Effect of phase change material tubes inclination angel on the performance of indirect hybrid thermal energy storage system: An experimental approach

•A modular indirect hybrid thermal energy storage tank is designed and investigated.•Temperature evolution of the PCM and the water in the tank are studied.•Thermal performance of the water tanks with and without PCM tubes are compared.•Effect of the PCM tubes inclination angel on the thermal performance is discussed. For an efficient use of indirect hybrid thermal energy storage, it is necessary to investigate the heat transfer characteristics of its charging and discharging processes. However, in previous studies, the inclination angel of phase change material (PCM) units is fixed. The influence of PCM units inclination angel on the thermal performance of the indirect hybrid thermal energy storage has not attracted much attention. To address this research gap, this study proposes a modular indirect hybrid thermal energy storage tank in which the PCM tubes inclination angel can be changed freely. The heat transfer characteristics of the tanks with horizontal, inclined, vertical PCM tubes, and without PCM tubes are studied by experiments. The experimental results show that the melting time of horizontal PCM tube is the shortest, which is 25% less than that of vertical PCM tube. However, the solidification time for all the three different inclination angel tubes is the same at 96 min. Compared to the tank without PCM tubes, the average overall heat transfer coefficient of the heat exchanger during charging/discharging processes for the tanks with PCM tubes are increased by 7.18%/7.58% (horizontal), 1.37%/10.05% (inclined), and 4.43%/8.96% (vertical), respectively. The maximum average thermal storage rate of 1.31 kW is observed in the tank with horizontal PCM tubes, which is an increase of 5.65% compared with the tank without PCM tubes. For the discharging process, the difference of the average thermal release rate among the four tanks is not noticeable. The results indicate that the PCM tubes inclination angel has significant impacts on the melting time of PCM, the heat transfer coefficient of the heat exchanger and average thermal storage rate of the tank. Optimizing the PCM tubes inclination angel is an effective way to improve the thermal storage rate of the indirect hybrid thermal energy storage system.