Study on the thermal storage performance of a new cascade structure phase change thermal storage tank

Due to the intermittent and fluctuating nature of solar energy, phase change thermal storage technology plays a crucial role in the field of solar thermal energy utilization. As a current research hotspot, the organic combination of a cascade setup and a thermal storage tank can significantly improve the performance of thermal storage. To this end, a new physical model of cascaded phase change thermal storage has been designed, and a corresponding numerical model of three-dimensional unsteady states has been established based on the conventional cascade structure. The equations were solved using the finite-difference method and the phase change material (PCM) melting process was simulated using MATLAB based on C language numerical programming and the simulation results were compared with literature for validation. The effects on the liquid phase fraction and heat storage capacity of the cascaded phase change storage tank were investigated for each level of PCM arrangement, the height-to-diameter ratio of the storage tank and the inlet temperature of the heat transfer fluid. The results show that compared to conventional cascade thermal storage tanks, the new cascade phase change thermal storage tank can decrease the thermal storage time by 33 % and increase the thermal storage rate by 42 %, optimizing the disadvantages of the conventional structure which significantly reduces the heat transfer rate in the late melting stage. When the arrangement of PCM in the tank is modified, the melting time is shorter for the solution that arranges the PCM in the order of decreasing melting temperature as the distance from the heat exchanger wall or from the inlet increases, and the complete melting time of the PCM is reduced by 28 % and 30 % respectively compared to other solutions, and the heat storage rate is increased by 35 % and 37 % respectively. The larger the height-to-diameter ratio, the faster the rate of heat transfer from the heat exchange pipe to the outermost side of the tank, resulting in a 19 % and 45 % increase in heat storage rate for the height-to-diameter ratio of 5.92 compared to 5.00 and 4.12, respectively. When the inlet temperature is increased from 85 °C to 95 °C, the melting rate is increased by 32 % due to the increased heat transfer temperature difference. The results presented in this paper can provide some theoretical reference for the optimal design of cascade phase change thermal storage units, etc.