Improvement of the thermal performance of a solar triple concentric-tube thermal energy storage unit using cascaded phase change materials

• The thermal performance of a cascaded triple concentric-tube storage unit combined with a flat-plate solar collector is investigated. • An optimization study of the lengths of the cascaded PCMs leading to an optimal charging of the storage unit is carried out. • The improvement of the thermal performance of the solar thermal energy storage system under the meteorological conditions of Marrakech city is assessed. • The use of cascaded PCM improves the melting process and leads to higher collection efficiency of the solar collector. In this study, the thermal performance of the storage unit of a solar domestic hot water (SDHW) system incorporating triple concentric-tube latent heat storage modules filled with cascaded phase change materials (cascaded PCMs) was numerically investigated. The triple concentric-tube storage modules were filled with three marks of phase change material (RT62HC, RT54HC and RT44HC) with melting points of 63 °C, 54 °C and 43 °C, respectively. The heat transfer fluid (HTF: water) was received solar thermal energy from the solar collector absorber and transferred it by forced convection to the cascaded PCMs which stored it as latent heat. A model based on the enthalpy method was elaborated to predict the combined heat transfer and phase change processes. The natural convection effect during phase change was taken into consideration through an effective thermal conductivity of liquid PCMs. The elaborated numerical model was verified and validated by comparing the predictions with the previously published experimental and numerical results. The thermal performance of the cascaded thermal energy storage tank was evaluated under the weather conditions of the month of July in Marrakesh city (Morocco), and compared with that for single-PCM storage tank during the charging process.