Numerical simulation study on discharging process of the direct-contact phase change energy storage system

• Discharging process of the direct-contact TES container are studied experimentally.
• Direct-contact solidification process are modeled and simulated by CFD.
• Effects of HTO flow rate and inlet temperature on solidification rate of PCM are clarified.

The mobilized thermal energy storage system (M-TES) has been demonstrated as a promising technology to supply heat using waste heat in industries to distributed users, where heat discharging determines whether M-TES system can satisfy the required heating rate. The objective of this work is to investigate the solidification mechanism of phase change materials (PCM) for heat discharging in a direct-contact thermal energy storage (TES) container for M-TES. A 2-dimensional (2D) numerical simulation model of the TES tank is developed in ANSYS FLUENT, and validated with the experimental measurement. Effects of flow rate and inlet temperature of heat transfer oil (HTO) were studied. Results show that (a) the discharging process includes the formation of solidified PCM followed by the sinking of solidified PCM; (b) the discharging time of M-TES can be reduced by increasing the flow rate of heat transfer oil. When the flow rate is increased from 0.46 m3/h to 0.92 m3/h, the solidified PCM is increased from 25 vol.% to 90 vol.% within 30 min; (c) the discharging time can be reduced by decreasing the inlet temperature of HTO. While the inlet temperature is reduced from 50 °C to 30 °C, the solidified PCM is increased from 60 vol.% to 90 vol.% within 30 min. This work provides engineering insights for the rational design of discharging process for M-TES system.