Temperature variation, heat storage and heat release characteristics of soil wall in solar greenhouse

Abstract: The solar greenhouse of soil wall, which has the advantages of good heat storage and low construction cost, is widely used in China. At present, the study on the heat storage and heat release performance of soil wall is an important hot spot, which can provide the theoretical basis for the simplification and thickness optimization of soil wall. In the paper, the sunken soil wall greenhouse serves as the research object in Taian, Shandong Province, China. The length is 50 m and the width is 10 m. The top and bottom thicknesses of the soil wall are 1.47 and 3.0 m, respectively. In order to get a better analysis of the heat storage and heat release performance of soil wall, the five testing layers of temperature were arranged in the north wall of greenhouse. The testing period was from Dec. 1st, 2014 to Jan. 30th, 2015. The outdoor weather conditions, indoor air temperature and interior wall temperature were monitored during the testing period. The variation regularity of the indoor and interior wall temperature was analyzed according to the monitoring data in the coldest season (from Dec. 20th, 2014 to Jan. 19th, 2015). The results showed that the testing point temperature of wall surface and interior wall (0.1-0.6 m) presented periodicity variation regularity with the indoor temperature variation in greenhouse, and temperature fluctuation amplitude reduced gradually, at the same time the phase was lags obviously along the wall thickness direction. Moreover, it showed that the interior wall (0-0.6 m) could be considered as the heat storage and heat release body in the soil wall. The heat storage body in the soil wall had large temperature fluctuation, which could be used for storing heat during daytime and releasing heat into the solar greenhouse during night. Besides that, the temperature fluctuation of testing point behind 0.7 m tended to the stabilization, which was in the process of the steady-state heat transfer from the soil wall to outdoors and could be considered as temperature stabilization body in the soil wall. The average temperatures of the testing points of 0.7 m from the first layer to the fifth layer were 8, 10.45, 12.79, 15.19 and 17.42 ℃ respectively, and the testing points behind 0.8 m were also the same temperature change regularity. It showed that the equidistance testing point between interior wall surface and interior soil wall on each testing layer had the same regularity of temperature change, that was, increased regularly with the thickening of the soil wall. In order to evaluate the performance of the heat storage and the heat release of soil wall better, the weighted average values of heat storage of all five testing layers were calculated, which were then converted to the heat storage and heat release relative to unit wall area so as to calculate the corresponding heat storage and heat release of the 0.6 m soil wall during a whole day. On the basis of the heat storage and the heat release results, the average efficiency of heat release for the soil wall during the coldest season period was calculated. The result showed that the average efficiency of heat release was 43% for solar greenhouse of the soil wall, which could release 43% of the heat storage of soil wall during the day to improve the thermal environment of the greenhouse nighttime. The study on heat storage calculation, heat release calculation and efficiency evaluation of average heat release will not only provide guidance for structural optimization and heat load calculation of greenhouse, but also provide theoretical basis for the simplification technology of soil wall greenhouse in different region of china.