Thermal performance test of solidified sand heat storage wall in Chinese solar greenhouse

Abstract: Solar greenhouse has been widely used in the north of China. Chinese solar greenhouse is made of north wall, east wall, west wall, front roof, back roof and heat preservation quilt. The main materials of north wall in traditional Chinese solar greenhouses are soil and brick. However, in the northwest of China, there are many non-cultivated lands with many sands instead of soil resources. In this research, we designed a new kind of north wall (W1) which was made of 2 layers of porous bricks and contained the solidified sands. The solidified sand was in the middle of W1 and the porous bricks were at the outside layers of the solidified sands. The wall thickness was 1000 mm, with 760 mm thick solidified sands and 240 mm thick porous bricks. In order to store more heat into the wall, a new active storage wall (W2) was also developed based on the W1. Compared with the W1, a sand-air heat transfer system was added with 2 axial flow fans and 3-layer air passages, and the air conduits were 80 m long, which was developed to store more heat. The thermal environmental properties of the 2 newly designed greenhouse walls were evaluated in Wuhai (39°39′N, 106°47′E), Inner Mongolia Autonomous Region, China, which were also compared with the local solar greenhouses with porous bricks and EPS (W3). During clear days (for instance, from 9:00 on January 7th, 2016 to the next 9:00), the average daily air temperature in W1, W2, and W3 was 17.2, 20.5 and 16.9 ℃, respectively with an average outside temperature of -11.3 ℃. The average night air temperature in W1, W2, and W3 was 13.7, 17.0 and 12.8 ℃, respectively, indicating that W2 had the best heat storage ability. During cloudy days (for instance, from 9:00 on January 16th, 2016 to the next 9:00), the average night air temperature in W1, W2, and W3 was 10.6, 13.8 and 10.0 ℃, respectively, indicating that W2 also had the best heat storage ability. In order to have a better understanding of the thermal properties of the 3 greenhouse walls, we selected the indoor temperature data of consecutive 27 days from January 1st to January 27th, 2016. During this period, the minimum average air temperature in W1, W2 and W3 was 10.48, 10.51 and 9.57 ℃, respectively. The daily average air temperature was 14.97, 17.30 and 14.89 ℃. Solar greenhouse wall was an important factor for maintaining greenhouse heat balance. Wall heat storage ability was important for the greenhouse performances. The distributions and changes of temperature inside the wall reflected the heat exchange process between the wall and indoor air and had great impacts on the indoor temperature in greenhouses. The internal constant temperature region of the wall W1 was between 500 and 740 mm, and the thickness of the thermal storage body was about 500 mm. For the solidified sand body, the effective thermal storage thickness was up to 380 mm. For the W2, the internal constant temperature region ranged from 740 to 1 000 mm, and the effective thermal storage body thickness exceeded 740 mm, among which, the effective thermal storage body thickness of the solidified soil exceeded 620 mm. For the W3, the effective thermal storage body thickness and temperature fluctuations were relatively smaller and the thermal storage capacity was also the smallest. Under sunny days, for the W2, the effective thermal storage body included the 500 mm thick wall body and the inside of the air tubes and their outside up to 200 mm, indicating that active heat storage fan system could significantly improve the wall heat storage capacities in greenhouses. Our results indicate that the new designed greenhouse wall W2 has some important application values.