up to 45% of the PCM-based HTF EUTC compared to commercially available HPETC. Moreover, the selection of erythritol + 1% MWCNT + Triethanolamine (TEA) as HTF shows superior heat transfer capability of the EUTC system, effectively providing higher water temperature to the demand side compared to traditional systems.