Thermal performance experiment on air convection heat storage wall with cavity in Chinese solar greenhouse

Abstract: Air convection wall, which was developed in recent years, is a new mode of Chinese solar greenhouse (CSG) wall. Two rows of vent holes, located in the upper and the lower part of the wall, make air exchange available between the environment in CSG and the hollow layer of the wall. During the daytime, the indoor temperature in CSG rises and keeps a higher level than the air temperature inside the hollow wall. The hot air enters the hollow layer through the upper vent holes and the cold air inside the wall enters the CSG through the lower vent holes with the increasing of temperature difference between inside and outside of the hollow wall. Hence, it forms a natural air convection. Then the heat can be transfered and stored into the wall from the flowing air during the daytime. Similarly, there is a different air convection with a reverse air flow at night, which transfers heat from the hollow wall into the flowing air and releases the stored heat into the CSG environment through the vent holes. So the heat storage and release properties of the wall are strengthened by the air convection between the CSG environment and the wall cavity. This study aims to investigate the characteristics of heat storage and release of the air convection hollow wall as well as its effects on the thermal environment in CSG. A comparison experiment was carried out in Tongzhou District, Beijing City. The tested CSG was seperated to control (without air convection holes) and test (with air convection holes) areas. The measurements included the temperature distribution inside the wall and its daily variation, the heat storage and release properties of the wall as well as the temperature variation during the day and night in the CSG. The environmental parameters were also investigated, like the temperature of wall and environment, and the surface heat flux of the wall. Additionally, the wind velocity and wet and dry bulb temperature of the vent holes were recorded to analyze the changes of air convection and heat exchange amount. The results showed that the temperature distribution of air convection wall was different from non-convection wall. Higher average temperature and magnified daily temperature variation inside the air convection wall indicated that the area of the wall (depth or volume) involved in heat storage and release was expanded under air convection conditions. The wind velocity showed that there was a good air convection and the speed of wind was corresponding to the temperature difference between the CSG and the hollow layer of the wall. The best convection performance appeared at noon with 0.6 m/s maximum wind velocity and 32.22 kW largest heat exchange amount. Compared with non-convection hollow wall, air convection wall increased the capacity of heat storage by 15.1% during the day and the capacity of heat release by 14.7% at night. The hollow layer of the wall contributed 1/3 of the total heat storage and release and improved the lowest temperature by 2.2 ℃ at night, so the heating performance during the night was clearly improved. Moreover, a cooling effect during the day appeared with the 6.2 ℃ maximum temperature difference at noon. In conclusion, the study shows that the new structure of air convection hollow wall enhances the heat storage and release properties of the wall effectively and improves the lowest temperature environment at night in CSG.