Investigation on heat transfer and phase transition in phase change material (PCM) balls and cold energy storage tank

The phase-change based energy storage provides an excellent solution for the mismatch of energy production and consumption. Cold energy storage tanks filled with PCM balls could be applied in energy-efficient air-conditioning systems. The main advantage lies in the reduction of energy storage space demand compared with water based cold energy storage. Clarifying the pattern of freezing process within the PCM balls is of critical importance for the design of energy storage tank. In the present work, experimental measurement is conducted to obtain the temperature variance and phase transition process of one PCM ball. With the aid of CFD simulation software, physical model of one PCM ball is established and well-validated with experimental data. The validated model is then used to simulate the thermal performance of cold energy storage tank filled with multiple PCM balls. Meanwhile, the influences of parameters involving in the diameter of the PCM balls (90, 90&60, and 60 mm), and the flow rate of chilled water in the tank are evaluated. The results indicated that higher freezing rate of PCM ball can be achieved by smaller diameter of the ball and faster flow rate of chilled water. Under the conditions set in this study, the optimal diameter of phase change balls is determined to be 90&60 mm. and the best inlet flow rate is 2.196 m/h. When PCM balls are concerned for cold storage systems, it is proposed to set the diameter carefully in terms of the capacity of cold energy (flow rate of chilled water) and the limit of freezing duration.