Numerical investigation of a triplex tube heat exchanger with phase change material: simultaneous charging and discharging

Thermal energy storage by phase change materials (PCMs) has received considerable attention in recent years. Its potential application is due to the issue of energy supply and demand time mismatch management. Thus, at the time of energy availability at supply side, it is stored in PCMs so as to be extracted later on when it is needed. However, in order to provide continuous operation, there might be some times that a system needs to be simultaneously charged and discharged. Most studies focused either on charging, discharging, or consecutive charging and discharging process, while limited work has been conducted for the case of simultaneous processes namely: simultaneous charging and discharging (SCD). This study presents the development of a numerical model to study the performance of a triplex tube heat exchanger equipped with a PCM under SCD. Governing equations were developed and numerically solved using ANSYS Fluent v16.2. Based on the solid-liquid interface evolution over time, the effect of natural convection on the heat transfer was investigated, and internal heating/external cooling mode was compared with the internal cooling/external heating for a triplex tube heat exchanger. According to the mode of heating and initial condition of the storage (fully melted or fully solidified), four different cases were compared based on the steady solid-liquid interface. The results indicated that the upward melted PCM motion had a great impact on the process. Interestingly, depending upon the initial PCM condition, different final solid-liquid interfaces were found. Finally, the pure conduction model was compared with combined conduction and natural convection to identify the dominant heat transfer mechanism. It was found that the former could be applied to the initially fully melted PCMs under SCD with small error, but for the initially solidified PCMs neglecting the natural convection would result in unacceptably large error.