Numerical study on the heat release capacity of the active-passive phase change wall affected by ventilation velocity

The north wall of Chinese solar greenhouses (CSGs) plays an important role in maintaining their indoor thermal environment without additional heating during the wintertime. To enhance the heat storage/release capacity of the CSG wall and further improve the indoor thermal environment, an active-passive phase change thermal storage wall system has been developed in this study. The system was composed of 5 concentrating solar air collectors (CSACs), 6 tanks that were embedded in the north wall of the CSG and filled by phase change material (PCM), tubes linking the tanks and the CSACs and a centrifugal fan with variable-frequency drive (VFD). During the daytime, the solar energy was collected by the CSACs and stored in the tanks, whereas during the nighttime, the stored energy was released into the indoor environment of the CSG through a passive heat mode of the north wall or an active heat mode of the system. Then, a numerical model of the active-passive phase change thermal storage wall system has been developed. The simulation results were validated by the experimental data with the maximum relative error and average relative error being 5.6% and 3.9%, respectively. Furthermore, the heat release capacity characteristics in three cases with the air velocities of 2 m/s (Case A), 3 m/s (Case B) and 4 m/s (Case C) at indoor outlet for the active heat mode and a passive heating case (Case D) were chosen as the control groups for study. In the proposed wall, the heat release capacity of ventilation increased and that of inner surface of the wall declined with an increasing ventilation velocity. The total heat release capacities of the cases A, B and C were 38.12 MJ, 40.26 MJ, 42.00 MJ, respectively, higher than that of the case D (33.76 MJ). On the other hand, the calculated temperature distribution indicated that there was no thermal-stable layer within depth of the 360 mm in the wall due to an apparent temperature variation of the PCM layer by ventilation. These results suggested that the proposed system could effectively promote the heat storage/release capacity of the middle layer of the wall.