Thermal Performance Modeling of a Novel Dual-PCM Evacuated U-Tube Solar Collector

Abstract

Solar water heating (SWH) systems are a well-established and eco-friendly way to generate hot water for household or commercial applications as it utilizes energy from solar radiation which is readily available all year round. However, there are drawbacks to this technology especially during times where solar intensity is inconsistent. To overcome this drawback, a backup booster unit is often required to power the SWH system. Recent studies have shown that utilizing phase change material (PCM) as an energy storage medium in SWH systems can alleviate the effect the booster unit. Nevertheless, current SWH technology has the PCM-based storage unit designed as a separate medium away from the solar collectors. The novelty of this study lies in the design of a new SWH that combines heat transfer and storage both in a single unit as well as the study of PCM as a heat transfer fluid (HTF). The selected type of collector for this purpose is an evacuated tube solar collector (ETC) and the new design of a dual-PCM evacuated u-tube collector (EUTC) has been developed by applying a U-tube inside the collector which contains the HTF. The solid-liquid PCM, Tritriacontane paraffin (C33H68), was integrated inside the dual-PCM EUTC for direct heat storage on the system and delayed release of heat.

Thermal analysis was first conducted to investigate the appropriate PCM as a HTF, in which HITEC molten salt and erythritol (C4H10O4) was chosen. This study also analyzes PCM doped with nanoparticles to be used as HTFs directly integrated in the dual-PCM EUTC. Preliminary analysis showed that erythritol would be a better combination as a HTF in moderating operating temperature conditions (150◦C) of this system due to its high specific heat capacity in liquid form, as well as its unique sub-cooling behavior. In order to overcome the low thermal conductivity of erythritol and further enhance specific heat capacity, a weight concentration of 1% multi-walled carbon-nanotubes (MWCNT) is added. Additionally, to insure even distribution of MWCNT and consistent properties of the HTF, triethanolamine (TEA) is proposed to be incorporated as a dispersant. Thermal analysis tests show 12.4% enhancement of specific heat capacity of the proposed HTF mixture as well as nearly 5◦C depression of freezing onset temperature.

Subsequently, a computational fluid dynamics (CFD) modeling of a single U-tube ETC is performed using ANSYS Fluent in stagnation (on-demand) operation. A 3D model of the ETC is developed and the appropriate boundary conditions was applied. The thermal performance comparison of the dual-PCM EUTC with erythritol as a HTF vs commercially available heat pipe ETC has been done. Simulation results shows a maximum fin temperature difference up to 24◦C enhancement of the dual-PCM EUTC compared with heat pipe ETC. A comparison of several different types of HTFs was also investigated in the new dual-PCM EUTC system. From this analysis, it can be predicted that the erythritol+MWCNT+TEA provides more hot water to the demand side.