A novel thermal energy storage unit (TESU) that can store the heat from lower solar radiation during daytime and preheat air in winter during nighttime for areas with poor solar resources is proposed. To achieve an efficient operation of this TESU for poor solar conditions, this study proposed a structure that innovatively combines tube bundle and spiral tube heat exchangers to enhance heat transfer with paraffin wax as the phase change material (PCM). The thermal performance of the TESU is then evaluated by experimental measurements and a three-dimensional Computational Fluid Dynamics (CFD) model. The numerical modelling provides details of the solidification behavior of the PCM and the two heat transfer processes - thermal heat extracted from PCM and the effective discharging process. Results show that the pure conduction model can suitably describe the solidification process of PCM. The possible maximum amount of heat extraction of 4.68 MJ can be achieved during the discharging process in 20h. Moreover, an air outlet temperature of 11.5 °C is considered the lowest effective temperature. The effective heat extraction efficiency is about 89.74% till the lowest effective temperature is reached.
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