Thermal analysis of a cascaded latent thermal storage tank in a solar domestic water heating system

To enhance the thermal characteristics of a solar collector storage system, this study investigates the performance of a rectangular thermal energy storage (TES) tank by incorporating cascade phase change materials (cascade-PCMs). Three different commercially available PCMs (RT44HC, RT54HC, and RT62HC) with distinct melting temperatures are employed. These cascadePCMs are utilized as slabs in vertical rectangular modules, which are integrated into the water TES tank. The heat transfer fluid (HTF) in the flat-plate solar collector captures solar energy and transfers it to the TES tank, where it is stored as latent thermal energy. A two-dimensional (2D) numerical model, utilizing the enthalpy-porosity approach and conservation equations, is developed to analyze the melting and heat transfer processes within the TES tank. The model is validated against previous experimental and numerical data. Performance evaluation of the cascade-PCMs tank is conducted during a 9-h charging period (from 8:00 AM to 5:00 PM) under Marrakesh weather conditions in Morocco. An optimization study is carried out to determine the optimal height of each cascade-PCM. The results suggest that an optimal design with heights of 23, 16, and 11 cm for RT44HC, RT54HC, and RT62HC respectively, offers improved melting and storage quality. The thermal characteristics of the TES tank incorporating cascade-PCMs are compared to those of a TES tank filled with a single phase change material (single-PCM) during the charging period. The findings indicate that the cascade-PCMs achieve complete melting, while the single-PCM only reaches a melting fraction of 0.903 at the end of the charging process. Furthermore, the optimal configuration of the cascade-PCMs storage tank exhibits slightly higher sensible and latent thermal energy storage capacity compared to the single-PCM tank. The combination of the solar collector with the cascade-PCMs storage tank results in a 3.47% higher average collection efficiency when compared to the single-PCM tank.