Experimental Study on Improving Thermal Environment in Solar Greenhouse with Active–Passive Phase Change Thermal Storage Wall

Chinese solar greenhouse (CSG) is an energy-saving production building that is used to grow off-season crops. The north wall of CSG plays an important part to maintain the indoor thermal environment without additional heating during wintertime. To enhance the heat storage/release capacity of the greenhouse wall and improve its indoor thermal environment, an active–passive phase change thermal storage wall system has been developed in this study. This system is composed of 5 concentrating solar air collectors (CSACs), 6 tanks that are embedded in the north wall of the CSG and filled by phase change material (PCM), pipes for linking tanks and CSACs, and a centrifugal fan. During the daytime, the solar energy is collected by the CSACs and then sent into the wall coupled with tanks to store the thermal energy by PCM and increase the temperature of the middle layer. During the nighttime, the energy is released into the indoor environment of the greenhouse north wall passively. By analyzing the air temperature at the inlet and outlet of the pipes, the results showed that the energy of about 8.17–11.52 MJ was stored in the north wall except heat gain through the inner surface for a typical sunny day in winter. Performances of the proposed system were also compared to the ordinary solar greenhouse of the same structure and dimensions. The system enhanced the wall’s heat storage capacity, resulting in an average increase in the CSG’s indoor air temperature, wall surface temperature, soil surface temperature, and daily effective accumulative temperature by 0.86–1.26 °C, 2.03–2.7 °C, 1.36–1.88 °C, and 48.1–55.5%, respectively.